JP7400331B2 - Manufacturing method of thermally conductive sheet - Google Patents

Description

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

2. Description of the Related Art In recent years, electronic components such as power semiconductors (IGBT modules, etc.) and integrated circuit (IC) chips have been increasing in heat generation as their performance has improved. As a result, in electronic devices using electronic components, it is necessary to take measures against functional failures caused by temperature rises in the electronic components.

Generally, as a countermeasure against functional failure due to temperature rise, a method is adopted to promote heat dissipation by attaching a heat dissipation body such as a metal heat sink, heat dissipation plate, or heat dissipation fin to a heat dissipation body such as an electronic component. ing. When using a heat sink, in order to efficiently transfer heat from the heat sink to the heat sink, a sheet-like member with high thermal conductivity (thermal conductive sheet) is used to connect the heat sink and the heat sink. are in close contact with each other. Therefore, a thermally conductive sheet used by being sandwiched between a heating element and a heat radiating element is required to have high thermal conductivity and high flexibility. Furthermore, from the viewpoint of safety, thermally conductive sheets are also required to exhibit high flame retardancy.

For example, Patent Document 1 discloses that a thermally conductive resin composition containing an electron beam (EB) crosslinked olefin resin and a thermally conductive filler is compression molded, and then crosslinked by EB irradiation. A method for manufacturing a thermally conductive sheet (heat dissipating sheet) which has excellent thermal conductivity and excellent heat resistance is disclosed.

Japanese Patent Application Publication No. 2012-54312

However, when the entire surface of the sheet is directly irradiated with an electron beam, a strong cross-linking reaction occurs on the sheet surface due to the influence of the electron beam irradiation, and the molecular chains on the sheet surface contract, resulting in distortion, deformation, and wrinkles on the surface. (hereinafter collectively referred to as wrinkles) (the stronger the electron beam and the higher the absorbed dose of electron beam, the stronger the cross-linking reaction on the surface occurs), the surface of the sheet becomes uneven, and metal coverings such as heat sinks There has been a problem in that a gap is created between the adhesive and the adherend, resulting in deterioration of thermal resistance and reduction in adhesion.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a technology for manufacturing a thermally conductive sheet having excellent sheet strength and thermal conductivity while suppressing the occurrence of wrinkles on the surface.

The present inventor conducted extensive studies in order to achieve the above object. The present inventor has also proposed that by irradiating at least one surface of a sheet containing an electron beam cross-linked resin and a thermally conductive filler with an electron beam such that regions with different electron beam absorbed doses are formed. The inventors have discovered that a thermally conductive sheet with excellent sheet strength and thermal conductivity can be obtained while suppressing the occurrence of wrinkles on the surface of the sheet, and has completed the present invention.

That is, the present invention aims to advantageously solve the above problems, and the method for producing a thermally conductive sheet according to the present invention provides a thermally conductive sheet containing an electron beam crosslinkable resin and a thermally conductive filler. A method for producing a sheet, the method comprising applying an electron beam to at least one surface of the sheet containing an electron beam crosslinkable resin and a thermally conductive filler so that regions with different electron beam absorption doses are formed. The method is characterized by including a step of irradiating with. In this way, if at least one surface of a sheet containing an electron beam cross-linked resin and a thermally conductive filler is irradiated with an electron beam so that regions with different electron beam absorbed doses are formed, the sheet surface can be It is possible to provide a thermally conductive sheet that has excellent sheet strength and thermal conductivity while suppressing wrinkles and the like.

Here, it is preferable that the step of irradiating the electron beam includes irradiating the surface with the electron beam through a mask member in which portions having different electron beam transmittances are arranged in a pattern. According to the above method, it becomes easy to form regions with different electron beam absorption doses on at least one surface of the heat conductive sheet.

It is preferable that the pattern of the mask member is a mesh structure pattern having a plurality of rectangular openings or a punching structure pattern having a plurality of circular openings. With such a configuration, it becomes easy to form regions with different electron beam absorbed doses on at least one surface of the thermally conductive sheet.

Further, the aperture ratio of the opening of the mask member is preferably 30% or more and 90% or less, more preferably 35% or more and 85% or less, and even more preferably 40% or more and 80% or less. . If the aperture ratio is within the above range, it is possible to suppress wrinkles in the resulting thermally conductive sheet and maintain a high level of sheet strength while maintaining adhesion between the sheet and the metal adherend. can.

It is preferable that the step of irradiating the electron beam includes irradiating the electron beam so that the irradiation dose per irradiation surface on the surface of the sheet is 50 kGy or more and 9000 kGy or less. If the electron beam irradiation dose per irradiation surface on the sheet surface is within the above range, it is possible to achieve both a high level of suppression of wrinkles and the like of the resulting thermally conductive sheet and sheet strength.

Here, the irradiation dose in electron beam irradiation is expressed as irradiation dose, and gray (Gy) is used as a unit. This is the "irradiation dose (1 Gy) when 1 joule of energy is absorbed per 1 kg."
A film dosimeter is used to measure the above-mentioned irradiation dose based on the amount of electron beam transmission. In other words, in film dosimeters, the film contains a substance that develops color when exposed to electron beams, and when the film for dosimetry is irradiated with an electron beam and absorbs energy, the spectral characteristics change and the film changes color. . There is a correlation between the degree of discoloration and the irradiation dose, and the irradiation dose can be measured by measuring the absorbance at a specific wavelength. In the present invention, the "irradiation dose" of the electron beam refers to a value measured using a radiochromic film dosimeter (manufactured by Far West Technology, FWT-60).

In the method for manufacturing a thermally conductive sheet of the present invention, it is preferable that the electron beam crosslinkable resin contains a thermoplastic fluororesin that is liquid at room temperature and pressure. If a sheet to be irradiated with electron beams contains a liquid thermoplastic fluororesin at normal temperature and pressure, it is possible to suppress an increase in thermal resistance and obtain a thermally conductive sheet having sufficiently high thermal conductivity. Furthermore, since wrinkles on the surface caused by electron beam irradiation can be suppressed, the strength of the sheet can be increased while maintaining the smoothness of the sheet.
In the present invention, "normal temperature" refers to 23° C., and "normal pressure" refers to 1 atm (absolute pressure).

Furthermore, in the method for manufacturing a thermally conductive sheet of the present invention, it is preferable that the thickness of the thermally conductive sheet obtained is 50 μm or more and 600 μm or less. If the thickness of the sheet is within the above range, the resulting thermally conductive sheet can be suitably used by being sandwiched between a heat generating element and a heat radiating element.
In the present invention, the "thickness" of the sheet refers to a value measured with an accuracy of 1/1000 mm using a Digimatic indicator (manufactured by Mitutoyo Co., Ltd., ID-C112X).

According to the present invention, it is possible to provide a technique for manufacturing a thermally conductive sheet that has excellent sheet strength and thermal conductivity while suppressing wrinkles and the like on the surface.

It is a schematic diagram which shows the mesh structure which is an example of the pattern of a mask member. It is a schematic diagram which shows the punching structure which is an example of the pattern of a mask member. It is a schematic diagram which shows the mesh structure which is an example of the pattern of a mask member. It is a schematic diagram which shows the elongated hole structure which is an example of the pattern of a mask member. It is a schematic diagram which shows the slit hole structure which is an example of the pattern of a mask member. FIG. 2 is a schematic diagram showing a structure in which circular openings and cross-shaped openings are alternately provided, which is an example of a pattern of a mask member. FIG. 7 is a schematic diagram of a thermally conductive sheet for explaining a thermal resistance test in Examples 8 and 9.

Embodiments of the present invention will be described in detail below. Here, the method for manufacturing a thermally conductive sheet of the present invention is used, for example, when manufacturing a thermally conductive sheet containing an electron beam crosslinked resin and a thermally conductive filler. The thermally conductive sheet manufactured by the manufacturing method of the present invention can be used, for example, by being sandwiched between a heat generating element and a heat radiating element when the heat radiating element is attached to the heating element. Note that the present invention is not limited to the following embodiments.

(Method for manufacturing thermally conductive sheet)
The method for producing a thermally conductive sheet of the present invention includes applying an electron beam to at least one surface of a sheet containing an electron beam crosslinkable resin and a thermally conductive filler so that regions with different electron beam absorbed doses are formed. (electron beam irradiation step). In the method for producing a thermally conductive sheet of the present invention, an electron beam crosslinkable resin containing a thermoplastic fluororesin that is liquid at room temperature and normal pressure is used as the electron beam crosslinkable resin of the sheet that is irradiated with an electron beam in the irradiation step. It is preferable to use
In addition, in the method for manufacturing a thermally conductive sheet of the present invention, the sheet irradiated with electron beams in the irradiation step may be used as a thermally conductive sheet as is, or the sheet irradiated with electron beams may be further subjected to post-treatment. may be used as a heat conductive sheet. That is, the method for manufacturing a thermally conductive sheet of the present invention may further include, after the irradiation step, a post-treatment step of performing a post-treatment on the sheet irradiated with the electron beam.

In the method for manufacturing a thermally conductive sheet of the present invention, electron beams are formed so that regions with different electron beam absorption doses are formed on at least one surface of the sheet containing the electron beam crosslinked resin and the thermally conductive filler. Since the surface layer of the sheet is treated with electron beams by irradiation with electron beams, it suppresses distortion, deformation, wrinkles, etc. (hereinafter collectively referred to as wrinkles) on the surface, and improves sheet strength and thermal conductivity. A thermally conductive sheet with excellent properties can be obtained.

<Irradiation process>
In the irradiation step, at least one surface of the sheet containing the electron beam crosslinkable resin and the thermally conductive filler is irradiated with an electron beam so that regions with different electron beam absorbed doses are formed.

[Sheet]
Here, as the sheet to be irradiated with the electron beam, a sheet containing an electron beam crosslinkable resin, a thermally conductive filler, and optionally further containing an additive can be used.

-Electron beam cross-linked resin-
As the electron beam crosslinkable resin constituting the sheet, it is preferable to use at least a thermoplastic fluororesin that is liquid at room temperature and pressure, and a thermoplastic fluororesin that is liquid at room temperature and pressure, and a thermoplastic fluororesin that is solid at room temperature and pressure. It is more preferable to use it together with a resin. If a thermoplastic fluororesin that is liquid at room temperature and pressure is not used as the electron beam crosslinked resin, the thermal resistance of the resulting thermally conductive sheet increases, making it impossible to obtain a thermally conductive sheet with excellent thermal conductivity. Furthermore, if a thermoplastic fluororesin that is liquid at room temperature and pressure and a thermoplastic fluororesin that is solid at room temperature and pressure are used together as electron beam crosslinkable resins, the sheet strength of the resulting thermally conductive sheet can be increased.
Note that in the present invention, rubber and elastomer are included in the "resin".

Examples of resins that are liquid at room temperature and pressure include thermoplastic resins that are liquid at room temperature and pressure, and thermosetting resins that are liquid at room temperature and pressure.
Examples of thermoplastic resins that are liquid at room temperature and pressure include acrylic resins, epoxy resins, silicone resins, and fluororesins.
In addition, examples of thermosetting resins that are liquid at normal 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; phenol resin; unsaturated polyester; diallyl phthalate resin; polyimide silicone resin; polyurethane; thermosetting polyphenylene ether; thermosetting Type-modified polyphenylene ether; etc.

Further, examples of resins that are solid at room temperature and normal pressure include thermoplastic resins that are solid at room temperature and normal pressure, and thermosetting resins that are solid at room temperature and normal pressure.
Examples of thermoplastic resins that are solid at room temperature and normal pressure include poly(2-ethylhexyl acrylate), a copolymer of acrylic acid and 2-ethylhexyl acrylate, polymethacrylic acid or its ester, polyacrylic acid or Acrylic resin such as its ester; silicone resin; fluororesin; polyethylene; polypropylene; ethylene-propylene copolymer; polymethylpentene; polyvinyl chloride; polyvinylidene chloride; polyvinyl acetate; ethylene-vinyl acetate copolymer; polyvinyl alcohol Polyacetal; Polyethylene terephthalate; Polybutylene terephthalate; Polyethylene naphthalate; Polystyrene; Polyacrylonitrile; Styrene-acrylonitrile copolymer; Acrylonitrile-butadiene-styrene copolymer (ABS resin); Styrene-butadiene block copolymer or its hydrogenation Styrene-isoprene block copolymer or its hydrogenated product; Polyphenylene ether; Modified polyphenylene ether; Aliphatic polyamides; Aromatic polyamides; Polyamideimide; Polycarbonate; Polyphenylene sulfide; Polysulfone; Polyether sulfone; Polyether nitrile ; polyetherketone; polyketone; polyurethane; liquid crystal polymer; ionomer; and the like.
In addition, examples of thermosetting resins that are solid at normal 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; phenol resin; unsaturated polyester; diallyl phthalate resin; polyimide silicone resin; polyurethane; thermosetting polyphenylene ether; thermosetting Type-modified polyphenylene ether; etc.

~Thermoplastic fluororesin that is liquid at room temperature and pressure~
Here, examples of thermoplastic fluororesins that are liquid at room temperature and normal pressure include vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, and vinylidene fluoride-hexafluoropentene-tetrafluorocarbon resin. Examples include ethylene terpolymer, perfluoropropene oxide polymer, and tetrafluoroethylene-propylene-vinylidene fluoride copolymer. These may be used alone or in combination of two or more.

～Thermoplastic fluororesin that is solid at room temperature and pressure～
In addition, examples of thermoplastic fluororesins that are solid at room temperature and pressure include polymerized fluorine-containing monomers such as vinylidene fluoride fluororesins, tetrafluoroethylene-propylene fluororesins, and tetrafluoroethylene-purple orovinyl ether fluororesins. Examples include elastomers obtained by More specifically, polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer, polyvinylidene fluoride, vinylidene fluoride Ride-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, polychlorotrifluoroethylene, ethylene-chlorofluoroethylene copolymer, tetrafluoroethylene-perfluorodioxole copolymer Polyvinyl fluoride, tetrafluoroethylene-propylene copolymer, acrylic modified polytetrafluoroethylene, ester modified polytetrafluoroethylene, epoxy modified polytetrafluoroethylene, and silane modified polytetrafluoroethylene etc. These may be used alone or in combination of two or more.

When a thermoplastic fluororesin that is liquid at room temperature and pressure is used together with a thermoplastic fluororesin that is solid at room temperature and pressure, the thermoplastic fluororesin is liquid at room temperature and pressure in 100% by mass of the electron beam crosslinkable resin. The proportion is preferably 5% by mass or more, more preferably 10% by mass or more, preferably 50% by mass or less, and more preferably 30% by mass or less. If the proportion of the thermoplastic fluororesin that is liquid at normal temperature and normal pressure is at least the above lower limit, the thermal conductivity of the resulting thermally conductive sheet can be further improved. Further, if the proportion of the liquid thermoplastic fluororesin at normal temperature and normal pressure is below the above upper limit, the sheet strength of the resulting thermally conductive sheet can be increased.

-Thermally conductive filler-
The thermally conductive filler constituting the sheet is not particularly limited, and examples thereof include particulate materials such as alumina particles, zinc oxide particles, inorganic nitride particles, silicon carbide particles, magnesium oxide particles, and particulate carbon materials. In addition, fibrous materials such as carbon nanotubes, vapor-grown carbon fibers, carbon fibers obtained by carbonizing organic fibers, and cut products thereof can be used. Among them, at least one type selected from the group consisting of particulate carbon materials, particulate materials such as inorganic nitride particles, and fibrous carbon nanomaterials such as carbon nanotubes (CNT) is used as the thermally conductive filler. It is preferable.
In addition, one type of thermally conductive filler may be used alone, or two or more types may be used in combination.

The amount of the thermally conductive filler contained in the sheet is not particularly limited, and can be, for example, 5 parts by mass or more and 200 parts by mass or less per 100 parts by mass of the above-mentioned electron beam crosslinkable resin.

～Particulate materials～
The particulate material is not particularly limited, and includes, for example, graphite such as artificial graphite, flaky graphite, exfoliated graphite, natural graphite, acid-treated graphite, expandable graphite, and expanded graphite; carbon black; and inorganic nitrided graphite. For example, particles such as carbon particles, alumina particles, zinc oxide particles, silicon carbide particles, magnesium oxide particles, etc. can be used. These may be used alone or in combination of two or more in any ratio.
Among these, it is preferable to use at least one type from the group consisting of expanded graphite and inorganic nitride particles. If expanded graphite or inorganic nitride particles are used, the thermal conductivity of the thermally conductive sheet can be further improved.

Expanded graphite can be obtained, for example, by chemically treating graphite such as flaky graphite with sulfuric acid or the like, heat-treating it to expand it, and then refining it. Examples of the expanded graphite include EC1500, EC1000, EC500, EC300, EC100, and EC50 (all trade names, manufactured by Ito Graphite Industries Co., Ltd.).

Examples of the inorganic nitride particles include boron nitride particles, aluminum nitride particles, and silicon nitride particles. These may be used alone or in combination of two or more in any ratio.
Among these, boron nitride particles are preferred in terms of imparting electrical insulation and thermal conductivity to the thermally conductive sheet.
Specific examples of commercially available boron nitride particles include the "PT" series (for example, "PT-110") manufactured by Momentive Performance Materials Japan; Dangdong Chemical Engineering Institute Co. , Ltd. Examples include "HSL" and "HS" manufactured by the company.

Here, the content of the particulate material in the thermally conductive filler is preferably 40% by mass, more preferably 70% by mass, and 130% by mass or less based on 100% by mass of the electron beam crosslinkable resin. It is preferably present, and more preferably 100% by mass or less. This is because when the content of the particulate material is within the above range, thermal conductivity and adhesion to adherends are excellent.

The volume average particle diameter of the particulate thermally conductive filler is preferably 0.1 μm or more, more preferably 1 μm or more, preferably 300 μm or less, and more preferably 250 μm or less. preferable. If the volume average particle diameter of the particulate thermally conductive filler is within the above range, the thermal conductivity of the thermally conductive sheet can be improved.
The volume average particle diameter can be measured using a laser diffraction/scattering particle diameter measuring device (manufactured by Horiba, Ltd., LA-960 series).

～Fibrous material～
Fibrous carbon nanomaterials containing CNTs that can be suitably used as fibrous materials may be composed only of CNTs, or may be a mixture of CNTs and fibrous carbon nanomaterials other than CNTs. Good too.
Note that CNTs in the fibrous carbon nanomaterial are not particularly limited, and single-walled carbon nanotubes and/or multi-walled carbon nanotubes can be used. Preferably, single-walled carbon nanotubes are used, and single-walled carbon nanotubes are more preferable.

Furthermore, the above-mentioned fibrous carbon nanomaterial is not particularly limited, and may be produced by chemical vapor deposition by supplying a raw material compound and a carrier gas onto a base material having a catalyst layer for CNT production on its surface. When synthesizing CNTs by (CVD method), the presence of a small amount of oxidizing agent (catalyst activating material) in the system dramatically improves the catalytic activity of the catalyst layer (super growth method; International Publication It is preferable to use a fibrous carbon nanomaterial containing CNTs manufactured according to the method of Japanese Patent No. 2006/011655). Note that, hereinafter, carbon nanotubes obtained by the super growth method may be referred to as "SGCNT".
Here, the fibrous carbon nanomaterial containing SGCNT produced by the super growth method may be composed only of SGCNT, or in addition to SGCNT, other carbon nanomaterials such as non-cylindrical carbon nanostructures may be used. May contain structures.

When the thermally conductive filler contains a fibrous material, the content of the fibrous material in the thermally conductive filler is preferably 0.001% by mass or more, and preferably 0.005% by mass or more. More preferably, it is 20% by mass or less, and more preferably 10% by mass or less. If the content ratio of the fibrous material is equal to or higher than the above lower limit, a heat transfer path can be formed satisfactorily, so that the thermal conductivity of the thermally conductive sheet can be further improved. In addition, if the content of the fibrous material is below the above upper limit, the combination of the fibrous material will prevent the flexibility of the heat conductive sheet from decreasing, and ), the thermally conductive sheet can exhibit excellent thermal conductivity.

-Additive-
Additives that can be optionally added to the sheet include, but are not particularly limited to, flame retardants, plasticizers, acid acceptors such as MgO, toughness improvers, moisture absorbers, adhesive strength improvers, and wettability improvers. , ion trapping agents, etc.
The blending amount of the additive can be adjusted as appropriate within a range that provides the desired effect.

-Method for preparing sheets-
A sheet containing an electron beam crosslinked resin and a thermally conductive filler can be prepared using any sheet forming method without particular limitation.
Among them, a sheet containing an electron beam crosslinkable resin and a thermally conductive filler is a sheet containing the above-mentioned electron beam crosslinkable resin and a thermally conductive filler, and is prepared by adding a composite material that may optionally further contain an additive. A step of pressing and forming into a sheet shape to obtain a composite material sheet, a step of laminating a plurality of composite material sheets in the thickness direction, or a step of folding or winding the composite material sheets to obtain a laminate, and a step of laminating It is preferable to manufacture the sheet through a step of slicing the body at an angle of 45° or less with respect to the stacking direction to obtain a sheet. The sheet manufactured through the above-mentioned process has a structure in which the sliced pieces of the composite material sheet that constituted the laminate are joined in parallel, and the thermally conductive filler is oriented in the thickness direction. This is because it has excellent thermal conductivity in the thickness direction.

Here, from the viewpoint of reducing the thermal resistance and improving the handling property of the obtained thermally conductive sheet, the thickness of the sheet containing the electron beam crosslinked resin and the thermally conductive filler is preferably 50 μm or more and 300 μm or less. . Note that the thermally conductive sheet obtained by irradiating the above sheet with an electron beam usually has the same thickness as the sheet. The thermally conductive sheet can be suitably used by being sandwiched between a heat generating element and a heat radiating element.

[Electron beam irradiation]
The above-mentioned irradiation of the sheet with an electron beam is not particularly limited, and is preferably carried out using an electron beam irradiation device in an environment with a low oxygen concentration, for example, an oxygen concentration of 1000 mass ppm or less. The low oxygen concentration environment can be prepared by reducing the pressure inside the chamber or by introducing nitrogen into the chamber, but the method of introducing nitrogen into the chamber is preferable in terms of production efficiency.

Here, the electron beam may be applied to at least one surface of the sheet. Further, the irradiation distance of the potential line is preferably 5 mm or more and 20 mm or less. Further, the conveyance speed is preferably 5 mm/s or more and 100 mm/s or less. A faster transport speed is preferable in order to improve production efficiency, but if the electron beam irradiation dose is to be the same, the beam current must be increased. However, simply increasing the beam current may cause wrinkles and the like, which may be undesirable. Therefore, by performing the conveyance multiple times, wrinkles can be suppressed with a relatively weak acceleration voltage and beam current. However, from the viewpoint of improving production efficiency, it is usually preferable that the number of times of transportation be small. Therefore, it is more preferable to adjust the beam current and conveyance speed to suppress the occurrence of wrinkles and the like.

Furthermore, in the present invention, the step of irradiating the sheet with the electron beam includes the step of irradiating at least one surface of the sheet with the electron beam so that regions having different absorbed doses of the electron beam are formed. Here, the "absorbed dose" of the electron beam refers to the amount that actually hits the sheet surface, not the amount irradiated onto the mask.
The method of irradiating the sheet with an electron beam so as to form regions with different electron beam absorption doses on at least one surface of the sheet is not particularly limited. What is necessary is to adopt a method in which a mask member made of 300 yen is placed (laminated) on a sheet and an electron beam is irradiated through the mask member. Regions with different electron beam absorbed doses can be formed by changing the electron beam irradiation dose for each location, but if a mask member is used as described above, the electron beam can be applied at a constant irradiation dose. Regions having different electron beam absorption doses can be easily formed during irradiation.
By irradiating the electron beam in such a way that regions with different electron beam absorption doses are formed, it is possible to suppress wrinkles on the surface caused by electron beam irradiation, so the sheet can be smoothed while maintaining its smoothness. can increase strength

The electron beam irradiation dose per irradiated surface is preferably 50 kGy or more, more preferably 100 kGy or more, even more preferably 1000 kGy or more, particularly preferably 2000 kGy or more, and 9000 kGy or less. It is preferable that it is, it is more preferable that it is 8000 kGy or less, and it is still more preferable that it is 6000 kGy or less. If the irradiation dose of the electron beam is equal to or greater than the above lower limit, the strength of the resulting thermally conductive sheet can be sufficiently increased. Moreover, if the irradiation dose of the electron beam is below the above upper limit value, wrinkles etc. on the surface of the resulting thermally conductive sheet can be suppressed.
Furthermore, since the electron beam is irradiated linearly, the sheet is conveyed at a constant speed in order to make the irradiation dose uniform. The irradiation dose of the electron beam is determined by the accelerating voltage and beam current.
Here, the accelerating voltage is preferably 50 kV or more, more preferably 70 kV or more, preferably 250 kV or less, and even more preferably 200 kV or less. Further, the beam current is preferably 1.0 mA or more, more preferably 1.7 mA, preferably 15 mA or less, and even more preferably 12 mA or less.
In addition, when irradiating both sides of the sheet with an electron beam, if the irradiation dose to at least one surface (irradiation surface) is within the above range, the irradiation dose to the other surface (irradiation surface) is outside the above range. However, it is preferable that the irradiation dose to both surfaces (irradiated surfaces) of the sheet is within the above range.

[Mask member]
The mask member can be a plate-like or sheet-like member in which portions having different electron beam transmittances are arranged in a pattern. Here, the parts with different electron beam transmittances are, for example, a "high transmittance part" with a high electron beam transmittance and a "non-transparent part" that does not transmit electron beams, or a "high transmittance part" with an electron beam transmittance higher than that of the "high transmittance part". Can be configured with a low "low transmittance area"

The pattern of the mask member can be, for example, a mesh structure pattern having a plurality of rectangular openings or a punching structure pattern having a plurality of circular openings, using a metal that does not transmit electron beams. Here, the openings can be formed in a pattern, for example, as "highly transparent parts", and other parts can be formed as "non-transparent parts".
Alternatively, a pattern may be formed using a mask member using a non-metal that transmits electron beams.
Alternatively, a pattern may be formed by combining two or more types of materials with different electron beam transmittances as a "highly transparent part" and a "non-transparent part" or a "lowly transparent part". Alternatively, it may be a concavo-convex pattern composed of a thin portion as a “highly transparent portion” and a thick portion as a “lowly transparent portion”.

Here, when a non-metal is used as the mask member, it is preferable that the electron beam transmittance in the high transmittance portion of the mask member is 50% or more and less than 100%. Further, the electron beam transmittance in the low transmittance portion of the mask member is preferably 10% or more and less than 50%.
In addition, in the present invention, the electron beam transmittance refers to the value obtained by dividing the electron beam absorbed dose when the mask member is installed, taking the electron beam absorbed dose when the mask member is not installed as 100%, Refers to the value measured using an electron beam absorption dosimeter.

Although the metal used for the mask member is not particularly limited, for example, stainless steel (SUS), iron, aluminum, and copper can be used.
Examples of the non-metal used in the mask member include any material that allows electron beams to pass through appropriately, such as a resin film with a thickness of about 1 to 20 μm, a pressure-sensitive adhesive, an adhesive, a resin nonwoven fabric, and the like.

Although FIGS. 1 to 6 show examples of preferred configurations of patterns of mask members, the patterns of mask members of the present invention are not limited thereto.

As shown in FIGS. 1 to 6, the opening (highly transparent portion) of the mask member can have, for example, a slit (elongated hole) shape, a circular shape (including an ellipse), a rectangular shape, or the like. Further, the low transmittance portion can be formed into a mesh shape or a grid shape, for example.
Furthermore, if a mask member with a circular, rectangular, or diamond pattern is transferred onto a sheet and removed after electron beam irradiation, regions with different absorbed doses will be formed in a pattern opposite to that shown in FIGS. 1 to 6. be able to.

For example, the pattern of the mask member can be a punching structure in which a plurality of circular openings are arranged as shown in FIG. The diameter of each opening is preferably 3 φ or more and 20 φ or less, and the interval between the openings is preferably 1 mm or more and 5 mm or less. It is preferable that the actual pattern of the mask member be sequentially designed in accordance with the thermal properties, adhesion, and sheet strength required depending on the size of the adherend used as a heat conductive sheet for electronic materials.

For example, the pattern of the mask member can have a structure in which a plurality of rectangular openings are provided, as shown in FIGS. 1 and 3. The rectangle includes, for example, a square, a rectangle, a diamond, and the like.

For example, the pattern of the mask member can have a structure in which a plurality of slit (long hole) shaped openings are provided as shown in FIGS. 4 and 5. In FIG. 4, the openings are arranged in a staggered manner in the short direction, and in FIG. 5, the openings are arranged in a staggered manner in the longitudinal direction. The slit shape can be particularly preferably used in that it is possible to form a pattern by aligning the longitudinal direction of the opening with the direction in which the sheet strength is desired to be increased.

Further, the pattern of the mask member may have a structure in which circular openings and cross-shaped openings are alternately provided as shown in FIG. 6, for example.

The mask member can be formed as a mesh structure made of wires. The mesh structure can be constructed using, for example, plain-woven wire mesh, twill-woven wire mesh, or the like.

In the mask member, it is preferable that the aperture ratio of the openings (highly transparent portions) is 50% or more and 90% or less. When the aperture ratio of the mask member is within the above range, it is possible to achieve both suppression of wrinkles and the like in the resulting thermally conductive sheet and a high level of sheet strength.

By providing a high transmission part and a non-transmission part or a low transmission part in the mask member, the electron beam absorption dose is high in the high transmission part of the mask member, and the electron absorption dose is high in the non-transmission part or low transmission part. Therefore, it is possible to provide a difference in the electron beam absorption dose on the surface of the heat conductive sheet. Since it is possible to create a pattern-like difference in the electron beam absorbed dose depending on the shape of the high transmittance part and the non-transparent part or low transmittance part of the mask member, there is a region where the electron beam absorbed dose differs and has a pattern similar to the pattern of the mask member. is transferred and formed on the sheet surface. Here, the area on the sheet corresponding to the high transmission part of the mask member is called the electron beam high absorption dose area, and the area on the sheet corresponding to the non-transmission part or low transmission part of the mask member is called the electron beam low absorption dose area. It's called.

Alternatively, the electron beam may be irradiated with the mask member separated from the sheet surface. In that case, the distance between the mask member and the sheet is preferably 0.1 mm or more and 1 mm or less. By setting it to the above lower limit value or more, it is possible to prevent the mask member and the sheet from coming into contact with each other, and by setting it to the above upper limit value or less, it is possible to transfer the pattern of the mask member more accurately.

In this way, by irradiating the sheet surface with an electron beam through the above mask member, regions with different electron beam absorption doses can be easily formed on the surface. In other words, it is possible to easily form regions on the sheet surface where the pattern of the mask member is substantially transferred and where the electron beam absorption dose differs. By irradiating the sheet with an electron beam so that regions with different electron beam absorption doses are formed in the above pattern, it is possible to increase the sheet strength while suppressing wrinkles in the resulting thermally conductive sheet. can.

EXAMPLES Hereinafter, the present invention will be specifically explained based on Examples, but the present invention is not limited to these Examples. In the following description, "%" and "part" representing amounts are based on mass unless otherwise specified. Further, in Examples and Comparative Examples, the thickness of the sheet, the thermal resistance value of the thermally conductive sheet, the wrinkle resistance, and the tensile strength were measured and evaluated by the following methods.

<Thickness of thermally conductive sheet>
The thickness of the thermally conductive sheet (total film thickness) was measured with an accuracy of 1/1000 mm using a Digimatic indicator (manufactured by Mitutoyo Co., Ltd., ID-C112X).

<Thermal resistance value>
The thermal resistance value of the thermally conductive sheet was measured using a resin material thermal resistance tester (manufactured by Hitachi Technology and Services, Ltd.). Here, a thermally conductive sheet cut into a 1.0 cm square is used as a sample, and the thermal resistance value (°C/W) when a relatively low pressure of 0.30 MPa is applied at a sample temperature of 50°C, and the sample Thermal resistance values (°C/W) when a relatively high pressure of 0.80 MPa was applied at a temperature of 50°C were measured and evaluated based on the following criteria.
A: 0.21 or less B: More than 0.21 and less than 0.30 C: 0.30 or more and less than 0.40 D: 0.40 or more

<Tensile strength>
Tensile strength (sheet strength) was measured as follows. First, a thermally conductive sheet was punched and molded using a dumbbell No. 2 in accordance with JIS K6251 to produce a sample piece. Using a tensile tester (manufactured by Shimadzu Corporation, product name: AG-IS20kN), pinch a point 1 cm from both ends of the sample piece, and at a temperature of 25°C, in a direction perpendicular to the normal line from the surface of the sample piece ( MD direction or TD direction) at a tensile speed of 500 mm/min, MD direction tensile strength (MPa) and TD direction tensile strength (MPa) were measured, respectively, and evaluated based on the following criteria.
A: 1.5 or more B: 1.0 or more and less than 1.5 C: 0.5 or more and less than 1.0 D: less than 0.5

<Sensitivity over wrinkles>
The wrinkle resistance of the sheet after electron beam irradiation was examined by visual observation of the degree of wrinkles on the entire surface of the thermally conductive sheet one day after electron beam irradiation. Based on the obtained test results, evaluation was made according to the following criteria.
A: No wrinkles B: Some weak wrinkles at the edges of the sheet C: Many strong wrinkles at the edges of the sheet, more gentle wrinkles throughout the sheet D: Many strong wrinkles throughout the sheet, undulations throughout, flattened Can not

(Example 1)
<Preparation of sheet>
400 mg of carbon nanotubes (manufactured by Nippon Zeon Co., Ltd., single-walled carbon nanotubes, specific surface area: 600 m ² /g) were weighed out, mixed in 2 L of methyl ethyl ketone as a solvent, and stirred for 2 minutes with a homogenizer to obtain a crude dispersion. Next, using a wet jet mill (manufactured by Jokosha Co., Ltd., product name: "JN-20"), the obtained crude dispersion was passed through a 0.5 mm channel of the wet jet mill for two cycles at a pressure of 100 MPa. The carbon nanotubes were dispersed in the methyl ethyl ketone. A dispersion liquid having a solid content concentration of 0.20% by mass was obtained.
Thereafter, the obtained dispersion liquid was filtered under reduced pressure using Kiriyama filter paper (No. 5A) to obtain a sheet-like easily dispersible carbon nanotube aggregate.
Next, add 70 parts of a thermoplastic fluororesin that is liquid at room temperature and pressure (manufactured by Daikin Industries, Ltd., product name: "Daiel G-101") and a thermoplastic fluororesin that is solid at room temperature and pressure (3M Japan Co., Ltd.). 30 parts of Dyneon FC-2211 (manufactured by Ito Graphite Industries Co., Ltd., trade name: Mooney viscosity: 27 ML ₁₊₄ , 100°C) and expanded graphite, a thermally conductive filler (manufactured by Ito Graphite Industries Co., Ltd., trade name: 90 parts of "EC300", volume average particle diameter: 50 μm, average particle diameter in short axis direction: 10 to 20 μm) and 0.5 part of the easily dispersible carbon nanotube aggregate obtained above were mixed in a pressure kneader (Japan Spindle). The mixture was stirred and mixed for 20 minutes at a temperature of 150° C. Next, the obtained mixture was put into a crusher and crushed for 10 seconds to obtain a composite material.
Next, 50 g of the obtained composite material was sandwiched between sandblasted PET films (protective films) with a thickness of 50 μm, and the conditions were as follows: roll gap 550 μm, roll temperature 50°C, roll linear pressure 50 kg/cm, and roll speed 1 m/min. The material was rolled and formed (primary pressing) to obtain a composite material sheet with a thickness of 0.5 mm.
Subsequently, the obtained composite material sheet was cut into pieces of length 150 mm x width 150 mm x thickness 0.5 mm, 300 composite material sheets were laminated in the thickness direction, and further, the composite material sheet was cut at a temperature of 120 ° C. and a pressure of 0.1 MPa for 3 minutes. A laminate with a height of about 150 mm was obtained by pressing (secondary pressing) in the stacking direction.
After that, while pressing the side surface of the secondary pressurized laminate with a pressure of 0.3 MPa, use a wood slicer (manufactured by Marunaka Iron Works Co., Ltd., product name: "Super Finish Planer Super Mecha S"). Then, by slicing at an angle of 0° to the lamination direction (in other words, in the normal direction of the main surface of the laminated composite material sheets), a sheet measuring 150 mm long x 150 mm wide x 0.10 mm thick was obtained. Obtained.
<Manufacture of thermally conductive sheets>
A wire mesh (manufactured by Hasegawa Wire Mesh Co., Ltd., 20 mesh, wire diameter 0.20 mm, opening 1.39 mm, aperture ratio 71.0%) having a pattern as shown in FIG. 1 was placed (laminated) on the above sheet. Using a low-energy electron beam irradiation device (manufactured by Hamamatsu Photonics, model number: L12978), the sheet was moved at a tube voltage of 70 kV, a tube current of 5.19 mA, and a conveyance speed of 15 mm/s, and the sheet was conveyed back and forth twice in total. Electron beam irradiation (irradiation dose on each irradiation surface: 1000 kGy) was carried out under the conditions of an irradiation distance of 10 mm and an oxygen concentration of 1000 ppm, so that the electron beam was uniformly irradiated onto the sheet. Further, the back side was also irradiated with an electron beam in the same manner.
Then, the thermal resistance value, tensile strength, and wrinkle resistance of the obtained thermally conductive sheet were measured and evaluated as described above. The results are shown in Table 1.

(Example 2)
A sheet and a thermally conductive sheet were manufactured in the same manner as in Example 1, except that the number of times the thermally conductive sheet was transported back and forth was 6 times in total, and the irradiation dose on each irradiation surface was changed to 3000 kGy. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Example 3)
A sheet and a thermally conductive sheet were manufactured in the same manner as in Example 1, except that the number of times of transportation was 12 times in total, and the irradiation dose on each irradiation surface was changed to 6000 kGy. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Example 4)
A sheet and a thermally conductive sheet were manufactured in the same manner as in Example 1, except that the number of times the thermally conductive sheet was transported back and forth was 18 times in total, and the irradiation dose on each irradiation surface was changed to 9000 kGy. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Example 5)
When preparing the sheet, instead of 90 parts of expanded graphite "EC300", expanded graphite "EC100" (manufactured by Ito Graphite Industries Co., Ltd., volume average particle diameter: 200 μm, average particle diameter in short axis direction: 10 to 20 μm) Using 50 parts, the slice film thickness was adjusted to 150 μm to produce a thermally conductive sheet, and when manufacturing the thermally conductive sheet, the number of times it was transported back and forth was 4 times in total, and the irradiation dose on each irradiation surface was changed to 2000 kGy. A thermally conductive sheet was manufactured in the same manner as in Example 1. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Example 6)
When preparing the sheet, instead of using the thermoplastic fluororesin "Dyneon FC-2211" which is solid at room temperature and pressure, 100 parts of the thermoplastic fluororesin "Daiel G-101" which is liquid at room temperature and pressure was used. A sheet and a thermally conductive sheet were manufactured in the same manner as in Example 2. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Example 7)
When preparing the sheet, boron nitride particles (manufactured by Dangdong Chemical Engineering Institute Co., Ltd., product name: "HSL"'', volume average particle size: 36 μm, average particle size in the long axis direction as observed by electron microscope: 30 μm, average particle size in the short axis direction: 0.5 to 3 μm, hexagonal boron nitride particles) 90 parts were used. A sheet and a heat conductive sheet were manufactured in the same manner as in Example 2. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Example 8)
When manufacturing the heat conductive sheet, a punching metal (manufactured by Ishikawa Kinami Co., Ltd., φ10, interval 0.5 mm, aperture ratio 64%) having a shape pattern in which circular openings were arranged in a staggered manner as shown in Figure 2 was used. Otherwise, a heat conductive sheet was manufactured in the same manner as in Example 2. In addition, in the thermal resistance test, as shown in Figure 7, the thermally conductive sheet was cut out into a 10 mm square with the electron beam irradiated area (electron beam high absorption dose area) on the surface of the sheet being at the left end. (FIG. 7), the thermal resistance was measured. The total area of the electron beam high absorption dose region during thermal resistance measurement was 50% of the entire area.

(Example 9)
In Example 8, the electron beam irradiation area (electron beam high absorption dose region) on the surface of the heat conductive sheet was arranged in a substantially square shape of 10 mm square so as to straddle three circles, as shown in FIG. (Fig. 7) and measured the thermal resistance. The total area of the electron beam high absorption dose region during thermal resistance measurement was 30% of the entire area.

(Comparative example 1)
A sheet and a heat conductive sheet were produced in the same manner as in Example 1, except that the wire mesh was not used during production of the heat conductive sheet. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Comparative example 2)
A sheet and a heat conductive sheet were produced in the same manner as in Example 8, except that no punching metal was used and only one side of the heat conductive sheet was irradiated with an electron beam. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Comparative example 3)
A sheet and a heat conductive sheet were produced in the same manner as in Example 3, except that the wire mesh was not used during production of the heat conductive sheet. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Comparative example 4)
A sheet and a heat conductive sheet were produced in the same manner as in Example 4, except that the wire mesh was not used during production of the heat conductive sheet. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Comparative example 5)
A sheet was produced in the same manner as in Example 1, and was used as a thermally conductive sheet without irradiation with an electron beam using a wire mesh. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

As shown in Table 1, Examples 1 to 9 in which the electron beam was irradiated through a metal mesh and punched metal as a mask member were compared with Comparative Examples 1 to 4 in which the electron beam was irradiated without using a mask member. It can be seen that the occurrence of wrinkles, etc. is well suppressed, and there is almost no decrease in sheet strength or thermal resistance value. Furthermore, in Comparative Example 5 in which no electron beam was irradiated, the sheet strength was lower than in Examples 1 to 9 in which electron beam was irradiated. In other words, it can be seen that in Examples 1 to 9 of the present invention, thermally conductive sheets with excellent sheet strength and thermal conductivity were obtained while suppressing the occurrence of wrinkles on the sheet surface caused by electron beam irradiation. .

According to the present invention, it is possible to obtain a thermally conductive sheet that has excellent sheet strength and thermal conductivity while suppressing wrinkles and the like on the surface.

Claims (7)

A method for producing a thermally conductive sheet comprising an electron beam crosslinked resin and a thermally conductive filler, the method comprising:
A step of irradiating at least one surface of a sheet containing an electron beam crosslinkable resin and a thermally conductive filler with an electron beam so that regions with different electron beam absorbed doses are formed ,
The method for manufacturing a thermally conductive sheet, wherein the step of irradiating the electron beam includes irradiating the surface with the electron beam through a mask member in which portions having different electron beam transmittances are arranged in a pattern. The method according to claim 1 , wherein the pattern of the mask member is a mesh structure-like pattern having a plurality of rectangular openings. The method of claim 1 , wherein the pattern of the mask member is a punching structure-like pattern having a plurality of circular openings. The method according to claim 2 or 3 , wherein the mask member has an aperture ratio of 30% or more and 90% or less. 1 . The step of irradiating the surface with an electron beam through the mask member includes irradiating the surface of the sheet with an electron beam such that the irradiation dose per irradiated surface is 50 kGy or more and 9000 kGy or less. - 4. The method for producing a thermally conductive sheet according to any one of 4 . 6. The method according to claim 1 , wherein the electron beam crosslinkable resin includes a thermoplastic fluororesin that is liquid at room temperature and pressure. The method according to any one of claims 1 to 6 , wherein the thickness of the thermally conductive sheet is 50 μm or more and 600 μm or less.

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