JP2002121292A - Method for producing heat conductive compound by using liquid metal-crosslinked particle cluster - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an improved method for forming a heat conductive bridge between opposing surfaces of a heat-generating semiconductor device and a surface for dissipating the heat. SOLUTION: This method for producing a heat conductive mechanically flexible pad comprises processes of (a) preparing a mixture of (1) a large amount of an alloy containing gallium and/or indium in a liquid state at <120 deg.C with (2) a heat conductive particle state solid consisting essentially of boron nitride, (b) mechanically mixing the above mixture for wetting the surface of the above particles with the above liquid alloy to form a uniform paste and also covering each of the above particles of boron nitride with the liquid alloy like capsules, and (c) mixing the above paste with a large amount of a free flowing plastic resin material to form a heat conductive material consisting of approximately 10-90 vol.% metal-covered particles and the rest of the free flowing plastic resin.

Description

DETAILED DESCRIPTION OF THE INVENTION

RELATED APPLICATIONS This application is assigned to the same assignee as the present application, entitled "Method of Manufacturing Thermally Conductive Compounds Using Liquid Metal Cross-Linked Particle Clusters" (METHOD OF PREPARING T
HERMALLY CONDUCTIVE COMPOUNDS BY LIQUID METAL BRID
GED PARTICLE CLUSTERS), filed April 5, 2000, and filed on April 5, 2000.
No. 09 / 543,661.

[0002]

The present invention relates generally to semiconductor devices that generate heat, from heat absorbers to heat spreaders.
ader) to improve heat transfer to a heat dissipator.
Improved methods and compositions for producing onductive mechanically compliant compounds). In particular,
The present invention relates to liquid metal coated percolating
The present invention relates to making improved formulations of highly thermally conductive polymer compounds, such as polymer liquids incorporating or filling particulate clusters, wherein the moisture resistance of the liquid metal is reduced by the hydrophobic alkyl-functional silane. , Especially octyl-triethoxysilane. Such compounds are highly effective due to the penetration promoted by the liquid metal, which liquid metal has an increased stability. The present invention uniformly coats the granular solid with the liquid metal,
Thereafter, the coated particles are combined with a liquid or fluid polymer,
A method comprising mixing with a composition comprising a mixture with a hydrophobic alkyl-functional silane, particularly octyl-triethoxysilane, to form a highly stable compliant pad having thermal pathways.

[0003]

2. Description of the Related Art Liquid metals have been proposed for use in thermally conductive pastes for semiconductor devices that generate heat. In most cases, applying liquid metals for this purpose has not been widely used.
This is primarily due to the problem that the liquid metal forms an alloy and / or amalgam, which tends to change and limit the physical properties of the liquid metal-containing mounting pad. In some applications, the liquid metal component becomes oxidized along the surface as in the body structure. While the prior art highly thermally conductive pastes are typically electrically conductive, this property is undesirable in certain applications and situations. In certain other situations, liquid metal and / or alloys of liquid metal are mixed with a polymer, and then the polymer is cured to provide a composite thermally conductive mounting pad. Although useful, these components have not been widely used, primarily due to the instability of the liquid metal component in the final product. This instability is due to the extremely high surface tension of the liquid metal component as well as other chemical and physical properties. By way of example, dispersed liquid metal droplets tend to agglomerate during the Ostwald ripening process, causing macroscopic separation of the metal from the polymer matrix. Furthermore, oxidation of the liquid metal is accelerated when exposed to a humid environment, resulting in the formation of a brittle oxide, which degrades the thermal properties of the compound.

[0004]

The main object of the present invention is to provide, in addition to being highly thermally conductive, an improved granular material which serves to fix and stabilize a liquid metal in a three-phase composite. Is to give the material.

It is yet another object of the present invention to provide an improved method of forming a thermally conductive bridge between opposing surfaces of a heat generating semiconductor device and a heat dissipating surface. The thermally conductive bridge is made of inorganic particulates
It consists of a three-phase composite consisting of a liquid metal / liquid silicone polymer / octyl-triethoxysilane mixture.

[0006] Still other objects of the present invention will become apparent to those skilled in the art from a study of the ensuing description, claims, and drawings.

[0007]

SUMMARY OF THE INVENTION The present invention uses a combination of particles coated with a liquid metal and a polymer carrier with octyl-triethoxysilane. The alkyl-functional silane binds to the surface oxide layer of the metal and forms a hydrophobic barrier, which prevents moisture attack on the metal. The preparation process described in the present invention also provides compounds which have improved stability, especially the tendency to undergo macroscopic phase separation. Furthermore, the formulation and the method of manufacture allow for the formation of large permeable clusters of liquid metal-coated particles that enhance heat transfer properties. The combination also has the desired mechanical properties, which makes it easier to use in manufacturing operations.

In accordance with the present invention, a particulate material such as boron nitride, alumina, or aluminum nitride is first dried and then liquid at a liquid metal, typically room temperature, or at a relatively low temperature. Melts, typically below 120 ° C,
Preferably, it is brought into contact with the melting metal at a temperature lower than 60C. The liquid metal is gallium, indium, tin, zinc alloy, bismuth, indium alloy, or tin, indium,
It preferably comprises an alloy of gallium and / or indium, such as a bismuth alloy. To properly wet the surface of the particles, the mixture of dry particles and liquid metal is subjected to a mixing operation until the particles are uniformly coated with the liquid metal. Although not absolutely necessary, it is desirable that the boron nitride particles be dry before mixing with the liquid metal alloy. In this mixing stage, a thixotropic paste of the liquid metal and the powder is obtained. The paste can also be viewed as a large permeable cluster.

[0009] Following the coating operation, the coated particles are mixed with a mixture of a liquid polymer carrier material such as, for example, a liquid silicone oil of the desired or selected viscosity and octyl-triethoxysilane. The liquid metal particles are preferably incorporated into the silicone / silane mixture to or near the filling limit. In the case of liquid metal-coated boron nitride, the loading fraction typically consists of about 60% to 65% by volume of the coated particles and the balance liquid silicone / octyl-triethoxysilane mixture. At these volume fractions, high packing densities result in mechanically compliant compounds with excellent thermal conductivity. This improves heat transfer by creating a compliant interface between the opposing spaced surfaces of the semiconductor device and the heat absorber.

In order to produce a highly thermally conductive, mechanically compliant bridge according to the present invention, thermally conductive particles are first selected, with boron nitride being the preferred particle. Materials such as aluminum oxide (alumina) and aluminum nitride have also proven useful if properly dried prior to contact with the liquid metal. For the purposes of the present invention, the particle size should be such that the average cross-sectional thickness is less than about 5μ. A liquid metal, preferably a low melting point alloy, is added to the granules and mechanically mixed until the surface of the particles is substantially uniformly wetted by the liquid metal and a uniform paste is formed. Thereafter, a liquid polymer mixture, preferably a liquid or fluid silicone polymer / octyl-triethoxysilane, is added to the liquid metal paste to form a working compound, and the working compound is subjected to a mechanical mixing operation. The operation typically involves a vigorous or high speed mixing step, which continues the vigorous mixing until a visually smooth paste is formed.

When incorporated into a liquid silicone / silane mixture, the addition of liquid metal-coated particles has been found to effectively reduce viscosity. The mechanism involved in this change in viscosity is thought to be due to a reduction in viscosity retardation at the "effective particles" -silicone oil / silane interface. Liquid metal coatings increase the spherical shape of the particle shape and also contribute to the effective "softening" of otherwise hard particles. These two factors work in concert with each other to reduce both the viscosity and the modulus of the resulting complex.

In addition to effectively transferring heat and / or thermal energy, the liquid metal-coated particles have also been found to solidify and stabilize the liquid metal into the three-phase composite and prevent significant migration. . The three phases are a particle / liquid metal / polymer mixture. Increasing the viscosity of the metal phase significantly delays the tendency of metal droplets to migrate, aggregate into large droplets, macroscopically separate, and leak from the composite. In addition, the liquid-coated particles impart Bingham plastic-like properties to the resulting composite, which keeps the paste stationary in the absence of external stress, easily adapts when further stressed, and / or ) It has been found to be able to flow.

[0013] Liquid metals do not mix well with polymeric liquids, including silicones, because they tend to undergo macroscopic liquid-liquid separation. However, according to the present invention, when a particular boron nitride particulate is first coated with a gallium alloy, the macroscopic separation phenomenon is reduced and the thixotropic nature of the metal phase is increased, resulting in the formation of particles coated with the liquid metal. Supported or maintained by Further, the coated particles, when added to the silicone / silane mixture, function to effectively form heat transfer paths in the composite. In some cases, the thermal conductivity of particles such as boron nitride can even exceed that of liquid metals, for example, eutectics of gallium, tin and indium.

[0014] In addition to thermal properties, it is yet another feature of the present invention that the composite also has desirable electrical properties. Formulations with optimal thermal properties are 10 ⁸ to 10 ¹² Ω
It has been found to have a volume resistivity in the range of cm.

[0015] Briefly, the method of the present invention comprises the step of first selecting a particulate material for the application. Boron nitride particles are particularly desirable, and particles having a BET specific surface area of 0.3 m ² / g have been found to be very useful. The boron nitride is typically in the form of anisotropic platelet-like particles, with a plate diameter in the range of about 5-50 μm and a plate thickness of about 2-3 μm. The next step is the coating of the particles. When coated with a liquid metal, these particles have a liquid metal / boron nitride volume ratio ranging from 4: 1 to 1: 1. Coating is achieved by mechanical mixing, as described above. Then an appropriate amount of the mixed liquid or fluid silicone and octyl-triethoxysilane is added to the coated particles and after the addition, high speed mixing is performed until a visually smooth paste is obtained.

As indicated above, although boron nitride is a preferred particulate, favorable results have been achieved with the use of alumina, which requires a pretreatment, including complete drying of the particles. This is typically done. Other particulates, such as aluminum nitride, after complete drying,
A liquid metal paste can be formed.

[0017]

The following examples are given to describe preferred embodiments:

Example 1

The granules selected were boron nitride, the particles having a normal platelet morphology with an average diameter of 40μ and a cross-sectional thickness of 2μ. The particles were easily wetted by the gallium alloy. When coated with a liquid gallium alloy, the BN powder did not form hard agglomerates, but rather formed a thixotropic paste. This configuration is desirable as long as the BN has a high thermal conductivity in the "in-plane" direction, which conductivity is substantially improved by the liquid metal bridge. BN has a specific gravity of 2.25 and 350
It has a thermal conductivity (in-plane) of W · m ⁻¹ · K ⁻¹ (the thermal conductivity averaged by direction is reported to be about 60 W · m ⁻¹ · K ⁻¹ ). The polymer matrix selected was a mixture of silicone oil and octyl-triethoxysilane, the silicone oil component having a dynamic viscosity of 100 centistokes, a specific gravity of 0.86, and 0.15 W · m ^{−1.
-It had} a thermal conductivity of K- ¹ . 6.5 for metal
And a thermal conductivity of 20 W · m ⁻¹ · K ⁻¹ .

The anisotropic platelet BN particles were first coated with a liquid gallium alloy. The liquid metal to BN volume ratio was selected in three different ranges, as described in Table I below:

[0021]

The coating is achieved by mechanically mixing the liquid gallium alloy of Example 1 with BN powder, which can be done by hand or by a high speed mixer. After mixing, an appropriate amount of silicone oil / octyl-
The triethoxysilane mixture was added, followed by high speed mixing until a visually smooth paste was obtained.

The mixing treatment stabilizes the compound. The surface tension of the silicone oil / silane mixture is about 20 mN / m, whereas for liquid metals it is 400-500 mN / m
Of the degree. This means that the ability of the silicone oil / silane mixture to wet the surface, the diffusion coefficient, is much greater than that of the liquid metal. Thus, the BN particles are coated with a liquid metal prior to contacting with the silicone oil / silane mixture so as to achieve proper and desirable wetting. In particular, the following advantages exist: 1. The material forms a liquid bridge; and The presence of the hydrophobic alkyl-functional silane in the mixture significantly reduces the amount of macroscopic separation of the liquid metal.

The tests were carried out without following the continuous steps of the present invention.
Mixing all the ingredients of the formulation together indicated that the powder was not properly wetted by the liquid metal. The order of the mixing steps is key to the successful production of stable and thermally conductive compounds in the present invention.

Example 2

The granules selected were aluminum oxide, ie, alumina, spherically symmetric particles having a diameter of 3 μm and a BET specific surface area of 2 m ² / g. Both the alumina and the alloy were heated to 100 ° C. (above the melting point of Alloy 2) and mixed. When coated with the liquid alloy, the alumina formed a smooth, thixotropic paste. Alumina has a specific gravity of 3.75 and a thermal conductivity of 25 W · m ⁻¹ · K ⁻¹ . The polymer matrix selected was a mixture of silicone oil and octyl-triethoxysilane, the silicone oil component having a dynamic viscosity of 100 centistokes, a specific gravity of 0.86, and 0.15 W · m ^{−1.
-It had} a thermal conductivity of 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 first coated with a liquid metal alloy. The metal to alumina volume ratio was selected in three different ranges, as described in Table II below:

[0028]

The coating is achieved by mechanically mixing the liquid alloy of Example 2 with the alumina powder, which can be done by hand or by a high speed mixer. After mixing, the appropriate amount of silicone oil / silane mixture was added, followed by high speed mixing until a visually smooth paste was obtained.

Example 3

The particulate selected was the alumina of Example 2. When coated with the liquid gallium alloy, the alumina formed a smooth, thixotropic paste. The polymer matrix chosen is silicone oil / octyl-triethoxysilane, which has a dynamic viscosity of 100 centistokes, a specific gravity of 0.86, and a viscosity of 0.15
It had a thermal conductivity of W · m ⁻¹ · K ⁻¹ . The liquid metal has a specific gravity of 6.5 and a thermal conductivity of 20 W · m ⁻¹ · K ⁻¹ .

The alumina particles were first coated with a liquid gallium alloy. The liquid metal to alumina volume ratio is shown in Table III below.
Selected in three different ranges as described in:

[0033]

The coating is achieved by mechanically mixing the liquid gallium alloy of Example 1 with the alumina powder, which can be done by hand or by a high speed mixer. After mixing, add an appropriate amount of silicone oil,
Next, high speed mixing was performed until a visually smooth paste was obtained.

Test Results Formulation 1 (Table I) was tested for thermal conductivity. AS
The TM D5470 method gave a thermal conductivity of 8.0 W · m ⁻¹ · K ⁻¹ . Controlled thermal impedance tests on industrial standard materials were also performed. One of these is a generic thermal interface compound from Dow Corning (DC-3).
40 thermal grease) and the other was a high performance compound (G-749 thermal grease) manufactured by Shin-Etsu Corporation. This formulation showed improved stability when exposed to a humid environment, as demonstrated by the impedance / humid environment exposure curve comparison of FIG. As illustrated, the response curve remains relatively stable for formulations containing octyl-triethoxysilane, as opposed to formulations without silane.

Properties of Liquid Metal Coated Particles As illustrated in the drawings, FIG. 1 illustrates how improved contact can be obtained between individual coated particles, particularly BN coated with a liquid gallium alloy. The surface properties or properties of the composite improve contact by forming liquid bridges. This sketch shows the feature of particle surface wetting, which usually gives a significant drop in surface resistivity that appears between adjacent particles. Liquid metals are stabilized by adding octyl-triethoxysilane to the silicone oil component.

FIG. 2 illustrates the characteristics of the improved infiltration obtained from near critical packing. Surface-to-surface contact as shown in the left part of FIG. 2 improves when near-critical filling is achieved by high concentrations.

The purpose of FIG. 3 is to demonstrate the reduction in aspect ratio achieved by liquid metal coating of the particles. Because boron nitride has an anisotropic platelet structure, its performance in applications contemplated by the present invention is improved. The liquid metal coating makes the shape of the "effective particles" more elliptical.

The purpose of FIG. 4 is to illustrate the advantageous feature of the present invention that reduces viscous dissipation by coating individual particles. Improved overall performance is predictable and has been obtained in practice.

FIG. 6 is a flow chart of the steps performed in accordance with the manufacture of a compliant pad according to the present invention. As shown in the figure and as evident from the process diagram, the granules and alloy are mixed until the surfaces of the particles are completely wetted, and then the paste blend is added by the addition of a liquid polymer. Form an object.

FIG. 7 is provided to illustrate the use of the compliant pad of the present invention in connection with a conventional form of heat generating semiconductor device. Thus, the assembly 10 shown in FIG. 7 has a heat-generating semiconductor device or package illustrated at 11 having a heat absorber, heat spreader, or other heat dissipating member illustrated at 12. Between the opposing surfaces of the semiconductor device 11 and the heat dissipation member 12, a mechanically compliant pad 13 manufactured according to the invention is inserted.

FIG. 8 gives a comparison to show the improved stability achieved by the addition of octyl-triethoxysilane. As shown, the performance is compared with the formulation (Table I) with and without octyl-triethoxysilane. The data is 85
It was obtained by exposure to an environment at a temperature of 85 ° C. and a relative humidity of 85%.

General Description As indicated earlier, BN or alumina particles can range in size up to about 1μ in diameter and up to about 40μ in cross-sectional thickness. In particular, it will be observed that the platelet morphology of boron nitride gives a highly desirable combination with the effective particles illustrated in FIG. 3 when wetted with liquid metal. This feature aids viscosity control.

The silicone oil used as a component in the examples is a typical liquid silicone, typically
EB100 (Sivento Inc., formerly Huls America), of course, these materials are commercially available. Silicones having a viscosity of up to about 1000 centistokes can be used satisfactorily. The presence of the silane changes the viscosity slightly, resulting in an oil composition having a slightly lower viscosity.

One unusual feature of the present invention is the resistivity. When Formulation 1 is formed as a pad between the opposing surfaces of the semiconductor and the heat absorber, the resistivity is very high, about 1
It has been found to have values up to 0 ¹² Ω · cm (Table I, Formulation 1).

It will be appreciated that the above embodiments are given for illustrative purposes only and are not to be considered as limiting as to the claims.

[Brief description of the drawings]

FIG. 1 is a schematic illustration showing an improved contact between liquid metal coated particles (BN). It is clear that wetting of the surface of the particles significantly reduces the surface resistance between adjacent particles.

FIG. 2 illustrates the change in polymer matrix filled with particles by forming clusters on a larger length scale, further when the volume fraction of liquid metal coated particles is near the packing limit as spherical particles. 3 is a schematic sketch illustrating a desirable state of the composite of Example 1, and a sketch illustrating a feature of high concentration such that heat infiltration close to a critical filling fraction is obtained.

FIG. 3 is an exemplary sketch similar to FIG. 2 illustrating the reduction of the aspect ratio using a liquid metal coating, particularly in the case of BN particle platelets.

FIG. 4 is a diagram exemplifying a feature in which a soft liquid gallium alloy is used as a coating of particles to reduce viscous dissipation.

FIG. 5 illustrates the aggregation and separation of individual liquid gallium metal droplets to achieve the results of the present invention.

FIG. 6 is a process diagram illustrating a process performed to manufacture a compliant pad of the present invention.

FIG. 7 shows a typical semiconductor mounted on a hinged heat absorber having a compliant pad made in accordance with the present invention interposed between the heat absorber and opposing surfaces of the semiconductor device. FIG. 2 is an exemplary view of a device.

FIG. 8 is a graph showing the performance of thermal impedance when exposed to a particular humid environment for various times.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Assembly 11 Semiconductor device 12 Heat dissipation member 13 Flexible pad

Claims (6)

[Claims] 1. A method of manufacturing a mechanically compliant thermally conductive pad, comprising: (a) (1) a large amount of gallium and / or indium containing alloy which is in a liquid state at a temperature lower than 120 ° C .; 2) preparing a mixture of a thermally conductive granular solid consisting essentially of boron nitride; and (b) mechanically mixing the mixture to wet the surface of the particles with the liquid alloy and form a uniform paste. Wherein said liquid alloy encapsulates said individual particles of said boron nitride; and (c) said paste is essentially made of a mixture of silicone oil and octyl-triethoxysilane. Combine with a large amount of flowable plastic resin material and mix with metal coated particles
Forming a thermally conductive material consisting of about 10% to 90% by volume and the balance of the flowable plastic resin. 2. The flowable plastic resin material mixture,
The method of claim 1, comprising about 70% to about 95% silicone oil and the balance octyl-ethoxysilane. 3. The method of claim 1, wherein the particles comprising the thermally conductive particulate solid have a diameter of about 1μ to 40μ. 4. The method according to claim 1, wherein the liquid metal alloy is in a liquid state at a temperature lower than 60 ° C. 5. A thermally conductive compliant pad manufactured according to the steps of claim 1. 6. A method of manufacturing a mechanically compliant thermally conductive pad, comprising: (a) (1) a component selected from the group consisting of gallium and indium, which is in a liquid state at a temperature lower than 120 ° C. Preparing a mixture of a liquid metal alloy and (2) a thermally conductive particulate solid selected from the group consisting of boron nitride, aluminum nitride and alumina; and (b) mechanically mixing the mixture to form a mixture. (C) wetting the surface of the particles with the liquid alloy to form a uniform paste, wherein the liquid alloy encapsulates the individual particles of the granular material in a capsule shape; And a large amount of a flowable plastic resin material consisting essentially of a mixture of a hydrophobic surface treatment agent, for example, an alkyl-functional silane or titanate, and about 10 vol. ~ 90
Forming a thermally conductive material comprising a volume% and a balance of a flowable plastic resin mixture.