Patent ID: 12233493

DESCRIPTION OF THE EMBODIMENTS

<Method for producing a heat transfer sheet>

A method for producing a heat transfer sheet according to the present art, the method including: a process of forming a mixture containing at least one of a carbon fiber and a boron nitride flake, an inorganic filler, and a binder resin into a molded body, and orienting at least one of the carbon fiber and the boron nitride flake in a thickness direction of the molded body (hereinafter referred to as process A1); a process of slicing the molded body into a sheet shape to obtain a molded sheet (hereinafter referred to as process B1); a process of pressing a sliced face of the molded sheet (hereinafter referred to as process C1); and a process of inserting the pressed molded sheet between films and vacuum packing it to cause an uncured component of the binder resin present inside the pressed molded sheet to be exuded to the pressed molded sheet surface (hereinafter referred to as process D1).

FIG.1is a cross-sectional view illustrating one example of a heat transfer sheet. According to the present production method, by inserting a pressed molded sheet between films and vacuum packing it, the mechanism by which an oil component (for example, an uncured component of a binder resin9of a heat transfer sheet3) present inside the heat transfer sheet3bleeds to an interface with the film is promoted by a reduced pressure state, and a tackiness of the heat transfer sheet3may be developed further. Therefore, ease of handling of the heat transfer sheet3obtained by this production method may be improved during use and, for example, adhesion to an adherend may be improved.

[Process A1]

In the process A1, a mixture containing at least one of the carbon fiber10and the boron nitride flake12, an inorganic filler11, and the binder resin9is formed into a molded body, and the carbon fiber10is oriented in a thickness direction of the molded body. For example, in the process A1, a thermally conductive resin composition containing at least one of the carbon fiber10and the boron nitride flake12, the inorganic filler11, and the binder resin9is first prepared. The thermally conductive resin composition may be uniformly mixed together with various additives or volatile solvents by a known method.

Next, in the process A1, a molded block is formed from the thermally conductive resin composition. An extrusion molding method, a die molding method, or the like is given as an example of the method for forming the molded block. The extrusion molding and die molding methods are not particularly limited, and may be adopted as appropriate from among various known extrusion molding and die molding methods depending on the viscosity of the thermally conductive resin composition, the properties required for the heat transfer sheet, or the like.

For example, when the thermally conductive resin composition is pushed out from a die in the extrusion molding method, or when the thermally conductive resin composition is pressed into a die in the die molding method, the binder resin9flows and at least one of the carbon fiber10and the boron nitride flake12are oriented along the flow direction thereof.

A size and shape of the molded block may be determined according to a required size of the heat transfer sheet. An example includes a cuboid having a cross section size of 0.5 to 15 cm in length and 0.5 to 15 cm in width. The length of the cuboid may be determined as needed. The extrusion molding method facilitates the formation of a columnar molded block, composed of a cured product of the thermally conductive resin composition wherein a major axis of the carbon fiber10and/or a major axis of the boron nitride flake12are oriented in an extrusion direction.

It is preferable that the obtained molded body is subjected to thermosetting. The curing temperature for thermosetting may be selected as appropriate according to the purpose; for example, when the binder resin9is a silicone resin, it may be in a range of 60° C. to 120° C. The curing time for thermosetting may be in a range of 30 minutes to 10 hours, for example.

Next, the carbon fiber10, the boron nitride flake12, the inorganic filler11, and the binder resin9used in the process A1will be described.

<Carbon fiber>

The carbon fiber10may be, for example, a pitch-based carbon fiber, PAN-based carbon fiber, graphitized PBO fiber, or a carbon fiber synthesized by an arc discharge method, laser evaporation method, CVD (chemical vapor deposition) method, CCVD (catalytic chemical vapor deposition) method, or the like. Among these, a pitch-based carbon fiber is preferred from the viewpoint of thermal conductivity.

An average fiber length (average major axis length) of the carbon fiber10may be, for example, 50 to 250 μm, or even 75 to 200 μm or 90 to 170 μm. Furthermore, an average fiber diameter (average minor axis length) of the carbon fiber10may be selected as appropriate according to the purpose; for example, it may be 4 to 20 μm or even 5 to 14 μm. An aspect ratio (average major axis length/average minor axis length) of the carbon fiber10may be selected as appropriate according to the purpose; for example, it may be 9 to 30. The average major axis length and average minor axis length of the carbon fiber10may be measured, for example, by a microscope or scanning electron microscope (SEM).

A surface of the carbon fiber10may be coated by an insulating film. Thus, an insulation-coated carbon fiber may be used as the carbon fiber10. The insulation-coated carbon fiber has the carbon fiber10and an insulating film on at least one portion of the surface of the carbon fiber10, and may contain other components as required.

The insulating film is composed of a material having an electrical insulating property, and is formed of, for example, a silicon oxide or a cured product of a polymerizable material. The polymerizable material is, for example, a radical polymerizable material, and examples include organic compounds having polymerizability, resins having polymerizability, and the like. A radical polymerizable material may be selected as appropriate according to the purpose, provided that the material utilizes energy for radical polymerization, and a compound having a radical polymerizable double bond may be given as an example. A vinyl group, an acryloyl group, a methacryloyl group, or the like may be given as examples of the radical polymerizable double bond. Two or more radical polymerizable double bonds in a compound having a radical polymerizable double bond is preferred from the viewpoint of heat resistance and strength including solvent resistance. Examples of compounds having two or more radical polymerizable double bonds include divinylbenzene (DVB) and compounds having two or more (meth)acryloyl groups. One type of radical polymerizable material may be used alone or two or more types may be used in combination. A molecular weight of the radical polymerizable material may be selected as appropriate according to the purpose, and may be, for example, in a range of 50 to 500. When the insulating film is formed by a cured product of a polymerizable material, a content of the constituent units derived from the polymerizable material in the insulating film may be, for example, 50 mass % or more, and may also be 90 mass % or more.

An average thickness of the insulating film may be selected as appropriate according to the purpose, and from the viewpoint of achieving high insulation, it may be 50 nm or more, or even 100 nm or more or 200 nm or more. An upper limit of the average thickness of the insulating film may be, for example, 1,000 nm or less or even 500 nm or less. The average thickness of the insulating film may be established, for example, by transmission electron microscopy (TEM) observation.

Examples of methods for coating the carbon fiber10using an insulating film include a sol-gel method, a liquid phase deposition method, a polysiloxane method, a method for forming an insulating film composed of a cured product of polymerizable material on at least one portion of the surface of the carbon fiber taught in JP 2018-98515 A, or the like.

From the viewpoint of thermal conductivity of the heat transfer sheet3, a content of the carbon fiber10in the thermally conductive resin composition may be, for example, 15 mass % or more, may be 18 mass % or more, may be 20 mass % or more, may be 21 mass % or more, or may be or 22 mass % or more. Furthermore, from the viewpoint of suppressing the viscosity of the heat transfer sheet3from becoming too high, the content of carbon fiber10in the thermally conductive resin composition may be, for example, 30 mass % or less, may be 28 mass % or less, may be 25 mass % or less, may be 24 mass % or less, or may be 23 mass % or less. One type of the carbon fiber10may be used alone or two or more types may be used in combination. When two or more types of the carbon fiber10are used in combination, it is preferable that the content thereof meets the aforementioned content.

<Boron nitride flake>

The boron nitride flake12is boron nitride having a major axis, a minor axis, and a thickness, and has a high aspect ratio (major axis/thickness) and isotropic thermal conductivity in a face direction including the major axis. The minor axis is a length of a shortest portion of the boron nitride flake12on a face including the major axis of the boron nitride flake12and in a direction intersecting the major axis at a substantial center portion of a particle of boron nitride flake12. Thickness refers to a value that is an average of measurements taken at ten points of the thickness of the face including the major axis of the boron nitride flake12.FIG.2is a perspective view schematically illustrating the boron nitride flake12having a hexagonal crystal shape. InFIG.2, a represents the major axis of the boron nitride flake12, b represents the thickness of the boron nitride flake12, and c represents the minor axis of the boron nitride flake12. From the viewpoint of thermal conductivity of the heat transfer sheet3, it is preferable to use boron nitride flake12having a hexagonal crystal shape as the boron nitride flake12, as illustrated inFIG.2.

An average particle diameter (D50) of the boron nitride flake12is not particularly limited and may be selected as appropriate according to the purpose. For example, the average particle diameter of a heat transfer material5having shape anisotropy may be 10 μm or more, or even 20 μm or more, 30 μm or more, 35 μm or more, or 40 μm or more. Furthermore, an upper limit of the average particle diameter of the boron nitride flake12may be 150 μm or less, or even 100 μm or less, 90 μm or less, 80 μm or less, 70 μm or less, 50 μm or less, or 45 μm or less. From the viewpoint of thermal conductivity of the heat transfer sheet3, the average particle diameter of the boron nitride flake12may be 20 to 100 μm, or may be 30 to 60 μm. A volume average particle diameter of the boron nitride flake12refers to a particle diameter—when a cumulative curve of the value of the particle diameter is found from a small particle diameter side of particle diameter distribution in the case where an overall particle diameter distribution of the boron nitride flake12is 100%—when a cumulative value thereof is 50%. Particle size distribution (particle diameter distribution) is found by a volume basis. An example of a method for measuring particle size distribution includes a method using a laser diffraction particle size distribution analyzer.

An aspect ratio of the boron nitride flake12is not particularly limited and may be selected as appropriate according to the purpose. For example, the aspect ratio of the boron nitride flake12may be in a range of 10 to 100. An average value of the ratio of the major axis to the minor axis (major axis/minor axis) of the boron nitride flake12may be, for example, in a range of 0.5 to 10, may be in a range of 1 to 5, or may be in a range of 1 to 3.

From the viewpoint of thermal conductivity of the heat transfer sheet3, a content of the boron nitride flake12in the thermally conductive resin composition may be, for example, 18 mass % or more, may be 20 mass % or more, may be 22 mass % or more, may be 24 mass % or more, may be 26 mass % or more, or may be 28 mass % or more. Furthermore, from the viewpoint of suppressing the viscosity of the heat transfer sheet3from becoming too high, a content of the boron nitride flake12in the thermally conductive resin composition may be, for example, 35 mass % or less, may be 33 mass % or less, may be 30 mass % or less, or may be 29 mass % or less. One type of the boron nitride flake12may be used alone or two or more types may be used in combination. When two or more types of the boron nitride flake12are used in combination, it is preferable that the content thereof meets the aforementioned content.

<Inorganic filler>

The inorganic filler11is an inorganic filler (heat transfer filler) other than the carbon fiber10or the boron nitride flake12. The inorganic filler11preferably includes at least one of aluminum oxide, aluminum nitride, and aluminum hydroxide, for example. A form that includes aluminum oxide or a form wherein aluminum oxide and aluminum nitride are used in combination is preferred as a specific example.

A specific surface area of the inorganic filler11may be selected as appropriate according to the purpose; for example, from the viewpoint of making the surface condition of the heat transfer sheet smoother after undergoing pressing, it may be 1.4 m2/g or more, or may be in a range of 1.4 to 3.3 m2/g. The specific surface area of the inorganic filler11may be measured, for example, by a BET method.

A shape or a volume average particle diameter of the inorganic filler11may be set as appropriate according to the purpose. Examples of the shape of the inorganic filler11include spherical, ellipsoidal, block-shaped, granular, flat, needle-shaped, and the like. Among these, spherical and ellipsoidal shapes are preferred from the viewpoint of fillability, and spherical is even more preferred.

From the viewpoint of more effectively developing the tackiness of the heat transfer sheet3, the volume average particle diameter of the inorganic filler11may be 0.1 μm or more, may be 0.5 μm or more, may be 1.0 μm or more, may be 2.0 μm or more, may be 3.0 μm or more, or may be 4.0 μm or more. Furthermore, an average particle diameter of the inorganic filler may be 8.0 μm or less, may be 7.0 μm or less, may be 6.0 μm or less, may be in a range of 3.0 to 7.0 μm, may be in a range of 4.0 to 6.0 μm, may be in a range of 0.5 to 2.0 μm, or may be in a range of 1.0 to 5.0 μm. The volume average particle diameter of the inorganic filler11refers to a particle size—when a cumulative curve of the value of the particle diameter is found from a small particle diameter side of particle diameter distribution in the case where an overall particle diameter distribution of the inorganic filler11is 100%—when a cumulative value thereof is 50%. An example of a method for measuring particle size distribution includes a method using a laser diffraction particle size distribution analyzer.

The inorganic filler11may undergo surface treatment. Examples of surface treatment include treating the inorganic filler11using a coupling agent, such as an alkoxysilane compound. The amount of coupling agent used for treatment may be in a range of 0.1 to 1.5 mass % of the total amount of inorganic filler, for example.

An alkoxysilane compound is a compound having a structure wherein one to three bonds from among four bonds having a silicon atom (Si) are bound to an alkoxy group and the remaining bond or bonds are bound to an organic substituent. Examples of the alkoxy group of the alkoxysilane compound include a methoxy group, an ethoxy group, a butoxy group, or the like. Specific examples of the alkoxysilane compound include a trimethoxysilane compound, a triethoxysilane compound, or the like.

From the viewpoint of more effectively suppressing an increase in thermal resistance of the heat transfer sheet3, a content of the inorganic filler11in the thermally conductive resin composition may be, for example, 40 mass % or more, may be 45 mass % or more, may be 50 mass % or more, may be 55 mass % or more, may be 60 mass % or more, or may be 65 mass % or more. Furthermore, from the viewpoint of further improving flexibility of the heat transfer sheet3, the content of the inorganic filler11in the thermally conductive resin composition may be, for example, 80 mass % or less, may be 75 mass % or less, may be 70 mass % or less, or may be 65 mass % or less. One type of the inorganic filler11may be used alone or two or more types may be used in combination. When two or more types of the inorganic filler11are used in combination, it is preferable that the content thereof meets the aforementioned content.

<Binder resin>

The binder resin9is not particularly limited and may be selected as appropriate according to the purpose; examples include a thermoplastic resin, a thermoplastic elastomer, a thermosetting polymer, or the like.

Examples of the thermoplastic resin include a polyethylene, a polypropylene, an ethylene-α olefin copolymer such as an ethylene-propylene copolymer or the like, a polymethylpentene, a polyvinyl chloride, a polyvinylidene chloride, a polyvinyl acetate, an ethylene-vinyl acetate copolymer, a polyvinyl alcohol, a polyvinyl acetal, a fluorinated polymer such as a polyvinylidene fluoride, a polytetrafluoroethylene, or the like, a polyethylene terephthalate, a polybutylene terephthalate, a polyethylene naphthalate, a polystyrene, a polyacrylonitrile, a styrene-acrylonitrile copolymer, an acrylonitrile-butadiene-styrene copolymer (ABS) resin, a polyphenylene-ether copolymer (PPE) resin, a modified PPE resin, an aliphatic polyamide, an aromatic polyamide, a polyimide, a polyamide-imide, a polymethacrylic acid, a polymethacrylate ester such as polymethacrylic acid methyl ester or the like, a polyacrylic acid, a polycarbonate, a polyphenylene sulfide, a polysulfone, a polyether sulfone, a polyether nitrile, a polyether ketone, a polyketone, a liquid crystal polymer, a silicone resin, an ionomer, or the like.

Examples of the thermoplastic elastomer include a styrene-butadiene block copolymer or a hydrogenated product thereof, a styrene-isoprene block copolymer or a hydrogenated product thereof, a styrene-based thermoplastic elastomer, an olefin-based thermoplastic elastomer, a vinyl chloride-based thermoplastic elastomer, a polyester-based thermoplastic elastomer, a polyurethane-based thermoplastic elastomer, a polyamide-based thermoplastic elastomer, or the like.

Examples of the thermosetting resin include a cross-linked rubber, an epoxy resin, a phenolic resin, a polyimide resin, an unsaturated polyester resin, a diallyl phthalate resin, or the like. Specific examples of the cross-linked rubber include a natural rubber, an acrylic rubber, a butadiene rubber, an isoprene rubber, a styrene-butadiene copolymer rubber, a nitrile rubber, a hydrogenated nitrile rubber, a chloroprene rubber, an ethylene-propylene copolymer rubber, a chlorinated polyethylene rubber, a chlorosulfonated polyethylene rubber, a butyl rubber, a halogenated butyl rubber, a fluoro rubber, a urethane rubber, a silicone rubber, or the like.

A silicone resin is preferred as the binder resin9, for example, from the viewpoint of moldability, weather resistance, and adhesion or followability between a heat-generating face of an electronic component and a heat sink face. A two-component addition-reaction silicone resin may be used as the silicon resin, composed of, for example, a main agent containing a silicone having an alkenyl group as a main component and a curing catalyst, and a curing agent having a hydrosilyl group (Si—H group). A polyorganosiloxane having a vinyl group may be used, for example, as the silicone having an alkenyl group. The curing catalyst is a catalyst for promoting an addition reaction between the alkenyl group in the silicone having the alkenyl group and the hydrosilyl group in the curing agent having the hydrosilyl group. Examples of the curing catalyst include a well-known catalyst as a catalyst used for a hydrosilylation reaction; for example, a platinum group-based curing catalyst—for example, a single platinum group metal such as platinum, rhodium, palladium, or the like, or platinum chloride—may be used. A polyorganosiloxane having a hydrosilyl group may be used, for example, as the curing agent having the hydrosilyl group.

The content of binder resin9in the thermally conductive resin composition is not particularly limited and may be selected as appropriate according to the purpose. For example, the content of binder resin9in the thermally conductive resin composition may be 11 mass % or more, or even 14 mass % or more, 20 mass % or more, or 25 mass % or more. Furthermore, the content of the binder resin9in the thermally conductive resin composition may be 60 mass % or less, or even 50 mass % or less, 40 mass % or less, 30 mass % or less, 20 mass % or less, or 15 mass % or less. One type of the binder resin9may be used alone or two or more types may be used in combination. When two or more types of the binder resin9are used in combination, it is preferable that the content thereof meets the aforementioned content.

[Process B1]

In the process B1, the molded body is sliced into a sheet shape to obtain a molded sheet. At least one of the carbon fiber10and the boron nitride flake12are exposed on the molded sheet obtained by slicing. There are no particular restrictions on the slicing method, which may be selected as appropriate from among known slicing devices according to a size or mechanical strength of the molded block. Examples of slicing devices include an ultrasonic cutter, a plane, or the like.

In terms of a slice direction of the molded block, when the molding method is an extrusion molding method, some of at least one of the carbon fiber10and the boron nitride flake12are oriented in an extrusion direction; therefore, for the extrusion direction, 60 to 120 degrees is preferred, a direction of 70 to 100 degrees is more preferred, and a direction of 90 degrees (vertical) is even more preferred. When a columnar molded block is formed by the extrusion molding method in the process A1, in the process B1, it is preferable to slice in a direction substantially perpendicular to a length direction of the molded block. An average thickness of the molded sheet may be selected as appropriate according to the purpose; for example, it may be in a range of 0.1 to 5.0 mm or may be in a range of 0.2 to 1.0 mm.

[Process C1]

In the process C1, a sliced face of the molded sheet is pressed. In the process C1, the sliced face of the molded sheet obtained in the process B1is pressed to obtain a molded sheet after pressing (hereinafter also referred to as “heat transfer sheet precursor”), containing at least one of the carbon fiber10and the boron nitride flake12, the inorganic filler11, and the binder resin9, wherein at least one of the carbon fiber10and the boron nitride flake12are oriented in the thickness direction. A surface of the heat transfer sheet precursor obtained in the process C1is made smoother, and adhesion may be further improved when it is inserted between films in the process D1described below.

A pair of press devices composed of a flat plate and a press head having a flat surface may be used to press the molded sheet. Furthermore, the molded sheet may also be pressed by a pinch roll. A pressure during pressing may be, for example, in a range of 0.1 to 100 MPa, or even a range of 0.1 to 1 MPa or a range of 0.1 to 0.5 MPa. A pressing time may be selected as appropriate according to the pressure, sheet area, or the like during pressing; for example, it may be in a range of 10 seconds to five minutes, or even in a range of 30 seconds to three minutes.

In one embodiment, a press head having a built-in heater may be used to perform pressing while heating. A pressing temperature may be in a range of 0 to 180° C., for example, and may even be in a range of room temperature (for example, 25° C.) to 100° C., or a range of 30 to 100° C. To further enhance the effect of pressing and reduce the pressing time, pressing may be performed at a glass transition temperature (Tg) or more of the binder resin9constituting the molded sheet.

The pressing process in the process C1is preferably performed using a spacer for compressing the molded sheet to a predetermined thickness. A thickness of the heat transfer sheet precursor may be selected as appropriate according to the purpose; for example, it may be in a range of 0.1 to 5.0 mm, or may be in a range of 0.2 to 1.0 mm.

Pressing the sliced face of the molded sheet in the process C1compresses the molded sheet in the thickness direction, which further increases a frequency of contact between at least one of the carbon fiber10and the boron nitride flake12and the inorganic filler11, thereby reducing a thermal resistance value of the heat transfer sheet precursor. The thermal resistance value of the heat transfer sheet precursor may be, at a load of 1 kgf/cm2, for example, 0.633° C.·cm2/W or less, may be 0.585° C.·cm2/W or less, may be 0.350° C.·cm2/W or less, may be 0.340° C.·cm2/W or less, may be 0.330° C.·cm2/W or less, may be 0.320° C.·cm2/W or less, may be 0.310° C.·cm2/W or less, may be 0.305° C.·cm2/W or less, may be in a range of 0.307 to 0.633° C.·cm2/W or less, may be in a range of 0.307 to 0.327° C.·cm2/W or less, or may be in a range of 0.585 to 0.633° C.·cm2/W or less. The thermal resistance of the heat transfer sheet precursor may be measured by a method described in an example below.

In the process C1, the molded sheet is in a state wherein it is interposed by films (for example, release films)—that is, a laminated body of a film, the molded sheet, and a film may be pressed. This may prevent the molded sheet from sticking to the press device when pressing the molded sheet. The film may be peeled off after the molded sheet is pressed.

[Process D1]

FIG.3is a perspective view for describing one example of inserting the pressed molded sheet between the films and vacuum packing it. The arrows inFIG.3indicate a direction of vacuum degassing.FIG.4is a cross-sectional view illustrating one example of the laminated body wherein the heat transfer sheet is interposed between the films. In the process D1, a heat transfer sheet precursor1is inserted between the films2and is vacuum packed to cause an uncured component of the binder resin9present inside the heat transfer sheet precursor1to exude to a surface of the heat transfer sheet precursor1. The uncured component of the binder resin9that seeps to the surface of the heat transfer sheet precursor1may be in an uncured state, or may be in a state where curing of the binder resin9has proceeded by about several percent.

In the present production method, in addition to imparting tackiness by exudation of the uncured component (residual component) of the binder resin9in the molded sheet by pressing in the process C1, in the process D1, the heat transfer sheet precursor1is inserted between the films2and is vacuum packed, causing the uncured component (residual component) of the binder resin9to be exuded further. The heat transfer sheet3, having tackiness on both faces, is thereby obtained in the present production method.

Examples of the film2include a PET (polyethylene terephthalate), a PEN (polyethylene naphthalate), a polyolefin, a polymethylpentene, a glassine paper, or the like. A thickness of the film2is not particularly limited and may be selected as appropriate according to the purpose; for example, it may be 0.01 to 0.15 mm. Furthermore, the thinner the thickness of the film2, the better adherence (followability) to the heat transfer sheet3, and the tack force of the heat transfer sheet3may develop more effectively. For example, from the viewpoint of more effectively developing the tack force of the heat transfer sheet3, a PET film having a thin thickness is preferred for the film2.

It is preferable that the tack force of the heat transfer sheet3obtained in process D1—that is, that of the heat transfer sheet3removed from the vacuum-packed state—satisfies condition 1 below.

Condition 1: The tack force of the heat transfer sheet3surface is 100 gf or more when a probe having a diameter of 5.1 mm presses in the heat transfer sheet3at a force of 200 gf at 2 mm/sec and pulls it off at 10 mm/sec.

The heat transfer sheet3has a tack force of 100 gf or more, as indicated by the aforementioned condition 1, and this may be 105 gf or more, 115 gf or more, 140 gf or more, 150 gf or more, 170 gf or more, 190 gf or more, in a range of 108 to 152 gf, in a range of 108 to 201 gf, or in a range of 176 to 201 gf.

A thermal resistance value of the heat transfer sheet3(the heat transfer sheet3removed from the vacuum-packed state) may be within ±5% of the thermal resistance value of the heat transfer sheet precursor1(the pressed molded sheet before vacuum packing), may be within ±5.0%, may be within ±4.7%, may be within ±4.1%, may be within ±3.1%, may be within ±2.2%, may be within ±1.0%, may be within ±0.5%, or may be within ±0.3%. Thus, the present production method may reduce a change in the thermal resistance value of the heat transfer sheet3before and after vacuum packing.

In the process D1, for example, it is preferable to insert the heat transfer sheet precursor1between the films2and hold it for a predetermined time or more in a predetermined reduced pressure state.

From the viewpoint of requiring the development of tack force, the predetermined reduced pressure state is preferably less than 400 torr, or even 350 torr or less, or 300 torr or less. Furthermore, a lower limit of the predetermined reduced pressure state is not particularly limited, but may be, for example, 150 torr or more, or even 200 torr or more, 250 torr or more, or 300 torr or more. A preferred range for the predetermined reduced pressure state is, for example, 150 to 300 torr.

Furthermore, a holding time of the predetermined reduced pressure state may be, for example, one minute or more, or even 10 minutes or more, 30 minutes or more, one hour or more, two hours or more, three hours or more, six hours or more, eight hours or more, 12 hours or more, 24 hours or more, 120 hours or more, 240 hours or more, or one minute or more and 240 hours or less.

In one embodiment of the process D1, it is preferable to insert the heat transfer sheet precursor1between the films2and hold it for one minute or more in a reduced pressure state of 150 to 300 torr.

Thus, the heat transfer sheet3, imparted with strong tack force on the surface, is obtained in the present production method. This kind of heat transfer sheet3may have improved adhesion to an adherend such as a heating element, heat dissipating element, or the like, and may be more securely fixed to the adherend. Furthermore, the heat transfer sheet3is imparted with a tack force, without worsening the thermal resistance value, such that it does not fall off even when attached to the adherend and inverted. Note that a method using a vacuum press is also conceivable as the method for imparting tackiness to the heat transfer sheet3; however, productivity tends to be poor because of the takt time required for creating a vacuum and returning to the atmosphere. On the other hand, according to the present production method, strong tackiness may be imparted without compromising a thermal performance of the heat transfer sheet3, while also reducing the labor and cost of the production process compared to the method using a vacuum press.

<Heat transfer sheet>

As illustrated inFIG.1, one embodiment of the heat transfer sheet3contains the binder resin9, at least one of the carbon fiber10and the boron nitride flake12, and the inorganic filler11. Furthermore, in the heat transfer sheet3, at least one of the carbon fiber10and the boron nitride flake12are oriented in a thickness direction B of the heat transfer sheet3. The heat transfer sheet3has good tackiness as described above, which improves handling during use and adhesion to the adherend. The binder resin9, the carbon fiber10, the inorganic filler11, and the boron nitride flake12are synonymous with the binder resin9, the carbon fiber10, the inorganic filler11, and the boron nitride flake12described in the section on the aforementioned method for producing the heat transfer sheet, and the preferred ranges are also the same.

Also, it is preferable that the heat transfer sheet3satisfies the aforementioned condition 1 in terms of tack force when removed from the sealed state obtained by holding the reduced pressure state of 150 to 300 torr for one minute or more. The sealed state is a tightly sealed state such that the heat transfer sheet3may be held at a reduced pressure state of 150 to 300 torr for one minute or more.

The thermal resistance value of the heat transfer sheet3after removal from the aforementioned sealed state may be within ±5% of the thermal resistance value of the heat transfer sheet before sealing, may be within ±4.1%, may be within ±3.1%, may be within ±2.2%, may be within ±1%, may be within ±0.5%, or may be within ±0.3%.

A thickness of the heat transfer sheet3is not particularly limited and may be selected as appropriate according to the purpose. For example, the thickness of the heat transfer sheet3may be 0.05 mm or more, or may be 0.1 mm or more. Furthermore, an upper limit of the thickness of the heat transfer sheet3may be 5 mm or less, or even 4 mm or less or 3 mm or less. It is preferable that the heat transfer sheet3has a thickness of 0.1 to 4 mm from the viewpoint of ease of handling. The thickness of the heat transfer sheet3may be found, for example, from an arithmetic average value of the thickness of the heat transfer sheet3measured at any five locations.

<Method for producing a heat transfer sheet package>

A method for producing the heat transfer sheet package according to the present art includes a process A2of forming a mixture containing at least one of the carbon fiber10and the boron nitride flake12, the inorganic filler11, and the binder resin9into the molded body, and orienting at least one of the carbon fiber10and the boron nitride flake12in the thickness direction of the molded body; a process B2of slicing the molded body into a sheet shape to obtain the molded sheet; a process C2of pressing a sliced face of the molded sheet; and a process D2of inserting the pressed molded sheet between thermoplastic resin films and vacuum packing it.

Since the process A2is the same as the process A1of the aforementioned method for producing the heat transfer sheet, a detailed description is omitted.

Since the process B2is the same as the process B1of the aforementioned method for producing the heat transfer sheet, a detailed description is omitted.

Since the process C2is the same as the process C1of the aforementioned method for producing the heat transfer sheet, a detailed description is omitted.

In the process D2, as illustrated inFIG.3, the heat transfer sheet package5is obtained by inserting the heat transfer sheet precursor1between thermoplastic resin films6and vacuum packing it. In the process D2, the mechanism by which an oil component (for example, the uncured component of the binder resin of the heat transfer sheet3) present inside the heat transfer sheet3bleeds to an interface with the thermoplastic resin film6is promoted by a vacuum state, and the tackiness of the heat transfer sheet3may be developed further. Therefore, the heat transfer sheet3constituting the heat transfer sheet package5obtained in the process D2has improved ease of handling during use and improved adhesion to an adherend.

In one embodiment of the process D2, the heat transfer sheet package5is obtained by sealing the laminated body4in which the heat transfer sheet3is interposed between the thermoplastic resin films6in a reduced pressure state inside a sealing film8.

In the process D2, in the same manner as the aforementioned process D1, it is preferable to insert the heat transfer sheet precursor1between the thermoplastic resin films6and hold it for a predetermined time or more in a predetermined reduced pressure state. This allows, for example, the tackiness of the heat transfer sheet3to be improved when the heat transfer sheet package5has the heat transfer sheet3vacuumed packed therein as a product, and also allows good thermal conductivity of the heat transfer sheet3to be maintained. Accordingly, when selling the heat transfer sheet3to a user, for example, the heat transfer sheet3may be supplied while maintaining the performance (for example, thermal conductivity) of the heat transfer sheet3when it is designed.

It is preferable that the tack force of the heat transfer sheet3—that is, the tack force of the heat transfer sheet3removed from the heat transfer sheet package5—satisfies the aforementioned condition 1. A preferred range of the tack force of the heat transfer sheet3is the same as the tack force of the heat transfer sheet3in the aforementioned process D1.

Even after the heat transfer sheet precursor1is inserted between the thermoplastic resin film6, vacuum packed, and held for a predetermined time—for example, 240 hours or less—the thermally conductive sheet3constituting the heat transfer sheet package5may exhibit good performance (for example, high tack force or thermal conductivity).

Examples of a material of the thermoplastic resin film6include a PET (polyethylene terephthalate), a polyolefin, a polymethylpentene, or the like. A thickness of the thermoplastic resin film6is not particularly limited and may be selected as appropriate according to the purpose; for example, it may be 0.01 to 0.15 mm. Furthermore, the thinner the thickness of the thermoplastic resin film6, the better the followability (adhesion) to the heat transfer sheet3, and the tack force of the heat transfer sheet3may be developed more effectively. For example, from the viewpoint of more effectively developing the tack force of the heat transfer sheet3, a PET film having a thin thickness is preferred for the thermoplastic resin film6.

A substrate7is not particularly limited in material or thickness, provided that the laminated body4may be placed thereon; for example, a thick paper may be used.

The sealing film8is not particularly limited, provided that it is capable of vacuum packing the laminated body4for a predetermined time or more; for example, a bag composed of a thermoplastic material may be used.

<Heat transfer sheet package>

The heat transfer sheet package5according to the present art, as illustrated inFIG.3, contains the heat transfer sheet3and holds the heat transfer sheet3in a vacuum state. The heat transfer sheet3contained in the heat transfer sheet package5has good tackiness as described above, which improves handling during use and adhesion to the adherend.

In one embodiment of the heat transfer sheet package5, as illustrated inFIGS.3and4, an example of the heat transfer sheet package5, obtained by sealing the laminated body4—wherein the heat transfer sheet3is interposed between the thermoplastic resin films6—in a vacuum state (preferably a reduced pressure state of 150 to 300 torr) inside the sealing film8. The number of heat transfer sheets3contained in the heat transfer sheet package5may be one, or may be two or more. When the heat transfer sheet package5has two or more heat transfer sheets3, each heat transfer sheet3may be disposed at a predetermined interval, as illustrated inFIG.3.

<Electronic device>

The heat transfer sheet3may be placed between a heating element and a heat dissipating element, for example, to create an electronic device (thermal device) having a structure arranged between these elements to release heat generated by the heating element to the heat dissipating element. The electronic device has at least the heating element, the heat dissipating element, and the heat transfer sheet3, and may further have other components as needed.

The heating element is not particularly limited, and examples include an integrated circuit element such as a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), a DRAM (Dynamic Random Access Memory), a flash memory, or the like, or an electronic component for generating heat in an electric circuit such as a transistor, resistor, or the like. Furthermore, the heating element also includes a component for receiving optical signals, such as an optical transceiver in a telecommunications device.

The heat dissipating element is not particularly limited, and examples include a heat sink or a heat spreader, which is used with an integrated circuit element, a transistor, an optical transceiver housing, or the like. Examples of the material for the heat sink or the heat spreader include copper, aluminum, or the like. In addition to a heat spreader or heat sink, the heat dissipating element may be any element that conducts and dissipates heat generated from a heat source to the outside, and examples include a radiator, a cooler, a die pad, a printed circuit board, a cooling fan, a Peltier element, a heat pipe, a vapor chamber, a metal cover, a housing, or the like. A heat pipe has a hollow structure having a cylindrical shape, a substantially cylindrical shape, or a flat cylindrical shape, for example.

FIG.5is a cross-sectional view illustrating one example of a semiconductor device to which the heat transfer sheet is applied. For example, as illustrated inFIG.5, the heat transfer sheet3is mounted on a semiconductor device50built-in various electronic devices and interposed between the heating element and the heat dissipating element. The semiconductor device50illustrated inFIG.5is provided with an electronic component51, a heat spreader52, and the heat transfer sheet3, wherein the heat transfer sheet3is interposed between the heat spreader52and the electronic component51. The heat transfer sheet3is interposed between the heat spreader52and a heat sink53, and together with the heat spreader52, thereby constitutes a heat-dissipating member for dissipating heat from the electronic component51. A mounting location of the heat transfer sheet3is not limited to between the heat spreader52and the electronic component51or between the heat spreader52and the heat sink53; it may be selected as appropriate according to a configuration of the electronic device or the semiconductor device. The heat spreader52has, for example, a main face52aformed as a square plate and facing the electronic component51, and a side wall52berected along a periphery of the main face main face52a. The heat spreader52is provided with a heat transfer sheet3on the main face52asurrounded by the side wall52b, and the heat sink53is provided through the heat transfer sheet3on an other face52con an opposite side to the main face52a.

EXAMPLES

Examples of the present art are described below. The present art is not limited to these examples.

Example 1

In Example 1, a silicon resin composition (thermally conductive resin composition) was prepared by dispersing 67 mass % aluminum oxide particles having a volume average particle diameter of 5 μm (manufactured by Denki Kagaku Kogyo) which was coupled by a 0.1 mass % coupling agent and 22 mass % pitch-based carbon fibers (average fiber length of 150 μm, average fiber diameter of 91 μm, manufactured by Nippon Graphite Fiber), in a 10.9 mass % two-component addition-reaction liquid silicone resin. Note that the two-component addition-reaction liquid silicone resin is a mixture of a silicone liquid A and a silicone liquid B at a ratio of 6.5 mass % and 4.4 mass %, respectively.

Next, a silicone mold was molded by extruding the resulting silicone resin composition into a cuboidal mold (50 mm×50 mm) having release-treated polyethylene terephthalate (PET) film applied to its inner walls.

A silicone cured product was created by curing the resulting silicone mold in an oven at 100° C. for six hours. A molded sheet having a thickness of 0.53 mm was obtained by cutting the resulting silicone cured product with an ultrasonic cutter. The slicing speed of the ultrasonic cutter was set to 50 mm per second. Furthermore, ultrasonic vibration imparted to the ultrasonic cutter was set at an oscillation frequency of 20.5 kHz and an amplitude of 60 μm.

A 0.50 mm thick heat transfer sheet having carbon fibers oriented in the thickness direction was obtained by inserting the resulting molded sheet between release-treated PET films and then subjecting it to pressing by applying a 0.5 mm thick spacer. The pressing conditions were 87° C. and 0.5 MPa for 180 seconds.

The heat transfer sheet obtained by pressing was again interposed between release-treated PET films and placed in a polyethylene bag, and then the polyethylene bag was sealed by reducing the pressure from normal (760 torr) to 200 torr using a degassing sealer (manufactured by Fuji Impulse). A heat transfer sheet package containing the heat transfer sheet and holding the heat transfer sheet in a vacuum state was thereby obtained. Also, in Example 1, the inside of the polyethylene bag constituting the heat transfer sheet package was returned to normal pressure one minute after sealing the polyethylene bag.

Example 2

Example 2 was conducted in the same manner as Example 1, except that the inside of the polyethylene bag constituting the heat transfer sheet package was returned to normal pressure 12 hours after sealing the polyethylene bag.

Example 3

Example 3 was conducted in the same manner as Example 1, except that the inside of the polyethylene bag constituting the heat transfer sheet package was returned to normal pressure 24 hours after sealing the polyethylene bag.

Example 4

Example 4 was conducted in the same manner as Example 1, except that the inside of the polyethylene bag constituting the heat transfer sheet package was returned to normal pressure 120 hours after sealing the polyethylene bag.

Example 5

Example 5 was conducted in the same manner as Example 1, except that the inside of the polyethylene bag constituting the heat transfer sheet package was returned to normal pressure 240 hours after sealing the polyethylene bag.

Example 6

Example 6 was conducted in the same manner as Example 2, except that the heat transfer sheet obtained by pressing was again interposed between release-treated PET film and placed in a polyethylene bag, and then the polyethylene bag was sealed by reducing the pressure from normal (760 torr) to 150 torr using a degassing sealer.

Example 7

Example 7 was conducted in the same manner as Example 2, except that the heat transfer sheet obtained by pressing was again interposed between release-treated PET film and placed in a polyethylene bag, and then the polyethylene bag was sealed by reducing the pressure from normal (760 torr) to 300 torr using a degassing sealer.

Example 8

In Example 8, a silicon resin composition (thermally conductive resin composition) was prepared by dispersing 32 mass % aluminum oxide particles having a volume average particle diameter of 11 μm coupled by a 0.1 mass % coupling agent, 28 mass % aluminum nitride particles having a volume average particle diameter of 11 μm, and 25 mass % boron nitride flake having a volume average particle diameter of 40 μm, in a 14.9 mass % two-component addition-reaction liquid silicone resin. Note that the two-component addition-reaction liquid silicone resin is a mixture of silicone liquid A and silicone liquid B at a ratio of 8.2 mass % and 6.7 mass %, respectively.

Next, a silicone mold was molded by extruding the resulting silicone resin composition into a cuboidal mold (50 mm×50 mm) having release-treated polyethylene terephthalate (PET) film applied to its inner walls.

A silicone cured product was created by curing the resulting silicone mold in an oven at 100° C. for six hours. A molded sheet having a thickness of 0.53 mm was obtained by cutting the resulting silicone cured product with an ultrasonic cutter. The slicing speed of the ultrasonic cutter was set to 50 mm per second. Furthermore, ultrasonic vibration imparted to the ultrasonic cutter was set at an oscillation frequency of 20.5 kHz and an amplitude of 60 μm.

A 0.50 mm thick heat transfer sheet having carbon fibers oriented in the thickness direction was obtained by inserting the resulting molded sheet between release-treated PET films and then subjecting it to pressing by applying a 0.5 mm thick spacer. The pressing conditions were 87° C. and 0.5 MPa for 180 seconds.

The heat transfer sheet obtained by pressing was again interposed between release-treated PET films and placed in a polyethylene bag, and then the polyethylene bag was sealed by reducing the pressure from normal (760 torr) to 200 torr using a degassing sealer (manufactured by Fuji Impulse). A heat transfer sheet package containing the heat transfer sheet and holding the heat transfer sheet in a vacuum state was thereby obtained. Also, in Example 8, the inside of the polyethylene bag constituting the heat transfer sheet package was returned to normal pressure 24 hours after sealing the polyethylene bag.

Example 9

Example 9 was conducted in the same manner as Example 8, except that the inside of the polyethylene bag constituting the heat transfer sheet package was returned to normal pressure 240 hours after sealing the polyethylene bag.

Comparative Example 1

In Comparative Example 1, the heat transfer sheet obtained by pressing using the same method as in Example 1 was used as is.

Comparative Example 2

In Comparative Example 2, the heat transfer sheet obtained by pressing using the same method as in Example 1 was interposed between release-treated PET films again, and then a load of 0.0176 kgf/cm2was applied and left for 48 hours.

Comparative Example 3

Comparative Example 3 was conducted in the same manner as Example 2, except that the heat transfer sheet obtained by pressing was again interposed between release-treated PET films and placed in a polyethylene bag, and then the polyethylene bag was sealed by reducing the pressure from normal (760 torr) to 400 torr using a degassing sealer.

Comparative Example 4

In Comparative Example 4, the heat transfer sheet obtained by pressing using the same method as in Example 8 was used as is.

<Tack force>

In Examples 1 to 9 and Comparative Example 3, the inside of the polyethylene bag constituting the heat transfer sheet package was returned to normal pressure, and the tack force of the heat transfer sheet was measured. In Comparative Examples 1 and 4, the tack force of the heat transfer sheets obtained by pressing was measured. In Comparative Example 2, the heat transfer sheet obtained by pressing was interposed between release-treated PET films again, a load of 0.0176 kgf/cm2was applied and left for 48 hours, and then the tack force was measured. Specifically, the tack force (gf) of the heat transfer sheet surface was found when a probe having a diameter of 5.1 mm pressed in the heat transfer sheet at a force of 200 gf at 2 mm/sec and pulled it off at 10 mm/sec. The tack force was measured in DEPTH mode using a tackiness tester (manufactured by Malcom). The results are shown in Table 1.

<Heat resistance>

In Examples 1 to 9 and Comparative Example 3, the thermal resistance (° C.·cm2/W) of the heat transfer sheet before heat sealing, wherein the external form was processed to 20 mm in diameter, was measured at a load of 1 kgf/cm2by a method according to ASTM-D5470. The results are shown in Table 1. Note that the thermal resistance of the heat transfer sheet was found according to the following formula.

ΔT=TH−Tc(TH: surface temperature [° C.] of the high temperature side of the heat transfer sheet; Tc: surface temperature [° C.] of the low temperature side of the heat transfer sheet)

R=ΔT/Q×A (R: thermal resistance (thermal impedance) [° C.·cm2/W]; Q: heat flux [W]; A: measured sample area [cm2])

<Rate of change of thermal resistance>

In Examples 1 to 9 and Comparative Example 3, the thermal resistance (° C.·cm2/W) of the heat transfer sheet after the inside of the polyethylene bag constituting the heat transfer sheet package was returned to normal pressure, wherein the external form was processed to 20 mm in diameter, was measured at a load of 1 kgf/cm2by a method according to ASTM-D5470. Then, the rate of change (%) of the thermal resistance of the heat transfer sheet after heat sealing was found based on the thermal resistance of the heat transfer sheet before heat sealing. The results are shown in Table 1.

Furthermore, in Comparative Example 2, after the heat transfer sheet obtained by pressing was interposed between release-treated PET films again, the thermal resistance (° C.·cm2/W) of the heat transfer sheet after a load of 0.0176 kgf/cm2was applied and left for 48 hours was measured in the same manner as Examples 1 to 9 and Comparative Example 3. Also, the rate of change (%) of the thermal resistance of the heat transfer sheet after a load was applied was then found based on the thermal resistance of the heat transfer sheet before a load of 0.0176 kgf/cm2was applied. The results are shown in Table 1.

TABLE 1Comp.Comp.Comp.Comp.Ex. 1Ex. 2Ex. 3Ex. 4Ex. 5Ex. 6Ex. 7Ex. 8Ex. 9Ex. 1Ex. 2Ex. 3Ex. 4Sheet thickness0.50.50.50.50.50.50.50.50.50.50.50.50.5[mm]Sealing200200200200200150300200200——400—pressure [torr]Reduced1/601224120240121224240——12—pressure sealingtime [hrs]Tack force [gf]11615415214815213810817620156575299Thermal0.3070.3150.3130.3130.3150.3270.3200.6330.5850.3140.2960.3170.614resistance[° C · cm2/W]Rate of change−2.20.3−0.3−0.30.34.11.93.1−4.7—−5.71.0—of thermalresistance [%]Ex. = Example, Comp. Ex. = Comparative Example

In Examples 1 to 9, it was found that a heat transfer sheet is obtained containing silicon resin, at least one of the carbon fiber and the boron nitride flake, and the inorganic filler, and at least one of the carbon fiber and the boron nitride flake are oriented in the thickness direction B, wherein the tack force satisfies the aforementioned condition 1 when removed from a sealed state created by holding a reduced pressure state of 150 to 300 torr for one minute or more. Thus, it was found that the heat transfer sheets obtained in Examples 1 to 9 were imparted with strong tack force on the surface.

Furthermore, it was found that the heat transfer sheets obtained in Examples 1 to 9 had improved tack force, and changes in thermal resistance were able to be suppressed. Specifically, it was found that the thermal resistance values of the heat transfer sheets obtained in Examples 1 to 9 after removal from the sealed state were within ±5% of the thermal resistance before sealing. Furthermore, it was found that the thermal resistance values of the heat transfer sheets obtained in Examples 2 to 5 and 7 after removal from the sealed state were within ±2% of the thermal resistance before sealing. Moreover, it was found that the thermal resistance values of the heat transfer sheets obtained in Examples 2 to 4 after removal from the sealed state were within ±1% of the thermal resistance before sealing.

It was found that heat transfer sheets having a tack force satisfying the aforementioned condition 1 could not be obtained in Comparative Examples 1 to 4. In Comparative Examples 1, 2, and 4, this is believed to be due to the fact that the pressed molded sheets were not inserted between release-treated PET film and vacuum packed. In Comparative Example 3, this is believed to be due to the fact that there was insufficient pressure reduction during vacuum packing of the heat transfer sheet obtained by pressing.

REFERENCE SIGNS LIST

1Pressed molded sheet (heat transfer sheet precursor),2film,3heat transfer sheet,4laminated body,5heat transfer sheet package,6thermoplastic resin film,7substrate,8sealing film, 9 binder resin,10carbon fiber,11inorganic filler,12boron nitride flake,50semiconductor device,51electronic component,52heat spreader,52amain face,52bside wall,52cother face,53heat sink