Thermally conductive dielectric interface

Abstract

Aspects of the present disclosure provide a thermally conductive dielectric interface comprising a multilayer structure that includes a first layer of a polymer matrix filled with 30–95 wt.% thermally conductive filler, and a second layer formed of a fluoropolymer such as a fluorinated ethylene propylene (FEP) film. The interface exhibits high thermal conductivity, excellent electrical insulation, and a high dielectric breakdown strength, while maintaining low hardness to ensure mechanical compliance.

Description

THERMALLY CONDUCTIVE DIELECTRIC INTERFACEFIELD

[0001] The present invention relates to thermally conductive interfaces generally, and more particularly to form-stable thermally conductive constructs with high electrical volume resistivity and low modulus, even at ambient temperatures such as 20 °C. The present invention further relates to methods for manufacturing such thermally conductive constructs.BACKGROUND

[0002] Thermal ly conductive and dielectric materials are highly desirable for various applications, including for power conversion in electric vehicles. In this application, battery packs generate significant excess heat that must be effectively dissipated in order to maintain desired battery performance. Although, some available thermally conductive materials possess the requisite heat transfer properties to accommodate such thermal dissipation loads, the material components that provide the thermal conductivity properties, thermally conductive particulate fillers, tend to reduce electrical volume resistivity. Electrical applications like inverters for power conversion use high voltages that require high volume resistivity characteristics in addition to good thermal conductivity; Conventional thermally conductive materials do not provide the necessary electrical volume resistivity and voltage breakdown properties required by modern power conversion sy stems.

[0003] It is therefore an aspect of the invention to provide a thermally conductive dielectric interface that exhibits a thermal conductivity of at least 1 W / nri K, a volume resistivity of at least 10’2Q*m, and a voltage breakdown level of at least 9kV / mm. The thermally conductive dielectric interface also preferably exhibits a low modulus at room temperature to aid in adhering and conforming to surfaces along a heat dissipation pathway between a heat, generating component and a heat dissipating component.SUMMARY[1)004] By means of the present invention, a thermally conductive interface provides hi gh electrical volume resistivity in a construct that presents a low-modulus mounting surface forconfortnability along a heat dissipation pathway. The thermally conductive interface may be utilized in applications that require both thermal dissipation and an electrical insulation barrier.

[0005] In one embodiment, a thermally conductive dielectric interface includes a first layer having thermally conductive filler dispersed in a polymeric matrix, wherein the thermally conductive filler is present at a concentration of between 30-95 wt.% of the first layer. The first layer has a first surface and a generally opposed second surface, and a hardness of between 20 - 90 Shore OO at 20 °C. The thermally conductive dielectric interface further includes a second layer disposed on at least one of the first and second surfaces of the first layer, wherein the second layer includes a fluoropolymer. A thickness of the interface is defined along a thickness direction that is substantially perpendicular to the first and second surfaces of the first layer. The interface exhibits athermal conductivity of at least 1 W / m*K, a volume resistivity of at least 10i2O*m, and a voltage breakdown of at least 9 kV / mm, each as measured through the interface thickness along the thickness direction.

[0006] In some embodiments, the second layer is disposed on the first surface, and has a second layer thickness of less than 250 p m as measured along the thickness direction. In some embodiments, the second layer thickness is between 20 and 100 pm.

[0007] In some embodiments, the fluoropolymer of the second layer includes fluorinated ethylene propylene.

[0008] In some embodiments, the polymeric matrix of the first layer includes sil icone formed from a resin composition having a molar ratio of hydride-functional siloxane Si~H groups to vinyl-functional siloxane Si-C=C groups in a range of between 0.5:1 and 5:1. In some embodiments, the molar ratio of hydride- functional siloxane Si-H groups to vinyl-functional siloxane Si-C=C groups is in a range of between 1 :1 and 3:1.

[0009] The thermally conductive filler dispersed in the first layer may be selected from metals, metal oxides, ceramics, and combinations thereof, In some embodiments, the thermally conductive filler is grafted with silane.

[0010] In some embodiments, a third layer is disposed on the second layer, such that the second layer is between the first and third layers, and wherein the third layer includes thermally conductive particulate filler dispersed in a polymer matrix, and exhibits a hardness of between 20-90 Shore OO at 20 °C.[00111 A thermally conductive dielectric interface may be arranged for transmitting heat along a thermal dissipation pathway from a heat generating component. In some embodiments, the thermally conductive dielectric interface includes a silicone polymer matrix formed from a resin composition having a molar ratio of hydride-functional siloxane Si-H groups to vinylfunctional si loxane Si-C~C groups in a range of between 0.5:1 and 5:1. The interface further includes thermally conductive filler dispersed in the silicone polymer matrix, and a fluorinated ethylene propylene film having a film thickness of less than 150 pm. The film is disposed on a surface of the silicone polymer matrix.

[0012] In some embodiments, the film thickness is in a range of between 25 and 80 pm.

[0013] The thermally conductive filler may be provided in a range of between 50 and 95 wt.% of the total interface, and may be selected from metals, metal oxides, ceramics, and combinations thereof.

[0014] In some embodiments, the molar ratio of hydride-fonctional siloxane Si~H groups to vinyl -functional siloxane Si-OC groups is in a range of between 1 :1 and 3:1.

[0015] In some embodiments, the thermally conductive dielectric interface may be disposed between the heat-generating component and a heat dissipating component. In some embodiments, the interface may be in contact with each of the heat-generating component and the heat dissipating component.

[0016] A method for forming a thermally conductive dielectric interface includes providing a curable resin composition including thermally conductive filler dispersed therein, and curing the curable resin composition. The method further includes applying a fluoropolymer film to a surface of the cured or curing resin composition. Subsequent to applying the fluoropolymer film to the surface, the method includes aging the thermally conductive dielectric interface at a temperature of between 20-30 °C and a relative humidity of between 40-60% for at least 48 hours.

[0017] In some embodiments, the fluoropolymer film is fluorinated ethylene propylene.

[0018] In some embodiments, the interface exhibits a first volume resistivity prior to the aging, and a second volume resistivity upon completion of the aging, wherein the second volume resistivity is greater than the first volume resistivity'. In some embodiments, the second volume resistivity is at least 1()12Q*m.

[0019] In some embodiments, the curable resin composition includes a vinyl-functional siloxane resin and a hydride-functional siloxane resin. The curable resin composition may include a molar ratio of the hydride-functional si loxane Si-H groups to the vinyl- functional siloxane Si-C^C groups in a range of between 1 :1 and 3:1.

[0020] In some embodiments, prior to dispersing the thermally conductive filler in the curable resin composition, the method includes deionizing the thermally conductive filler. In some embodiments, the thermally conductive filler is deionized to an extent sufficient so that the concentration of sodium ion associated with the thermally conductive filler is less than 15 ppm.

[0021] In some embodiments, the thermally conductive filler is grafted with a silane.BRIEF DESCRIPTION OF THE FIGURES

[0022] Figure 1 is a schematic illustration of an electronic package incorpora ting a thermally conductive dielectric interface.

[0023] Figure 2 is an enlarged isolation view of the thermally conductive dielectric interface illustrated in Figure 1.|0024] Figure 3 is an isolation view of an embodiment of a thermally conductive dielectric interface.

[0025] Figure 4 is an isolation view of an embodiment of a thermally conductive dielectric interface.DETAILED DESCRIPTION

[0026] The inventive thermally conductive dielectric interface may be provided as a gap filler for placement along a thermal dissipation pathway, typically to remove excess heat from a heat-generating electronic component or a power conversion system. The cured thermally conductive dielectric construct exhibits a desired thermal conducti vity of at least 1 W / m*K, as well as desired electrical volume resistivity of at least 10nH*m, and a voltage breakdown of at least 9 kV / mm. The construct preferably exhibits sufficient flexibility and cohesive strength to provide a stable interface.

[0027] Generally, the thermally conductive dielectric interface may be prepared from the following components:(A) a curable resin composition;(B) thermally conductive filler;(C) a fluoropolymer film; and(D) optional additional components.Resin

[0028] The curable resin composition for forming the polymeric matrix of the thermally conductive dielectric interface typically includes a liquid monomer, oligomer, or a polymer that is capable of undergoing a cross-linking reaction to form a network. The cure reaction may occur upon the mixing of two initially separated reactable components, and / or upon exposure of the curable resin to a cure initiator. The cure initiator may be a chemical cure initiator and / or an environmental cure initiator. Preferably, a cure reaction is initiated with exposure between the curable component(s) and the cure initiator, in some cases when in the presence of an environmental cure reaction facilitator, such as water, heat, pressure, electromagnetic radiation, and the like. For the purposes hereof, the presence of an environmental cure facilitator is assumed if necessary to the cure reaction initiated with exposure between the curable component(s) and the cure initiator.

[0029] In some embodiments, the chemical cure initiator is an agent that is initially separated from the curable component to avoid a cure reaction, and subsequently introduced to the curable component when the cure reaction is desired, In some embodiments, the chemical cure initiator includes a cross-linking agent. The chemical cure initiator may also or instead include a monomer, oligomer, a polymer, a reaction initiator, a catalyst, and combinations thereof. The curable component may include a first resin, and the cure initiator may include a second resin that is reactive with the first resin. In some embodiments, the .matrix may comprise an organic matrix including at least one of a thermoplastic and / or a thermoset resin. Some example resins for forming the polymer matrix include elastomers comprising one or more of a silicone, an acrylic, a natural rubber, a synthetic rubber, or other appropriate elastomeric materials.

[0030] In some embodiments, one or more of the curable resins may be in liquid form at 20 °C and 1 bar pressure, and exhibit a viscosity of less than 500 cP at 20 °C at a shear rate of 1 s'!. In some embodiments, one or more of the curable resins may be in liquid form at 20 °C and 1 bar pressure, and exhibit a viscosity of less than 200 cP at 20 °C at a shear rate of 1 s"1.

[0031] The curable resin system may be selected from a wide variety of monomers and oligomers wherein the terra “resin” may include any natural or synthetic organic compound or mixture that is convertible into a polymer. An example cure reaction suitable for the curable systems of the present invention is the hydrosilylation from vinyl and hydride functional siloxane resins. An example silicone may be an organosiloxane having the structural formula:wherein “x” represents an integer ranging from between 1 and 1,000. The thermally conductive interface material may be prepared as a reaction product of the organosil oxane together with a chain extender / cross-linker such as a hydride functional polydimethyl siloxane having the structural formula:wherein “x” and “y” each represent an integer having a value of between 1 and 1,000.

[0032] Generally, the polymer matrix is formed from a curable composition of a first reactant including siloxane, and a second reactant composition that is reactive with the first reactant to form a silicone. Organosiloxanes useful in a first reactant composition may include at least two aliphatic, unsaturated organic groups such as vinyl, allyl, butenyl, hexenyl, ethenyl, and propenyl. The unsaturated functional groups may be located at terminal or pendant positions.

[0033] An example first reactant composition for a curable mixture of the present invention includes polydiorganosiloxanes, such as various vinyl or siloxy -terminated polydimethylsiloxanes (PDMS), Example commercially-available PDMS materials include Nusil PLY-7500 and 7905 available from Avantor, Inc.; Evonik VS 100, 200, 500, 10000, 20000, and 65000 available from Evonik Industries AG; and Gelest DMS-V21, V22, V41, V42, and V43available from Gelest, Inc. The curable component may include one or more polymers that differ in, for example, molecular weight, viscosity, and molecular structure,

[0034] The chemical cure initiator may itself be reactive with the first reactant, and may include a cross-linker for a hydrosilylation reaction. The second reactant, or chemical cure initiator, may include a dihydroxy aliphatic chain extender such as a bydride-terminated polydimethylsiloxane. The silicon-bonded hydrogen atoms may be located at terminal, pendant, or at both terminal and pendant positions. The second reactant may include one or more organohydrogenpolysiloxanes that, may differ in at least one of molecular weight, viscosity, and molecular structure. Example commercially-available niethylhydropolydimethylsiloxanes useful as a chemical cure initiator that is reactive with the first reactant composition include Nusil XE- 112 and XL-7505 available from Avantor, Inc.; Gelest HMS-071, 082, and 991 available from Gelest, Inc,; and Andisil XL- IB and 1340 available from AB Specialty Silicones.

[0035] In some embodiments, the first reactant resin composition includes a vinyl-functional siloxane resin, and the second reactant resin composition includes a hydride-functional siloxane resin. The reactants may be provided in a relative molar ratio, wherein the molar ratio of hydride- functional siloxane Si-H groups to vinyl-functional siloxane Si-OC groups is in a range of between 0.5: 1 to 5:1. In some embodiments, the molar ratio of hydride-functional siloxane Si-H groups to vinyl-functional siloxane Si-C-C groups is in a range of between 0.7:1 to 4:1, In some embodiments the molar ratio of hydride- functional siloxane Si-H groups to vinyl-fiinctiona! siloxane Si-OC groups is in a range of between 1 : 1 to 3:1.

[0036] The relative molar ratios of the hydride-functional siloxane groups to vinyl-functional siloxane groups described herein are useful in producing polymer matrices that exhibit desired physical properties. For example, the polymer matrices of the present invention preferably exhibit relatively low modulus values, to provide conformability to uneven thermal surfaces along the thermal dissipation pathway, as well as to provide tackiness for the film layer of the present constructs, while nevertheless being form stable at room and operating temperatures.

[0037] In some embodiments, the polymeric matrix of the present thermally conductive dielectric interface may exhibit a hardness of between 20-90 Shore OO at 20 °C. In some embodiments, the polymeric matrix exhibits a hardness of between 30-90 Shore OO at 20 °C. In some embodiments, the polymeric matrix exhibits a hardness of between 50-90 Shore OO at 201°C. In some embodiments, the polymeric matrix exhibits a hardness of between 60-90 Shore OO at 20 °C,

[0038] In some embodiments, the polymeric matrix of the present thermally conductive dielectric interface may exhibit high temperature stability, such as being resistant to breakdown at elevated temperatures up to 250 °C. The temperature stability of the polymeric matrix may be characterized by thermogravimetric analysis, which measures the temperature at the onset of degradation (5% mass loss). In some embodiments, the polymeric matrix of the present invention exhibits a temperature stability of at least 250 °C. The vinyl-hydride silicones described above may preferably exhibit such a temperature stability.

[0039] A variety of silane or silanol terminated resins may be employed in the matrices of the present invention. Condensation-curable silane- or silanol-terminated resins participate in a hydrolysis-condensation cure pathway, preferably at and above ambient temperatures. In some embodiments, the resins are non-silicone, wherein no more than a trace amount of silicone is contained in the composition. In some embodiments, no silicone is contained in the composition.

[0040] Example resins suitable for the curable component of the present invention include reactive polymer resins with at least one silyl-reactive functional group, including at least one bond that may be activated with water. Example silyl-reactive functional groups include alkoxy silane, acetoxy silane, and ketoxime silane.

[0041] The reactive polymer resin can be any polymer capable of participating in a silyl hydrolyzation reaction. For example, the reactive polymer resin can be selected from a wide range of polymers as polymer systems that possess reactive silyl groups, for example a silyl- modified reactive polymer. Preferably, the silyl-modified reactive polymer has a flexible, backbone for lower modulus and glass transition temperature. Preferably, the silyl-modified reactive polymer has a flexible backbone of poiyether, polyester, polyurethane, polyacrylate, polyisoprene, polybutadiene, polystyrene-butadiene, or polybutylene-.isopre.ne,

[0042] The silyl-modified reactive polymer can be obtained by reacting a polymer with at least one ethylenically unsaturated silane in the presence of a radical starter, the ethylenically unsaturated silane carrying at least one hydrolyzable group on the silicon atom. For example, the silyl modified reactive polymer can be dimethoxysiiane modified polymer, trimethoxysilane modified polymer, or triethoxysilane modified polymer, Tor example, the silyl modified reactivepolymer may include a silane modified polyether, polyester, polyurethane, polyacrylate, polyisoprene, polybutadiene, polystyrene-butadiene, or polybutylene-isoprene.

[0043] The ethylenically unsaturated silane may be selected from the group made up of vinyltrimethoxysilane, vinyltriethoxysilane, vinyldimethoxymethylsilane, vinyldiethoxymethylsilane, trans-p-methylacrylic acid trimethoxysilylmethyl ester, and trans-jf- methyiacrylic acid trimethoxysilylpropyl ester.

[0044] The silyl -modified reactive polymer preferably comprise^) silyl groups having at least one hydrolyzable group on the silicon atom in a statistical distribution. For example, the silyl-modified reactive polymer can be a silane-modified polymer of general formula:in which; R is a mono- to tetravalent polymer radical, R1,R2,R3independently is an alkyl or alkoxy group having 1 to 8 carbon atoms, and A represents a carboxy, carbamate, amide, carbonate, ureido, urethane or sulfonate group or an oxygen atom, x = 1 to 8 and n = 1 to 4.

[0045] The silyl-modified reactive polymer can also be obtained by reacting a polymer with hydroxy group and alkoxysilane with isocyanate group. For example, the silyl modified reactive polymer can be dimethoxysilane modified polyurethane polymer, trimethoxysilane modified polyurethane polymer, or triethoxy silane modified polyurethane polymer. Further, the silyl- modified reactive polymer can be a a-ethoxysilane modified polymer of the average general formula:in which R is a mono- to tetravalent polymer residue, at most one third of the polymer of formula contained residues Rl> R2and RJare independently alkyl radicals having 1 to 4 carbon atoms, at least one-quarter of the polymer of the formula residues contained R1, R'\ and R3are independently ethoxy residues that any remaining radicals Rl, R2, and RJindependently of one another are methoxy radicals, and wherein n 1 to 4,

[0046] Silyl -modified reactive polymers are available, for example, as dimethoxysilane modified MS polymer with polyether backbone and XMAPTMpolymer with polyacrylate backbone from Kaneka Belgium NV, trimethoxysilane modified ST polymer from Evonik, triethoxy silane modified Tegopac™ polymer from Evonik, silane modified Desmoseal™ polymer from Covestro, or di- or tri- methoxy silane modified Geniosil™ polymer from Wacker.Filler

[0047] In order to provide desired thermal conductivity properties to the interfaces of the present invention, thermally conductive filler is preferably dispersed in the polymeric matrix. The thermally conductive fillers contemplated for use in the preparations of the present invention include metals, metal oxides, ceramics, and combinations thereof. Example conductive fillers include boron nitride, aluminum nitride, alumina, alumina trihydrate, silicon, silicon carbide, graphite, diamond, magnesium oxide, magnesium hydroxide, zinc oxide, gold, silver, copper, platinum, palladium, nickel, aluminum, indium, alloy of nickel (e.g., alloy 42), alloy of zinc, alloy of iron, alloy of indium, silver-plated copper, silver-plated aluminum, bismuth, tin, bismuth-tin alloy, silver-plated fiber, silver-plated graphite, silver-plated silicon carbide, silver- plated boron nitride, silver-plated diamond, silver-plated alumina, silver-plated alloy 42, graphene, si lver-plated graphene, silver-coated polymer, cadmium and alloys of cadmium, lead and alloys of lead, antimony and alloys of antimony, and the like, as well as mixtures of any two or more thereof.

[0048] In some embodiments, the thermally conductive fi ller is electrically insulating in order to promote an overall electrical resistivity to the interface construct. Example electrically insulating and thermally conductive fillers include boron nitride, aluminum nitride, alumina, and alumina trihydrate. In some embodiments, the thermally conductive filler is alumina.

[0049] The thermally conductive fillers may be of various shapes and size, and typically has a mean particle size (dso) in a range of between 0.1 and 200 pm. In some embodiments, the thermally conductive fillers have a mean particle size (dso) in a range of between 1 and 100 pm. In some embodiments, the distribution of thermally conductive filler particles is not a mono dispersion, but rather a particle size distribution. In some embodiments, the particle size distribution is multi-modal, including a mixture of relatively small particles and relatively large particles, within the size ranges described above. For the purposes hereof, the term “meanparticle size” refers to a cumulati ve weight average value (djo) 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.

[0059] In some embodiments, of the total weight of thermally conductive filler, at least 30 wt.% of the filler has a mean particle size in a range of between 50 and 1.00 pm, and at least 30 wt.% of the filler has a mean particle size in a range of between 0>l and 25 pm. In some embodiments, at least 40 wt.% of the filler has a mean particle size in a range of between 50 and 100 pm, and at least 30 wt.% of the filler has a mean particle size in a range of between 0.1 and 25 pm. In some embodiments, at least 50 wt.% of tile filler has a mean particle size in a range of between 50 and 100 pm, and at least 30 wt.% of the filler has a mean particle size in a range of between 0.1 and 25 pm. In some embodiments, at least 30 wt.% of the filler has a mean particle size in a range of between 50 and 100 pm, and at least 40 wt.% of the filler has a mean particle size in a range of between 0.1 and 25 pm. In some embodiments, at least 40 wt.% of the filler has a mean particle size in a range of between 50 and 100 pm, and at least 40 wt.% of the filler has a mean particle size in a range of between 0.1 and 25 pm. In some embodiments, at least 50 wt.% of the filler has a mean particle size in a range of between 50 and 100 pm, and at least 40 wt.% of the filler has a mean particle size in a range of between 0.1 and 25 pm. In some embodiments, at least 30 wt.% of the filler has a mean particle size in a range of between 50 and 100 pm, and at least 50 wt.% of the filler has a mean particle size in a range of between 0.1 and 25 pm. In some embodiments, at least 40 wt.% of the filler has a mean particle size in a range of between 50 and 100 pm, and at least 50 wt.% of the filler has a mean particle size in a range of between 0.1 and 25 pm.

[0051] The shape of the particulate filler may be spherical, aspherical, and combinations thereof. Example aspherical shapes include flake-like, plate-like, rod-like, and so on. Spherical particulate filler may have an aspect ratio of between 0.8-1 .2.

[0052] The thermally conductive filler is present in an amount sufficient to provide the dielectric interface with a thermal conductivity of at least 1 W / m*K by ASTM 5470. I n some embodiments, the dielectric interface exhibits a thermal conductivity of at least 2 W / m*K. In some embodiments, the dielectric interface exhibits a thermal conductivity of at least 3 W / m*K.

[0053] The thermally conductive particles may be dispersed in at least one of the first and second reactant compositions to achieve a loading concentration of between 10-95 wt.% of thecurable composition. In some embodiments, the thermally conductive particles may be dispersed, in at least one of the first and second reactant compositions to achieve a loading concentration of between 30-95 wt.% of the curable composition. In some embodiments, the thermally conductive particles may be dispersed in at least one of the first and second reactant compositions to achieve a loading concentration of between 50-95 wt.% of the curable composition.

[0054] In some embodiments, the thermally conductive filler may be deionized prior to dispersion in the polymer matrix. In some embodiments, the thermally conductive filler may be deionized prior to dispersion in the polymer matrix by washing the filler particles with deionized water. The particles may be washed by deionized water by placing the particles in a container, and adding deionized water with a temperature of between 25-99 °C to the container and stirring the mixture. In some embodiments, the deionized water is between 25-50 °C. In some embodiments, the extent of deionization can be set so that tire wash water exhibits an electrical conductivity of less than 25 pS after stirred contact with the particles for 5 minutes. Repeated washings of the particles by decanting and re-exposure to fresh deionized water may be necessary to achieve sufficient deionization of the particles. In some embodiments, the thermally conductive filler is deionized to an extent sufficient so that the concentration of sodium ion associ ated with the wash water of the thermally conducive filler is less than 15 ppm.

[0055] In some embodiments, the thermally conductive filler may be surface treated with organic modifiers to improve the dispersion properties of the particulate filler in the curable compositions. In some embodiments, the thermally conductive filler may be surface treated with a silane, such as one or more of organosilane, organosilazane, and organopolysiloxane. In some embodiments, the silane may contain between 3 and 20 carbon atoms. In some embodiments, the thermally conductive filler is grafted with silane to an extent sufficient so that the thermally conductive filler is hydrophobic. For the purposes hereof, the term “hydrophobic” is intended to mean that the thermally conductive filler tends to repel water. Film[0056 J A fluoropolymer film is used in combination with the polymeric matrix as described above to yield the desired interface properties of the present invention. Applicant has found that, by applying a thin fluoropolymer film to the base polymeric matrix, surprisingly high electrical volume resistivities and voltage breakdown values can be achieved for the thermally conductiveinterface. Moreover, the surprising dielectric properties can be achieved without significant sacrifice to the thermal conductivity and conformability of the interface.

[0057] In some embodiments, the fluoropolymer used in the film is a copolymer of fluorocarbon. Example fluoropolymers useful in the film of the present interfaces include copolymers of tetrafluoroethylene and hexafluoropropylene, copolymers of tetrafluoroethylene and perfluoroethers, and copolymers of ethylene and tetrafluoroethylene. In an embodiment, the fluoropolymer film is a fluorinated ethylene propylene (FEP). The FEP film may be, in some embodiments, made from Teflon™ FEP- 100 by The Chemours Company FC, LLC.

[0058] The fluoropolymer film may a thickness as applied to the base polymeric matrix of less than 250 pm. In some embodiments, the fluoropolymer film has a thickness of between 5- 250 pm. In some embodiments, the fluoropolymer film has a thickness of between 10-150 pm. In some embodiments, the fluoropolymer film has a thickness of between 20-100 pm. In some embodiments, the fluoropolymer film has a thickness of between 25-80 pm.

[0059] The fluoropolymer film is preferably of a thickness and material that exhibits a flexural modulus by ASTM D790 in a range of between 0.1 -5 GPa at 20 °C. In some embodiments, the fluoropolymer film exhibits a flexural modulus in a range of between 0,3-3.5 GPa at 20 °C. In some embodiments, the fluoropolymer film exhibits a flexural modulus in a range of between 0.5-3.0 GPa at 20 °C.

[0060] The fluoropolymer film preferably exhibits a volume resistivity by ASTM D257 of at least 10!5£2*cm. In some embodiments, the fluoropolymer film exhibits a volume resistivity of at least 10i&£2*cm. In some embodiments, the fluoropolymer film exhibits a volume resistivity of al least 1017O*cm.

[0061] The fluoropolymer film may include fillers, such as thermally conductive fillers, electrically insulating fillers, and the like to aid in providing desired thermal, electrical, and physical properties.Additional Components

[0062] In some embodiments, the curable compositions may include a cure activator such as a reaction catalyst. A reaction catalyst may. for example, be employed to further facilitate the hydrosilylation reactions described above. Example reaction catalysts useful in the compositions of the present invention include platinum compounds, iridium compounds, titanium compounds,rhodium compounds, and organotin and organo-zinc and organo-titanium compounds (together referred to herein as “organo-metal catalyst”) that facilitate moisture cure of the reactive resins.

[0063] Reaction catalysts used in the compositions of the present invention may be present in a catalytic amount, including in the range of 0 up to 1 percent by weight. In some embodiments, the compositions comprise in the range of 0.01 up to 0,5 percent by weight reaction catalyst. In some embodiments, the compositions comprise in the range of 0.01 up to 0.1 percent by weight reaction catalyst. In some embodiments, the compositions comprise in the range of 0.01 up to 0.05 percent by weight reaction catalyst. Typically, the reaction catalyst is present in an amount required for curing the composition, which is referred to as the catalytic amount. In some embodiments, the cataly st may be present in an amount of between 0,1 and 1000 ppm mass of the curable siloxane components. In some embodiments, the catalyst may be present in an amount of between 0.1 and 500 ppm mass of the curable siloxane components. In some embodiments, the catalyst may be present in an amount of between 0.1 and 100 ppm mass of the curable siloxane components. In some embodiments, the catalyst may be present in an amount of between 1 and 50 mass of the curable siloxane components.

[0064] The curable compositions of the present invention may be curable in the presence of water (moisture curable) at ambient temperature. Depending upon the application, the moisture may be available from the ambient environment or from water released from the object(s) to which the composition is applied. In some embodiments, the compositions of the invention are curable without addition of environmental moisture. In some embodiments, water may be included as an ingredient in a non-resin part of the multiple part curable composition, for mixture with the reactive constituents in situ. Preferably, however, the amount of water required in the composition itself is minor so as not to interfere with functional properties of the thermal material. In some embodiments, water is present in the compositions of the invention in the range of 0 up to 0.5 wt%. In some embodiments, the compositions comprise in the range of 0.01 up to 0.3 wt% water. In some embodiments, the compositions comprise in the range of 0.01 up to 0.2 wt% water.

[0065] For the purposes hereof the term “ambient temperature” is intended to mean the temperature of the environment within which the reaction occurs, and within a temperature range of 15-30 °C, and preferably 25 °C. The thermally conductive compositions are curable at ambient temperature within 72 hours, and preferably within 24 hours. The thermally conductivecompositions may also be curable at elevated temperatures. For the purposes hereof, the term ‘‘curable” is intended to mean reactable to form a solidous body.

[0066] In order to control the curing speed of the curable compositions, a curing reaction inhibitor may be included. Inhibitors useful in the present invention include inhibitors for hydrosilylation, such as acetylene-based compounds including acetylenic alcohols, fumarate- based compounds, and maleate-based compounds. Although there are no limitations on the amount of curing reaction inhibitor used in the compositions of the present invention, example amounts include between 1 and 10,000 ppm mass of the siloxane components. In some embodiments, the curing reaction inhibitor is present in an amount of between .10 and 5,000 ppm mass of the siloxane components. In some embodiments, the curing reaction inhibitor is present in an amount of between 20 and 1,000 ppm mass of the siloxane components.

[0067] The thermally conductive dielectric interfaces of the present invention may further include one or more additional components, such as an adhesion promoter, a silicone diluent, a reactive diluent, a colorant, a corrosion inhibitor, an acid receptor, silica, carbon black, glass beads, and combinations thereof. In some embodiments, one or more adhesion promoters may be present in an amount effective for building chemical bonding at a joining location between the thermally conductive interface and a substrate, such as a thermal dissipation surface. Examples of suitable adhesion promoters include organosilanes, including monosilanes, dipodal silanes, tripodal silanes, and oligomer silanes with methoxy, ethoxy, and / or propyloxyl structure. An example alkoxysilane is an epoxy-functional alkoxysilane. Other suitable adhesion promoters include organotitanates and mercapto- functional compounds.

[0068] Adhesion promoters may be present in the compositions in an amount of between 0 and 5 wt.% of the composition. In some embodiments, an adhesion promoter is present in an amount of between 0 and 2 wt.% of the composition. In some embodiments, an adhesion promoter is present in an amount of between 0.1 and 1 .5 wt.% of the composition.

[0669] The compositions of the present invention may be used after being cured. In some embodiments, the thermally conductive compositions may be cured at a temperature from about room temperature to about 250 °C. In some embodiments, the thermally conductive compositions may be cured when heated to a tempera ture of between 70 °C to about 200 °C. In some embodiments, the thermally conductive compositions may be cured when heated to a temperature of between 125 °C to about 190C,C.1.5

[0070] A curing time may be at least one minute. In some embodiments, the curing time is less than 250 minutes. In some embodiments, the curing time is between 1 and 200 minutes. In some embodiments, the curing time is between 1 and 150 minutes.

[0071] An electronic package 110 incorporating a thermally conductive dielectric interface 120 of the present invention is illustrated in Figure 1 . Electronic package 110 may be a battery system including a heat-generating component as a battery cell 112. Electronic package 110 further includes a heat dissipater 118, wherein thermally conductive dielectric interface 120 may be positioned in a heat dissipation pathway (designated by dashed arrow 122) between batery cell 112 and heat dissipater 118. Heat dissipation pathway 122 may, in some embodiments, be parallel to a thickness direction 128 of thermally conductive dielectric interface 120. Electronic package 110 is arranged to dissipate thermal energy generated by the battery cell 112 by providing a highly thermally conductive path from battery cell 11.2 to heat dissipater 118, while also electrically insulating battery cell 112 from other components of the system, including heat dissipater 118. Electronic package 110 is an example arrangement that may be modified as appropriate to accommodate a variety of electronic applications, such as data processors, power converters, data memory, communication boards, antennae, and the like.

[0072] An isolation view of thermally conductive dielectric interface 120 is illustrated in Figure 2, and includes first layer 130 having a first surface 132 and a generally opposed second surface 134. First layer 130 is comprised of thermally conductive filler dispersed in a polymeric matrix. A second layer 136 is illustrated as being disposed on first surface 132 of first layer 130. Second layer 136 is comprised of a fluoropolymer, and in some embodiments, a fluorinated ethylene propylene. Second layer 136 is provided with a thickness Tj as measured along thickness direction 128, which thickness direction 128 may be substantially perpendicular at least one of the first and second surfaces 132, 134 of first layer 130.

[0073] In some embodiments, thickness Ti may be less than 250 pm as measured along thickness direction. 128. In some embodiments, thickness Ti may be less than 200 pm as measured along thickness direction 128. In some embodiments, thickness Ti may be between 20 and 100 pm as measured along thickness direction 128. Applicant has found that thickness TJ. of less than 20 pm may fail to provide sufficient dielectric properties to dielectric interface 120. Moreover, applicant, has found that Thickness Ti of greater than 250 pin may interfere with the thermal conductivity and modulus properties of dielectric interface 120.

[0074] Another thermally conductive dielectric interface 320 is illustrated in Figure 3, and includes first layer 330 having a first surface 332 and a generally opposed second surface 334. A second layer 336 is disposed on first surface 332 and second surface 334 of first layer 330. Such an embodiment may be utilized to provide increased volume resistivity and voltage breakdown of the dielectric interface 320. Second layer 336 may exhibit lower tackiness than first layer 334, wherein interface 320 may be suitable in applications that do not require strong adherence of interface 320 to other surfaces along a thermal dissipation pathway.

[0075] A further embodiment is illustrated in Figure 4. wherein thermally conductive dielectric interface 420 includes a second layer 436 disposed between portions of first layer 430. in this embodiment, first layer 430 may be divided into two portions 430a, 430b, which are at least partially separated by second layer 436. In some embodiments, thickness T2 of first portion 430a may be different from thickness Ta of second portion 430b. Moreover, first portion 430a may contain different filler material and concentration than that of second portion 430b.

[0076] In some embodiments, a thermally conductive dielectric interface may be formed by curing a curable resin composition including thermally conductive filler dispersed therein, and applying a fluoropolymer film to a surface of the cured or curing resin composition. The curable resin composition may be cured by an appropriate curing mechanism. In some embodiments, the curable resin composition may be cured in a hydrosilylation reaction by mixing a two-part composition in the presence of a cure reaction catalyst and ambient moisture. In some embodiments, additional water may be added to facilitate the cure reaction.

[0077] The fluoropolymer film may be applied to the resin composition during the cure process by dispensing the curable resin composition upon the fluoropolymer film, and the combination is subsequently placed in a hot press for curing. Due to an inherent tackiness of the curable resin composition after curing, the fluoropolymer film remains adhered to a surface of the cured composition.

[0078] In some embodiments, the fluoropolymer film may be surface modified through various methods such as plasma treatment prior to lamination in order to increase the wettability and therefore improve the adhesion with cured resin compositions.

[0079] The fluoropolymer film may be applied to the resin composition subsequent to curing by laminating the fluoropolymer film to a surface of the cured composition with, for example, a film roller.

[0080] It has been found that, by aging the thermally conductive dielectric interface, initial dielectric properties of the interface are surprisingly enhanced. In particular, the thermally conductive interface, upon cure of the curable resin composition with a secured fluoropolymer film in accordance with this disclosure, but prior to aging, the thermally conductive dielectric interlace exhibits a first electrical volume resistivity, as measured by ASTM D257. Subsequent to exposing the thermal ly conductive dielectric interface to aging condi tions of between 20-30 °C and a relative humidity of between 40-60% for at least 48 hours, the thermally conductive dielectric interface exhibits a second electrical volume resistivity that is greater than the first volume resistivity. In some embodiments, the increase in electrical volume resistivity subsequent to aging is at least 5%. In some embodiments, the increase in electrical volume resistivity subsequent to aging is at least 10%. In some embodiments, the increase in electrical volume resistivity subsequent to aging is at least 30%. In some embodiments, the increase in volume resistivity subsequent to aging is at least one order of magnitude inunits.

[0081] A particularly surprising aspect is that, as the molar ratio of hydride-functional siloxane Si-H groups to viny l -functional siloxane Si-C^C groups in the curable resin composition increases to within a range, a rate of increase in volume resistivity upon aging of the thermally conductive dielectric interface correspondingly increases. In some embodiments, a molar ratio of hydride-functional siloxane Si-H groups to vinyl-functional siloxane Si-C=C groups in a range of between 1 :1 and 3:1 demonstrates increased dielectric properties, including electrical volume resistivity, subsequent to aging of the thermally conductive dielectric interface. For the purposes hereof, the term ‘'aging” is intended to mean storing the cured thermally conductive dielectric interface in aging conditions for a designated period of time. In some embodiments, the aging conditions include a temperature of between 20-30 °C, a relati ve humidity of between 40-60%, and a storage time of at least 48 hours.EXAMPLESExample 1

[0082] A two-part curable resin composition was prepared as follows:Part A Component Wt.%Vinyl-terminated polydimethylsiloxane 8.4Platinum catalyst 0.08Reaction inhibitor 0.07Alumina (spherical; dso = 70 pm) 45.9Alumina (spherical; dso - 10 pm) 14.9Alumina (spherical; dso:;T7 pm.) 30.6Part B Component Wt.%Di-fonctional hydride-terminated siloxane 8.2Pendant hydride multi-functional siloxane 0.2Vinyl terminated siloxane 0.2Alumina (spherical; dso = 70 pm) 45.9Alumina (spherical: dso ~ 10 urn) 14.9Alumina (spherical; dso ~ 7 pm) 30.6

[0083] Prior to dispersing the alumina particles in the curable resin, 50 kg of the alumina particles were placed in a 30-gallon drum. The drum was then filled with deionized water at 50 °C, and the contents stirred for 5 minutes. The water was then tested with a multimeter for electrical conductivity, with a “pass” threshold of 25 pS. If the water tested at or above 25 pS, the mixture was allowed to settle, the water decanted, and the process repeated with new deionized water at 50 °C. Once the water tested below 25 pS, the mixture was allowed to settle, the water decanted, and the alumina particles were moved to an oven at 93 °C for 2 hours to dry.

[0084] Following deionization, the alumina particles were treated with silane. A silane solution was prepared by blending 1 wt.% octyltriethoxysilane with distilled water at ambient temperature. The pH of the silane solution was adjusted to 5.0 with acetic acid, and stirred for 60 minutes for the hydrolysis of the methoxy groups. Thereafter, the alumina particles were added to the si lane solution and stirred for 10 minutes. The alumina particles were then moved to an oven at 93 °C for 1 hour to dry. Subsequent to drying, the silanized particles were dispersed in the curable resin composition by hand mixing.

[0085] A fluorinated ethy lene propylene (FEP-100) film with a thickness of 40 pm was placed in an 8 inch x 8 inch mold. Parts A and B were dispensed from a 2K cartridge onto the film to a thickness of I -3mm. The mold was then closed and placed between pl atens of a hot press set at 85 °C. The platens of the hot press were applied to the mold with 5 tons of force for 2hours, and thereafter the mold was cooled to 25 °C under ambient conditions. The cured resin with attached film was then aged for 48 hours al 25 (+ / - 3) °C and 50 (+ / - 10)% relative humidity.Dielectric Property TestingThe following samples were prepared for testing of dielectric properties:A = Polymer matrix prepared in accordance with Example 1 ; no film layer.B:= Polymer matrix prepared in accordance with Example 1 ; ceramic-filled polyimide filmC Polymer matrix prepared in accordance with Example I; polyimide filmD Polymer matrix prepared in accordance with Example 1; FEP-100 film

[0086] The samples were tested by ASTM D257 for electrical volume resistivity at. the following intervals: (i) upon cooling to 25 °C after cure; (ii) upon 48 hours aging as set forth above; and (iii) upon 216 hours aging as set forth above.Table 1

[0087] The volume resistance testing demonstrated that electrical volume resistivity of the fluoropolymer FEP-100 film / polymer matrix combination (Sample D) surprisingly dramatically increased with aging, while the control samples (Samples B and C) did not show appreciable volume resistivity increase with aging.

[0088] Additional samples were prepared to analyze the effect of the molar ratio of hydride- functional siloxane groups to vinyl-functional siloxane groups (h:v ratio) on volume resistivity (ASTM D257) and voltage breakdown value (ASTM D 149) over different aging periods:E polymer matrix with h:v ratio of 3.07, FEP-100 film layer F = polymer matrix with h:v ratio of 1.96, FEP-100 film layer G ~ polymer matrix with h:v ratio of 0.84, FEP-100 film layerTable 2Table 3

[0089] The sample with an H / V ratio of 1 .96 showed the largest increa.se in volume resistivity over the Aging period.

Claims

CLAIMS1. A thermally conductive dielectric interface, comprising: a first layer having thermally conductive filler dispersed in a polymeric matrix, wherein the thermally conductive filler is present at a concentration of between 30-95 wt.% of the first layer, the first layer having a first surface and a generally opposed second surface, and a hardness of between 20-90 Shore 00 at 20 °C; a second layer disposed on at least one of the first and second surfaces of the first layer, the second layer comprising a fluoropolymer, and a thickness of the interface being defined along a thickness direction that is substantially perpendicular to the first and second surfaces of the first layer, wherein the interface exhibits a thermal conductivity of at least 1 W / m*K» a volume resistivity of at least. IO12Q*m, and a voltage breakdown of at least 9 kV / mm, each as measured through the interface thickness along the thickness direction.

2. The thermal ly conductive dielectric interface as in Claim 1 wherein the second layer is disposed on the first surface, and has a second layer thickness of less than 250 pm as measured along the thickness direction.

3. The thermally conductive dielectric interface as in Claim 2 wherein the second layer thickness is between 20 and 100 pm.

4. The thermally conducti ve dielectric interlace as in Claim 2 wherein the fluoropolymer of second layer includes fluorinated ethylene propylene.

5. The thermally conductive dielectric interface as in Claim 1 wherein the polymeric matrix of the first layer includes si licone formed from a resin composition having a molar ratio of hydride- functional siloxane Si-H groups to vinyl-functional siloxane Si-C::::C groups in a range of between 0.5:1 and 5:1.

6. The thermally conductive dielectric interface as in Claim 5 wherein the molar ratio of Si-H groups to Si-OC groups is in a range of between 1:1 and 3:1.

7. The thermally conductive dielectric interface as in Claim 1 wherein the thermally conductive filler is selected from metals, metal oxides, ceramics, and combinations thereof.<8. The thermally conductive dielectric interface as in Claim 7 wherein the thermally conductive filler is grafted with silane.

9. The thermally conductive dielectric interface as in Claim .1 , including a third layer disposed on the second layer, such that the second layer is between the first and third layers, wherein the third layer includes thermally conductive particulate filler dispersed in a polymer matrix, and exhibits a hardness of between 20-90 Shore 00 at 20 °C.

10. A thermally conductive dielectric interface for transmitting heat along a thermal dissipation pathway from a heat generating component, the interface comprising: a silicone polymer matrix formed from a resin composition having a molar ratio of hydride-functional siloxane Si-H groups to vinyl-functional siloxane Si-C::::C groups in a range of between 0,5:1 and 5:1 ; thermally conductive filler dispersed in the silicone polymer matrix; and a fluorinated ethy lene propylene film having a fi lm thickness of less than 150 pm and disposed on a surface of the silicone polymer matrix.11 . The thermally conductive dielectric interface as in Claim 10 wherein the film thickness is in a range of between 25 and 80 pm.

12. The thermally conductive dielectric interface as in Claim 10, including between 50 and 95 wt.% of the thermally conductive filler, selected from metals, metal oxides, ceramics, and combinations thereof.

13. The thermally conductive dielectric interface as in Claim 10 wherein the molar ratio of hydride-functional siloxane Si~H groups to vinyl-functional siloxane Si-OC groups is in a range of between 1 :1 and 3:1 .

14. The thermally conductive dielectric interface as in Claim 10 wherein the interface is disposed between the heat-generating component and a heat dissipating component.

15. The thermally conductive dielectric interface as in Claim 14 wherein the interface is in contact with each of the heat-generating component and the heat dissipating component.

16. A method for forming a thermally conductive dielectric interface, the method comprising:(a) providing a curable resin composition including thermally conductive filler dispersed therein;(b) curing the curable resin composition;(c) apply ing a fluoropolymer film to a surface of the resin composition;(d) subsequent to step (c), aging the thermally conductive dielectric interface at a temperature of between 20-30 °C and a relative humidity of between 40-60% for at least 48 hours.

17. The method as in Claim 16 wherein the interface exhibits a first electrical volume resistivity prior to the aging, and a second electrical volume resistivity upon the completion of the aging, wherein the second electrical volume resistivity is greater than the first electrical volume resistivity.

18. The method as in Claim 1.7 wherein the second volume resistivity is at least 1012Q*m.

19. The method as in Claim 18 wherein the curable resin composition includes a vinyl -functional siloxane resin and a hydride-functional siloxane resin.

20. The method as in Claim 19 wherein the curable resin composition includes a molar ratio of Si-H groups of the hydride-functional siloxane resin to Si-C C groups of the vinyl-functional siloxane resin in a range of between 1:1 and 3:1.21 . The method as in Claim 17, including, prior to dispersing the thermally conductive filler in the curable resin composition, deionizing Che thermally conductive filler.

22. The method as in Claim 22 wherein the thermally conductive filler is grafted with a silane to an extent sufficient so that the thermally conductive filler is hydrophobic.

23. The method as in Claim 16, including applying the fluoropolymer film to the surface of the resin composition subsequent to cure of the curable resin composition.

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