U.S. patent number RE39,992 [Application Number 11/122,210] was granted by the patent office on 2008-01-01 for morphing fillers and thermal interface materials.
This patent grant is currently assigned to The Bergquist Company. Invention is credited to GM Fazley Elahee, Sanjay Misra.
United States Patent |
RE39,992 |
Misra , et al. |
January 1, 2008 |
Morphing fillers and thermal interface materials
Abstract
A thermally conductive mechanically compliant pad including a
quantity of gallium and/or indium alloy liquid at temperatures
below about 120.degree. C. and a boron nitride particulate solid
blended into the liquid metal alloy to form a paste. The paste is
then combined with a quantity of a matrix forming flowable plastic
resin such as microwax, silicone wax, or other silicone polymer to
form the thermally conductive mechanically compliant pad, the
compliant pad comprising from between about 10% and 90% of metal
alloy coated particulate, balance flowable plastic resin.
Inventors: |
Misra; Sanjay (Shoreview,
MN), Elahee; GM Fazley (St. Paul, MN) |
Assignee: |
The Bergquist Company
(Chanhassen, MN)
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Appl.
No.: |
11/122,210 |
Filed: |
May 4, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09543661 |
Apr 5, 2000 |
6339120 |
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09690994 |
Oct 17, 2000 |
6624224 |
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09865778 |
May 25, 2001 |
6649325 |
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Reissue of: |
09946879 |
Sep 5, 2001 |
06797758 |
Sep 28, 2004 |
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Current U.S.
Class: |
524/404;
257/E23.106; 257/E23.107; 257/E23.109; 524/434; 524/439 |
Current International
Class: |
C08K
3/08 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0696630 |
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Feb 1996 |
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EP |
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0708582 |
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Apr 1996 |
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EP |
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0813244 |
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Dec 1997 |
|
EP |
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Other References
IBM Technical Disclosure Bulletin, vol. 19, No. 8, Jan. 1977
"Thermal Enhancement of Modules", E.B. Hultmark et al. cited by
other .
IBM Technical Disclosure Bulletin, vol. 20, No. 11B, Apr. 1978,
"Liquid-Metal-Cooled Integrated Circuit Module Structures",
Berndlmaier et al., pp. 4817-4818. cited by other .
IBM Technical Disclosure Bulletin, vol. 20, No. 11B, Apr. 1978,
"Electronic Packaging Structure", Arnold et al. pp. 4820-4822.
cited by other .
Harman Hard Gallium Alloys for Use as Low Contact Resistance
Electrodes and for Bonding Thermocouples into Samples, The Review
of Scientific Instruments, Jul. 1960, vol. 31, No. 7, pp. 717-720.
cited by other .
IEEE Transactions on Components, Hybrids, and Mfg. Tech., vol. 13,
No. 4, Dec. 1990 "Materials/Processing Approaches to Phase
Stabilization of Thermally Conductive Pastes", Anderson, Jr. et
al., pp. 713-717. cited by other.
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Primary Examiner: Cain; Edward J.
Attorney, Agent or Firm: Haugen Law Firm FLLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
The present invention is a continuation-in-part of application Ser.
No. 09/543,661, filed Apr. 5, 2000, .Iadd.now U.S. Pat. No.
6,339,120 .Iaddend.entitled "METHOD OF PREPARING THERMALLY
CONDUCTIVE COMPOUNDS BY LIQUID METAL BRIDGED PARTICLE CLUSTERS",
and continuation-in-part application Ser. No. 09/690,994, filed
Oct. 17, 2000, .Iadd.now U.S. Pat. No. 6,624,224 .Iaddend.entitled
"METHOD OF PREPARING THERMALLY CONDUCTIVE COMPOUNDS BY LIQUID METAL
BRIDGED PARTICLE CLUSTERS", and application Ser. No. 09/865,778,
filed May 25, 2001, .Iadd.now U.S. Pat. No. 6,649,325
.Iaddend.entitled "THERMALLY CONDUCTIVE DIELECTRIC MOUNTS FOR
PRINTED CIRCUITRY AND SEMICONDUCTOR DEVICES AND METHOD OF
PREPARATION", all of which are assigned to the same assignee as the
present invention.
Claims
What is claimed is:
1. A method of preparing thermally conductive mechanically
compliant pads comprising the steps of: (a) selecting a quantity of
an indium containing alloy which has a melt temperature of between
about 40.degree. C. and 120.degree. C. (b) treating said alloy to
cause dispersal into divided form; (c) combining said dispersed
alloy with a compatible surface active agent and thermally
conductive particles and blended to form a paste; and (d) combining
said dispersed alloy containing paste with a quantity of a flowable
plastic resin material to form a thermally conductive mechanically
compliant pad with said thermally conductive mechanically compliant
pad comprising from between about 10% and 90% by volume of the
combined dispersed metal alloy and thermally conductive
particulate, balance flowable plastic resin.
2. The method of claim 1 wherein the particles making up said
thermally conductive particulate solid have a diameter of between
about 1 and 40 microns.
3. The method of claim 1 wherein said liquid alloy substantially
encapsulates said thermally conductive particles to form a coating
thereon, and wherein the liquid metal to thermally conductive
particle volume ratio is at least 3:1.
4. The method of claim 1 wherein said blended paste is further
blended with microwax or silicone wax to form a conformable pad,
with the pad comprising between about 10% and 90% by volume of
homogeneous paste, balance microwax or silicone wax.
5. The method of claim 1 being particularly characterized in that
said liquid metal alloy is in liquid state at temperatures above
60.degree. C.
6. The compliant thermally conductive pad prepared in accordance
with the steps of claim 1.
.Iadd.7. A method of preparing thermally conductive mechanically
compliant pads comprising the steps of: (a) selecting a quantity of
an indium containing alloy which has a melt temperature of between
about 40.degree. C. and 120.degree. C.; (b) treating said alloy to
cause dispersal into divided form; (c) combining said dispersed
alloy with thermally conductive particles and blending the
combination to form a paste; and (d) combining said dispersed alloy
containing paste with a quantity of a flowable plastic resin
material to form a thermally conductive mechanically compliant pad
with said thermally conductive mechanically compliant pad
comprising between about 10% and 90% by volume of the combined
dispersed metal alloy and thermally conductive particulate, balance
flowable plastic resin..Iaddend.
.Iadd.8. A thermally conductive electrically-resistive mechanically
compliant pad comprising a mixture of: (a) between about 10% and
90% by volume of a paste, which paste includes a blend of: (i) an
indium containing alloy having a melt temperature of between about
40.degree. C. and 120.degree. C.; (ii) thermally conductive
particles, at least a portion thereof being dispersed within at
least a portion of said indium containing alloy; and (b) balance
flowable plastic resin..Iaddend.
.Iadd.9. A thermally conductive electrically-resistive mechanically
compliant pad comprising: (a) an indium containing alloy having a
melt temperature of between about 40.degree. C. and 120.degree. C.,
and being in divided particulate form of between about 1 and 100
.mu.m in diameter; and (b) a polymeric resin material having a melt
temperature of between about 40.degree. C. and 120.degree.
C..Iaddend.
.Iadd.10. A thermally conductive mechanically compliant pad as in
claim 9, including a compatible surface active agent..Iaddend.
.Iadd.11. A thermally conductive mechanically compliant pad as in
claim 9 wherein the melt temperature of said polymeric resin
material is about 10.degree. C. lower than the melt temperature of
said indium containing alloy..Iaddend.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to a method of preparing
thermally conductive interface materials and compounds for
improving heat transfer from a heat generating semiconductor device
to a heat dissipator device such as a heat sink or heat spreader.
More specifically, the present invention relates to a method and/or
technique for preparing a mixture of an indium alloy blended with a
polymer matrix, the polymer being in the solid phase at room
temperatures and with both the alloy and the polymer having a
melting temperature of between about 40.degree. C. and 120.degree.
C., preferably between about 40.degree. C. and 100.degree. C. These
blends of metal alloy and polymer have been found to sharply reduce
the thermal resistance or impedance which typically arises from a
less-than-perfect contact between the boundaries or surfaces of a
thermal interface positioned between the components of the
assembly. More particularly, the present invention involves a
process for blending a normally solid polymeric matrix with a low
melting alloy of indium metal for forming an improved thermal
management system for use in combination with high performance
semiconductor devices.
The thermal impedance or resistance created between two components
in a typical electronic thermal management assembly is increased
when surface imperfections are present on the opposed surfaces of
the two components. The causes of poor physical contact typically
lie with macroscopic warpage of one or both surfaces, surface
roughness, or other non-flat characteristics created on one or both
of the opposed contact surfaces. Areas of non-intimate surface
contact result in the creation of air-filled voids which are, of
course, exceptionally poor conductors of heat. High thermal
impedance resulting from poor thermal contact results in
undesirable heating of electronic components which in turn
accelerates the rate of failure of the components such as
semiconductor components and comprising the assembly. Replacement
of air gaps or voids with a thermally conducting medium comprising
a good thermal management system has been found to sharply reduce
the thermal impedance and/or resistance.
In the past, liquid metals have been proposed for incorporation in
thermally conductive pastes for use with heat generating
semiconductor devices. In some cases, liquid metals were not
readily adapted for this purpose, primarily because of problems
created with the tendency of the liquid metal to form alloys and/or
amalgams, which altered or modified the thermal and other physical
properties of the mounting systems. Other thermal interface
materials are made by dispersing thermally conductive fillers in a
polymer matrix. While most polymer matrices range in thermal
conductivity from 0.1-0.2 W-m.sup.-1-K.sup.-, the properties of the
fillers are quite varied. They include silica (2
W-m.sup.-1-K.sup.-), zinc oxide (10-20 W-m.sup.-1-K.sup.-), alumina
(20-30 W-m.sup.-1-K.sup.-), aluminum nitride (100
W-m.sup.-1-K.sup.-), and boron nitride (200 W-m.sup.-1-K.sup.-).
When placed in the thermal joint, these compounds are intended to
displace air and reduce overall thermal impedance. Addition of
thermally conductive fillers, generally consisting of fine
particulates, improved the thermal conductivity of the compound
filling the voids.
In our copending application Ser. No. 09/543,661, a number of low
melting alloys are disclosed which are highly effective for use as
thermal interfaces in thermal management systems for enhancement of
percolation of thermal energy. The present invention provides
additional advantages in thermal interfaces through the use of
certain selected polymer matrices for retention of the low melting
alloy, the matrices having melting points which are also low and,
preferably, relatively close to the melting points of the retained
alloys. These polymers as well as the alloys are in solid phase at
room temperature, and this feature facilitates ease of handling of
the thermal interface particularly during production and use.
In accordance with the present invention, improved interface
materials have been developed based on incorporation of low melting
alloys as fillers capable of altering their shape in response to
heat and pressure. At room temperature, these fillers are in solid
phase, as is the polymer matrix, with this combination of features
facilitating ease of handling. In addition, these morphing fillers
respond to heat and pressure by their ability to flow into and fill
air gaps or voids that may be present in the matrix, thereby
avoiding creation of standoff or poor particle-to-particle contact
(see FIG. 2).
In those applications where the opposed surface areas are small, or
alternatively are relatively flat, interfaces having thin
cross-sections may be employed. Typically, in such applications,
those dispersions utilizing only polymeric matrices having
dispersed low melting alloys function well (see FIG. 3). For
interfaces employing a laterally disposed mechanical standoff, or
those subject to large warpage, it is normally desirable to utilize
highly thermally conductive particulate fillers in combinations
with the low melting alloys in order to create large heat
percolating clusters (see for example FIG. 4).
SUMMARY OF THE INVENTION
In accordance with the present invention, an indium-containing
alloy is selected which is in the solid phase at room temperature,
while having a melt temperature of between about 40.degree. C. and
120.degree. C. The alloy is then subjected to a size reduction
operation--typically by emulsifying, while in molten phase, in the
polymer matrix of interest. A surface active agent may be added
during the emulsification to enhance the rheological properties and
dispersion stability. Alternatively, the size reduction of the
metal alloy may be accomplished by blow or impact, or alternatively
by grinding or abrasion, under cryogenic conditions. Depending upon
the particular type of equipment and conditions under which the
particulate is formed, it may be possible to add the surface active
agent to the working material while undergoing size reduction
process. The metallic powder can then be blended with a quantity of
a matrix polymer which is likewise in the solid phase at room
temperature, having a melt point of between about 40.degree. and
100.degree. C. to form a compliant pad. The polymer matrix is
preferably selected from the group consisting of paraffin,
microwax, and silicone waxes. The low melting alloy may also be
blended with a particulate filler such as, for example, boron
nitride or alumina with the resultant mixture being mechanically
agitated in the presence of a compatible wetting agent to form a
stable dispersion for ultimate blending with the polymer
matrix.
It should be noted that while the melt temperatures for the polymer
matrix and the metal alloy are both indicated as being between
about 40.degree. C. and 120.degree. C., it is desirable that a
differential be maintained between the actual melt temperatures.
For example, it has been found desirable to select a polymer matrix
having a melting temperature which is approximately 10.degree. C.
lower than that of the metal alloy. Other differential
relationships may also be useful. While certain other metal alloys
may be found useful, indium-based alloys are generally preferred
for utilization in the present invention.
The physical properties of thermal interface compounds prepared in
accordance with the present invention are such that conventional
production handling techniques may be employed during assembly
operations. In this connection, the compounds may be handled or
formed into an interface device by stamping or they may be printed
directly onto heat-transfer surfaces. Alternatively, they may be
made into tapes that can be die-cut so as to be later applied
directly onto the heat transfer surfaces.
Therefore, it is a primary object of the present invention to
provide compositions of materials useful as thermal interface
compounds, wherein a low melting metallic alloy is retained within
a polymer matrix, and wherein each of these components is in the
solid phase at room temperature, and has a melting temperature of
between about 40.degree. C. and 120.degree. C. and preferably
between about 40.degree. C. and 100.degree. C.
It is a further object of the present invention to provide an
improved combination of components utilized to form a composition
which is useful as thermal interface compounds, and wherein hard
particulate fillers such as boron nitride and/or alumina may be
employed in combination with an indium alloy, and thereafter
blended into and retained within a polymeric matrix.
It is yet a further object of the present invention to provide an
improved thermal interface compound which is dry and solid at room
temperature, and which changes to liquid phase at moderately
elevated temperatures, thereby permitting the compounds to be
easily handled utilizing conventional handling techniques and yet
respond effectively in a thermal management application.
Other and further objects of the present invention will become
apparent to those skilled in the art upon a study of the following
specification, appended claims, and accompanying drawings.
IN THE DRAWINGS
FIG. 1 is a demonstrative display of the performance of a prior art
thermal interface utilizing a hard particulate within a
conventional polymeric matrix, and demonstrating the non-responsive
or non-compliant nature of the combination when subjected to the
application of heat and pressure;
FIG. 2 is a view similar to FIG. 1 illustrating the response of a
low melting point alloy within a conventional polymeric matrix, and
showing the response when subjected to the application of heat and
pressure;
FIG. 3 is a demonstrative sketch illustrating a metal alloy
dispersed within a conventional polymeric matrix;
FIG. 4 is a demonstrative sketch illustrating the arrangement of
percolating clusters of a metal alloy in which a thermally
conducting inorganic particulate is dispersed, with the
alloy/particulate clusters being in turn disposed within a
polymeric matrix;
FIG. 5 is a demonstrative sketch illustrating a low melting point
alloy dispersed within a polymeric matrix and designed for
accommodating surface areas which are small and/or flat and which
lie between a heat generating semiconductor device and a heat
sink;
FIG. 6 is a demonstrative sketch illustrating a percolating cluster
of low melting point metal alloy blended with particulate, and held
in place within a laterally disposed mechanical standoff for
application as a thermal interface between surfaces of large
warpage, it being noted that the presence of high thermal
conductivity fillers assists in the creation of large heat
percolating clusters;
FIG. 7 is a flow diagram illustrating the steps involved in a
typical operation for preparing thermal interface devices in
accordance with the present invention;
FIG. 8 is a graph demonstrating the change in thermal impedance
versus temperature for the metal alloy and polymeric components of
phase-change interface materials prepared in accordance with the
present invention; and
FIG. 9 is an illustration of a typical semiconductor mounted on a
finned heat sink, and having the thermal interface of the present
invention interposed between opposed surfaces of the semiconductor
device and the heat sink.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In carrying out the steps of the present invention, an
indium-containing alloy is initially selected with this alloy
having a melt temperature of between about 40.degree. C. and
100.degree. C., it being understood that alloys having melt
temperatures of up to about 120.degree. C. may also find
application. Preferably, the low melting indium alloy comprises
indium alloys containing quantities of bismuth, tin, and/or zinc as
set forth below.
The selected indium alloy is subjected to an emulsification step
wherein the metal is reduced to a finely divided form. It is
preferred that the metal alloy be reduced to particles which
average about 1-100 .mu.m in diameter. The size reduction or
emulsification may be undertaken in a high shear mixer, with the
addition of a compatible surface active agent at a point in this
step.
Following size reduction, the metal particulate is blended with a
polymer, with the blend being subsequently cured to form the
polymeric matrix retainer. Alternatively, the materials may be
compounded in liquid state creating an emulsion with metal droplets
dispersed in the polymer.
Specific Preferred Embodiments
In order to describe the preferred embodiments, the following
examples are given:
TABLE-US-00001 TABLE I Alloys which are prepared for use in the
present invention having the composition and melting points as
follows: Melting Indium Bismuth Sn Zinc Point Alloy (%) (%) (%) (%)
(.degree. C. 1 51 32.5 16.5 0 60 2 66.3 33.7 0 0 70 3 26 57 17 0 79
4 52.2 0 46 1.8 108
Surface Active Agents
As surface active agents, silanes, titanates, zirconates and/or
assorted surface active agents are preferred to improve rheology
and stability of the dispersion, and particularly for creating a
hydrophobic barrier. Surface treatments with surface active agents
that work well for improving rheology as well as stability of the
dispersion, especially against moisture, are alkyl functional
silanes, such as for example octyl triethoxy silane (OTES). Another
example is methyltrimethoxy (MTMS) silane. These silanes bind to
the oxides on the surface of the metal particles, creating a
durable hydrophobic barrier. Additionally, these silanes
compatibilize the particles with the polymer matrix and reduce
particle aggregation.
The following compositions have been prepared, with numbers being
by weight:
TABLE-US-00002 TABLE II 40 .mu.m Boron Matrix Alloy 1 Nitride OTES
Parts Parts Parts Parts by by by by Formula weight Vol % weight Vol
% weight Vol % weight Vol % 1 100.sup.1 30 1200 52 100 15 12 3 2
100.sup.1 34 1000 48 83 14 10 4 3 100.sup.1 35 1200 61 0 0 12 4 4
100.sup.1 40 1000 56 0 0 10 4 5 100.sup.2 35 1200 61 0 0 12 4 6
100.sup.3 30 1200 52 100 15 12 3 .sup.1silicone wax consisting of
siloxane backbones with pendant alkyl chains and having a melting
point of 60.degree. C. .sup.2microwax, melting point 60.degree. C.
.sup.3soft silicone polymer consisting of a reactive siloxane
elastomer.
Typical properties of the formulations are set forth in Table
III:
TABLE-US-00003 TABLE III Thermal Conductivity Thermal
Impedance.sup.4 Formula (W/m-K) (K-cm.sup.2/W 1 >7 0.25 2 5.0
0.20 3 1.8 0.20 6 >7 0.25 .sup.4ASTM D5470, flat surfaces, no
mechanical standoff.
Thermal Management Applications
Compounds prepared pursuant to the formulations of Table III are
varied. Formulations 3, 4 and 5, in particular, may be applied as
coatings by typical coating techniques including hot stamp, screen
printing, or applied to the heat transfer surface directly by other
means. These coatings will typically have a cross-sectional
thickness of less than about 10 mils.
For coatings of larger cross-section, those formulations containing
a particulate filler, such as Formulations 1, 2 and 6 may find
particular application. These coatings may be applied to carriers
such as glass or polymer fabrics, plastic films or metal foils.
When supported, the coatings may be handled with ease, thereby
facilitating their use in production.
Heat Transfer Modes
For those applications which require intimate contact, i.e., where
the contact line is desired to be as thin as possible, Formula 3 is
recommended, although those of Formula 4 and 5 are highly suited as
well. In each event, the metal droplet will deform completely so as
to reduce contact resistance without increasing standoff. See for
example the demonstrative dispersions illustrated in FIG. 5.
For those applications requiring mechanical, standoff, formulations
pursuant to Formula 1 are well suited, it being noted that this
formulation has highly desirable thermal conductive properties. In
addition, the metal droplets present in the formulation will
continue to function for reduction of contact resistance, while
portions of the metallic component will be present in larger
percolating clusters for enhanced transfer of thermal energy. See,
for example, the demonstrative percolating cluster dispersions of
FIG. 6.
Device Application
With attention now being directed to FIG. 9 of the drawings, a
thermal interface is prepared pursuant to any one selected
formulation of Formulas 1 through 6 of Table II, with a thermal
interface so prepared being employed in combination with a heat
generating semiconductor device of conventional configuration.
Accordingly, the assembly 30 shown in FIG. 9, includes a heat
generating semiconductor device or package illustrated at 31 having
a heat sink, heat spreader, or other finned heat dissipating member
illustrated at 32. Interposed between the opposed surfaces of
semiconductor device 31 and heat dissipating member 32 is a
mechanically compliant thermally conductive interface 33, prepared
in accordance with the present invention.
FIG. 7 is a flow diagram setting forth the steps typically
undertaken in accordance with the creation of thermally conductive
interfaces in accordance with the present invention. As indicated,
and as is apparent from the flow diagram, the alloy/particulate
mixture is blended until the surfaces of the particulate are
thoroughly wetted with a surface active agent, and thereafter an
alloy/particulate/matrix formulation is prepared through the
addition to a selected polymer, preferably one which is heated to a
highly flowable condition or in the "B" stage of cure.
Typical Preparation Operation
As indicated above, FIG. 7 is a flow chart illustrating the steps
undertaken in preparing the thermal interfaces of the present
invention commencing with the initial milling of the indium alloy,
and identifying the steps that follow.
Conversion of Alloy to Powdered Form
The preferred method is emulsification of the metal in molten form.
This can either be done in-situ in the polymer matrix of interest
or in another liquid medium, followed by separation and
purification of the powder. Utilizing typical operating parameters,
the powdered alloy is available in sizes ranging up to about 100
microns.
Surface Treatment
Surface treatment includes, preferably, the addition of a surface
active agent such as, for example, octyl triethoxy silane (OTES) or
methyl triethoxy silane (MTMS). These silanes bind to the oxides
which readily form of the surface of the metallic particles to
create a hydrophobic barrier. Additionally, they compatibilize the
particles with the polymer matrix and reduce particle aggregation.
Alternatively, or additionally, titanates or zirconates such as,
for example, the barium or calcium salt forms, may be used.
Blending with Thermally Conductive Particulate
As indicated hereinabove, particulate materials such as boron
nitride and alumina may typically be employed to improve the
thermal conductivity and stability of the blend. These particulate
components may be present in a range up to about 15% by volume,
although blends containing up to about 50% by volume may be
employed successfully. When blended, the alloy coats the
particulate, with the blending operation being undertaken with the
alloy in the liquid phase.
The Polymer Matrix
As indicated, the polymer matrix is preferably selected from
paraffin, microwax, and silicone waxes comprising alkyl silicones.
For most purposes microwax having a melting point of about
50-60.degree. C. has been found particularly suited for this
application. As indicated above, it is generally desirable to
utilize a polymer matrix which undergoes a phase change at a
temperature of about 10.degree. C. lower than the phase change
temperature of the alloy.
Blending Alloy with Polymer Matrix
It is generally preferred that this step by undertaken with both
components in the liquid phase. As such, the materials are blended
in a high shear mixer until the metal becomes thoroughly dispersed
in the polymer, at which time it may be formed into the
configuration desired for the thermal interface. Conventional
techniques for preparing the coating may be utilized, with this
operation being compatible with most liquid phase treatment
operations.
Properties of Thermal Interfaces
As illustrated in FIG. 1, prior art thermal interfaces utilizing
hard particulate within a conventional hard or firm polymeric
matrix lacks the ability to flow under heat and pressure, and
therefore results in a standoff between the adjacent or opposed
surfaces.
FIG. 2 illustrates the performance and activity when a phase change
filler is employed in a polymeric matrix, with the filler deforming
and modifying its configuration under heat and pressure, thereby
permitting the opposed surfaces to mate.
FIG. 3 demonstrates the dispersal of metal alloy particles within a
polymer, with the configuration of the particulate being determined
primarily by surface tension phenomena.
With reference to FIG. 4, this figure demonstrates the presence of
percolating clusters of inorganic particulate such as boron nitride
confined within metal alloy, with the percolating effect being
achieved through the merger of various individual particulate.
With attention being directed to FIG. 5, this figure demonstrates
the utilization of a low melting metal alloy as a dispersion for
small and flat surfaces, it being noted that the metal alloy
conforms under the influence of heat and pressure to enhance the
contact areas.
With reference to FIG. 6, it will be observed that a percolating
cluster of dispersions of metal alloy/inorganic particulate
retained within the confines of laterally dispersed mechanical
standoff elements 40-40 in order to accommodate larger area
surfaces or those subject to large warpage.
With attention now being directed to FIG. 8 of the drawings, it
will be noted that the curves illustrate the performance and
properties of the polymer taken together with the metal alloy
component in a typical thermal interface. As indicated, the phase
change for the metal alloy component occurs at a temperature
approximately 10.degree. higher than that for the polymeric matrix.
This has been found to be a workable arrangement with respect to
temperature differentials pursuant to the present invention.
General Commentary
Boron nitride or alumina particulate preferably ranges in size from
about 1 micron and up to about 40 microns in diameter or
cross-sectional thickness. It will be observed that the
platelet-like configuration of boron nitride in particular provides
a highly desirable configuration and combination when wetted with
liquid metal. The effective boron nitride particle is illustrated
in FIG. 4 of the drawings. Viscosity control is also aided by this
feature or property of boron nitride.
One silicone wax utilized in the formulations of the examples is
GP-533 (M.P. of 60.degree. C.) (Genesee Polymer of Flint, Mich.),
with these materials being, of course, commercially available. A
microwax employed is M-7332 (M.P. of 55.degree. C.) (Moore and
Munger of Shelton, Conn.). Another polymer matrix used is a
one-part soft reactive silicone elastomer (GE Toshiba Silicones of
Tokyo, Japan).
One unusual and unexpected property or feature of formulations of
the present invention is the electrical resistivity. When
Formulation 1 is formed in a pad of thickness of 3-5 mils and
interposed between opposed surfaces of a semiconductor device and a
heat sink, the electrical resistivity of the pad has been found to
be highly significant, having a value of up to about 10.sup.12
.OMEGA.cm (Formulation 1, Table II).
It will be appreciated that the above examples are given for
purposes of illustration only and are not to be otherwise construed
as a limitation upon the scope of the following appended
claims.
* * * * *