U.S. patent application number 09/757897 was filed with the patent office on 2001-05-31 for microelectronic connections with liquid conductive elements.
Invention is credited to Smith, John W..
Application Number | 20010001989 09/757897 |
Document ID | / |
Family ID | 24573495 |
Filed Date | 2001-05-31 |
United States Patent
Application |
20010001989 |
Kind Code |
A1 |
Smith, John W. |
May 31, 2001 |
Microelectronic connections with liquid conductive elements
Abstract
A method of making a microelectronic assembly includes providing
a first microelectronic element and a second microelectronic
element with confronting, spaced-apart surfaces defining a space
therebetween and providing one or more masses of a fusible
conductive material having a melting temperature below about
150.degree. C. in said space, whereby the fusible conductive masses
connect the first and second microelectronic elements to one
another. Next, a flowable material is introduced between the
confronting surfaces of the first and second microelectronic
elements and around the one or more fusible conductive masses and
the flowable material is then cured to provide a compliant layer
disposed between said confronting surfaces and intimately
surrounding each fusible conductive mass. The fusible conductive
masses are capable of electrically interconnecting the contacts on
microelectronic elements confronting one another and/or conducting
heat between confronting microelectronic elements.
Inventors: |
Smith, John W.; (Palo Alto,
CA) |
Correspondence
Address: |
LERNER, DAVID, LITTENBERG,
KRUMHOLZ & MENTLIK
600 SOUTH AVENUE WEST
WESTFIELD
NJ
07090
US
|
Family ID: |
24573495 |
Appl. No.: |
09/757897 |
Filed: |
January 10, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09757897 |
Jan 10, 2001 |
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08962693 |
Nov 3, 1997 |
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6202298 |
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08962693 |
Nov 3, 1997 |
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08641698 |
May 2, 1996 |
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5808874 |
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Current U.S.
Class: |
174/565 ;
257/E21.503; 257/E23.01; 257/E23.021; 257/E23.069; 29/835; 29/840;
29/841; 361/303 |
Current CPC
Class: |
H01L 2224/73204
20130101; H01L 2224/16225 20130101; H01L 2224/32225 20130101; H01L
2924/00 20130101; H01L 2224/16225 20130101; H01L 2924/01005
20130101; H01L 2224/73253 20130101; H01L 2924/01049 20130101; H01L
2924/15311 20130101; H01L 2924/3011 20130101; Y10T 29/49135
20150115; H01L 2224/13 20130101; H01L 23/49816 20130101; H01L
2924/01082 20130101; H01L 2224/32225 20130101; H01L 2924/01013
20130101; H01L 2224/73203 20130101; H01L 2924/01075 20130101; H01L
2924/014 20130101; H01L 2224/73253 20130101; H01L 2924/15174
20130101; H01L 2224/8192 20130101; H01L 2924/351 20130101; H01L
2924/00 20130101; H01L 2924/00 20130101; H01L 2924/00 20130101;
H01L 2224/13099 20130101; H01L 2224/13022 20130101; H01L 2224/32225
20130101; H01L 2924/01006 20130101; H01L 2924/01322 20130101; H01L
2924/16152 20130101; H01L 21/563 20130101; H01L 2224/13 20130101;
H01L 2924/01047 20130101; H01L 2224/73204 20130101; H01L 2224/16
20130101; H01L 2924/01079 20130101; H01L 2924/00013 20130101; H01L
2224/73204 20130101; H01L 2224/14181 20130101; H01L 2924/0103
20130101; H01L 2924/01033 20130101; H01L 2924/01087 20130101; H01L
2224/16225 20130101; H01L 2924/01022 20130101; H01L 2924/00013
20130101; H01L 2924/01074 20130101; H01L 2924/19105 20130101; Y10T
29/49126 20150115; H01L 2924/351 20130101; H01L 2224/0401 20130101;
H01L 24/10 20130101; H01L 2924/01042 20130101; H01L 24/13 20130101;
Y10T 29/49146 20150115; H01L 2224/17051 20130101; H01L 2924/3025
20130101; Y10T 29/49144 20150115; H01L 2924/01019 20130101; H01L
2224/02333 20130101; H01L 23/055 20130101; H01L 2224/32245
20130101; H01L 2924/01078 20130101; H01L 2924/01015 20130101; H01L
2924/01039 20130101; H01L 2924/15311 20130101; H01L 2924/16195
20130101; H01L 2924/19041 20130101; H01L 2924/01029 20130101; H01L
23/48 20130101; H01L 2924/16152 20130101 |
Class at
Publication: |
174/52.1 ;
29/841; 29/840; 29/835; 361/303 |
International
Class: |
H01G 004/005; H05K
003/30 |
Claims
What is claimed is:
1. A method of making a microelectronic assembly comprising the
steps of: (a) providing a first microelectronic element and a
second microelectronic element with confronting, spaced-apart
surfaces defining a space therebetween; (b) providing one or more
masses of a fusible conductive material having a melting
temperature below about 150.degree. C. in said space; then (c)
introducing a flowable material between/said confronting surfaces
of said first and second microelectronic elements and around said
one or more conductive masses; and (d) curing said flowable
material to form a compliant layer disposed between said
confronting surfaces and intimately surrounding each said
conductive mass.
2. A method as claimed in claim 1, wherein said one or more fusible
conductive masses are maintained in a substantially solid condition
during said introducing a flowable material and said curing
steps.
3. A method as claimed in claim 1, wherein said one or more fusible
conductive masses are maintained in a substantially liquid
condition during said introducing a flowable material step.
4. A method as claimed in claim 3, wherein said one or more
conductive masses are maintained in a substantially liquid
condition during said curing step.
5. A method as claimed in claim 1 wherein said first and second
microelectronic elements are circuit elements.
6. A method as claimed in claim 5, wherein said fusible conductive
masses are capable of conducting electrical signals and heat
between said first and second microelectronic elements.
7. A method as claimed in claim 1, wherein said first and second
microelectronic elements include opposing contacts on said
confronting surfaces, the step of providing one or more masses of a
fusible conductive material step including the step of disposing
said fusible conductive masses between said opposing contacts for
electrically interconnecting said first and second microelectronic
elements to one another.
8. A method as claimed in claim 7, further comprising the step of
providing a barrier layer between said contacts and said fusible
conductive masses disposed thereon.
9. A method as claimed in claim 8, wherein said barrier layer
includes polycrystalline silicon.
10. A method as claimed in claim 5, wherein said second
microelectronic element includes a flexible dielectric sheet having
an exterior surface facing away from said first microelectronic
element, said second microelectronic element including conductive
terminals accessible at said exterior surface, the method further
including the step of electrically connecting said terminals to
said first microelectronic element.
11. A method as claimed in claim 10, wherein said connecting step
includes the step of connecting said terminals to said first
microelectronic element through flexible leads extending between
said microelectronic elements.
12. A method as claimed in claim 11 wherein said microelectronic
elements have contacts on their confronting surfaces, said step of
electrically connecting said terminals to said first
microelectronic element including the step of positioning at least
some of said conductive masses between the contacts on such
confronting surfaces so that such masses electrically connect
contacts on said first and second microelectronic elements with one
another.
13. A method as claimed in claim 10, further comprising the step of
forcing said conductive terminals into substantially coplanar
disposition while maintaining said fusible conductive masses in an
at least partially molten condition.
14. A method as claimed in claim 11, wherein said fusible
conductive masses are maintained in a substantially solid condition
while said flowable material is introduced.
15. A method as claimed in claim 11, wherein said fusible
conductive masses are maintained in a substantially liquid
condition while said flowable material is introduced.
16. A method as claimed in claim 10, wherein said first
microelectronic element includes a plurality of semiconductor chips
and said connecting step includes the step of electrically
connecting said plurality of chips to conductive elements on to
said flexible dielectric sheet.
17. A method as claimed in claim 16 wherein said conductive
elements on said flexible dielectric sheet include conductive
traces and said connecting step is performed so that said traces
interconnect at least some of said chips with one another.
18. A method as claimed in claim 16, wherein said first
microelectronic element includes a plurality of semiconductor
chips, each said chip being aligned with a portion of said
dielectric sheet, the method further including the step of severing
said portions of said sheet and separating said chips from one
another to thereby form individual chip assemblies each including
one said chip and the portion of said dielectric sheet aligned
therewith.
19. A method as claimed in claim 18, wherein said first
microelectronic element includes a unitary wafer incorporating said
plurality of semiconductor chips, said step of separating said
chips from one another including the step of severing said
wafer.
20. A method as claimed in claim 1 wherein said first
microelectronic element includes one or more semiconductor chips
and said second microelectronic element includes a thermally
conductive package element, and wherein said conductive masses are
thermally conductive.
21. A method as claimed in claim 20 further comprising the step of
moving said one or more semiconductor chips relative to said
package element while said conductive masses are in a substantially
liquid condition.
22. A method as claimed in claim 1, wherein said first
microelectronic element has a front face and a rear face, said step
of providing said first and second element s being performed so
that said second microelectronic element confronts said front face
of said first microelectronic element and defines a front space
therebetween, said conductive masses being provided in said front
space, the method further comprising the steps of: providing a
third microelectronic element confronting said rear surface so that
said first microelectronic element and said third microelectronic
element define a rear space between said rear surface and said
third microelectronic element; disposing one or more fusible
conductive masses having a melting temperature below about
150.degree. C. in said rear space; and introducing a flowable
material into said rear space and around said conductive masses in
said rear space.
23. A method as claimed in claim 22, further comprising the step of
curing said flowable material in said rear space to form a rear
compliant layer between said third microelectronic element and said
first microelectronic element, said rear compliant layer intimately
surrounding said conductive masses in said rear space.
24. A method as claimed in claim 23 wherein said step of
introducing a flowable material into said rear space and said step
of introducing a flowable material between the confronting surfaces
of said first and second microelectronic elements are preformed by
introducing simultaneously using a single flowable material.
25. A method as claimed in claim 23 wherein said first
microelectronic element includes a semiconductor chip, said second
microelectronic element includes a circuit panel, and said third
microelectronic element includes a thermally conductive
element.
26. A method as claimed in claim 22 further comprising the step of
moving said second and third microelectronic elements toward one
another while said masses in said front and rear spaces are in a
molten condition to thereby compress material in said front and
rear spaces.
27. A method as claimed in claim 26 wherein said second
microelectronic element is a flexible dielectric sheet having an
exterior surface facing away from said first microelectronic
element, said second microelectronic element including conductive
terminals accessible at said exterior surface, the method further
including the step of electrically connecting said terminals to
said first microelectronic element.
28. A method as claimed in claim 1, wherein said fusible conductive
material has a melting temperature below about 125.degree.C.
29. A method as claimed in claim 1, wherein said fusible conductive
material has a melting temperature below about 65.degree.C.
30. A method as claimed in claim 1, wherein said fusible conductive
material has a melting temperature between about 25.degree.C. and
about 65.degree.C.
31. A method as claimed in claim 1, wherein said fusible conductive
material includes a metal.
32. A method as claimed in claim 1, wherein said fusible conductive
material includes a metal alloy.
33. A method as claimed in claim 1, further comprising the step of
applying a polymer coating to said one or masses of a fusible
conductive material before the introducing a flowable material
step.
34. A method as claimed in claim 33, wherein said polymer coating
includes polyparaxylene.
35. A method of making a microelectronic assembly comprising the
steps of: (a) providing a metallic plate; (b) juxtaposing said
metallic plate with a surface of a microelectronic element; (c)
providing one or more masses of a fusible conductive material so
that said conductive masses extend between a surface of said
metallic plate and a surface of said microelectronic element; (d)
injecting a flowable material between said metallic plate and said
microelectronic element and curing said flowable material to form a
compliant dielectric layer intimately surrounding said fusible
conductive masses; and (e) subdividing said metallic plate to form
separate portions connected to separate ones of said conductive
masses.
36. A method as claimed in claim 35, wherein said step of
subdividing said metallic plate is performed by etching said plate
after the curing step.
37. A method as claimed in claim 35, wherein said microelectronic
element includes an array of semiconductor chips.
38. A method as claimed in claim 37, wherein said microelectronic
element includes a wafer.
39. A method as claimed in claim 38, further comprising the step of
severing said wafer and said compliant layer after said step of
subdividing said metallic sheet to form individual units, each
including one or more chips.
40. A method as claimed in claim 35 wherein said step of providing
said conductive masses is performed by providing said masses at
predetermined locations on a surface of said metallic plate before
said plate is juxtaposed with said microelectronic element.
41. A method as claimed in claim 40, wherein said step of providing
said metallic plate with said conductive masses thereon includes
the steps of forming a layer on a first side of said metallic
plate, said layer having apertures therein and including a material
which is non-wettable by said conductive masses and exposing said
first side of said plate to said conductive masses while said
conductive masses are in molten condition so that drops of said
fusible conductive masses adhere to said metallic plate at said
apertures.
42. A method as claimed in claim 41, wherein the exposing step
includes the step of dipping said metallic plate into a bath of
said fusible conductive material.
43. A method as claimed in claim 42, further comprising the step of
covering a second side of said metallic plate opposite from said
first side thereof during the exposing step.
44. A microelectronic package comprising: (a) a first
microelectronic element having a front surface including contacts
and a rear surface, said first microelectronic element being
operable in a range of operating temperatures; (b) a package
element having a surface confronting the rear surface of said
microelectronic element; (c) a first layer of a compliant material
having top and bottom surfaces extending in lateral directions
between said confronting surfaces of said first microelectronic
element and said package element; (d) one or more masses of a
thermally conductive material dispersed in said first compliant
layer so that said thermally conductive masses are spaced apart
from one another in lateral directions, said thermally conductive
masses having a melting temperature within or below the range of
operating temperatures of said first microelectronic element, each
said thermally conductive mass being adjacent to said confronting
surfaces of said first microelectronic element and said package
element for transferring heat therebetween during operation of said
first microelectronic element; and (e) a second microelectronic
element overlying said front surface of said first microelectronic
element so that said first microelectronic element is disposed
between said package element and said second microelectronic
element, said second microelectronic element being electrically
connected to said first microelectronic element.
45. A package as claimed in claim 44, wherein said second
microelectronic element has a contact bearing surface confronting
the contact bearing front surface of said first microelectronic
element; , the package further comprising: a second layer of a
compliant material having top and bottom surfaces extending in
lateral directions between said confronting surfaces of said first
and second microelectronic elements; one or more masses of a
fusible electrically conductive material dispersed in said second
compliant layer so that said electrically conductive masses are
spaced apart from one another and extend between contacts of said
first and second microelectronic elements, said electrically
conductive masses having a melting temperature within or below said
range of operating temperatures.
46. A package as claimed in claim 45, wherein said first and second
compliant layers are contiguous with one another.
47. A package as claimed in claim 44, wherein said second
microelectronic element includes a flexible dielectric sheet having
a first surface confronting the contact bearing front surface of
said first microelectronic element and a second surface facing away
from said first microelectronic element, said flexible dielectric
sheet including conductive terminals accessible at said second
surface, the package further comprising flexible leads electrically
connected between said conductive terminals said contacts of said
first microelectronic element for electrically interconnecting said
first and second microelectronic elements.
48. A package as claimed in claim 47 further comprising: a second
layer of a compliant material having top and bottom surfaces
extending in lateral directions between said confronting surfaces
of said first and second microelectronic elements; one or more
masses of a thermally conductive fusible material dispersed in said
second compliant layer so that said thermally conductive masses are
spaced apart from one another in lateral directions, said thermally
conductive masses having a melting temperature within or below the
range of operating temperatures of said first and second
microelectronic elements, each said mass of thermally conductive
material having opposite ends adjacent to said confronting surfaces
of said first and second microelectronic elements for conducting
heat therebetween.
49. A package as claimed in claim 45, wherein said thermally
conductive material and said electrically conductive material
comprise one or more metals.
50. A package as claimed in claim 44, wherein said first
microelectronic element includes one or more semiconductor
chips.
51. A package as claimed in claim 44, wherein said package element
includes a heat sink.
52. A structure for making a microelectronic assembly comprising: a
layer of a matrix material having top and bottom surfaces extending
in lateral directions, said matrix material being selected from the
group consisting of (a) compliant materials having a degradation
temperature higher than the melting temperature of said conductive
masses and (b) flowable, curable precursor materials; and one or
more individual masses of a fusible conductive material dispersed
in said layer so that said individual conductive masses are spaced
apart from one another in said lateral directions and separated
from one another by said matrix material, said conductive masses
having a melting temperature below about 150.degree.C.; and a
removable release layer overlying at least one of said
surfaces.
53. A structure as claimed in claim 52, further comprising a second
removable release layer overlying the other one of said
surfaces.
54. A structure as claimed in claim 52, further comprising one or
more microelectronic elements in engagement with one of said
surfaces.
55. A structure as claimed in claim 54, wherein said one or more
microelectronic elements include one or more semiconductor chips,
each said chip having a surface in engagement with said one of said
surfaces of said layer.
56. A structure as claimed in claim 54, wherein said one or more
microelectronic elements include one or more thermally conductive
package elements in engagement with said one of said surfaces of
said layer.
57. A structure as claimed in claim 52 wherein said fusible
conductive material includes one or more metals.
58. A structure as claimed in claim 52, wherein said fusible
conductive material has a melting temperature below about
65.degree.C.
59. A structure as claimed in claim 52, wherein at least some of
said one or more individual masses of a fusible conductive material
extend over a major portion of the distance between said top and
bottom surfaces of said layer.
60. A structure as claimed in claim 52, wherein at least some of
said one or more individual masses of a fusible conductive material
extend from said top surface to said bottom surface of said layer
to provide a continuous conduction path from the top surface of
said layer to the bottom surface of said layer.
61. A structure as claimed in claim 43, further comprising an
adhesive at at least one of said top and bottom surfaces of said
layer.
62. A method of making microelectronic assemblies comprising the
steps of: (a) providing a plurality of microelectronic elements
each having a rear surface and a front surface; (b) providing a
conductive layer including masses of a fusible conductive material
having a melting temperature less than about 150.degree. C. and a
matrix material surrounding said masses so that said conductive
layer overlies said rear surfaces of said microelectronic elements;
and (c) severing said layer and separating said microelectronic
elements from one another to provide a plurality of units, each
said unit incorporating one said microelectronic element and a
portion of said layer.
63. A method as claimed in claim 62, wherein said step of providing
a plurality of microelectronic elements is performed by providing a
unitary wafer including a plurality of semiconductor chips, and
wherein said severing step includes the step of severing said wafer
to separate said chips from one another.
64. A method as claimed in claim 63, further comprising the step of
providing connection components on the front faces of said chips
before severing said wafer, each said connection component having
terminals thereon connected to one said chip and movable with
respect to such chip.
65. A method as claimed in claim 62, wherein said fusible
conductive material has a melting temperature less than the maximum
operating temperature of said plurality of microelectronic
elements.
Description
[0001] This is a continuation-in-part of U.S. patent application
Ser. No. 08/641,698, filed May 2, 1996, the disclosure of which is
hereby incorporated by reference herein.
FIELD OF THE INVENTION
[0002] The present invention relates to mounting and connection
devices and techniques for use with microelectronic elements such
as semiconductor chips.
BACKGROUND OF THE INVENTION
[0003] Complex microelectronic devices such as semiconductor chips
require numerous connections to other electronic components.
Typically, the microelectronic devices are mounted on substrates or
external circuit elements, such as printed circuit boards, having
electrical contacts, and the contacts on the chip are electrically
connected to the contacts of the external circuit element. The
external circuit element may have pins or other connectors adapted
to accommodate other components, including additional semiconductor
chips. Also, the external circuit element may have pins or other
connectors adapted to connect the contacts or internal circuitry of
the external circuit element to a larger assembly, thereby
connecting the chip to the larger assembly.
[0004] Connections between microelectronic elements and substrates
must meet several demanding and often conflicting requirements.
They must provide reliable, low-impedance electrical
interconnections. They must also withstand stresses caused by
thermal effects during manufacturing processes such as soldering.
Other thermal effects occur during operation of the device. As the
system operates, it evolves heat and the components of the system,
including the chip and the substrate expand. When operation ceases,
the components cool and contract. When the assembly is heated and
cooled during manufacture or in operation, the chip and the
substrate expand and contract at different rates, so that portions
of the chip and substrate move relative to one another. Also, the
chip and the substrate can warp as they are heated and cooled,
causing further movement of the chip relative to the substrate.
These and other effects cause repeated strain on electrical
elements connecting the chip and the substrate. The interconnection
system should withstand repeated thermal cycling without breakage
of the electrical connections. The interconnection system should
provide a compact assembly, and should be suitable for use with
components having closely-spaced contacts. Moreover, the
interconnection should be economical.
[0005] Various solutions have been proposed to meet these needs. In
particular, as disclosed in U.S. Pat. Nos. 5,148,265; 5,148,266;
5,455,390 and in International Publication WO 96/02068, flexible
leads may be provided between the contacts on a chip or other
microelectronic element and the contact pads of a substrate.
According to preferred embodiments taught in these documents, a
compliant layer, such as an elastomer or a gel may be provided
between the chip and the substrate. Flexible leads connecting the
chip and substrate may extend through the compliant layer. In these
preferred arrangements, the chip is mechanically decoupled from the
substrate, so that the chip and substrate can expand and move
independently of one another without excessive stress on the
electrical connections between the chip contacts and the contact
pads of the substrate. Moreover, the assemblies disclosed in these
patents and publications meet the other requirements discussed
above. In certain preferred embodiments according to these
documents, the chip and the interconnections to the substrate can
occupy an area of the substrate about the same size as the chip
itself.
[0006] Microelectronic elements such as semiconductor chips
generate considerable amounts of heat during use. For example, a
complex, high-speed chip only a few centimeters square in area may
produce tens of watts of heat. This heat must be dissipated to
maintain the chip at a safe operating temperature. Improvements in
chip mountings and electrical connections, and in related assembly
methods, have made it possible to reduce the distance between chips
so as to achieve a more compact assembly. Such assemblies,
typically referred to as "multichip modules," incorporate one or
more substrates with chips disposed close to one another on the
substrate. The heat dissipation problems discussed above are
particularly extreme in such compact multichip modules.
[0007] Considerable effort has been devoted in the art towards
meeting these needs for cooling. A general outline of the
approaches taken heretofore is set forth in the text Multichip
Module Technologies and Alternatives--The Basics, Doane, D. A. and
Franzon, P. D., EDS 1993 Van Nostrand Reinhold, New York, N.Y. at
chapter 12, pp. 569-613, entitled "Thermal Design Considerations
For Multichip Module Applications" (Azar, K., chapter author) and
at pages 109-111 of the same reference. As described therein, heat
transfer problems in electronic packaging can be addressed in terms
of "thermal resistance" of the elements involved. The thermal
resistance of any element in the heat transfer path refers to the
ratio between the temperature difference across such element and
the rate of heat flow through the element. Thermal insulators have
high thermal resistance whereas elements which convey heat
effectively by conduction or convection have low thermal
resistance. The overall thermal resistance of the package is the
sum of the individual thermal resistances in series in the heat
path between the chip and the ambient environment. The overall
thermal resistance in turn provides a ratio between the temperature
rise of the chips above ambient temperature and the amount of heat
produced in the chips.
[0008] As described in the aforementioned reference, the heat
conduction pathway may include an element commonly referred to as a
"heat sink." There is normally a low thermal resistance connection
from the heat sink to the environment. For example, the vanes of
the heat sink may be bathed in a flow of forced air or liquid.
However, there is generally an appreciable thermal resistance
between the semiconductor chip, or other microelectronic
components, and the heat sink. Stated another way, it is difficult
to provide a low thermal resistance connection between the chip and
the heat sink while still meeting all of the other requirements for
such a connection. This is because the thermal connection must
accommodate relative movement between the chips or other components
and the heat sink during use of the device. Such relative movement
arises in part from movement of the components and the substrate
bearing the components as the assembly undergoes temperature
changes during use. When the unit is first supplied with power, the
temperature of the chips or other components rises faster than the
temperature of the substrate, causing differential thermal
expansion, warpage and distortion. Further, the coefficients of
thermal expansion of the chips and the substrate normally are not
matched with the coefficient of thermal expansion of the heat sink,
causing further differential thermal expansion and contraction.
[0009] Moreover, the connection between the components and the heat
sink should accommodate dimensional tolerances in the components,
the substrate and the heat sink itself. For example, the chips
themselves may be of different thicknesses. Also, the chips can be
supported at different levels above the face of the substrate by
solder balls or other mountings. The surfaces of the chips may be
tilted from their nominal positions, so that the chip surfaces are
out of alignment with the surface of the heat sink. The heat sink
itself may not be perfectly flat or parallel to the nominal plane
of the chip surfaces. Any elements used to connect the heat sink
with the chip or the components should be capable of accommodating
these tolerances and misalignments. Considerable efforts have been
made in the art heretofore towards satisfying these
requirements.
[0010] Nonetheless, still further improvement would be desirable.
For example, it would be desirable to provide additional connection
components and methods which provide effective mechanical
decoupling and high resistance to thermally induced stresses, while
also providing low cost and high reliability. It would also be
desirable to provide a microelectronic package including improved
assemblies and methods for dissipating heat therefrom to minimize
thermally induced stresses, while also providing high reliability
and low cost.
SUMMARY OF THE INVENTION
[0011] One aspect of the present invention provides a method of
making a microelectronic assembly including the steps of providing
a first microelectronic element, such as a semiconductor chip and a
second microelectronic element with confronting spaced-apart
surfaces defining a space therebetween and providing one or more
masses of a fusible conductive material having a melting
temperature below about 150.degree. C. in the space. Next, a
flowable liquid material, typically a liquid, is introduced between
the confronting surfaces of the first and second microelectronic
elements and around the one or more fusible conductive masses. The
flowable material is then cured to form a compliant layer disposed
between the confronting surfaces of the first and second
microelectronic elements and surrounding each of the fusible
conductive masses. The one or more conductive masses may be
maintained in a substantially solid condition or in a substantially
liquid condition when the flowable material is introduced. The
fusible conductive masses may also be maintained in either a
substantially solid condition or in a substantially liquid
condition during the curing step.
[0012] The first microelectronic element preferably is a circuit
element such as a semiconductor chip. The second microelectronic
element may be a further circuit element such as a dielectric
element having conductors thereon, or else may be a package element
such as a casing, heat spreader or heat sink. The conductive
material is thermally conductive, electrically conductive or both.
Where the second microelectronic element is a circuit element, the
method desirably includes the step of electrically connecting the
first and second elements to one another. For example, the step of
providing the masses of fusible conductive material may be
performed so that the masses extend between contacts on the
confronting surfaces of the first and second microelectronic
element, so that the masses electrically interconnect the first and
second elements. The conductive masses desirably provide a thermal
conduction path between the first and second microelectronic
elements. Preferably, the one or more fusible conductive masses are
contiguous with both elements, and connect the first and second
microelectronic elements to one another.
[0013] Preferably, the fusible conductive masses are contiguous
with and are contained by the compliant material, so that the
conductive material remains in place when in a liquid state. Thus,
the compliant layer keeps the fusible conductive masses separate
and electrically insulated from one another. Alternatively or
additionally, a polymer coating such as a polyparaxylene coating
may be provided over the fusible conductive masses. The
polyparaxylene coating is a conformal coating which preferably
fully encompasses the fusible conductive masses and desirably
extends to the neighboring portions of the confronting surfaces of
the first and second microelectronic elements. The coating enhances
the electrical isolation of the fusible conductive masses and also
protects the masses from contamination, e.g. prevents the fusible
conductive material and the compliant layer from diffusing into one
another. The polyparaxylene coating can also aid in maintaining the
masses in place when the masses are in the liquid state.
[0014] In additional preferred methods according to this aspect of
the invention, the step of providing said first and second elements
being performed so that said second microelectronic element
confronts a front surface of the first microelectronic element and
these elements define a front space therebetween, the conductive
masses being provided in said front space. A third microelectronic
element is provided so that this element confront a rear surface.
Thus, the first and third microelectronic elements define a rear
space between said rear surface and said third microelectronic
element. The first microelectronic element is sandwiched between
the second and third microelectronic elements, with front and rear
spaces on opposite sides of the first microelectronic element.
Methods according to this aspect of the invention desirably include
the step of disposing one or more additional fusible conductive
masses in the rear space and introducing additional flowable
material into said rear space and around said conductive masses in
said rear space. Most preferably, this additional flowable material
is cured to form a rear compliant layer between the third
microelectronic element and said first microelectronic element,
said rear compliant layer intimately surrounding said conductive
masses in said rear space. The step of introducing a flowable
material into the rear space, and the step of introducing a
flowable material into the front space can be preformed by
simultaneously using a single flowable material.
[0015] A related aspect of the invention provides a microelectronic
package comprising a first microelectronic element such as a
semiconductor chip operable in a range of operating temperatures.
The chip or other microelectronic element has a front face
including contacts and a rear surface. A compliant layer is
disposed between the confronting surfaces of the first
microelectronic element and the package element. Masses of a
fusible, thermally conductive material having a melting temperature
within or below the range of operating temperatures of the first
microelectronic element are also provided between the package
element and the first microelectronic element. Each such mass
extends adjacent to the confronting surfaces of the first
microelectronic element and the package element for transferring
heat therebetween during operation of the microelectronic package.
The package may also include a second microelectronic element such
as a circuit element electrically connected to the first
microelectronic element, most preferably a flexible dielectric
sheet with terminals thereon, overlying the front face of the first
microelectronic element or chip. A compliant layer most preferably
is provided between the front face and the second microelectronic
element. Here again, masses of a fusible conductive material,
preferably a material capable of conducting both electrical signals
and heat, are dispersed in the second compliant layer so that the
fusible conductive masses are spaced apart from one another in
lateral directions parallel to the surfaces of the microelectronic
elements. These masses may be disposed between the opposing
contacts on the first microelectronic element and the second
microelectronic element for electrically interconnecting the first
and second microelectronic elements. In the final package the first
and second compliant layers are preferably contiguous with one
another. The package mechanically isolates the first
microelectronic element or chip and effectively decouples it from
mechanical stresses and differential thermal expansion, while also
providing effective heat transfer from the chip and effective
interconnection between the chip and external circuitry.
[0016] The masses of the fusible conductive material used in
various aspects of the invention may comprise one or more metals or
may comprise a metal alloy and are preferably capable of conducting
electrical signals, heat or both. The fusible conductive material
preferably has a melting temperature below about 125.degree. C.,
and in more preferred embodiments the fusible conductive material
has a melting temperature below about 65.degree. C. Most preferably
the fusible conductive material has a melting temperature between
about 25.degree. C. and about 65.degree. C. However, lower melting
temperatures can be employed if the production process is altered
to accommodate the lower melting temperature. The conductive masses
preferably are liquid at temperatures within the range of
temperatures encountered during normal operation of microelectronic
elements, and may have a melting temperature below the range of
operating temperatures of the microelectronic elements. The fusible
conductive masses may be in a solid state or a liquid state when
the assembly is inactive; however, during operation, the fusible
conductive masses may be wholly or partially liquid so that
essentially no forces will be transmitted between the
microelectronic elements through the conductive masses. Stated
another way, the conductive masses in their liquid state have
spring constants at or close to zero and do not resist movement of
the microelectronic elements relative to one another.
Alternatively, the conductive material may be a fusible material
which melts at temperatures slightly above the range of
temperatures encountered during normal operation. In this case, the
assembly relieves mechanical stress in the electrical connections
or thermal connections, and repairs defects in the connections,
when the assembly is exposed to high temperatures during abnormal
operating conditions or during processing operations.
[0017] Preferably, the compliant dielectric layer also allows the
microelectronic elements to move relative to one another. Thus, the
dielectric layer desirably is formed from an elastomer, gel, foam
or other material having relatively low resistance to deformation.
Preferred assemblies according to the present invention thus allow
electrical signals to pass between microelectronic elements through
the fusible conductive masses. In addition, preferred assemblies
according to the present invention also allow heat generated by the
microelectronic elements to be effectively dissipated through the
package and allow the confronting faces of the microelectronic
elements to move relative to one another to compensate for movement
and distortion during thermal cycling of the package. The compliant
connection between the microelectronic elements also helps to
compensate for tolerances encountered during manufacturing and can
be provided even where each conductor has substantial
cross-sectional area. Thus, low resistance, low impedance
conductors can be utilized without impairing the flexible
connection. Moreover, when the fusible material melts, cracks or
other defects in the conductive masses are repaired.
[0018] One or more of the microelectronic elements may include a
flexible dielectric sheet having an exterior surface facing away
from the other elements and having conductive terminals accessible
at the exterior surface. Thus, where the first microelectronic
element includes a semiconductor chip, the second element may
include a flexible dielectric sheet overlying a surface of the chip
and having terminals facing away from the surface of the chip.
Thus, the front space lies between the flexible sheet and the chip
contact bearing face of the chip. The method may further include
the step of forcing the terminals into substantially coplanar
disposition while maintaining the masses of fusible conductive
material in an at least partially molten condition. Preferably,
this step is performed prior to curing the flowable material,
either before or after introduction of the flowable material into
the space. After the conductive masses have been provided between
the confronting surfaces of the chip and the dielectric sheet, the
conductive terminals are forced into substantially coplanar
alignment with one another as the flowable material is introduced
therebetween and the conductive masses are allowed to freeze.
Alternatively, the flowable material is introduced between the
confronting surfaces before the conductive terminals are forced
into substantially coplanar alignment. In certain preferred methods
according to this aspect of the invention, the flexible sheet and
the chip or other element have contacts on their opposing surfaces.
Fusible conductive masses are provided between the opposing
contacts to electrically interconnect the chip and the dielectric
sheet. Additional fusible conductive masses, aside from those used
for electrical interconnection, may be provided between the
confronting faces of the chip and the dielectric sheet to provide
additional conduct heat therebetween. In an alternative embodiment,
the dielectric sheet includes conductive terminals accessible at
the exterior surface and flexible leads are used to connect the
conductive terminals of the dielectric sheet with the contacts of
the semiconductor chip. Fusible conductive masses are provided
between the confronting faces of the chip and the dielectric sheet
for conducting heat therebetween during operation.
[0019] In methods according to further aspects of the invention,
one or more of the microelectronic elements may include a plurality
of semiconductor chips. For example, the first microelectronic
element may include a unitary wafer incorporating a plurality of
semiconductor chips, whereas the second microelectronic element may
include a flexible dielectric sheet as described above. Each chip
is preferably aligned with a portion of the sheet and the contacts
on each chip may be connected by the fusible conductive masses to
the terminals in the aligned portion of the sheet. The method
according to this aspect of the invention may include the further
step of severing individual portions of the sheet and wafer to form
individual units, each including one or more chips and the portion
of the sheet aligned therewith. The step of providing the
microelectronic elements and the fusible conductive masses may
include the step of providing the masses attached to contacts on
one of the microelectronic elements and then juxtaposing the
elements with one another and at least partially melting the masses
to thereby bond the masses to the contacts on the other element.
For example, where one of the elements is a wafer, the masses may
be provided on the wafer, and the wafer may be juxtaposed with the
flexible dielectric sheet. In methods according to a further aspect
of the invention, the second microelectronic element has
electrically conductive traces thereon, and a plurality of chips
are electrically connected to the second element, as by connection
through conductive masses as discussed above, so that the chips are
interconnected to one another to form a multichip module.
[0020] Further methods of making a microelectronic assemblies
according to the invention include the steps of providing a
metallic plate, juxtaposing the plate with a surface of a
microelectronic element and providing one or more masses of a
fusible conductive material so that the masses extend in a space
between the plate and the microelectronic element, and preferably
extend all the way from the plate to the microelectronic element.
In a particularly preferred arrangement, the plate is provided with
the more masses of a fusible conductive material disposed at
predetermined locations on a surface thereof before juxtaposing the
plate with the microelectronic element. Methods according to this
aspect of the invention desirably include the step of injecting a
flowable material between the metallic plate and the
microelectronic element and curing the flowable material to form a
compliant dielectric layer which intimately surrounds the fusible
conductive masses. The predictable, isotropic thermal expansion
properties of the metallic plate help to provide precise alignment
of the fusible conductive masses with the contacts on the
microelectronic element. The metallic plate is then subdivided to
form separate portions connected to separate ones of the fusible
conductive masses. The microelectronic element according to this
particular embodiment may include an array of semiconductor chips
or a semiconductor wafer. The metallic plate may be subdivided by
etching the plate after the flowable material is cured. After the
metallic sheet has been subdivided, the wafer and the compliant
layer may be severed to form individual units whereby each unit
includes one or more chips. In certain embodiments, the metallic
plate with the fusible conductive masses may be provided by forming
a layer on a first side of the metallic plate whereby the layer has
apertures therein and includes a material, such as a polymer, which
is non-wettable by the conductive masses. The first side of the
plate is then exposed to the fusible conductive masses while the
conductive masses are in a molten condition so that drops of the
fusible conductive masses adhere to the metallic plate at the
apertures therein. The first side of the plate is preferably
exposed to the conductive masses by dipping the metallic plate into
a bath of the fusible conductive material. A second side of the
metallic plate opposite from the first side thereof may be covered
by a protective coating during the exposing step to prevent the
second side of the metallic plate from coming into contact with the
fusible conductive material.
[0021] Barrier layers, such as a layers of polysilicon, may be
provided on surfaces of the microelectronic elements which are in
contact with the fusible masses, such as on contacts or on surfaces
of package elements. The barrier layer prevents the material in the
fusible conductive masses from diffusing into the microelectronic
element, the opposing contacts and/or the package element. The
barrier metal layer also prevents contamination of the fusible
material by the microelectronic element and/or the package
element.
[0022] Yet a further embodiment of the invention provides
structures which can be used as components in fabrication of
microelectronic assemblies. A structure according to this aspect of
the present invention includes a layer of a matrix material having
top and bottom surfaces extending in lateral directions. One or
more masses of a fusible conductive material are dispersed in the
layer so that the individual conductive masses are spaced apart
from one another in the lateral directions and are separated from
one another by the matrix material. The fusible conductive masses
preferably include one or more metals and preferably have a melting
temperature below about 125.degree. C. and more preferably below
about 65.degree. C. In certain embodiments, at least some of the
fusible conductive masses extend over a major portion of the
distance between the top and bottom surfaces of the layer. In still
other embodiments, at least some of the fusible conductive masses
extend from the top surface to the bottom surface of the layer to
provide a continuous conductive path from the top surface to the
bottom surface of the layer. The matrix material is selected from
the group consisting of (a) compliant materials having a
degradation temperature higher than the melting temperature of the
fusible conductive masses and (b)flowable, curable precursor
materials. This structure may be provided as a prefabricated
structure, without microelectronic elements. The prefabricated
structure may be used with one or more microelectronic elements to
provide a compliant, thermally conductive connection, and may also
be used to provide a compliant electrically conductive connection.
The structure may also include one or more removable release layers
overlying at least one of the surfaces of the layer of matrix
material. The release layer protects the structure from
contamination during storage and may be removed from the structure
shortly before final assembly of the structure with a
microelectronic package. The structure may also include an adhesive
overlying at least one of the top and/or bottom surfaces of the
matrix layer so that the structure may be easily assembled to the
surface(s) of one or more microelectronic elements. Alternatively,
the matrix material itself may be capable of bonding to a surface
of a microelectronic element. Related aspects of the invention
provide assembly methods using such components. For example, a
layer as aforesaid may be assembled to one or more semiconductor
chips, whereby each semiconductor chip has a surface in engagement
with a surface of the layer. Thus, the layer may be applied to a
surface of a wafer or to an assemblage of individual chips, and the
resulting assembly may be severed to provide individual units
including one or more chips and a portion of the layer.
[0023] Other objects and advantages of the present invention will
be pointed out in the following detailed description and
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a fragmentary, diagrammatic sectional view
depicting a stage in a manufacturing process according to one
embodiment of the invention.
[0025] FIG. 2 is a diagrammatic perspective view depicting a wafer
utilized in the process of FIG. 1.
[0026] FIG. 3 is a diagrammatic perspective view depicting a
subassembly made by the process of FIG. 1.
[0027] FIG. 4 is a diagrammatic, fragmentary sectional view
depicting an assembly made using the subassembly of FIG. 2.
[0028] FIG. 5 is a diagrammatic, sectional view similar to FIG. 4,
but depicting a portion of an assembly in accordance with a further
embodiment of the invention.
[0029] FIG. 6 is a fragmentary sectional view depicting a component
in accordance with yet another embodiment of the invention.
[0030] FIG. 7 is a diagrammatic elevational view of an assembly
incorporating the subassembly of FIG. 6.
[0031] FIG. 8 is a diagrammatic perspective view depicting a
component in accordance with a further embodiment of the
invention.
[0032] FIG. 9 is a fragmentary view on an enlarged scale depicting
portions of an assembly in accordance with a further embodiment of
the invention.
[0033] FIG. 10 is a diagrammatic sectional view of an assembly in
accordance with a further embodiment of the invention.
[0034] FIGS. 11 through 14 are fragmentary diagrammatic sectional
views of an assembly during successive stages in a fabrication
process in accordance with another embodiment of the invention.
[0035] FIG. 15 is a view similar to FIGS. 11-14 but depicting an
assembly in accordance with yet another embodiment of the
invention.
[0036] FIGS. 16 is a fragmentary diagrammatic sectional view
depicting elements according to further embodiments of the
invention.
[0037] FIG. 17 is a fragmentary diagrammatic-sectional view
depicting elements according to yet another embodiment of the
invention.
[0038] FIG. 18 is a further fragmentary diagrammatic sectional view
depicting elements during a process according to another embodiment
of the invention.
[0039] FIG. 19 is a fragmentary diagrammatic sectional view
depicting elements according to still another embodiment of the
invention.
[0040] FIG. 20 is a diagrammatic sectional view depicting an
assembly in accordance with yet another embodiment of the
invention.
[0041] FIG. 21 is a diagrammatic, partially cutaway, perspective
view depicting an assembly in accordance with yet another
embodiment of the invention.
[0042] FIG. 22 is a diagrammatic sectional view depicting an
assembly in accordance with yet another embodiment of the
invention.
[0043] FIG. 23 is a fragmentary diagrammatic sectional view
depicting an assembly in accordance with a further embodiment of
the invention.
[0044] FIG. 24 is a diagrammatic sectional view of an assembly in
accordance with another embodiment of the invention.
[0045] FIG. 25 is a diagrammatic sectional view of an assembly in
accordance with still another embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0046] An assembly in accordance with one embodiment of the
invention includes as a first element a unitary semiconductor wafer
22 incorporating a large number of semiconductor chips 28 disposed
side by side. The wafer has numerous contacts 24 disposed on its
top or front surface 26. A rear surface 27 is devoid of contacts.
Each contact includes a small spot of a metal such as aluminum,
gold, copper, zinc or tin. The wafer is formed in the normal
fashion, with numerous semiconductor devices within each chip 28
connected to one another and to contacts 24 by internal circuitry
(not shown) formed within the wafer. The wafer also has narrow
strip-like regions 30, commonly referred to as "saw lanes" or
"scribe streets" extending between adjacent chips 28.
[0047] The second element of the assembly includes a flexible, but
substantially inextensible dielectric sheet 10 having a first
surface 12 and a second surface 14. For example, sheet 10 may be a
sheet of polyimide about 25 microns or less thick. Vias are formed
through sheet 10 and filled with a metallic material to form solid
via liners or terminal assemblies 15 extending through the sheet at
predetermined locations. Each via liner defines a contact 16 at the
first surface 12 of sheet 10 and a terminal 17 at the second
surface of the sheet. The terminal assemblies 15 may be formed from
any suitable metal which can be conveniently deposited in the vias
as, for example, copper and copper alloys. Each contact 16 is
provided with a layer 18 of a barrier material, such as a
polycrystalline silicon, on the exposed face of the contact. The
barrier material is selected to resist dissolution in the
low-melting fusible conductive metal discussed below, and to
prevent difflusion of the underlying material of the contact in the
fusible metal. The composition of the barrier layer will depend in
part upon the composition of the fusible metal. Metals such as
nickel, tungsten, titanium and their alloys normally can be used as
barrier layers with typical fusible metals such as ultra-low
melting solders of the types discussed below. The barrier layer 18
may also include a polycrystalline silicon or polysilicon. Barrier
layer 18 need only be thick enough to inhibit dissolution of the
underlying metal in the contact 16. A layer of barrier material
about 1 micron thick typically is sufficient. The barrier layer
material desirably is wettable by the fusible conductive metal when
the fusible conductive metal is in its liquid state. The barrier
layer may also include a plurality of layers of different
compositions. Terminals 17 have bonding material layers or masses
20 thereon. Essentially any conventional bonding material can be
employed, including conventional solders, conductive polymers, and
eutectic bonding materials, also referred to as diffusion-bonding
alloys. The bonding material is selected to provide satisfactory
connection to the contact pads engaged with the terminals in
service, as further discussed below.
[0048] In a first step of a process according to one embodiment of
the invention, masses 42 of a fusible conductive material such as
an ultra-low melting point solder are deposited on the contacts 24
of wafer 22. This step of the process may be performed by
conventional equipment and techniques commonly used to deposit
solder masses on microelectronic elements. Thus, the masses may be
applied individually or, by screening the fusible conductive
material onto the surface of the wafer using a mask or screen with
perforations corresponding to the contacts, and then removing the
mask. The mask may be formed separately from the wafer or may be
formed in place on the top surface of the wafer by
photolithographic techniques. The mask may be removed from the
wafer by mechanically separating the mask and wafer or by
dissolving the mask after deposition of the fusible metal. Suitable
fluxes may be employed during deposition of the fusible metal on
the wafer. The flux may be removed after deposition of the fusible
metal. After the fusible metal has been deposited on the contacts
of the wafer and the mask has been removed, the fusible metal may
be briefly re-melted so as to reflow the fusible metal and bring it
into even more intimate contact with the contacts of the wafer.
[0049] The melting temperature of the fusible conductive material
desirably is within or below the normal operating temperature of
the semiconductor elements in the wafer, or only slightly above the
normal operating temperature range. The normal, expected range of
operating temperatures of the semiconductor elements will depend
upon the configuration and composition of the element, and upon the
operating environment encountered in service. Typical silicon-based
semiconductor elements are designed to operate at about
40.degree.C. to about 85.degree.C. Where the fusible conductive
material melts or freezes over a range of temperatures, the term
"melting temperature" as used in this disclosure should be
understood as referring to the solidus temperature, i.e., the
temperature at which the metal begins to melt (when heated slowly)
or completes freezing (when cooled slowly). Preferably, the melting
temperature of the fusible conductive material is above normal room
temperature (20.degree.C.) so that the fusible conductive material
can be handled conveniently in solid form during the steps
discussed below. Thus, the fusible conductive material desirably
has a melting temperature of less than about 150.degree.C.,
preferably less than about 125.degree.C. and more preferably less
than about 100.degree.C. Melting temperatures below about
85.degree.C. are more preferred, and melting temperatures below
about 65.degree.C. are even more preferred. The range of melting
temperatures between about 25.degree.C. and 65.degree.C. is
particularly preferred, and melting temperatures between about
35.degree.C. and about 55.degree.C. are especially preferred.
However, lower melting temperatures can be employed if the
production process is altered to accommodate the lower melting
temperature. For example, where a conductive material which melts
at a temperature below room temperature is employed, the conductive
material and the adjacent parts can be kept at sub-ambient
temperatures during those process steps where the conductive
material must remain solid. Conversely, where the operating
temperature of the microelectronic elements is higher than the
typical ranges mentioned above, higher melting fusible materials
can be employed.
[0050] Among the suitable low-melting point solders are the
following:
1 COMPOSITION 1 COMPOSITION 2 ELEMENT WEIGHT % WEIGHT % Sn 18.5
10.5 Bi 45 40 Pb 24 21.5 In 10 20 Cd 9.5 8 Melting 55.degree. C.
50.degree. C. Temperature
[0051] Solders having compositions intermediate between the two
low-melting point solders illustrated in Table 1 can be used. Other
suitable low-melting solders include the solder sold under the
trademark Indalloy by the Indium Corporation of America, in
Clinton, N.Y. For example, Indalloy Number 8 has a melting point of
about 93.degree.C., whereas Indalloy Number 117 has a melting point
of about 47.degree.C. Still other low-melting solders include other
combinations of metals selected from the group consisting of
cadmium, bismuth, tin, lead and indium in various proportions, with
or without other metals. Additional fusible metals include mercury
and mercury containing alloys.
[0052] In the next stage of the process, the dielectric sheet or
second element 10 is assembled to the wafer or first element 22 so
that the first surface 12 of the sheet faces toward the front
surface 26 of the wafer and these confronting surfaces define a
space 46 between them. The contacts 16 of the dielectric sheet or
second element are aligned with the contacts 24 of the first
element and aligned with masses 42 of fusible conductive material
disposed on the first element contacts. The alignment between the
contacts 16 of the second element and the fusible conductive masses
42 need not be perfect. The alignment need only be close enough
that each contact 16 on the second element touches the correct
fusible conductive mass 42 during the melting step discussed below,
and so that each contact 16 on the sheet or second element does not
touch any other fusible conductive mass 42.
[0053] Processes for aligning a sheet and a wafer are disclosed in
commonly owned International Patent Publication WO 96/02068, the
disclosure of which is hereby incorporated by reference herein as
well as in copending, commonly assigned U.S. Provisional Patent
Application 60/001,718, filed Jul. 31, 1995, and commonly assigned
U.S. Provisional Patent Application entitled "Framed Sheet
Processing," filed Oct. 17, 1997, the disclosures of which are also
incorporated by reference herein. As disclosed in these
applications, sheet 10 can be stretched taut by bonding it to a
ring of a material such as aluminum having a coefficient of thermal
expansion higher than the coefficient of thermal expansion of the
sheet and then heating the assembly. While the assembly is in this
taut condition at elevated temperature, the sheet is then bonded to
a frame formed from a material such as molybdenum having a
coefficient of thermal expansion close to that of the sheet, and
the ring is removed. The assembly of the frame and the sheet can
then be cooled to room temperature and the sheet will remain taut.
The taut sheet can be aligned with wafer using a manually
adjustable device such as a micrometer-actuated microscope stage by
an operator while the operator observes the sheet and wafer under
magnification. The alignment step also can be performed
robotically, using generally conventional machine-vision systems.
Preferably, both the dielectric sheet and the wafer are provided
with fiducial marks to be used as a reference in alignment. These
marks are arranged so that when the fiducial marks are aligned with
one another, the contacts are also properly aligned. The sheet
typically is transparent and hence the fiducial marks on the wafer
can be observed through the sheet by a human operator or by a
machine vision system.
[0054] While the sheet and the wafer are aligned with one another,
the sheet is pressed inwardly, toward the wafer so that the exposed
surfaces of the contacts on the first or inwardly facing surface 12
of sheet 10 engage the fusible conductive masses 42. This can be
accomplished by placing the elements between a pair of plates 48
and 50 so that a first plate engages wafer 22 on its bottom or
outwardly facing surface whereas a second plate engages sheet 14
along its top or outwardly facing surface 14, and urging plates 50
and 48 towards one another. Depending upon the configuration of
terminals 17 and the bonding materials thereon, plate 50 may be
provided with pockets or recesses corresponding to the terminals.
The sheet may be held in engagement with plate 50 by application of
vacuum through ports 52 in the plate. Alternatively, plate 50 can
be provided with a resilient covering such as a foam on the surface
of the plate which engages the sheet surface. In yet another
alternative, a rigid stiffening plate (not shown) may be provided
between plate 50 and the sheet surface. A gas or other fluid may be
introduced between plate 50 and the stiffening plate so that the
fluid pressure urges the stiffening plate and the sheet inwardly,
towards the wafer until the stiffening plate and sheet reach a stop
(not shown).
[0055] While the contacts are held in engagement with the fusible
conductive masses, the conductive masses are brought to a
temperature above their melting temperature, so that the conductive
material at least partially liquefies and flows into intimate
engagement with the exposed surfaces of the contacts 16 on the
sheet. This may be accomplished by heating the assemblage after the
second element or sheet 10 has been engaged with the wafer and
conductive masses. Alternatively, the wafer and the conductive
masses may be at a temperature above the melting temperature of the
fusible conductive material prior to engagement of the sheet. The
sheet and hence the contacts 16 can also be preheated to a
temperature above the melting temperature of the fusible conductive
masses before engagement with masses 42. In yet another
alternative, only the sheet 10 and contacts 16 are at a temperature
above the melting temperature of the masses, whereas the masses
themselves and the wafer are at a temperature slightly below the
melting temperature. In this arrangement, each mass will be only
partially melted in the region adjacent the mating contact 16 on
the sheet.
[0056] The molten conductive material wets the barrier metal 18 on
the surfaces of contacts 16. A flux may be employed in this step as
well. Any flux used in the process may be removed by flushing space
46 with a suitable solvent and removing the solvent. While the
masses are in at least a partially molten condition, plate 50 holds
the sheet and hence terminals in a substantially planar condition,
with the exposed surfaces of the terminals on the second or
outwardly facing surface 14 of the sheet in substantially coplanar
alignment with one another. The plates also maintain the alignment
of the contacts 16 and masses 42 in horizontal directions, parallel
to the opposed surfaces 26 and 12 of the first and second elements.
While the elements are aligned in this manner, the conductive
masses are cooled to below their melting temperature, as by cooling
the entire assemblage, including plates 50 and 48. If the fusible
masses 42 were only partially melted, as where the masses were
originally at below-melting temperatures and the contact 16 were at
temperatures above the melting temperature, the partially melted
portion of the masses can be cooled by heat transfer to the
remaining portions of the mass.
[0057] Optionally, a polymer such as a polyparaxylene coating 43
may be provided over the fusible conductive masses 42. As shown in
the drawings, the polyparaxylene coating 43 is a conformal coating
which fully encompasses the fusible conductive masses 42 and the
adjacent regions of the contacts 16 and 24. The coating extends
between the confronting surfaces of the dielectric sheet 10 and the
wafer 22, and may cover portions of these surfaces as well. The
coating 43 helps to retain the fusible masses 42 in place when they
are in a molten condition and helps preserve the electrical
isolation of the fusible conductive masses 42 from one another. The
coating also helps to prevent cross-contamination; the coating
prevents the fusible conductive material and the material of the
compliant layer from diffusing into one another. The polymeric
coating may be provided by a vapor-phase coating process of known
type.
[0058] After the masses have been completely frozen and after the
polymer coating (if used) has been applied, a flowable, preferably
liquid material is introduced into space 46 between confronting
surfaces 12 and 26 so that the flowable material fills the space
and intimately surrounds masses 42 and the adjacent surfaces of
contacts 16 and 24. The flowable material also intimately contacts
the inwardly facing surfaces 12 and 26 of the sheet and wafer.
During injection of the flowable material, the contact 16 and sheet
10 are maintained in substantially planar disposition, and the
contacts are maintained in alignment with masses 42. Alignment and
planarity can be maintained by adhesion between the frozen masses
42 and contacts 16, without external fixturing at this stage.
Preferably, however, plates 48 and 50 of the fixture used during
the melting and freezing steps discussed above remain in place.
Alternatively, the assemblage of the wafer can be removed from this
fixture after freezing and placed into another, similar fixture
prior to injection of the flowable material. In either case, the
fixtures will help to maintain the coplanarity and alignment during
injection of the flowable material. In alternative embodiments, the
flowable material may be introduced while the fusible conductive
masses are in their liquid state. In this alternative system, the
flowable material desirably has relatively low viscosity and is
essentially immiscible with the molten fusible conductive material.
For example, many organic materials, such as silicone gel
components, are immiscible with liquid metals. The surface tension
of the liquid fusible material on the opposing contacts tends to
maintain the molten masses in place. The flowable material may wet
the opposing surfaces of the sheet and wafer, so that flow of the
material into space 46 is aided by capillary action.
[0059] After space 46 has been completely filled by the flowable
material, the flowable material is cured to form a compliant
resilient layer 54 occupying space 46 and intimately surrounding
the fusible conductive masses 42 and contacts. The compliant layer,
after curing, should have some resistance to deformation. The
compliant layer may be a solid or a gel. The compliant layer may
incorporate voids, and indeed may take the form of a solid or gel
foam. However, the compliant layer should form substantially
continuous surfaces surrounding the conductive material masses 42.
Preferably, the compliant material has an elastic modulus less than
about 100,000 pounds per square inch, and still lower values of
elastic modulus are more preferred. The compliant layer desirably
has relatively low resistance to shear between opposed surfaces 12
and 26. Preferably, the compliant layer is between about 50 and 250
microns thick. The compliant layer desirably has a low spring
constant per unit area when stressed in shear between opposing
surfaces 12 and 26. The compliant layer desirably also has a
relatively low spring constant with respect to displacement of
surfaces 12 and 26 towards and away from one another. The compliant
layer 54 desirably remains soft and cohesive over a range of
temperatures encompassing at least the range from the melting
temperature of the fusible conductor material 42 to above the
normal operating temperatures of the chips 28 constituting the
wafer. The compliant layer desirably retains these properties from
about 20.degree.C. or below to about 45.degree. C. or higher.
Preferably, the compliant layer retains properties in the aforesaid
ranges from about 0.degree.C. or below to about 60.degree.C. or
above. Most preferably, the compliant layer retains the desired
properties from about -65.degree.C. or below to about 150.degree.C.
or above. In an alternative embodiment, the flowable material may
be introduced around the fusible conductive masses 42 when the
masses are in a liquid state. The fusible masses 42 may then be
frozen before the flowable material cures or after the flowable
material cures.
[0060] The flowable material used to form layer 54 should be
capable of flowing, prior to cure, at temperatures below the
melting temperature of the fusible conductive material in masses
42. To assure complete filling of space 46 by the flowable
material, the flowable material may be injected under pressure.
Also, space 46 may be evacuated prior to injection of the flowable
material. Techniques for evacuation of a space between a flexible
sheet and wafer and for injection of flowable, curable materials
into such a space are further disclosed in the aforementioned
International Patent Publication 96/02068 and United States
Provisional Patent Application 60/001,718. Suitable flowable
materials for forming the compliant layer include polymer
compositions which are initially in the form of liquids but which
cure by chemical reaction of their ingredients to form a solid or
gel. Among the compositions that can be used are silicones, epoxies
and urethanes. Particularly suitable compositions include silicone
gels of the type sold under the designation Sylgard 577 Curable
Silicone Gel by the Dow-Corning Corporation of Midland, Mich. Other
suitable silicon gels are available from the Shin-etsu Corporation
and from the General Electric Corporation of Schenectady, N.Y. The
reaction-curable material may be provided as two mutually reactive
components which are mixed immediately prior to introduction of the
material into space 46 and which react spontaneously with one
another at ambient temperature. Other reactive polymer compositions
can be activated by application of ultraviolet light. The curing
step can also be initiated or accelerated by heating the reactive
polymer composition. Some or all of the curing step may entail
temperatures above the melting temperature of the fusible material
in masses 42. In this case, it is desirable to maintain alignment
of the elements, and maintain planarity of the contacts 16 on the
sheet by holding the assemblage in a fixture during at least the
elevated temperature portions of the curing step.
[0061] After curing to form the compliant layer, wafer 22, sheet 10
and the compliant layer 54 are severed by cutting along saw lanes
30, using a saw of the type commonly used for dicing wafers. This
subdivides the assemblage into individual units 58 (FIG. 3). Each
unit includes one chip 28 as well as a portion of compliant layer
54 overlying the chip and a portion of sheet 10 overlying the chip.
Each unit has terminals 17 with exposed bonding material 20 on the
side of sheet 10 facing away from the chip, and each unit has its
contact 16 connected to the corresponding contacts of the chip by
fusible conductive masses 42. These units can be handled and placed
like other surface mountable electronic devices. As seen in FIG. 4,
unit 58 can be placed on a substrate 60. Substrate 60 has internal
electrical circuitry 62 and contact pads 64 connected to such
circuitry and disposed on a surface of the substrate. The outwardly
facing second surface 14 of flexible layer or second element 10 is
juxtaposed with the surface of the substrate, so that the exposed
surfaces of terminal 17 and bonding material 20 are engaged with
contact pads 64. In this condition, the assembled parts are brought
to an elevated temperature so as to activate the bonding material
20 and bond terminals 17 to contact pads 64 on the substrate.
During this elevated-temperature bonding process, the fusible
conductive masses 42 melt. However, the fusible conductive material
of each mass is contained by the surrounding compliant layer, as
well as by the first element or chip 28 and the second element or
flexible sheet 10 and the contacts on these elements. In those
embodiments where polymer coating 43 is used, the fusible
conductive masses 42 may also be contained by the polymer coating
around the masses. Therefore, the fusible conductive material
remains in position and maintains electrical continuity between
contact 16 and contact 24 on the chip. While unit 58 is in this
condition, contacts 16 and terminals 17 can be displaced readily
relative to chip 28. For example, if the contact pads on substrate
60 are out of plane, or if substrate 60 is tilted out of
parallelism with chip 28, all terminals 17 can still be brought
into engagement with contact pads 64 without applying destructive
forces to the unit. After bonding, the assembly can be cooled,
whereupon the flowable material in masses 42 will freeze.
[0062] Typically, substrate 60 incorporates additional electronic
components, such as additional semiconductor chips and other
components electrically interconnected with chip 28 through the
conductors 62 and contact pads 64 of the substrate and through
terminals 17, contacts 16 and fusible conductive masses 42.
Alternatively or additionally, substrate 60 may include further
connectors such as contact pads 66 or other devices such as
sockets, pins for engagement in sockets, wires or other
conventional interconnection devices for connecting circuitry 62 of
substrate 60 with a still larger circuit. The assembly is quite
compact; each unit 58 occupies an area on the surface of substrate
60 about the same size as the area of chip 42 itself.
[0063] The assembly may be incorporated in an electronic device
such as a computer, a communications device, or an electronic
device associated with a non-electronic machine such as an
automobile or an industrial machine. During use of the device,
electrical signals pass through the substrate and chip via the
contacts and fusible conductive masses. Electrical power is
converted to heat in the device, principally in semiconductor chip
28 and in other electronic elements of the device. The heat raises
the temperature of the chip and the surrounding elements. As the
temperature of the device rises, the fusible conductive masses 42
melt and are contained by the surrounding elements of the
structure, including compliant layer 54, chip or first element 28
and the flexible layer or second element 10 and the contacts 16 and
24 on those elements.
[0064] As the assembly is heated, each contact 24 on the chip
typically moves with respect to the corresponding contact 16 of the
flexible sheet 10 or second element. Thus, as the temperature of
the chip rises, the chip 28 tends to expand, thereby moving
contacts 24 relative to the contact pads 64 of the substrate. For
example, where the chip and substrate are formed from materials
having different coefficients of thermal expansion, the contacts on
the chip will move relative to the contact pads of the substrate as
the entire assembly is heated. Even where the coefficients of
thermal expansion are the same, differential movement will occur if
the temperature of the chip rises or falls at a different rate than
the temperature of the substrate. Also, the chip, the substrate or
both can warp as they undergo thermal expansion and contraction.
Because terminals 17 on the flexible sheet or second element 10 are
bonded to the contact pads 64 of the substrate, contacts 16 will
also move relative to the contacts 24 of the chip. However, while
the fusible conductive material in each mass 42 is at least
partially liquid, the conductive masses have essentially no
resistance to deformation. The only mechanical interconnection
between the first element or chip 28 and the second element or
flexible layer 10, and hence the only mechanical interconnection
between the chip and substrate 60, is provided by the compliant
layer 54. This compliant layer can accommodate substantial movement
of the chip surface relative to the surface of the second element
or layer 10 without applying high forces between these elements.
Accordingly, relative movement of the chip contacts 24 and
substrate contact pads 64 do not apply appreciable forces at the
bonds between the terminals 17 of the second element and the
contact pads 64 of the substrate so that the bonds are not subject
to thermally-induced fatigue as the system operates. Because the
fusible conductive masses 42 are liquid, they are not subject to
fatigue during operation at normal operating temperatures.
[0065] When power to the system is turned off, the device cools and
the fusible conductive masses 42 may freeze again. The cycle of
melting and freezing may be repeated numerous times during the
service life of the device. Defects which may occur in masses 42
are automatically repaired when the masses melt and freeze.
Alternatively, where the device is stored in a relatively warm
environment, the conductive material and the masses 42 may remain
liquid indefinitely. The metal in barrier layer 18 of contact 16 is
selected to prevent dissolution of the base metal of the contact
into the molten conductive material. Similarly, the contacts 24 of
the chip are formed from metals which will not dissolve in the
fusible conductive material. This assures that the composition of
the conductive material will remain essentially unchanged and hence
its melting temperature will not vary during continued use of the
device. Compliant layer 54 protects the fusible conductive material
from contamination and helps to assure reliability of the device.
Additional packaging may be provided around the chip and substrate.
For example, the chip and substrate may be encapsulated in a
flexible encapsulant. The encapsulant may also penetrate between
layer 10 and substrate 60. Other conventional packaging elements,
such as metallic shields or "cans", heat spreaders and the like may
be included in the assembly. As discussed in more detail below,
further fusible conductive masses may be provided between the chip
or other first microelectronic element and the packaging element to
conduct heat for dissipating heat from the assembly.
[0066] In a variant of the assembly process discussed above, the
finished unit 58 may be tested by engaging it with a test substrate
so as to engage the exposed surface of each terminal 17 with a
contact on the test substrate and then operating chip 28 by
applying signals through the terminals. Prior to or during such
engagement, unit 58 is heated to a temperature high enough to melt
the fusible conductive masses 42, but not high enough to activate
the bonding material 20 on the contacts. This allows the compliant
layer 54 and masses 42 to deform and hence allows the terminals 17
on the exposed surface of the unit to engage the contacts of the
test substrate even where the test substrate and/or terminals 17
are not precisely coplanar.
[0067] An assembly according to a further embodiment of the
invention (FIG. 5) includes a substantially rigid first element
such as a chip 128 with contacts 124 on a front surface 126, and
also includes a substantially rigid second element such as a
substrate 160 with contacts 164 on an interior surface. The
contact-bearing surface 126 of the chip overlies the
contact-bearing surface of the substrate. Here again, masses of a
fusible conductive material 142 are disposed between contacts 124
and contacts 164. Masses 142 are surrounded by a compliant layer
154 substantially filling the space between the confronting
surfaces of the first and second elements and intimately
surrounding masses 142. In this embodiment as well, the contacts
are provided with barrier layers to avoid dissolution of the
contact metals in the fusible conductive material. Structures
according to this embodiment may be fabricated by assembling the
first and second elements with the fusible conductive material
masses, momentarily melting the masses by heating the assembly and
then freezing the masses. These steps may be performed using
techniques similar to those used in the so-called controlled
collapse chip connection technique, commonly referred to as "C4"
bonding. C4 bonding is described in detail in Multi-Chip Module
Technologies and Alternatives-the Basics, Doane and Franzon, eds;
1993, pp. 450-476 and 434-446, the disclosure of which is hereby
incorporated by reference herein. However, the steps of C4 bonding
involving melting of the solder typically would be performed at a
far lower temperature in preferred embodiments according to this
aspect of the invention than in conventional C4 bonding processes
employing ordinary solder. After joining the chip and substrate by
C4 bonding, compliant layer 154 is formed by injecting a flowable
material as an encapsulant into the space between the confronting
surfaces of the chip and substrate and curing the flowable to form
a solid, gel or form as discussed above. Assemblies according to
this aspect of the present invention provide benefits similar to
those discussed above. Once again, at operating temperature, the
chip is mechanically connected to the substrate only through the
compliant layer 154. Masses 142 are molten and hence provide
essentially no resistance to relative movement between the chip and
substrate contacts or between the chip 128 and the substrate 160 as
a whole. By contrast, in conventional assemblies fabricated by C4
processing, the solder joints remain solid at operating temperature
and are subjected to fatigue stresses during thermal cycling.
[0068] As illustrated in FIG. 6, a further embodiment of the
invention provides a connection component 200 including a compliant
layer 254 with a first surface 253 and a second surface 255. Holes
or cavities 243 extend through the compliant layer. A mass 242 of a
fusible conductive material is disposed within each hole or cavity
243. A connection component according to this aspect of the
invention, can be fabricated by procedures similar to those
discussed above. However, in this instance the first element 210
and second element 222 are both held in a taut condition and
aligned with one another with fusible conductive masses 242
disposed therebetween. Once again, after the conductive material
has been reflowed into contact with contacts 258 and 262, it is
frozen and the flowable material is injected and cured to form
compliant layer 254. Connection components according to this aspect
of the invention can be used for interconnecting other
microelectronic elements such as a chip and a substrate. Thus, the
connection component can be placed between confronting surfaces of
an element such as a chip 280 and another element such as a
substrate 282 (FIG. 7) so that first terminals 260 face the
contacts of chip 280 whereas second terminals 261 face the contacts
of substrate 282. The assemblage is heated to a temperature
sufficient to activate the solder or bonding materials 263, 264 on
the terminals, thereby fusing the terminals to the contacts of the
chip and substrate. During this step, the fusible conductive
material 242 melts, but is retained in position by compliant layer
254. The compliant layer can bend and compress locally as required
during this bonding process, to assure good engagement between
terminals 260 and 261 and the contacts of the chip and substrate.
After completion of the bonding process, first terminals 260 and
hence contacts 258 are fixed to the chip, whereas second terminals
261 and the associated contacts 262 are fixed to the substrate. A
further encapsulant (not shown) may be introduced between sheet 222
and chip 280, and between sheet 222 and the substrate, to fill
voids in these regions. In use, the fusible conductive masses 242
melt and allow contacts 258, fixed to the chip, to move relative to
contacts 262 on the substrate.
[0069] As shown in FIG. 8, a simpler connection component includes
a layer of a matrix material 354 having oppositely directed top and
bottom surfaces 353, 355 extending in lateral directions. The
matrix material may include compliant materials having a
degradation temperature higher than the melting temperature of the
fusible conductive masses or curable precursor materials. The
compliant material may be of the same types as discussed above. A
curable precursor matrix material may be formed from the flowable
materials discussed above, but preferably has somewhat greater
cohesion. For example, a curable precursor matrix material can be
formed by partially curing a flowable liquid material to provide a
material which is still deformable but which has some cohesion and
which will remain in place. Alternatively or additionally, fillers
such as silica may be added to a flowable material to make it
highly thixotropic. The matrix layer defines holes or cavities 343
extending between these surfaces. Masses 342 of a flowable
conductive material are disposed within cavities 343. The
individual fusible conductive masses are dispersed in the layer of
matrix material so that the masses are spaced from one another in
the lateral directions and separated from one another by the matrix
material. The fusible conductive masses preferably are formed from
fusible materials of the types discussed above. Each mass 342 has
an exposed portion 352 at the first surface of matrix layer 354 and
a similar exposed portion 354 at the second surface. A first
removable release liner 357A overlies the top surface 353 of the
layer 354 and a second release liner 357B overlies the bottom
surface 355 of the layer 354. A thin layer of an adhesive 359
optionally may be provided over the top surface 353 or the bottom
surface 355, as by coating these surfaces before the release liners
357A and 357B are provided and preferably before masses 342 are
provided. The release liners isolate the layer 354 and the masses
342 from contaminants during storage.
[0070] Components according to this embodiment of the invention can
be fabricated by a variety of processes. Thus, the compliant layer
354 can be injection molded to form cavities 343 and then filled
with the flowable conductive material. To facilitate such filling,
the interior surfaces of holes 343 may be treated to improve
wettability of the compliant material by the conductive material,
as by electroless plating of the interior surfaces of the holes.
Alternatively, the masses 342 can be placed into a mold and a
liquid material may be introduced into the mold and solidified
around the masses in the manner discussed above.
[0071] Components according to FIG. 8 may be provided, handled and
stored separately from the microelectronic elements. These
components can be used to interconnect opposed microelectronic
elements similar to those shown and described above. In such an
assembly process, the component is assembled with one
microelectronic element, so that the top or bottom surface of the
matrix layer is engaged with the microelectronic element. Where the
conductive masses are to provide circuit interconnection, this
assembly procedure should be performed so that the conductive
masses are engaged with contacts on the microelectronic element.
The opposite microelectronic element is engaged with the other
surface of the matrix layer in similar fashion. Preferably, the
matrix layer is bonded to the surfaces of the microelectronic
elements. The adhesive layer provided over the top and bottom
surfaces 353 and 355 bonds and assures a void-free interface
between layer 354 and the microelectronic elements. Alternatively
or additionally, compliant layer 354 itself may be arranged to
adhere to the surfaces of the mating elements. For example, matrix
layer 354 may be formed as a partially cured or "B-stage" material.
When the component is engaged between mating elements and heated to
melt masses 342, the partially cured material fully cures and bonds
with the surfaces of the mating elements. Once the component is
assembled with the opposed microelectronic elements, the contacts
on the opposed elements bear on the fusible conductive masses 342
and help to contain the masses within holes 343. The resulting
assembly may have a configuration similar to the assembly of FIG.
5, in that the fusible conductive material bears directly on the
contacts of the elements such as a chip and a substrate, and the
surfaces of compliant layer 354 bear directly on these elements.
Separately-formed components as shown in FIG. 8 are particularly
useful in providing thermally conductive connections between
elements. In this use, the components can be applied to surfaces of
microelectronic elements without regard for locations of electrical
contacts. In one process, a large component as shown in FIG. 8 is
applied to the rear surface 27 of a wafer (FIG. 2) before the wafer
is severed to separate the individual chips from the wafer. Thus,
each severed chip includes a matrix layer conductive masses on its
rear surface, which can be used to mount the rear surface on a
circuit board or to connect a package element such as a heat
spreader or heat sink to the chip. Such a process can be performed
in conjunction with processes as discussed above for providing
similar masses on the front or contact-bearing surface.
[0072] Further embodiments, not shown in the drawings, can
incorporate combinations of the features discussed above. For
example, a component as depicted in FIG. 8 may include terminal
assemblies as depicted in FIGS. 6 and 7 on one or both sides of the
compliant layer without the flexible sheets 210, 222, or with such
a flexible sheet on only one side.
[0073] One consideration in design is the effect of alpha radiation
emitted by the conductive materials. Alpha radiation is known to
damage the electronic components incorporated in semiconductor
chips and to cause momentary errors in operation of such
components. Fusible conductive materials which contain heavy metals
typically contain small amounts of radioactive isotopes which emit
alpha particles. Several measures may be taken to control the
effects of alpha radiation on the underlying chip. One such
approach is to limit the amount of alpha particle radiation emitted
by controlling the radioactive isotope content of the fusible
conductive material. Alternatively or additionally, the physical
configuration of the contacts and fusible conductive masses may be
selected to limit the effects of alpha radiation. Ordinarily, the
contacts themselves provide effective shielding against alpha
radiation. Alpha particles normally cannot pass directly through a
metallic contact into the underlying electronic components of the
chip. For example, as shown in FIG. 9, the alpha radiation emitted
by fusible conductive mass 442a normally cannot pass directly
through contact 424a. Any deleterious effects of alpha radiation on
the chip are caused by alpha particles passing around the edges of
the contact along paths such as path 443. Simply increasing the
thickness of contact 424a limits the effect of such alpha
radiation. Because the fusible material mass 442a is spaced at a
substantial distance above the surface of chip 428, alpha particles
passing along path 443 and along other paths around the periphery
of contact 424a must pass through a substantial thickness of
compliant layer 454 so that the compliant material absorbs most of
the alpha radiation. Alternatively or additionally, the contact may
have a larger diameter than the fusible conductive mass. For
example, contact 424b has a diameter D.sub.c substantially larger
than the diameter D.sub.m of mass 442b at the juncture of the mass
and contact. This assures that any alpha radiation passing around
the periphery of the contact will pass along a path 443b at a
relatively low angle to the chip surface. Here again, the length of
a straight path from the fusible conductive mass to the chip
surface is markedly increased. To assure that the fusible
conductive mass remains at or near the center of contact 424b, the
contact is provided with a ring of a material which is not wettable
by the fusible conductive material. The polyparaxylene coating
described above may maintain the fusible conductive mass in place
when the mass is in its liquid state. In still further embodiments,
the contact or the barrier metal at the surface of the contact
adjacent the fusible conductive mass may be non-wettable by the
fusible material. A small spot adjacent to center of the contact
may be plated with a metal which is wettable by the fusible
material. Also, the mass 442b is tapered inwardly towards its
central axis in the vertical direction upwardly, away from the
contact 424b. This further assures a long path length from the mass
surface to the chip surface. Such tapered masses can be produced by
processes such as that discussed above with reference to FIG. 8.
Also, the masses can be tapered by techniques commonly used in the
C4 bonding art, as by momentarily moving chip 428 away from the
mating element 410 while the fusible conductive material is in a
molten state.
[0074] As shown in FIG. 10, an assembly in accordance with a
further embodiment of the invention includes a chip or first
element 522 and a second element 510 including a flexible
multilayer sheet. Sheet 510 has contacts 516 on a first side facing
inwardly, toward the chip or first element, and has terminals 517
on the opposite, outwardly-facing side. As in the embodiments
discussed above, contacts 516 are disposed in a pattern
corresponding to the pattern of contacts 524 on the chip. Terminals
517 are not integral with contacts 516. Instead, the terminals are
distributed on the outwardly-facing side of sheet 510 in an array
different from the pattern of contacts 516. In the depicted
embodiment, terminals 517 occupy a larger area of the sheet than
contacts 516. Terminals 517 are connected to contacts 516 by leads
519 extending within sheet 510. Sheet 510 may be a multilayer
structure, with the leads disposed between layers. The assembly
further includes additional electrical elements 580 mounted to
sheet 510 and electrically connected to leads 519, so that the
additional elements are connected between some of contacts 516 and
terminals 517. The additional elements may include any circuit
element, but most typically include capacitors. The capacitors
typically are connected to the terminals and contacts which form
the power and ground connections to the chip. As in the
arrangements discussed above, the electrical contacts 524 of the
first element or chip 522 are connected to the contacts 516 by
fusible, electrically-conductive masses 542.
[0075] The assembly further includes a package element adapted to
physically support and protect the chip and additional electrical
elements. The package element is depicted schematically as a heat
sink 584 defining a back wall and a separate ring 586 surrounding
the chip and additional circuit elements. Heat sink 582 thus forms
a back wall of the package, whereas ring 586 forms side walls.
Flexible sheet 510 extends across the front of the package, and
overlies ring 586. Ring 586 and heat sink 582 can also be formed
integrally with one another to provide a unitary shell. The back
wall or heat sink 584 has a region 592 confronting the rear surface
590 of the chip. A thermally conductive adhesive 594 forms a bond
between the rear surface 590 of the chip and region 592 of heat
sink 584, and provides enhanced thermal conductance between the
rear surface 590 and the heat sink or back wall. Other devices for
providing enhanced thermal conductance may be used. For example,
arrangements of flexible thermal conductors as taught in copending,
commonly-assigned U.S. patent application Ser. No. 08/342,222,
filed Nov. 18, 1994, the disclosure of which is also incorporated
by reference herein, may be employed between the chip and the heat
sink. Alternatively, as further discussed below with reference to
FIGS. 24-26, fusible conductive masses having properties identical
to or substantially similar to those described above may be
provided between the rear surface 590 and the heat sink to conduct
heat therebetween during operation of the assembly.
[0076] As in the embodiments discussed above, the fusible
conductive masses 542 disposed between the contacts are intimately
surrounded by a layer of a compliant material 554. The compliant
layer 554 may be formed integrally with encapsulant filling the
space cooperatively enclosed by the package elements 584 and 586
and sheet 510. In fabrication of the assembly, chip 522, sheet 510
and fusible masses 542 may be assembled as discussed above, and
additional circuit elements 580 may be assembled to the sheet. A
first portion of the encapsulant may be introduced into the space
between the chip and sheet and cured to form compliant layer 554
while leaving rear surface 590 exposed. After this step, thermal
adhesive 594 and heat sink 582 may be added. The reverse process
may also be employed, in which the chip is assembled to the heat
sink with thermal adhesive 594, followed by assembly of sheet 510
and masses 542 and formation of layer 554.
[0077] The assembly can be handled and mounted using ordinary
surface-mounting techniques. Here again, terminals 517 on sheet 510
are bonded to contact pads 564 of a substrate 568 to form the
electrical connections between the chip 522 and other components.
During the surface mounting procedure, the assembly may be exposed
to temperatures in excess of the melting temperature of the
conductive masses 642 so that the masses melt. As described above,
the resulting liquid masses are contained by the compliant layer
554. After the surface mounting procedure, the masses are allowed
to freeze. The assembly may also be exposed to high temperatures
during other manufacturing procedures, such as during soldering of
substrate 568 to other components; during high-temperature
encapsulation processes such as molding or high-temperature curing
of an encapsulant around the assembly; or during testing, storage
or shipment. During each such high-temperature exposure, fusible
conductive masses 542 melt and allow movement of terminals 517
relative to the chip. During operation of the completed assembly,
masses 542 will melt, but will remain in place to provide a
stress-free electrical interconnection as discussed above.
[0078] Referring to FIG. 11, a process according to a further
embodiment of the present invention utilizes a plate 602 of a
metal, preferably copper or a copper alloy. The metallic plate is
large enough to cover an entire wafer; however, only a small
portion of the metallic plate is seen in FIG. 11. A first surface
of the plate is covered by a first resist layer 604 with apertures
606 disposed in locations corresponding to the locations of
contacts on a wafer. The locations and sizes of apertures 606 can
be controlled precisely using conventional photographic techniques
for forming resist patterns. The second surface of the plate is
covered by a uniform layer of a resist 608. Spots 610 of a barrier
metal are then applied on the first surface of the plate in
apertures 606. After the barrier metal is applied, the plate is
then exposed to the fusible material in molten form, as by dipping
the plate into the molten material; by passing the plate through a
flowing curtain or shower of the molten material; or by exposing
the first surface of the plate to a wave of molten material using
conventional wave-soldering equipment. The dipping procedure is
preferred. Because the molten material does not wet the resist
layers 604 and 608, but does wet barrier metal 610, a drop 612 of
molten material will cling to the plate at each aperture 606 after
the plate is withdrawn from the molten material. The drops freeze
to form fusible material masses. Numerous masses can be formed
simultaneously at extremely low cost; there is no need for
controlled application of the molten material.
[0079] In the next stage of the process, resist layer 604 and 608
are stripped using conventional removal techniques. Metallic plate
602, with masses 612 on it, is heated to a temperature sufficient
to remelt fusible material 612 and the plate 602 and masses are
assembled to a wafer 622. The plate is aligned with the wafer so
that each mass 612 is aligned with a contact 624 on the surface of
the wafer. The fusible conductive masses 612 bonds with the
contacts 624 of the wafer. After the bonds have formed, the
assembly is cooled to below the melting temperature of the fusible
conductive masses, thereby refreezing masses 612.
[0080] Because plate 602 is metallic, its thermal expansion
properties are quite uniform and isotropic so that the distances
between the masses vary in a predictable manner with the
temperature of the plate. Moreover, the metal plate resists
stretching and compression in directions parallel to its surfaces.
These factors greatly facilitate precise alignment of masses 612
with contacts 624. During assembly with the wafer 622, plate 602
can be supported and engaged with the wafer by a press plate,
similar to the press plates 50 discussed above with reference to
FIG. 1, which supports plate 602 over substantially its entire
surface and reinforces plate 602 against bending. However, where
plate 602 is thick enough to resist bending, it can be handled and
assembled to the wafer using other equipment which does not support
the plate over its surface.
[0081] Referring to FIG. 13, after masses 612 have frozen, a
curable material is injected between plate 602 and wafer 622 and
cured to form a compliant material layer 626 intimately surrounding
masses 612. After curing of the compliant material, an
etch-resistant metal, such as gold, is applied in spots 628 aligned
with masses 612. The etch-resistant metal can be applied using
conventional plating techniques with a conventional photoresist
(not shown). After the photoresist is stripped, plate 602 is
exposed to an etchant, such as an acid, which removes the plate
except in the regions 630 protected by spots 628. During the
plating, resist-stripping and etching steps, wafer 622 is protected
from chemical contamination by the overlying compliant layer
626.
[0082] The etching process thus subdivides the plate into separate
regions 630, leaving each region attached to a mass of fusible
conductive material 612. Each region 630, with the overlying metal
spot 628, forms a separate terminal assembly 632, mechanically
decoupled from the other terminal assemblies. A further bonding
material 634 may be applied on the terminal assemblies, and the
wafer may be severed to form individual units, each including one
chip and the associated terminal assemblies and fusible masses,
together with a portion of the compliant layer. The finished unit
thus has a microelectronic element 622; a layer 626 of a compliant
material overlying the microelectronic element, and terminal
assemblies 632 disposed on the side of layer 626 opposite from the
microelectronic element. Each terminal assembly is connected to a
contact 624 on the microelectronic element by a fusible mass 612.
The units may be handled, tested and bonded to substrates in the
same manner as the units 58 discussed above with reference to FIG.
3.
[0083] In a variant of this process, the patterned resist 604 (FIG.
11) is replaced by a flexible dielectric sheet 650 (FIG. 15)
defining the same pattern of apertures. Sheet 650 may be formed
from a polymeric material such as a polyimide. The sheet may be
formed in situ on the surface of the metal plate, as by coating the
surface with a liquid precursor and curing the coating to form the
sheet. The polymeric sheet may be provided with the apertures by
selectively etching or ablating the sheet using processes which do
not substantially affect the underlying metal plate. The remaining
steps of the process are conducted in substantially the same way as
discussed above with reference to FIGS. 11-14, except that sheet
650 is not removed. Thus, sheet 650 remains during and after the
etching process to provide additional protection to the wafer.
During the severing step, sheet 650 is severed along with the
compliant layer 626.
[0084] Referring to FIG. 16, a great variety of terminal shapes and
types may be provided using the processes discussed above. The
etching step used to subdivide the metallic plate may form numerous
posts 655, each constituting a single terminal assembly associated
with one fusible metal mass 612. The techniques used for forming
posts disclosed in copending, commonly-assigned U.S. patent
application Ser. No. 08/366,236, filed Dec. 29, 1994, the
disclosure of which is hereby incorporated by reference herein. As
set forth in said '236 application, a continuous metallic plate can
be formed into a plurality of posts by applying a photoresist to
the exposed surface of the plate and selectively treating the
photoresist to leave a pattern of spots covered by etch-resistant
regions, and then exposing the surface to an etchant. As also
described in the '236 application, a microelectronic assembly
having an array of such posts can be engaged with a mating unit
having sockets adapted to engage the posts.
[0085] A different form of terminal assembly, also illustrated in
FIG. 16, has a solid-core solder ball including a core 660 formed
from a high-melting, highly conductive metal such as copper,
surrounded by a layer 662 of a solder overlying a region 630 formed
by severing the sheet. Another form of terminal assembly has a mass
of conventional solder 668 overlying each region 630. Yet another
terminal assembly has a metal bump 670 formed from gold, copper or
other solderable metal on each region. As will be appreciated, the
various types of terminal assemblies illustrated in FIG. 16
normally are not found in a single unit; they are illustrated
together for ease of comparison. The terminal assemblies on
dielectric sheet or element are disposed on the side of the sheet
facing away from the opposite element or chip 622. However, each
terminal assembly defines a contact surface, covered by barrier
metal spot 610, facing toward the opposite element or chip 622 and
exposed through an aperture 651 in sheet 650. Thus, each fusible
conductive mass extends through an aperture in the sheet to the
contact surface of the associated terminal assembly.
[0086] In the embodiment of FIG. 17, one element is a multilayer
dielectric sheet 700 with conductors 702 and potential planes 704
disposed in and on the sheet. Each terminal assembly includes a
metallic via liner 706 extending through the sheet and defining a
contact surface on the side of the sheet facing toward the opposite
element 722. Each via liner defines a terminal 708 on the surface
facing away from the opposite element 722, and a bonding material
710 may be provided on such terminal surface. The contacts and
terminals discussed above with reference to FIGS. 1-4 may have the
configuration illustrated in FIG. 17.
[0087] As illustrated in FIG. 18, it may be desirable to form
fusible conductive masses by applying a first mass 800 in contact
with the terminal assembly or contact 802 of one element; applying
a second mass 804 in contact with the terminal assembly or contact
806 of the opposite element, and then merging these masses with one
another by bringing the elements towards one another while the
masses are both liquid or molten. Masses 800 and 804 can be applied
to the individual elements while the surfaces of the elements
remain open and accessible. Thus, fluxes may be applied to
facilitate wetting of the contacts by masses 800 and 804, and can
be removed readily by rinsing. Once masses 800 and 804 have wet
their respective contacts, they can be readily united with one
another; the fusible material will merge with itself without
difficulty. As illustrated in FIG. 18, the fusible materials on
each element may be surrounded by partially cured compliant
material layers 810, 812 on each element, and these layers may be
united with one another when the elements are brought together, so
as to form the compliant material layer surrounding the united
fusible conductive masses. Alternatively, the compliant layer may
be formed in place between the elements, in the manner described
above, after uniting the fusible conductive masses.
[0088] As shown in FIG. 19, there need not be one-to-one
association between the fusible masses and the terminal assemblies
or contacts. Thus, a large terminal assembly or contact 904 on one
element may be connected to several fusible masses 912. Conversely,
a large mass 914 may be connected to several terminal assemblies
905. Thus, each mass may be associated with one or more contacts or
terminal assemblies on each element, and each terminal assembly or
contact may be associated with one or more fusible masses.
[0089] As shown in FIG. 20, an assembly according to a further
embodiment of the invention includes a chip 1022 connected to the
substrate 1068 through fusible material masses 1042 which are again
surrounded by a compliant material 1054. Thermal insulation 1070
may be provided around the assembly, particularly in the areas
adjacent the fusible masses, so as to assure that the fusible
masses reach their melting temperature when the chip is in
operation.
[0090] As will be readily appreciated in light of the foregoing
discussion, numerous variations and combinations of the features
discussed above can be utilized without departing from the present
invention. For example, fusible conductive materials other than
metals can be employed. These include aqueous and non-aqueous
electrolytes. Low-melting conductive compositions including
polymeric materials can also be employed. Moreover, the fusible
conductive material need not be uniform in composition and need not
be entirely molten even at the operating temperature of the device.
For example, the fusible conductive material may include particles
of a first conductive material such as copper, silver or graphite
having a high melting temperature dispersed in a second conductive
material, such as a low-melting solder or an electrolyte, having a
lower melting temperature. At the operating temperature of the
device, the second conductive material is liquid but the first
conductive material remains solid, so that the fusible material as
a whole is in the form of a conductive slurry. As used in this
disclosure, the term "liquid" should be understood as including a
slurry unless otherwise specified. The first and second conductive
materials should be insoluble in one another and non-reactive with
one another. The particles of the first conductive material can be
plated or otherwise coated with a barrier material as discussed
above to inhibit solution and reaction between the particles and
the second conductive material.
[0091] In the embodiments discussed above, the fusible conductive
masses melt during normal operation of the assembly. In a further
variant of the invention, the melting temperature of the fusible
material is above the normal operating temperature of the
microelectronic elements, but below the temperatures encountered by
the assembly during manufacturing, storage or shipment. In this
variant, the fusible conductive material acts to limit stress
applied to the electrical connections due to high temperature
exposure in processing steps such as manufacturing, storage or
shipment. Indeed, the assembly may be deliberately heated so as to
melt the fusible material and thus repair any defects in the
masses. Where the assembly is part of a larger device such as a
multichip module or circuit board, the melting temperature of the
fusible material desirably is below the maximum temperature which
can be tolerated by the remainder of the device, so that the
assembly can be repaired by heating without removing it from the
remainder of the device.
[0092] According to further variants of the invention, the elements
which are electrically connected to one another need not have
confronting surfaces, and the masses of liquid or fusible material
need not be physically disposed between the elements. For example,
as shown in FIG. 21, a microelectronic element such as a
semiconductor chip 1122 may be provided with beam leads 1125
connected to the contacts 1124 of the chip. The beam leads project
outwardly away from the chip. An outboard end of each beam lead,
remote from the chip, is embedded in a mass 1142 of a fusible
conductive material as discussed above, and electrically connected
through such mass to a contact 1164 on a substrate 1168. Thus, each
beam lead is electrically connected in series with a mass of the
fusible conductive material to form an electrical interconnection
between the chip and substrate . The beam leads and fusible masses
are covered by a mass of soft, compliant dielectric encapsulant
1154 which intimately surrounds and protects the fusible masses. In
this embodiment as well, the contacts 1124 of the chip or first
element 1122 are electrically connected to the contacts 1164 of the
substrate or second element 1168. When the assembly is exposed to
high temperatures, the fusible material melts, allowing beam leads
1125 to move relative to the substrate. This action relieves stress
on the beam leads and on the connections between the beam leads and
the contacts 1124 of the first element. Here again, the surrounding
compliant dielectric material 1154 contains the liquid masses 1124
and maintains them electrically isolated from one another. Other
physical configurations may be used, provided that the contacts of
the elements are electrically connected to one another through
masses of liquid or fusible material, and provided that the fusible
masses are contained by the surrounding compliant dielectric
material. For example, the fusible masses may be used in
conjunction with flexible leads as taught in the aforementioned
U.S. Pat. Nos. 5,148,265; 5,148,266; 5,455,390 and International
Publication WO 96/02068, the disclosures of which are hereby
incorporated by reference herein. Indeed, a fusible material as
referred to herein may serve as a bonding material for connecting
the flexible leads as taught in these documents to a
microelectronic element.
[0093] Referring to FIG. 22, an assembly in accordance with a
further embodiment of the invention provides a chip package
including a rigid panel 1210 such as a conventional fiber
reinforced epoxy panel of the type commonly referred to as a "FR-4"
circuit board or a ceramic circuit panel; a chip 1222 disposed
above the panel and a combined thermal spreader and protective
shield 1282 formed from a metal or a metal compound such as
aluminum nitride disposed above the chip. The chip contacts 1224
are electrically connected to panel contacts 1216 by masses of
fusible conductive material 1242 in the manner discussed above. The
masses of fusible material are surrounded by a compliant material
1254 forming a layer between the chip and panel. Further portions
1255 of the compliant material fill the space around the chip,
between the shield 1282 and the panel 1210. Panel contacts 1216,
and hence the chip contacts, are electrically connected to
terminals 1228 by leads on the panel (not shown). Terminals 1228
include solder balls for mounting the packaged chip to a larger
circuit panel.
[0094] Yet another embodiment of the invention provides a connector
(FIG. 23) similar to that discussed above with reference to FIG. 8
having terminal assemblies similar to those discussed above with
reference to FIGS. 14-17 on both surfaces. The connector thus
includes a layer 1354 of compliant material having first terminal
assemblies 1332 on a first surface. Each first terminal assembly
defines a contact surface 1310 facing toward the compliant layer
and a terminal 1328 facing away from the compliant layer.
Similarly, second terminal assemblies 1333 on the second surface of
layer 1354 define contact surfaces 1311 facing toward the compliant
layer and terminals 1329 facing away from the compliant layer.
Masses 1342 of fusible material extend between the contact surfaces
and electrically interconnect each first terminal with a second
terminal. A connector of this type may be connected between a pair
of microelectronic elements to provide an assembly as discussed
above. The connector can be fabricated by a process as discussed
above with reference to FIGS. 12-15, utilizing two metallic plates.
The fusible masses are provided between the metallic plates,
followed by formation of the compliant layer between the plates.
After the compliant layer is formed, both metallic plates are
subdivided, as by etching, to form the separate terminal
assemblies.
[0095] Still another embodiment of the present invention provides a
microelectronic package as shown in FIG. 24. The microelectronic
package 1410 includes a first microelectronic element such as a
semiconductor chip 1424 which has a generally planar front surface
1426 including electrical parts or contacts 1428 formed on
peripheral regions of the front face 1426. A second microelectronic
element or circuit element 1412 is provided in the form of a
sheet-like dielectric film 1412 having a first surface 1414 and a
second surface 1416. The dielectric film 1412 has electrically
conductive parts including conductive terminals 1418. Although
terminals 1418 are physically disposed on the first surface 1414 of
the sheet, they are accessible at the second surface 1416 for
connection through vias 1419 extending through the sheet. The
second microelectronic element or sheet 1412 overlies the front or
contact-bearing surface of chip 1424, so that these elements define
a front space 1434 therebetween. A plurality of thermally
conductive fusible masses 1422, formed from fusible materials as
discussed above, are disposed in the front space 1434a. A compliant
dielectric material fills front space 1434a, and surrounds masses
1422 as discussed above. However, masses 1422 do not electrically
connect the chip with terminals 1418. Rather flexible leads 1420
extending from the terminals 1418 electrically connecting the
terminals to the contacts 1428 of the chip. These flexible leads
may be provided by conventional processes such as wire bonding, or
else may be formed by processes in as shown in U.S. Pat. Nos.
5,398,863; 5,390,844; 5,536,909 and 5,491,302, utilizing leads
which are initially formed on the flexible sheet. The leads may be
connected to contacts 1428 before the compliant material is applied
in the front space, while fusible masses 1422 are in a solid state
and the sheet is supported above the surface of chip 1424 by
masses. As discussed in the aforementioned patents, the leads may
initially extend across a bond window 1432 in the sheet, and the
bond window may be sealed by a mask or coverlay 1436 prior to
introduction of the compliant material.
[0096] The semiconductor chip 1424 also has a rear surface 1430
which faces away from the front surface 1426 and faces away from
dielectric film 1412. In order to dissipate heat from the chip
1424, as well as support and protect the chip 1424, a third
microelectronic element 1442 which is a package element, such as a
heat sink, is provided. Third element or heat sink 1442 includes a
back wall 1444 and side walls 1446 forming a unitary shell and
surrounding the chip 1424. The dielectric sheet 1412 extends across
the front of the heat sink 1442. The heat sink 1442 has a central
region 1448 confronting the rear surface 1430 of the chip 1424.
Thus, the first element or chip 1424 is sandwiched between the
second element or dielectric sheet 1412 and the third element--the
package element or heat sink 14442. A rear space 1434b is defined
between the surface of central region 1448 of the third element or
heat sink 1442 and the rear surface of the first element 1430
Masses of a fusible conductive material 1450 are disposed in this
rear space, between the rear face 1430 of the semiconductor chip
1424 and the central region 1448 of the heat sink 1442 to provide
heat or thermal conductance between the chip 1424 and the heat sink
1442. The thermally conductive material incorporated in the masses
1450 comprises an ultra-low melting point solder which is similar
to the fusible conductive material described above in reference to
FIGS. 1-23. Thermally conductive masses 1450 may be positioned in
the rear space by depositing them on the surface of the heat sink
or on the rear surface of the chip before assembling these
elements. For example, the heat sink, with the masses thereon in a
molten condition, may be assembled to the back of the chip. This
may be performed before or after assembly of the dielectric sheet
or first element and the chip.
[0097] A barrier layer 1452, similar to the barrier layers
described above, may be disposed between the rear face 1430 of the
semiconductor chip 1424 and the fusible conductive masses 1450. The
barrier metal layer 1452 is preferably wettable by the fusible
conductive masses 1450 when the latter is in its liquid state. The
barrier metal may be formed as spots which are coextensive with
each fusible conductive mass so that the fusible conductive masses,
when in the liquid state, will only wet to the spots.
Alternatively, the barrier metal may be provided as a contiguous
layer which substantially covers the rear face 1430 of the chip
1424. When the barrier metal is formed as a contiguous layer, then
a non-wettable solder mask or screen should be employed to contain
the masses in place and separate from one another when the masses
are in a molten condition. Barrier layer 1452 may also act to block
alpha radiation from the masses As discussed above, the barrier
layer composition should be selected to prevent diffusion of the
barrier metal into the chip 1424 and/or to prevent contamination of
the thermally conductive masses by the material of the chip 1424.
Selection of an appropriate barrier metal will assure that the
composition of the fusible conductive masses 1450 will remain
essentially unchanged and hence its melting temperature will not
vary during continued use of the device. If the material of the
heat sink is not compatible with the fusible material, a second
barrier layer 1454 may also be disposed between the heat sink 1442
and the fusible conductive masses 1450 to avoid the problems set
forth above.
[0098] After the fusible conductive masses 1450 and 1422 have been
disposed in the front and rear spaces as discussed above, the
fusible conductive masses are surrounded by a curable liquid which
fills the front space 1434a and which also fills the rear space
1434b. Thus, a single flowable material, such as a curable liquid
is introduced simultaneously into the front and rear spaces, and
forms compliant layers in both of these spaces. This material is
then cured to form the compliant layers. Desirably, the cured
compliant material also extends between the edges of the chip and
the side walls 1446 of the package element, so that the compliant
material encapsulates the chip 1424. The compliant material
desirably also encapsulates the flexible leads 1420. The
encapsulant used to form the compliant layer should be capable of
flowing, prior to cure, at temperatures below the melting
temperature of the fusible conductive masses 1450 and 1422. To
insure complete filling of the spaces by the flowable material, the
flowable material may be injected under pressure.
[0099] The assembly described above may be incorporated in an
electronic device such as a computer or communications device by
connecting the terminals 1418 of the semiconductor package 1410 to
contacts 1440 on the substrate 1438. To facilitate such connection,
sheet 1412 and particularly terminals 1418 on the sheet desirably
are coplanar or substantially coplanar. Such planarity can be
provided by engaging the sheet or first element 1412 with a planar
platen (not shown) and forcing it towards the package element or
heat sink 1442 while masses 1422 in the front space, and preferably
rear-space masses 1450 as well are in a molten condition. This also
causes compression of the compliant materials in the front and rear
spaces. A planarization process may also occur during bonding of
the terminals to a substrate, as during a surface mounting
operation. Thus, under the conditions used for surface mounting,
the molten masses 1422 and the soft compliant layer allow the
terminals and sheet to move into engagement with the contact pads
1440 of the substrate.
[0100] During operation as the assembly is heated and cooled, each
contact 1428 on the chip 1424 typically moves with respect to the
corresponding contact 1440 on the substrate 1438. Also, the chip
1424, the substrate 1438 or both can warp as they undergo thermal
expansion and contraction. Because the terminals 1418 on the
dielectric film 1412 are bonded to the contacts 1440 of the
substrate 1438, the dielectric film terminals 1418 will also tend
move relative to the contacts 1428 on the chip 1424. At operating
temperature, however, the masses 1422 are molten. The compliant
layer in the front space 1434a, and the molten masses 1422 can
accommodate substantial movement of the chip 1424 relative to the
dielectric film 1412 and terminals 1418 without applying high
forces between these elements. The flexible leads provide
electrical interconnection while still permitting such movement.
Masses 1422 will provide good heat transfer from the chip 1424 to
dielectric sheet 1412 and thus to the substrate 1438. Also, thermal
effects will cause movement of chip 1424 relative to the heat sink
1442. The molten thermally conductive masses, in conjunction with
the compliant layer in rear space 1434b will allow such movement
while still providing effective heat transfer between the chip and
the heat sink.
[0101] In an assembly according to a further embodiment of the
invention, the thermally conductive masses 1422 in the front space
are omitted. In this case, a compliant layer may be provided in the
front space to mechanically decouple the dielectric sheet 1412 from
the chip. Such a compliant layer may be formed by providing
compliant posts (not shown) on the dielectric sheet to support the
sheet on the chip, and injecting a compliant material around these
posts, or else may be formed or assembled on the sheet before
assembly of the sheet to the chip.
[0102] As shown in FIG. 25, an assembly and method in accordance
with yet further embodiments of the invention provides a multichip
module. The assembly comprises a first element which includes
plural semiconductor chips 1524a and 1524b. A second element 1512
includes a flexible dielectric sheet having a first surface 1514
facing inwardly toward the chips 1524 and a second surface 1516
facing away from the chips. The flexible dielectric sheet 1512 has
contacts 1519 on the first surface 1514, and has terminals 1518
disposed on the second surface 1516 and hence accessible at the
second surface. The contacts 1519 on the dielectric sheet 1512 are
disposed in a pattern corresponding to the pattern of contacts 1528
on the chips 1524; however, the contacts 1519 are not integral with
the terminals 1518. Instead, the terminals 1518 are distributed on
the second surface 1516 of the sheet 1512 in an array different
from the pattern of contacts 1519 and may occupy a larger area of
the sheet 1512 than do the contacts 1519. The terminals 1518 are
connected to the contacts 1519 by traces 1520 extending within the
sheet 1512. Thus, the sheet 1512 may be a multilayer structure,
with the traces 1520 disposed between layers as well as on the
surfaces of the structure. The assembly further includes additional
electrical elements 1580 mounted to the sheet 1512 and electrically
connected to the leads 1520, so that the additional elements 1580
are connected between some of the contacts 1519 and the terminals
1518. The additional elements 1580 may include any common circuit
element, but most typically include capacitors. The capacitors
typically are connected to the terminals 1518 and the contacts 1519
which form the power and ground connections to the chips 1524.
Traces 1520 electrically interconnect chips 1524a and 1524b with
one another, and with additional electrical elements 1580.
[0103] The assembly according to this particular embodiment further
comprises a third microelectronic element in the form of a package
element 1542 which serves to dissipate heat from the chips 1524 as
well as to physically support and protect the chips 1524. The
package element is depicted schematically as a heat sink 1542
defining a back wall and a separate ring 1546 surrounding the chips
1524 and the additional circuit elements 1580. The dielectric sheet
1512 extends across the front of the package element, and overlies
the ring 1546. The heat sink 1542 has a central region 1544
confronting the rear face 1530 of the chips 1524. Masses of a
fusible, thermally conductive material 1550b extend between the
rear faces 1530 of the chips 1524 and the central region 1544 for
providing thermal conductance between the chips 1524 and the heat
sink 1542. The thermally conductive masses 1550b typically comprise
substantially identical materials as described above. The thermally
conductive masses 1550b may be larger in diameter than the
electrically conductive masses 1550a, and each thermally conductive
mass 1550b may cover a substantial portion of the rear surface 1530
of the a chip.
[0104] The thermally conductive masses 1550b may also be surrounded
by a compliant layer 1555 which is substantially similar to the
compliant layer 1556 surrounding the electrically conductive masses
1550a. The compliant layers between the chips 1524 and the
dielectric sheet 1512 and between the rear face 1530 of the chips
1524 and the heat sink 1542 may be formed from the same materials,
and both such layers may be formed integrally with the flowable
material 1534 filling the space cooperatively enclosed by the heat
sink 1542, the ring 1546 and the dielectric sheet 1512.
[0105] In one method of manufacture, masses of a fusible,
electrically conductive material 1550a, such as the ultra-low
melting point solder described above, are deposited on the contacts
1528 of the chips 1524, as described above. In the next stage of
the process, the dielectric sheet 1512 is assembled to the chips
1524 so that the first surface 1514 of the dielectric sheet 1512
faces toward the front faces 1526 of the chips 1524 and these
confronting surfaces define front space 1560 between them. The
contacts 1519 of the dielectric sheet 1512 are aligned with the
contacts 1528 of the chips 1524 and aligned with the electrically
conductive masses 1550a disposed on the chips contacts 1528.
[0106] While the dielectric sheet 1512 and the chips 1524 are
aligned with one another, the sheet 1512 is pressed toward the
chips 1524 so that the exposed surfaces of the contacts 1519 on the
first surface 1514 of the sheet 1512 engage the electrically
conductive masses 1550a. While the contacts 1519 and 1528 are held
in engagement with the electrically conductive masses 1550a, the
conductive masses 1550a are brought to a temperature above their
melting temperature, so that the conductive material at least
partially liquefies and flows into intimate engagement with the
exposed surfaces of the contacts 1519 on the dielectric sheet
1512.
[0107] While the masses 1550a are in at least a partially molten
condition, the sheet 1512 and hence the terminals 1518 are held in
a substantially planar condition so that the terminals 1518 on the
second surface 1516 of the dielectric sheet 1512 are in
substantially coplanar alignment with one another. While the
terminals 1518 are aligned in this manner, the electrically
conductive masses 1550a are cooled to below their melting
temperature.
[0108] After the electrically conductive masses 1550a have been
completely frozen, a flowable, preferably liquid material or
encapsulant 1534a is allowed to flow into the front space 1560
between confronting surfaces 1514 and 1526 so that the flowable
material 1534a fills the space 1560 and intimately surrounds the
electrically conductive masses 1550a and the surfaces adjacent
contacts 1519 and 1528. During introduction of the flowable
material 1534a, the contacts 1519 and the sheet 1512 are maintained
in substantially planar disposition, and the contacts 1519 are
maintained in alignment with the electrically conductive masses
1550a. After the front space 1560 has been completely filled by the
flowable material 1534a, the flowable material is cured to form a
compliant resilient layer 1556 occupying the space 1560 and
intimately surrounding the electrically conductive masses 1550a and
contacts 1519 and 1528, as described above.
[0109] Next, thermally conductive masses 1550b and heat sink 1542
are assembled to the rear surfaces of the chips. Here again, the
conductive masses are melted momentarily during the assembly
process, so that the thermally conductive masses bond to the rear
surfaces of the chips and to the heat sink. After these elements
have been added, and preferably after the thermally conductive
masses 1550b are frozen, a further flowable material is added to
fill the rear space 1555 between the chip rear surfaces and the
heat sink. The reverse process may also be employed, in which the
chips 1524 are first assembled to the heat sink 1542 and compliant
layer 1555 is formed, followed by assembly of the sheet 1512 and
the electrically conductive masses 1550a and formation of compliant
layer 1556 between the chips 1524 and the dielectric sheet 1512.
Here again, the flowable material used to form both front and rear
compliant layers may be applied simultaneously.
[0110] The assembly can be handled and mounted using ordinary
surface-mounting techniques. In one embodiment, the terminals 1518
on the dielectric sheet 1512 are bonded to the contacts 1540 on the
substrate 1538 to form electrical connections between the chips
1524 and other devices. Additional packaging may also be provided
around the chips 1524 and the substrate 1538. For example, the
chips 1524 and the substrate 1538 may be encapsulated in a flexible
encapsulant which may also flow between the dielectric sheet 1512
and the substrate 1538.
[0111] In this embodiment as well, masses 1550a and 1550b melt
during operation, and offer essentially resistance to mechanical
deformation. The only mechanical interconnection between the chips
1524 and the flexible, dielectric sheet 1512, and hence the only
mechanical interconnection between the chips 1524 and substrate
1538, is provided by the compliant layer. This compliant layer can
accommodate substantial movement of the chips 1524 relative to the
dielectric sheet 1512 without applying high forces between these
elements. Similarly, the thermally conductive masses 1550b will
melt, but will be contained by the compliant layer 1555 to provide
a highly conductive but highly compliant, flexible thermal pathway
between the chips 1524 and the heat sink 1542. When power to the
system is turned off, the device cools and the electrically
conductive masses 1550a and/or the thermally conductive masses
1550b may freeze again. The cycle of melting and freezing may be
repeated numerous times during the service life of the device.
[0112] In an alternative embodiment, the melting temperature of the
thermally conductive masses 1550b may be different than that of the
electrically conductive masses 1550a. The higher temperature
melting masses are placed first and frozen. The lower temperature
melting masses are then placed and melted while the higher
temperature melting masses hold the chips 1524 in position relative
to the heat sink 1542 or relative to the dielectric sheet 1512.
After both sets of fusible conductive masses have been frozen, the
flowable material is introduced and cured to form compliant layers
1555 and 1556 simultaneously. The flowable material may then be
cured to provide a unitary, homogenous compliant layer between the
package element and the dielectric sheet.
[0113] As these and other variations and combinations of the
features discussed above can be employed, the foregoing description
of the preferred embodiments should be taken by way of illustration
rather than by way of limitation of the invention as defined by the
claims.
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