Classifications

• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
  • C09K5/00—Heat-transfer, heat-exchange or heat-storage materials, e.g. refrigerants; Materials for the production of heat or cold by chemical reactions other than by combustion
  • C09K5/08—Materials not undergoing a change of physical state when used
  • C09K5/14—Solid materials, e.g. powdery or granular
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
  • C08J5/18—Manufacture of films or sheets
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K3/00—Use of inorganic substances as compounding ingredients
  • C08K3/02—Elements
  • C08K3/04—Carbon
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K3/00—Use of inorganic substances as compounding ingredients
  • C08K3/02—Elements
  • C08K3/04—Carbon
  • C08K3/042—Graphene or derivatives, e.g. graphene oxides
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K5/00—Use of organic ingredients
  • C08K5/04—Oxygen-containing compounds
  • C08K5/13—Phenols; Phenolates
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K7/00—Use of ingredients characterised by shape
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W40/00—Arrangements for thermal protection or thermal control
  • H10W40/10—Arrangements for heating
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W40/00—Arrangements for thermal protection or thermal control
  • H10W40/20—Arrangements for cooling
  • H10W40/22—Arrangements for cooling characterised by their shape, e.g. having conical or cylindrical projections
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W40/00—Arrangements for thermal protection or thermal control
  • H10W40/20—Arrangements for cooling
  • H10W40/231—Arrangements for cooling characterised by their places of attachment or cooling paths
  • H10W40/242—Arrangements for cooling characterised by their places of attachment or cooling paths comprising thermal conductors between chips and the and the arrangements for cooling, e.g. compliant heat-spreaders
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W40/00—Arrangements for thermal protection or thermal control
  • H10W40/20—Arrangements for cooling
  • H10W40/25—Arrangements for cooling characterised by their materials
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W72/00—Interconnections or connectors in packages
  • H10W72/30—Die-attach connectors
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J2323/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
  • C08J2323/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
  • C08J2323/18—Homopolymers or copolymers of hydrocarbons having four or more carbon atoms
  • C08J2323/20—Homopolymers or copolymers of hydrocarbons having four or more carbon atoms having four to nine carbon atoms
  • C08J2323/22—Copolymers of isobutene; butyl rubber
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J2433/00—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers
  • C08J2433/04—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers esters
  • C08J2433/06—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers esters of esters containing only carbon, hydrogen, and oxygen, the oxygen atom being present only as part of the carboxyl radical
  • C08J2433/08—Homopolymers or copolymers of acrylic acid esters
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K3/00—Use of inorganic substances as compounding ingredients
  • C08K3/02—Elements
  • C08K3/08—Metals
  • C08K2003/0806—Silver
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K2201/00—Specific properties of additives
  • C08K2201/001—Conductive additives
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K2201/00—Specific properties of additives
  • C08K2201/002—Physical properties
  • C08K2201/005—Additives being defined by their particle size in general

Definitions

• This disclosure relates to a thermally conductive sheet, a heat dissipation device, and a method for manufacturing a thermally conductive sheet.
• Heat dissipation devices are commonly used to dissipate heat by sandwiching thermally conductive grease or a thermally conductive sheet between a heat source such as a semiconductor package and a heat sink such as aluminum or copper. Thermally conductive sheets are usually easier to work with than thermally conductive grease when assembling heat dissipation devices.
• Resin sheets filled with thermally conductive fillers are also known as thermally conductive sheets.
• Various resin sheets filled with thermally conductive fillers and excellent in thermal conductivity have been proposed, in which inorganic particles with high thermal conductivity are selected as the thermally conductive filler and the inorganic particles are further oriented perpendicular to the sheet surface.
• a thermally conductive sheet in which thermally conductive filler (boron nitride) is oriented in a direction approximately perpendicular to the sheet surface see, for example, Patent Document 1
• a thermally conductive sheet in which carbon fibers dispersed in a gel-like substance are oriented perpendicular to the sheet surface see, for example, Patent Document 2
• Patent Documents 1 and 2 discuss a method of suppressing thermal resistance by orienting thermally conductive fillers, carbon fibers, etc. in a direction perpendicular to the sheet surface.
• the object of one aspect of the present invention is to provide a thermally conductive sheet with low thermal resistance, a heat dissipation device including the same, and a method for manufacturing a thermally conductive sheet that can produce a thermally conductive sheet with low thermal resistance.
• a thermal conductive sheet comprising: ⁇ 2> The thermal conductive sheet according to ⁇ 1>, wherein the resin component includes at least one selected from the group consisting of a curable resin component, an adhesive resin component, and a thermoplastic resin component.
• thermoplastic resin component contains a thermoplastic epoxy resin.
• thermally conductive sheet according to any one of ⁇ 1> to ⁇ 5>, wherein the thermally conductive filler is at least one type of particle selected from the group consisting of silver, copper, aluminum, aluminum oxide, aluminum hydroxide, magnesium oxide, beryllium oxide, boron nitride, aluminum nitride, silicon nitride, silicon carbide, silicon dioxide, aluminum fluoride, calcium fluoride, and zinc oxide.
• the thermal conductive filler is silver particles.
• thermoly conductive sheet according to any one of ⁇ 1> to ⁇ 7> wherein the content of the thermally conductive filler is 70% by mass to 99% by mass with respect to the total amount of the adhesive layer.
• the adhesive layer has an average thickness of 2 ⁇ m to 50 ⁇ m.
• a method for producing a thermal conductive sheet according to any one of ⁇ 1> to ⁇ 9> comprising the steps of: preparing a composition containing the graphite particles (A); forming the thermal conductive layer using the composition; and forming an adhesive layer on at least a portion of a main surface of the thermal conductive layer.
• the present disclosure provides a thermally conductive sheet with low thermal resistance, a heat dissipation device including the same, and a method for manufacturing a thermally conductive sheet that can produce a thermally conductive sheet with low thermal resistance.
• FIG. 1 is a schematic diagram of a thermally conductive sheet according to one embodiment of the present invention
• 1 is a schematic cross-sectional view of a heat dissipation device according to an embodiment of the present invention, in which a heat generating body is a semiconductor chip and a heat dissipating body is a heat spreader.
• 1 is a schematic diagram of a thermally conductive sheet incorporated in a heat dissipation device according to one embodiment of the present invention
• FIG. 2 is a diagram showing the state of the interface by image analysis in Examples 1 and 2 and Comparative Example 1.
• FIG. 1 is a diagram showing the state of the interface by image analysis in Examples 3 to 7 and Comparative Examples 2 to 4.
• the present invention is not limited to the following embodiment.
• the components including element steps, etc.
• the term "step” includes not only a step that is independent of other steps, but also a step that cannot be clearly distinguished from other steps as long as the purpose of the step is achieved.
• the numerical range indicated using “to” includes the numerical values before and after "to” as the minimum and maximum values, respectively.
• each component may contain multiple types of the corresponding substance.
• the content or amount of each component means the total content or amount of the multiple substances present in the composition, unless otherwise specified.
• the particles corresponding to each component may include multiple types.
• the particle size of each component means the value for a mixture of the multiple types of particles present in the composition, unless otherwise specified.
• the terms “layer” and “film” include cases where the layer or film is formed over the entire area when the area in which the layer or film is present is observed, as well as cases where the layer or film is formed over only a portion of the area.
• laminate refers to stacking layers, where two or more layers may be bonded together or two or more layers may be removable.
• the thermal conduction sheet of the present disclosure comprises a thermal conduction layer containing at least one type of graphite particles (A) selected from the group consisting of scale-like particles, ellipsoidal particles, and rod-shaped particles, with the planar direction in the case of the scale-like particles, the major axis direction in the case of the ellipsoidal particles, and the major axis direction in the case of the rod-shaped particles being oriented in the thickness direction, and an adhesive layer containing a resin component and a thermally conductive filler and located on at least a part of a main surface of the thermal conduction layer.
• A graphite particles
• the thermally conductive sheet of the present disclosure is believed to have excellent thermal conductivity in the thickness direction and low thermal resistance due to the thermally conductive layer in which the graphite particles (A) are oriented in the thickness direction.
• the thermally conductive sheet exhibits lower thermal resistance by providing the above-mentioned adhesive layer together with the thermally conductive layer containing graphite particles (A).
• the reason for this is presumed to be as follows. Note that the present disclosure is not limited to the following presumption. In a thermally conductive sheet in which the graphite particles (A) are oriented in the thickness direction, unevenness exists on the surface that contacts the adherend, and most of the thermal resistance is derived from resistance due to gaps generated by contact between the thermally conductive sheet and the adherend, such as a heating body or a heat sink, which contacts the thermally conductive sheet (also referred to as "contact thermal resistance").
• an adhesive layer containing a resin component and a thermally conductive filler is disposed on at least a part of the main surface of the thermally conductive layer, so that the adhesive layer softens, deforms, etc. due to heat, pressure, etc., when the thermally conductive sheet and the adherend, such as a heating body or a heat sink, are heat-pressurized and bonded together.
• the adhesive layer softens, deforms, etc., gaps generated when the thermally conductive sheet and the adherend are heat-pressurized and bonded together (for example, gaps resulting from the unevenness of the thermally conductive sheet) are filled with the adhesive layer. This reduces the gap between the thermally conductive sheet and the adherend while allowing the thermally conductive sheet and the adherend to be in close contact with each other via the adhesive layer, significantly reducing the contact thermal resistance.
• thermally conductive sheet of the present disclosure the thermally conductive sheet can be attached to the adherend, which has an uneven surface, via the adhesive layer. At this time, gaps that occur when the thermally conductive sheet and the adherend are heated and pressed together (for example, gaps resulting from the unevenness of the adherend) are filled with the adhesive layer, so contact thermal resistance is significantly reduced.
• the thermally conductive layer included in the thermally conductive sheet of the present disclosure contains at least graphite particles (A) and may contain the components described below within the range in which the effects of the present disclosure are achieved.
• the materials used in the thermally conductive layer of the present disclosure are described below.
• the thermally conductive layer included in the thermally conductive sheet contains graphite particles (A).
• the graphite particles (A) are considered to function mainly as a highly thermally conductive filler.
• the graphite particles (A) are at least one selected from the group consisting of scaly particles, ellipsoidal particles, and rod-shaped particles.
• the graphite particles (A) are oriented in the thickness direction in the planar direction in the case of scaly particles, in the major axis direction in the case of ellipsoidal particles, and in the major axis direction in the case of rod-shaped particles.
• the six-membered ring plane in the crystal of the graphite particles (A) is oriented in the planar direction in the case of scaly particles, in the major axis direction in the case of ellipsoidal particles, and in the major axis direction in the case of rod-shaped particles.
• the six-membered ring plane is a plane on which a six-membered ring is formed in a hexagonal crystal system, and means a (0001) crystal plane.
• the graphite particles (A) are preferably flaky in shape.
• thermal conductivity tends to be improved. This is thought to be because, for example, flaky graphite particles are more easily oriented in a specific direction in the thermally conductive layer.
• the hexagonal ring planes in the crystals of graphite particles (A) are oriented in the planar direction of scale-shaped particles, the long axis direction of ellipsoidal particles, or the long axis direction of rod-shaped particles can be confirmed by X-ray diffraction measurement. Specifically, the orientation direction of the hexagonal ring planes in the crystals of graphite particles (A) is confirmed by the following method.
• a sample sheet for measurement is prepared in which the planar direction of the scale-like particles, the long axis direction of the ellipsoidal particles, or the long axis direction of the rod-like particles of the graphite particles (A) are oriented along the planar direction of the sheet.
• Specific methods for preparing the sample sheet for measurement include, for example, the following methods.
• a mixture of resin and graphite particles (A) in an amount of 10% by volume or more relative to the resin is formed into a sheet.
• the "resin” used here is not particularly limited as long as it is a material that does not exhibit peaks that interfere with X-ray diffraction and can be formed into a sheet.
• amorphous resins that have the cohesive power to act as a binder such as acrylic rubber, NBR (acrylonitrile butadiene rubber), and SIBS (styrene-isobutylene-styrene copolymer), can be used.
• a sheet of this mixture is pressed to 1/10 or less of its original thickness, and several of the pressed sheets are stacked to form a laminate.
• This laminate is further compressed to 1/10 or less, and this operation is repeated three or more times to obtain a sample sheet for measurement.
• This operation results in the graphite particles (A) in the sample sheet for measurement being oriented in the planar direction in the case of scale-like particles, in the major axis direction in the case of ellipsoidal particles, and in the major axis direction in the case of rod-like particles, along the planar direction of the sample sheet for measurement.
• X-ray diffraction measurement is performed on the surface of the measurement sample sheet prepared as described above.
• the value obtained by dividing H1 by H2 is 0 to 0.02.
• X-ray diffraction measurements are performed under the following conditions.
• Apparatus For example, Bruker AXS "D8DISCOVER" X-ray source: CuK ⁇ with wavelength of 1.5406 nm, 40 kV, 40 mA Step (measurement step size): 0.01° Step time: 720 sec
• graphite particles are oriented in the planar direction in the case of scaly particles, in the major axis direction in the case of ellipsoidal particles, and in the thickness direction in the case of rod-shaped particles” means that the angle (hereinafter also referred to as "orientation angle") between the planar direction in the case of scaly particles, the major axis direction in the case of ellipsoidal particles, and the major axis direction in the case of rod-shaped particles and the surface (principal surface) of the heat conduction layer is 60° or more.
• the orientation angle is preferably 80° or more, more preferably 85° or more, and even more preferably 88° or more.
• the orientation angle is the average value of the angle (orientation angle) between the surface (principal surface) of the thermally conductive layer and the planar direction in the case of scale-shaped particles, the major axis direction in the case of ellipsoidal particles, and the major axis direction in the case of rod-shaped particles, for 50 random graphite particles (A) observed with a cross section of the thermally conductive layer using a SEM (scanning electron microscope).
• the particle size of the graphite particles (A) is not particularly limited.
• the average particle size of the graphite particles (A), as the mass average particle size, is preferably at least 1/2 the average thickness of the heat conduction layer and not more than the average thickness.
• the mass average particle size of the graphite particles (A) is at least 1/2 the average thickness of the heat conduction layer, an efficient heat conduction path is formed in the heat conduction layer, and the thermal conductivity tends to improve.
• the mass average particle size of the graphite particles (A) is not more than the average thickness of the heat conduction layer, the protrusion of the graphite particles (A) from the surface of the heat conduction layer is suppressed, and the adhesion of the surface of the heat conduction layer tends to be excellent.
• the method for producing a thermally conductive layer so that the planar direction in the case of scale-shaped particles, the major axis in the case of ellipsoidal particles, and the major axis in the case of rod-shaped particles are oriented in the thickness direction is not particularly limited, and for example, the method described in JP 2008-280496 A can be used. Specifically, a method can be used in which a sheet is produced using the composition, the sheets are stacked to produce a laminate, and the side end surface of the laminate is sliced (for example, at an angle of 0° to 30° with respect to the normal line extending from the main surface of the laminate) (hereinafter also referred to as the "lamination slicing method").
• the particle diameter of the graphite particles (A) used as the raw material is preferably at least 1/2 the average thickness of the heat conduction layer as the mass average particle diameter, and may exceed the average thickness.
• the reason why the particle diameter of the graphite particles (A) used as the raw material may exceed the average thickness of the heat conduction layer is, for example, that even if the heat conduction layer contains graphite particles (A) with a particle diameter exceeding the average thickness of the heat conduction layer, the heat conduction layer is formed by slicing the graphite particles (A) together, so that the graphite particles (A) do not protrude from the surface of the heat conduction layer. Furthermore, slicing the graphite particles (A) together in this way produces a large number of graphite particles (A) that penetrate the heat conduction layer in the thickness direction, forming an extremely efficient heat conduction path and tending to further improve heat conductivity.
• the particle diameter of the graphite particles (A) used as the raw material is preferably 1 to 5 times, and even more preferably 2 to 4 times, the mass average particle diameter of the average thickness of the heat conduction layer. If the mass average particle diameter of the graphite particles (A) is 1 or more times the average thickness of the heat conduction layer, a more efficient heat conduction path is formed, and the thermal conductivity is further improved. If it is 5 or less times the average thickness of the heat conduction layer, the area of the surface of the graphite particles (A) is prevented from becoming too large, and a decrease in adhesion can be suppressed.
• the mass average particle size (D50) of graphite particles (A) is measured using a laser diffraction particle size distribution device that applies the laser diffraction/scattering method (for example, Nikkiso Co., Ltd.'s "Microtrack Series MT3300"), and corresponds to the particle size at which the mass accumulation is 50% when the mass accumulation particle size distribution curve is plotted from the small particle size side.
• a laser diffraction particle size distribution device that applies the laser diffraction/scattering method (for example, Nikkiso Co., Ltd.'s "Microtrack Series MT3300"), and corresponds to the particle size at which the mass accumulation is 50% when the mass accumulation particle size distribution curve is plotted from the small particle size side.
• the thermally conductive layer may contain graphite particles other than scaly particles, ellipsoidal particles, and rod-shaped particles, and may contain spherical graphite particles, artificial graphite particles, exfoliated graphite particles, acid-treated graphite particles, expanded graphite particles, carbon fiber flakes, etc.
• graphite particles (A) scaly particles are preferred, and from the viewpoint of high crystallinity and ease of obtaining large-diameter flakes, scaly expanded graphite particles obtained by pulverizing sheet-formed expanded graphite are preferred.
• the content of the graphite particles (A) in the thermal conductive layer is, for example, preferably 15 vol% to 50 vol%, more preferably 20 vol% to 45 vol%, and even more preferably 25 vol% to 40 vol%, from the viewpoint of the balance between thermal conductivity and adhesion.
• the content of the graphite particles (A) is 15% by volume or more, the thermal conductivity tends to be improved.
• the content of the graphite particles (A) is 50% by volume or less, the decrease in adhesiveness and adhesion tends to be suppressed.
• the thermally conductive layer contains graphite particles other than scaly particles, ellipsoidal particles and rod-shaped particles, it is preferable that the total content of the graphite particles is within the above range.
• the thermally conductive layer included in the thermally conductive sheet of the present disclosure may contain a component that is liquid at 25° C. (hereinafter, also referred to as “liquid component (B)").
• liquid at 25° C.” means a substance that exhibits fluidity and viscosity at 25° C. and has a viscosity, which is a scale indicating viscosity, of 0.0001 Pa ⁇ s to 1000 Pa ⁇ s at 25° C.
• viscosity is defined as a value measured at 25° C. at a shear rate of 5.0 s- 1 using a rheometer.
• viscosity is measured as shear viscosity at a temperature of 25° C. using a rotary shear viscometer equipped with a cone plate (diameter 40 mm, cone angle 0°).
• the viscosity of the liquid component (B) at 25°C is preferably 0.001 Pa ⁇ s to 100 Pa ⁇ s, and more preferably 0.01 Pa ⁇ s to 10 Pa ⁇ s.
• the liquid component (B) is not particularly limited as long as it is liquid at 25°C, and is preferably a polymer.
• the liquid component (B) include polybutene, polyisoprene, polysulfide, acrylonitrile rubber, silicone rubber, hydrocarbon resin, terpene resin, acrylic resin, etc. Among these, from the viewpoint of heat resistance, it is preferable that the liquid component (B) contains polybutene.
• the liquid component (B) may be used alone or in combination of two or more types.
• polybutene refers to a polymer obtained by polymerizing isobutene or normal butene. It also includes a polymer obtained by copolymerizing isobutene and normal butene. As for the structure, it refers to a polymer having a structural unit represented by "-CH 2 -C(CH 3 ) 2 -" or "-CH 2 -CH(CH 2 CH 3 )-". It is sometimes called polyisobutylene. Polybutene may contain the above structure, and other structures are not particularly limited.
• Polybutene includes homopolymers of butene and copolymers of butene with other monomer components.
• Examples of copolymers with other monomer components include copolymers of isobutene and styrene and copolymers of isobutene and ethylene.
• the copolymers may be random copolymers, block copolymers, or graft copolymers.
• polybutene examples include NOF Corporation's "NOF Polybutene TM Emawet (registered trademark),” JXTG Nippon Oil & Energy Corporation's “Nippon Oil Polybutene,” JXTG Nippon Oil & Energy Corporation's “Tetrax,” JXTG Nippon Oil & Energy Corporation's “Himol,” and Tomoe Engineering Co., Ltd.'s "Polyisobutylene.”
• the liquid component (B) is thought to function primarily, for example, as a stress relaxant with excellent heat and humidity resistance, and as a tackifier. In addition, by using it in combination with the hot melt agent (D) described below, it tends to be possible to further increase the cohesive strength and fluidity when heated.
• the content of the liquid component (B) in the thermal conductive layer is preferably 10 vol. % to 55 vol. %, more preferably 15 vol. % to 50 vol. %, and even more preferably 20 vol. % to 50 vol. %.
• the content of the liquid component (B) is 10% by volume or more, the adhesiveness and adhesion tend to be further improved, and when the content of the liquid component (B) is 55% by volume or less, the decrease in the sheet strength and thermal conductivity tend to be more effectively suppressed.
• the thermally conductive layer included in the thermally conductive sheet may contain an acrylic acid ester-based polymer (C).
• the acrylic acid ester-based polymer (C) is considered to mainly function, for example, as both a tackifier and an elasticity imparting agent that restores the thickness to accommodate warping.
• Acrylic acid ester polymers (C) that are preferably used are, for example, acrylic acid ester polymers (so-called acrylic rubbers) that have butyl acrylate, ethyl acrylate, acrylonitrile, acrylic acid, glycidyl methacrylate, 2-ethylhexyl acrylate, etc. as the main raw material components and, if necessary, are copolymerized with methyl acrylate, etc.
• Acrylic acid ester polymers (C) may be used alone or in combination of two or more types.
• the weight average molecular weight of the acrylic acid ester polymer (C) is preferably 100,000 to 1,000,000, more preferably 250,000 to 700,000, and further preferably 400,000 to 600,000. When the weight average molecular weight is 100,000 or more, the film strength tends to be excellent, and when it is 1,000,000 or less, the flexibility tends to be excellent.
• the weight average molecular weight can be measured by gel permeation chromatography using a calibration curve of standard polystyrene.
• the glass transition temperature (Tg) of the acrylic acid ester polymer (C) is preferably 20° C. or lower, more preferably ⁇ 70° C. to 0° C., and even more preferably ⁇ 50° C. to ⁇ 20° C. When the glass transition temperature is 20° C. or lower, the flexibility and adhesiveness tend to be excellent.
• the glass transition temperature (Tg) can be calculated from tan ⁇ derived by performing dynamic viscoelasticity measurement under tension.
• the acrylic acid ester polymer (C) may be added internally so that it is present throughout the entire thermally conductive layer, or it may be localized on the surface by coating or impregnating the surface.
• coating or impregnating one side is preferable in that it gives strong tackiness to only one side, resulting in a sheet with good handleability.
• the content of the acrylic ester polymer (C) in the thermally conductive layer is preferably 3% to 25% by volume, more preferably 5% to 20% by volume, and even more preferably 7% to 15% by volume.
• the thermally conductive layer included in the thermally conductive sheet may contain a hot melt agent (D).
• the hot melt agent (D) has the effect of improving the strength of the thermally conductive layer and improving the fluidity when heated.
• hot melt agent (D) examples include aromatic petroleum resins, terpene phenol resins, and cyclopentadiene petroleum resins.
• the hot melt agent (D) may also be a hydrogenated aromatic petroleum resin or a hydrogenated terpene phenol resin.
• the hot melt agent (D) may be used alone or in combination of two or more types.
• the hot melt agent (D) preferably contains at least one selected from the group consisting of hydrogenated aromatic petroleum resins and hydrogenated terpene phenol resins.
• These hot melt agents (D) are highly stable and have excellent compatibility with polybutene, so when used to form a thermally conductive layer, they tend to achieve better thermal conductivity, flexibility, and handleability.
• hydrogenated aromatic petroleum resins include, for example, “Alcon” from Arakawa Chemical Industries Co., Ltd. and “Imarve” from Idemitsu Kosan Co., Ltd.
• commercially available hydrogenated terpene phenol resins include, for example, “Clearon” from Yasuhara Chemical Co., Ltd.
• commercially available cyclopentadiene petroleum resins include, for example, “Quinton” from Zeon Corporation and "Marukarets” from Maruzen Petrochemical Co., Ltd.
• the hot melt agent (D) is preferably solid at 25°C and has a softening temperature of 40°C to 150°C.
• a thermoplastic resin is used as the hot melt agent (D)
• the softening fluidity during thermocompression is improved, which tends to improve adhesion.
• the softening temperature is 40°C or higher, the cohesive force can be maintained at around room temperature, which tends to make it easier to obtain the required sheet strength and improve handling.
• the softening temperature is 150°C or lower, the softening fluidity during thermocompression is high, which tends to improve adhesion. It is more preferable that the softening temperature is 60°C to 120°C.
• the softening temperature is measured by the ring and ball method (JIS K 2207:1996).
• the content of the hot melt agent (D) in the thermally conductive layer is preferably 3 vol% to 25 vol%, more preferably 5 vol% to 20 vol%, and even more preferably 5 vol% to 15 vol%.
• the content of the hot-melt agent (D) is 3% by volume or more, the adhesive strength, heat flowability, sheet strength, etc. tend to be sufficient, and when it is 25% by volume or less, the flexibility tends to be sufficient, and the handling properties and thermal cycle resistance tend to be excellent.
• the thermal conductive layer included in the thermal conductive sheet may contain an antioxidant (E) for the purpose of imparting thermal stability at high temperatures, for example.
• the antioxidant (E) include phenol-based antioxidants, phosphorus-based antioxidants, amine-based antioxidants, sulfur-based antioxidants, hydrazine-based antioxidants, and amide-based antioxidants.
• the antioxidant (E) may be appropriately selected depending on the temperature conditions used, and a phenol-based antioxidant is more preferred.
• the antioxidant (E) may be used alone or in combination of two or more kinds.
• phenolic antioxidants include, for example, Adeka STAB AO-50, Adeka STAB AO-60, and Adeka STAB AO-80 from ADEKA CORPORATION.
• the content of the antioxidant (E) in the thermally conductive layer is not particularly limited, and is preferably 0.1 vol.% to 5 vol.%, more preferably 0.2 vol.% to 3 vol.%, and even more preferably 0.3 vol.% to 1 vol.%.
• the content of the antioxidant (E) is 0.1 vol.% or more, there is a tendency for a sufficient antioxidant effect to be obtained.
• the content of the antioxidant (E) is 5 vol.% or less, there is a tendency for a decrease in the strength of the thermally conductive layer to be suppressed.
• the heat conductive layer included in the heat conductive sheet may contain other components other than the graphite particles (A), the liquid component (B), the acrylic acid ester-based polymer (C), the hot melt agent (D), and the antioxidant (E) depending on the purpose.
• the heat conductive layer may contain a flame retardant from the viewpoint of flame retardancy.
• the flame retardant is not particularly limited and can be appropriately selected from commonly used flame retardants.
• red phosphorus-based flame retardants and phosphate ester-based flame retardants can be mentioned.
• phosphate ester-based flame retardants are preferred from the viewpoint of excellent safety and improved adhesion due to the plasticization effect.
• red phosphorus-based flame retardants in addition to pure red phosphorus particles, those that have been coated with various coatings for the purpose of increasing safety or stability, or those made into masterbatches, etc., may also be used. Specific examples include Nova Red, Nova Excel, Nova Ban, and Nova Pellet (all trade names) manufactured by Rin Kagaku Kogyo Co., Ltd.
• phosphate ester-based flame retardant examples include aliphatic phosphate esters such as trimethyl phosphate, triethyl phosphate, and tributyl phosphate; aromatic phosphate esters such as triphenyl phosphate, tricresyl phosphate, cresyl diphenyl phosphate, trixylenyl phosphate, cresyl di-2,6-xylenyl phosphate, tris(t-butylated phenyl)phosphate, tris(isopropylated phenyl)phosphate, and triaryl isopropyl phosphate; and aromatic condensed phosphate esters such as resorcinol bisdiphenyl phosphate, bisphenol A bis(diphenyl phosphate), and resorcinol bisdixylenyl phosphate.
• bisphenol A bis(diphenyl phosphate) is preferred from the viewpoints of excellent hydrolysis resistance and excellent effect
• the content of the flame retardant in the thermally conductive layer is not limited and can be used in an amount that exhibits flame retardancy, preferably about 30% by volume or less, and preferably 20% by volume or less from the viewpoint of preventing deterioration of thermal resistance due to the flame retardant components seeping out onto the surface of the thermally conductive layer.
• the average thickness of the thermally conductive layer is not particularly limited and can be appropriately selected according to the purpose.
• the thickness of the thermally conductive layer can be appropriately selected according to the specifications of the semiconductor package to be used. The smaller the thickness, the lower the thermal resistance tends to be, and the larger the thickness, the higher the warpage compliance tends to be.
• the average thickness of the thermally conductive layer may be 20 ⁇ m to 3000 ⁇ m, and from the viewpoint of thermal conductivity and adhesion, it is preferably 30 ⁇ m to 500 ⁇ m, and more preferably 50 to 400 ⁇ m.
• the average thickness of the thermally conductive layer is determined by observing a cross section of the measurement target using an electron microscope, or by measuring the thickness at three random locations using a micrometer, and taking the arithmetic mean value thereof.
• the thermally conductive sheet of the present disclosure includes an adhesive layer that contains a resin component and a thermally conductive filler and is located on at least a portion of a main surface of the thermally conductive layer.
• the resin component may be a curable resin component, a sticky resin component, a thermoplastic resin component, etc.
• the curable resin component may be a thermosetting resin component, a photocurable resin component, etc.
• the resin component may contain one or more curable resin components, may contain one or more sticky resin components, or may contain one or more thermoplastic resin components.
• the resin component may also be a mixture of two or more resin components.
• the curable resin component examples include epoxy resins, phenolic resins, melamine resins, urea resins, unsaturated polyester resins, alkyd resins, urethane resins, polyimide resins such as bismaleimide resins, polyamide resins, polyamideimide resins, silicone resins, and thermosetting (meth)acrylic resins.
• epoxy resins are preferred from the viewpoint of adhesion.
• thermoplastic resin components examples include polyethylene (PE), polypropylene (PP), polycarbonate (PC), polystyrene, polyvinyl chloride, vinyl polymers, polyesters, polyamides, acrylonitrile-butadiene-styrene copolymer resins (ABS resins), thermoplastic (meth)acrylic resins, acrylonitrile-ethylene-propylene-diene-styrene copolymer resins (AES resins), thermoplastic epoxy resins, phenoxy resins, and thermoplastic elastomers.
• PE polyethylene
• PP polypropylene
• PC polycarbonate
• ABS resins acrylonitrile-butadiene-styrene copolymer resins
• AES resins thermoplastic (meth)acrylic resins
• AES resins acrylonitrile-ethylene-propylene-diene-styrene copolymer resins
• thermoplastic epoxy resins phenoxy resins
• thermoplastic resin component has excellent storage stability compared to a thermally conductive sheet using a curable resin component (especially a thermosetting resin component), and can be stored at room temperature.
• a thermoplastic epoxy resin is preferable from the viewpoint that the viscosity of the thermoplastic resin component can be reduced by heating and the gap between the adherend and the thermally conductive sheet can be further reduced.
• the thermoplastic resin component may be an amorphous thermoplastic resin, which may be, for example, at least one of a thermoplastic epoxy resin and a phenoxy resin.
• An amorphous thermoplastic resin is a resin that has a melting point (Tm), but in measurements using a differential scanning calorimeter (DSC), the endothermic peak associated with melting is not clearly observed as an endothermic peak, or the endothermic peak is very small.
• the heat of fusion of the amorphous thermoplastic resin is preferably 15 J/g or less, more preferably 11 J/g or less, even more preferably 7 J/g or less, particularly preferably 4 J/g or less, and most preferably the melting peak is below the detection limit.
• the heat of fusion is calculated from the area of the endothermic peak in a DSC (differential scanning calorimeter) and the mass of the thermoplastic resin component. At this time, the heat of fusion is calculated using the mass of the thermoplastic resin component excluding components contained in the adhesive layer such as a thermally conductive filler.
• the heat of fusion can be determined as follows. First, 2 mg to 10 mg of a sample is weighed out and placed in an aluminum pan, and the sample is heated to 200° C. or higher at 10° C./min using a DSC (e.g., DSC8231 manufactured by Rigaku Corporation) to obtain a DSC curve. Next, the heat of fusion is calculated from the area of the endothermic peak at the time of melting obtained from the DSC curve and the weighed value.
• a DSC Differential scanning calorimeter
• the thermoplastic epoxy resin is preferably a polymer of (a) a bifunctional epoxy resin monomer or oligomer and (b) a bifunctional compound containing two identical or different functional groups selected from the group consisting of a phenolic hydroxyl group, a carboxyl group, a mercapto group, an isocyanate group, and a cyanate ester group.
• the aforementioned polymer can be produced by heating the components (a) and (b) in the presence of a catalyst such as an imidazole catalyst.
• the (a) bifunctional epoxy resin monomer or oligomer refers to an epoxy resin monomer or oligomer having two epoxy groups in the molecule.
• Specific examples of the (a) include bisphenol A type epoxy resins, bisphenol F type epoxy resins, bifunctional phenol novolac type epoxy resins, bisphenol AD type epoxy resins, biphenyl type epoxy resins, bifunctional naphthalene type epoxy resins, bifunctional alicyclic epoxy resins, bifunctional glycidyl ester type epoxy resins (e.g., diglycidyl phthalate, diglycidyl tetrahydrophthalate, and dimer acid diglycidyl ester), bifunctional glycidyl amine type epoxy resins (e.g., diglycidyl aniline and diglycidyl toluidine), bifunctional heterocyclic epoxy resins, bifunctional diarylsulfone type epoxy resins, hydroquinone type epoxy resins (e.g., hydroquinone diglycidyl
• bisphenol A type epoxy resins, bisphenol F type epoxy resins and biphenyl type epoxy resins are preferred in terms of reactivity and workability.
• bifunctional compounds containing a phenolic hydroxyl group in (b) above include mononuclear aromatic dihydroxy compounds having one benzene ring such as catechol, resorcin, and hydroquinone; bisphenols such as bis(4-hydroxyphenyl)propane (bisphenol A), bis(4-hydroxyphenyl)methane (bisphenol F), bis(4-hydroxyphenyl)ethane (bisphenol AD), and bis(4-hydroxyphenyl)sulfone (bisphenol S); compounds having a condensed ring such as dihydroxynaphthalene; bifunctional phenol compounds having an allyl group introduced therein such as diallyl resorcin, diallyl bisphenol A, and triallyl dihydroxybiphenyl; and dibutyl bisphenol A.
• mononuclear aromatic dihydroxy compounds having one benzene ring such as catechol, resorcin, and hydroquinone
• bisphenols such as bis(4-hydroxyphenyl
• Examples of the (b) bifunctional compound containing a carboxyl group include adipic acid, succinic acid, malonic acid, cyclohexanedicarboxylic acid, phthalic acid, isophthalic acid, and terephthalic acid.
• Examples of the (b) bifunctional compound containing a mercapto group include ethylene glycol bisthioglycolate and ethylene glycol bisthiopropionate.
• bifunctional compound containing an isocyanate group examples include diphenylmethane diisocyanate (MDI), isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HMDI), tolylene diisocyanate (TDI), etc.
• MDI diphenylmethane diisocyanate
• IPDI isophorone diisocyanate
• HMDI hexamethylene diisocyanate
• TDI tolylene diisocyanate
• bifunctional compound containing a cyanate ester group (b) examples include 2,2-bis(4-cyanatophenyl)propane, 1,1-bis(4-cyanatophenyl)ethane, bis(4-cyanatophenyl)methane, etc.
• bifunctional compounds containing a phenolic hydroxyl group are preferred from the viewpoint of obtaining a thermoplastic polymer
• bifunctional compounds containing two phenolic hydroxyl groups and a bisphenol structure or a biphenyl structure are preferred from the viewpoint of heat resistance and adhesiveness
• bisphenol A, bisphenol F or bisphenol S are preferred from the viewpoint of heat resistance and cost.
• the polymer obtained by polymerization of the (a) and (b) preferably has a main chain having a paraphenylene structure and an ether bond as a main skeleton, the main chain being linked by an alkylene group, and a structure in which a hydroxyl group generated by polyaddition is arranged in a side chain.
• the linear structure consisting of the paraphenylene skeleton can increase the mechanical strength of the polymer after polymerization, and the hydroxyl groups arranged in the side chains can improve adhesion. As a result, high adhesive strength can be achieved while maintaining the workability of thermosetting resins.
• the epoxy equivalent (molecular weight/number of epoxy groups) of the thermoplastic epoxy resin may be 1600 g/eq or more, 2000 g/eq or more, 5000 g/eq or more, or 9000 g/eq or more.
• the epoxy equivalent of the epoxy resin is a value measured by a method in accordance with JIS K 7236:2009.
• the adhesive layer may contain a phenoxy resin as a thermoplastic resin component.
• the phenoxy resin is, for example, a polyhydroxy polyether synthesized from bisphenols and epichlorohydrin, and has thermoplastic properties.
• a method by direct reaction of dihydric phenols with epichlorohydrin and a method by addition polymerization reaction of diglycidyl ether of dihydric phenols with dihydric phenols are known.
• examples of dihydric phenols include phenols such as bisphenol A, bisphenol F, bisphenol S, biphenol, biphenylenediol, and fluorenediphenyl; and aliphatic glycols such as ethylene glycol, propylene glycol, and diethylene glycol.
• the phenoxy resin preferably contains a chemical structure similar to that of an epoxy resin, and has a main skeleton formed of a paraphenylene structure and an ether bond, a main chain connecting these, and a structure in which hydroxyl groups are arranged in the side chains.
• the weight average molecular weight which is a value converted into polystyrene by GPC (gel permeation chromatography), is preferably 10,000 to 500,000, more preferably 18,000 to 300,000, and even more preferably 20,000 to 200,000.
• the weight average molecular weight is calculated from the elution peak position detected by GPC, and each is a molecular weight value converted into standard polystyrene. If the weight average molecular weight is within this range, a good balance between thermoplasticity and heat resistance is achieved. If the weight average molecular weight is 10,000 or more, the heat resistance is excellent, and if it is 500,000 or less, the viscosity when melted is low and the adhesiveness is high.
• Thermal conductive fillers include metal-containing particles and non-metal particles with excellent thermal conductivity, such as metals, metal oxides, metal nitrides, metal hydroxides, metal carbides, metal fluorides, and carbon.
• the thermal conductive filler may be, for example, a filler with a thermal conductivity of 10 W/(m ⁇ K) or more.
• the thermal conductive filler may be insulating or conductive.
• the thermally conductive filler may be at least one type of particle selected from silver, copper, aluminum, aluminum oxide, aluminum hydroxide, magnesium oxide, beryllium oxide, boron nitride, aluminum nitride, silicon nitride, silicon carbide, silicon dioxide, aluminum fluoride, calcium fluoride, and zinc oxide.
• silver particles are preferred from the viewpoint of thermal conductivity.
• boron nitride is an insulating particle, it is preferably used in applications where insulation is required.
• boron nitride is scaly in shape and has anisotropic thermal conductivity, and tends to be oriented in the plane direction, which results in a tendency for thermal conductivity to be low in the thickness direction. Therefore, when using boron nitride, from the standpoint of thermal conductivity, it is necessary to form an adhesive layer so that the boron nitride is oriented in the thickness direction, which can make the process of forming the adhesive layer complicated.
• metal particles such as silver, copper, and aluminum have higher thermal conductivity and smaller anisotropy of thermal conductivity than ceramic fillers such as boron nitride. Therefore, there is no need to orient the metal particles in the thickness direction when forming the adhesive layer, and an adhesive layer with excellent thermal conductivity can be easily formed, and the thermal resistance of the thermal conductive sheet is suitably reduced. Furthermore, by using silver particles as the metal particles, sintering of the particles proceeds at a relatively low temperature. Therefore, a network of silver particles is formed by sintering, and a higher thermal conductivity tends to be obtained compared to other thermally conductive fillers. In addition, sintered silver particles have excellent adhesion to adherends such as heating elements and heat sinks, so the thermal resistance tends to be further reduced.
• the thermally conductive filler preferably contains silver particles.
• the content of the silver particles may be 50% by mass to 100% by mass, 80% by mass to 100% by mass, or 90% by mass to 100% by mass, relative to 100% by mass of the thermally conductive filler.
• the particle size of the thermally conductive filler may be 0.1 ⁇ m to 50 ⁇ m, 0.2 ⁇ m to 20 ⁇ m, or 0.5 ⁇ m to 10 ⁇ m, from the viewpoint of excellent thermal conductivity and further reducing the gap between the adherend and the thermally conductive sheet.
• the particle size (D50) of the thermally conductive filler is measured using a laser diffraction particle size distribution device that applies the laser diffraction/scattering method (for example, the Microtrac Series MT3300 manufactured by Nikkiso Co., Ltd.), and corresponds to the particle size at which the mass accumulation is 50% when the mass accumulation particle size distribution curve is plotted from the small particle size side.
• a laser diffraction particle size distribution device that applies the laser diffraction/scattering method (for example, the Microtrac Series MT3300 manufactured by Nikkiso Co., Ltd.), and corresponds to the particle size at which the mass accumulation is 50% when the mass accumulation particle size distribution curve is plotted from the small particle size side.
• the adhesive layer may be located on at least a portion of the main surface of the thermally conductive layer, and the adhesive layer may be located on the entire main surface, or on a portion of the main surface (e.g., the portion that comes into contact with an adherend such as a heating element or a heat sink).
• the adhesive layer may be located on one main surface, or on two main surfaces.
• the content of the resin component contained in the adhesive layer is preferably 1% by mass to 30% by mass, more preferably 3% by mass to 25% by mass, and even more preferably 5% by mass to 20% by mass, based on the total amount of the adhesive layer, from the viewpoint of the balance between thermal conductivity and adhesion, for example.
• the content of the thermoplastic resin component may be 50 mass% or more, 60 mass% or more, 80 mass% or more, or 90 mass% or more, relative to the total amount of the resin component.
• the upper limit of the content of the thermoplastic resin component is not particularly limited, and may be 100% by mass or may be 95% by mass or less.
• the content of the thermally conductive filler in the adhesive layer is preferably 70% by mass to 99% by mass, more preferably 75% by mass to 97% by mass, and even more preferably 80% by mass to 95% by mass, based on the total amount of the adhesive layer, from the viewpoint of the balance between thermal conductivity and adhesion.
• the total content of the resin component and the thermally conductive filler contained in the adhesive layer may be 80% by mass to 100% by mass, or 90% by mass to 100% by mass, based on the total amount of the adhesive layer.
• the adhesive layer may or may not contain components other than the resin component and the thermally conductive filler.
• components other than the resin component and the thermally conductive filler include the hot melt agent (D), antioxidant (E), and other components described above in the section on the thermally conductive layer.
• an adhesive layer in a semi-cured state can be obtained by subjecting a resin composition containing a thermosetting resin component and a thermally conductive filler to a process such as heating or drying.
• the semi-cured state means that the viscosity of the adhesive layer is 10 4 Pa ⁇ s to 10 6 Pa ⁇ s at room temperature (25 to 30° C.).
• the viscosity is measured by a DMA (dynamic viscoelasticity measuring device; frequency 1 Hz, load 10 g, heating rate 20° C./min).
• the method for obtaining a semi-cured adhesive layer is not particularly limited.
• a resin composition containing a thermosetting resin component, a thermally conductive filler, a solvent, etc. may be applied to the main surface of the thermally conductive layer, and the applied resin composition may be heated, dried, etc.
• a resin composition may be applied to a release film, which may then be heated, dried, etc., and the adhesive layer made of the heated, dried, etc. resin composition may then be hot roll laminated onto the main surface of the thermally conductive layer.
• Methods for heating, drying, etc. include hot vacuum pressing and hot roll lamination.
• the average thickness of the adhesive layer is preferably 2 ⁇ m to 50 ⁇ m, more preferably 2 ⁇ m to 30 ⁇ m, and even more preferably 2 ⁇ m to 20 ⁇ m.
• the average thickness of the adhesive layer is 2 ⁇ m or more, the gap between the adherend and the thermal conductive sheet tends to be further reduced, and the contact thermal resistance tends to be further reduced.
• the average thickness of the adhesive layer is 50 ⁇ m or less, the thermal conductivity of the thermal conductive sheet tends to be more excellent.
• the thickness may be measured at three random locations by observing the cross section of the thermally conductive layer to be measured using an electron microscope, and the arithmetic average value may be used as the average thickness of the adhesive layer.
• the thickness of the thermally conductive layer not including an adhesive layer and the thermally conductive sheet including an adhesive layer may be measured at three random locations using a micrometer, and the arithmetic average values may be used as the average thickness of the thermally conductive layer and the average thickness of the thermally conductive sheet, respectively.
• the average thickness of the adhesive layer may then be calculated by subtracting the average thickness of the thermally conductive sheet from the average thickness of the thermally conductive layer.
• the average thickness of the adhesive layers means the total thickness of the adhesive layers formed on the two main surfaces.
• the surface roughness Ra of the thermally conductive sheet (before bonding to the adherend) may be 10 ⁇ m or less from the viewpoint of further reducing the gap between the adherend and the thermally conductive sheet when bonded to the adherend, and may be 2 ⁇ m to 20 ⁇ m from the viewpoint of productivity of the thermally conductive sheet.
• the surface roughness Ra refers to a value measured based on JIS B0601:2013.
• the thermal conductivity of the thermal conductive layer in the thickness direction is preferably greater than that of the adhesive layer in the thickness direction.
• the ratio of the thermal conductivity of the thermal conductive layer in the thickness direction to the thermal conductivity of the adhesive layer in the thickness direction may be greater than 1, and may be, for example, greater than 1 and 20 or less, greater than 1 and 10 or less, or may be 2 to 8.
• the thermal conductivity of the thermally conductive layer in the thickness direction and the thermal conductivity of the adhesive layer can be measured by a xenon flash (Xe-flash) method.
• the thermal conductivity of the adhesive layer means the thermal conductivity of the adhesive layer after curing.
• the thermal conductivity of the adhesive layer may be 5.0 W/(m ⁇ K) or more, may be 5.0 W/(m ⁇ K) to 20 W/(m ⁇ K), or may be 7.0 W/(m ⁇ K) to 15 W/(m ⁇ K).
• the thermally conductive sheet may have a protective film on at least one side, and preferably has protective films on both sides. This makes it possible to protect the adhesive surface of the thermally conductive sheet.
• the protective film may be, for example, a resin film such as polyethylene, polyester, polypropylene, polyethylene terephthalate, polyimide, polyetherimide, polyether naphthalate, or methylpentene, coated paper, coated cloth, or metal foil such as aluminum. These protective films may be used alone or in combination of two or more types to form a multi-layer film. It is preferable that the protective film is surface-treated with a release agent such as a silicone-based or silica-based release agent.
• a release agent such as a silicone-based or silica-based release agent.
• the use of the thermally conductive sheet is not particularly limited.
• the thermally conductive sheet of the present disclosure is particularly suitable as a thermally conductive sheet (TIM1; Thermal Interface Material 1) that interposes a semiconductor chip and a heat spreader when the semiconductor chip is the heat generating body and the heat spreader is the heat dissipating body, or as a film-like adhesive material for semiconductor chips such as a die bonding film.
• TIM1 Thermal Interface Material 1
• the thermally conductive sheet When the thermally conductive sheet is used as a die bonding film, it may be laminated with a dicing tape to form an integrated dicing/die bonding film.
• the thermally conductive sheet 1A shown in FIG. 3 comprises a thermally conductive layer 11A and adhesive layers 12A and 13A, with the adhesive layer 12A located on one main surface of the thermally conductive layer 11A and the adhesive layer 13A located on the other main surface of the thermally conductive layer 11A.
• the thermally conductive layer 11A in the thermally conductive sheet 1A may have two main surfaces that are not flat, but may have an uneven shape.
• the thermally conductive layer 11A in the thermally conductive sheet 1A may have two main surfaces that are flat (see, for example, Fig. 3).
• the method for producing the thermal conductive sheet is not particularly limited as long as it is a method that can obtain a thermal conductive sheet having the above-mentioned configuration.
• the method for producing the thermal conductive sheet includes a step of preparing a composition containing the graphite particles (A) (also referred to as a "preparation step"), a step of forming the thermal conductive layer using the composition (also referred to as a “formation step”), and a step of forming an adhesive layer on at least a part of the main surface of the thermal conductive layer (also referred to as an "adhesive layer formation step").
• a composition containing graphite particles (A) and any other components for example, a component (B) that is liquid at 25° C., an acrylic acid ester polymer (C), a hot melt agent (D), an antioxidant (E), and other components
• a component (B) that is liquid at 25° C., an acrylic acid ester polymer (C), a hot melt agent (D), an antioxidant (E), and other components is prepared. Any method may be used for blending the components as long as it is possible to uniformly mix the components, and there is no particular limitation.
• the composition may be prepared by obtaining a commercially available product. For details on the preparation of the composition, refer to paragraph [0033] of JP2008-280496A.
• the thermally conductive layer is formed using a composition containing the graphite particles (A) and any other components.
• the thermally conductive layer may be formed by forming the composition into a sheet.
• the forming process preferably includes a process of forming the composition into a sheet (also referred to as a "sheet preparation process”), a process of preparing a laminate of the sheets (also referred to as a “laminate preparation process”), and a process of slicing the side end faces of the laminate (also referred to as a "slicing process").
• the sheet preparation step may be performed by any method as long as the composition obtained in the previous step can be made into a sheet, and is not particularly limited.
• the sheet preparation step refer to paragraph [0034] of JP2008-280496A.
• a laminate of the sheets obtained in the previous step is formed.
• the laminate may be prepared, for example, by stacking a plurality of independent sheets in order, by folding a single sheet, or by rolling up a single sheet.
• details of the laminate preparation step refer to paragraphs [0035] to [0037] of JP 2008-280496 A.
• the slicing step may be any method as long as it can slice the side end surface of the laminate obtained in the previous step, and is not particularly limited. From the viewpoint of forming an extremely efficient heat conduction path by the graphite particles (A) penetrating the heat conduction layer in the thickness direction and further improving the heat conductivity, it is preferable to slice to a thickness of not more than twice the mass average particle diameter of the graphite particles (A). For details of the slicing step, refer to paragraph [0038] of JP2008-280496A.
• the adhesive layer forming step may be any method as long as an adhesive layer can be formed on at least a part of the main surface of the thermally conductive layer (for example, a sliced sheet obtained by slicing), and is not particularly limited.
• a resin composition containing a thermosetting resin component, a thermally conductive filler, a solvent, etc. may be applied to the main surface of the thermally conductive layer, and the applied resin composition may be heated, dried, etc. to volatilize the solvent.
• a resin composition may be applied to a release film, and then heated, dried, etc., and then an adhesive layer made of the heated, dried, etc.
• the resin composition may be hot roll laminated on the main surface of the thermally conductive layer.
• methods for heating, drying, etc. include hot vacuum pressing and hot roll lamination.
• the adhesive layer formed on the main surface of the thermally conductive layer may be in a semi-cured state.
• the method for producing a thermally conductive sheet may further include a step of laminating the thermally conductive sheet by attaching a protective film thereto after the adhesive layer forming step (also called a "lamination step").
• the lamination step is not particularly limited and may be performed by any method as long as the thermally conductive sheet obtained in the adhesive layer forming step can be attached to the protective film.
• thermally conductive sheet By manufacturing a thermally conductive sheet in this way, efficient thermal conduction paths tend to be formed, which tends to result in a thermally conductive sheet with high thermal conductivity and excellent adhesion.
• the heat dissipation device of the present disclosure is a device including a heat generating body, a heat dissipation body, and a heat conductive sheet of the present disclosure disposed between the heat generating body and the heat dissipation body, and the adhesive layer is located on at least a part of at least one of the main surfaces of the heat conductive layer located on the heat generating body side and the main surface located on the heat dissipation body side.
• the adhesive layer is located on at least a part of the main surface located on the heat generating body side and at least a part of the main surface located on the heat dissipation body side, and it is more preferable that the adhesive layer is located on the area of the main surface located on the heat generating body side facing the heat generating body and the area of the main surface located on the heat dissipation body side facing the heat dissipation body.
• Heat generating elements include semiconductor chips, semiconductor packages, power modules, etc.
• Heat dissipating elements include heat spreaders, heat sinks, water-cooled pipes, etc.
• a heat dissipation device using a semiconductor chip as a heat generating body and a heat spreader as a heat dissipation body will be described.
• the semiconductor chip and the heat spreader are examples of a heat generating body and a heat dissipation body, respectively, and the present disclosure is not limited thereto.
• the thermally conductive sheet 1 is used by adhering one side to the semiconductor chip 2 and the other side to the heat spreader 3.
• the semiconductor chip 2 is fixed to the substrate 4 using an underfill material 5, and the heat spreader 3 is fixed to the substrate 4 by a sealing material 6, and the adhesion between the thermally conductive sheet 1 and the semiconductor chip 2 and the heat spreader 3 is improved by pressing.
• multiple semiconductor chips 2 may be provided to one thermally conductive sheet 1, one semiconductor chip 2 may be provided to multiple thermally conductive sheets 1, or multiple semiconductor chips 2 may be provided to multiple thermally conductive sheets 1.
• An adhesive layer is located on the main surface of the thermal conductive sheet 1 facing the semiconductor chip 2 and on the main surface of the thermal conductive sheet 1 facing the heat spreader 3.
• the adhesive layer 13 is located on the main surface of the thermal conductive sheet 1 facing the semiconductor chip 2, and the adhesive layer 12 is located on the main surface of the thermal conductive sheet 1 facing the heat spreader 3. Furthermore, the adhesive layer 13 may be in contact with the semiconductor chip 2, and the adhesive layer 12 may be in contact with the heat spreader 3.
• the heat dissipation device is formed by disposing the thermally conductive sheet of the present disclosure between a heating element and a heat dissipation element.
• heat from the heating element can be efficiently conducted to the heat dissipation element.
• Efficient heat conduction improves the lifespan of the heat dissipation device when in use, and a heat dissipation device that functions stably even when used for long periods of time can be provided.
• the temperature range in which the thermally conductive sheet can be particularly suitably used may be, for example, -10°C to 150°C, -10°C to 100°C, or -10°C to 80°C.
• suitable examples of heat generating elements include semiconductor packages, displays, LEDs, electric lights, automotive power modules, and industrial power modules.
• Heat dissipators include, for example, heat sinks using aluminum or copper fins or plates, aluminum or copper blocks connected to heat pipes, aluminum or copper blocks inside which cooling liquid is circulated by a pump, and Peltier elements and aluminum or copper blocks equipped with these.
• the heat dissipation device is constructed by contacting the heating element and the heat sink with each side of a thermally conductive sheet.
• a thermally conductive sheet There are no particular limitations on the method of contacting the heating element with one side of the thermally conductive sheet, and the method of contacting the heat sink with the other side of the thermally conductive sheet, as long as the method can secure them in a sufficiently tight contact state.
• a heat conductive sheet is placed between the heat generating element and the heat sink, and fixed with a tool capable of applying pressure of about 0.05 MPa to 1 MPa.
• the heat generating element is heated, or heated to about 80°C to 200°C using an oven or the like.
• Another method is to use a press machine capable of applying heat and pressure at 80°C to 200°C and 0.05 MPa to 1 MPa.
• the preferred pressure range is 0.10 MPa to 1 MPa
• the preferred temperature range is 100°C to 180°C.
• the reliability of adhesion tends to be improved. This is thought to be because the heat conductive sheet is prevented from being excessively compressed, resulting in a thin thickness, or the distortion or residual stress of the surrounding components becoming too large.
• the thermally conductive sheet to be placed between the heating element and the heat sink is not particularly limited as long as it is the thermally conductive sheet described above.
• the thermally conductive sheet shown in FIG. 1 may be placed between the heating element and the heat sink.
• the adhesive layers 12A and 13A located on the main surface of the thermal conductive sheet 1A are softened or deformed by heating and pressing the thermal conductive sheet 1A between the heating element and the heat sink.
• the gaps that occur when the thermal conductive sheet 1A is heated and pressed to the heating element and the heat sink are filled with the softened, deformed, etc. adhesive layer. This allows the thermal conductive sheet and the adherend to be closely attached via the adhesive layer while reducing the gap between the thermal conductive sheet and the adherend.
• the thermal conductive sheet 1A with an uneven surface as shown in FIG. 1 is used, the thermal conductive sheet 1A is heated and pressed with the thermal conductive sheet 1A placed between the heating element and the heat sink.
• the uneven thermal conductive sheet 1A is flattened by pressing, and when the heat sink is manufactured, the thermal conductive sheet 1 is flat as shown in FIG. 3, so that the occurrence of gaps between the thermal conductive sheet and the adherend is suppressed.
• the adhesive layer containing a thermosetting resin component by applying an adhesive layer containing a thermosetting resin component, the adhesive layer deformed by heating and pressing fills the gap between the thermally conductive sheet and the adherend, and the thermosetting resin component hardens by heating, thereby allowing the thermally conductive sheet and the adherend to be closely attached via the adhesive layer.
• the adhesive layer softened by heating and pressing fills the gap between the thermally conductive sheet and the adherend.
• the gap between the thermally conductive sheet and the heat generating body or heat dissipating body is reduced, and the contact thermal resistance is significantly reduced.
• the thermally conductive sheet may be arranged between the heating element and the heat sink and the ratio of the thickness reduction after compression to the initial thickness before compression (compression rate) may be 1% to 35%.
• the void ratio calculated as the ratio of the area of the gas region to the area of the measurement region is preferably 0% to 25.0%, more preferably 0% to 20.0%. This reduces gaps (for example, gaps due to the unevenness of the heat conductive sheet and gaps due to the unevenness of the heat generating body or heat dissipating body) that occur when the heat conductive sheet and the heat generating body or heat dissipating body are heat-pressed together. As a result, it is presumed that the contact thermal resistance is significantly reduced.
• the porosity satisfies the above-mentioned numerical range both in the case of the main surface side of the thermally conductive layer on which the adhesive layer is located and in the case of the main surface side of the thermally conductive layer on which the adhesive layer is not located.
• the aforementioned porosity is preferably 0% to 10.0%, more preferably 0% to 5.0%, and even more preferably 0% to 2.0%.
• the numerical range of the porosity of the interface can be adjusted by, for example, adjusting the thickness of the adhesive layer, the composition of the resin composition forming the adhesive layer, the compression rate of the thermal conductive sheet, etc.
• Examples 1 and 2 The materials listed below were charged into a kneader (Moriyama Corporation, DS3-SGHM-E type pressure twin-arm kneader) so as to have the mixing ratio (volume %) shown in Table 1, and kneaded at a temperature of 150° C. to obtain a composition.
• a kneader Moka Corporation, DS3-SGHM-E type pressure twin-arm kneader
• the kneaded composition was placed in an extrusion molding machine (Parker Corporation, product name: HKS40-15 type extruder) and extruded into a flat plate shape with a width of 20 cm and a thickness of 1.5 mm to 1.6 mm to obtain a primary sheet.
• the obtained primary sheet was press-punched using a 40 mm x 150 mm die blade, and 61 punched sheets were stacked, and pressure was applied in the stacking direction at 90 ° C. for 30 minutes with a spacer of 80 mm in height so that the height was 80 mm, to obtain a 40 mm x 150 mm x 80 mm laminate.
• the 80 mm x 150 mm side end surface of this laminate was sliced with a wood slicer to obtain a thermally conductive layer with a thickness of 0.11 mm.
• Example 1 A resin composition containing 6 parts by mass of a thermosetting resin component (bisphenol F type epoxy resin), 4 parts by mass of a cresol novolac type epoxy resin, 8 parts by mass of a phenolic resin, 6 parts by mass of an acrylic acid ester resin, 76 parts by mass of a thermally conductive filler (silver particles, particle size 1.5 ⁇ m) and 39 parts by mass of a solvent (cyclohexanone) was prepared. The prepared resin composition was applied onto a release film, and the resin composition was dried in an oven at 130 ° C. A thermally conductive sheet having an adhesive layer (adhesive layer 1 in Table 2) formed on one main surface was obtained by hot roll lamination at 70 ° C.
• a thermosetting resin component bisphenol F type epoxy resin
• a cresol novolac type epoxy resin 8 parts by mass of a phenolic resin
• 6 parts by mass of an acrylic acid ester resin 6 parts by mass of an acrylic acid ester resin
• Example 2 A resin composition containing 4 parts by mass of a thermosetting resin component (bisphenol F type epoxy resin), 3 parts by mass of a cresol novolac type epoxy resin, 6 parts by mass of a phenolic resin, 5 parts by mass of an acrylic acid ester resin, 83 parts by mass of a thermally conductive filler (silver particles, particle size 1.5 ⁇ m) and 39 parts by mass of a solvent (cyclohexanone) was prepared. The prepared resin composition was applied onto a release film, and the resin composition was dried in an oven at 130 ° C. A thermally conductive sheet having an adhesive layer (adhesive layer 2 in Table 2) formed on one main surface was obtained by hot roll lamination at 70 ° C. on one main surface of the thermally conductive layer obtained as described above.
• a thermosetting resin component bisphenol F type epoxy resin
• cresol novolac type epoxy resin 6 parts by mass of a phenolic resin
• an acrylic acid ester resin 83 parts by mass of a thermally conductive filler (s
• Example 1 The materials shown in Table 1 were kneaded, laminated, pressed, and sliced in the same manner as in Examples 1 and 2 to produce a thermally conductive layer, so that the mixture ratio (volume %) of each material shown in Table 1 was the same as that of Examples 1 and 2.
• the thermally conductive layer was used as a thermally conductive sheet without forming an adhesive layer.
• thermal conductive sheets of Examples 1 and 2 and Comparative Example 1 were evaluated using the following methods. The results are shown in Table 2 and Figure 4.
• Thermal resistance was measured using a tabletop xenon flash analyzer (LFA 467 Hyper Flash). A ⁇ 14 mm thermal conductive sheet was sandwiched between 1 mm copper plates to prepare a three-layered sample. The sample was prepared under the conditions of 150°C and 0.14 MPa pressure for 3 minutes. When a thermosetting resin was included, the resin was heated in an oven at 175°C for 1 hour to harden the resin, and then cooled sufficiently at room temperature. In addition, the copper surface was blackened with carbon spray as a pretreatment for the measurement, and the measurement was performed.
• the thermal conductivity ⁇ excluding the influence of the copper plate was obtained from the three-layered structure, and the thermal resistance value X (K ⁇ cm 2 /W) per unit area (1 cm 2 ) was calculated from the obtained thermal conductivity ⁇ and thickness t using the following formula as follows.
• X (10 x t) / ⁇ t: thickness (mm) of the thermal conductive sheet of Examples 1 to 4 or Comparative Example 1 ⁇ : Thermal conductivity (W/(m ⁇ K))
• thermal conductive layer 2 was prepared in the same manner as the thermal conductive layer 1, except that the blending ratio of the graphite particles (A) was increased to 40.0 volume % with respect to the blending ratio in Table 1 and the blending ratios of the other components were changed as shown in Table 3.
• Example 3 A resin composition containing 16 parts by mass of bisphenol A type epoxy resin, 1 part by mass of bisphenol S, 83 parts by mass of thermally conductive filler (silver particles, particle size 1.5 ⁇ m), 0.1 parts by mass of triphenylphosphine, and 32 parts by mass of solvent (methyl ethyl ketone) was prepared. The prepared resin composition was applied onto a release film, heated at 70 ° C. for 5 minutes, and then heated at 160 ° C. for 2 hours to react the bisphenol A type epoxy resin and bisphenol S to form a thermoplastic epoxy resin. One main surface of the thermally conductive layer 1 obtained as described above was hot roll laminated at 140 ° C.
• thermally conductive sheet having an adhesive layer (adhesive layer 3 in Table 4) formed on one main surface.
• the thickness of the adhesive layer before hot roll lamination as measured with a micrometer, was 10 ⁇ m.
• the pressure when preparing a three-layer structure sample for thermal resistance measurement described later was set to 0.14 MPa.
• Example 4 In the same manner as in Example 3, a thermally conductive sheet having an adhesive layer (adhesive layer 3 in Table 4) formed on one main surface was obtained. The thermal resistance was measured in the same manner as in Example 3, except that the pressure condition when preparing the three-layer structure sample for measuring the thermal resistance was changed to 0.28 MPa.
• Example 5 A thermally conductive sheet having an adhesive layer (adhesive layer 3 in Table 4) formed on two main surfaces was obtained in the same manner as in Example 3, except that the two main surfaces of the thermally conductive layer 1 obtained as described above were hot roll laminated.
• the pressure when preparing a three-layer structure sample for thermal resistance measurement described later was set to 0.14 MPa.
• Example 6 In the same manner as in Example 5, a thermally conductive sheet having adhesive layers (adhesive layer 3 in Table 4) formed on the two main surfaces was obtained. The thermal resistance was measured in the same manner as in Example 5, except that the pressure condition when preparing the three-layer structure sample for measuring the thermal resistance was changed to 0.28 MPa.
• Example 7 A thermally conductive sheet having adhesive layers (adhesive layer 3 in Table 4) formed on two main surfaces was obtained in the same manner as in Example 6, except that thermally conductive layer 1 was changed to thermally conductive layer 2.
• the pressure when preparing a three-layer structure sample for thermal resistance measurement described later was set to 0.28 MPa.
• Comparative Examples 2 and 3 In Comparative Examples 2 and 3, no adhesive layer was formed and the thermally conductive layer 1 was used as the thermally conductive sheet.
• the pressures applied when preparing three-layered samples for thermal resistance measurement described later were 0.14 MPa and 0.28 MPa, respectively.
• Comparative Example 4 In Comparative Examples 2 and 3, no adhesive layer was formed and the thermally conductive layer 2 was used as a thermally conductive sheet.
• the pressure when preparing a three-layer structure sample for thermal resistance measurement described later was set to 0.28 MPa.
• the thickness of the thermally conductive sheets after compression was determined in the same manner as described above, and the thermal resistance of the thermally conductive sheets was determined as follows. The results are shown in Table 4.
• Thermal resistance was measured using a tabletop xenon flash analyzer (LFA 447 Hyper Flash). A 15 mm square thermally conductive sheet was sandwiched between 1 mm copper plates to prepare a three-layered sample. The sample was prepared under the following conditions: temperature 150°C, pressure 0.14 MPa or 0.28 MPa. In order to ensure sufficient adhesion of the resin to the copper plate, the sample was heated in an oven at 150°C for 30 minutes, then heated in an oven at 180°C for 1 hour, and cooled sufficiently at room temperature. In addition, as a pretreatment for the measurement, the copper surface was blackened with carbon spray, and the measurement was performed.
• the thermal conductivity of the heat conductive layer/thermal conductivity of the adhesive layer were determined in Examples 3 to 7 in the same manner as in Examples 1 and 2. Furthermore, the porosity of the interface was determined as follows. The results are shown in Table 5 and FIG. When adhesive layers were disposed on both sides of the thermally conductive layer, the porosity of the interface (the surface with the adhesive layer) was taken as the arithmetic average of the porosities of both surfaces. When an adhesive layer was not disposed on both sides of the thermally conductive layer, the porosity of the interface (the side without an adhesive layer) was taken as the arithmetic average of the porosities of both sides.

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