U.S. patent application number 09/935432 was filed with the patent office on 2002-02-07 for semiconductor flip-chip package and method for the fabrication thereof.
Invention is credited to Capote, Miguel A., Zhu, Xiaoqi.
Application Number | 20020014703 09/935432 |
Document ID | / |
Family ID | 26731833 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020014703 |
Kind Code |
A1 |
Capote, Miguel A. ; et
al. |
February 7, 2002 |
Semiconductor flip-chip package and method for the fabrication
thereof
Abstract
A simplified process for flip-chip attachment of a chip to a
substrate is provided by pre-coating the chip with an encapsulant
underfill material having separate discrete solder columns therein
to eliminate the conventional capillary flow underfill process.
There is also provided a flip-chip configuration having a flexible
tape lamination for underfill encapsulation. With this
configuration, the complaint solder/flexible encapsulant
understructure absorbs the strain caused by the difference in the
thermal coefficients of expansion between the chip and the
substrate and provides enhanced ruggedness.
Inventors: |
Capote, Miguel A.;
(Carlsbad, CA) ; Zhu, Xiaoqi; (Vista, CA) |
Correspondence
Address: |
BURNS, DOANE, SWECKER & MATHIS, L.L.P.
P.O. Box 1404
Alexandria
VA
22313-1404
US
|
Family ID: |
26731833 |
Appl. No.: |
09/935432 |
Filed: |
August 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09935432 |
Aug 20, 2001 |
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09137971 |
Aug 21, 1998 |
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6297560 |
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60056043 |
Sep 2, 1997 |
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60053407 |
Jul 21, 1997 |
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Current U.S.
Class: |
257/778 ;
257/E21.503; 257/E21.511; 438/107 |
Current CPC
Class: |
B23K 35/025 20130101;
H01L 2224/13099 20130101; H01L 2224/1403 20130101; H01L 2924/01006
20130101; H01L 24/81 20130101; H01L 2224/1147 20130101; H01L
2224/27436 20130101; H01L 2924/01039 20130101; H01L 2224/13111
20130101; H01L 2224/27614 20130101; H01L 2924/01046 20130101; H01L
2924/01327 20130101; H01L 2924/10253 20130101; H01L 2224/83856
20130101; H01L 2924/0103 20130101; H01L 24/29 20130101; H01L
2224/2929 20130101; H01L 2224/83191 20130101; H01L 2924/01025
20130101; H01L 2224/16105 20130101; H01L 2924/01049 20130101; H01L
2924/0105 20130101; H01L 2924/01078 20130101; H01L 2924/1579
20130101; B23K 35/3618 20130101; H01L 23/293 20130101; H01L 2224/29
20130101; H01L 2924/01029 20130101; B32B 7/12 20130101; H01L
2924/01015 20130101; H01L 2924/01079 20130101; H01L 2924/01082
20130101; H01L 21/563 20130101; H01L 2924/0102 20130101; H01L
2224/02379 20130101; H01L 2924/01322 20130101; H01L 2224/81024
20130101; H01L 2224/8121 20130101; H01L 2224/83192 20130101; H01L
2924/0132 20130101; H01L 2924/3511 20130101; H01L 2224/81815
20130101; H01L 2224/32225 20130101; H01L 2224/73104 20130101; H01L
2224/29299 20130101; H01L 2924/01018 20130101; H05K 3/321 20130101;
H01L 2224/06102 20130101; H01L 2224/16225 20130101; H01L 2224/81011
20130101; H01L 2224/0401 20130101; H01L 2924/01042 20130101; H01L
23/49883 20130101; H01L 24/11 20130101; H01L 2224/29082 20130101;
H01L 2924/01027 20130101; H01L 2924/0781 20130101; H01L 2924/14
20130101; H01L 2224/16108 20130101; H01L 2224/2919 20130101; H01L
2224/29562 20130101; H01L 2924/01023 20130101; H01L 2924/01045
20130101; H01L 2924/12042 20130101; H01L 2224/1148 20130101; H01L
2224/73204 20130101; H01L 2924/00013 20130101; H01L 2924/01047
20130101; H01L 2224/1182 20130101; H01L 2924/014 20130101; H01L
2224/83193 20130101; H05K 1/095 20130101; H01L 2924/01033 20130101;
H01L 2924/01051 20130101; C09J 4/00 20130101; H01L 2224/2969
20130101; H01L 2924/0665 20130101; B23K 35/3613 20130101; H01L
23/4334 20130101; H01L 23/49827 20130101; H01L 2224/29101 20130101;
H01L 2224/29111 20130101; H01L 2924/01004 20130101; H01L 2924/15311
20130101; H01L 2224/27618 20130101; H01L 24/83 20130101; H01L
2224/274 20130101; H01L 2224/83101 20130101; H01L 2224/27602
20130101; H01L 2924/01005 20130101; C09J 4/00 20130101; C08F 222/20
20130101; H01L 2924/0665 20130101; H01L 2924/00 20130101; H01L
2224/29101 20130101; H01L 2924/014 20130101; H01L 2924/00 20130101;
H01L 2224/73204 20130101; H01L 2224/16225 20130101; H01L 2224/32225
20130101; H01L 2924/00 20130101; H01L 2224/83192 20130101; H01L
2224/32225 20130101; H01L 2924/0132 20130101; H01L 2924/01029
20130101; H01L 2924/0105 20130101; H01L 2924/0132 20130101; H01L
2924/01028 20130101; H01L 2924/0105 20130101; H01L 2924/0132
20130101; H01L 2924/0105 20130101; H01L 2924/01082 20130101; H01L
2224/16225 20130101; H01L 2224/13111 20130101; H01L 2924/00
20130101; H01L 2224/83191 20130101; H01L 2224/83101 20130101; H01L
2924/00 20130101; H01L 2224/32225 20130101; H01L 2924/00 20130101;
H01L 2224/83192 20130101; H01L 2224/83101 20130101; H01L 2924/00
20130101; H01L 2224/32225 20130101; H01L 2924/00 20130101; H01L
2224/13111 20130101; H01L 2924/01082 20130101; H01L 2924/00012
20130101; H01L 2224/32225 20130101; H01L 2924/00 20130101; H01L
2924/3512 20130101; H01L 2924/00 20130101; H01L 2924/00014
20130101; H01L 2224/32225 20130101; H01L 2924/00 20130101; H01L
2224/13111 20130101; H01L 2924/01028 20130101; H01L 2924/00014
20130101; H01L 2224/32225 20130101; H01L 2924/00 20130101; H01L
2224/13111 20130101; H01L 2924/01029 20130101; H01L 2924/00014
20130101; H01L 2224/32225 20130101; H01L 2924/00 20130101; H01L
2924/15311 20130101; H01L 2224/73204 20130101; H01L 2224/16225
20130101; H01L 2224/32225 20130101; H01L 2924/00 20130101; H01L
2224/2919 20130101; H01L 2924/0665 20130101; H01L 2924/00014
20130101; H01L 2224/29299 20130101; H01L 2924/00014 20130101; H01L
2924/00013 20130101; H01L 2224/29099 20130101; H01L 2924/00013
20130101; H01L 2224/29199 20130101; H01L 2924/00013 20130101; H01L
2224/29299 20130101; H01L 2924/00013 20130101; H01L 2224/2929
20130101; H01L 2924/10253 20130101; H01L 2924/00 20130101; H01L
2224/83192 20130101; H01L 2224/73204 20130101; H01L 2224/16225
20130101; H01L 2224/32225 20130101; H01L 2924/00 20130101; H01L
2924/12042 20130101; H01L 2924/00 20130101; H01L 2224/27614
20130101; H01L 2924/00014 20130101; H01L 2224/27618 20130101; H01L
2924/00014 20130101; H01L 2224/27602 20130101; H01L 2924/00014
20130101; C09J 4/00 20130101; C08F 222/104 20200201 |
Class at
Publication: |
257/778 ;
438/107 |
International
Class: |
H01L 021/44; H01L
021/48; H01L 021/50; H01L 023/48 |
Goverment Interests
[0002] The U.S. Government has a paid-up license in this invention
and the right in limited circumstances to require the patent owner
to license others on reasonable terms as provided for by the terms
of contract no. N00164-96-C-0089 awarded by Defense Advanced
Research Projects Agency.
Claims
We claim:
1. An electrical component assembly, comprising: a substrate having
a plurality of pads on a first surface thereof; an integrated
circuit chip having a film laminated on an active surface thereof,
the film having a plurality of holes therethrough filled with an
electrically conductive material that extends from contacts on the
active surface aligned with the holes through the film to the
plurality of pads on the substrate.
2. The electrical component assembly of claim 1 wherein the
electrically conductive material in the plurality of holes are
discrete solder bumps pre-assembled on the integrated circuit
chip.
3. The electrical component assembly of claim 1 further comprising
a layer of flux adhesive between a subassembly comprising the film
and the electrically conductive material and the substrate.
4. An electrical component assembly, comprising: an integrated
circuit chip having a plurality of pads on an active surface
thereof; a substrate having a film laminated on a first surface
thereof, the film having a plurality of holes therethrough filled
with an electrically conductive material that extends from contacts
on the first surface aligned with the holes through the film to the
plurality of pads on the integrated circuit chip.
5. The electrical component assembly of claim 4 wherein the
electrically conductive material in the plurality of holes are
discrete solder bumps pre-assembled on the substrate.
6. The electrical component assembly of claim 4 further comprising
a layer of flux adhesive between a subassembly comprising the film
and the electrically conductive material and the integrated circuit
chip.
7. A method for making an electrical component assembly, comprising
the steps of: laminating a film on an active surface of an
integrated circuit; producing holes in the film to expose contact
pads on the active surface of the integrated circuit chip; filling
the holes with an electrically conductive material; placing the
integrated circuit chip on a substrate with the film located
between the integrated circuit chip and the substrate; and
reflowing the electrically conductive material in order to attach
the integrated circuit chip to the substrate.
8. The method of claim 7 further comprising the step of: coating a
surface of the film which faces the substrate with a flux
adhesive.
9. The method of claim 7 wherein the filling step comprises filling
the holes with molten solder.
10. The method of claim 7 wherein the filling step comprises
filling the holes with solder paste.
11. The method of claim 7 wherein the holes in the encapsulant are
produced by laser drilling.
12. The method of claim 7 wherein the holes in the encapsulant are
produced by plasma etching.
13. The method of claim 7 wherein the holes in the encapsulant are
produced by chemical etching.
14. The method of claim 7 wherein the holes in the encapsulant are
produced by photoimaging.
15. A method for making an electrical component assembly,
comprising the steps of: laminating a film on a substrate;
producing holes in the film to expose contact pads on the
substrate; filling the holes with an electrically conductive
material; placing an integrated circuit chip on a substrate with
the film located between the integrated circuit chip and the
substrate; and reflowing the electrically conductive material in
order to attach the integrated circuit chip to the substrate.
16. The method of claim 15 further comprising the step of: coating
a surface of the film which faces the integrated circuit chip with
a flux adhesive.
17. The method of claim 15 wherein the filling step comprises
filling the holes with molten solder.
18. The method of claim 15 wherein the filling step comprises
filling the holes with solder paste.
19. A method for making an electrical component assembly,
comprising the steps of: laminating a film on a substrate having
discrete solder bumps thereon; placing an integrated circuit chip
on a substrate with the film located between the integrated circuit
chip and the substrate; and reflowing the solder bumps in order to
attach the integrated circuit chip to the substrate.
20. The method of claim 18 further comprising the step of: coating
a surface of the film which faces the integrated circuit chip with
a flux adhesive.
21. A method for making an electrical component assembly,
comprising the steps of: laminating a film on an active surface of
an integrated circuit chip having discrete solder bumps thereon;
coating a substrate with a portion of an encapsulant; placing the
integrated circuit chip on the substrate with the film and
encapsulant portion located between the integrated circuit chip and
the substrate; curing the encapsulant portion; and reflowing the
solder bumps in order to attach the integrated circuit chip to the
substrate.
22. A method for making an electrical component assembly,
comprising the steps of: laminating a film on an active surface of
an integrated circuit chip having discrete solder bumps thereon;
coating the film with a portion of an encapsulant; placing the
integrated circuit chip on the substrate with the film and
encapsulant portion located between the integrated circuit chip and
the substrate; curing the encapsulant portion; and reflowing the
solder bumps in order to attach the integrated circuit chip to the
substrate.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/056,043, filed Sep. 2, 1997, and U.S. patent
application Ser. No. 09/120,172 filed Jul. 21, 1998, Ser. No.
08/926,159 filed Sep. 9, 1997, and Ser. No. 09/012,382 filed Jan.
23, 1998, and incorporates herein the disclosures of those
applications in their entirety.
FIELD OF THE INVENTION
[0003] This invention relates generally to semiconductor chips
electrically and mechanically connected to a substrate,
particularly to flip-chip configurations.
BACKGROUND OF THE INVENTION
[0004] Flip-chip technology is well known in the art. A
semi-conductor chip having solder bumps formed on the active side
of the semi-conductor chip is inverted and bonded to a substrate
through the solder bumps by reflowing the solder. Structural solder
joints are formed between the semi-conductor chip and the substrate
to form the mechanical and electrical connections between the chip
and substrate. A narrow gap is left between the semi-conductor chip
and the substrate.
[0005] One obstacle to flip-chip technology when applied to polymer
printed circuits is the unacceptably poor reliability of the solder
joints due to the mismatch of the coefficients of thermal expansion
between the chip, having a coefficient of thermal expansion of
about 3 PPM/.degree. C., and the polymer substrate, e.g.
epoxy-glass having a coefficient of thermal expansion of about 16
to 26 PPM/.degree. C., which causes stress build up in the solder
joints. Because the structural solder joints are small, they are
thus subject to failures. In the past, the solder joint integrity
of flip-chip interconnects to a substrate has been enhanced by
underfilling the volume between the chip and the substrate with an
underfill encapsulant material comprised of a suitable polymer. The
underfill material is typically dispensed around two adjacent sides
of the semiconductor chip, then the underfill material slowly flows
by capillary action to fill the gap between the chip and the
substrate. The underfill material is then hardened by baking for an
extended period. For the underfill encapsulant to be effective, it
is important that it adhere well to the chip and the substrate to
improve the solder joint integrity. Underfilling the chip with a
subsequently cured encapsulant has been shown to reduce solder
joint cracking caused by thermal expansion mismatch between the
chip and the substrate. The cured encapsulant reduces the stresses,
induced by differential expansion and contraction, on the solder
joints.
[0006] The underfill process, however, makes the assembly of
encapsulated flip-chip printed wire boards (PWB) a time consuming,
labor intensive and expensive process with a number of
uncertainties. To join the integrated circuit to the substrate, a
flux, generally a no-clean, low residue flux, is placed on the chip
or substrate. Then the integrated circuit is placed on the
substrate. The assembly is subjected to a solder reflowing thermal
cycle, soldering the chip to the substrate. The surface tension of
the solder aids to self align the chip to the substrate terminals.
After reflow, due to the close proximity of the chip to the
substrate, removing flux residues from under the chip is such a
difficult operation that it is generally not done. Therefore the
flux residues are generally left in the space between the chip and
the substrate. These residues are known to reduce the reliability
and integrity of the encapsulant.
[0007] After reflow, underfill encapsulation of the chip generally
follows. In the prior art, the polymers of choice for the underfill
encapsulation have been epoxies, the coefficient of thermal
expansion and moduli of the epoxies being adjusted with the
addition of inorganic fillers. To achieve optimum reliability, a
coefficient of thermal expansion in the vicinity of 25 PPM/.degree.
C. is preferred and a modulus of 4 GPa or more. Since the preferred
epoxies have coefficient of thermal expansions exceeding 80
PPM/.degree. C. and moduli of less than 4 GPa, the inorganic
fillers selected generally have much lower coefficient of thermal
expansions and much higher moduli so that in the aggregate, the
epoxy-inorganic mixture is within the desired range.
[0008] The underfill encapsulation technique of the prior art has
four principal disadvantages:
[0009] 1. The reflowing of the solder bump and then underfilling
and curing the encapsulant is a multi-step process that results in
reduced production efficiency;
[0010] 2. To underfill a flip-chip assembly takes too long because
the material must flow through the tiny gap between the chip and
the substrate;
[0011] 3. The flux residues remaining in the gap reduce the
adhesive and cohesive strengths of the underfill encapsulating
adhesive, affecting the reliability of the assembly; and
[0012] 4. As the size of chips increase, the limiting effect of
capillary action becomes more critical and makes the encapsulation
procedure more time consuming, more susceptible to void formation
and to the separation of the polymer from the fillers during
application.
[0013] Clearly, many improvements to this process are feasible to
increase reliability, reduce the time required and decrease the
likelihood of producing a void in the encapsulant while providing
the required low coefficient of thermal expansion and high
modulus.
[0014] Other prior art methods of encapsulating the chip have
attempted to overcome the above limitations by applying the
encapsulating resin through a hole in the substrate located near
the center of the chip. After the soldering and cleaning
operations, the encapsulating resin is forced through the hole and
around the periphery of the chip to ensure complete coverage of the
chip surface. This method suffers from the need to reserve an area
in the center of the substrate that is free of circuitry in order
to provide an unused space for the hole. It also does not eliminate
the problems of entrapped air bubbles.
[0015] Another prior art method in U.S. Pat. No. 5,128,746
(Pennisi) teaches a method wherein an adhesive material including a
fluxing agent is applied to the chip or substrate. The chip is
positioned on the substrate and the solder bumps are reflowed.
During the reflow step, the fluxing agent promotes wetting of the
solder to the substrate metallization pattern and the adhesive
material is cured, mechanically interconnecting and encapsulating
the substrate to the component. The limitation of this technique is
that in order for the molten solder to readily wet the substrate
metallization and also to allow the solder, through surface
tension, to self-align the chip bumps to the substrate
metallization pattern, the material must maintain very low
viscosity during the reflow step. But the viscosity of these
materials is severely increased by the presence of the required
inorganic fillers. As a result, this approach has failed to produce
a material that can serve as both the flux and the encapsulant with
the required low coefficient of thermal expansion and high modulus
for optimum reliability.
[0016] Referring to FIGS. 1 and 2, underfilling the chip 100 with a
subsequently hardened encapsulant 102 has been shown to reduce
solder joint cracking caused by thermal expansion mismatch between
the chip and the substrate 104. The hardened encapsulant 102
transfers the stresses, induced by differential expansion and
contraction, from the solder joints 106 to deformation of the chip
100 and substrate 104 as shown in FIG. 1 for expansion-induced
strain at elevated temperatures and FIG. 2 for contraction-induced
strain at reduced temperatures. In other words, the main effect of
the hardened encapsulant during thermal expansion or contraction is
to effectively force the chip and the substrate to take up the
stress caused by the coefficients of thermal expansion mismatch by
bending and bulging the chip and substrate. This bending and
bulging reduces the stress on the solder joints and virtually
eliminates solder fatigue failure.
[0017] Unfortunately, a limitation of the prior art is the expense
of applying solder bumps to a chip. The solder bumps have been
applied to chips by one of several methods. Coating the solder on
the chip bumps by evaporation of solder metals through a mask is
one such method. This method suffers from 1) long deposition times,
2) limitations on the compositions of solder that can be applied to
those metals that can be readily evaporated, and 3) evaporating the
metals over large areas where the solder is ultimately not wanted.
Also, since most solders contain lead, a toxic metal, evaporation
involves removal and disposal of excess coated lead from equipment
and masks. Another common method in the prior art is electroplating
of the solder onto the chip pads through a temporary sacrificial
mask. Electroplating is a slow and expensive process that also
deposits the solder over large areas where the solder is ultimately
not wanted. Another method is to screen print solder paste on the
chip pads through a stencil, then reflowing the solder to form a
ball or bump on the pad. This technique is limited to bump
dimensions that can be readily stencil printed, so it is not
practical in bump pitches of 50 microns or less. Yet another method
is to apply a thick layer of photoresist on the chip, expose
through a mask, and develop to create openings through the thick
photoresist to the chip pads beneath. Subsequently, the openings
are filled with solder paste by printing through a stencil and then
reflowing the solder to create a solder column on the chip pads.
The final step is removal of the thick photoresist and reflowing
the solder to create a bump or ball on the chip pads. This method
allows fabrication of chips with bump pitches of 200 microns or
less and is preferable to the other methods described due to its
lower cost. Yet the removal of the thick photoresist from the chips
after solder reflow is a cumbersome procedure that often damages
the chips and the solder bumps. All these methods are generally
performed prior to dicing the wafer on which the semiconductor
chips are fabricated, so the application of bumps is done on many
chips simultaneously.
SUMMARY OF THE INVENTION
[0018] In one aspect of the present invention there is provided a
chip with underfiling encapsulant and separate discrete solder
bumps pre-coated and pre-assembled on the chip for assembly to a
substrate. This configuration provides a simple, cost-effective
assembly procedure wherein the chip/encapsulant/discrete solder
bump combination is placed on the substrate and subsequently heat
is applied so that the solder is reflowed while simultaneously the
encapsulant hardens, without the labor intensive underfill steps of
the prior art.
[0019] In another aspect of the present invention there is provided
a chip precoated with underfilling encapsulant having holes therein
which expose metallized contact pads on the active surface of the
chip. The holes are subsequently filled with solder paste and
reflowed to create the chip/encapsulant/discrete solder bump
assembly. The assembly can be placed on a substrate and
subsequently, the solder is reflowed again while simultaneously the
encapsulant hardens, eliminating the labor intensive underfill
steps of the prior art. Alternatively, the solder paste is not
reflowed at the time the holes are filled, but left in paste form
until the chip/encapsulant/solder paste assembly is placed on a
substrate. Subsequently, the solder is reflowed while
simultaneously the encapsulant hardens. In yet another alternative
approach, the chip/encapsulant/solder assembly is coated with a
thin layer of a flux adhesive and, subsequently, the solder is
reflowed while simultaneously the flux adhesive and encapsulant
harden.
[0020] The present invention also provides a substrate precoated
with the encapsulant having holes therein which expose the
metallized solder pads on the substrate. The holes are subsequently
filled with solder which is then hardened prior to attachment of
the chip to the substrate by reflow. In another embodiment, the
substrate has encapsulant and separate discrete solder columns
pre-assembled thereon.
[0021] In one aspect of the present invention, there is provided a
first portion of an underfilling encapsulant and separate discrete
solder bumps pre-coated and pre-assembled on a chip for assembly to
a substrate. The first portion of the encapsulant comprises a solid
film which has been either (1) laminated to the chip in solid film
form, or (2) printed on the chip in liquid form and subsequently
hardened. In the first case, the film can be laminated to the chip
by means of a thin layer of an adhesive or by adhesive properties
intrinsic to the film itself At least the second portion of the
encapsulant comprises an adhesive material with solder fluxing
properties, for example, an adhesive flux. The invention provides a
simple, cost-effective assembly procedure wherein the chip/first
portion of encapsulant/discrete solder bump combination is placed
on the substrate/second portion of encapsulant combination and
subsequently heat is applied so that the solder is reflowed while
simultaneously the encapsulant cures, without the labor intensive,
time-consuming underfill steps of the prior art. An advantage of
the present invention is that the lower viscosity of the unfilled
or lightly filled second portion during the reflow process allows
the solder to flow without impediment from the hard nature of the
first portion of the encapsulant.
[0022] In another aspect of the present invention, the chip/first
portion of encapsulant/discrete solder bump assembly described
above is coated with a thin layer of the second portion of the
encapsulant. Placement of the chip, solder reflow and adhesive cure
follows as described above. In this instance, the second portion
comprises a liquid which is applied to the chip/first portion of
encapsulant/solder bump combination or to the substrate in situ at
the time they are assembled and the solder is reflowed.
Alternatively, the second portion comprises a solid or viscous
liquid which is applied to the chip/first portion of
encapsulant/solder bump combination or to the substrate a priori,
prior to the time the chip and substrate are assembled together,
the second portion subsequently melting temporarily into a low
viscosity liquid when the chip/first portion of encapsulant/solder
bump/substrate are assembled and the solder is reflowed. This
melting of the second portion provides a low-viscosity liquid at
the reflow temperature of the solder so as not to impede the flow
of the solder as it melts. Subsequently, the second portion hardens
as previously described.
[0023] In a preferred embodiment of the present invention, the
first portion of the encapsulant consists of a thick film having a
reduced coefficient of thermal expansion and an increased modulus
as compared to the second portion. To achieve this, the first
portion comprises an encapsulant material with filler having a
lower coefficient of thermal expansion and higher modulus than the
encapsulant material without filler to increase the encapsulant's
modulus and reduce its coefficient of thermal expansion.
Alternatively, the first portion comprises an encapsulant material
having an intrinsically high modulus and low coefficient of thermal
expansion as compared to the second portion, such as polyimide
films sold under the trade name UPILEX.RTM. and available from
Oxychem, Grand Island, N.Y. Preferably, the second portion
constitutes a relatively thin layer in the overall encapsulant
structure which partially intermixes with the first portion during
cure and has minimal effect on the reliability of the flip-chip
structure, despite the second portion having generally a lower
modulus and higher coefficient of thermal expansion than the first
portion. The preferred invention provides a low coefficient of
thermal expansion and high modulus in the first portion of the
encapsulant while at the same time achieving good solder wetting
and chip self-aligning in the second portion of the encapsulant.
The higher modulus and lower coefficient of expansion enhance the
ability of the chip/underfill/solder/substrate assembly to
withstand thermal shock and extreme thermal cycles.
[0024] Yet another aspect of the present invention provides a
simplified method for creating the solder bumps. In the embodiment
where the first portion of the encapsulant comprises a solid film,
the first portion is applied to the chip prior to the solder bumps
being applied. The solid first portion is subsequently imaged and
developed or drilled by lasers, plasmas, chemicals, or other
methods known in the art, to create openings in the first portion
exposing underlying chip pads. The openings being subsequently
cleared of debris and contaminants by etching or other means known
in the art, solder paste is then screen printed on the chip pads.
This method is similar to the prior art method of bumping chips
through a thick photoresist layer, but has the advantage of leaving
the thick layer permanently attached to the chip so that it becomes
part of the chip underfill encapsulant. This method also has the
advantage of permitting bump pitches of 50 microns or less.
[0025] Preferably, all embodiments of the invention are applied
prior to dicing the wafer or substrate on which the semiconductor
chips are fabricated, so the application of the encapsualting
layers and solder is done on many chips sites simultaneously.
[0026] The semiconductor chip package structures of the present
invention provide, among other advantages, simple chip placement
followed by reflow without labor intensive underfill steps; a
solder bumped chip or substrate with an encapsulant pre-attached,
with the encapsulant performing a mechanical function and the
solder performing an electrical function; a low-cost method for
applying the solder bumps to a flip chip or flip chip substrate by
creating holes in a pre-coated or pre-laminated encapsulant; and a
low-cost method for applying the solder bumps to a flip chip or
substrate by creating holes in a pre-coated or pre-laminated
encapsulant.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a diagrammatic representation of a prior art
underfilled flip-chip structure under expansion-induced strain at
elevated temperatures.
[0028] FIG. 2 is a diagrammatic representation of a prior art
underfilled flip-chip structure under contraction-induced strain at
reduced temperatures.
[0029] FIG. 3 is an assembled flip-chip structure in accordance
with a first embodiment of the present invention.
[0030] FIG. 4 is a diagrammatic representation of the first
embodiment for forming a flip-chip structure prior to assembly.
[0031] FIGS. 5-7 are diagrammatic representations of another
embodiment for forming a flip-chip structure.
[0032] FIGS. 8 and 9 are diagrammatic representations of yet
another embodiment for forming a flip-chip structure.
[0033] FIG. 10 is a diagrammatic representation of a flip-chip
structure wherein the first portion of the encapsulant material is
applied to the bumped chip and the second portion is applied to the
substrate.
[0034] FIG. 11 is a diagrammatic representation of a flip-chip
structure wherein the first portion of the encapsulant material is
applied to the bumped chip and the second portion is applied over
the first portion.
[0035] FIGS. 12 and 13 illustrate the assembly of the flip-chip
structure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] Referring to FIG. 3, an integrated circuit chip 10 is shown
mounted on a substrate 20. A plurality of solder pads 12 on top
surface 26 of the substrate 20 are arranged to receive
corresponding solder bumps 14 connected to the contact pads 24 of
the chip 10. Each of the solder pads 12 is metallized so as to
become solderable and electrically conductive to provide an
electrical interconnection between the chip 10 and the substrate
20. With this flip-chip mounting arrangement, a gap 18 is formed
around the solder bumps 14 between the top surface 26 of the
substrate 20 and the bottom surface 16 of the chip 10. The gap 18
typically varies from 50 to 200 microns. The gap 18 is completely
filled with an encapsulant material 22. The encapsulant material 22
is applied to the chip in either liquid form that is then hardened
or in adhesive tape form, which is adhered to the chip. In one
preferred embodiment of the invention, the encapsulant material 22
is a film, which has been laminated to the chip with an adhesive
layer therebetween. (See FIG. 12). One preferred film, which is not
meant to limit the invention but only by way of example, is a hot
melt adhesive-coated polyimide tape such as SUMIOXY.RTM. ITA-53 15
available from Oxychem, Grand Island, N.Y. Other types of
encapsulants known to those skilled in the art are possible.
[0037] In another embodiment, the chip 10 having separate discrete
solder bumps 14 pre-assembled thereon is precoated with the
encapsulant material 22 prior to assembly to the substrate 20 (FIG.
4) to alleviate the underfill problems of the prior art processes
and to overcome the performance limitations of substrates which are
pre-coated with a homogeneous combination of adhesive material,
fluxing agent and curing agent or chips which are pre-coated with a
homogeneous combination of adhesive material, fluxing agent, curing
agent and metal particles. The separate discrete solder bumps with
encapsulant material therearound provide superior electrical
performance compared to a distribution of metal particles spread
throughout an encapsulating material. The encapsulating material 22
is uniformly spread across the surface 16 of the chip 10 between
the solder bumps 14 covering the remainder of the chip 10. The chip
10 is then positioned so that the solder bumps 14 are facing the
substrate 20 and aligned with the solder pads 12 of the substrate.
In one embodiment, the solder bumps 14 protrude beyond the
encapsulant after the encapsulant-coating step. In an alternate
embodiment, the solder bumps 14 are covered by the encapsulant 22
wherein the encapsulant is ground, melted away, shaved off or
otherwise removed to expose the solder bumps prior to attachment to
the substrate. The encapsulant 22 and solder bumps 14 are moved
into intimate contact with the substrate 20 and solder pads 12,
respectively. The assembly is heated to cure the encapsulant 22 and
reflow the solder using reflow oven technology, preferably in a
nitrogen blanket to attach the solder bumps 14 to the contact pads
12 of the substrate 20. Other heating and reflow techniques, known
to those skilled in the art, are possible. The encapsulant 22
provides a continuous seal between the chip 10 and the substrate
20.
[0038] In yet another embodiment (FIG. 5), the circuitry on the
bottom surface 16 of the chip 10 is coated with the encapsulant 22,
comprised of an adhesive 19 (such as a high temperature
thermoplastic adhesive) and a film 21 (FIG. 12), then the contact
pads 24 are exposed by making vias 28 through the encapsulant 22
(e.g., either with a laser, plasma etching, chemical etching, a
drill or by photo-imaging and development or any other method known
to one skilled in the art) (FIG. 6). The vias 28 within the
encapsulant 22 are then filled with solder 30 (FIG. 7) which is
forced into the holes by solder injection molding, solder jetting,
screen printing solder paste, or other methods known to those
skilled in the art. With any of these embodiments, the solder 30 is
reflowed to form the electrical connection between the chip and the
substrate while the encapsulant 22 bonds to the substrate 20
(usually with a polymer flax layer 23. See FIG. 13) and the chip 10
to form the structural connection. As can be easily appreciated by
one of ordinary skill in the art, any of the above-described
embodiments can be modified by precoating the substrate 20 (rather
than the chip) with the encapsulant 22 or encapsulant 22 and solder
30 combination as shown in FIGS. 8 and 9, respectively.
[0039] FIG. 10 illustrates one embodiment for forming the flip-chip
package illustrated in FIG. 3 using two pre-coated portions of
encapsulant. The chip 10 having separate discrete solder bumps 14
pre-assembled thereon is pre-coated with the first portion 37 of an
encapsulant material 22 (FIG. 10) prior to assembly to the
substrate 20. The first portion 37 comprises a film that has been
laminated to the chip 10. The substrate 20, having a pattern of
separate discrete solderable metal pads 12 thereon, is pre-coated
with the second portion 39 of the encapsulant material prior to
assembly with the chip 10. The two-layer configuration alleviates
the underfill problems of the prior art processes and overcomes the
performance limitations of substrates which are pre-coated with a
homogeneous combination of adhesive material, fluxing agent and
curing agent or chips which are pre-coated with a homogeneous
combination of adhesive material, fluxing agent, curing agent and
metal filler particles.
[0040] The first portion 37 (FIG. 10) which makes up part of the
encapsulating material 22 (FIG. 3) extends uniformly across the
surface 16 of the chip 10 between the solder bumps 14 to cover the
remainder of the chip surface. The second portion 39, which makes
up part of the encapsulating material 22 (FIG. 3). is spread across
the surface 26 of the substrate 20 over the solderable metal pads
12 covering the chip region of the substrate 20. The chip 10 is
then positioned so that the solder bumps 14 are facing the
substrate 20 and aligned with the solder pads 12 of the substrate
20. The solder bumps 14 can protrude beyond the first portion 37
(as shown in FIG. 10) of the encapsulant after the encapsulant
coating step of the chip 10. The encapsulant portion 37 and solder
bumps 14 are moved into intimate contact with the encapsulant
portion 39 and solder pads 12, respectively. The combination of
portions 37 and 39 form the encapsulant 22 (FIG. 3). The assembly
is heated to cure the encapsulant 22 and reflow the solder using
infrared reflow technology, preferably in a nitrogen blanket to
attach the solder bumps 14 to the contact pads 12 of the substrate
20. Other heating and reflow techniques, known to those skilled in
the art, can be used in the present invention. The encapsulant 22
provides a continuous seal between the chip 10 and the substrate
20.
[0041] FIG. 11 illustrates another embodiment for forming the
flip-chip package illustrated in FIG. 3 using two pre-coated
discrete portions of encapsulant. The chip 10 having separate
discrete solder bumps 14 pre-assembled thereon is pre-coated with
the first portion 37 of an encapsulant material 22 (FIG. 3) prior
to assembly to the substrate 20. The first portion 37 comprises a
film which has been laminated to the chip. The first portion 37 is
then pre-coated with the second portion 39 of the encapsulant
material prior to assembly with the substrate 20. The first portion
37 (FIG. 11) which makes up part of the encapsulating material 22
(FIG. 3) extends uniformly across the surface 16 of the chip 10
between the solder bumps 14 to cover the remainder of the chip
surface. The second portion 39 (FIG. 1) which makes up part of the
encapsulating material 22 (FIG. 10) is uniformly spread over the
prior applied first portion 37. The chip 10 is then positioned so
that the solder bumps 14 are facing the substrate 20 and aligned
with the solder pads 12 of the substrate 20 as described before.
The encapsulant portions 37 and 39 and solder bumps 14 are moved
into intimate contact with the substrate 20 and solder pads 12. The
combination of portions 37 and 39 forms the encapsulant 22 (FIG.
3). The assembly is heated to cure the encapsulant 22 and reflow
the solder as described before to attach the solder bumps 14 to the
contact pads 12 of the substrate 20.
[0042] The encapsulating material of the first portion 37 can
comprise a flexible film which has been laminated to the chip 10.
In the preferred embodiment, in order to provide the most durable
assembly, especially during severe thermal cycling, the significant
properties of the first portion 37 are:
[0043] 1. After cure, a coefficient of thermal expansion in the
vicinity of 25 PPM/.degree. C.;
[0044] 2. After cure, a Tg above 120.degree. C.;
[0045] 3. After cure, a modulus greater than 0.1 GPa, preferably
greater than 4 GPa;
[0046] 4. After cure, high adhesion to the chips passivation layer
that usually consists of silicon nitride, polyimide, or
benzocyclobutene;
[0047] 5. Solventless;
[0048] 6. A chemical composition such that it does not interfere or
adversely affect the properties of the second portion 39 of the
encapsulant to which it will be mated; and
[0049] 7. After cure, high adhesion to the second portion 39 of the
encapsulant.
[0050] In general terms, the adhesive flux 39 comprises a liquid or
solid composition which acts as both a primary fluxing agent and a
crosslinking monomer or polymer. More specifically, the adhesive
fluxes comprise the following:
[0051] A. chemical components with carboxylic acid moieties for
fluxing;
[0052] B. chemical components with polymerizable moieties for
crosslinking the composition;
[0053] C. a chemical or mechanical mechanism for impeding or
preventing the onset of polymerization of the composition until the
solder has melted and wetted all the surfaces to be soldered;
and
[0054] D. optional solvents, fillers, moderating agents,
neutralizing agents, surfactants, modifiers, resins and other
additives performing desirable functions and generally known to
those skilled in the art.
[0055] A number of compositions are known in the prior art
comprising these features, such as described in U.S. Pat. Nos.
5,376,403, 5,088,189, 5,136,365 and 5,128,746. A preferred
composition is directed to fluxing adhesive compositions that
include a fluxing agent comprising a single active component which
is capable of functioning as both a primary fluxing agent and a
crosslinking monomer. Generally, depending upon the intended end
use, the inventive thermally curable adhesive composition comprises
(a) a fluxing agent having a carboxylic acid group and one or more
carbon-carbon double bonds, (b) a carboxylic acid neutralizing
agent; (c) optionally, a crosslinkable diluent, (d) optionally, a
free-radical initiator, and (e) optionally, a resin.
[0056] In addition the thermally curable adhesive composition may
include a solvent for adjusting the viscosity. Other viscosity
modifiers, thickeners and thixotropic agents may also be added.
Fillers, such as silica powder, can be employed for increased
modulus and lower thermal coefficient of expansion.
[0057] 1. Fluxing Agents.
[0058] The fluxing agent is a carboxyl containing compound that has
the structure RCOOH, wherein R comprises a moiety which include two
or more carbon-carbon double bonds. For high flux activity due to
the presence of multiple carboxylic acids, the preferred fluxing
agent is a carboxylic acid that is selected from the group
consisting of compounds represented by Formulae I, II, and III, and
mixtures thereof,
HOOCCH.dbd.CH(O)COR.sup.18OC(O)CH.dbd.CHCOOH (I)
R.sup.2H.sub.2C(HCOR.sup.n).sub.nCH.sub.2OR.sup.3 (II)
[X.sup.1X.sup.2X.sup.3X.sup.4]C (III)
[0059] wherein R.sup.18 is an alkyl having 1 to 16 carbons,
preferably 1 to 9 carbons, and more preferably 1 to 3 carbons,
wherein n is an integer from 1 to 16 preferably an integer from 1
to 9, and more preferably an integer from 1 to 3, wherein each of
R.sup.1, R.sup.2, . . . R.sup.n, is independently selected from
--C(O)CH.dbd.CHCOOH, and H, wherein X.sup.1, X.sup.2, X.sup.3, and
X.sup.4, are each independently selected from --CH.sub.2OH,
--CH.sub.2OC(O)CH.dbd.CHCOOH, and H, with the proviso that not all
of X.sup.1, X.sup.2, X.sup.3, and X.sup.4 are H, and preferably
only one of said X.sup.1, X.sup.2, X.sup.3, and X.sup.4 is H.
[0060] A preferred fluxing adhesive composition that has a lower
curing temperature, faster curing rate and increased moisture
resistance includes a fluxing agent that has the general structure
R COOH, wherein R comprises a moiety having two or more
carbon-carbon double bonds, of which preferably at least one is
within an acrylate or methacrylate moiety, that is, R contains at
least one acrylate (--C(O)CH.dbd.CH.sub.2) or methacrylate
(--C(O)C(CH.sub.3).dbd.CH.sub.2) group. (Preferably, there are 1 to
5 groups.) For high flux activity due to the presence of multiple
carboxylic acids, a preferred fluxing agent is a carboxylic acid
that is selected from the group consisting of compounds represented
by Formulae IV, V, VI and mixtures thereof,
HOOCCH.dbd.CH(O)COR .sup.18OC(O)CH.dbd.CHCOOH (IV)
R.sup.2H.sub.2C(HCOR.sup.n).sub.nCH.sub.2OR.sup.3 (V)
[Y.sup.1Y.sup.2Y.sup.3Y.sup.4]C (VI)
[0061] where R.sup.18 is a substituted alkyl moiety containing at
least one acrylate or methacrylate moiety and said substituted
alkyl moiety comprising a chain having 1 to 16 carbons, preferably
1 to 9 carbons, and more preferably 1 to 3 carbons, and wherein n
is an integer from 1 to 16, preferably an integer from 1 to 9, and
more preferably an integer from 1 to 3, wherein each of R.sup.1,
R.sup.2, . . . R.sup.n, is independently selected from
--C(O)CH.dbd.CHCOOH, --C(O)CH.dbd.CH.sub.2,
--C(O)C(CH.sub.3).dbd.CH.sub.2, and H, and wherein Y.sup.1,
Y.sup.2, Y.sup.3, and Y.sup.4, are each independently selected from
--CH.sub.2OH, --CH.sub.2OCOCH.dbd.CH.sub.2,
--CH.sub.2OCOC(CH.sub.3).dbd.CH.sub.2, --CH.sub.2OC(O)CH.dbd.CHCOOH
and H with the proviso that not all of Y.sup.1, Y.sup.2, Y.sup.3,
and Y.sup.4 are H, and preferably not more than one of said
Y.sup.1, Y.sup.2, Y.sup.3, and Y.sup.4 is H.
[0062] Incorporating an acrylate and/or methacrylate in the
structure can reduce the curing temperature of the adhesive polymer
since the carbon-carbon double bonds in the acrylate or
methacrylate tend be more reactive than most other such double
bonds. Reducing the crosslink temperature of the double bonds tends
to make the adhesive flux more compatible with current practice in
the field wherein solder reflow is achieved in a thermal cycle
having a peak temperature of 230 C. The lower crosslink temperature
can produce an adhesive flux formulation that is fully crosslinked
after only one or two passes through the solder reflow thermal
cycle. The acrylate and methacrylate will also tend to repel
moisture and reduce the absorption of water in the cured polymer. A
particularly preferred acrylate containing fluxing agent is
glycidyl methacrylate which exhibits low viscosity, high flux
activity and excellent curing characteristics, as further described
in Example 4.
[0063] The fluxing agent typically comprises about 0.01%-100%,
preferably about 5%-80%, and more preferably about 10%-70% by
volume of the thermally curable adhesive composition.
[0064] The fluxing agents of the present invention exhibit flux
activities that are superior to that of prior art polymer-fluxing
agent mixtures. Since the inventive fluxing agents are
intrinsically self-crosslinking, the thermally curable adhesive
composition does not require the use of epoxy resins for
crosslinking, though an epoxy may be used in the neutralizing
agent. As a corollary, the shelf life or pot life of the
composition is long and its flux activity high relative to
conventional polymer-fluxing mixtures that include epoxy
resins.
[0065] Further, the adhesion properties, mechanical integrity, and
corrosion resistance achieved with the fluxing agents are superior
to those achieved with prior art polymer fluxing agents because
there is no need to add aggressive fluxing activators. The
inventive fluxing agents are fully crosslinked and all components
thereof are chemically immobilized upon curing. Even the reaction
by-products of flux deoxidization of the metals may be chemically
bound in the polymer matrix.
[0066] Carboxylic acids function well as fluxing agents to remove
oxides from metals. In addition, carboxylic acids are also very
effective crosslinking moieties when present in their reactive form
in a fluxing composition containing a suitable thermosetting resin,
such as an epoxy. For this reason, in the prior art, chemical
protection of the carboxylic acid was essential to achieving
stability and preventing premature reactions. Protection was
achieved by binding the fluxing agent with a chemically- or
thermally-triggered species so that it becomes reactive only at or
near the time that the solder melts. However, with the present
invention, no such protection is necessary because the compositions
can be formulated with only minimal amounts of components that can
crosslink with the carboxylic acid moiety. This results in a
fluxing agent that can function at near its full strength with the
metal oxides to produce fluxing that is superior to any heretofore
polymerizable fluxing agent. The flux activity of the inventive
fluxing agent in some applications may be too high thereby
requiring dilution of the fluxing agent to prevent formation of
undesirable gaseous by-products.
[0067] With the inventive fluxing agent, the principal crosslinking
mechanism occurs at the carbon-carbon double bonds existing in the
fluxing agent molecule and not at the carboxylic acid groups. The
carboxylic acids do not react with the double bonds, therefore on
its own, in the absence of other molecules that can react with the
carboxylic acid, the fluxing agent does not polymerize at ambient
temperatures. It is at elevated temperatures that the double bonds
begin to open and react with other opened double bonds to
crosslink. Since each fluxing agent molecule contains at least two
double bonds, the molecules crosslink into polymeric networks.
[0068] By reducing or eliminating the need for a separate
thermosetting resin in the flux composition, as is required in the
prior art, the flux activity can be kept very high without concern
about pre-maturely crosslinking the thermosetting resin. By
crosslinking the fluxing agent itself, an adhesive having a higher
glass transition temperature and lower coefficient of thermal
expansion can be created without sacrificing fluxing activity.
[0069] Another preferred fluxing-adhesive composition, one that has
very high moisture resistance, comprises a fluxing agent with the
general structure R COOH, wherein R comprises a moiety having two
or more carbon-carbon double bonds, of which preferably at least
one is within an acrylate or methacrylate moiety and R further
contains at least one aromatic moiety, which is an unsaturated
aromatic carbocylic group having a single ring (e.g., phenyl) or
multiple condensed rings (e.g., naphthyl) which condensed rings may
or may not be aromatic. The aromatic moiety also includes
substituted aromatic moieties. The R group can also be fluorinated.
For high flux activity due to the presence of multiple carboxylic
acids, the preferred fluxing agent is a carboxylic acid that is
selected from the group consisting of compounds represented by
Formulae VII and mixtures thereof. A particularly preferred
aromatic-containing fluxing agent is one made from bisphenol A
epoxy, as described in Example 1, which exhibits significant
hydrophobicity.
[0070] The generalized structure for carboxylic acids containing
two or more carbon-carbon double bonds and also containing aromatic
moieties is:
R.sup.19--Ar--R.sup.20 (VII)
[0071] in which Ar is 1
[0072] and R.sup.19 and R.sup.20 are 2
[0073] in which R.sup.21 is --C(O)CH.dbd.CH--COOH,
--C(O)CF.sub.2CF.sub.2C- F.sub.2COOH, or H.
[0074] 2. Carboxylic Acid Neutralizing Agent.
[0075] The carboxylic acid neutralizing agent is a compound that
has the structure R.sup.1--X--R.sup.2, wherein X comprises a
carboxylic-neutralizing moiety such as, for example, epoxide,
--NH-- or --CH(OH)-- group and wherein R.sup.1 and R.sup.2 are
independently selected from (i) H, (ii) alkyl or alkylene moiety
having 1 to 18 carbons, preferably 1 to 9 carbons, and more
preferably 1 to 3 carbons, and (ii) aromatic moiety which is an
unsaturated aromatic carbocylic group having a single ring (e.g.,
phenyl) or multiple condensed rings (e.g., naphthyl) which
condensed rings may or may not be aromatic. The neutralizing agent
may also be a compound containing isocyanate or cyanate ester
groups, or any other group that can react with the carboxylic acid.
The aromatic moiety also includes substituted aromatic moieties.
The alkyl, alkylene, or aromatic moieties can include one or more
carbon-carbon double bonds and/or one or more of X groups. To
neutralize the carboxylic acids without the formation of
condensation by-products, the preferred neutralizing agent is an
epoxide that is selected from the group consisting of compounds
represented by Formulae VIII, IX, and X, and mixtures thereof,
H(CHOCH)R.sup.1 (VIII)
H(CHOCH)R.sup.4(CHOCH)H (IX)
R.sup.1(CHOCH)R.sup.2 (X)
[0076] wherein R.sup.1 and R.sup.2 are defined above and R.sup.4 is
preferably selected an alkyl, alkylene, or aromatic group.
Preferably R.sup.1, R.sup.2, and R.sup.4 is selected from:
CH.sub.2.dbd.C(CH.sub.3)COOCH.sub.2--,
CH.sub.2.dbd.C(CH.sub.3)CH.sub.2(CH- .sub.2).sub.4CH.sub.2--, 3
[0077] and 4
[0078] Another preferred neutralizing agent has the general
structure H(CHOCH)R.sup.5, wherein R.sup.5 comprises a moiety
having one or more carbon-carbon double bonds, of which preferably
at least one is contained in an acrylate or methacrylate moiety,
that is, R.sup.5 contains at least one acrylate
(--C(O)CH.dbd.CH.sub.2) or methacrylate
(--C(O)C(CH.sub.3).dbd.CH.sub.2) group. (Preferably, there are 1 to
5 groups, for example, glycidyl methacrylate; 1,2-epoxy-7-octene;
and 1,2-epoxy-9-decene.)
[0079] In the process of neutralizing the carboxylic acid of the
fluxing agent, the neutralizing agent becomes incorporated, i.e.,
crosslinked, into the cured adhesive composition. Furthermore,
incorporation an acrylate and/or methacrylate in the structure of
the carboxylic neutralizing agent will also to reduce the curing
temperature of the adhesive polymer since the carbon-carbon double
bonds in the acrylate or methacrylate tend to be more reactive than
most other such double bonds. Reducing the crosslink temperature of
the double bonds tends to make the adhesive flux more compatible
with current practice in the field wherein solder reflow is
achieved in a thermal cycle having a peak temperature of 230 C. The
lower crosslink temperature can produce an adhesive flux
formulation that is fully crosslinked after only one or two passes
through the solder reflow thermal cycle. The acrylate and
methacrylate will also tend to repel moisture and reduce the
absorption of water in the cured polymer. A particularly preferred
acrylate containing neutralizing agent is glycidyl methacrylate
which exhibits low viscosity and high flux activity as further
described in Example 4.
[0080] The amount of neutralizing agent employed is preferably
proportional to the amount of fluxing agent present and the
neutralizing agent typically comprise about 0.01%-90% preferably
about 5%-50%, and most preferably about 10%-50% by volume of the
thermally adhesive composition. Preferably, no more than a
stoichiometric amount (with the carboxylic acid prior to fluxing
reactions) is employed to neutralize the carboxylic acid in the
fluxing agent. Thermally curable adhesive compositions where the
amount of neutralizing agent is substantially higher than the
required stoichiometric amount will generally exhibit inferior
fluxing properties, whereas compositions with substantially less
than stoichiometric amounts of neutralizing agent will generally
exhibit poor electrical insulation and high metallic
electromigration when exposed to humid environments.
[0081] Since the neutralizing agents are chemically linked with the
carboxylic acid in the fluxing agent only slowly and at elevated
temperatures, they reduce the fluxing activity of the adhesive flux
minimally. As a corollary, the shelf life or pot life of the
composition is long and its flux activity high relative to
conventional polymer-fluxing mixtures that require epoxy
resins.
[0082] With the inventive fluxing agent, the principal crosslinking
mechanism still occurs at the carbon-carbon double bonds existing
in the fluxing agent molecule and not at the carboxylic acid
groups.
[0083] 3. Diluents.
[0084] The presence of carbon-carbon double bond(s) in the fluxing
agent molecule allows much flexibility in the formulation of a flux
composition with exceptional thermomechanical properties. This is
achieved by the addition of double bond containing diluents that
can also crosslink with the flux to create a superior adhesive.
This technique permits the design of fluxing adhesive compositions
that can attain high crosslink densities, which are desirable for
good thermomechanical properties and good adhesion. Moreover, this
is accomplished without the concern of premature crosslinking and
reduced pot life associated with the prior art. Preferred diluents
include, for example, (a) penta eryethritol tetraacrylate,
C(CH.sub.2OOCCH.dbd.CH.sub.2).sub.4, (b)
triallyl-1,3,5-triazine-2,4,6 (1H,3H,5H)-trione, (c)
tris[2-(acryloxy)ethyl]isocyanurate, (d) glycerol propoxylate
triacrylate and mixtures thereof. Diluents (b) and (c) have the
following structures: 5
[0085] Other double bond compounds, many of which are commercially
available, including, for example, diallyl phthalate and divinyl
benzene can also be used. Hydrophobic diluents as described are
preferred but hydrophilic diluents can also be employed when
appropriate. The diluent when employed typically can comprise up to
about 90%, preferably between about 5%-80%, and more preferably
between about 50%-80% by volume of the thermally curable adhesive
composition.
[0086] One benefit of employing hydrophobic diluents is that their
presence tends to reduce the amount of water which the cured
adhesive composition will absorb. The reason is that the fluxing
agent, when crosslinked, will have active carboxylic groups that
can attract water, even though these carboxylic groups, being part
of a network, are immobile. Water acts as a plasticizer which
softens the cured adhesive composition. The use of hydrophobic
diluents which are crosslinked to the fluxing agent will counteract
the hydrophilic effects of the carboxylic acid groups. Indeed, the
cured adhesive compositions containing hydrophobic diluents can
have less than 2% (wt) moisture when exposed to ambient
conditions.
[0087] 4. Free Radical Initiators.
[0088] While the thermally curable adhesive composition can be
cured using heat alone, the cross linking reaction can be initiated
and facilitated by the presence of free-radicals, including, for
example, those generated by benzoyl peroxide, butyl hydroperoxide,
2,2'-azobisisobutyronitrile, and mixtures thereof. These free
radical initiators or sources are commercially available.
[0089] Free-radicals can be created in-situ by exposure of the
free-radical initiator to heat, radiation, or other conventional
energizing sources. Introduction of an appropriate free-radical
initiator can accelerate the onset of crosslinking to the desired
moment in a solder reflow operation. The presence of a small amount
of free-radical crosslinking initiator in the fluxing agent can be
used to control the rate and the temperature of crosslinking of the
fluxing agent, ensuring effective fluxing action and strong
adhesion of the fluxing agent to the substrates upon curing.
[0090] The free radical initiator when employed typically comprises
up to about 5%, preferably between about 0%-3%, and more preferably
about 0.3%-1% by weight of the thermally curable adhesive
composition.
[0091] 5. Resins.
[0092] The thermally curable adhesive composition does not require
resins; further, compositions that do not include resins tend to
have longer pot lives and lower viscosities during solder reflow.
However, as an option, a resin can be employed and it functions to
increase the adhesion of the cured composition to the substrate and
to increase the cohesive strength and glass transition temperature
of the cured composition. The resin may be any suitable resin that
is compatible (i.e., blendable) with the fluxing agent. By
blendable is meant that the resins do not have to be chemically
bonded to the fluxing agent and/or diluent. Resins which meet these
requirements include, but are not limited to, epoxies, phenolics,
novalacs (both phenolic and cresolic), polyurethanes, polyimides,
bismaleimides, maleimides, cyanate esters, polyvinyl alcohols,
polyesters, and polyureas. Preferred resins
1,4-cyclohexanedimethanol diglycidyl ether,
3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxyla- te,
N,N-diglycidyl-4-glycidyl-oxyanilline, bisphenol A based epoxy
resins, and mixtures thereof. These are commercially available.
[0093] Suitable compounds (including polymers) can also be modified
to form resins that are blendable with the diluent and/or the
carboxylic acid fluxing agent. Examples of such compounds are
acrylics, rubbers (butyl, nitrile, etc.), polyamides,
polyacrylates, polyethers, polysulfones, polyethylenes,
polypropylenes, polysiloxanes, polyvinyl acetates/polyvinyl esters,
polyolefins, cyanoacrylates, and polystyrenes. Generally, any
compound can function as a resin if it can be modified to contain
at least one of the following illustrative functional groups that
act as reactive sites for polymerization: anhydrides, carboxylic
acids, amides, amines, alcohols/phenols, nitrites, carbamates,
isocyanates, sulfonamides, semicarbazones, oximes, hydrazones,
cyanohydrins, ureas, phosphoric esters/acids, thiophosphoric
esters/acids, phosphonic esters/acids, phosphites, phosphonamides,
and sulfonic esters/acids. For example, a polyolefin which has no
reactive sites for binding and has poor adhesive properties is
typically not a suitable resin, however, a carboxylated polyolefin
functions well when matched with a suitable crosslinking agent. A
combination of these and other resins, such as non-crosslinkable
thermoplastic resins, may also be used as resins. Resins when
employed can comprise up to about 80%, preferably between about
10%-80%, and more preferably about 60%-70% by volume of the
thermally curable adhesive composition.
[0094] In preparing the fluxing composition, the proportions of the
five components may be varied over a considerable range and still
yield acceptable fluxing activity as well as good post cured
material properties. Preferably, the fluxing composition employed
does not produce gaseous byproducts that can result in the
formation of bubbles in the final cured composition. This can be
achieved with thermally curable adhesive compositions preferably
formulated as follows:
[0095] a) Fluxing agent comprising about 5%-80% (vol.) of the
composition;
[0096] b) Neutralizing agent comprising about 0.1-90% (vol.) of the
composition;
[0097] c) Diluent comprising about 5%-80% (vol.) of the
composition;
[0098] d) Free radical initiator comprising about 0%-3% (wt) of the
composition; and
[0099] e) Resin comprising about 0%-80% (vol.) of the
composition.
[0100] It should be noted that some neutralizing agents can also
function as resins. Therefore, when resins are employed, the amount
can be kept to a minimum. Some thermally curable adhesive
compositions having components within these ranges may exhibit
undesirably high moisture absorption, low glass transition
temperatures, or high coefficients of thermal expansions after
cured, but they remain useful as fluxing compositions in
applications where these characteristics are not critical.
[0101] Most preferably, the thermally curable adhesive composition
after being cured has a coefficient of thermal expansion of about
25 PPM/.degree. C., a glass transition temperature in excess of
150.degree. C., electrical insulation resistance greater that 100
MegOhms according to IPC-TM-650 testing on the IPC-B-24 test board
and moisture content of less than 2%. These characteristics can be
achieved with thermally curable adhesive compositions preferably
comprising about 5%-35% (vol.) fluxing agent, a stoichiometric
quantity of neutralizing agent and about 20%-80% (vol.)
diluent.
[0102] While, again, some of the fluxing agents within these ranges
may exhibit high coefficient of thermal expansion or low glass
transition temperature when cured, they remain useful as fluxes in
applications where these characteristics are not critical.
[0103] In order for the thermally curable adhesive composition to
achieve the largest spreading and wetting by the solder, it must
achieve and maintain low viscosity up to the temperature at which
the solder melts and wets the metallizations. If the composition
becomes too thick before the solder has melted, it will impede the
flow of the solder melt and reduce the degree of metal soldering.
For this reason, the curing of the composition must occur slowly
relative to the time required to reach the melting point of the
solder powder. This can be achieved by selection of the components
with appropriate crosslinking temperatures and formulating the
appropriate proportions by use of a differential scanning
calorimeter to control reaction rates and times.
[0104] The inventive thermally curable composition exhibit the
following features:
[0105] a) provides sufficient flux activity to promote the solder
bump to readily wet the metallization on the substrate during
solder reflow, without the presence of corrosive flux activators
that can contaminate the silicon chip;
[0106] b) promotes solder wetting and self-alignment of the chip to
the pads on the substrate by action of the wetting force of the
molten solder, during the solder reflow cycle, no curing of the
flux composition occurs until the solder bump has been melted;
[0107] c) reduces or eliminates gaseous evolution during the reflow
cycle that would otherwise create voids;
[0108] d) cures quickly and soon after solder bump melts;
[0109] e) demonstrates little shrinkage of the composition during
curing to minimize the stress resulting from the curing process and
subsequent cooling; and
[0110] f) forms strong adhesion of the cured composition to the
chip, substrate and solder joints.
Synthesis of Fluxing Agents
EXAMPLE 1
Preparation of bisphenol A glycerolate di(2-octen-1-ylsuccinic)
acid monoester
[0111] 10.54 g of bisphenol A diglycidyl ether was heated to 70 C
under stirring and then 4.46 g acrylic acid was added slowly under
nitrogen atmosphere. After maintaining the reaction at 70 C for 2
hours, 13.0 g of 2-octen-1-ylsuccinic anhydride was added and then
the temperature was raised to 80 C under mechanical stirring. The
reactants are stirred at 80 C for 2-3 hours to complete the
reaction.
[0112] The reactions involved in this synthesis include: 6
EXAMPLE 2
Preparation of Bisphenol A Diglycerolate Dimaleic Acid
Monoester
[0113] 34.8 g of bisphenol A diglycidyl ether was heated to 70 C
under continuous stirring and then 14.4 g of acrylic acid was added
slowly under nitrogen atmosphere. After maintaining the reaction at
70 C for 2 hours, 19.6 g of maleic anhydride was added and then the
temperature was raised to 80 C under mechanical stirring for 2-3
hours to complete the reaction.
[0114] The reaction involved in this synthesis is: 7
EXAMPLE 3
Synthesis of Pentaerythritol Triacrylate Maleic Acid Monoester
[0115] 9.8 g of maleic anhydride was heated to 80 C under nitrogen
atmosphere until all the maleic anhydride is melted before 29.8 g
of pentaerythritol triacrylate was added slowly under continuous
stirring. The reaction was then maintained at 80 C for 3 hours
followed by cooling to room temperature.
[0116] The reaction involved in this synthesis is: 8
EXAMPLE 4
Formulation of Fluxing Adhesive
[0117] Stoichiometric amounts of glycidyl methacrylate was added to
the flux agent to neutralize the carboxyl groups. 24.6 g of
glycidyl methacrylate was added to 75.4 g of bisphenol A
glycerolate di(2-octen-1-ylsuccinic) acid monoester and the mixture
was thoroughly stirred at 60 C for 10 min. The reaction that occurs
after reflow is: 9
EXAMPLE 5
Formulation of Fluxing Adhesive
[0118] Stoichiometric amounts of bisphenol A based epoxy was added
to the flux agent to neutralize the carboxyl groups. 30.3 g
bisphenol A based epoxy was added to 71.5 g bisphenol A glycerolate
di(2-octen-1-ylsuccinic- ) acid monoester and the mixture was
thoroughly stirred at 60 C for 10 min. The reaction that occurs
after reflow is: 10
EXAMPLE 6
Formulation of Flexing Adhesive
[0119] 15.3 g bisphenol A based epoxy and 10 g glycidyl
methacrylate was added to 74.5 g bisphenol A glycerolate
di(2-octen-1-ylsuccinic) acid monoester and the mixture was
thoroughly stirred at 60 C for 10 min. Then the mixture was cooled
to room temperature for use.
EXAMPLE 7
Formulation of Fluxing Adhesive
[0120] 21.6 g glycidyl methacrylate and 10 g pentaerythritol
tetraacrylate was added to 68.4 g bisphenol A glycerolate
di(2-octen-1-ylsuccinic) acid monoester and the mixture was
thoroughly stirred at 60 C for 10 min. The mixture was cooled to
room temperature for use.
EXAMPLE 8
Formulation of Fluxing Adhesive
[0121] 21.6 g glycidyl methacrylate and 10 g pentaerythritol
tetraacrylate was added to 68.4 g bisphenol A glycerolate dimaleic
acid monoester and the mixture was thoroughly S stirred at 60 C for
10 min. The mixture was cooled to room temperature for use.
EXAMPLE 9
[0122] The inventive compositions were formulated as follows:
1 % Fluxing Agent in Composition Number: Fluxing Agent 1 2 3 4 5 6
bisphenol A glycerolate di(2- 100 70 75 75 60 65
octen-1-ylsuccinic) acid monoester bisphenol A diglycidyl ether --
30 -- 15 40 25 glycidyl methacrylate -- -- 25 10 -- --
pentaerythritol tetraacrylate -- -- -- -- -- 10
[0123] Physical characteristics of the inventive curable adhesive
compositions were measured. The results are set forth in the
following table. SIR is surface insulation resistance.
2 Shear Glass Thermal SIR (Ohms) strength Shear strength Solder
Transition Degradation (85 C., 85% for gold after 110 Hrs. Spread
Temperature Temperature RH after surface humidity (85 C.,
Composition (Area) Tg(C.) (C.) 168 Hrs.) (MPa) 85% RH) A 1.28 -- --
-- -- -- 1 3.80 >200 >350 2.28E8 20.57 -- 2 3.45 >200
>270 2.35E8 30.16 -- 3 3.62 >200 >300 2.42E8 22.31 36.31 4
3.56 >200 >320 2.30E8 33.84 35.42 5 3.27 >200 >290
2.46E8 32.94 44.86 6 3.25 >200 >300 2.25E8 28.62 32.58
[0124] Composition A consisted of an adhesive material that is
described in U.S. Pat. No. 5,128,746 (Example 4) which contains a
fluxing agent and hardener. The composition was prepared with the
following components (by weight): 50% Shell EPON 825 epoxy resin
(Shell Chemical Co.), 7% malic acid, 42% methylhexahydrophthalic
anhydride (MA) and 1% imidazole. The malic acid and the epoxy resin
were mixed and heated to about 150 C with stirring until the
solution was clear. The solution was allowed to cool to room
temperature before the MA and imidazole were added and the mixture
was stirred until uniform.
[0125] As is apparent, the inventive compositions yield superior
physical properties. The solder spread was measured by placing a
ball of solder on a surface and then applying a small amount of the
curable adhesive composition (or composition A) to the solder. The
surface was then heated to about 200 C and the area that the melted
solder covered was measured. The solder spread values are
normalized, that is, the solder spread in the case where no fluxing
agent was employed is equal to 1.
[0126] The surface insulation resistance (SIR) test was performed
according to the IPC-TM-650 test method and using the IPC-B-24 test
board, both available from the IPC, Lincolnwood, Ill. The test was
conducted at 85 C and 20% relative humidity at the start. The
chamber is stabilized under these conditions for 3 hours, then the
humidity is slowly increased to 85% over a 15 minute period and the
specimens were allowed to come to equilibrium for at least 1 hour
before applying a bias voltage. The bias voltage for this test is
50 v. The test voltage is -100 v. Insulation resistance is measured
at the start and at 168 hours. Using this test method, all the
formulations listed above exhibited significantly lower insulation
resistance values if prepared without a carboxylic acid
neutralizing agent. Many test boards made with the above
formulations, but without the neutraliing agent exhibited
electromigration.
[0127] It will now be apparent to those skilled in the art that
various modifications, variations, substitutions, and equivalents
exist for various elements of the invention but which do not
materially depart from the spirit and scope of the invention.
Accordingly, it is expressly intended that all such modifications,
variations, substitutions and equivalents which fall within the
spirit and scope of the invention as defined by the appended claims
be embraced thereby.
* * * * *