U.S. patent application number 09/208790 was filed with the patent office on 2001-09-13 for thermal paste preforms as a heat transfer media between a chip and a heat sink and method thereof.
Invention is credited to DAVES, GLENN G., EDWARDS, DAVID L, FAROOQ, SHAJI, IRUVANTI, SUSHUMNA, POMPEO, FRANK L.
Application Number | 20010021102 09/208790 |
Document ID | / |
Family ID | 22776077 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010021102 |
Kind Code |
A1 |
EDWARDS, DAVID L ; et
al. |
September 13, 2001 |
THERMAL PASTE PREFORMS AS A HEAT TRANSFER MEDIA BETWEEN A CHIP AND
A HEAT SINK AND METHOD THEREOF
Abstract
The present invention relates generally to a new apparatus and
method for introducing thermal paste into semiconductor packages.
More particularly, the invention encompasses an apparatus and a
method that uses at least one preform of thermal paste for the
cooling of at least one chip in a sealed semiconductor package. The
thermal paste preform is subcooled, and is transferred onto a
module component from a separable transfer sheet, or is placed onto
the module component using an attached and/or imbedded mesh. The
preform of thermal paste may be of simple or complex shape, and
enables cooling of one or more chips in a module.
Inventors: |
EDWARDS, DAVID L;
(POUGHKEEPSIE, NY) ; DAVES, GLENN G.; (BEACON,
NY) ; FAROOQ, SHAJI; (HOPEWELL JUNCTION, NY) ;
IRUVANTI, SUSHUMNA; (WAPPINGERS FALLS, NY) ; POMPEO,
FRANK L; (MONTGOMERY, NY) |
Correspondence
Address: |
AZIZ M AHSAN
INTELLECTUAL PROPERTY LAW
IBM CORPORATION 1580 ROUTE 52
HOPEWELL JUNCTION
NY
125336531
|
Family ID: |
22776077 |
Appl. No.: |
09/208790 |
Filed: |
December 10, 1998 |
Current U.S.
Class: |
361/704 |
Current CPC
Class: |
H01L 2924/16152
20130101; H01L 2924/16152 20130101; H01L 2224/73253 20130101; H01L
2224/73253 20130101; H01L 23/42 20130101; H01L 2224/16225 20130101;
H05K 7/20454 20130101 |
Class at
Publication: |
361/704 |
International
Class: |
H05K 007/20 |
Claims
What is claimed is:
1. A heat transfer structure comprising at least one preformed slug
of at least one thermal paste sandwiched between at least one chip,
acting as at least one heat source, and at least one thermal
dissipation device.
2. The structure of claim 1, wherein said at least one thermal
dissipation device is selected from a group consisting of thermal
cap and heat spreader.
3. The structure of claim 1, wherein said slug of at least one
thermal paste is temporarily attached to at least one removable
transfer sheet.
4. The structure of claim 1, wherein said slug of at least one
thermal paste has at least one imbedded mesh.
5. The structure of claim 1, wherein said slug of at least one
thermal paste has at least one imbedded mesh and is temporarily
secured to at least one removable transfer sheet.
6. The structure of claim 1, where shape of said at least one slug
of at least one thermal paste is selected from a group consisting
of triangular shape, square shape, rectangular shape, circular
shape, elliptical shape, polygonal shape, X-shape, Y-shape, Z-shape
or any odd shape.
7. A heat transfer apparatus comprising at least one heat
generating device with at least one thermal paste preform secured
to at least one surface of said at least one heat generating
device.
8. The apparatus of claim 7, wherein said at least one thermal
paste preform transfers heat from said heat generating device to at
least one thermal dissipation device.
9. The apparatus of claim 7, wherein said at least one thermal
paste preform transfers heat from said heat generating device to at
least one thermal dissipation device, and wherein at least a
portion of the surface of said thermal dissipation device has a
plurality of embedded first particles covering the area which is in
thermal contact with said thermal paste preform.
10. The apparatus of claim 9, where said at least one first
particle is embedded in said thermal dissipation device by means of
casting, pressing or grit blasting.
11. The apparatus of claim 8, where said thermal dissipation device
is of the same material as at least one solid in said at least one
thermal paste preform.
12. The apparatus of claim 8, wherein the thickness of said thermal
paste preform is greater than the maximum spacing between said chip
and said thermal dissipation device.
13. The apparatus of claim 7, wherein said at least one heat
generating device is secured to a substrate.
14. The apparatus of claim 7, wherein said at least one heat
generating device is secured to a substrate, and wherein said
substrate is secured to a thermal dissipation device, and wherein
said at least one thermal paste preform provides a thermal path
between said substrate and said thermal dissipation device.
15. The apparatus of claim 7, wherein said at least one heat
generating device is secured to a substrate and wherein said
substrate is secured to a thermal dissipation device, and wherein
said thermal paste preform provides a thermal path between said
heat generating device and said thermal dissipation device, and
wherein said heat generating device is secured to said substrate by
at least one means selected from a group consisting of solder
balls, C4s, solder columns or solder mass.
16. The apparatus of claim 7, wherein said at least one heat
generating device is secured to a substrate, and wherein said
substrate is secured to a thermal dissipation device, and wherein
said at least one thermal paste preform provides a thermal path
between said substrate and said thermal dissipation device, and
wherein said thermal dissipation device has at least one protrusion
in said thermal path.
17. The apparatus of claim 7, wherein said at least one heat
generating device is secured to a substrate, and wherein said
substrate is secured to a thermal dissipation device, and wherein
said at least one thermal paste preform provides a thermal path
between said substrate and said thermal dissipation device, and
wherein said thermal dissipation device has at least one blind hole
in said thermal path.
18. The apparatus of claim 7, wherein at least one shape of said
thermal paste preform is selected from a group consisting of
triangular shape, square shape, rectangular shape, circular shape,
elliptical shape, polygonal shape, X-shape, Y-shape, Z-shape or any
odd shape.
19. The apparatus of claim 7, wherein at least one of said thermal
paste preform is temporarily attached to at least one removable
transfer sheet.
20. The apparatus of claim 7, wherein at least one of said thermal
paste preform has at least one embedded mesh.
21. The apparatus of claim 7, wherein at least one of said thermal
paste preform has at least one imbedded mesh, and said thermal
paste preform with said imbedded mesh is temporarily secured to at
least one transfer sheet.
22. A process for providing heat transfer to at least one chip,
comprising the step of placing at least one preformed slug of at
least one thermal paste on said chip to provide said heat
transfer.
23. The process of claim 22, where said thermal paste preform is
temporarily attached to a transfer sheet, and wherein said transfer
sheet is removed after said transfer.
24. The process of claim 22, wherein said thermal paste preform has
at least one imbedded mesh.
25. The process of claim 22, wherein two or more thermal paste
preforms are applied simultaneously to two or more chips.
26. The process of claim 22, wherein at least one thermal paste
preform is applied simultaneously onto two or more chips.
27. The process of claim 22, wherein said thermal paste/mesh
preform is applied onto said thermal dissipation device and
sub-cooled.
28. The process of claim 22, wherein said thermal paste preform is
subcooled at time of placement, and is subsequently allowed to warm
above subcooling temperature prior to any application of any
compression force.
29. A process of forming at least one preformed slug of at least
one thermal paste, comprising the steps of: (a) filling at least
one mold with at least one thermal paste, (b) subcooling said mold
and said thermal paste to a temperature that temporarily stiffens
and reduces the tackiness of said thermal paste to form said at
least one preformed slug, and (c) removing said formed subcooled
preform slug from said mold.
30. The process of claim 29, wherein said thermal paste preform is
sub-cooled to a temperature of between about 20.degree. C. and
about minus 100.degree. C.
31. The process of claim 29, wherein said thermal paste preform is
sub-cooled to a temperature of at least about minus 20.degree.
C.
32. The process of claim 29, wherein said preformed thermal paste
is temporarily secured to at least one removable transfer
sheet.
33. The process of claim 29, wherein said preformed thermal paste
has at least one embedded mesh.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to a new apparatus
and method for introducing thermal paste into semiconductor
packages. More particularly, the invention encompasses an apparatus
and a method that uses at least one preform of thermal paste for
the cooling of at least one chip in a sealed semiconductor package.
The thermal paste preform is subcooled, and is transferred onto a
module component from a separable transfer sheet, or is placed onto
the module component using an attached and/or imbedded mesh. The
preform of thermal paste may be of simple or complex shape, and
enables cooling of one or more chips in a module.
BACKGROUND OF THE INVENTION
[0002] Semiconductor devices are becoming smaller and more dense
with the evolution of new technology. However, increases in circuit
density produce a corresponding emphasis on overall chip packaging
strategies in order to remain competitive. Chip and substrate
manufacturers are therefore constantly being challenged to improve
the quality of their products by identifying and eliminating
problems, reducing package size and weight, decreasing package
costs, providing improved thermal efficiencies and better and more
advanced chips. Whereas significant improvements are being made to
eliminate systematic problems by reducing process variability,
process improvements alone are not sufficient to eliminate all the
problems which effect both performance and reliability.
[0003] One way to allow high degrees of integration is to provide a
highly efficient internal cooling design. A preferred way of
cooling high performance SCMs (single chip modules) and MCMs
(multi-chip modules) is using thermal paste. Thermal paste is often
used as a high thermal conductivity interface material to fill the
gaps between the back-side of chips, such as, flip chips, and the
inside surfaces of the caps or heat spreaders. Producing modules
that use thermal paste has multiple challenges. Module components,
like the back surface of the chip and the inside of the cap must be
chemically compatible with the thermal paste, so that the paste can
adhere to them. The package must be designed such that the thermal
paste filled chip-to-cap gap has sufficient thickness that it will
form a reliable structure.
[0004] During assembly, sufficient paste must be dispensed to
completely cover the chip. Care must also be taken to use a
dispense pattern that will cause the entire chip to be covered.
Thermal paste is often very viscous and difficult to handle or
dispense. The paste dispense may be accomplished by either use of
expensive automated equipment, or by manually screening the thermal
paste through a template.
[0005] U.S. Pat. No. 3,993,123 (Chu et al.) discloses the thermal
conduction module (TCM). This was a major advancement in the
cooling of MCMs. It utilized a complex cooling hat. Over each of
the flip chips on the substrate was a hole in the hat. From each
hole extended a spring loaded piston that contacted the back of the
chip. The module was hermetic, and filled with helium. The primary
cooling path was from the circuit side of the chip, through the
thickness of the chip, to the face of the piston, up the piston, to
the inside of the hat, to the back of the hat, across an interface
to an attached cold plate, and to the water circulating through the
cold plate. The high helium content of the gas in the module
greatly reduced the thermal resistance of the chip-to-piston
interface and the piston-to-hat interface.
[0006] U.S. Pat. No. 4,193,445 (Chu et al.) discloses another
enhancement on the TCM. Here solder was included with each of the
pistons so that it could be reflowed after assembly, to fill the
chip-to-piston and piston-to-hat gaps, for improved thermal
performance.
[0007] U.S. Pat. No. 4,226,281 (Chu) disclosed an enhancement to
the TCM. Rather than just one piston per chip, each chip now had a
matrix of pistons that contacted the back of the chip for cooling.
To maintain almost full coverage of the back of the chip, headers
are used on the faces of each of the pistons.
[0008] U.S. Pat. No. 5,005,638 (Goth et al.) disclose an improved
piston geometry. Barrel shaped pistons were used to allow tighter
piston to hat gaps, while maintaining the ability to accommodate
chip tilt. Material changes also improved thermal performance over
the traditional TCM. Pistons were now made of copper rather than
aluminum, and the module was now filled with oil rather than
helium.
[0009] U.S. Pat. No. 5,023,695 (Umezawa et al.) discloses a flat
plate cooling (FPC). In this structure, a flat cooling plate is
just above the array of chips. Thermal paste is used to fill the
gaps between the chips and the flat plate.
[0010] U.S. Pat. No. 5,098,609 (Iruvanti et al.) the disclosure of
which is incorporated herein by reference, discloses stable high
solids, high thermal conductivity pastes. The pastes include a
thermally conducting solid filler, a non-aqueous liquid carrier and
a stabilizing dispersant. The resulting pastes are highly
concentrated, of low viscosity, electrically resistive, highly
thermally conducting and stable.
[0011] U.S. Pat. No. 5,291,371 (Gruber et al.) disclose a thin high
performance cooling structure. The purpose of the structure is to
establish and maintain a thin solder interface between the chips
and the cooling hat. In order for this to be functional and
reliable, the solder and the hat have to be very flat, and a thin
lubricant is between them. The solder can be positioned onto the
chip as a preform (and then reflowed or plastically deformed).
[0012] U.S. Pat. No. 5,325,265 (Turlik et al.) discloses a higher
performance version of flat plate cooling. In this invention
thermally conductive cushions conduct heat from the backs of the
chips to the inside surface of the hat. The hat has shallow
cavities above each of the chips. The cushions are low melting
point solder, preferably indium. The indium may be placed between
the chips and the hat as preforms.
[0013] U.S. Pat. No. 5,591,789 (Iruvanti et al.) the disclosure of
which is incorporated herein by reference, discloses a polyester
dispersant for use in high thermal conductivity pastes.
[0014] U.S. Pat. Nos. 5,604,978, 5,623,394 and 5,724,729, (Sherif
et al.) the disclosure of which is incorporated herein by
reference, disclose a method and apparatus for the customized
cooling of chips on an MCM with a range of cooling requirements. It
uses flat plate cooling, and uses pastes of different thermal
conductivities on chips to customize the cooling of the chips. The
paste is either between the chips and a flat cooling hat, or
between the chip and a blind hole in the hat. Surplus paste may
also fill some or all of the rest of the inside of the module.
[0015] U.S. Pat. No. 5,718,361 (Braun, et al.) the disclosure of
which is incorporated herein by reference, discloses an apparatus
and method for forming mold for metallic material. Braun also
discloses the use of a mold to form heat sinks with fins.
[0016] U.S. Pat. No. 5,718,367 (Covell, et al.) the disclosure of
which is incorporated herein by reference, discloses a mold
transfer apparatus and method. Covell also discloses the use of a
mold transfer to form a high density heat sink.
[0017] U.S. Pat. Nos. 5,757,620 and 5,819,401, (Edwards, et al.)
the disclosure of which is incorporated herein by reference,
discloses a method and apparatus for cooling of chips using blind
holes with customized depth. It uses flat plate cooling, and varies
the depth of the thermal paste filled gap to customize the cooling
to each of the chips on a module, such as, a MCM (Multi-Chip
Module).
[0018] U.S. patent application Ser. No. 08/758,789, filed on Dec.
3, 1996, (Iruvanti, et al.) the disclosure of which is incorporated
herein by reference, discloses a hermetically sealed module where
the internal surface of the module has a roughened surface by grit
blasting or machined to have parallel and/or crossing grooves. The
paste penetrates the roughened surface and inhibits the flow of the
paste out of the gap.
[0019] U.S. patent application Ser. No. 09/140,583, filed on Aug.
26, 1998, Attorney Docket No. F19-98-111, (Edwards, et al.) the
disclosure of which is incorporated herein by reference, discloses
a structure and a method that uses surface chemistry modification
of the inside of the thermal cooling cap where it contacts the
thermal paste. This is done by modifying the internal surface of
the cap by embedding particles that preferably have the same
chemical composition as one or more of the solids in the thermal
paste.
[0020] As stated earlier a variety of internal cooling means have
been disclosed, using complicated hardware designs, helium filled
modules, solder interfaces, thermal paste filled interfaces, etc.
Thermal paste offers the combination of low cost hardware, high
thermal performance, and module reworkability. What is needed are
improved methods of introducing thermal paste into semiconductor
modules.
[0021] One way to improve the application of thermal paste in
semiconductor modules would be to use thermal paste preforms. The
preforms can be formed with conventional thermal paste, and then
subcooled to temporarily increase stiffness and decrease tackiness.
The thermal paste may be on a transfer sheet, or attached to a mesh
and would be transferred onto or associated with the chip that
needs to be cooled.
PURPOSES AND SUMMARY OF THE INVENTION
[0022] The invention is a novel method and an apparatus for making
and using at least one preform of thermal paste for the cooling of
at least one chip in a capped module.
[0023] Therefore, one purpose of this invention is to provide an
apparatus and a method that will allow the making and using of at
least one preform of thermal paste for the cooling of at least one
chip in a capped module.
[0024] Another purpose of this invention is to provide thermal
paste preforms that are of simple or complex shapes.
[0025] Yet another purpose of this invention is to provide preforms
of thermal paste that are used to cool one or more chips in a
module.
[0026] Still yet another purpose of this invention is to provide
thermal paste preforms that are transferred onto modules from
separable transfer sheets, or are placed onto modules using an
attached and/or imbedded mesh.
[0027] Still another purpose of this invention is to provide
thermal paste preforms to enhance the thermal path between a chip
and a heat sink.
[0028] One advantage of the current invention is that the thermal
paste may be incorporated into the module without the requirement
for expensive automated tooling.
[0029] Another advantage of the current invention is that complex
templates are not required.
[0030] Yet, another advantage of this invention is the improved
control of the quantity of paste applied to each chip.
[0031] Still, another advantage of this invention is the ability to
use complex patterns.
[0032] Still, yet another advantage of this invention is the
improved reliability, when the mesh is used and is left inside the
module.
[0033] Another advantage of this invention is eliminating the need
for manual screening of paste.
[0034] Yet another advantage of this invention is the reduced cycle
time for assembly of a module.
[0035] Therefore, in one aspect this invention comprises a heat
transfer structure comprising at least one preformed slug of at
least one thermal paste sandwiched between at least one chip,
acting as at least one heat source, and at least one thermal
dissipation device.
[0036] In another aspect this invention comprises a heat transfer
apparatus comprising at least one heat generating device with at
least one thermal paste preform secured to at least one surface of
said at least one heat generating device.
[0037] In yet another aspect this invention comprises a process for
providing heat transfer to at least one chip, comprising the step
of placing at least one preformed slug of at least one thermal
paste on said chip to provide said heat transfer.
[0038] In still another aspect this invention comprises a process
of forming at least one preformed slug of at least one thermal
paste, comprising the steps of:
[0039] (a) filling at least one mold with at least one thermal
paste,
[0040] (b) subcooling said mold and said thermal paste to a
temperature that temporarily stiffens and reduces the tackiness of
said thermal paste to form said at least one preformed slug,
and
[0041] (c) removing said formed subcooled preform slug from said
mold.
DESCRIPTION OF THE DRAWINGS
[0042] The features of the invention believed to be novel and the
elements characteristic of the invention are set forth with
particularity in the appended claims. The drawings are for
illustration purposes only and are not drawn to scale. Furthermore,
like numbers represent like features in the drawings. The invention
itself, however, both as to organization and method of operation,
may best be understood by reference to the detailed description
which follows taken in conjunction with the accompanying drawings
in which:
[0043] FIG. 1, illustrates a mold containing the thermal paste
preform of this invention.
[0044] FIG. 2A, shows one embodiment of the invention with the
thermal paste preform on a transfer sheet.
[0045] FIG. 2B, shows another embodiment of the invention with the
thermal paste preform having an imbedded mesh.
[0046] FIG. 3, illustrates the transfer of a thermal paste preform
onto a heat generating device.
[0047] FIG. 4, illustrates a cross-sectional view of a module using
the thermal paste preform of this invention.
[0048] FIG. 5, illustrates a cross-sectional view of a MCM showing
the deformed thermal paste preforms of this invention which are
used to cool chips in the module.
DESCRIPTION OF THE INVENTION
[0049] This invention describes one way to improve the assembly of
multilayered ceramic (MLC) electronic packages without any loss or
degradation of their performance. Packaging methods which improve
assembly increase the desirability of such electronic packages in
the marketplace. As a person skilled in the art knows, increases in
packaging density typically require high performance cooling
solutions. There are numerous advantages to cooling with thermal
paste.
[0050] FIG. 1, illustrates at least one mold or template 10,
containing at least one thermal paste preform or disc 12. The
thermal paste preform 12, can be of a simple or a complex shape,
typically, the shape of at least one surface of the thermal paste
preform could be selected from a group comprising, triangular
shape, square shape, rectangular shape, circular shape, elliptical
shape, polygonal shape, X-shape, or some odd shape, to name a few.
It is preferred that the material for the thermal paste preform
should be such that it has a temporary increase in stiffness and
decrease in tackiness between about 20.degree. C. and about minus
100.degree. C. Wall angle 11, of the openings in the mold 10, can
be 90 degrees or more than 90 degrees to help facilitate the
removal of the preform 12.
[0051] FIG. 2A, shows one embodiment of the invention with the
thermal paste disk or preform 12, on a transfer sheet 20. The disk
of thermal paste 12, is preferably temporarily attached to a
transfer sheet 20. However, for some applications the transfer
sheet 20, could itself have good thermal conductivity and could be
integrated into or be a part of the heat transfer structure or
preform 12.
[0052] FIG. 2B, shows another embodiment of the invention with the
thermal paste preform 12, which has been integrated into at least
one imbedded mesh 21, such as, a mesh screen 21.
[0053] For some applications the thermal paste preform 12, may be
prepared by placing a blob of thermal paste onto a transfer sheet
20, or a mesh 21, and subcooling thermal paste to reduce tackiness
and increase viscosity and thereby forming the thermal paste
preform 12.
[0054] The thermal paste preform 12, could be a disc shaped, or as
stated earlier, it could have many different shapes.
[0055] A single transfer sheet 20, containing a single preform 12,
has been shown in FIG. 2A, however, one could have one or more
transfer sheet 20, having one or more thermal paste preforms 12.
Similarly, one or more imbedded mesh 21, shown in FIG. 2B, could
have one or more attached preforms 12. Multiple preforms 12, on a
transfer sheet 20, or mesh 21, would of course help facilitate the
simultaneous placement of these preforms 12, such as, on an array
of chips in a module.
[0056] Similarly, the preforms for the different chips could vary
in shape and/or composition, or the preforms themselves on a given
transfer sheet 20, or imbedded mesh 21, could be similar or
different. For example, one preform 12, could be taller, while
another preform 12, could be wider, while another could be of a
different thermal paste.
[0057] The thermal paste preform 12, could also be attached to a
sheet 20, or a thin mesh 21. In this case the thermal paste preform
12, could become integrated with the thin mesh 21, such as, a thin
wire mesh 21.
[0058] The mesh 21, can be of conductive or non-conductive
materials, however, it needs to be thin enough to fit between a
chip 30, and a heat dissipation device 50, as shown in FIG. 5. The
thermal or heat dissipation device 50, could be a thermal cap 50, a
heat spreader 50, to name a few. The thermal paste preform 12,
could be attached to a transfer sheet 20, and have an imbedded mesh
21.
[0059] One benefit of the imbedded mesh 21, over a transfer sheet
20, is that it is has higher throughput since the mesh 21, will be
left inside a module, while the transfer sheet 20, usually is
removed. Another benefit is the enhanced reliability, since the
imbedded mesh 21, impedes the flow of the thermal paste 12, out of
the chip-to-cap gap, and is thus more forgiving of larger gaps.
[0060] FIG. 3, illustrates the transfer of a thermal paste preform
12, onto a heat generating device 30, such as, a chip 30. The
transfer sheet 20, may be used to place the thermal paste preform
12, on top of a chip 30, and then the transfer sheet 20, may be
removed. However, as stated earlier, for some applications the
transfer sheet 20, may be left inside the final module, i.e.,
between the cap 50, and the chip 30.
[0061] Typically, a substrate 36, with I/O 37, has one or more
chips 30, connected to it by solder balls or C4 (Controlled
Collapse Chip Connection) or solder mass 32. The physical
dimensions of the thermal paste preform 12, will depend upon the
application, for example, for a chip that needs a shorter heat
transfer path, the thermal paste preform 12, could be thinner,
however, if the gap between the chip and the thermal cap is wider,
then a thicker thermal paste preform 12, would be needed. It is
preferred that in the final assembly the deformed thermal paste
preform 12, cover the entire back surface of the chip 30. However,
for some applications the thermal paste preform 12, could leave a
portion of the periphery of the back surface of the chip 30,
exposed, and for some applications the thermal paste preform 12,
could extend beyond the periphery of the back surface of the chip
30, as more clearly shown in FIG. 4.
[0062] FIG. 4, illustrates a cross-sectional view of a module using
the thermal paste preform of this invention. The thermal paste
preform 12, provides a direct heat transfer path between the chip
30, and a thermal cap 40, and provides, and deforms during the
assembly to assure contact with both the chip 30, and the cap 40.
It is preferred that the thermal paste preform 12, provide a good
thermal contact between the chip 30, and the cap 40. To assure
this, the initial thickness of the preform needs to be greater than
or equal to the chip-to-cap gap in the finished assembly. However,
for some applications under pressure a bulge 13, appears, instead
of the load or pressure being transmitted to the adjacent
features.
[0063] FIG. 5, illustrates a cross-sectional view of a MCM showing
the thermal paste preforms 12, of this invention which are used to
cool chips 30, in the module. Typically, the substrate 36, with I/O
37, has one or more chips 30, connected to it by solder balls or
solder mass 32. The thermal paste preforms 12, thermally connect
the chips 30, to a thermal cap 50, and provide a direct thermal
transfer path. The chips 30, may have similar or different sizes
and may have similar or different performance characteristics. The
thermal paste preform 12, that is used at different chip sites may
be similar or different. The thermal cap 50, could have an inner
surface 51, over each chip, which inner surface 51, may be flat
(not shown), or have a protrusion or pedestal 56, or may have a
blind hole or cavity 54.
[0064] For some applications the inner surface 51, of a thermal cap
50, could have a plurality of particles 52, embedded into the inner
surface 51. At least one of the heat transfer particles 52, that is
embedded is preferably of the same material as a particle that will
be used in making the thermal paste that will subsequently come in
thermal contact with the inner surface 51, of the thermal cap 50,
and provide the thermal transfer.
[0065] It is preferred that the thermal paste preform 12, should be
sufficient in placement and volume to completely cover the surface
of the chips 30, and also fill the thermal path to the cap 40 and
50.
[0066] It is also preferred that the thermal paste preform 12, have
a thickness that is greater than or equal to the assembled
chip-to-cap gap. However, for some applications under pressure a
bulge 13, appears, instead of the load or pressure being
transmitted to the adjacent features.
[0067] The thermal paste preforms 12, can be manufactured in
several ways. The preforms may be made using conventional thermal
pastes. The thermal paste is formed to the desired shape at room
temperature or at an elevated temperature (to decrease viscosity).
The desired shape may be achieved by using a mold, template, or an
automated dispense tool. If a mold or template is used, the paste
may be removed from the mold or template at elevated temperature,
room temperature, or after sub-cooling, depending on the type of
thermal paste used. Before subcooling, the thermal paste preform is
attached to a transfer sheet, or a mesh, or it has a mesh embedded
in it. The preform is subcooled to stiffen it and decrease
tackiness. The preforms are normally shipped and stored sub-cooled.
With optimal material selection and packaging, subcooling, is not
required for storage or shipping.
[0068] In the preferred embodiment, the preforms are subcooled
(e.g. less than room temperature) when they are placed on the chips
or on the lid. If a transfer sheet is being used, it is normally
removed before the thermal paste begins to warm. If a mesh is being
used, it is left with the thermal paste. To assure proper thermal
performance, the minimum thickness of the preform must exceed the
maximum possible chip to lid gap (for that chip site). To maximize
adhesion of the paste to the chip and lid, and to avoid damage, the
paste is allowed to warm and soften before the lid is clamped to,
and permanently attached to the substrate.
[0069] The particles 52, may be embedded by any of several methods
known in the art, for example casting, grit blasting, or pressing,
to name a few. The entire internal surface 41 or 51, of the thermal
cap 40 or 50, may be altered, or only those areas 41 or 51,
immediately above the flip chips 30, where the enhanced thermal
transfer path is desired is altered.
[0070] Since each chip 30, on an MCM 75, may have differing cooling
requirements, two or more different thermal paste preforms 12, may
be used in the same module 75, and in those cases if the thermal
paste preforms 12, have different chemical compositions it may be
desirable to use different surface modifications at the different
sites, to correspond to the different thermal paste preforms 12,
that may be used.
[0071] The current invention uses preforms of thermal paste 12, in
the assembly of SCMs and MCMs. The temporarily stiffened thermal
paste 12, (by sub-cooling) may be temporarily attached to a
transfer sheet 20, or it may have an imbedded mesh 21, as shown in
FIG. 5.
[0072] The paste preform may have many different shapes to enable
paste placement and chip coverage. The preforms may be separate
shapes for each chip, larger shapes to cover multiple smaller
chips, or multiple shapes (to merge upon assembly) for large
chips.
[0073] The preforms may have paste for just one chip or for
multiple chips on a module. For the case of preforms for multiple
chip sites, the pastes used in the preforms may be similar for all
chip sites, or it may be different for different chip sites.
[0074] One reason for varying the thermal paste used on the preform
would be to vary the thermal conductivity of the pastes in order to
customize the cooling of the chips.
[0075] Another potential reason to vary the thermal pastes from one
site to another is to take advantage of the different adhesive
properties for different materials used in the chips or cap.
[0076] The thermal paste preform may be formed and attached to
either a mesh 21, or transfer sheet 20, by any of several ways
known in the art.
[0077] In a preferred embodiment, the thermal paste is formed into
the desired shape and thickness and then precooled to increase
stiffness and decrease tackiness. A mesh 21, may be imbedded in the
thermal paste preform by a variety of techniques. In a preferred
embodiment, the mesh 21, is imbedded by applying thermal paste to
both sides of the mesh 21.
[0078] In an alternate embodiment, the mesh is imbedded by applying
paste on one side of the mesh under pressure, so that some of the
thermal paste extrudes through the mesh.
[0079] In a preferred embodiment, the current invention uses
surface modification of the caps used in the assembly of flip chip
SCMs and MCMs, to enhance the reliability.
[0080] For the case of multiple thermal pastes used in an MCM, the
same surface treatment may be applied at all chip sites when the
pastes use at least one of the same solids, or different surface
modifications may be employed when the pastes are dissimilar.
[0081] In an alternative embodiment, the thermal paste preform can
adhere well to both the chip and the lid, and no particles are
imbedded in the lid.
[0082] This invention further allows the ability to build with
higher throughput, and therefore at lower cost.
[0083] In the preferred embodiment, the particles embedded into the
surface of the thermal cap could be of the same size or material
characteristics as the solids in the thermal paste, or could be of
a different size or characteristics than the solids in the thermal
paste.
[0084] The chips may have similar or different sizes. The thermal
paste preforms used at different chip sites may be similar or
different. The cap over each chip may be flat (not shown), or have
a pedestal 56, as shown over chip 30, or may have a cavity 54, as
shown over chip 30. The thermal paste preform 12, is sufficient in
placement and volume to completely cover chip 30, and fill the
thermal path to the cap 40 or 50. The thermal paste preform 12, at
room temperature is typically very viscous, and difficult to
handle. Therefore, the thermal paste preform 12, is normally
dispensed onto the transfer sheet 20 or mesh 21, by either manually
screening thermal paste through a template, or by use of an
automated dispense tool.
[0085] During the process of forming the preform 12, it is
preferred that the thermal paste preform 12, is sub-cooled to a
temperature of between about 20.degree. C. and about minus
100.degree. C. However, of some applications the thermal paste
preform is sub-cooled to a temperature of at least about minus
20.degree. C.
[0086] It is preferred that least one preform 12, makes a good
thermal contact with the heat or thermal dissipation device 50, and
wherein at least one of the heat dissipation device 50, could be
selected from a group consisting of thermal cap, heat spreader, to
name a few.
[0087] Additional throughput increases can be achieved on MCMs when
multiple preforms are applied simultaneously to all the flip chips
on the module by using a single mesh 21, or transfer sheet 20.
EXAMPLES
[0088] The following examples are intended to further illustrate
the invention and are not intended to limit the scope of the
invention in any manner.
Example 1
[0089] In a preferred embodiment, the cooling cap is made of
aluminum, and on at least a portion of its internal surface alumina
particles are embedded to be compatible with thermal paste preforms
containing an alumina filler. A subcooled preform of thermal paste
with attached transfer sheet is placed over the chips on the
module. The transfer sheet is peeled off, leaving the subcooled
thermal paste preforms on the chips. The lid is placed on the
assembly, and the thermal paste is allowed to warm to approach room
temperature so that the thermal paste adheres to the chip and lid,
and softens prior to compression. The lid is then pushed down onto
the substrate, deforming the preform to fill the chip to cap gap,
and the lid is attached to the substrate.
Example 2
[0090] In an alternative embodiment, different thermal paste
preforms utilizing different solids are used, and the thermal cap
is not made from the same material as one of the solids in the
thermal paste preforms, but the surface is embedded with particles
of the same material as used for one of the solids in the thermal
paste preform. A subcooled preform of thermal paste with an
embedded mesh is placed over the chips on the module. The lid is
placed on the assembly, and the thermal paste is allowed to warm to
approach room temperature so that the thermal paste adheres to the
chip and lid, and softens prior to compression (of the preform).
The lid is then pushed down onto the substrate, deforming the
preform to fill the chip to cap gap, and the lid is permanently
attached to the substrate. The mesh stays between the chip and lid,
and is thin enough so that it will not damage the chip during (the
compression part of) the module assembly.
[0091] While the present invention has been particularly described,
in conjunction with a specific preferred embodiment, it is evident
that many alternatives, modifications and variations will be
apparent to those skilled in the art in light of the foregoing
description. It is therefore contemplated that the appended claims
will embrace any such alternatives, modifications and variations as
falling within the true scope and spirit of the present
invention.
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