Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF PREPARING THERMALLY CONDUCTIVE COMPOUNDS BY
LIQUID METAL BRIDGED PARTICLE CLUSTERS
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of our
prior co-pending application Serial No. 09/543,661, filed
April 5, 2000, entitled "METHOD OF PREPARING THERMALLY
CONDUCTIVE COMPOUNDS BY LIQUID METAL BRIDGED PARTICLE
CLUSTERS", and assigned to the same assignee as the
present application.
BACKGROUND OF THE INVENTION
The present intention relates generally to an improved
method and composition for preparing thermally conductive
mechanically compliant compounds for improving heat
transfer from a heat generating semiconductor device to a
heat dissipator such as w heat sink or heat spreader. More
specifically, the present invention relates to the
preparation of improved formulations of highly thermally
conductive polymer compounds such as a polymer liquid
loaded or filled with percolating particulate clusters
coated with a liquid metal and wherein the humidity
resistance of the liquid metal is :stabilized through the
addition of a hydrophobic alky7_ functional silane,
specifically octyl-triethoxysilane. Such compounds are
highly effective through liquid metal enhanced percolation;
with the liquid metal having enh~.nced stability. The
present invention involves a process for uniformly coating
particulate solids with a liquid metal, and thereafter
blending the coated particulate with a composition
comprising a blend of a liquid or fluid polymer and a
hydrophobic alkyl functional silane, specifically octyl-
triethoxysilane to form a highly stable compliant pad with
thermal vias therein.
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In the past, liquid metals have been proposed for
incorporation in thermally conductive pastes for heat
generating semiconductor devices. In most cases, the
application of liquid metals for this purpose was not
widely used, primarily because of problems created with the
tendency of the liquid metal to form alloys and/or
amalgams, thereby altering and modifying the physical
properties of the liquid metal conta_ning mounting pad. In
certain applications, the liquid metal component would
become oxidized, both along the sur:Eace as well as in the
bulk structure. While the highly thermally conductive
pastes of the prior art are typically electrically
conductive, this property may not b~s desirable in certain
applications and situations. In certain other situations,.
liquid metals and/or alloys of liquid metal were blended
with a polymer, with the polymer thereafter being cured in
order to provide a composite thermally conductive mounting
pad. While useful, these devices did not find widespread
application due primarily to the in~~tability of the liquid
metal component in the finished product. This instability
is due to the extremely high surface tension as well as
other chemical and physical properties of the liquid metal
component. By way of example, the dispersed liquid metal
droplets had a tendency to coalesce, a process of Ostwald
ripening, and cause macroscopic separation of the metal
from the polymer matrix. In additic>n the oxidation of the
liquid metal was accelerated upc>n exposure to humid
environments - leading to the formation of brittle oxides
that diminished the thermal properties of the compound.
The present invention utilizes the combination of a
liquid metal coated particulate w_Lth a polymer carrier
along with octyl-triethoxysilane. The alkyl functional
silane binds to the surface oxide 7_ayer of the metal and
creates a hydrophobic barrier that resists moisture attack
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on the metal. The method of preparation described in the
invention also provides the compounds with enhanced
stability, particularly regarding any tendency toward
macroscopic phase separation. In addition, the formulation
and method of preparation renders po:~sible the formation of
large percolating clusters of liquid metal coated particles
which enhances the heat transfer properties. The
combination also possesses desirable mechanical properties
which facilitate its use in production operations.
SUMMARY OF THE INVENTION.
In accordance with the present invention, a
particuhate such as boron nitride, alumina or aluminum
nitride is initially dried, and thereafter placed in
contact with a liquid metal, typically a metal that is
liquid at room temperature or melting at a relatively low
temperature, typically below 120°C and preferably below
60°C. Preferably, the liquid metal comprises an alloy of
gallium and/or indium, such as a gallium-indium-tin-zinc
alloy, a bismuth-indium alloy or a tin-indium-bismuth
alloy. In order to appropriately wet the surfaces of the
particulate, a mixture of dried particulate and liquid
metal is subjected to a mixing operation until the
particulate is uniformly coated with the liquid metal.
While not absolutely necessary, it is desirable that the
boron nitride particulate be dry before blending with the
liquid metal alloy. At this~stage of mixing one obtains a
thixotropic paste of liquid metal and the powder. One can
also visualize the paste as a large percolating cluster.
Following the coating operation, the coated
particulate is mixed with a blend of a liquid polymeric
carrier material such as, for examp7_e, liquid silicone oil
of a desired or selected viscosity with octyl
triethoxysilane. It is preferred that the liquid metal
particulate be incorporated in the silicone/silane blend at
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or near the packing limit. For liquid metal coated boron
nitride, the packing fraction is t;rpically between about
60% and 65% by volume coated particles, balance liquid
silicone/octyl-triethoxysilane blend. At these volume
fractions, one obtains mechanically compliant compounds
that have excellent thermal conductivity due to high
packing density. This improves heat transfer due to the
creation of a compliant interface between the opposed
spaced-apart surfaces of the semiconductor device and the
heat sink.
In preparing the mechanically compliant highly
thermally conductive bridges in accordance with the present
invention, the thermally conductive particulate is
initially selected, with boron nitride being the preferred
particulate. Materials such as aluminum oxide (alumina?,
and aluminum nitride have also been :found to be useful when
properly dried prior to contact with the liquid metal. For
the application of the present invention, the particle size
should be such that the average cross-sectional thickness
is less than about 5 microns. A liquid metal, preferably
a low melting alloy, is added to the particulate and
mechanically mixed until the particulate surface is
'substantially uniformly wet by the liquid metal and a
uniform paste is formed. There.aft.er, a liquid polymer
blend, preferably a liquid or fluid silicone polymer/octyl-
triethoxysilane is added to the liquid metal paste to form
a working formulation, with this working formulation being
subjected to a mechanical mixing operation which typically
includes a vigorous or high-speed mixing step, with
vigorous mixing being continued until a visually smooth
paste is formed.
When incorporated into liquid silicone/silane blend,
it has been found that the addition of the liquid metal
coated particulate effectively reduces viscosity. The
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mechanism involved in this alteration of viscosity is
believed to be due to the reduction of viscous drag at the
"effective particle"-silicone. oil/silane interface. The
liquid metal coating increases t.~e sphericity of the
configuration of the particulate, and also contributes to
an effective "softness" of the otherwise hard particles.
These two factors function in a mutually cooperative
fashion so as to reduce both viscosity and modulus of the
resulting composite.
It has been further found that the liquid metal coated
particulate, in addition to effectively transferring heat
and/or thermal energy, also stabilizes and anchors the
liquid metal into a three phase composite to prevent gross
migration. The three phases are particle-liquid metal-
polymer blend. By increasing the viscosity of the metal
phase, the tendency of metal drc>plets to migrate and
coalesce into large drops that could macroscopically
separate and leak from the composite is severely retarded.
Furthermore, it has been found that the liquid coated
particulate provides a Bingham-plastic like character in
the resultant composite, this allow:i.ng the paste to remain
static in the absence of external stress, and yet conform
and/or flow. easily when subjected to stress.
Because of the tendency to undergo liquid-to-liquid
macroscopic separation, liquid metals do not blend well
with polymer liquids, including silicones. In accordance
with the present invention, however, when particulate, in
particular boron nitride, is initially coated with a
gallium alloy, the microscopic separation phenomena is
reduced, with the liquid metal being supported or retained
in coated particulate form, due to the increased thixotropy
of the metal phase. In addition, t:he coated particulate,
when added to the silicone/silane blend, functions
effectively to form thermal vial within the composite. In
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certain cases, the thermal conductivity of the particulate
such as boron nitride, may even exc~°ed that of the liquid
metal, for example, a eutectic alloy of gallium, tin and
indium.
It is a further feature of t=he invention that in
addition to its thermal properties, i~he composite possesses
desirable electrical properties a~~ well. Formulations
having the optimal thermal properties have been found to
possess electrical volume resistivit~T in the range of 10a to
1012 S2-cm.
Briefly, the technique of t:he present invention
involves the steps of initially selecting a particulate
material for the application. Boron nitride particles are
particularly desirable, with those particles having a
BET surface area of 0.3 m2-g-1 have been found quite useful.
Boron nitride is typically configured in the form of
anisotropic platelet-like particle~~, with plate diameter
ranging from about 5-50 ~m and the plate thickness being
from about 2-3 Vim. The next step is coating of the
particulate. When coated with liquid metal, these
particles have liquid metal/boron nitride volume ratios
ranging from 4:1 to 1:1. Coating is achieved by
mechanically mixing as previously stated. This is followed
by the addition of the appropriate amount of blended liquid
or fluid silicone and octyl-triethoxysilane to the coated
particulate, with this addition being followed by high-
speed mixing until a visually smooth paste is obtained.
As indicated above, while boron nitride is the
preferred particulate, favorable results have been achieved
through the utilization of alumina, with the alumina
typically requiring a pre-treatment which involves thorough
drying of the particulate. Other particulates such as
aluminum nitride can also form liquid metal pastes after
thorough drying.
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Therefore, it is a primary object of the present
invention to provide an improved particulate material which
in addition to being highly thermally conductive, functions
to anchor and stabilize the liquid metal into a three phase
composite.
It is a further object of the present invention to
provide an improved method of preparing a thermally
conductive bridge between the opposed surfaces of a heat
generating semiconductor device and a heat dissipating
surface, with the thermally conductive bridge comprising a
three phase composite consisting of inorganic particulate-
liquid metal-liquid silicone polymer/octyl-triethoxysilane
blend.
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
Figure 1 is a diagrammatic or demonstrative display of
improved contact between particulate (BN) coated with
liquid metal. It is clear that the surface wetting of the
particulate provides a significant reduction in surface
resistivity between adjacent particles;
Figure 2 is a demonstrative sketch illustrating the
response of the polymer matrix filled with particulate by
creating clusters on a larger length scale, and further
illustrating the desirable response of the composite when
the volume fraction of the liquid metal coated particles
near the packing limit for spherical particles, with this
sketch illustrating the feature of high concentration so as
to obtain thermal percolation near the critical packing
fraction;
Figure 3 is a demonstrative sketch similar to Figure
2 illustrating the reduction of aspect ratio utilizing
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liquid metal coating, particularly with the platelet
configuration of BN particulate;
Figure 4 illustrates the feature of utilizing a soft
liquid gallium alloy as a coating for particle, so as to
lower viscous dissipation;
Figure 5 is a showing of aggregation and separation of
discrete liquid gallium metal droplets so as to achieve the
results of the present invention;
Figure 6 is .a flow chart illustrating the steps
undertaken in preparation of the compliant pads of the
present invention;
Figure 7 is an illustration of a typical semiconductor
mounted on a hinged heat sink, and having the compliant pad
prepared in accordance with the present invention
interposed between opposed surface; of the semiconductor
device and heat sink; and
Figure 8 is a performance grad>h of thermal impedance
versus hours of exposure to a specific humid environment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In order to describe the preferred embodiments, the
following examples are given:
EXAMPLE I
Alloy Melting Gallium Indium Tin Bismuth Zinc
Point C C) C o) ( ~) C a) C o) C%)
1 7 61 25 13 0 1
The particulate selected was boron nitride, with the
particulate having the normal platelet-like configuration
and averaging 40 microns in diameter; and 2 microns in
cross-sectional thickness. This ;particulate is readily
wetted by the gallium alloy. When coated with the liquid
gallium alloy, the BN powder did not form hard aggregates,
but rather formed a thixotropic paste. This configuration
is desirable inasmuch as BN has a high thermal conductivity
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in the "in-plane" direction, with the conductivity being
substantially improved with liquid metal bridging. BN has
a specific gravity of 2.25 and a thermal conductivity (in-
plane) of 350 W-m-1-K-1 (orientationally averaged thermal
conductivity is reported around 60 W-m-1-K-1) . The polymer
matrix chosen was a blend of silicone oil with octyl-
triethoxysilane, the silicone oiZ. component having a
kinematic viscosity of 100 centisto~:es, a specific gravity
of 0.86 and a thermal conductivity of 0.15 W-m-1-K-1. The
metal has a specific gravity of. 6.5 and a thermal
conductivity of 20, W-m-1-K-1.
. The anisotropic platelet BN particles were initially
coated with the liquid gallium alloy. The liquid metal-to-
BN volume ratios were selected in three different ranges as
set forth in Table I hereinbelow:
TABLE I
Formulation:
-
3
1 2
Parts Volume Parts Volume Parts Volume
Material Wt. o Wt. o Wt.
BN (40 Vim) 100 14 0 0 0 0
BN (10 ~Cm) 0 0 100 14 100 15
[Liquid 1000 49 1000 49 800 43
gal l ium]
Alloy 1 [of
Example I]
Silicone 90. 33.5 90 33.5 90 38
oil
4cty1-tri- 10 3.5 10 3.5 10 4
ethoxy-
silane
The coating was accomplished by mechanically mixing
the BN powder with the liquid galliuri~ alloy of Example I,
and this may be achieved either by hand or in a high-speed
mixer. Mixing was followed by addil~ion of the appropriate
amount of the silicone oil/octyl-triethoxysilane blend
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followed by high-speed mixing until <~ visually smooth paste
was obtained.
The mixing procedure stabilizes the compound. The
surface tension of silicone oil/silane blend is around (20)~
mN-m-1 whereas for the liquid metal it is of the order of
400-500 mN-m-1. This means that the spreading coefficient
or the ability of silicone oil/silane blend to wet the
surface is far greater than that of a liquid metal. Thus,
the BN particulate is coated with liquid metal prior to
contact with silicone oil/silane blend so as to achieve
proper and desirable wetting. Specifically, the following
advantages are present:
1. The material will form liquid bridges; and
2. There is a significant reduction in the amount of
macroscopic separation of the liquid metal due to the
presence of the hydrophobic alkyl functional silane in the
blend.
Tests have indicated that when all materials of the
formulation are mixed together without following the
sequential steps of the present invention, the powder is
not properly wetted with the liquid metal. The sequencing
of the mixing steps is key to successfully making the
stable, thermally conductive compounds in the present
invention.
EXAMPLE II
Alloy Melting Gallium Indium Tin Bismuth Zinc
Point (%) ( ~) ( o) ( ~) ( o)
(C)
2 60 0 I 51 16.5 32.5 0
The particulate selected was aluminum oxide or
alumina, a particulate of spherical symmetry, with a
diameter of 3 ~.m and a BET surface area of 2 m2/g. Both
alumina and the alloy were heated to 100 °C (above melt
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point of Alloy 2) and mixed. When ~~oated with the liquid
alloy, the alumina formed a smooth, thixotropic paste.
Alumina has a specific gravity of. 3.75 and a thermal
conductivity 25 W-m-1-K-1. The polymer matrix chosen was a
blend of silicone oil and octyl-triethoxysilane, with
silicone oil component having a kinematic viscosity of 100
centistokes, a specific gravity of (0.86) and a thermal.
conductivity of 0.15 W-m-1-K-1. The liquid metal has a
specific gravity of 7.88 and a thermal conductivity of 25
W-m-1-K 1.
The alumina particles were initially coated with the
liquid metal alloy. The metal-to--alumina volume ratios
were selected in three different ranges as set forth in
Table II hereinbelow:~
TABLE II
Formulation:
.
.-_. _ 1 _.- 2 3
Parts Volume Parts Volume Parts Volume
Material Wt. % Wt. o Wt. o
Alumina 160 15 220 20 375 30
( 3 ~,m)
Alloy 2 1050 45 900 40 800 30
Silicone oil 90 36 90 36 90 36
Octyl-tri- 10 4 10 4 10 4
ethoxy-
silane
The coating was accomplished :by mechanically mixing
the alumina powder with the liquid alloy of Example II, and
this may be achieved either by hand or in a high-speed
mixer. Mixing was followed by addition of the appropriate
amount of the silicone oil/silane blend followed by high-
speed mixing until a visually smooth paste was obtained.
EXAMPLE III
Alloy Melting~Gallium Indium Tin Bismuth Zinc
Point (°C) (%) (%) (a) (%)
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1 7 I 61 ~ 25 ~ 13 ~ 0 ~ 1
The particulate selected was alumina of Example II.
When coated with the liquid gallivzm alloy, the alumina
formed a smooth, thixotropic paste. The polymer matrix
chosen was a silicone oil/octyl-triethoxysilane with the
silicone oil having a kinematic viscosity of 100
centistokes, a specific gravity o:f 0.86 and a thermal
conductivity of 0.15 W-m-1-K-1. The liquid metal has a
specific gravity of 6.5 and a therma=L conductivity of 20 W
.m-1- K-1 .
The alumina particles were initially coated with the
liquid gallium alloy. The liquid metal-to-alumina volume
ratios were selected in three different ranges as set forth
in Table I hereinbelow:
TABLE III
Formulation:
1 2 3
Parts Volume Parts Volume Parts Volume
Material Wt. % Wt. % Wt. o
Alumina 100 8 150 13 200 18
(3 ~Cm)
Alloy 1 1100 55 1000 50 900 45
Silicone oil 90 33.5 90 33.5 90 33.5
Octyl-tri- 10 3.5 10 3.5 10 3.5
ethoxy- I,
silane ~ '
The coating was accomplished by mechanically mixing
the alumina powder with the liquid g<~llium alloy of Example
I, and this may be achieved either by hand or in a high-
speed mixer. Mixing was followed by addition of the
appropriate amount of the silicone oil followed by high-
speed mixing until a visually smooth paste was obtained.
TEST RESULTS
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The formulation 1 (Table I) was tested for thermal
conductivity. The ASTM D5470 method yielded a thermal
conductivity of 8.0 W-m-1-K-1. Controlled thermal impedance
testing against industry standarcL materials was also
undertaken. One of these is a generic thermal interface
compound from Dow Corning (DC-340 Thermal grease) and
another is a high performance compound made by Shin-Etsu
Corporation (G-749 Thermal Grease). This formulation
demonstrated enhanced stability when exposed to a humid
environment as demonstrated by the comparison of the
impedance/humid environment exposure curves of Figure 8. As
illustrated, the response curve remains relatively stable
for the formulation including octyl-triethoxysilane, when
contrasted with the formulation in which the silane is not'
included.
PROPERTIES OF LIQUID METAL COATED PARTICULATE
As is illustrated in the drawings, Figure 1
illustrates the manner in which imp=roved contact is
obtained between individual coated ~?articulate,
particularly BN coated with a liquid gallium alloy. The
surface characteristics or propertiE=_s of the composite
improve the contact through the formation of liquid
bridges. This sketch demonstrates l~he feature of surface
wetting of the particulate providin<~ a significant
reduction in surface resistivity no=rurally encountered
between adjacent particles. The liquid metal is
stabilized through the addition of the octyl-
triethoxysilane to the silicone oil component.
Figure 2 illustrates the feature of improved
percolation resulting from near-critical packing
fraction. The surface-to-surface contact as shown in the
portion to the left of Figure 2 is enhanced when a near-
critical packing fraction is achieved through higher
concentrations.
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It is the purpose of Figure 3 t:b demonstrate the
reduction in aspect ratio achieved with liquid metal
coating of particulate. Since boron nitride has an
anisotropic platelet structure, its performance in
applications contemplated by the prE:sent invention are
enhanced. With the liquid metal coating, the "effective
particle" configuration becomes more ellipsoidal.
It is the purpose of Figure 4 t;o demonstrate the
advantageous feature of the present invention for coating
the individual particles, thus lowering viscous
dissipation. Improved overall performance can be
' expected and is. accordingly obtained.
Figure 6 is a flow diagram of the steps undertaken
in accordance with the creation of compliant pads in
accordance with the present invention. As indicated, and
as is apparent from the flow diagram, the particulate and,
alloy are blended until the surface~~ of the particulate .
are thoroughly wetted, and thereafter a paste formulation
is prepared through the addition of a liquid polymer.
Figure 7 is provided to demonstrate the utilization
of the compliant pad of the present invention in
connection with a heat generating semiconductor device of
conventional configuration. Accordingly, the assembly 10
shown in Figure 7, includes a heat generating
semiconductor device or package illustrated at ll having
a heat sink, heat spreader, or other heat dissipating
member illustrated at 12. Interposed between the opposed
surfaces of semiconductor device 11 and heat dissipating
member 12 is a mechanically compliant pad 13 prepared in
accordance with the present invention.
Figure 8 provides a comparison for demonstrating the
improved stability achieved through the addition of
octyl-triethoxysilane. As indicated, the performance is
compared between the formulation of (Table I) in the
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presence of and in the absence of oc:tyl-triethoxysilane.
The data is taken from exposure in an environment at
85°C. and 85o RH.
GENERAL COMMENTP,RY
As previously indicated, BN or alumina particulate can
range in size from up to about 1 micron diameter and up to
about 40 microns in cross-sectional thickness. It will be
observed that the platelet-like configuration of boron
nitride in particular provides a highly desirable
combination when wetted with lie~uid metal, with the
effective particle being illustrated _in Figure 3 of. the
drawings. Viscosity control is aided by this feature.
The silicone oils utilized as a component in the
examples are typical liquid silicones, typically vEB 100
(Sivento Inc., previously Huls America), with these
materials being, of course, commercially available.
Silicones having viscosities up to about 1000 centistokes
may be satisfactorily utilized. The presence of the silane
modifies the viscosity slightly, producing an oil
composition with slightly lower viscosities.
One unusual feature of the present invention was
electrical resistivity. When Formulation 1 is formed in a
pad between opposed surfaces of a semiconductor and a heat
sink, the resistivity has been found to be highly
significant, having a value of up to about 1012 S2-cm
(Formulation 1, Table I).
It will be appreciated that t:he 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.
What is claimed is:
