JP7687371B2 - Thermally conductive sheet and method for producing same - Google Patents

Description

The present invention relates to a thermally conductive sheet and a method for manufacturing the thermally conductive sheet.

In recent years, the amount of heat generated by electronic components such as plasma display panels (PDPs) and integrated circuit (IC) chips has increased as their performance has improved. As a result, electronic devices that use electronic components need to take measures to prevent malfunctions caused by temperature increases in the electronic components.

To prevent malfunctions of electronic components due to temperature rise, a common method is to promote heat dissipation by attaching a heat sink, heat sink plate, heat sink fin, or other heat sink made of metal to the heat generating body of the electronic component. When using a heat sink, a sheet-like member having thermal conductivity (thermal conductive sheet) is used to efficiently transfer heat from the heat generating body to the heat sink. For example, a thermal conductive sheet containing resin and particulate filler is sandwiched between the heat generating body and the heat sink, and the heat generating body and the heat sink are brought into close contact with each other via this thermal conductive sheet, thereby transferring heat from the heat generating body to the heat sink. Attempts have been made to improve the properties of thermal conductive sheets (see, for example, Patent Document 1).

Patent Document 1 discloses a method for obtaining a thermally conductive sheet by slicing a laminate obtained by stacking primary sheets containing resin and particulate carbon material in the thickness direction at an angle of 45° or less to the stacking direction, and then pressurizing the sheet obtained by slicing. According to Patent Document 1, the thermally conductive sheet obtained by the above-mentioned method can exhibit excellent thermal conductivity even when used at a relatively low clamping pressure.

JP 2018-67695 A

In recent years, there has been a demand for improving the thickness accuracy of the thermally conductive sheet in order to achieve good adhesion between the heat generating element and the heat dissipating element via the thermally conductive sheet and uniform heat transfer from the heat generating element to the heat dissipating element.
However, with the above-mentioned conventional methods, it is difficult to produce a thermally conductive sheet having a uniform thickness while allowing the thermally conductive sheet to exhibit excellent thermal conductivity in the thickness direction.

Therefore, the present invention aims to provide a thermally conductive sheet that has sufficient thickness accuracy while allowing good heat transfer in the thickness direction, and a method for manufacturing the thermally conductive sheet.

The present inventors have conducted intensive research to achieve the above object. First, the present inventors attempted to improve the thickness accuracy of the thermally conductive sheet by applying pressure after slicing as described in the above Patent Document 1. However, according to the research of the present inventors, it has become clear that it is difficult to ensure sufficient efficiency of heat transfer per unit thickness (i.e., thermal conductivity in the thickness direction), presumably because it is difficult to orient the particulate filler in the thickness direction in the obtained thermally conductive sheet when the sliced sheet is strongly pressed. Based on this, the present inventors have found that a thermally conductive sheet that can transfer heat well in the thickness direction while having sufficient thickness accuracy can be obtained by setting the standard deviation of the thickness to a predetermined value or less and the thermal conductivity in the thickness direction to a predetermined value or more in a thermally conductive sheet containing a resin and a particulate filler, and have completed the present invention.

That is, the present invention aims to advantageously solve the above problems, and the thermally conductive sheet of the present invention is characterized in that it contains a resin and a particulate filler, has a thermal conductivity in the thickness direction of 12 W/m·K or more, and has a standard deviation of thickness of 7.0 μm or less. Thus, a thermally conductive sheet that contains a resin and a particulate filler, and has a thermal conductivity in the thickness direction of the above value or more and a standard deviation of thickness of the above value or less has sufficient thickness precision and can transfer heat well in the thickness direction.
In the present invention, the "thermal conductivity in the thickness direction" can be calculated using the method described in the Examples of this specification.
In addition, in the present invention, the "standard deviation of thickness" is a value obtained by measuring the thickness at any five points on the thermal conductive sheet and from these measured values, and can be calculated, for example, using the method described in the examples of this specification.

Here, it is preferable that the thermally conductive sheet of the present invention contains the particulate filler at a ratio of 45 volume percent or less. If the volume ratio of the particulate filler in the thermally conductive sheet is equal to or less than the above value, the thickness precision can be further improved while maintaining the flexibility of the thermally conductive sheet.

The thermally conductive sheet of the present invention can have a main surface area of 30 ^cm2 or more.
In the present invention, the term "principal surface" refers to the surface having the largest area in a thermally conductive sheet or the like.

The thermally conductive sheet of the present invention preferably has an average thickness of 200 μm or less. When the average thickness of the thermally conductive sheet is equal to or less than the above value, heat can be more effectively transferred in the thickness direction of the thermally conductive sheet.
In the present invention, the "average thickness" is a value obtained by measuring the thickness at any five points of the thermal conductive sheet and calculating the value from these measurements, for example, using the method described in the examples of this specification.

Furthermore, the thermal conductive sheet of the present invention preferably has a surface roughness Sa of 2.80 μm or less on at least one of its main surfaces. If the surface roughness Sa of at least one of its main surfaces is equal to or less than the above value, heat can be transferred more effectively in the thickness direction of the thermal conductive sheet.
In the present invention, the "surface roughness Sa" is a value obtained in accordance with the international standard ISO 25178, and can be measured, for example, by the method described in the examples of this specification.

The present invention also aims to advantageously solve the above problems, and the method for producing a thermally conductive sheet of the present invention is characterized by comprising a step of slicing a block body containing resin and particulate filler and having an Asker C hardness of 35 to 90 using a blade having a tip radius of curvature R of 1.0 μm to 12.0 μm. In this way, by slicing a block body containing resin and particulate filler and having an Asker C hardness within the above range using a blade having a tip radius of curvature R within the above range, a thermally conductive sheet can be obtained that has sufficient thickness accuracy and is capable of transferring heat well in the thickness direction.
In the present invention, the "Asker C hardness" is a value measured at a temperature of 25°C using a hardness meter in accordance with the Asker C method of the Society of Rubber Industry, Japan (SRIS), and can be measured, for example, using the method described in the examples of this specification.
In the present invention, the "tip curvature radius R" can be measured using the method described in the examples of this specification.

In the method for producing a thermally conductive sheet of the present invention, the slicing step is preferably performed with a slice width of 210 μm or less. By slicing the block body with a slice width of the above value or less, the thickness of the thermally conductive sheet can be reduced, and heat can be transferred more effectively in the thickness direction of the thermally conductive sheet.
In the present invention, the term "slice width" refers to the average distance that the block body and the blade move relatively in a fixed direction from the nth slice (n is a natural number of 1 or more) to the n+1th slice when continuously slicing the block body while moving the block body and the blade relatively in the above fixed direction to produce multiple thermal conductive sheets.

Furthermore, the manufacturing method for a thermal conductive sheet of the present invention can further include a step of stacking multiple primary sheets containing resin and particulate filler in the thickness direction, or folding or rolling the primary sheets, prior to the slicing step, to obtain the block body.
In this specification, the terms "stacking", "folding" and "winding" may be collectively abbreviated as "stacking, etc."

In the method for producing a thermally conductive sheet of the present invention, the tensile strength of the primary sheet is preferably 0.3 MPa or more and 1.9 MPa or less. If the tensile strength of the primary sheet is within the above range, the thickness precision of the thermally conductive sheet can be further improved.
In the present invention, the tensile strength is a value obtained in accordance with JIS K6251, and can be measured, for example, by the method described in the examples of this specification.

The present invention provides a thermally conductive sheet that has sufficient thickness accuracy while allowing good heat transfer in the thickness direction, and a method for manufacturing the thermally conductive sheet.

1A to 1C are explanatory diagrams showing a process for producing a thermally conductive sheet using an example of a method for producing a thermally conductive sheet according to the present invention.

Hereinafter, an embodiment of the present invention will be described in detail.
The thermally conductive sheet of the present invention can be used, for example, by being sandwiched between a heat generating body and a heat dissipating body when attaching the heat dissipating body to the heat generating body. That is, the thermally conductive sheet of the present invention can be used together with a heat dissipating body such as a heat sink, a heat dissipating plate, or a heat dissipating fin to form a heat dissipating device.
The thermally conductive sheet of the present invention can be produced, for example, according to the method for producing a thermally conductive sheet of the present invention.

(Thermal Conduction Sheet)
The thermal conductive sheet of the present invention includes a resin and a particulate filler, and may further include an additive. The thermal conductive sheet of the present invention has a thermal conductivity in the thickness direction of 12 W/m·K or more and a standard deviation of the thickness of 7.0 μm or less. The thermal conductive sheet of the present invention has a thermal conductivity in the thickness direction of 12 W/m·K or more and a standard deviation of the thickness of 7.0 μm or less, so that the thermal conductive sheet of the present invention can transfer heat well in the thickness direction while having sufficient thickness accuracy.

<Resin>
The resin contained in the thermal conductive sheet is not particularly limited, and any resin can be used. For example, the resin can be either a liquid resin or a solid resin. The resin may be used alone or in combination of two or more types. For example, the thermal conductive sheet can contain at least one of a liquid resin and a solid resin, but from the viewpoint of further improving the thickness accuracy of the thermal conductive sheet and more favorably transferring heat in the thickness direction, it is preferable that the thermal conductive sheet contains both a liquid resin and a solid resin.

<<Liquid resin>>
The liquid resin is not particularly limited as long as it is liquid at room temperature and normal pressure, and for example, a thermoplastic resin that is liquid at room temperature and normal pressure can be used.
In the present invention, "normal temperature" refers to 23° C., and "normal pressure" refers to 1 atm (absolute pressure).

Examples of liquid resins include fluororesins, silicone resins, acrylic resins, and epoxy resins. These may be used alone or in combination of two or more. Among them, silicone resins and fluororesins are preferred as liquid resins, and fluororesins are more preferred. If at least one of silicone resins and fluororesins is used as the liquid resin, the flame retardancy of the thermal conductive sheet can be improved. Furthermore, if a fluororesin is used as the liquid resin, the heat resistance, oil resistance, and chemical resistance of the resulting thermal conductive sheet can be improved.

<<Solid resin>>
The solid resin is not particularly limited as long as it is not liquid at room temperature and normal pressure. For example, a thermoplastic resin that is solid at room temperature and normal pressure, or a thermosetting resin that is solid at room temperature and normal pressure can be used.

[Thermoplastic resin that is solid at room temperature and pressure]
Examples of thermoplastic resins that are solid at room temperature and normal pressure include acrylic resins such as poly(2-ethylhexyl acrylate), a copolymer of acrylic acid and 2-ethylhexyl acrylate, polymethacrylic acid or its ester, and polyacrylic acid or its ester; silicone resins; fluororesins; polyethylene; polypropylene; ethylene-propylene copolymers; polymethylpentene; polyvinyl chloride; polyvinylidene chloride; polyvinyl acetate; ethylene-vinyl acetate copolymers; polyvinyl alcohol; polyacetal; polyethylene terephthalate; polybutylene terephthalate; polyethylene naphthalate; polystyrene; and polyacrylonitrile. Examples of the polymerizable compound include styrene-acrylonitrile copolymers, acrylonitrile-butadiene copolymers (nitrile rubbers), acrylonitrile-butadiene-styrene copolymers (ABS resins), styrene-butadiene block copolymers or hydrogenated products thereof, styrene-isoprene block copolymers or hydrogenated products thereof, polyphenylene ethers, modified polyphenylene ethers, aliphatic polyamides, aromatic polyamides, polyamideimides, polycarbonates, polyphenylene sulfides, polysulfones, polyethersulfones, polyethernitriles, polyetherketones, polyketones, polyurethanes, liquid crystal polymers, and ionomers. These may be used alone or in combination of two or more.
In the present invention, rubber is included in the "resin".

[Thermosetting resin that is solid at room temperature and pressure]
Examples of thermosetting resins that are solid at room temperature and normal pressure include natural rubber, butadiene rubber, isoprene rubber, nitrile rubber, hydrogenated nitrile rubber, chloroprene rubber, ethylene propylene rubber, chlorinated polyethylene, chlorosulfonated polyethylene, butyl rubber, halogenated butyl rubber, polyisobutylene rubber, epoxy resin, polyimide resin, bismaleimide resin, benzocyclobutene resin, phenolic resin, unsaturated polyester, diallyl phthalate resin, polyimide silicone resin, polyurethane, thermosetting polyphenylene ether, thermosetting modified polyphenylene ether, etc. These may be used alone or in combination of two or more.

<<Resin content>>
The content of the resin in the thermal conductive sheet is not particularly limited, but is preferably 35% by mass or more, more preferably 45% by mass or more, even more preferably 55% by mass or more, and preferably 95% by mass or less, more preferably 85% by mass or less, and even more preferably 75% by mass or less. If the content of the resin is 35% by mass or more, the thickness precision of the thermal conductive sheet can be further improved while ensuring the flexibility of the thermal conductive sheet. On the other hand, if the content of the resin is 95% by mass or less, the heat can be transferred more effectively in the thickness direction of the thermal conductive sheet.

<<Liquid resin content>>
In addition, the content ratio of the liquid resin in the resin (in other words, the ratio of the liquid resin in the total of the solid resin and the liquid resin) is not particularly limited, but is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, particularly preferably 60% by mass or more, preferably 95% by mass or less, more preferably 90% by mass or less, even more preferably 85% by mass or less, and particularly preferably 80% by mass or less. If the content ratio of the liquid resin in the resin is 30% by mass or more, the thickness accuracy can be further improved while ensuring the flexibility of the thermal conductive sheet. On the other hand, if the content ratio of the solid resin in the resin is 95% by mass or less, the thickness accuracy of the thermal conductive sheet can be further improved.

<Particulate filler>
The particulate filler contained in the thermal conductive sheet is not particularly limited as long as it can impart thermal conductivity to the thermal conductive sheet. As such a particulate filler, a particulate carbon material having high thermal conductivity can be suitably used. The particulate filler may be used alone or in combination of two or more kinds.

<<Carbon material particles>>
The particulate carbon material is not particularly limited, and examples thereof include graphite such as artificial graphite, scaly graphite, exfoliated graphite, natural graphite, acid-treated graphite, expandable graphite, and expanded graphite; carbon black; etc. These may be used alone or in combination of two or more kinds.

Among the above, it is preferable to use expanded graphite as the particulate carbon material. By using expanded graphite, the thermal conductivity in the thickness direction of the thermal conductive sheet is increased, and heat can be transferred more effectively in the thickness direction of the thermal conductive sheet. Here, expanded graphite can be obtained by, for example, chemically treating graphite such as flake graphite with sulfuric acid or the like to obtain expandable graphite, which is then expanded by heat treatment and then refined. Examples of expanded graphite include EC1500, EC1000, EC500, EC300, EC100, and EC50 (all product names) manufactured by Ito Graphite Industries Co., Ltd.

<<Properties of particulate filler>>
The particulate filler preferably has a volume average particle diameter of 30 μm or more, more preferably 50 μm or more, even more preferably 100 μm or more, particularly preferably 150 μm or more, preferably 500 μm or less, more preferably 400 μm or less, even more preferably 300 μm or less, and particularly preferably 230 μm or less. If the volume average particle diameter of the particulate filler is 30 μm or more, it is presumed that the heat transfer path of the particulate filler in the heat conductive sheet can be well formed, and the thermal conductivity in the thickness direction of the heat conductive sheet is increased. As a result, the heat can be transferred more well in the thickness direction of the heat conductive sheet. On the other hand, if the volume average particle diameter of the particulate filler is 500 μm or less, the thickness precision of the heat conductive sheet can be further improved.
In the present invention, the "volume average particle size" can be measured in accordance with JIS Z8825, and represents a particle size at which the cumulative volume calculated from the small diameter side is 50% in a particle size distribution (volume basis) measured by a laser diffraction method.

The particulate filler preferably has an aspect ratio (major axis/minor axis) of more than 1 and not more than 10, more preferably more than 1 and not more than 5. If the particulate filler has an aspect ratio of more than 1 and not more than 10, it is presumed that the particulate filler is easily oriented in the thickness direction in the thermal conductive sheet, and the thermal conductivity in the thickness direction of the thermal conductive sheet is increased. As a result, heat can be transferred more effectively in the thickness direction of the thermal conductive sheet.
In the present invention, the "aspect ratio" can be determined by observing the particulate filler with a SEM (scanning electron microscope), measuring the maximum diameter (long diameter) and the particle diameter (short diameter) in a direction perpendicular to the maximum diameter for any 50 particulate fillers, and calculating the average value of the ratio of the long diameter to the short diameter (long diameter/short diameter). In the above, for example, when the particulate filler is scaly, the "long diameter" refers to the length in the direction of the long axis of the main surface of the scaly shape, and the "short diameter" refers to the length in the direction perpendicular to the long axis of the main surface.

<<Particulate filler content>>
The content of the particulate filler in the thermal conductive sheet is not particularly limited, but is preferably 5% by volume or more, more preferably 10% by volume or more, even more preferably 20% by volume or more, preferably 45% by volume or less, more preferably 35% by volume or less, and even more preferably 30% by volume or less. If the content of the particulate filler is 5% by volume or more, the thermal conductivity in the thickness direction of the thermal conductive sheet is increased, and the heat can be transferred more effectively in the thickness direction of the thermal conductive sheet. On the other hand, if the content of the particulate filler is 45% by volume or less, the thickness precision can be further improved while maintaining the flexibility of the thermal conductive sheet.

The content of particulate filler in the thermally conductive sheet is not particularly limited, but is preferably 5% by mass or more, more preferably 15% by mass or more, even more preferably 25% by mass or more, and preferably 70% by mass or less, more preferably 55% by mass or less, and even more preferably 45% by mass or less. If the content of particulate filler is 5% by mass or more, the thermal conductivity in the thickness direction of the thermally conductive sheet is increased, and heat can be transferred more effectively in the thickness direction of the thermally conductive sheet. On the other hand, if the content of particulate filler is 70% by mass or less, the thickness precision of the thermally conductive sheet can be further improved while maintaining the flexibility of the thermally conductive sheet.

In addition, the content of the particulate filler in the thermal conductive sheet is not particularly limited, but is preferably 20 parts by mass or more, more preferably 30 parts by mass or more, even more preferably 40 parts by mass or more, and preferably 100 parts by mass or less, more preferably 80 parts by mass or less, and even more preferably 60 parts by mass or less per 100 parts by mass of resin. If the content of the particulate filler is 20 parts by mass or more per 100 parts by mass of resin, the thermal conductivity in the thickness direction of the thermal conductive sheet is increased, and heat can be transferred more effectively in the thickness direction of the thermal conductive sheet. On the other hand, if the content of the particulate filler is 100 parts by mass or less per 100 parts by mass of resin, the thickness precision of the thermal conductive sheet can be further improved while maintaining the flexibility of the thermal conductive sheet.

<Additives>
The heat conductive sheet of the present invention may further contain known additives that can be used to form the heat conductive sheet, if necessary. The additives that can be contained in the heat conductive sheet are not particularly limited, and include, for example, plasticizers such as fatty acid esters such as sebacic acid esters; flame retardants such as red phosphorus flame retardants and phosphate ester flame retardants; toughness improvers such as urethane acrylates; moisture absorbents such as calcium oxide and magnesium oxide; adhesion improvers such as silane coupling agents, titanium coupling agents, and acid anhydrides; wettability improvers such as nonionic surfactants and fluorine surfactants; ion trapping agents such as inorganic ion exchangers; and the like. The additives may be used alone or in combination of two or more.

If the thermally conductive sheet further contains an additive, the amount of the additive can be, for example, 0.1 parts by mass or more and 20 parts by mass or less per 100 parts by mass of the above-mentioned resin, and preferably 10 parts by mass or less.

<Properties of thermal conductive sheet>
The thermal conductivity of the thermal conductive sheet in the thickness direction must be 12 W/m·K or more, more preferably 13 W/m·K or more, and even more preferably 14 W/m·K or more. If the thermal conductivity of the thermal conductive sheet in the thickness direction is less than 12 W/m·K, the thermal conductive sheet cannot transfer heat well in the thickness direction. The upper limit of the thermal conductivity of the thermal conductive sheet in the thickness direction is not particularly limited, but is, for example, 50 W/m·K or less.
The thermal conductivity of the thermal conductive sheet in the thickness direction can be adjusted by changing the type and content of the material (resin, particulate filler, etc.) used in the production of the thermal conductive sheet, as well as the production conditions of the thermal conductive sheet. For example, the thermal conductivity of the thermal conductive sheet in the thickness direction can be increased by changing the volume average particle size and/or content of the particulate filler in the thermal conductive sheet. In addition, the thermal conductivity of the thermal conductive sheet in the thickness direction can be increased by producing the thermal conductive sheet using the production method of the thermal conductive sheet of the present invention described later.

In addition, the thermal conductive sheet needs to have a standard deviation of thickness of 7.0 μm or less, preferably 6.0 μm or less, more preferably 5.0 μm or less, and even more preferably 4.0 μm or less. If the standard deviation of thickness exceeds 7.0 μm, the thickness precision of the thermal conductive sheet is impaired. Therefore, it becomes difficult to make the heat generating body and the heat dissipating body adhere well through the thermal conductive sheet, and the heat transfer from the heat generating body to the heat dissipating body cannot be performed uniformly. The lower limit of the standard deviation of the thickness of the thermal conductive sheet is not particularly limited, but is, for example, 1.0 μm or more.
The standard deviation of the thickness of the thermally conductive sheet can be adjusted by changing the type and content of the materials (resin, particulate filler, etc.) used in the manufacture of the thermally conductive sheet, as well as the manufacturing conditions of the thermally conductive sheet. For example, the standard deviation of the thickness of the thermally conductive sheet can be increased by manufacturing the thermally conductive sheet using the manufacturing method of the thermally conductive sheet of the present invention described below. More specifically, in the manufacturing method of the thermally conductive sheet of the present invention, the standard deviation of the thickness of the thermally conductive sheet can be reduced by changing the tensile strength of the primary sheet, the Asker C hardness of the block body, and/or the radius of curvature R of the tip of the blade used for slicing.

In addition, the thermally conductive sheet preferably has an average thickness of 70 μm or more, more preferably 80 μm or more, and preferably 200 μm or less, more preferably 160 μm or less, even more preferably 130 μm or less, and particularly preferably 110 μm or less. If the average thickness is 70 μm or more, the strength of the thermally conductive sheet can be ensured, and if it is 200 μm or less, heat can be transferred more effectively in the thickness direction of the thermally conductive sheet.

Furthermore, the area of the main surface of the thermally conductive sheet can be, for example, 30 ^cm2 or more, 50 ^cm2 or more, 80 ^{cm2 or more, 100 cm2 or} more, and 1000 ^cm2 or less.

The thermal conductive sheet has at least one main surface having a surface roughness Sa of 2.80 μm or less, more preferably 2.60 μm or less, even more preferably 2.20 μm or less, and particularly preferably 2.00 μm or less. If the surface roughness Sa of the main surface is 2.80 μm or less, the main surface is sufficiently smooth, so that when the thermal conductive sheet is sandwiched between a heat generating body and a heat dissipating body, the thermal conductive sheet and the heat generating body and/or the heat dissipating body are well adhered to each other, and the interface resistance is reduced. Therefore, the thermal conductive sheet can be more effectively transferred in the thickness direction. The lower limit of the surface roughness Sa of the main surface of the thermal conductive sheet is not particularly limited, but is, for example, 1.00 μm or more.
Furthermore, from the viewpoint of achieving better heat transfer in the thickness direction of the thermally conductive sheet, it is preferable that the surface roughness Sa of both main surfaces (front and back surfaces) of the thermally conductive sheet is equal to or less than the above-mentioned suitable upper limit value.
The surface roughness Sa of the main surface of the thermal conductive sheet can be adjusted by changing the type and content of the materials (resin, particulate filler, etc.) used in the manufacture of the thermal conductive sheet, as well as the manufacturing conditions of the thermal conductive sheet. For example, the surface roughness Sa of the thermal conductive sheet can be reduced by manufacturing the thermal conductive sheet using the manufacturing method of the thermal conductive sheet of the present invention described below. More specifically, in the manufacturing method of the thermal conductive sheet of the present invention, the surface roughness Sa of the main surface of the thermal conductive sheet can be reduced by changing the tensile strength of the primary sheet, the Asker C hardness of the block body, and/or the curvature radius R of the tip of the blade used for slicing.

(Method of manufacturing thermally conductive sheet)
The thermally conductive sheet of the present invention described above can be manufactured, for example, by using the method for manufacturing a thermally conductive sheet of the present invention, which includes at least a step of slicing a block body containing a resin and a particulate filler and having an Asker C hardness of 35 to 90 using a blade having a tip curvature radius R of 1.0 μm to 12.0 μm (slicing step).
Furthermore, according to the method for producing a thermally conductive sheet of the present invention, it is possible to obtain a thermally conductive sheet that has sufficient thickness accuracy and is capable of transferring heat well in the thickness direction.

<Slicing process>
In the slicing step, as described above, the block body having an Asker C hardness of 35 or more and 90 or less is sliced using a blade having a tip curvature radius R of 1.0 μm or more and 12.0 μm or less, to cut out a thermally conductive sheet from the block body.

<<Block letters>>
The block body includes a resin and a particulate filler, and may further include an optional additive. The block body has an Asker C hardness of 35 or more and 90 or less.

[Resin, Particulate Filler, and Additives]
The suitable types, properties and content ratios of the resin and particulate filler contained in the block body, as well as the optional additives contained therein, may be the same as the suitable types, properties and content ratios described above for the thermal conductive sheet of the present invention.

[Asker C hardness]
Here, the block body needs to have an Asker C hardness of 35 or more and 90 or less, preferably 85 or less, more preferably 80 or less, preferably 40 or more, more preferably 50 or more, and even more preferably 60 or more. If the Asker C hardness exceeds 90, it becomes difficult to ensure sufficient thickness precision of the heat conductive sheet obtained by slicing the block body (particularly the thickness precision when the slice width is narrowed to reduce the thickness of the heat conductive sheet). On the other hand, if the Asker C hardness is below 35, it is not possible to suppress the vibration of the blade tip during slicing due to the stickiness of the block body, etc., and the thickness precision of the heat conductive sheet decreases.
The Asker C hardness of the block body can be adjusted by changing the type and content of materials (resin, particulate filler, etc.) used in manufacturing the block body, or the manufacturing method of the block body.

<<Blade>>
The shape of the blade used to slice the block body is not particularly limited, and may be a single-edged, double-edged, or asymmetric blade, but a double-edged blade is preferable from the viewpoint of ensuring sufficient thickness accuracy of the obtained heat conductive sheet. In addition, the material of the blade is not particularly limited, but it is preferably made of metal.

The blade used to slice the block body must have a tip curvature radius R of 1.0 μm or more and 12.0 μm or less, as described above, preferably 1.5 μm or more, more preferably 4.5 μm or more, preferably 10.0 μm or less, more preferably 9.0 μm or less, even more preferably 7.0 μm or less, and particularly preferably 5.0 μm or less. If the tip curvature radius R of the blade is less than 1.0 μm, the blade may chip during slicing, which may reduce the manufacturing efficiency of the thermal conductive sheet. On the other hand, if the tip curvature radius R of the blade exceeds 12.0 μm, it becomes difficult to ensure the thickness accuracy of the thermal conductive sheet obtained by slicing the block body (especially the thickness accuracy when the slice width is reduced to reduce the thickness of the thermal conductive sheet), or the obtuse blade does not bite into the block body, making slicing difficult.

<<Slice>>
The method for slicing the block is not particularly limited as long as it is a method using a blade whose tip has a radius of curvature R within the above-mentioned range. A cutting tool having a blade whose tip has a radius of curvature R within the above-mentioned range can be used for slicing. Examples of such cutting tools include a cutter, a plane, and a slicer.

Here, when the block body is a laminate obtained by stacking the primary sheets by the stacking process described later, the angle at which the block body is sliced is preferably 45° or less with respect to the stacking direction, more preferably 30° or less with respect to the stacking direction, even more preferably 15° or less with respect to the stacking direction, and particularly preferably approximately 0° with respect to the stacking direction (i.e., in the direction along the stacking direction). When the block body is a laminate obtained by stacking the primary sheets, it is presumed that the particulate filler is oriented in a direction approximately perpendicular to the stacking direction inside the block body. And, if such a block body is sliced at an angle of 45° or less with respect to the stacking direction, it is presumed that the particulate filler is oriented in the thickness direction (i.e., in a direction approximately perpendicular to the stacking direction of the primary sheets) in the obtained thermally conductive sheet, and the heat transfer path formed by the contact of the particulate filler is mainly formed well in the thickness direction of the thermally conductive sheet, but the heat can be transferred even better in the thickness direction of the thermally conductive sheet.

From the viewpoint of easily slicing the block body and ensuring sufficient thickness precision of the resulting thermally conductive sheet, the temperature of the block body during slicing is preferably −20° C. or higher and 80° C. or lower, and more preferably −10° C. or higher and 50° C. or lower.
Furthermore, from the viewpoint of easily slicing the block body and ensuring sufficient thickness accuracy of the resulting thermally conductive sheet, it is preferable to fix the block body by applying pressure or the like when slicing. In such a pressurizing process, the surface to which the pressure is applied is not particularly limited.

<Other processes>
Other steps that may be optionally included in the method for producing a thermally conductive sheet of the present invention are not particularly limited.
For example, in the manufacturing method of the thermal conductive sheet of the present invention, before the above-mentioned slicing step, a step (stacking step) can be carried out in which multiple primary sheets containing resin and particulate filler are stacked in the thickness direction, or the primary sheets are folded or rolled to obtain a block body.
In the method for producing a thermally conductive sheet of the present invention, a step of heating the block body (heating step) can be carried out prior to the above-mentioned slicing step.
In the method for producing a thermally conductive sheet of the present invention, a step of pressing the thermally conductive sheet obtained after the slicing step in the thickness direction (pressing step) may be carried out as long as the effect of the present invention is not significantly impaired. However, from the viewpoint of suppressing a decrease in the thermal conductivity of the obtained thermally conductive sheet in the thickness direction, it is preferable that the method for producing a thermally conductive sheet of the present invention does not include a pressing step.
The lamination step and the heating step as other steps will be described in detail below.

<<Lamination process>>
As described above, in the lamination step, a plurality of primary sheets are laminated in the thickness direction, or the primary sheets are folded or rolled to obtain a block body which is a laminate.

[First sheet]
The primary sheet comprises a resin and a particulate filler, and may optionally further comprise additives.

-Resin, particulate fillers, and additives-
The suitable types, properties and content ratios of the resin and particulate filler contained in the primary sheet, as well as the optional additives contained therein, can be the same as the suitable types, properties and content ratios of each component described above for the block body and the thermally conductive sheet of the present invention.

-Characteristics of the first sheet-
The tensile strength of the primary sheet is preferably 0.3 MPa or more, more preferably 0.4 MPa or more, even more preferably 0.6 MPa or more, and preferably 1.9 MPa or less, more preferably 1.6 MPa or less, and even more preferably 1.4 MPa or less. If the tensile strength is 0.3 MPa or more, the Asker C hardness of the block body obtained by stacking the primary sheets is increased. Therefore, it is possible to obtain a thermally conductive sheet with even better thickness accuracy by suppressing the vibration of the blade tip when slicing the block body. On the other hand, if the tensile strength is 1.9 MPa or less, the Asker C hardness of the block body obtained by stacking the primary sheets is not excessively increased. Therefore, it becomes easy to slice the block body, and the thickness accuracy of the obtained thermally conductive sheet (especially the thickness accuracy when the slice width is reduced to reduce the thickness of the thermally conductive sheet) can be sufficiently ensured.
The tensile strength of the primary sheet can be adjusted by changing the type and content of materials (resin, particulate filler, etc.) used in the manufacture of the primary sheet, or the manufacturing method of the primary sheet. For example, the tensile strength of the primary sheet can be increased by increasing the content of resin in the primary sheet.

Furthermore, the thickness (average thickness) of the primary sheet is not particularly limited, and can be, for example, 0.05 mm or more and 2 mm or less.
The "thickness (average thickness)" of the primary sheet can be measured in the same manner as the "average thickness" of the thermally conductive sheet.

- How to prepare the primary sheet -
The method for preparing the primary sheet is not particularly limited. The primary sheet can be obtained, for example, by molding a composition containing a resin, a particulate filler, and any additives by a known molding method such as press molding, rolling molding, or extrusion molding.

[Formation of block body by lamination etc.]
The formation of a block body by stacking the primary sheets is not particularly limited, and may be performed using a stacking device or manually. The formation of a block body by folding a thermally conductive sheet is not particularly limited, and may be performed by folding the primary sheet at a fixed width using a folding machine. The formation of a block body by winding the primary sheet is not particularly limited, and may be performed by winding the primary sheet around an axis parallel to the short side or long side of the primary sheet.

<<Heating process>>
Here, for example, the block body obtained through the lamination process described above may be subjected to the slicing process as it is, or may be subjected to the slicing process after being further heated. The heating temperature in the heating process may be, for example, 50° C. or higher and 170° C. or lower, and the heating time may be, for example, 1 minute or higher and 8 hours or lower. By passing through the heating process, the Asker C hardness of the block body can be adjusted. For example, when the block body contains a thermoplastic resin, the Asker C hardness of the block body can be reduced by carrying out the heating process.

The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the following description, "%" and "parts" expressing amounts are based on mass unless otherwise specified.
In the examples and comparative examples, the radius of curvature R of the blade tip, the tensile strength of the primary sheet, the Asker C hardness of the block body, and the average thickness, standard deviation of the thickness, surface roughness Sa, thermal conductivity in the thickness direction, and thermal resistance in the thickness direction of the thermal conductive sheet were each measured or evaluated according to the following methods.

<Radius of curvature R>
The radius of curvature R of the tip of the blade was measured using an all-focus 3D surface shape measuring device (manufactured by Alicona Imaging, product name "Infinite Focus G5").
Specifically, a spot light was applied from the direction of the blade tip by coaxial epi-illumination, and the tip shape was measured using a lens of 50x magnification. The blade was fixed with the entire blade tilted at an angle of 20 degrees relative to the spot light.
For the obtained 3D image of the blade tip, cross-section observation was performed at an arbitrary cross section, and an inner circle was drawn so that approximately half of its circumference was in contact with the blade tip, and the radius of this circle was defined as the radius of curvature R of the blade tip.
<Tensile strength>
The primary sheet was punched out using a dumbbell No. 2 in accordance with JIS K6251 to prepare a test piece. Using a tensile tester (manufactured by Shimadzu Corporation, product name "AG-IS20kN"), the test piece was pinched at points 1 cm from both ends and pulled at a temperature of 23°C in a direction perpendicular to the normal line extending from the surface of the test piece at a pulling speed of 500 mm/min to measure the breaking strength (tensile strength).
<Asker C hardness>
The Asker C hardness of the block body was measured at 25° C. using a hardness tester (manufactured by Kobunshi Keiki Co., Ltd., product name "ASKER CL-150LJ") in accordance with the Asker C method of the Society of Rubber Industry Standards of Japan (SRIS). Specifically, the obtained laminate was left to stand in a thermostatic chamber maintained at a temperature of 25° C. for 48 hours or more to prepare a test specimen. Next, a hardness tester was placed so that the distance of the needle tip was 2 cm from the laminate surface, and a damper was lowered to cause the laminate and the damper to collide. The Asker C hardness of the laminate 60 seconds after the collision was measured twice using a hardness tester (manufactured by Kobunshi Keiki Co., Ltd., product name "ASKER CL-150LJ"), and the average value of the measurement results was used.
<Average thickness>
Using a film thickness gauge (manufactured by Mitutoyo, product name "Digimatic Indicator ID-C112XBS"), the thickness was measured at five points, approximately the center and each of the four corners (squares), and the average value (μm) of the measured thicknesses was calculated.
<Standard deviation of thickness>
Using a film thickness meter (manufactured by Mitutoyo, product name "Digimatic Indicator ID-C112XBS"), the thickness was measured at five points, approximately the center and each of the four corners (squares), and the standard deviation (μm) of the measured thicknesses was calculated.
<Surface roughness Sa>
The surface roughness Sa of the thermal conductive sheet was measured using a three-dimensional shape measuring instrument (manufactured by Keyence Corporation, product name "One Shot 3D Measurement Macroscope"). Here, a thermal conductive sheet cut into an arbitrary size of approximately 1 cm or more was used as a sample, the analysis range was 1 cm x 1 cm, and the three-dimensional shape was measured on the front and back sides of the sample. The measurement results of the three-dimensional shape were further filtered (2.5 mm) by software to remove the waviness component, and the surface roughness Sa (μm) was automatically calculated.
<Thermal conductivity in the thickness direction>
The thermal diffusivity α (m ² /s) in the thickness direction, specific heat at constant pressure Cp (J/g·K), and specific gravity ρ (g/m ³ ) of the thermally conductive sheet were measured by the following methods.
[Thermal diffusivity α in the thickness direction]
The measurements were made using a thermal diffusion/thermal conductivity measuring device (manufactured by Ai-Phase Corporation, product name "Ai-Phase Mobile 1u") in accordance with the provisions of ISO 22007-3.
[Constant pressure specific heat Cp]
The specific heat was measured at 25° C. under a temperature increase rate of 10° C./min using a differential scanning calorimeter (manufactured by Rigaku, product name "DSC8230").
[Specific gravity ρ (density)]
The measurement was performed using an automatic specific gravity meter (manufactured by Toyo Seiki Co., Ltd., product name "DENSIMETER-H").
Each measured value was calculated using the following formula (I):
λ=α×Cp×ρ...(I)
The thermal conductivity λ (W/m·K) in the thickness direction of the thermal conductive sheet at 25° C. was calculated.
<Thermal resistance value>
The thermal resistance value of the thermal conductive sheet was measured using a thermal resistance tester (Hitachi Technology & Services Co., Ltd., product name "Resin Material Thermal Resistance Measuring Device"). Here, the thermal conductive sheet cut into a roughly 1 cm square was used as a sample, and the thermal resistance value (°C/W) was measured when a pressure of 0.1 MPa and 0.9 MPa was applied at a sample temperature of 50°C. The smaller the thermal resistance value, the better the thermal conductivity of the thermal conductive sheet, and for example, the better the heat dissipation characteristics when interposed between a heat generating body and a heat sink.

Example 1
<Formation of the primary sheet>
70 parts of a thermoplastic fluororesin (manufactured by Daikin Industries, Ltd., product name "Daiel G-101") that is liquid at room temperature and normal pressure as a resin, 30 parts of a thermoplastic fluororesin (manufactured by 3M Japan Ltd., product name "Dyneon FC2211") that is solid at room temperature and normal pressure, and 50 parts of expanded graphite (manufactured by Ito Graphite Industries Co., Ltd., product name "EC100", volume average particle size: 200 μm) as a particulate filler were mixed and stirred at a temperature of 150 ° C. for 20 minutes using a pressure kneader (manufactured by Nippon Spindle). Next, the resulting mixture was put into a crusher (manufactured by Osaka Chemical Co., Ltd., product name "Wonder Crush Mill D3V-10") and crushed for 10 seconds.
50 g of the crushed mixture was sandwiched between 50 μm thick polyethylene terephthalate (PET) films (protective films) that had been sandblasted, and rolled under the conditions of a roll gap of 550 μm, a roll temperature of 50° C., a roll linear pressure of 50 kg/cm, and a roll speed of 1 m/min to obtain a primary sheet with a thickness of 0.8 mm. The tensile strength of the primary sheet was then measured. The results are shown in Table 1.
<Lamination process>
The obtained primary sheet was cut into a size of 150 mm length x 150 mm width x 0.8 mm thickness, 100 sheets were stacked in the thickness direction of the primary sheet, and pressed in the stacking direction at a temperature of 120°C and a pressure of 0.1 MPa for 3 minutes to obtain a block (laminate) with a height of about 80 mm. The Asker C hardness of the obtained block was then measured. The results are shown in Table 1.
<Slicing process>
Then, leaving a length required for slicing, the entire upper surface of the obtained block was pressed with a metal plate, and a pressure of 0.1 MPa was applied in the stacking direction (i.e., from above) to fix the block. Note that the side and back of the block were not fixed. At this time, the temperature of the block was 25°C.
Next, a blade 10 (double-edged, blade angle 2θ: 20°, maximum thickness of blade: 3.5 mm, material: super steel, Rockwell hardness: 91.5, silicon processing of blade surface: none, total length: 200 mm, radius of curvature R of tip: 1.7 μm) having the shape shown in FIG. 1 was attached to the press part of a servo press (manufactured by Electric Discharge Precision Machining Research Institute), and sliced in the stacking direction of the block body (laminated body) (in other words, in the direction that coincides with the normal line of the main surface of the laminated primary sheet) under the conditions of slice width: 85 μm and slice speed: 200 mm/sec to obtain a thermally conductive sheet having a main surface of 150 mm length x 80 mm width. Note that the orientation of the blade during slicing was such that the angle α shown in FIG. 1 was 10°, and the extension direction of the blade surface 11 was parallel to the slice surface 21 of the block body 20.
The average thickness, standard deviation of thickness, surface roughness Sa, thermal conductivity in the thickness direction, and thermal resistance of the obtained thermal conductive sheet were measured. The results are shown in Table 1.

Example 2
In the slicing process, a blade with a tip curvature radius R of 4.7 μm (double-edged, blade angle 2θ: 20°, maximum thickness of blade: 3.5 mm, material: super steel, Rockwell hardness: 91.5, silicon processing on blade surface: none, total length: 200 mm) was used, and the primary sheet, block body, and heat conductive sheet were prepared and various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

Example 3
A primary sheet was formed in the same manner as in Example 1. Next, in the lamination step, the primary sheet was laminated and pressed in the same manner as in Example 1, and the resulting laminate was further heated at 100° C. for 3 hours (heating step) and allowed to cool at room temperature for 3 hours. The Asker C hardness of the block body after this heating and cooling was measured. The results are shown in Table 1.
The slicing process was carried out in the same manner as in Example 1, except that the block body after heating and cooling was used, and a thermally conductive sheet was produced and subjected to various evaluations. The results are shown in Table 1.

Example 4
In forming the primary sheet, 100 parts of a thermoplastic fluororesin (manufactured by Daikin Industries, Ltd., product name "Dai-el G-101") that is liquid at room temperature and normal pressure was used as the resin, but the primary sheet, block body, and thermally conductive sheet were produced in the same manner as in Example 1, and various evaluations were performed. The results are shown in Table 1.

Example 5
In the slicing process, a blade with a tip curvature radius R of 4.7 μm (double-edged, blade angle 2θ: 20°, maximum thickness of blade: 3.5 mm, material: super steel, Rockwell hardness: 91.5, silicon processing on blade surface: none, total length: 200 mm) was used, but the primary sheet, block body, and heat conductive sheet were produced in the same manner as in Example 3, and various evaluations were performed. The results are shown in Table 1.

Example 6
In the slicing process, a blade with a tip curvature radius R of 9.1 μm (double-edged, blade angle 2θ: 20°, maximum thickness of blade: 3.5 mm, material: carbide, Rockwell hardness: 91.5, silicon processing on blade surface: none, total length: 200 mm) was used, and the slice width was changed to 90 μm. Except for this, a primary sheet, a block body, and a heat conductive sheet were produced in the same manner as in Example 1, and various evaluations were performed. The results are shown in Table 1.

(Example 7)
<Formation of the primary sheet>
45 parts of a thermoplastic fluororesin that is liquid at room temperature and normal pressure (manufactured by Daikin Industries, Ltd., product name "Daiel G-101") as a resin, 40 parts of a thermoplastic fluororesin that is solid at room temperature and normal pressure (manufactured by 3M Japan Ltd., product name "Dyneon FC2211") as a resin, 85 parts of expanded graphite (manufactured by Ito Graphite Industries Co., Ltd., product name "EC100", volume average particle size: 250 μm) as a particulate filler, and 5 parts of a sebacic acid ester (manufactured by Daihachi Chemical Industry Co., Ltd., product name "DOS") as a plasticizer were stirred and mixed at a temperature of 150 ° C. for 20 minutes using a pressure kneader (manufactured by Nippon Spindle). Next, the resulting mixture was put into a crusher (manufactured by Osaka Chemical Co., Ltd., product name "Wonder Crush Mill D3V-10") and crushed for 10 seconds.
50 g of the crushed mixture was rolled under the same conditions as in Example 1 to obtain a primary sheet having a thickness of 0.8 mm. The tensile strength of the primary sheet was then measured. The results are shown in Table 1.
<Lamination process>
A block was obtained in the same manner as in Example 1, except that the above-mentioned primary sheet was used. The Asker C hardness of the obtained block was then measured. The results are shown in Table 1.
<Slicing process>
A thermally conductive sheet was produced and various evaluations were carried out in the same manner as in Example 1, except that the above-mentioned block was used and the slice width was changed to 160 μm. The results are shown in Table 1.

(Comparative Example 1)
A primary sheet and a block were produced in the same manner as in Example 1. Then, in the slicing process, a blade with a tip curvature radius R of 13.5 μm (double-edged, blade angle 2θ: 20°, maximum thickness of blade: 3.5 mm, material: carbide, Rockwell hardness: 91.5, silicon processing on blade surface: none, total length: 200 mm) was used, and slicing of the block was attempted under the same conditions as in Example 1, except that the slicing width was changed to 150 μm. However, slicing was not possible due to the poor sharpness of the blade, and a thermally conductive sheet was not obtained.

(Comparative Example 2)
A primary sheet was formed in the same manner as in Example 4. Next, in the lamination step, the primary sheet was laminated and pressed in the same manner as in Example 4, and the resulting laminate was further heated at 100° C. for 3 hours (heating step) and allowed to cool at room temperature for 3 hours. The Asker C hardness of the block body after this heating and cooling was measured. The results are shown in Table 1.
Then, except for using the block body after heating and cooling, the slicing process was carried out in the same manner as in Example 4, and a thermally conductive sheet was produced and various evaluations were carried out. The results are shown in Table 1.

From Table 1, it can be seen that the thermal conduction sheets of Examples 1 to 7 have low thermal conductivity in the thickness direction and low thermal resistance in the thickness direction, so that they can transfer heat well in the thickness direction. In addition, it can be seen that the thermal conduction sheets of Examples 1 to 7 have excellent thickness precision because the standard deviation of the thickness is small.
On the other hand, in Comparative Example 1, as described above, the blade was not sharp enough to slice the material, and therefore no thermally conductive sheet was obtained.
Furthermore, from Table 1, it can be seen that the thermal conductive sheet of Comparative Example 2 has a large standard deviation of thickness and therefore has poor thickness precision.

The present invention provides a thermally conductive sheet that has sufficient thickness accuracy while allowing good heat transfer in the thickness direction, and a method for manufacturing the thermally conductive sheet.

10 Blade 11 Blade surface 20 Block body 21 Slicing surface 30 Heat conductive sheet

Claims (4)

A thermally conductive sheet comprising a thermoplastic resin that is solid at room temperature and normal pressure and a particulate filler,
The particulate filler is oriented at an angle of 45° or more with respect to the main surface of the thermal conductive sheet,
The thermal conductivity in the thickness direction is 12 W/m K or more,
The standard deviation of the thickness is 7.0 μm or less,
A thermally conductive sheet, at least one of whose main surfaces has a surface roughness Sa of 2.60 μm or less. The thermal conductive sheet according to claim 1, wherein the particulate filler content is 45 volume % or less. The thermal conductive sheet according to claim 1 or 2, wherein the area of the main surface is 30 ^cm2 or more. The thermal conductive sheet according to any one of claims 1 to 3, having an average thickness of 200 μm or less.

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