B-stage thermally conductive adhesive

Abstract

A thermally conductive curable composition is dispensable to form a thin film and subsequently B-staged to reduce the flowability and to thereby prevent overflow of the composition during the die attach process. The thermally conductive curable composition includes a matrix precursor that is curable at a cure temperature, a thermally conductive particulate filler, and an organic solvent or solvent mixture. The thermally conductive particulate filler is sinterable at a sintering temperature that is equal to or lower than the cure temperature, and the organic solvent or solvent mixture has a flash point temperature equal to or less than 200 oC.

Description

2024P00051B-STAGE THERMALLY CONDUCTIVE ADHESIVEFIELD

[0001] The present invention relates to thermally conductive adhesives generally, and more particularly to B-stage die attach adhesives and a process of applying the B-stage adhesive to bond electrical components such as integrated circuit chips without excessive adhesive overflow.BACKGROUND

[0002] Conductive compositions are widely used in the fabrication and assembly of semiconductor packages and microelectronic devices. For example, metallic solder alloy paste and die attach paste with high thermal conductivity are used to bond die onto the substrate (i.e. lead frame). With the miniaturization of electronic packages, the die is required to be progressively thinner. Conventional die attach past / solder paste often overflows onto the top of thin dies during die attach procedures, which can cause short circuiting of the electronic circuits located on the top of the die.

[0003] Film adhesives are known for limiting flow during die attach. However, highly thermally conductive fdm adhesives typically contain high filler loadings and relatively low organic content. This causes the film adhesive to be brittle, which negatively affects the processability of the films during lamination, dicing, die pick-up, and die attach procedures. In addition, many manufacturers prefer to use paste adhesives as they don’t have the equipment necessary to handle film adhesive assembly processes.

[0004] As demands for thermal dissipation performance in electronic packages continue to increase, fabricators often seek to reduce die thickness to reduce the thermal impedance. Assembling the components with thin die, such as less than 100pm, however often present other issues. For instance, fabricators face the issue of die attach paste / solder paste overflow onto the top of the die surface, which can cause short-circuiting of the integrated circuit devices.

[0005] It would therefore be desirable to provide a B-stage thermally conductive adhesive that is capable of application in thin die assemblies without harmful overflow.2024P00051SUMMARY

[0006] AB-stage process is used to control the flowability of a curable paste adhesive so as to reduce overflow of the curable past adhesive during assembly procedures. The curable paste adhesive is dispensed into a thin film, and then subjected to a B-stage process to increase its viscosity within a range that is useful to reduce overflow while remaining sufficiently flexible and tacky to accommodate die attach procedures.

[0007] In one embodiment, a thermally conductive curable composition includes a matrix precursor that is curable at a cure temperature, a thermally conductive particulate filler, and a solvent or solvent mixture. The thermally conductive particulate filler may be sinterable at a sintering temperature that is equal to or lower than the cure temperature. The thermally conductive particulate filler may be present in a range of between 60 wt.% and 99 wt.% of the curable composition. The solvent or solvent mixture has a flash point temperature that is equal to or less than 200 °C.

[0008] In some embodiments of the thermally conductive curable composition, the matrix precursor is in a liquid phase at 20 °C. The matrix precursor may include an epoxy. The thermally conductive curable composition may include a curing agent that is effective to cure the epoxy at the cure temperature. In some embodiments, the cure temperature is between 120 °C and 350 °C.

[0009] In some embodiments, the thermally conductive particulate filler of the thermally conductive curable composition includes at least one of silver and copper.

[0010] The solvent mixture of the thermally conductive curable composition may include a first organic solvent having a flash point temperature above 40 °C, and a second organic solvent having a flash point temperature below 40 °C. The first and second organic solvents may be present in the composition in a weight ratio of first organic solvent : second organic solvent between 1 : 1 and 20: 1. In some embodiments, the weight ratio of the first organic solvent : second organic solvent may be between 3: 1 and 10:1.

[0011] In some embodiments, the thermally conductive curable composition includes non-reactive organic polymer particles having a maximum particle size of less than 50 pm.

[0012] A thermally conductive curable composition may include an epoxy resin matrix precursor, a thermally conductive particulate filler, and first and second solvents. The epoxy resin matrix precursor may be present in a range of between 0.1 wt.% and 15 wt.% of the composition.2024P00051The thermally conductive particulate filler may include thermally conductive particles that are sinterable at a sintering temperature that is less than 350 °C. The first solvent may be selected from diethylene glycol butyl ether, diethylene glycol monoethyl ether, dibasic ester 9, diethylene glycol monoethyl ether acetate, trietheylene glycol monomethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and combinations thereof. The second solvent has a flash point temperature of less than 40 °C at 760 mm Hg.

[0013] In some embodiments of the thermally conductive curable composition, the thermally conductive particulate filler includes at least one of silver and copper. In some embodiments, the thermally conductive particulate filler is present in a range of between 60 wt.% and 99 wt.% of the composition. In some embodiments, the first and second solvents are together present in a range of between 1 wt.% and 10 wt.% of the composition.

[0014] A method for forming a thermally conductive adhesive from a thermally conductive curable composition includes providing the thermally conductive curable composition with a resin matrix precursor that is curable at a cure temperature, a thermally conductive particulate filler, and an organic solvent or solvent mixture. The thermally conductive particulate filler may be sinterable at a sintering temperature that is equal to or lower than the cure temperature. The thermally conductive curable composition exhibits an initial viscosity at 25 °C. The method for forming the thermally conductive adhesive further includes processing the thermally conductive curable composition to adjust a viscosity of the thermally conductive curable composition from the initial viscosity to a B-stage viscosity at 25 °C that is at least 10% greater than the initial viscosity. The method for forming the thermally conductive adhesive further includes, subsequent to the processing step, exposing the thermally conductive curable composition to the cure temperature sufficient to sinter at least a portion of the thermally conductive particulate filler.

[0015] The processing step may include partially drying the thermally conductive curable composition at a B-stage temperature that is less than the cure temperature to remove some of the organic solvent or solvent mixture.

[0016] In some embodiments of the method, the B-stage temperature is between 20 °C and 110 °C. In some embodiments of the method, the cure temperature is less than 300 °C.

[0017] In some embodiments, the method further includes exposing the thermally conductive curable composition to the cure temperature for a cure time period sufficient to fully2024P00051 cure the resin matrix precursor. In some embodiments, the method further includes applying an external pressure to the thermally conductive curable composition while exposing the thermally conductive curable composition to the cure temperature.

[0018] In some embodiments of the method, the solvent mixture includes a first organic solvent and a second organic solvent, with the second organic solvent having a flash point temperature that is below the B-stage temperature. In some embodiments, the first organic solvent has a flash point temperature that is above the B-stage temperature.DETAILED DESCRIPTION

[0019] A composition, and in some embodiments, an adhesive composition for use in electronic apparatus is provided. The composition includes a curable resin system having a matrix precursor, thermally conductive particulate filler, an organic solvent or solvent mixture, and a curing agent that is effective in curing the matrix precursor. The curable composition has an initial viscosity that is suitable for dispensation onto a substrate, and thereafter processed to increase viscosity prior to die attach. Such processing is referred to as B-stage processing, wherein a raw material is transformed from initial properties to an intermediate state without complete curing. In this application, the raw curable composition is readily dispensed onto a substrate, and then B-stage processed in situ to establish an increased, B-stage viscosity that promotes overall handleability and aids in preventing overflow of the composition during assembly procedures, such as die attach procedures. Subsequent to the electronic device assembly procedure in which a die or other component is secured to the substrate via the adhesive composition, the curable composition is exposed to cure conditions to fully cure the matrix precursor.

[0020] The B-stage process, overall, is used to control the flowability of the curable composition in order to reduce overflow of the composition during assembly procedures onto a top surface of a die. The B-stage process may include, for example, thermal treatment to increase the viscosity of the composition through flashing off a portion of the solvent.

[0021] The materials of the present invention may exhibit thermal conductivities in the range of at least 1 W / m*K, and preferably at least 20 W / m*K, and in some embodiments at least 30 W7m*K.2024P00051

[0022] The compositions of the present invention may, in some embodiments, be provided in a one-part system including a curing agent that is effective with exposure to an activator such as actinic radiation and / or heat. In some embodiments, the one-part systems may include reaction inhibitors that are denatured with exposure to the activator. In other embodiments, the compositions of the present invention may be provided in a two-part system, reactable upon mixture of the two initially separate parts.

[0023] As used herein to describe components of the binder resin, “thermoplastic” is differentiated from “thermosetting” and refers to a resin which softens and melts when exposed to heat and re-solidifies to an often brittle and glassy state when cooled sufficiently. On the other hand, a thermosetting polymer irreversibly solidifies when heated. Thermosetting resins materials are typically resins that attain this set or solid state through being “dried” under the action of heat, through being “cured” via a chemical reaction requiring a curing agent, or through curing under irradiation.

[0024] As used herein, a “die” is a singular, semi -conductive element disposed on a semiconductor wafer and generally separated from its neighboring die(s) by scribe lines. After semiconductor wafer fabrication steps are completed, the die is generally separated into elements or units by a die singulation process, such as sawing.Matrix Precursor

[0025] The matrix precursor of the present invention commonly contains a thermosetting resin. Typically, such a thermosetting resin will be selected from the group consisting of epoxy resin; oxetane resins; oxazoline resins; benzoxazine; resole; maleimides; cyanate esters; acrylate resins; methacrylate resins; maleates; fumarates; itaconates; vinyl esters; vinyl ethers; cyanoacrylates; styrenics; and, combinations thereof. Preferably, the thermosetting resin comprises one or more of: an epoxy resin, an acrylate resin, and a methacrylate resin. In particular, the thermosetting resin comprises an epoxy resin.

[0026] An epoxy resin is any compound containing at least one or more reactive oxirane groups, referred to herein as “epoxy group(s)” or “epoxy functionality”. Epoxy resins as used herein may include mono-functional epoxy resins, multi- or poly-functional epoxy resins, and combinations thereof. The epoxy resins may be pure compounds but equally may be mixtures2024P00051 epoxy functional compounds, including mixtures of compounds having different numbers of epoxy groups per molecule. An epoxy resin may be saturated or unsaturated, aliphatic, cycloaliphatic, aromatic or heterocyclic and may be substituted. Further, the epoxy resin may also be monomeric or polymeric.

[0027] Suitable polymeric epoxies for use in the present invention include but are not limited to: linear polymers having terminal epoxy groups, for example a diglycidyl ether of a polyoxyalkylene glycol; polymer skeletal oxirane units, for example polybutadiene polyepoxide; and, polymers having pendant epoxy groups, for example a glycidyl methacrylate polymer or copolymer.

[0028] In an embodiment, the matrix precursor of the composition comprises an epoxy resin selected from the group consisting of cycloaliphatic epoxy resins, cycloaliphatic epoxy resins modified with glycols, hydrogenated aromatic epoxy resins, epoxy phenolic novolac resins and cresol novolac type epoxy resins, bisphenol A-based epoxy resins, bisphenol F-based epoxy resins, and mixtures thereof.

[0029] A cycloaliphatic epoxy resin according to the present invention is a hydrocarbon compound containing at least one non-aryl hydrocarbon ring structure and containing one, two or more epoxy groups. The cycloaliphatic epoxy compound may include an epoxy group fused to the ring structure and / or an epoxy group residing on an aliphatic substituent of the ring structure. The cycloaliphatic epoxy resin may have at least one epoxy group residing on an aliphatic substituent of the ring. An example cycloaliphatic glycidyl ester is EPALLOY 5200 from Emerald Performance Materials.

[0030] In some embodiments, the matrix precursor of the composition may include a hydrogenated aromatic epoxy resin, a cycloaliphatic epoxy resin or a mixture thereof. In particular, the matrix precursor may comprise an epoxy resin selected from the group consisting of 1,2-cy cl ohexanedi carboxylic acid diglycidyl ester, bis(4-hydroxycylohexyl)methanediglycidyl ether, 4-methylhexahydrophthalic acid diglycidyl ester, 2,2-bis(4-hydroxycyclohexyl)propane diglycidyl ether, 3,4-epoxycyclohexylmethyl-3', 4'-epoxycylohexane carboxylate, bis(3,4- epoxycyclohexylmethyl)adipate, and mixtures thereof. Good results have, in particular, been obtained where the cycloaliphatic epoxy resins include 1,2-cy cl ohexanedicarboxylic acid diglycidyl ester, 2,2-bis(4-hydroxycyclohexyl)propane diglycidyl ether, or mixtures thereof.2024P00051

[0031] The epoxy resin materials of the present invention may preferably be any of a liquid, semi-solid, and a solid dissolved or suspended in a liquid such as a solvent, each at 25 °C. In some embodiments, the epoxy resin materials of the present invention may preferably by in a liquid state at 25 °C to minimize pre-cure viscosity. The epoxy resin may include a combination of distinct epoxy resins that are curable into a material of desired properties.

[0032] Other epoxies suitable for use herein include polyglycidyl derivatives of phenolic compounds, such as those available commercially under the tradename 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 epoxies include polyepoxides prepared from polyols and the like and polyglycidyl derivatives of phenol-formaldehyde novolacs, the latter of such as DEN 431, DEN 438, and DEN 439 from Dow Chemical. Cresol analogs are also available commercially under the tradename ARALDITE, such as ARALDITE ECN 1235, ARALDITE ECN 1273, and ARALDITE ECN 1299 from Ciba Specialty Chemicals Corporation, as well as EPICLON N-665 from DIC Corporation. SU-8 is a bisphenol-A-type epoxy novolac available from Resolution.Polyglycidyl adducts of amines, aminoalcohols and polycarboxylic acids are also useful in this invention, commercially available resins of which include GLYAMINE 135, GLYAMINE 125, and GLYAMINE 115 from F.I.C. Corporation; ARALDITE MY-720, ARALDITE 0500, and ARALDITE 0510 from Ciba Specialty Chemicals and PGA-X and PGA-C from the Sherwin- Williams Co.

[0033] The matrix precursor of the present invention may be present in a range of between 0.1 wt.% and 15 wt.% of the total composition. In some embodiments, the matrix precursor of the present invention may be present in a range of between 0.5 wt.% and 10 wt.% of the total composition. In some embodiments, the matrix precursor of the present invention may be present in a range of between 1 wt.% and 5 wt.% of the total composition. The matrix precursors of the present invention may be provided in a single part composition, or in a first part of a multi-part composition, such as in a first part of a two-part composition. In the case of a two-part composition, the first part may be mixed with the second part of the composition to cure the precursor with a curing agent that is initially provided in the second part of the composition. For the purposes hereof, the term “cure” is intended to mean a cross-linking reaction to form a tri-dimensional polymer network.2024P00051Curing Agent

[0034] The matrix precursors of the present invention may typically require a curing agent in order to cure. The choice of hardener / curing agent is not particularly limited, except that it must comprise functional groups suitable for reacting with the functional groups on the matrix precursors in order to achieve cross-linking. Preferably, the curing agents utilized in the curable compositions of the present invention become reactive at an elevated temperature. For the purposes hereof, such elevated temperature at which the curing agents of the present compositions are reactive with the matrix precursors to initiate the cross-linking cure reaction is considered the “cure temperature”. In some embodiments, the cure temperature is between 100 °C and 400 °C. In some embodiments, the cure temperature is between 120 °C and 350 °C. In some embodiments, the cure temperature is between 180 °C and 300 °C. In some embodiments, the cure temperature is between 200 °C and 250 °C.

[0035] Various curing agents contemplated for use in the compositions of the invention include anhydride, ureas, aliphatic and aromatic amines, polyamides, imidazoles, dicyandiamides, hydrazides, urea-amine hybrid curing systems, free radical initiators, organic bases, transition metal catalysts, phenols, acid anhydrides, Lewis acids, Lewis bases, and the like. In some embodiments, an anhydride-based curing agent may be preferred to promote sintering of the particulate filler. Acid curing agents such as an anhydride-based curing agent provides cure shrinkage, which brings the particulates of the filler closer together in the cured product. Example anhydride-based curing agents useful in the curable compositions of the present invention include dodecynylsuccinic anhydride and methylhexahydrophthalicacid anhydride.

[0036] Curing agents may be present in the compositions of the invention in an amount effective to cure the matrix precursor at the cure temperature. In some embodiments, the curing agent component is effective to cure an epoxy resin at the cure temperature. In some embodiments, the curing agent component is present in a range of between 0.1 wt.% and 10 wt.% of the total composition. In some embodiments, the curing agent component may be present in a range of between 0.1 wt.% and 5 wt.% of the total composition. In some embodiments, the curing agent component may be present in a range of between 0.1 wt.% and 32024P00051 wt.% of the total composition. In some embodiments, the curing agent component may be present in a range of between 0.5 wt.% and 2 wt.% of the total composition.Thermally Conductive Particulate Filler

[0037] The compositions of the present invention include thermally conductive particulate filler, which is dispersed in the matrix formed by the cured matrix precursor. Preferably, the thermally conductive particulate filler of the present compositions is sinterable at a sintering temperature. As used herein, the term “sintering” is a method for making objects from particles or powder by heating the material — below its melting point — until its particles adhere and / or fuse to each other. “Sinterable” refers to materials that can be sintered. “Sintered” refers to particles or powder that have undergone a sintering process. The “sintering temperature” is the temperature at which the thermally conductive particles become sintered. A sintered mass refers to the formed shape that is the result of the sintering of powders or particulate. In the sintered mass, formerly discrete particles or powder grains retain a core, and the interstitial area from one core to another core is at least partially filled with a grain boundary layer that separates the cores.

[0038] The sinterable particles of the thermally conductive particulate filler are sinterable at a sintering temperature. In some embodiments, the sintering temperature of the thermally conductive particulate filler is less than 350 °C. In some embodiments, the sintering temperature of the thermally conductive particulate filler is less than 300 °C. In some embodiments, the sintering temperature of the thermally conductive particulate filler is less than 250 °C. In some embodiments, the sintering temperature of the thermally conductive particulate filler is less than 220 °C.

[0039] An aspect of the present invention is obtaining a sintered thermally conductive filler. It has been found that a sintered filler enhances the thermal conductivity properties of the adhesive composition. To achieve a sintered filler, the sintering temperature of the thermally conductive particulate filler may preferably be equal to or less than the cure temperature of the matrix precursor. In this manner, the filler may be sintered as the curable composition is heated to its cure temperature.

[0040] As used herein, the sinterable particles can be a pure material powder, a metal particle coated with silver or copper on its surface, or a mixture thereof. The sinterable particles2024P00051 can be a commercially available product or may be prepared methods known in the art, such as mechanical milling, reduction, electrolysis and vapor phase processes. In some embodiments, the sinterable particles are silver.

[0041] Where a metal particle coated with silver on its surface is used as at least a portion of the sinterable particulate filler, the core of the particle may be constituted by copper, iron, zinc, titanium, cobalt, chromium, tin, manganese or nickel or alloys of two or more of said metals, and a coating of silver should constitute at least 5 wt. %, preferably at least 20 wt. % and more preferably at least 40 wt. % based on the weight of the particle. Such a silver coating may be formed by electroless Ag-plating, electroplating or vapor deposition, as is known in the art.

[0042] Subject to meeting the defined parameters of particle size, surface area and tap density, there is no intention to limit the actual physical shape of the particles within the particulate filler. The particles may be, for instance, spheres, flakes, leaf-like particles, dendritic particles or combinations thereof. A preference for flakes and spheres might be mentioned.

[0043] The sinterable particulate filler of the present invention is characterized by having a polydisperse particle population: it is a population of particles in which there is a range of particle sizes. The particles have thus been defined by specific “D-values” which herein provides a “mass division diameter”: it is the diameter which, when all particles in a sample are arranged in order of ascending mass, divides the sample's mass into specified percentages. The percentage mass of particles below the diameter of interest is the number expressed after the “D”. For example, the D10 diameter is the diameter at which 10% of a sample's mass is comprised of smaller particles, and D50 (mass median diameter) is the diameter at which 50% of a sample's mass is comprised of smaller particles. The maximum diameter is the maximum value in the particle diameter distribution and herein designated as DI 00.

[0044] In some applications of the present invention, the maximum particle diameter (D100) of the sinterable particulate filler is not critical. However, it is noted that the sinterable particulate filler will generally have a maximum particle diameter (D100) of less than 75 pm, for example less than 60 pm, less than 50 pm, less than 30 pm, or of less than 25 pm, for example less than 10 pm or less than 7.5 pm. Alternatively or additionally, the sinterable particulate filler may have a D90 diameter of less than 7 pm, for example less than 6 pm or less than 5.5 pm.

[0045] The D10, D50 (mass median diameter), D90 and DI 00 particle sizes may be obtained using conventional light scattering techniques and equipment, such as: Hydro 20002024P00051MU, available from: Malvern Instruments, Ltd., Worcestershire, United Kingdom; or Sympatec Helos, Clausthal-Zellerfeld, Germany.

[0046] The “tap density” of the particles recited herein is determined in accordance International Organization for Standardization (ISO) Standard ISO 3953. The principle of the method specified is tapping a specified amount of powder in a container — typically a 25 cm3graduated glass cylinder — by means of a tapping apparatus until no further decrease in the volume of the powder takes place. The mass of the powder divided by its volume after the test gives its tap density.

[0047] As used herein, the term “specific surface area” refers to the surface area per unit mass of the particles concerned. As is known in the art, the Brunauer, Emmett, and Teller (BET) method may be employed to measure the specific surface area of said particles, which method include the steps of flowing gas over a sample, cooling the sample, and subsequently measuring the volume of gas adsorbed onto the surface of the sample at specific pressures.

[0048] For completeness, commercially available silver powders suitable for inclusion in the present invention include but are not limited to: FA-SAB-534, and Ag-SAB-307 available from Dowa; P554-19, P620-22, P698-1, F741-6, F747-3 and F781-1, available from Metalor; and SF134, available from Ames-Goldsmith.

[0049] In some embodiments, the particulate thermally conductive filler may be substantially spherical, plate-like, rod-like, or combinations thereof. It is contemplated that a particle size distribution may be employed to fit the parameters of any particular application, although certain particle size distributions may be found to be more effective than others.

[0050] Thermally conductive particles used in the compositions of the present invention may be present in the range of between 60 and 99 wt.%. In some embodiments, the thermally conductive particles may be present in the range of between 65 and 97 wt.% of the total composition. In some embodiments, the thermally conductive particles may be present in the range of between 70 and 95 wt.% of the total composition. In some embodiments, the thermally conductive particles may be present in the range of between 75 and 95 wt.% of the total composition. In some embodiments, the thermally conductive particles may be present in the range of between 80 and 90 wt.% of the total composition. In some embodiments, the thermally conductive particles may be present in the range of at least 80 wt.% of the total composition.2024P00051

[0051] In a useful embodiment, the particulate thermally conductive fdler may be a multi-modal particle size distribution, having discrete concentrations of particles with different average particle sizes. In some embodiments, a first portion of the thermally conductive filler may have an average particle size (dso) of less than 1 micrometer, and a second portion of the thermally conductive filler may have an average particle size (dso) of greater than 1 micrometer. In some embodiments, the first portion of the thermally conductive filler may comprise between 20 and 40 percent by weight of the total thermally conductive particulate filler. In some embodiments, the first portion of the thermally conductive filler may comprise between 25 and 35 percent by weight of the total thermally conductive particulate filler. Such particle size distributions can promote high thermal conductivity values without detracting from the desired physical properties of the composition.

[0052] In some embodiments, the thermally conductive particulate filler may be subjected to surface treatment or surface modification. Example surface treatment and surface modification include treatment with a silane coupling agent, phosphoric acid or a phosphoric acid compound, or a surfactant. The silane coupling agent may include at least one hydrolysable group such as an alkoxy group and an aryloxy group bonded to a silicon atom. Other examples include an alkyl group, and alkenyl group, and any aryl group bonded to the silicon atom. The thermally conductive particulate filler, whether or not surface treated or modified, may be blended with the curable resin and the curing agent by mixing the components in a mechanical mixer as needed to achieve a desired extent of dispersion of the particulate filler with the curable resin.Solvent

[0053] The thermally conductive curable compositions of the present invention preferably include solvent in a range of between 0.1 wt.% and 15 wt.%. In some embodiments, the solvent is present in a range of between 1 wt.% and 10 wt.% of the total composition. In some embodiments, the solvent is present in a range of between 1 wt.% and 8 wt.% of the total composition.

[0054] The solvent or solvent mixture used in the thermally conductive curable compositions of the present invention is preferably compatible with the matrix precursor, and2024P00051 preferably aids in dispensation of the composition onto a substrate by reducing or maintaining a relatively low initial viscosity for the composition. Moreover, the solvent or solvent mixture used in the thermally conductive curable compositions of the present invention may facilitate an embodiment of B-stage processing, wherein the thermally conductive curable composition is partially dried at a B-stage temperature that is less than the cure temperature to adjust the initial viscosity of the thermally conductive curable composition to a B-stage viscosity that is higher than the initial viscosity. The B-stage viscosity is preferably sufficiently high to promote handleability of the thermally conductive curable composition, and particularly to limit or avoid overflow of the curable composition during assembly procedures, such as die attach procedures. For the purposes hereof, the term “B-stage processing” is intended to mean treatment of the thermally conductive curable composition in situ to transition the composition to an intermediate, B-stage condition. The “B-stage temperature” is a treatment temperature in the B- stage processing. The “B-stage viscosity” is the viscosity of the thermally conductive curable composition at the intermediate, B-stage condition. For the purposes hereof, the B-stage condition may include a partially dried condition of the curable composition. For the purposes hereof, “partially dried” or “partially drying” means the evaporation of a portion, less than 100%, of the solvent of the thermally conductive curable composition.

[0055] Preferably, the B-stage processing does not fully dry the thermally conductive curable composition, such that the B-stage viscosity, while higher than the initial viscosity, remains somewhat flowable and tacky to assist in securing components, such as dies, to the substrate.

[0056] To facilitate B-stage processing involving solvent drying, the solvent or solvent mixture preferably has a sufficiently low flash point temperature to permit partial, but incomplete, drying at a B-stage temperature that is less than the cure temperature. For the purposes hereof, the term “flash point temperature” is intended to mean the lowest temperature at which, under standardized conditions, a liquid gives off vapors in a quantity such as to be capable of forming an ignitable vapor / air mixture. The flash point is therefore dependent on the boiling point and consequent vapor pressure of the liquid. The boiling point of a solvent is when the vapor pressure is equal to the atmospheric pressure, and the flash point is when the ratio between the atmospheric pressure and the vapor pressure correspond to the ratio required to ignite the combustible.2024P00051

[0057] In some embodiments, the solvent or solvent mixture has a flash point temperature of less than 200 °C. In some embodiments, the solvent or solvent mixture has a flash point temperature of less than 175 °C. In some embodiments, the solvent or solvent mixture has a flash point temperature of less than 160 °C. In some embodiments, the solvent or solvent mixture has a flash point temperature of less than 150 °C. In some embodiments, the solvent or solvent mixture has a flash point temperature of less than 125 °C. In some embodiments, the solvent or solvent mixture has a flash point temperature of less than 100 °C. In some embodiments, the solvent or solvent mixture has a flash point temperature of less than 80 °C. In some embodiments, the solvent or solvent mixture has as flash point temperature of less than 70 °C. In some embodiments, the solvent or solvent mixture has a flash point temperature of less than 60 °C. In some embodiments, the solvent or solvent mixture has a flash point temperature of less than 50 °C. In some embodiments, the solvent or solvent mixture has a flash point temperature of less than 40 °C. For the purposes of this disclosure, the flash point temperature is measured by ASTM D93, Pensky -Martens closed cup flash point. For solvent mixtures including more than one solvent, the relevant flash point temperature for the mixture is the lowest temperature that meets the ASTM D93 criteria for flash point.

[0058] In some embodiments, the solvent or solvent mixture has a boiling point temperature of less than 200 °C. In some embodiments, the solvent or solvent mixture has a boiling point temperature of less than 175 °C. In some embodiments, the solvent or solvent mixture has a boiling point temperature of less than 160 °C. In some embodiments, the solvent or solvent mixture has a boiling point temperature of less than 150 °C. In some embodiments, the solvent or solvent mixture has a boiling point temperature of less than 125 °C. In some embodiments, the solvent or solvent mixture has a boiling point temperature of less than 100 °C. In some embodiments, the solvent or solvent mixture has a boiling point temperature of less than 80 °C. In some embodiments, the solvent or solvent mixture has as boiling point temperature of less than 70 °C. In some embodiments, the solvent or solvent mixture has a boiling point temperature of less than 60 °C. In some embodiments, the solvent or solvent mixture has a boiling point temperature of less than 50 °C. In some embodiments, the solvent or solvent mixture has a boiling point temperature of less than 40 °C. For the purposes of this disclosure, the boiling point temperature is measured at standard atmospheric pressure (760 mmHg). For2024P00051 solvent mixtures including more than one solvent, the relevant boiling point for the mixture is the lowest temperature at which a vapor pressure of the mixture is equal to the atmospheric pressure.

[0059] In some embodiments, the thermally conductive curable compositions of the invention include a first solvent and a second solvent, wherein the first solvent has a first flash point temperature and a first boiling point temperature, and the second solvent has a second flash point temperature and a second boiling point temperature, and wherein the first flash point temperature is greater than the second flash point temperature. In some embodiments, the first boiling point temperature is greater than the second boiling point temperature. In some embodiments, the first flash point temperature is equal to greater than the B-stage temperature. In some embodiments, the first flash point temperature is equal to or greater than the B-stage temperature, the second flash point temperature is equal to or less than the B-stage temperature, and the first flash point temperature is greater than the second flash point temperature. In some embodiments, the first boiling point temperature is equal to or greater than the B-stage temperature. In some embodiments, the first boiling point temperature is equal to or greater than the B-stage temperature, the second boiling point temperature is equal to or less than the B-stage temperature, and the first boiling point temperature is greater than the second boiling point temperature.

[0060] As described herein, the thermally conductive curable composition may be B- stage processed to partially dry the composition at a B-stage temperature. In some embodiments, the partial drying of the thermally conductive curable composition is facilitated by including at least two distinct solvents having different flash point temperatures and different boiling point temperatures. As the curable composition is heated to the B-stage temperature, the solvent with the lower flash point / boiling point temperature may be partially or completely evaporated without completely evaporating the solvent with the higher flash point / boiling point temperature. In some embodiments, the flash point and / or boiling point temperature of the second solvent may be less than the B-stage temperature to facilitate selective evaporation of the second solvent as the thermally conductive curable composition is heated to and maintained at the B-stage temperature for B-stage processing. Because the flash point temperature and / or boiling point temperature of the first solvent is equal to or higher than the B-stage temperature, at least some of the first solvent may not be evaporated, and may act to maintain lower viscosity of the curable composition than would otherwise be exhibited by a fully dried composition. A combination of2024P00051 two or more solvents with diverse evaporation characteristics preferably provides a curable composition with good dispensability in an initial condition with a full compliment of solvent, and good B-stage processability post-dispensation to partially dry the composition, leaving a B- staged composition that remains somewhat flexible and tacky, but not overly soft that can cause uncured adhesive overflow during pre-cure assembly procedures.

[0061] In some embodiments, the thermally conductive curable compositions of the present invention may include a first solvent having a flash point temperature above 40 °C, and a second solvent having a flash point temperature below 40 °C. In some embodiments, the first solvent has a first flash point temperature above 50 °C, 60 °C, 80 °C, 100 °C, 120 °C, 150 °C, 180 °C, or 200 °C, and the second solvent has a second flash point temperature below the first flash point temperature, and below 180 °C, 150 °C, 120 °C, 100 °C, 80 °C, 60 °C, or 50 °C.

[0062] In some embodiments, the thermally conductive curable compositions of the present invention may include a first solvent having a boiling point temperature above 40 °C, and a second solvent having a boiling point temperature below 40 °C. In some embodiments, the first solvent has a first boiling point temperature above 50 °C, 60 °C, 80 °C, 100 °C, 120 °C, 150 °C, 180 °C, or 200 °C, and the second solvent has a second boiling point temperature below the first flash point temperature, and below 180 °C, 150 °C, 120 °C, 100 °C, 80 °C, 60 °C, or 50 °C.

[0063] In some embodiments, the thermally conductive curable compositions of the present invention may include first and second solvents with different flash point temperatures and / or different boiling point temperatures. When the second solvent has a lower flash point temperature and / or boiling point temperature than the first solvent, the first and second solvents may preferably be present in the curable compositions in a weight ratio of first solvent : second solvent between 1 :1 and 20:1. In some embodiments, the weight ratio of first solvent : second solvent is between 2: 1 and 15: 1. In some embodiments, the weight ratio of first solvent : second solvent is between 3 : 1 and 10: 1. A weight ratio having too little of the first solvent with respect to the second solvent has the effect of making the B-staged material too brittle and insufficiently tacky to accommodate die attach procedures. A weight ratio having too much of the first solvent with respect to the second solvent has the effect of making the B-staged material too soft and susceptible to overflow during die attach procedures.

[0064] The solvent or solvent mixture of the thermally conductive curable compositions of the present invention may be organic for compatibility with the matrix precursor. Example2024P00051 solvents useful in the compositions of present invention, and particularly as high flash point / boiling point solvents include dipropylene glycol; ethylene glycol, diethylene glycol, tri ethylene glycol, hexylene glycol, l-methoxy-2-propanol, diacetone alcohol, 2-ethyl-l,3- hexanediol, tridecanol, 1,2-octanediol, butyldiglycol, alpha-terpineol or beta-terpineol, 2-(2- butoxyethoxy)ethyl acetate, 2,2,4-trimetyl-l,3-pentanediol diisobutyrate, 1,2-propylene carbonate, carbitol acetate, butyl carbitol acetate, butyl carbitol, ethyl carbitol acetate, 2-phenoxy ethanol, hexylene glycol, dibutylphthalate, dibasic ester (DBE), dibasic ester 9 (DBE-9), dibasic ester 7 (DBE-7), diethylene glycol butyl ether, diethylene glycol monoethyl ether, diethylene glycol monoethyl ether acetate, triethylene glycol monomethylether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and mixtures thereof. An example solvent useful as a low flash point / boiling point solvent includes methyl ethyl ketone. However, it is contemplated that other solvents may be used in the present compositions as understood by those of ordinary skill in the art.Other Additives

[0065] The thermally conductive curable compositions of the present invention may further include compatible additives and modifiers which serve to stabilize the composition and / or to control the composition’s rheology, substrate adhesion, and appearance. Example additives and modifiers for use in the present invention include thickeners, viscosity modifiers, rheology modifiers, wetting agents, leveling agents, adhesion promoters, de-foaming agents, and thermal conductivity promoters.

[0066] In some embodiments, a non-reactive filler may be included for benefit to the viscosity and / or rheology of the curable compositions. In some embodiments, the non-reactive filler may be included for the benefit of reducing the shrinkage of the matrix formed by the matrix precursor upon cure. The non-reactive filler is preferably of a size that does not interfere with a goal of a thin bond line, as well as a size that has functional benefit to the physical and operational properties of the composition. The non-reactive filler may therefore be smaller than the desired bond line thickness. In some embodiments, the non-reactive filler may have a maximum particle size of less than 100 pm, preferably less than 75 pm, preferably less than 60 pm, preferably less than 50 pm, preferably less than 40 pm, preferably less than 30 pm,2024P00051 preferably less than 20 pm, and preferably less than 10 pm. In some embodiments, the non- reactive filler may be substantially mono-disperse. In some embodiments, the non-reactive filler may be polymeric particles, with an aspect ratio of between 0.8 - 1.2, preferably 0.9 - 1.1, and preferably 0.95 - 1.05. An example non-reactive polymeric filler useful in the compositions of the present invention includes MicroPearl SP-203 from Sekisui Chemical.

[0067] The non-reactive filler may be present in a range of between 0.1 wt.% and 10 wt.% of the curable composition. In some embodiments, the non-reactive filler may be present in a range of between 0.5 wt.% and 8 wt.% off the curable composition. In some embodiments, the non-reactive filler may be present in a range of between 0.8 wt.% and 5 wt.% of the curable composition.Cured Composition

[0068] The thermally conductive curable compositions of the present invention, when cured, may form a film-like adhesive. For the purposes hereof, the term “film” means a thin film having a thickness of less than 250 pm, preferably less than 150 pm, preferably less than 100 pm, preferably less than 75 pm, and preferably less than 50 pm. In some embodiments, the film formed from the curable compositions of the present invention, alone or in combination, may be disposed at a surface of a substrate, such as a silicon substrate of an electronic package. The film may also be applied only a surface of a release-treated substrate liner for ease of handling and application to the package substrate. Example release-treated films include release-treated polypropylene, release-treated polyethylene, and release-treated polyethylene terephthalate. In other embodiments, the curable compositions of the present invention, when cured, may form adhesives in non-film form.

[0069] The curable compositions of the present invention exhibit good flowability at relatively low temperatures, such as room temperature. This facilitates processing of the compositions by liquid dispensation onto various substrates, including those which are temperature-sensitive.

[0070] To form the thermally conductive adhesives from the thermally conductive curable compositions, the thermally conductive curable composition is deposited on a surface. Techniques such as dispensing and printing can facilitate the application of the composition to a2024P00051 specific locus on the substrate. It is envisaged that the present composition can be applied to conventional high temperature substrates such as glass, silicon, silicon oxides, cadmium telluride, copper, silver, gold, indium gallium selenide and gallium arsenide. The application to low temperature substrates such as paper or polymer substrates is also not precluded. The thermally conductive curable composition exhibits an initial viscosity at 25 °C under a low shear environment as deposited on the substrate, such as a shear rate developed with a Brookfield DV- III type viscometer operated with a CP-51 spindle at 5 rpm. In some embodiments, the initial viscosity is in a range of between 1,000 and 70,000 cP at 25 °C. In some embodiments, the initial viscosity is in a range of between 5,000 and 50,000 cP at 25 °C. In some embodiments, the initial viscosity is in a range of between 7,000 and 30,000 cP at 25 °C. In some embodiments, the initial viscosity is in a range of between 10,000 and 20,000 cP at 25 °C.

[0071] The deposited thermally conductive curable composition is processed to adjust the viscosity to a B-stage viscosity. In some embodiments, the thermally conductive curable composition partially dried at a B-stage temperature. In some embodiments, the partially dried composition loses at least 10 wt.% of the initial total solvent present in the thermally conductive curable composition at the time of deposition, due to the drying step. In some embodiments, the partially dried composition loses at least 20 wt.% of the initial total solvent. In some embodiments, the partially dried composition loses at least 30 wt.% of the initial total solvent. In some embodiments, the partially dried composition loses at least 40 wt.% of the initial total solvent. In some embodiments, the partially dried composition loses at least 50 wt.% of the initial total solvent. In some embodiments, the partially dried composition loses at least 60 wt.% of the initial total solvent. In some embodiments, the partially dried composition loses at least 70 wt.% of the initial total solvent.

[0072] The B-stage temperature may be less than the cure temperature of the matrix precursor. In some embodiments, the B-stage temperature is less than 200 °C. In some embodiments, the B-stage temperature is less than 180 °C. In some embodiments, the B-stage temperature is less than 150 °C. In some embodiments, the B-stage temperature is in a range of between 20 °C and 150 °C. In some embodiments, the B-stage temperature is in a range of between 20 °C and 110 °C. In some embodiments, the B-stage temperature is in a range of between 20 °C and 80 °C. In some embodiments, the B-stage temperature is in a range of between 20 °C and 60 °C. In some embodiments, the B-stage temperature is in a range of2024P00051 between 20 °C and 50 °C. In some embodiments, the B-stage temperature is in a range of between 20 °C, 30 °C, 40 °C, 50 °C, 60 °C, 70 °C and 150 °C, 140 °C, 130 °C, 120 °C, 100 °C, 90 °C, 80 °C.

[0073] The step of processing the thermally conductive curable composition is suitable to adjust viscosity of the curable composition from its initial viscosity to a B-stage viscosity at 25 °C that is greater than the initial viscosity. In some embodiments, the B-stage viscosity is at least 10% greater than the initial viscosity. In some embodiments, the B-stage viscosity is at least 25% greater than the initial viscosity. In some embodiments, the B-stage viscosity is at least 50% greater than the initial viscosity. In some embodiments, the B-stage viscosity is at least 100% greater than the initial viscosity. In some embodiments, the B-stage viscosity is at least 200% greater than the initial viscosity. In some embodiments, the B-stage viscosity is at least 300% greater than the initial viscosity. In a particular embodiment, the B-stage viscosity is between 20,000 cP and 200,000 cP at 25 °C.

[0074] Once the thermally conductive curable composition has undergone B-stage processing as described herein, the device may be assembled by applying a component, such as a die, to the curable composition. An example assembly process is referred to as the “die attach” procedure, wherein a die is placed at the curable composition with an attachment force. The die attach procedure is known in the art. Subsequent to the assembly process, the curable composition on the substrate is exposed to the cure temperature that is sufficient to cure the matrix precursor. The composition may be heated to the cure temperature in a conventional heating furnace, by infra-red irradiation, laser, microwave radiation, and any other photonic radiation. In some embodiments, the atmosphere within which the curable composition is exposed to the cure temperature sufficient to cure the matrix precursor is appropriate for the selected matrix precursor, as determined by those of ordinary skill in the art. In some embodiments, the environment may be reducing, oxygen-containing, or inert. Moreover, the heating can be conducted with or without the application of pressure. In some embodiments, a pressure of between 1 and 5 ATM may be typical. The curable composition is heated at the cure temperature for a sufficient time to permit a desire cure of the matrix precursor. In some embodiments, the curable composition is heated at the cure temperature for a sufficient time to fully cure the matrix precursor. For the purposes hereof, the term “fully cure” means at least 90% of the cross-linkable sites in the matrix precursor are cross-linked.2024P00051

[0075] In some embodiments, the curable composition is heated at the cure temperature for a sufficient time to permit the sintering of at least a portion of the thermally conductive filler. In some embodiments, an external pressure is applied to the curable composition while the curable composition is exposed to the cure temperature. The external pressure may aid in sintering the thermally conductive filler. In some embodiments, the external applied pressure is a force of at least 50g. In some embodiments, the external applied pressure is a force of at least 100g. In some embodiments, the external applied pressure is a force of at least 250g. In some embodiments, the external applied pressure is a force of 300g. In some embodiments, the external applied pressure is a force of at least 500g. In some embodiments, the external applied pressure is a force of at least 1000g. In some embodiments, the external applied pressure is a force of at least 2000g.EXAMPLES

[0076] To form the thermally conductive compositions described in Table 1 below, the silver particles, the epoxy resin(s), solvents, curing agent, non-reactive polymer particles, and any additives were mixed under sufficient agitation to prevent observable silver particle aggregations. The compositional values given in Table 1 are weight % based on the total weight of the composition.2024P00051Table 1

[0077] The Example compositions were stencil printed onto a metal substrate using a 5x5mm stencil to a coating thickness of about 50-100 pm. The coated substrates were then placed in an oven pre-set to 50 °C, and held in the oven for 15 minutes for B-stage processing of the curable composition. A 3x3mm Si-Ag die was subsequently bonded to each B-staged sample by applying various die attach (DA) bonding force at various bonding temperatures for 50ms. Subsequent to assembly of the dies to the coated substrates, the compositions were cured by placing in an oven in a nitrogen environment at room temperature, and ramping the temperature to 220 °C over 30 minutes, and thereafter holding the temperature at 220 °C for 1 hour.

[0078] Following cooling of the cured samples for 24 hours at room temperature, the samples were tested for die shear strength in accordance with test method MIL STD883G, method 2019.7. Die shear strength and adhesive failure mode results are set forth below in Table 2.2024P00051Table 2

[0079] Examples 3-6 showed good results in the die shear / adhesion testing, likely due to the presence of high-flash point solvent (di ethylene glycol butyl ether) that survives B-stage processing, such that the B-staged composition remains tacky for good adhesion and sinterability during the die attach process. Examples 3-6 also demonstrated good fillet control under 50g of die attach force.

Claims

2024P00051What is claimed is:

1. A method for processing adhesive from a thermally conductive curable composition, the method comprising: providing the thermally conductive curable composition with: a resin matrix precursor that is curable at a cure temperature; a thermally conductive particulate filler that is sinterable at a sintering temperature that is equal to or lower than the cure temperature; an organic solvent or solvent mixture, wherein the thermally conductive curable composition exhibits an initial viscosity at 25 °C; processing the thermally conductive curable composition to adjust the viscosity of the thermally conductive curable composition from the initial viscosity to a B-stage viscosity at 25 °C that is at least 10% greater than the initial viscosity; and subsequent to step (b), exposing the thermally conductive curable composition to the cure temperature sufficient to sinter at least a portion of the thermally conductive particulate filler.

2. The method as in claim 1 wherein the processing includes partially drying the thermally conductive curable composition at a B-stage temperature to remove some of the solvent or solvent mixture.

3. The method as in claim 2 wherein the B-stage temperature is between 20 °C and 110 °C.

4. The method as in claim 3 wherein the cure temperature is equal to or less than 300 °C.

5. The method as in claim 4, including exposing the thermally conductive curable composition to the cure temperature for a cure time period sufficient to fully cure the resin matrix precursor.

6. The method as in claim 5, including applying an external pressure to the thermally conductive curable composition while exposing the thermally conductive curable composition to the cure temperature.2024P000517. The method as in claim 2 wherein the solvent mixture includes a first organic solvent and a second organic solvent, with the second organic solvent having a flash point temperature that is below the B-stage temperature.

8. The method as in claim 6 wherein the first organic solvent has a flash point temperature above the B-stage temperature.

9. The method as in claim 1 wherein the initial viscosity is between 1,000 cP and 70,000 cP at 25 °C, and the B-stage viscosity is at least 100% greater than the B-stage viscosity.

10. A thermally conductive curable composition, comprising: a. a matrix precursor that is curable at a cure temperature; b. a thermally conductive particulate filler that is sinterable at a sintering temperature that is equal to or lower than the cure temperature, the thermally conductive particulate filler being present in a range of between 60 wt.% and 99 wt.% of the total composition; and c. a solvent or solvent mixture having a flash point temperature equal or less than 200 °C.

11. The thermally conductive curable composition as in claim 10 wherein the matrix precursor is in a liquid phase at 25 °C.

12. The thermally conductive curable composition as in claim 11 wherein the matrix precursor includes an epoxy.

13. The thermally conductive curable composition as in claim 12, including a curing agent effective to cure the epoxy at the cure temperature.

14. The thermally conductive curable composition as in claim 13 wherein the cure temperature is between 120 °C and 350 °C.2024P0005115. The thermally conductive curable composition as in claim 10 wherein the thermally conductive particulate filler includes at least one of silver and copper.

16. The thermally conductive curable composition as in claim 10 wherein the solvent mixture includes a first organic solvent having a flash point temperature above 40 °C, and a second organic solvent having a flash point temperature below 40 °C, both at 760 mm Hg.

17. The thermally conductive curable composition as in claim 16 wherein the first and second organic solvents are present in the composition in a weight ratio of first organic solvent : second organic solvent between 1 : 1 and 20: 1.

18. The thermally conductive curable composition as in claim 17 wherein the weight ratio is between 3: 1 and 10: 1.

19. The thermally conductive curable composition as in claim 10, including non-reactive organic polymer particles having a maximum particle size of less than 50 pm.

20. A thermally conductive curable composition, comprising: a. an epoxy resin matrix precursor present in a range of between 0.1 wt.% and 15 wt.% of the composition; b. a thermally conductive particulate filler including thermally conductive particles that are sinterable at a sintering temperature that is less than 350 °C; c. a first solvent selected from diethylene glycol butyl ether, diethylene glycol monoethyl ether, dibasic ester 9, diethylene glycol monoethyl ether acetate, trietheylene glycol monomethylether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and combinations thereof; and d. a second solvent having a flash point temperature of less than 40 °C at 760 mm Hg.2024P0005121 . The thermally conductive curable composition as in claim 20 wherein the thermally conductive particulate fdler includes at least one of silver and copper.

22. The thermally conductive curable composition as in claim 21 wherein the first and second solvents are together present in a range of between 1 wt.% and 10 wt.% of the composition.

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