Low melting alloy thermal interface materials

Abstract

A thermal interface material includes a matrix material; a first component including a plurality of metal droplets, the plurality of metal droplets having a melting point temperature of less than about 30 °C; and a second component having a solid phase including particles having an aspect ratio greater than about 2:1, at least a portion of the particles being dispersed in the matrix material, wherein the second component has a melting point temperature of between about 30 °C and about 90 °C at which the second component transitions from the solid phase to a liquid phase, wherein the second component in the liquid phase may be soluble in the matrix material.

Description

2024P00116 LOW MELTING ALLOY THERMAL INTERFACE MATERIALS TECHNICAL FIELD

[0001] The subject matter disclosed herein relates to thermal interface materials generally, and more particularly to low melting point metal-containing thermal interface materials. The present disclosure further relates to methods for manufacturing such thermally conductive constructs. BACKGROUND

[0002] Conventional thermal interface materials typically include polymeric matrices with dispersed thermally conductive solid particles that impart thermal conductivity to the bulk material. However, traditional thermal interface materials exhibit certain limitations in conductive performance due to the size exclusion at the boundaries of the interface material caused by hard filler particles. The exclusion zone near the interface is often polymer rich with a thickness approximately equal to the harmonic mean particle size of the thermally conductive particles. This exclusion layer exhibits low, polymer like thermal conductivity, and is an impediment to heat transfer. As thermal applications require reduced bond line thicknesses, the heat transfer impediment becomes more pronounced, leading to decreased effectiveness of conventional thermal interface materials.

[0003] To increase the heat transfer efficiency for certain applications, low melting alloys (LMAs) have been used in place of solid particles in thermal interface materials. These low melting alloys exhibit excellent heat dissipation performance in high heat flux applications such as for use in conjunction with central processing units (CPUs), graphics processing units (GPUs), and multichip modules. However, low melting alloys can be susceptible to oxidation, especially when in the presence of water vapor. Conventionally, low melting alloys can also be difficult to stabilize in dispersions. It is therefore an object of the invention to stabilize low melting alloy dispersions for enhanced shelf stability while maintaining good dispensability as thermal interface materials.2024P00116 SUMMARY

[0004] According to one aspect, a thermal interface material includes a matrix material; a first component including a plurality of metal droplets, the plurality of metal droplets having a melting point temperature of less than about 30 °C; and a second component having a solid phase including particles having an aspect ratio greater than about 2:1, at least a portion of the particles being dispersed in the matrix material, wherein the second component has a melting point temperature of between about 30 °C and about 90 °C at which the second component transitions from the solid phase to a liquid phase, wherein the second component in the liquid phase is soluble in the matrix material.

[0005] According to another aspect, a thermal interface material includes a matrix material; a first component including a plurality of metal droplets having a melting point temperature of less than about 30 °C; and a second component having a solid phase including a plurality of particles dispersed in the matrix material, wherein an average particle size of the plurality of particles is less than about 200 μm, wherein the second component has a melting point temperature of between about 30 °C and about 90 °C.

[0006] According to another aspect, a method of forming a thermal interface material includes (a) mixing a first material with a second material, the second material having a melting point temperature of between about 30 °C and about 90 °C; (b) heating at least the first material and the second material, sufficient to disperse the second material in the first material; and (c) mixing the first material with a plurality of metal droplets, the plurality of metal droplets having a melting point temperature of less than about 30 °C. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG.1 illustrates a method 100 for forming a thermal interface material, according to some embodiments.

[0008] FIG.2 illustrates a heat transfer assembly 10, according to some embodiments.

[0009] FIG.3 illustrates a method 300 for forming a heat transfer assembly, according to some embodiments.2024P00116 DETAILED DESCRIPTION

[0010] Embodiments of the present disclosure provide thermal interface materials and methods of forming these thermal interface materials. These thermal interface materials can be utilized for various heat dissipation applications, such as for central processing units (CPUs), graphics processing units (GPUs), and multichip modules. Low melting point materials with high thermal conductivity values can be used to increase the overall thermal conductivity of the thermal interface material. However, these low melting materials, such as low melting point alloys, can suffer from dispersion instability. Therefore, it is desirable to provide stabilized thermal interface materials, such that the materials can be stable under storage conditions, while nevertheless exhibiting desirable thixotropic rheology properties during dispensation, and desirable thermal conductivity properties during operation.

[0011] The thermal interface materials of the present disclosure generally include at least one of a matrix material, a first component, a second component, and a third component. Matrix materials can include polymers that may be blended with one or more components of the present disclosure to form the thermal interface material, such as in the form of an emulsion or dispersion. The matrix material may act as a binder to hold the composition together and to prevent outflow in operation. In one example, the matrix material includes a material capable of holding and / or at least partially surrounding a dispersed material. The matrix material can include an organic material. In one example, the matrix material can include at least one of a polymeric material and a non-polymeric material. In one non-limiting example, polymeric materials used for the matrix material include at least one of silicone materials, perfluoropolyether materials, and polyphenyl ether materials. Examples of non-polymeric materials include aromatic ethers and esters.

[0012] An example silicone polymer includes an organosiloxane having the structural formula:2024P00116 wherein “x” represents an integer ranging from between 1 and 1,000. In some embodiments, the matrix material may be prepared as a reaction product of the organosiloxane together with a chain extender / cross-linker such as a hydride-functional polydimethylsiloxane having the structural formula: wherein “x” and “y”1 and 1,000.

[0013] The polymers may be thermoplastic or thermosetting polymers that may be blended with other component(s) of the present disclosure to form the thermal interface material, such as in the form of a liquid-liquid emulsion. Thermosetting polymers can refer to polymers that can set irreversibly upon curing or cross-linking, often induced by heat or radiation. For example, the thermosetting polymer may include an acrylic polymer, a polyester polymer, a polyurethane polymer, a polyamide polymer, a polyether polymer, a polysiloxane polymer, a fluoropolymer, a polyisoprene polymer, a copolymer thereof, or a combination thereof. Matrix materials of the present disclosure can be used in conjunction with a metal filler, discussed in further detail herein, to reduce or prevent oxidation of the metal filler.

[0014] Curing of the thermosetting polymer can be achieved by applying heat to the thermosetting polymer. In one example, the thermosetting polymer is at least partially cured by applying a temperature of greater than about 20 °C. In another example, the thermosetting polymer is at least partially cured by applying a temperature of greater than about 40 °C. In another example, the thermosetting polymer is at least partially cured by applying a temperature of greater than about 60 °C. In another example, the thermosetting polymer is at least partially cured by applying a temperature of greater than about 30 °C, greater than about 50 °C, greater than about 90 °C, or greater than about greater than about 100 °C.

[0015] The matrix material, such as in the uncured state, can have a viscosity greater than about 10 mPa‧s. In one example, the matrix material, such as in the uncured state, has a viscosity greater than about 100 mPa‧s. The matrix material, such as in the uncured state, has a viscosity ranging from about 10 mPa‧s to about 10000 mPa‧s. In one example, the matrix material, such as2024P00116 in the uncured state, can have a viscosity ranging from about 10 mPa‧s to about 1000 mPa‧s. For example, the matrix material, such as in the uncured state, can include an organic fluid having a viscosity ranging from about 10 mPa‧s to about 10000 mPa‧s. In another example, the matrix material, such as in the uncured state, has a viscosity ranging from about 100 mPa‧s to about 1000 mPa‧s. Viscosity values may be measured at about room temperature, such as at about 20 °C. Viscosity values may be measured at a shear rate of 1 / s.

[0016] The first component includes metal filler. The metal filler can be utilized to increase the overall thermal conductivity of the thermal interface material. In one example, the metal filler is in the form of a plurality of metal droplets. The metal material or materials forming the metal filler may exhibit a melting point temperature of between -25oC and 100oC, and preferably below an operating temperature of a heat generating device to which the heat transfer apparatus of the present invention is thermally coupled. In some embodiments, the metal filler may exhibit a melting point temperature of between -25oC and 40oC. In some embodiments, the metal filler may exhibit a melting point temperature of between -25oC and 15oC. In some embodiments, the metal filler may exhibit a melting point temperature of between -25oC and 0oC. The metal filler can exhibit a melting point temperature of less than about 30oC. In some embodiments, the metal filler can exhibit a melting point temperature of less than about 15oC. In some embodiments, the metal filler can exhibit a melting point temperature of less than about 0oC. Melting point temperatures may be determined at a pressure of about 1 bar.

[0017] The metal filler may be in a liquid phase at temperatures above about 20 °C. In some embodiments, the metal filler is in the liquid phase at temperatures above 15 °C. In some embodiments, the metal filler is in the liquid phase at temperatures above 0 °C. In some embodiments, the metal filler is in the liquid phase at temperatures above -19 °C. The determination of whether the liquid phase is achieved at the respective temperature may be made at a pressure of about 1 bar. As discussed, in contrast to hard filler particles that exhibit certain limitations in conductive performance due to the size exclusion at the boundaries of the interface material, low melting point metal filler of the present disclosure can improve heat dissipation characteristics by not limiting a reduction in the bondline thickness.

[0018] Although the metal filler may comprise a single metal material, typical metal fillers useful in the thermal interface materials of the present disclosure include alloys of two or more metal materials such as gallium, indium, bismuth, tin, and zinc. An example alloy metal filler of2024P00116 the present invention comprises 50-75% by weight gallium, 10-30% by weight indium, and 5- 20% by weight tin. A particular example alloy metal filler of the present invention comprises 68.5% by weight gallium, 21.5% by weight indium, and 10% by weight tin, with a melting point of -19oC. An additional example alloy metal filler comprises 66% by weight gallium, 20.5% by weight indium, and 13.5% by weight tin, with a melting point of 10.5oC. Other alloy blends, however, are contemplated for use as the metal filler in the thermal interface material of the present invention.

[0019] The volume percentage of the metal filler in the thermal interface material can be greater than about 20 vol%. In some embodiments, a volume percentage of the metal filler in the thermal interface material can be greater than, or equal to, about 50 vol%, about 60 vol%, or about 70 vol%. In some embodiments, a volume percentage of the metal filler in the thermal interface material can be greater than, or equal to, about 80 vol%. In some embodiments, a volume percentage of the metal filler in the thermal interface material ranges from about 50 vol% to about 90%. In some embodiments, a volume percentage of the metal filler in the thermal interface material ranges from about 60 vol% to about 90%. In some embodiments, a volume percentage of the metal filler in the thermal interface material ranges from about 70 vol% to about 90%.

[0020] The metal filler may be present in the thermal interface material in a weight ratio of the metal filler to the matrix material of between 20:1 and 60:1. In some embodiments, the metal filler may be present in the thermal interface material in a weight ratio of the metal filler to the matrix material of between 30:1 and 55:1. The weight percentage of the metal filler in the thermal interface material may be greater than about 80 wt.%. In one example, the weight percentage of the metal filler in the thermal interface material is greater than about 90 wt.%. In another example, the weight percentage of the metal filler in the thermal interface material is greater than about 95 wt.%.

[0021] As discussed, the metal filler can be in the form of a plurality of metal droplets, which are often in a liquid state at room temperature. The metal filler may be subjected to a sizing operation to particulate the metal / alloy material into a desired droplet size distribution, including monodisperse, polydisperse, gaussian, multi-modal, and the like. The sizing operation may perform upon the metal filler alone, or with the metal filler mixed with the matrix material. In some embodiments, the sizing operation may be performed by a high shear mixer. An2024P00116 example approach includes blending the metal filler with the matrix material in a high shear mixer until the metal filler becomes thoroughly dispersed in the polymer, at which time it may be formed into the configuration desired for the thermal interface.

[0022] The metal filler and matrix material may be blended in a liquid phase, a solid phase, or a combination of liquid and solid phases. In the case of mixing the metal filler and matrix material when both components are in a liquid phase, the metal filler may be particulated into discrete small droplets and dispersed in the matrix material as an emulsion. In some embodiments, the thermal interface material includes liquid metal droplets in a maximum amount that nonetheless remains stable in an emulsion or dispersion with the matrix material by remaining encapsulated by the matrix material and not separating out from the matrix material.

[0023] Droplet size of the metal droplets is preferably controlled to achieve a droplet size distribution with a mean droplet size that is associative with a mean gap width. In some embodiments, droplet size may be controlled with shear imparted upon the metal filler. For the purposes hereof, the term “droplet” is intended to mean a discrete volume of metal filler, whether in a solid or liquid state. Metal droplets with a desired droplet size distribution and mean droplet size may also be obtained from commercial sources. An example process involves placing a mixture of metal filler and matrix material under shear force until a dispersion or emulsion is formed with desired metal filler mean particle size. For the purposes hereof, the mean droplet size of the metal droplets may be measured with the metal droplets in a dispersed state, such as encapsulated by the matrix material. Mean droplet size may be ascertained by various techniques, including volume, area, or weight-based measurement techniques.

[0024] Accordingly, materials and / or mixing techniques can be selected to achieve a desired particle size for the plurality of metal droplets in the thermal interface material. In some embodiments, an average droplet size of the plurality of metal droplets can range from about 10 μm to about 200 μm. In some embodiments, an average droplet size of the plurality of metal droplets can range from about 25 μm to about 200 μm. In some embodiments, an average droplet size of the plurality of metal droplets can range from about 50 μm to about 200 μm. In some embodiments, an average droplet size of the plurality of metal droplets can range from about 50 μm to about 150 μm. In some embodiments, an average droplet size of the plurality of metal droplets can range from about 75 μm to about 100 μm. The average droplet size of the plurality2024P00116 of metal droplets can be greater than 50 μm. The average droplet size of the plurality of metal droplets can be less than 200 μm.

[0025] The density of the metal filler may be greater than about 5 g / cm3. In one example, the density of the metal filler is greater than about 6 g / cm3. The thermal conductivity of the metal filler may be greater than about 5 W / mK. In one example, the thermal conductivity of the metal filler may be greater than about 10 W / mK. In another example, the thermal conductivity of the metal filler may be greater than about 15 W / mK. The viscosity of the metal filler at 20 °C at a shear rate of 1 / s may be less than 500 Pa‧s. The viscosity of the metal filler at 20 °C at a shear rate of 1 / s may be less than 100 Pa‧s. In one example, the viscosity of the metal filler at 20 °C at a shear rate of 1 / s is less than 1 Pa‧s. In another example, the viscosity of the metal filler at 20 °C at a shear rate of 1 / s is less than 0.1 Pa‧s.

[0026] The second component can at least improve the stability of the thermal interface material and can reduce settling of the metal filler. For example, the second component can increase the viscosity of the thermal interface material to reduce settling of the metal filler, thereby increasing shelf-stability at room temperature. In some embodiments, the second component can increase low-shear viscosity of the thermal interface material to increase shelf- stability at room temperature. The second component may include a phase-change material that is solid at room temperature and has a melting point that is below a typical operating temperature of the thermal interface. Accordingly, the second component can have a solid phase including particles, where the second component can have solid phase particles at or below melting point temperatures (discussed further herein).

[0027] The second component can include at least one of a wax ester, a polyolefin wax, a silicone wax, and a paraffin wax. Examples of wax esters include stearyl palmitate, cetyl palmitate, and myristoyl myristate. For example, stearyl palmitate has a melting point temperature of about 55 °C. In one example, the silicone wax includes an alkylated silicone wax. Examples of suitable alkylated silicone waxes include cerotyl dimethicone wax and stearyl dimethicone wax. In one example, cerotyl dimethicone wax has a melting point temperature of about 43oC. In another example, stearyl dimethicone wax has a melting point temperature of about 28oC. The paraffin wax may include one or more components following the formula CnH2n+2.For example, n may range from 20 to 40. In one example, the second component includes wax particles. At least a portion of the second component, such as at least a portion of2024P00116 the particles, can be dispersed in the matrix material. In one example, the second component is soluble in the matrix materials at certain temperatures, and preferably at temperatures above room temperature.

[0028] The second component can be miscible and dissolvable in the matrix material, such as at temperatures at or above the melting point temperature of the second component. A melting point temperature can include temperatures where at least a portion of the material begins the phase change to a liquid phase. The melting point temperature can be the temperature at which the second component transitions from the solid phase to the liquid phase. In one example, the second component has a melting point temperature of between about 30 °C and about 90 °C. In another example, the second component has a melting point temperature of between about 40 °C and about 90 °C. In another example, the second component has a melting point temperature of between about 40 °C and about 70 °C. In yet another example, the second component has a melting point temperature of between about 40 °C and about 60 °C. The second component may exhibit a melting point temperature above about 50 °C. The second component in the liquid phase can be dispersible and / or soluble in the matrix material.

[0029] The second component may exhibit a melting point temperature below about 80 °C. The second component may exhibit a melting point temperature below about 95 °C, below about 90 °C, below about 85 °C, below about 80 °C, or below about 75 °C. The inclusion of the second component, with melting point temperatures of the present disclosure, improves the shelf- stability of the thermal interface material at non-operating temperatures, such as room temperature, by increasing its viscosity through interaction between the solid-phase second component and the matrix material. The second component is meltable and dissolvable at heat generating device operating temperatures to avoid interference with optimal bondline thickness and to therefore maximize heat dissipation properties. Conventional particulate thickeners restrict bondline thickness reduction. By melting and dispersing / dissolving at temperatures at or below device operating temperatures, the second component does not present a physical impediment to thin bondlines. Example heat generating device operating temperatures include temperatures above about 60 °C, above about 80 °C, above about 100 °C, above about 120 °C, or above about 140 °C. A melting point of the present disclosure (such as less than about 90 °C) ensures that the second component can melt at or below typical heat generating device operating temperatures.2024P00116

[0030] The second component can include a first group and a second group, wherein the first group has a first average melting point temperature that is lower than a second average melting point temperature of the second group. The first group and the second group include wax components of the present disclosure, such as a wax ester, a polyolefin wax, a silicone wax, and a paraffin wax. The first group and the second group can exhibit melting point temperatures of the present disclosure. In one example, the first group has a melting point temperature below about 50oC, and the second group has a melting point temperature above about 50oC. The first average melting point temperature can be at least 5oC less than the second average melting point temperature. The first average melting point temperature can be at least 10oC less than the second average melting point temperature.

[0031] The volume percentage of the second component in the thermal interface material can range from about 0.1 vol% to about 10 vol%. In one example, the volume percentage of the second component in the thermal interface material can range from about 0.1 vol% to about 5 vol%. In another example, the volume percentage of the second component in the thermal interface material can range from about 0.5 vol% to about 3 vol%. The volume percentage of the second component in the thermal interface material can be greater than 0.1 vol%. The volume percentage of the second component in the thermal interface material can be greater than 0.5 vol%. The volume percentage of the second component in the thermal interface material can be less than 5 vol%. The volume percentage of the second component in the thermal interface material can be less than 3 vol%.

[0032] The concentration of the second component dispersed in the matrix material can range from about 1 part per hundred resin (phr) to about 20 phr. In one example, the concentration of the second component dispersed in the matrix material can range from about 1 part per hundred resin (phr) to about 15 phr. In another example, the concentration of the second component dispersed in the matrix material can range from about 1 part per hundred resin (phr) to about 10 phr. The concentration of the second component dispersed in the matrix material may be greater than 2 phr. The concentration of the second component dispersed in the matrix material may be greater than 4 phr. The inclusion of the second component, with volume percentages and / or phr concentrations of the present disclosure, can increase the viscosity of the thermal interface material at room temperature, sufficient to improve storage stability, while maintaining desirable dispensability.2024P00116

[0033] The weight percentage of the second component in the thermal interface material can be tuned according to desired properties. In one example, the weight percentage of the second component in the thermal interface material is greater than about 0.05 wt.%. In another example, the weight percentage of the second component in the thermal interface material is greater than about 0.1 wt.%. In yet another example, the weight percentage of the second component in the thermal interface material is less than about 10 wt.%. In one example, the weight percentage of the second component in the thermal interface material ranges from about 0.1 wt.% to about 10 wt.%. In another example, the weight percentage of the second component in the thermal interface material ranges from about 0.1 wt.% to about 5 wt.%.

[0034] The density of the second component can range from about 0.8 g / cm3to about 1.1 g / cm3. In one example, the density of the second component ranges from about 0.8 g / cm3to about 1.1 g / cm3. In one example, the density of the second component ranges from about 0.9 g / cm3to about 1.1 g / cm3. The density of the second component may be less than 1.5 g / cm3. The density difference between the density of the second component and the density of the matrix material may be less than about 20%. The density difference between the density of the second component and the density of the matrix material may be less than about 15%. The density difference between the density of the second component and the density of the matrix material may be less than about 10%. The inclusion of the second component, with density values and density differences of the present disclosure, provides a stable thermal interface material by reducing settling. In contrast to additives prone to settling issues with density values above about 2 g / cm3, above about 3 g / cm3, or above about 4 g / cm3, the second component exhibits a density close to the matrix material to reduce or prevent particle settling.

[0035] The aspect ratio of the second component can be greater than about 1.5:1. The aspect ratio may be defined as the ratio of the length to the width of the individual particle. In one example, the aspect ratio of the second component is greater than about 2:1. In another example, the aspect ratio of the second component is greater than about 10:1. In yet another example, the aspect ratio of the second component is greater than about 20:1. The aspect ratio of the second component can greater than about 3:1, greater than about 5:1, greater than about 6:1, greater than about 8:1, or greater than about 10:1. Aspect ratios of the present disclosure can be aspect ratios of individual particles. Aspect ratios of the present disclosure can be average aspect ratios of the plurality of particles. The shape of the second component may be aspherical.2024P00116 Example aspherical shapes include flake-like, plate-like, rod-like, and so on. For example, the second component may be shaped as a plate-like material having aspect ratios of the present disclosure.

[0036] The aspect ratio of the second component may range from about 2:1 to about 100:1. The aspect ratio of the second component may range from about 10:1 to about 100:1. In one example, without the second component, the plurality of liquid metal droplets have a tendency to reaggregate and / or separate out of the matrix material. This can place limits on manufacturing, storage, and transportation. The inclusion of the second component, with aspect ratios of the present disclosure (such as greater than, or equal to, 10:1), can be used to block or reduce the tendency for the plurality of liquid metal droplets from merging together, reducing aggregation of the plurality of liquid metal droplets and ensuring a desirable droplet size. By preventing aggregation of the plurality of liquid metal droplets into bigger droplets (such as maintaining a mean droplet size of less than 200 μm, less than 150 μm, or less than 100 μm, the stability of the pre-installed thermal interface material is improved by limiting the settling of the liquid metal droplets.

[0037] The second component may have a mean particle size of less than about 200 μm. For the purposes hereof, the term “mean particle size” refers to a cumulative weight average value (d50) in which 50% of the particles are larger than the value, and 50% of the particles are smaller than the value, as determined by laser light diffraction. In one example, the mean particle size of the second component ranges from about 5 μm to about 200 μm. In another example, the mean particle size of the second component ranges from about 5 μm to about 100 μm. In another example, the mean particle size of the second component ranges from about 20 μm to about 150 μm. In another example, the mean particle size of the second component ranges from about 10 μm to about 75 μm. The inclusion of the second component, with particle sizes of the present disclosure, can be used to tune the stability and the dispensability of the thermal interface material. In one non-limiting example, the second component has a mean particle size of less than about 200 μm to ensure desirable dispensability, since particles above 200 μm can undesirably decrease the dispense rate.

[0038] Compositions of the present disclosure can include a third component, such as a hydrophobic surface modifier. The hydrophobic surface modifier of the present disclosure is capable of enhancing the hydrolytic stability of one or more components of the composition, at2024P00116 least in part by resisting chemical decomposition in the presence of water. Accordingly, the hydrophobic surface modifier can improve the hydrophobicity of one or more components in the thermal interface material. In addition, or alternatively, the hydrophobic surface modifier can act as a surfactant or compatibilizer. In one example, the third component is a silane-based hydrophobic surface modifier. In another example, the third component includes one or more of an alkyl silane material, a fluoroalkyl silane material, a thiol material, and a phosphate material. In one non-limiting example, the third component includes dodecyltriethoxysilane.

[0039] Materials of the present disclosure can be utilized to permit a decrease in bondline thickness between a heat generating component and a heat dissipating component. For example, the heat generating component can be electronic component 12, and the heat dissipating component can be heat dissipater 18, both discussed in further detail herein. The thermal interface material can be in contact with, and compressed between, the heat generating component and the heat dissipating component. Due to at least in part from the inclusion of the second component of the present disclosure, the stability of the stored material can be improved without interfering with the operational bondline thickness, improving heat dissipation properties. In one example, a bondline thickness of the thermal interface material is less than 50 μm. In another example, a bondline thickness of the thermal interface material is less than 30 μm. In another example, a bondline thickness of the thermal interface material is less than 10 μm. In another example, a bondline thickness of the thermal interface material is less than 5 μm. In another example, a bondline thickness of the thermal interface material is less than 1 μm.

[0040] The viscosity of the thermal interface material at a shear rate of 1 / s can be greater than about 50000 mPa‧s. The viscosity of the thermal interface material at a shear rate of 1 / s can be greater than about 100000 mPa‧s. The viscosity of the thermal interface material at a shear rate of 1 / s can be greater than about 500000 mPa‧s. The viscosity of the thermal interface material at a shear rate of 1 / s can be less than about 1500000 mPa‧s. The viscosity of the thermal interface material at a shear rate of 1 / s can be less than about 1000000 mPa‧s. The viscosity of the thermal interface material at a shear rate of 10 / s can be less than about 100000 mPa‧s. The viscosity of the thermal interface material at a shear rate of 10 / s can be less than about 85000 mPa‧s. The viscosity of the thermal interface material at a shear rate of 10 / s can be less than about 70000 mPa‧s. The viscosity of the thermal interface material at a shear rate of 10 / s can be greater than about 10000 mPa‧s. The viscosity of the thermal interface material at a shear rate of2024P00116 10 / s can be greater than about 20000 mPa‧s. Viscosity values may be measured at about room temperature, such as at about 20 °C.

[0041] In one example, the overall thermal conductivity of the thermal interface material is greater than about 1 W / mK. In another example, the overall thermal conductivity of the thermal interface material is greater than about 2 W / mK. In yet another example, the overall thermal conductivity of the thermal interface material is greater than about 4 W / mK. The overall thermal conductivity of the thermal interface material can range from about 1 W / mK to about 10 W / mK. In one non-limiting example, the overall thermal conductivity of the thermal interface material ranges from about 5 W / mK to about 10 W / mK.

[0042] Referring to FIG.1, method 100 for forming a thermal interface material is illustrated, according to some embodiments. Method 100 includes one or more of the following steps (with various orders possible):

[0043] Referring to Step 110, at least a first material is mixed with a second material. The first material and the second material include the matrix material and the second component of the present disclosure, respectively. Mixing the first material with the second material can be sufficient to at least partially disperse the second material in the first material. Mixing can be performed using a high shear mixer. In one example, the second material is dispersed, but not dissolved in the first material.

[0044] Referring to Step 120, at least the first material and the second material are heated. The first material and the second material can be sufficient to further disperse the second material in the first material. Heating first material and the second material can be heated sufficiently to partially, or completely, dissolve the second material in the first material. The first material and the second material can be heated at temperatures above about 40 °C. In one example, the first material and the second material are heated at temperatures above 60 °C. In another example, first material and the second material are heated at temperatures above 70 °C. The first material and the second material can be heated at / to temperatures above the melting point temperature of the second material.

[0045] Subsequent to heating, the mixture of the first material and the second material can be cooled. Heating and / or cooling the first material and the second material can be sufficient to form particles of the second material having aspect ratios of the present disclosure. In one example, subsequent to heating, the mixture is cooled to room temperature, such as about 20 °C.2024P00116 The mixture may be in the form of a dispersion. In one non-limiting example, Step 120 is performed simultaneously with Step 110. In another non-limiting example, Step 120 is performed subsequently to Step 110.

[0046] Referring to Step 130, the first material is mixed with a plurality of liquid metal droplets. The liquid metal droplets can include metal fillers of the present disclosure. Mixing the first material with the plurality of liquid metal droplets may be sufficient to form an emulsion or dispersion. After mixing, the formed mixture can be degassed. In one non-limiting example, Step 130 is performed simultaneously with Step 110. In another non-limiting example, Step 130 is performed subsequently to Step 110. Step 130 can be performed simultaneously with, or subsequent to, Step 120.

[0047] Method 100 may further include mixing at least one of the first material, the second material, and the plurality of liquid metal droplets with a hydrophobic surface modifier of the present disclosure. According to method 100, the first material may be mixed with the second material prior to addition of the plurality of liquid metal droplets, or the first material, second material, and the plurality of liquid metal droplets may be mixed and heated simultaneously. The hydrophobic surface modifier may be added at various steps, such as with the plurality of liquid metal droplets.

[0048] An example heat transfer assembly 10 incorporating a thermal interface material 16 is illustrated in FIG.2. Thermal interface material 16 includes thermal interface materials of the present disclosure. Heat transfer assembly 10 includes an electronic component 12 or a substrate to which may be mounted one or more heat-generating electronic components. Heat transfer assembly 10 further includes a heat dissipater 18 in the form of a heat sink or heat spreader. A first surface 13 is associated with electronic component 12, and a second surface 19 is associated with heat dissipater 18. First surface 13 may form a portion of electronic component 12, or may form a portion of a first body that is thermally coupled to electronic component 12. The first body may be separate from, connected to, or integrally formed with electronic component 12. Second surface 19 may form a portion of heat dissipater 18, or may form a portion of a second body that is thermally coupled to heat dissipater 18. The second body may be separate from, connected to, or integrally formed with heat dissipater 18.

[0049] Second surface 19 is spaced or separated from first surface 13 by a gap 26 having a mean gap width 28 (bondline). For the purposes hereof, the term “mean gap width” is intended to2024P00116 be the mean distance between first and second surfaces 13, 19, as measured along a thermal dissipation pathway 22 from the heat-generating electronic component 12 and heat dissipater 18. Thermal interface material 16 is disposed in gap 26 along the thermal dissipation pathway 22, such that the mean gap width may be determined as the mean distance between first and second surfaces 13, 19 where thermal interface material 16 is disposed along thermal dissipation pathway 22 in gap 26. In some embodiments, thermal interface material 16 is disposed in gap 26 in contact with first and second surfaces 13, 19. In some embodiments, thermal interface material 16 substantially fills gap 26 along thermal dissipation pathway 22.

[0050] Heat transfer assembly 10 is arranged to dissipate thermal energy generated by electronic component 12 (and / or an array of electronic components 12) by providing a highly thermally conductive path from electronic component 12 to a heat-absorbing fluid media 24 in contact with heat dissipater 18. In typical applications, fluid media 24 may be a gas, such as air, that may be motivated by an air mover to absorb thermal energy from heat dissipater 18. Heat transfer assembly 10 is an example arrangement that may be modified as appropriate to accommodate a variety of electronic applications, such as data processors, data memory, communication boards, antennae, and the like. Such devices may be utilized in computing devices, communication devices, and peripherals thereof. In a particular example embodiment, heat transfer assembly 10 may be employed to support various functions in a cellular communication device.

[0051] A substrate may serve one or more of a variety of functions in addition to being a support for one or more electronic components 12. For example, a substrate may be a circuit board, such as a printed circuit board with electrically conductive traces for electrically connecting electronic components 12 as needed in the assembly. The substrate may also or instead by a heat spreader, fabricated with at least a layer of thermally conductive material. In operation, electronic components 12 generated significant excess thermal energy which must be dissipated in order to maintain optimal performance. Electronic components 12 may be any of a variety of elements useful in an electronic process, and may include, for example, integrated circuits, resistors, transistors, capacitors, inductors, and diodes. Thermal interface material 16 provides a thermally conductive bridge between first and second surfaces 13, 19 along thermal dissipation pathway 22. Thermal interface material 16 includes a matrix material and a metal filler dispersed in the matrix material.2024P00116

[0052] Heat transfer assembly 10 may be assembled, as set forth in FIG.3, to define gap 26 having a mean gap width 28 that separates first surface 13 from second surface 19 by applying thermal interface material 16 to at least one of first and second surfaces 13, 19, and positioning first and second surfaces 13, 19 so that thermal interface material 16 is disposed therebetween, and so that first and second surfaces are spaced apart by gap 26 with mean gap width 28. In some embodiments, thermal interface material 16 is contacted to both of first and second surfaces 13, 19.

[0053] Importantly, the thermal interface materials of the present disclosure can be used to form a heat transfer pathway between a heat generating component and a heat dissipating component. Prior to installation, the second component is capable of reducing particle aggregation of the plurality of liquid metal droplets, without decreasing the heat transfer efficiency at operating conditions. Due at least in part to the inclusion of the second component, these thermal interface materials can exhibit improved stability at storage temperatures while exhibiting desirable dispense rates and a decreased, installed bondline thickness. Thermal interface materials of the present disclosure can be utilized for various applications, such as for central processing units and graphics processing units. Example 1

[0054] 5 grams of stearyl palmitate wax added to 95 grams of 100 cSt silicone fluid and stirred at 80oC until the stearyl palmitate was dissolved in the silicone fluid. The resulting mixture was subsequently allowed to cool at room temperature, such as about 20oC.2.5 grams of the silicone mixture and 0.5 grams of dodecyltriethoxysilane were added to 97 grams of metal alloy. The metal alloy included 68.5% by weight gallium, 21.5% by weight indium, and 10% by weight tin, with a melting point of -19oC. The resulting material was then degassed. Due at least in part to the inclusion of the stearyl palmitate, the formulation was stable at room temperature even after shaking and agitation. The formulation remains substantially homogeneous after shaking and agitation. Further, the formulation is capable of exhibiting a bondline thickness of less than 1 µm at device operating conditions, improving heat transfer characteristics. In contrast, a formulation without stearyl palmitate was formed, and this distinct formulation was easily separated using shaking and agitation. Further, this distinct formulation had a lower viscosity at shear rates of 1 / s and a temperature of about 20oC. For example, a formulation without stearyl2024P00116 palmitate, or a similar component of the present disclosure, can have a shelf-life of less than one month.

[0055] While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

2024P00116 CLAIMS:

1. A thermal interface material, the thermal interface material comprising: a matrix material; a first component including a plurality of metal droplets, the plurality of metal droplets having a melting point temperature of less than about 30 °C; and a second component having a solid phase including particles having an aspect ratio greater than about 2:1, at least a portion of the particles being dispersed in the matrix material, wherein the second component has a melting point temperature of between about 30 °C and about 90 °C at which the second component transitions from the solid phase to a liquid phase, wherein the second component in the liquid phase is soluble in the matrix material.

2. The thermal interface material of claim 1, wherein the matrix material includes an organic fluid having a viscosity at about 20 °C and a shear rate of 1 / s ranging from about 10 mPa‧s to about 10000 mPa‧s.

3. The thermal interface material of claim 1, wherein the matrix material includes a silicone.

4. The thermal interface material of claim 1, wherein an average droplet size of the plurality of metal droplets ranges from about 25 μm to about 200 μm.

5. The thermal interface material of claim 1, wherein the plurality of metal droplets includes at least one of gallium, indium, and tin.

6. The thermal interface material of claim 1, wherein a volume percentage of the second component in the thermal interface material ranges from about 0.1 vol% to about 10 vol%.

7. The thermal interface material of claim 1, wherein a volume percentage of the second component in the thermal interface material ranges from about 0.5 vol% to about 3 vol%.2024P00116 8. The thermal interface material of claim 1, wherein a concentration of the second component dispersed in the matrix material ranges from about 1 part per hundred resin (phr) to about 20 phr.

9. The thermal interface material of claim 1, wherein the second component includes one or more of a wax ester, a polyolefin wax, and silicone wax.

10. The thermal interface material of claim 1, wherein the aspect ratio of the second component particles ranges from about 10:1 to about 100:

1.

11. The thermal interface material of claim 1, wherein a density of the second component is less than about 1.5 g / cm3.

12. The thermal interface material of claim 1 further comprising a third component including a hydrophobic surface modifier.

13. A thermal interface material, the thermal interface material comprising: a matrix material; a first component including a plurality of metal droplets having a melting point temperature of less than about 30 °C; and a second component having a solid phase including a plurality of particles dispersed in the matrix material, wherein an average particle size of the plurality of particles is less than about 200 μm, wherein the second component has a melting point temperature of between about 30 °C and about 90 °C.

14. The thermal interface material of claim 13, wherein the matrix material includes an organic fluid having a viscosity at about 20 °C and a shear rate of 1 / s ranging from about 10 mPa‧s to about 10000 mPa‧s.2024P00116 15. The thermal interface material of claim 13, wherein the plurality of metal droplets includes at least one of gallium, indium, and tin, and wherein the plurality of particles include one or more of a wax ester, a polyolefin wax, and silicone wax.

16. The thermal interface material of claim 13, wherein an average aspect ratio of the particles is greater than about 10:

1.

17. A method of forming a thermal interface material, the method comprising: (a) mixing a first material with a second material, the second material having a melting point temperature of between about 30 °C and about 90 °C; (b) heating at least the first material and the second material, sufficient to disperse the second material in the first material; and (c) mixing the first material with a plurality of metal droplets, the plurality of metal droplets having a melting point temperature of less than about 30 °C.

18. The method of claim 17, wherein step (c) is performed simultaneously with step (a).

19. The method of claim 17 further comprising mixing at least one of the first material, the second material, and the plurality of metal droplets with a hydrophobic surface modifier.

20. The method of claim 17, further comprising cooling at least the first material and the second material, sufficient to form particles of the second material having an average aspect ratio ranging from about 10:1 to about 100:

1.

21. The method of claim 17, wherein heating at least the first material and the second material is sufficient to dissolve the second material in the first material.

22. The method of claim 17, wherein step (b) is performed at temperatures above about 60 °C.

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