Thermally conductive wafer bonding film

Abstract

The present invention relates to a thermally conductive curable composition useful as a conductive wafer bonding adhesive. The curable compositions exhibit high thermal conductivity in excess of 5 W / m * K while also exhibiting relatively low melt viscosities over a wide temperature range. The low melt viscosity facilitates good substrate wettability and adhesion in semiconductor wafer bonding applications.

Description

TECHNICAL FIELD

[0001] The present invention relates generally to thermally conductive adhesives, and more particularly to thermally conductive die attach films and curable compositions for producing thermally conductive die attach films. The present invention also relates to electronic packages using the thermally conductive adhesives. BACKGROUND

[0002] Attaching electronic components, such as semiconductor dies, to substrates in electronic packages is typically accomplished by using an adhesive that bonds the electronic component to the surface of the substrate. Over the years, numerous adhesives have been used for this purpose and are known as die attach films. As electronic packages have become more complex and the density of components, such as semiconductor chips, has increased, the excess heat generated by the electronic components has become a challenging problem. Dissipating the excess heat is critical to maintaining optimal electronic package performance.

[0003] Developers have employed inorganic particulate fillers in adhesive resin compositions, particularly in commonly used thermoset polymer adhesive compositions, to enhance the otherwise limited thermal conductivity performance. As the power and density of electronic components increase, the need to increase the heat dissipation capability has become increasingly urgent, and thus the thermal conductivity of die attach films has been enhanced by increasing the filler concentration. However, doing so decreases the flowability of the curable adhesive composition and increases the brittleness of the film after curing. Such characteristics present challenges to the lamination of the film to the surface of the substrate, dicing, and die pick-up processes.

[0004] Accordingly, it is an aspect of the present invention to provide a thermally conductive curable composition that exhibits high thermal conductivity while maintaining low melt viscosity.

[0005] It is another aspect of the present invention to provide a die attach film that exhibits high thermal conductivity and good processability to manufacturing processes including lamination, substrate wet-out, dicing, and die pick-up. SUMMARY

[0006] With the present invention, a thermally conductive curable composition can be shaped into a heat sink that can be positioned between a heat source and a heat sink. The curable composition can be shaped into an adhesive film for securing components within an electronic package. In one example, the composition can provide a thermally conductive die attach film with high thermal conductivity. The curable composition exhibits high flowability and low melt viscosity relative to conventional compositions with comparable thermal conductivity, thus enhancing wet-out to large surface area substrates.

[0007] In one embodiment, a thermally conductive curable composition comprises: a curable resin component such as a curable epoxy resin; a thermally conductive particulate filler present at less than 95 wt% of the composition solids; and a curing agent effective to cure the curable resin component. The thermally conductive particulate filler can be selected from metals, metal alloys, metal oxides, and combinations thereof. The curing agent can be selected such that the composition exhibits a polymerization onset temperature of at least 140°C. When cured, the composition exhibits a thermal conductivity of at least 5 W / m*K.

[0008] In some embodiments, the thermally conductive particulate filler is present at less than 85 wt% of the composition solids.

[0009] In some embodiments, the thermally conductive particulate filler comprises a metal selected from copper, silver, aluminum, and combinations thereof. In some embodiments, the thermally conductive particulate filler has a melting point of at least 300°C.

[0010] In some embodiments, the curing agent is a latent curing agent that does not react with the curable resin until a temperature threshold is exceeded. In some embodiments, the latent curing agent does not react with the curable resin at temperatures below 140°C.

[0011] In some embodiments, the curing agent comprises at least one of an aromatic substituted urea and an aromatic amine.

[0012] In some embodiments, the curable composition has a melt viscosity that varies by less than 75% between 50-130°C. In some embodiments, the curable composition has a melt viscosity of less than 50,000 Pa*s at 50°C.

[0013] In some embodiments, when cured, the composition exhibits a thermal conductivity of at least 10 W / m*K.

[0014] In another embodiment, a thermally conductive adhesive composition comprises: 5 to 40 wt% of a curable resin based on solids; 0.1 to 2 wt% of a curing agent effective to cure the resin based on solids; and less than 95 wt% of a thermally conductive particulate metal filler based on solids. The curing agent can be selected such that the composition exhibits a polymerization onset temperature of at least 140°C. When cured, the composition can exhibit a thermal conductivity of at least 6 W / m*K.

[0015] In some embodiments, the curable resin comprises an epoxy resin. In some embodiments, the curable resin comprises at least one of an acrylate, a bismaleimide, and combinations thereof.

[0016] In some embodiments, the thermally conductive particulate metal filler comprises silver.

[0017] In some embodiments, the curing agent comprises at least one of an aromatic amine and an aromatic substituted urea.

[0018] In some embodiments, the thermally conductive adhesive composition comprises less than 85 wt% of the solids of thermally conductive particulate metal filler.

[0019] An electronic package of the present application can include a silicon substrate, a thermally conductive curable composition applied to a surface of the silicon substrate, and an electronic component attached to the silicon substrate by the thermally conductive curable composition in a cured state. The thermally conductive curable composition can include a curable resin, a thermally conductive particulate filler present at less than 95 wt% of the solids of the composition, and a curing agent effective to cure the resin. The thermally conductive filler can be selected from the group consisting of metals, metal alloys, metal oxides, and combinations thereof. The curing agent can be selected such that the composition exhibits a polymerization onset temperature of at least 140°C. When cured, the composition can exhibit a thermal conductivity of at least 5 W / m*K.

[0020] In some embodiments, the thermally conductive curable composition can be disposed on the surface of the silicon substrate in the form of a thin film. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 FIG. 1 is a flow chart illustrating a process flow for die attach to a semiconductor wafer using a thermally conductive adhesive.

[0022] Figure 2 FIG. 2 is a schematic of an assembly using a thermally conductive curable composition of the present application.

[0023] Figure 3 FIG. 3 is a graph comparing the melt viscosity of a thermally conductive curable composition of the present application and a conventional thermally conductive curable composition as a function of temperature.

[0024] Figure 4 FIG. 4 is a schematic of a semiconductor chip employing a thermally conductive curable composition of the present application.

[0025] Figure 5 FIG. 5 is a schematic of an electronic package employing a die attach film formed from a thermally conductive curable composition of the present application. DETAILED DESCRIPTION

[0026] The advantages, such as those listed above, are now described with respect to specific embodiments. However, other embodiments and aspects of the present application are contemplated as within the skill of the art. In particular, features described with respect to the compositions and devices of the present application can be combined in ways other than those described in the claims and in ways other than those described in the specification.

[0027] A composition is provided, and in some embodiments, an adhesive composition, such as for wafer bonding applications. The adhesive composition includes a curable resin component, a curing agent, and a thermally conductive particulate filler. The curable resin can include an epoxy resin, and in such embodiments, the curing agent is effective in curing the epoxy resin.

[0028] The thermally conductive compositions of the present invention can be formed into a coating or film on a substrate, typically as an adhesive along a heat dissipation path, such as between a heat generating electronic component and the substrate to which the component is attached. The thermally conductive compositions of the present invention can be suitable for use as a wafer bonding film in electronic packaging, sometimes referred to as a conductive die attach film (CDAF) for securing a "die" (such as a processor, power semiconductor device, etc.) to a silicon wafer substrate. The compositions preferably exhibit sufficient wettability to coat the corresponding substrate surface and subsequently cure. When cured, the thermally conductive film preferably exhibits a high thermal conductivity of at least 5 W / m*K, in some embodiments at least 10 W / m*K.

[0029] The thermally conductive compositions of the present invention preferably can be one-component curable compositions that are activated upon the application of heat. Thus, the curable compositions are stable at room temperature and curable upon the application of sufficient heat. The curable compositions can be cured through a reagent-promoted route, wherein the one or more curing agents present in the composition only become active above a minimum temperature threshold. Such curing agents are sometimes referred to as latent curing agents.

[0030] Resin matrix material The thermally conductive interface material of the present invention comprises a matrix formed from at least one curable resin component and a chemical curing agent, wherein the term "resin" can include any natural or synthetic organic compound or mixture that can be converted into a polymer. Preferably, the curing reaction is initiated upon contact of the curable resin component with the curing agent, in some cases in the presence of an environmental curing reaction promoter such as water, heat, pressure, electromagnetic radiation, etc. In preferred embodiments, the curing reaction is initiated upon contact of the curable resin component with the curing agent in the presence of heat, particularly a temperature above a minimum threshold.

[0031] The curable resin component can be any curable resin, and is preferably a thermoset polymeric material that is cured by the application of heat. Examples of useful curable resins include epoxy resins, maleimides, (meth)acrylates, etc. If the resin component selected comprises an epoxy resin, the resin can be any of a liquid, a solid, a semi-solid, or a solid dissolved or suspended in a liquid such as a solvent at room temperature. The curable resin component can comprise a combination of epoxy resins that can be cured into a film having desired properties.

[0032] A wide variety of epoxy-functionalized resins are contemplated for use in the curable compositions of the present application, for example, liquid epoxy resins based on bisphenol A, liquid epoxy resins based on bisphenol F, multifunctional epoxy resins based on novolac resins, dicyclopentadiene-type epoxy resins, naphthalene-type epoxy resins, and the like. Other exemplary epoxy-functionalized resins contemplated for use herein include diepoxides of cycloaliphatic alcohols, difunctional cycloaliphatic glycidyl esters of hexahydrophthalic anhydride (commercially available as Epalloy 5200), Epiclon EXA-835LV, Epiclon HP-7200L, and the like, as well as mixtures of any two or more thereof.

[0033] In some embodiments, the curable composition can comprise a combination of two or more different epoxy-functionalized resins, including two or more different bisphenol-based epoxy resins. The bisphenol-based epoxy resins can be selected from bisphenol A, bisphenol F, or bisphenol S epoxy resins, and combinations thereof. Further, two or more different bisphenol epoxy resins within the same type of resin (such as A, F, or S type) can be used.

[0034] Commercially available examples of bisphenol epoxy resins contemplated for use herein include bisphenol F-type epoxy resins (such as RE-404-S from Nippon Kayaku, EPICLON 830 (RE1801), 830S (RE1815), 830A (RE1826), and 830W from Dai Nippon Ink & Chemicals, Inc., and RSL 1738 and YL-983U from Resolution) and bisphenol A-type epoxy resins (such as YL-979 and 980 from Resolution). Other examples of commercially available epoxy resins include Epon 828, Epon 826, Epon 862 (all from Hexion Co., Ltd.); DER 331, DER 383, DER 332, DER 330-EL, DER 331-EL, DER 354, DER 321, DER 324, DER 29, DER 353 (all from Dow Chemical Co.); JER YX8000, JER RXE21, JER YL 6753, JER YL 6800, JER YL 980, JER 825, and JER 630 (all from Epoxy Resins Co.).

[0035] The bisphenol epoxy resins commercially available from Dai Nippon and mentioned above are claimed to be liquid undiluted epichlorohydrin-bisphenol F epoxy resins having much lower viscosity than conventional epoxy resins based on bisphenol A epoxy resins and having similar physical properties to liquid bisphenol A epoxy resins. The viscosity of the bisphenol F epoxy resins is lower than the bisphenol A epoxy resins under otherwise identical conditions of the two epoxy resins, which provides lower viscosity for use as fast-flowing underfill encapsulants. The epoxy equivalent weight (EEW, i.e., molecular weight divided by the number of epoxy groups) of the four bisphenol F epoxy resins is between 165 and 180. The viscosity at 25°C is between 3,000 and 4,500 cps (except for RE1801, which has an upper limit of 4,000 cps). The hydrolyzable chloride content is reported to be 200 ppm for RE1815 and 830W, and 100 ppm for RE1826.

[0036] The bisphenol epoxy resins commercially available from Resolution and mentioned above are claimed to be low chloride content liquid epoxy resins. The EEW (g / eq) of the bisphenol A epoxy resins is between 180 and 195, and the viscosity at 25°C is between 100 and 250 cP. The total chloride content is reported to be between 500 and 700 ppm for YL-979, and between 100 and 300 ppm for YL-980. The EEW (g / eq) of the bisphenol F epoxy resins is between 165 and 180, and the viscosity at 25°C is between 30 and 60. The total chloride content is reported to be between 500 and 700 ppm for RSL-1738, and between 150 and 350 ppm for YL-983U.

[0037] In addition to bisphenol epoxy resins, other epoxy compounds are contemplated for use as the epoxy component of the formulations of the present application. For example, cycloaliphatic epoxy resins such as 3,4-epoxycyclohexylmethyl carbonate 3,4-epoxycyclohexyl ester can be used. In addition, monofunctional, difunctional, or polyfunctional active diluents can be used to adjust viscosity and / or lower the glass transition temperature (Tg) of the resulting resin material. Exemplary active diluents include butyl glycidyl ether, cresyl glycidyl ether, o-cresyl glycidyl ether, polyethylene glycol glycidyl ether, polypropylene glycol glycidyl ether, and the like.

[0038] Other epoxy resins applicable to this document include polyglycidyl derivatives of phenolic compounds, such as those commercially available under the trade name EPON, such as EPON 828, EPON 1001, EPON 1009, and EPON 1031 from Resolution; DER 331, DER 332, DER 334, and DER 542 from Dow Chemical Co.; and BREN-S from Nippon Kayaku. Other suitable epoxy resins include polyepoxides prepared from polyols, etc., and polyglycidyl derivatives of phenolic varnishes, such as DEN 431, DEN 438, and DEN 439 from Dow Chemical. Cresol analogs are also commercially available under the trade name ARALDITE, such as ARALDITE ECN 1235, ARALDITE ECN 1273, and ARALDITE ECN 1299 from Ciba Specialty Chemicals Corporation. SU-8 is a bisphenol A type epoxy phenolic varnish from Resolution. Polyglycidyl adducts of amines, amino alcohols, and polycarboxylic acids can also be used in this invention, with commercially available resins including GLYAMINE 135, GLYAMINE 125, and GLYAMINE 115 from FIC Corporation; ARALDITE MY-720, ARALDITE 0500, and ARALDITE 0510 from Ciba Specialty Chemicals; and PGA-X and PGA-C from Sherwin-Williams Co.

[0039] In some embodiments, the epoxy component used herein is a silane-modified epoxy resin, such as a composition comprising: (A) Epoxy components represented by the following structures: in: Y may or may not exist. When Y exists, it is a direct bond, CH2, CH(CH3)2, C=O, or S. R here 1 It can be alkyl, alkenyl, hydroxyl, carboxyl, or halogen, and Here, x ranges from 1 to 4; (B) Epoxy-functionalized alkoxysilanes represented by the following structures: in R 1 It is an oxygen-containing heterocyclic butane moiety, and R 2 is an alkyl or alkoxy-substituted alkyl, aryl, or aralkyl group having 1 to 10 carbon atoms; and (C) a reaction product of components (A) and (B).

[0040] One such example of a silane-modified epoxy resin is the reaction product of an aromatic epoxy resin, such as bisphenol A, E, F, or S epoxy resin or a biphenyl epoxy resin, with an epoxy silane, wherein the epoxy silane is represented by the following structure: wherein R 1 is an oxetane moiety, examples of which include 2-(ethoxymethyl)oxetane, 2- (propoxymethyl)oxetane, 2-(methoxymethyl)oxetane, and 2-(3-methoxypropyl)oxetane, and R 2 is an alkyl or alkoxy-substituted alkyl, aryl, or aralkyl group having 1 to 10 carbon atoms.

[0041] In one embodiment, R 1 is 2-(ethoxymethyl)oxetane and R 2 is methyl.

[0042] An idealized structure of an aromatic epoxy resin used to make the silane-modified epoxy resin includes wherein Y can be present or can be absent, when Y is present, it is a direct bond, CH2, CH(CH3)2, C=0, or S, R1is an alkyl, alkenyl, hydroxyl, carboxyl, or halogen, and x is 1-4.

[0043] Of course, when x is 2-4, it is contemplated that this structure encompasses a chain-extended version of the aromatic epoxy resin as well.

[0044] For example, a chain-extended version of the aromatic epoxy resin can be represented by the following structure: .

[0045] In some embodiments, the siloxane-modified epoxy resin has the following structure: -(O-Si(Me)2-O-Si(Me)(Z)-O-Si(Me)2-O-Si(Me)2) n - wherein: Z is -0-(CH2)3-0-Ph-CH2-Ph-0-(CH2-CH(OH)-CH2-0-Ph-CH2-Ph-0-) n -CH2-oxetane, and n is in the range of about 1-4.

[0046] In some embodiments, the siloxane-modified epoxy resin is produced by contacting a combination of the following components under conditions suitable to promote reaction thereof: Me2Si(OMe)2 + (MeO)3Si-(CH2)3-0-CH2-oxetane + oxetane-CH2-0-Ph-CH2-Ph-0-(CH2-CH(OH)-CH2-0-Ph-CH2-Ph-0-) n -CH2-oxetane, wherein "n" is in the range of about 1-4.

[0047] The silane-modified epoxy resin can also be a combination of the aromatic epoxy resin, the epoxy silane, and a reaction product of the aromatic epoxy resin and the epoxy silane. The reaction product can be prepared from the aromatic epoxy resin and the epoxy silane in a weight ratio of 1 : 100 to 100: 1, such as 1 : 10 to 10: 1.

[0048] Acrylates useful as the curable resin component can be selected from a variety of different compounds. The terms acrylate and acrylic as used herein are used interchangeably when referring to monomers and monomer-containing components. It is contemplated that such terms also include (meth)acrylate and (meth)acrylic. Exemplary polymerizable acrylate monomers include triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, pentaerythritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate, dipentaerythritol monohydroxypenta(meth)acrylate, pentaerythritol tri(meth)acrylate, bisphenol A-ethoxylate di(meth)acrylate, trimethylolpropane ethoxylate tri(meth)acrylate, trimethylolpropane propoxylate tri(meth)acrylate, and bisphenol A-diepoxy di(meth)acrylate.

[0049] In addition, monofunctional (meth)acrylate monomers can be used, including tetrahydrofurfuryl (meth)acrylate and di(meth)acrylates, hydroxypropyl (meth)acrylate, tetrahydrodicyclopentadienyl (meth)acrylate, triethylene glycol (meth)acrylate, and combinations thereof.

[0050] The curable composition of the present application can comprise 1 to 40 weight percent of the total composition, preferably 5 to 40 weight percent of the total composition, preferably 5 to 30 weight percent of the total composition, preferably 10 to 30 weight percent of the total composition, preferably 10 to 25 weight percent of the total composition, of the curable matrix-forming resin.

[0051] For purposes of the present disclosure, weight percent of the composition or weight percent of the total composition is calculated based on the non-solvent components of the composition. This is sometimes referred to as the "solid" components of the composition, where the non-solvent components remain in the final solid film. However, it should be understood that certain components of the composition can be in liquid form, but are still counted as "solids" because they form part of the final solid film product.

[0052] In some embodiments, the curable matrix-forming resin can comprise one or more epoxy resins. The one or more epoxy resins can be present at 1 to 40 weight percent of the total composition, preferably 1 to 30 weight percent of the total composition, more preferably 1 to 20 weight percent of the total composition, preferably 1 to 10 weight percent of the total composition, preferably 5 to 10 weight percent of the total composition.

[0053] Curing agent The composition of the present application further comprises one or more curing agents, such as latent curing agents, which only effectively promote polymerization of the curable resin when the temperature exceeds a threshold temperature. In some embodiments, the curing agent of the present application is activated at temperatures exceeding 140 °C. In some embodiments, the curing agent of the present application is activated at temperatures exceeding 150 °C. For purposes herein, the term "curing" is intended to mean the crosslinking reaction that forms a three-dimensional polymer network.

[0054] Applicants have found that limiting the formation of the polymer matrix until sufficient temperature parameters are met helps to enhance the effective thermal conductivity of the inorganic particulate filler dispersed in the system. In particular, certain thermally conductive inorganic particulate fillers can have a surface coating that impairs metal-metal interactions. Examples of such particulate fillers include metals and metal alloys, in some embodiments including copper, silver, aluminum, and combinations thereof. Removal of the surface coating from these particulate materials results in better contact between the metal fillers, and it has been found that the coating can be reduced or eliminated by exposure to sufficiently high temperatures. Once the surface coating is removed from the particulate fillers, the particles can interact more effectively, thereby more effectively transferring thermal energy. However, it is an aspect of the present application that the debonding of the surface coated particles be accomplished prior to the formation of the polymer matrix, so as to facilitate metal-metal interactions while the fillers have at least some mobility within the monomeric resin. Thus, the formation of the polymer matrix is preferably initiated at a temperature above that which results in debonding of the surface coating from the particles.

[0055] Applicants have found that the surface layer coating can be debonded from the particles when the temperature exceeds 140-150°C, thereby enhancing the heat transfer performance. As a result of the enhanced heat transfer performance, the thermally conductive film can achieve comparable or higher thermal conductivity performance without increasing the concentration of the thermally conductive particulate filler.

[0056] For purposes of the present disclosure, the term "debonding" means dissociating the surface coating from the surface of the particle.

[0057] In some embodiments, the curing agent is effective to promote polymerization of the curable resin only when the temperature is sufficient to substantially debond the surface coating from the thermally conductive particulate filler dispersed in the curable composition. Preferably, the curing agent is effective to promote polymerization of the curable resin only when the temperature is sufficient to debond the surface coating from the thermally conductive particulate filler.

[0058] An epoxy resin curing agent can be used in combination with the epoxy monomer. Exemplary epoxy resin curing agents include ureas, aliphatic and aromatic amines, amine hardeners, polyamides, acid anhydrides, polyhydric phenols, imidazoles, dicyandiamides, hydrazides, urea-amine hybrid curing systems, free radical initiators (e.g., peroxy esters, peroxy carbonates, hydroperoxides, alkyl peroxides, aryl peroxides, azo compounds, etc.), organic bases, transition metal catalysts, phenols, acid anhydrides, Lewis acids, Lewis bases, and the like.

[0059] Specific examples of curing agents useful in the compositions of the present application include the following structures:

[0060] The curing agent can be present from 0.1 to 10 weight percent of the total composition, preferably from 0.1 to 5 weight percent of the total composition. Generally, the curing agent can be present from 0.1 to 40 weight percent of the curable resin in the composition, preferably from 0.2 to 30 weight percent of the curable resin in the composition, preferably from 0.5 to 20 weight percent of the curable resin in the composition, preferably from 1 to 15 weight percent of the curable resin in the composition. As noted above, it is preferred that one or more curing agents be present in the curable composition effective to provide a resin polymerization onset temperature of at least 140°C, preferably at least 150°C.

[0061] Thermally conductive particulate filler To enhance thermal conductivity, the thermally conductive composition of the present application preferably includes a thermally conductive particulate dispersed therein. The particulate can be both thermally conductive and electrically conductive. Alternatively, the particulate can be thermally conductive but electrically insulating.

[0062] Electrically conductive fillers contemplated for use herein include, for example, gold, silver, copper, platinum, palladium, nickel, aluminum, indium, nickel alloys (e.g., Alloy 42), zinc alloys, iron alloys, indium alloys, silver-coated copper, silver-coated aluminum, bismuth, tin, bismuth-tin alloys, silver-coated fibers, silver-coated graphite, silver-coated silicon carbide, silver-coated boron nitride, silver-coated diamond, silver-coated aluminum oxide, silver-coated Alloy 42, graphene, silver-coated graphene, graphene nanoplatelets, single- and multi-walled carbon nanotubes, silver-coated polymers, cadmium and cadmium alloys, lead and lead alloys, antimony and antimony alloys, boron nitride, aluminum nitride, aluminum oxide, aluminum oxide trihydrate, silicon, silicon carbide, graphite, diamond, magnesium oxide, magnesium hydroxide, zinc oxide, and the like, as well as mixtures of any two or more thereof. In some embodiments, the thermally conductive particulate filler has a melting point of at least 300°C, preferably at least 400°C, preferably at least 500°C.

[0063] In some embodiments, the particulate thermally conductive filler can be substantially spherical, flaky, rod-like, or a combination thereof. Certain particle size distributions can be contemplated to suit the parameters of any particular application, although certain particle size distributions can be more effective than others.

[0064] The thermally conductive particles used in the compositions of the present application can be present in less than 95% by weight of the total composition. In some embodiments, the thermally conductive particles can be present in 40 to 94% by weight of the total composition. In some embodiments, the thermally conductive particles can be present in 50 to 94% by weight of the total composition. In some embodiments, the thermally conductive particles can be present in 60 to 94% by weight of the total composition. In some embodiments, the thermally conductive particles can be present in less than 85% by weight of the total composition. In some embodiments, the thermally conductive particles can be present in 40 to 84% by weight of the total composition. In some embodiments, the thermally conductive particles can be present in 50 to 84% by weight of the total composition. In some embodiments, the thermally conductive particles can be present in 60 to 84% by weight of the total composition.

[0065] The thermally conductive particles used in the compositions of the present application can have an average particle size (d 50 ) of between 0.1 and 100 microns. In some embodiments, the average particle size is between 0.1 and 50 microns. In some embodiments, the average particle size is between 0.1 and 25 microns. In some embodiments, the average particle size is between 0.1 and 10 microns. In some embodiments, the average particle size is between 0.1 and 5 microns.

[0066] In one useful embodiment, the particulate thermally conductive filler can comprise a multimodal particle size distribution having discrete concentrations of particles of different average particle sizes. In some embodiments, a first portion of the thermally conductive filler has an average particle size (d 50) can be less than 1 micron, the average particle size (d 50 ) can be greater than 1 micron. In some embodiments, the first portion of thermally conductive filler can comprise 20 to 40 percent by weight of the total thermally conductive particulate filler. In some embodiments, the first portion of thermally conductive filler can comprise 25 to 35 percent by weight of the total thermally conductive particulate filler. Applicant has found that such a particle size distribution can facilitate high thermal conductivity values while not compromising the physical properties required of the composition.

[0067] It is desirable that the curable composition of the present application exhibit a thermal conductivity of at least 5 W / m*K, more preferably at least 10 W / m*K, more preferably at least 15 W / m*K when cured.

[0068] In some embodiments, the thermally conductive particulate filler can be surface treated or surface modified. Exemplary surface treatments and surface modifications include treatment with silane coupling agents, phosphoric acid or phosphoric acid compounds, or surfactants. The silane coupling agents can include at least one hydrolysable group such as an alkoxy group and an aryloxy group bonded to a silicon atom. Other examples include alkyl, alkenyl, and any aryl groups bonded to a silicon atom. Whether surface treated or modified, the thermally conductive particulate filler can be blended with the curable resin and curing agent by mixing the components in a mechanical mixer to achieve the desired degree of dispersion of the particulate filler in the curable resin.

[0069] Reaction catalyst A reaction catalyst can be employed to facilitate the cross-linking polymerization reaction of the curable resin. In some embodiments, the reaction catalyst facilitates the polymerization reaction at a temperature of at least 140°C, more preferably at least 150°C. The reaction catalyst can be referred to as a cure accelerator. In some embodiments, the cure accelerator can be selected from the group consisting of tertiary amines, imidazole derivatives, and combinations thereof. Exemplary tertiary amines include: trimethylamine, triethylamine, tetraethylenemethylendiamine, tetramethylpropane-1,3-diamine, tetramethylhexane-1,6-diamine, pentamethyldiethylenetriamine, bis(2-dimethylaminoethyl)ether, ethyleneglycol(3-dimethyl)aminopropyl ether, dimethylaminoethanol, dimethylaminoethoxyethanol, triethylenediamine, and hexamethylenetriamine.

[0070] Other exemplary catalysts that can be used in the curable composition of the present application include the following structures: .

[0071] As described herein, a curable composition is formulated to delay the initiation of polymerization to high temperatures, such as temperatures in excess of 140°C, to facilitate particle-to-particle interaction prior to network formation. This interaction is facilitated by reducing or eliminating oxidation at the surface of the particles, which is achieved by exposure to high temperatures prior to network formation. With this reduction or elimination of oxidation, particle-particle interaction can be maintained after network formation. For purposes herein, the term "reducing or eliminating oxidation" means reducing the average concentration of oxygen atoms associated with the thermally conductive particles in the curable composition by at least 20%.

[0072] Other Additives In addition to the curable resin, curing agent, and thermally conductive particulate filler, the compositions of the present invention can also include additives such as organic solvents (such as methyl ethyl ketone), curing catalysts (as described above), viscosity modifiers, antioxidants, adhesion promoters, wetting agents, dispersants, flame retardants, corrosion inhibitors, pigments, and stress relaxants. Additives that remain in the cured film are considered "solid."

[0073] Cured Composition The curable compositions of the present invention, when cured, can form a film-like adhesive. For purposes herein, the term "film" means a thin film having a thickness of less than 250 microns. In some embodiments, the film formed from the curable compositions of the present invention (alone or in combination) can be disposed at the surface of a substrate, such as a silicon substrate of an electronic package. The film can also be applied to a release treated substrate liner surface to facilitate handling and application to a package substrate. Examples of release treated films include release treated polypropylene, release treated polyethylene, and release treated polyethylene terephthalate.

[0074] The compositions of the present invention exhibit good flowability at relatively low temperatures, such as temperatures below 80°C. This facilitates processing of the composition into a wafer bonding film, including the step of laminating the composition to a substrate. Figure 1 The flow chart illustrates an exemplary process for bonding a wafer to a substrate using a wafer bonding film of the present invention. The bonding film is prepared from a curable resin composition that is formed by mixing the components, including the thermally conductive particulate filler, into a slurry in a mixing device.

[0075] Typically, as Figure 2The illustrated assembly 101 can be prepared by applying a curable resin composition to a semiconductor substrate or silicon wafer 102, such as by inkjet printing, stencil printing, screen printing, or spray coating. The application preferably uniformly coats the application surface of the substrate and can be partially cured or B-staged into an adhesive layer 104. Alternatively, the curable composition can be applied to a support layer 106, such as a dicing tape, and then bonded by heat pressing to the application surface 103a of the substrate 102. In some embodiments, the support layer 106 can be a polyolefin film, such as one or more layers of polyethylene, polyvinyl chloride, polybutylene, polybutadiene, polyurethane, polyester, polyamide, and copolymers thereof.

[0076] Examples of the substrate 102 include semiconductor wafers, the circuit surface 103b in which can support semiconductor circuits, where the wafer substrate is formed of silicon, SiC, or GaN. In some embodiments, the adhesive layer 104 can be formed of a single layer of curable composition, or two or more layers can be laminated onto the substrate 102. One or more layers of curable composition can be laminated to one or both of the substrate 102 and the support layer 106 under lamination conditions, such as a temperature of 50 °C to 130 °C and an applied pressure, for example, onto the circuit surface 103b of the substrate 102, to achieve a desired thickness. During lamination, the curable composition softens and wets the application surface 103a to adhere the substrate 102 to the support layer 106.

[0077] When the curable composition is heated according to ASTM D4440, the melt viscosity varies by less than 100% between 50 °C and 130 °C. In some embodiments, the melt viscosity varies by less than 90% between 50 °C and 130 °C. In some embodiments, the melt viscosity varies by less than 80% between 50 °C and 130 °C. In some embodiments, the melt viscosity varies by less than 75% between 50 °C and 130 °C. In some embodiments, the melt viscosity at 50 °C is less than 50,000 Pa*s when tested according to ASTM D4440. In some embodiments, the melt viscosity at 50 °C is less than 20,000 Pa*s when tested according to ASTM D4440.

[0078] Figure 3 A comparison of the melt viscosity of the curable composition of the present invention to conventional adhesive compositions that require significantly higher particle filler loadings to achieve suitable thermal conductivity performance is shown. The results of this comparison illustrate how the relatively lower particle filler loading of the present invention can still exhibit high thermal conductivity performance while maintaining a low melt viscosity at low temperature.

[0079] After lamination, the substrate 102 and adhesive layer 104 are cut using a dicing saw to form semiconductor dies 108, which wafer has the semiconductor substrate 102 and adhesive layer 104 divided into different regions or dies 110 on the support layer 106. After cutting is complete, the die wafer 110 can be removed from the support layer 106 by a process known as die pick-up or die placement. Die pick-up is typically performed by using a placement apparatus to lift the individual dies from the support layer 106 using suction. The placement apparatus then mounts the dies 110 onto a wiring board 112, typically by thermal compression, where the adhesive layer 104 acts as a die attach medium. Exemplary die attach conditions include a temperature between 100°C and 130°C, with an applied pressure of about 10 N. The wiring board 112 can be used to form a semiconductor circuit, where one or more conductive wires 114 are bonded between the circuit surface 103b of the substrate 102 and the wiring board 112. The completed wiring connections to the wiring board 112 conductors form an electronic package 116.

[0080] To fully secure the dies 110 to the wiring board 112, the adhesive layer 104 is preferably cured by thermal curing, optionally including the application of pressure to one or both of the dies 110 and the wiring board 112. Thermal curing can include raising the temperature of the adhesive layer 104 to at least 140°C, preferably at least 150°C. Curing conditions can be set in a curing oven, where the temperature is between 140°C and 180°C, preferably between 150°C and 175°C, for a duration sufficient to bring the temperature of the adhesive layer 104 to or above the threshold values described above. In some embodiments, the time is between 10 and 120 minutes.

[0081] In some embodiments, the wiring board 112 and dies 110 can be sealed with a sealing resin in the form of a mold. Sealing the electronic package 116 can be performed by methods known to those of ordinary skill in the art.

[0082] Examples The following examples are intended to help those of ordinary skill in the art better understand and practice the present application. The scope of the present application is not limited by the examples, but is defined by the appended claims. Unless otherwise indicated, all parts and percentages are on a solids weight basis, excluding any solvent or other liquid material that can be present for the convenience of storage and dispensing of the settable composition. The thermal conductivity values provided in each example represent the thermal conductivity of a cured film of the settable composition after polymerization.

[0083] Raw Materials Epalloy 5200 is an epoxy resin.

[0084] Epoxy Resin A is a chain-extended epoxy resin containing CTBN rubber.

[0085] EP7 is a solution of a chain-extended epoxy resin in methyl ethyl ketone solvent with a solids content of 50%.

[0086] Epoxy B is a liquid epoxy resin at room temperature.

[0087] N665 is a solution of an epoxy resin in methyl ethyl ketone solvent with a solids content of 70%.

[0088] Erisys RDGE / H is a low viscosity aromatic difunctional epoxy resin containing resorcinol diglycidyl ether.

[0089] SG-80H is a polyacrylate resin.

[0090] Resin A is a bismaleimide resin.

[0091] Nano Dicy is a dicyandiamide epoxy resin curing agent with a particle size of 1-2 µm.

[0092] Dodecenyl succinic anhydride is a long chain aliphatic anhydride epoxy resin reaction catalyst.

[0093] Curing agent A is a 2,4'-toluene bisdimethylurea curing agent.

[0094] Curing agent B is a dicyandiamide epoxy resin curing agent.

[0095] Curing agent C is an aromatic amine curing agent.

[0096] Filler 1 is particulate silver with an average particle size of 1.9 µm.

[0097] Filler 2 is particulate silver with an average particle size of 1.5 µm.

[0098] Filler 3 is particulate silver with an average particle size of 0.8 µm.

[0099] Silane Z 6040 is a silane adhesion promoter.

[0100] MEK is methyl ethyl ketone solvent.

[0101] The method of preparation of each example composition is as follows: the weighed ingredients are added to a container and the mixture is stirred at room temperature using a high speed mixer at 2000 rpm for 2-3 minutes until the composition is thoroughly mixed into a slurry. The slurry is then spread onto a release liner film to a thickness of 10-100 µm using a doctor blade and dried in an oven at 190 °F for 4 minutes to remove solvent. After the solvent has evaporated, the film is cured at 200 °C for 60 minutes.

[0102] The polymerization onset temperature of each of the example compositions was measured using a differential scanning calorimeter (DSC) according to ASTM E793 over a temperature range from room temperature to 300°C.

[0103] The thermal conductivity of each of the example compositions was measured using the laser flash method according to ASTM E 1461.

[0104] Example 1

[0105] Example 2

[0106] Example 3

[0107] Example 4

[0108] Example 5

[0109] Example 6

[0110] Table 1 summarizes the data from Examples 1-6 above.

[0111] Table 1

[0112] Table 1 shows that the compositions of Examples 3-6, which exhibited a polymerization onset temperature of at least 140°C, unexpectedly exhibited significantly higher thermal conductivity values than the compositions of Examples 1 and 2. Specifically, the compositions of Examples 4-6 exhibited significantly higher thermal conductivity values than the composition of Example 1, even though the compositions of Examples 4-6 contained a comparable or even slightly lower concentration of thermally conductive particulate filler. The same was true for the composition of Example 3, which exhibited a significantly higher thermal conductivity than the composition of Example 2, even though the composition of Example 3 contained a comparable concentration of thermally conductive particulate filler.

[0113] In the example compositions, the curing agents "B" and "NanoDicy" suppressed the polymerization onset temperature compared to the use of curing agents A and C alone or in combination. The elevated polymerization onset temperature of Examples 3-6 had the unexpected benefit of enhancing thermal conductivity without increasing the concentration of particulate filler. Because the compositions of Examples 3-6 did not require an increase in particulate filler loading, it is expected that the compositions and films of the present application will exhibit good handling properties, wet-out, and adhesion comparable to known films, while also providing excellent thermal conductivity.

Claims

1. A thermally conductive curable composition comprising: an epoxy resin; a thermally conductive particulate filler present at less than 95 wt% of the composition solids, the thermally conductive particulate filler selected from the group consisting of metals, metal alloys, metal oxides, and combinations thereof; and a curing agent effective to cure the epoxy resin, wherein the curing agent and epoxy resin are selected such that the composition exhibits a polymerization onset temperature of at least 140°C, and when cured, the composition exhibits a thermal conductivity of at least 5 W / m*K.

2. The thermally conductive curable composition of claim 1, wherein the thermally conductive particulate filler is present at less than 80 wt% of the composition solids.

3. The thermally conductive curable composition of claim 1, wherein the thermally conductive particulate filler comprises a metal selected from the group consisting of copper, silver, aluminum, and combinations thereof.

4. The thermally conductive curable composition of claim 3, wherein the thermally conductive particulate filler has a melting point of at least 300°C.

5. The thermally conductive curable composition of claim 1, wherein the curing agent comprises one or more of an aromatic substituted urea and an aromatic amine.

6. The thermally conductive curable composition of claim 1, which has a melt viscosity variation of less than 75% between 50-130°C.

7. The thermally conductive curable composition of claim 1, which has a melt viscosity of less than 20,000 Pa at 50°C.

8. The thermally conductive curable composition of claim 1, wherein when cured, the composition exhibits a thermal conductivity of at least 10 W / m*K.

9. A thermally conductive adhesive composition comprising: a curable resin at 1-40 wt% of solids; a curing agent effective to cure the resin at 0.1-2 wt% of solids, wherein the curing agent and the resin are selected such that the composition exhibits a polymerization onset temperature of at least 140°C; and a thermally conductive particulate metal filler at less than 95 wt% of solids, wherein when cured, the composition exhibits a thermal conductivity of at least 5 W / m*K.

10. The thermally conductive adhesive composition of claim 9, wherein the curable resin comprises an epoxy resin.

11. The thermally conductive adhesive composition of claim 10, wherein the curable resin comprises an acrylate.

12. The thermally conductive adhesive composition of claim 11, wherein the thermally conductive particulate metal filler comprises silver.

13. The thermally conductive adhesive composition of claim 12, wherein the curing agent comprises one or more of an aromatic substituted urea and an aromatic amine.

14. The thermally conductive adhesive composition of claim 9, comprising less than 80 wt% of the thermally conductive particulate metal filler of solids.

15. An electronic package comprising: a silicon substrate; a thermally conductive curable composition according to claim 1 applied to a surface of the silicon substrate; and an electronic component connected to the silicon substrate by the thermally conductive curable composition in a cured state.

16. The electronic package of claim 15, wherein the thermally conductive curable composition is disposed on the surface of the silicon substrate in a thin film form.

17. A method of forming a thermally conductive film, the method comprising: providing a thermally conductive curable composition comprising: an epoxy resin; thermally conductive particulate filler present at less than 95% by weight of the solids of the curable composition; and a curing agent effective to cure the epoxy resin only at a curing temperature of at least 140°C; applying the thermally conductive curable composition to a surface of a substrate in a layer having a thickness of less than 250 microns; heating the curable composition to a temperature sufficient to de-oxidize the thermally conductive particulate filler; curing the thermally conductive curable composition.

18. The method of claim 17, wherein curing the thermally conductive curable composition comprises heating the curable composition to a curing temperature of at least 140°C.

19. The method of claim 18, wherein the temperature sufficient to de-oxidize the thermally conductive particulate filler is less than the curing temperature.

20. The method of claim 19, wherein the substrate is a silicon wafer plate and the combination of the silicon wafer plate and the curable composition is a die.

21. The method of claim 20, comprising bonding the die to a wiring board prior to heating the curable composition to the curing temperature. ​

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