JP6814384B2 - Method of manufacturing heat conductive sheet - Google Patents

Description

The present invention relates to a method for producing a heat conductive sheet containing inorganic particles and a resin.

In recent years, heat conductive sheets made of heat conductive inorganic particles and resins have been used for parts requiring heat dissipation and insulation such as substrates and insulating layers of heat-generating electronic circuits.
Among the inorganic particles used in the heat conductive sheet, for example, scaly hexagonal boron nitride, which has anisotropy in particle shape and thermal conductivity, the major axis direction (plane direction) is the minor axis direction. Some have higher thermal conductivity than (thickness direction). In a heat conductive sheet using such anisotropic inorganic particles, the inorganic particles are oriented so that the major axis direction is parallel to the direction in which heat conductivity is required (that is, the thickness direction of the sheet) (hereinafter, , "Vertical orientation".) Is preferable.
However, if the composition containing the inorganic particles and the resin serving as the binder is simply coated in the form of a sheet with a doctor blade or the like, the major axis direction of each particle is oriented parallel to the surface direction of the sheet.

In this regard, in Non-Patent Document 1, one point of potting is formed by dropping a paste containing hexagonal boron nitride particles and a resin using a discharge nozzle, and a large number of these pottings are repeatedly formed to form an aggregate. As a result, it is disclosed that a sheet in which hexagonal boron nitride particles are vertically oriented is prepared.
Further, in Patent Document 1, after laminating primary sheets in which the major axis direction of hexagonal boron nitride particles is oriented parallel to the plane direction to obtain a laminated body, the normal line emitted from the surface of the primary sheet is 0. It is disclosed that by slicing the laminate at ~ 30 degrees, a sheet in which the hexagonal boron nitride particles are vertically oriented is prepared.
Further, Patent Document 2 discloses that hexagonal boron nitride particles are positively charged and vertically oriented by utilizing electrostatic force.
However, all of these methods have complicated sheet manufacturing methods and have a problem in mass productivity.

Japanese Unexamined Patent Publication No. 2012-38763 Japanese Unexamined Patent Publication No. 5-174623

"2) Development of heat sink material with orientation control of ceramic particles", Saga Ceramics Technology Center 2013 Report, pp. 9-13

In view of the above problems, it is an object of the present invention to efficiently produce an inorganic particle-containing heat conductive sheet having high thermal conductivity by a simple method.

As a result of diligent research to solve the above problems, the present inventors can easily print a coating film in which inorganic particles are vertically oriented by simply stencil printing an ink containing inorganic particles having an amorphous shape and a resin. We found that it could be formed, and completed the present invention by utilizing such findings.

That is, the present invention is as follows.
[1] A method for producing a heat conductive sheet, which comprises a step of stencil printing an ink containing inorganic particles and a resin to form a coating film.
[2] The method for producing a heat conductive sheet according to [1], further comprising a step of heating the coating film to form a semi-cured sheet.
[3] The method for producing a heat conductive sheet according to [1] or [2], wherein the inorganic particles have an anisotropic shape.
[4] The method for producing a heat conductive sheet according to any one of [1] to [3], wherein the average particle size of the inorganic particles is 0.1 μm to 500 μm.
[5] The method for producing a heat conductive sheet according to any one of [1] to [4], wherein the inorganic particles are scaly hexagonal boron nitride particles.
[6] The method for producing a heat conductive sheet according to any one of [1] to [5], wherein the pore diameter of the plate used in the stencil printing is 1.1 times or more the maximum particle size of the inorganic particles.
[7] The method for producing a heat conductive sheet according to any one of [1] to [6], wherein the stencil printing is screen printing.
[8] The method for producing a heat conductive sheet according to [7], wherein the space ratio of the plate used in the screen printing is 0.1 to 90%.
[9] The method for producing a heat conductive sheet according to any one of [1] to [8], wherein the ink further contains a solvent having an evaporation rate of 4.5 times or less that of butyl acetate.
[10] The heat conduction according to any one of [1] to [9], further comprising a step of forming a plurality of coating films and laminating the plurality of coating films in the step of forming the coating film. How to make a sheet.
[11] In the step of laminating the coating film, the step of forming an uneven pattern on the surface of the coating film is further included between the step of forming the coating film and the step of laminating the coating film. The method for producing a heat conductive sheet according to [10], wherein a plurality of coating films are laminated so that the surfaces on which the patterns are formed face each other.
[12] The method for producing a heat conductive sheet according to [10], wherein a discontinuous coating film having a regular pattern is formed in the step of forming the coating film.

According to the method of the present invention, a sheet in which inorganic particles are vertically oriented can be easily manufactured in a large area in a batch without using special equipment, whereby a heat conductive sheet having high thermal conductivity can be easily produced. Can be efficiently manufactured.

Hereinafter, embodiments for carrying out the present invention (hereinafter, referred to as “the present embodiment”) will be described in more detail, but the present invention is not limited to this, and various aspects are not deviated from the gist thereof. It can be transformed.

The method for producing a heat conductive sheet of the present embodiment is stencil printing of a coating film of ink (heat conductive sheet forming material) containing inorganic particles and resin, that is, a large number of holes provided in the plate by applying pressure to the ink. It is formed by a printing method in which ink is transferred to a printed matter by passing the ink.
As described above, it is difficult to produce a sheet in which inorganic particles are vertically oriented by coating, and conventionally such a sheet has been produced by a method other than simple coating such as a potting method.
However, the present inventors have found that when an ink containing inorganic particles and a resin is stencil-printed, the particles are vertically oriented in the formed coating film.
The reason why the particles are vertically oriented in the coating film by stencil printing is not clear, but when the ink passes through the pores, a flow occurs in the thickness direction of the plate, and each particle flows along this flow in the major axis direction. It is presumed that this is because they are arranged so as to be parallel. However, the mechanism does not depend on this.

First, an ink containing inorganic particles and a resin used in the method for producing a heat conductive sheet of the present embodiment will be described.
In the present embodiment, examples of the inorganic particles include particles made of an inorganic compound such as carbon, an inorganic oxide, an inorganic nitride or an inorganic carbide, and specific examples thereof include boron nitride, aluminum nitride, aluminum oxide, and zinc oxide. Spherical, powdery, fibrous, needle-like, made of metal oxides such as silicon carbide and aluminum hydroxide, metal nitrides, metal carbides, metal hydroxides; metals and alloys; carbon materials such as graphite and diamond. Examples include scaly and whisker-like particles.
Of these, hexagonal boron nitride particles are preferable from the viewpoint of thermal conductivity. Examples of the particle shape of the hexagonal boron nitride particles include flat, scaly, plate-shaped, flat plate-shaped, granular, spherical, fibrous, whiskers, etc. Among them, scaly hexagonal boron nitride particles are included. preferable.

As described above, the production method of the present embodiment can be particularly preferably used when such inorganic particles have anisotropy in shape (when the diameter differs depending on the direction).
The aspect ratio (ratio of the longest diameter to the shortest diameter) of the inorganic particles used in the present embodiment is not limited, but may be, for example, 2 or more, 5 or more, or 10 or more. You may. There is no upper limit to the aspect ratio, but it is usually 1000 or less.
The shape and aspect ratio of the inorganic particles can be determined by observing the particles with an electron microscope. For the aspect ratio, 200 or more particles were randomly selected from images taken with a scanning electron microscope (SEM) (for example, FE-SEM-EDX (SU8220): manufactured by Hitachi High Technology Co., Ltd.). It is determined by obtaining the aspect ratio, which is the ratio of the longest diameter and the shortest diameter of each, and calculating the average value thereof.
Preferred examples of the inorganic particles having anisotropy in such a shape include flat (for example, scaly), fibrous, and whisker-like particles of hexagonal boron nitride.

Further, in the present embodiment, only one kind of inorganic particles may be used, or two or more kinds of inorganic particles may be contained as long as the effects of the present invention are not impaired. For example, boron nitride particles and other thermally conductive inorganic particles can be used in combination.

By the way, the heat conductive sheet may be used at a high temperature of 180 ° C. or higher when it is incorporated into a heat-generating electronic component or the like. Even when used at a high temperature of 180 ° C. or higher, in order to extend the life of peripheral members and ensure reliability such as high thermal conductivity and withstand voltage characteristics, the average particle size of the inorganic particles should be a thermal conductive sheet. It is preferably about 1/2 to 1/3 or less of the thickness of.
If the average particle size exceeds 1/2 of the thickness of the sheet, inorganic particles protrude on the surface of the sheet, the surface shape of the heat conductive sheet deteriorates, and the adhesion when producing a bonded sheet with other members deteriorates. However, the withstand voltage characteristics may deteriorate.
On the other hand, if the average particle size of the inorganic particles is too small, the probability that the heat conduction path is connected from top to bottom in the thickness direction of the heat conduction sheet becomes small, and the heat conductivity in the thickness direction of the heat conduction sheet becomes insufficient. Sometimes.

From the above viewpoint, although the particle size of the inorganic particles is not limited, the average particle size is preferably about 0.1 to 500 μm, more preferably 0.5 to 200 μm, and 1 to 100 μm. It is particularly preferable to have.
Here, the average particle size of the inorganic particles refers to the median diameter (diameter) measured by a laser diffraction / scattering type particle size distribution measuring device (for example, manufactured by Microtrac Bell, MT3000II series, etc.).

The maximum particle size of the inorganic particles is not limited, but is preferably 1000 μm or less, preferably 500 μm, from the viewpoint of preventing clogging of the stencil during printing and ensuring reliability such as high thermal conductivity and withstand voltage characteristics. It is more preferably 300 μm or less, and further preferably 300 μm or less.
Here, the maximum particle size of the inorganic particles means the maximum particle size observed by a laser diffraction / scattering type particle size distribution measuring device (for example, manufactured by Microtrac Bell, MT3000II series).

In addition, since large inorganic particles that are not observed by the particle distribution measuring device may be present in the raw material, in the present embodiment, the inorganic particles having a diameter equal to or larger than a predetermined size are removed from the raw material in advance by a sieve or the like. It is preferably added to the ink.

In the present embodiment, the content of the inorganic particles of the ink can be appropriately set according to the desired characteristics, and is, for example, 30% by volume or more and 80% by volume or less.
If the amount of inorganic particles is less than 30% by volume, the desired thermal conductivity may not be obtained because the amount of inorganic particles is too small.
On the contrary, when the content of the inorganic particles exceeds 80% by volume, the heat conductive sheet may become brittle due to too many inorganic particles, or the electrical insulation property of the heat conductive sheet may decrease. In addition, the viscosity of the ink may increase, making it difficult to obtain a thin and flat heat conductive sheet. Here, the volume of the inorganic particles is a value obtained from the specific gravity of the inorganic compounds constituting the inorganic particles and the mass of the inorganic particles contained in the ink.

In the present embodiment, the resin used for the ink is not limited, but for example, a thermoplastic resin (polyolefin, vinyl chloride resin, methyl methacrylate resin, nylon, fluororesin), a curable resin (epoxy resin, phenol resin, urea resin). , Melamine resin, unsaturated polyester resin, silicon resin), synthetic rubber and the like. Of these, thermosetting resins are preferable.
Further, in the present embodiment, the curable resin may be added to the ink in the form of a resin composition mixed with a curing agent or a curing catalyst, and further, a precursor of a corresponding raw material (monomer, dimer, oligomer, etc.). ) May be added to the ink.

Examples of the thermosetting resin include polyimide resins, polyaminobismaleimide (polybismaleimide) resins, bismaleimide / triazine resins, polyamideimide resins, polyetherimide resins and other polyimide resins; polybenzoxazole resins; polyethers. Resins; benzocyclobutene resins; silicone-based resins; phenol-based epoxy resins, alcohol-based epoxy resins and other epoxy-based resins and the like can be mentioned.
Of these, epoxy resins, polyether resins, benzocyclobutene resins, and silicone resins are preferable, and epoxy resins are particularly preferable, because they have high thermal conductivity and good solubility in organic solvents.
One of these resins may be used alone, or two or more of these resins may be used in any combination and ratio.

In the present embodiment, a solvent can be further used for the ink, if necessary. The type of solvent is not particularly limited, and for example, ketones such as acetone, methyl ethyl ketone, methyl cellsolve, and cyclopentanone; aromatic hydrocarbons such as toluene and xylene; cycloalkanes such as cyclohexane and cyclopentane; Alcohols such as methanol, ethanol and n-butyl alcohol; esters such as ethyl acetate, propyl acetate and butyl acetate; amides such as dimethylformamide; organic solvents such as propylene glycol monomethyl ether and acetate thereof. As the solvent, one type may be used alone, or two or more types may be used in combination.
From the viewpoint of adjusting the viscosity of the ink to be constant during printing and quickly drying the ink after printing, it is preferable to use a solvent having an evaporation rate of 4.5 times or less that of butyl acetate. If the evaporation rate is too fast, a large amount of solvent will volatilize during printing, and the viscosity of the ink will change during printing. Further, if the evaporation rate is too slow, it becomes difficult to dry the solvent, and if the solvent is dried at a high temperature for a long time, the curing will proceed when a thermosetting resin is used as the resin. From this point of view, it is more preferable to use a solvent having a rate of 0.05 to 4.5 times the evaporation rate of butyl acetate.
The content of the solvent is not limited, and can be appropriately determined in consideration of, for example, the viscosity of the ink, and is usually about 10 to 50% by mass of the ink. If the amount of solvent is insufficient, the pores of the stencil may be clogged, or the bubbles may not disappear sufficiently and the coated surface may become uneven. On the other hand, if the amount of the solvent is excessive, the orientation retention of the inorganic particles in the coating film after printing is inferior, and repelling is likely to occur during printing.

In the present embodiment, in addition to the inorganic particles, the resin and the solvent used as needed, various additives conventionally used for the heat conductive sheet and the ink can be added to the ink.
Examples of such additives include a coupling agent for improving the adhesiveness between inorganic particles and a resin, the above-mentioned curing agent and curing catalyst, resin curing accelerator, viscosity modifier, and dispersion stabilizer. , Surfactants, emulsifiers, low elastic agents, diluents, defoamers, ion trapping agents and the like. One of these may be used alone, or two or more thereof may be mixed and used in any combination and ratio.

As a preferable example of the coupling agent, those generally used for surface treatment of inorganic substances can be preferably used, and the type thereof is not particularly limited. Specifically, aminosilane systems such as γ-aminopropyltriethoxysilane and N-β- (aminoethyl) -γ-aminopropyltrimethoxylane; γ-glycidoxypropyltrimethoxysilane, β- (3,4). -Epoxysilanes such as (epoxycyclohexyl) ethyltrimethoxysilane; vinylsilanes such as γ-methacryloxipropyltrimethoxysilane, vinyl-tri (β-methoxyethoxy)silane; N-β- (N-vinylbenzylaminoethyl)- Cationic silanes such as γ-aminopropyltrimethoxysilane hydrochloride; silane coupling agents such as phenylsilanes can be mentioned.
These silane coupling agents may be used alone or in combination of two or more.

The coupling agent is preferably added in an amount of 0.01 to 10% by mass, more preferably 1 to 2% by mass, based on the surface area of the inorganic particles.

The method of adding the coupling agent to the inorganic particles is not particularly limited. A wet method of immersing and stirring is preferable.

In the present embodiment, the method for preparing the ink is not particularly limited, and each component may be mixed. At that time, in order to uniformly mix each component, known treatments such as stirring, mixing, and kneading can be performed.
Further, in the present embodiment, the viscosity of the ink is not limited, but is preferably 0.1 to 100 Pa · s, for example. A viscoelastic ink of 5 to 20 Pa · s is desirable from the viewpoint of having high orientation retention of inorganic particles in the coating film after printing and preventing clogging of mesh holes.

Next, stencil printing in the method for manufacturing a heat conductive sheet of the present embodiment will be described.
Stencil printing refers to a printing method in which ink is transferred to a printed matter by applying pressure to the ink to pass through the holes of a stencil (a plate having a large number of holes). In the present embodiment, the above-mentioned heat conductive sheet forming material is used as the ink, thereby forming a coating film for the heat conductive sheet by stencil printing.

In the present embodiment, the stencil is not particularly limited as long as it has a large number of holes, but from the viewpoint of producing a uniform sheet, the holes having a uniform size and shape are in a square grid shape or in a houndstooth pattern. It is preferable that they are evenly arranged like a child. The aperture ratio is not limited, but when forming a continuous film, the aperture ratio is preferably 0.1 to 90 (area)%, more preferably 20 to 80 (area)%.
The thickness of the plate is also not limited, but for the purpose of causing the ink to flow in the vertical direction and vertically aligning the inorganic particles, it is preferable that the ink has a certain thickness, for example, a plate having a thickness of about 40 to 300 μm is preferable. Can be used.
The pore size is also not limited, and can be appropriately selected depending on the maximum particle size of the inorganic particles.
However, if it is smaller than the maximum particle size of the inorganic particles, it may cause clogging of the stencil. Therefore, it is preferably larger than the maximum particle size of the inorganic particles, and preferably 1.1 times or more the maximum particle size. It is more preferably 2 times or more, and further preferably 3 times or more.
Further, from the viewpoint of causing the ink to flow in the vertical direction, the pore diameter is preferably 1100 μm or less, more preferably 500 μm or less, further preferably 300 μm or less, and further preferably 100 μm or less. Is still preferable.
Here, the hole diameter means the diameter when the hole is circular, the average value of the major axis and the minor axis when the hole is rectangular, and the hole (opening) in the case of other shapes. It shall mean the diameter of the smallest circle circumscribing.
The material of the stencil is not limited, and examples thereof include punching metal (aluminum, steel, stainless steel, etc.).

In the present embodiment, it is preferable to perform screen printing using a screen mesh (a mesh woven with synthetic fibers or metal fibers) as the stencil printing.
The opening and thickness of the screen mesh used at that time are not limited, and the preferable range is about the same as the above-mentioned hole diameter and plate thickness. Further, the wire diameter and pitch of the screen mesh may be within a range in which such an opening can be provided, and are not particularly limited. The space ratio corresponding to the aperture ratio of the screen mesh is preferably 0.1 to 90%, more preferably 20 to 80%, and particularly preferably 20 to 60%.
Here, the spatial ratio (X) of the screen mesh is a value expressed by the following equation.
X = (a / (a + b)) ² x 100 (%)
(However, a: mesh opening (mm), b: mesh wire diameter (mm))
The material of the screen mesh is not limited, and stainless steel can be preferably used from the viewpoint of strength and rust prevention.

In the present embodiment, the distance (clearance) between the stencil and the surface to be printed is not limited, and may be, for example, 0.3 to 10.0 mm, preferably 0.5 mm to 5.0 mm.
In the present embodiment, the thickness of the coating film (printing film thickness) formed by stencil printing is not limited and can be appropriately determined according to the desired thickness of the heat conductive sheet, for example, 10 to 1000 μm. It may be 10 to 500 μm, or 50 to 300 μm.
In the present embodiment, the heat conductive sheet may be formed by one coating film alone, or a plurality of thin coating films may be laminated to form the heat conductive sheet.

In the present embodiment, during stencil printing, pressure is applied to the ink to allow the ink to pass through the stencil holes. The method of applying pressure at this time is not limited, and for example, the squeegee may be slid from one side of the stencil to the other side to apply pressure to the ink, or is it the same as the hole-forming portion of the stencil? A plate having a larger area may be used to uniformly apply pressure to the ink perpendicular to the stencil.
The printing pressure can be, for example, 300 grams or more per 1 cm of squeegee length. It may be 400 grams or more per 1 cm, or 500 grams or more. In general, if the squeegee printing pressure is too low, the uniformity of the printing film thickness tends to be impaired, and particularly in the case of high viscoelastic ink printing, sufficient printing pressure is required for scraping the ink on the screen plate. Is.

When a squeegee is used for the pressure load on the composition during stencil printing, the material and hardness thereof are not limited, and the same material as that used for general screen printing can be used. A squeegee made of an elastic body such as urethane rubber having a temperature of about 50 ° to 95 ° can be used.
Further, the squeegee speed (sliding speed) and the squeegee angle (angle formed by the squeegee and the plate) are not limited, and can be, for example, 0.01 to 30 m / min and about 10 to 70 degrees.

In the present embodiment, the thickness of the coating film formed by printing is not limited, but it is preferably larger than the average particle size of the inorganic particles. By making the particle size of the sheet larger than the average particle size of the inorganic particles, the smoothness of the sheet surface is ensured, and when the sheets are laminated as described later, voids are formed between the laminated surfaces. Can be prevented.

In the present embodiment, the coating film can be once formed on the carrier (printed surface).
Such a carrier preferably has a strength capable of retaining the coating film formed by stencil printing and has good releasability when the coating film (sheet) is peeled off from the carrier after the coating film is cured. Specific examples include metal foils such as copper and aluminum, metal plates, plastic films, plastic plates, and the like.

The coating film formed as described above can be made into a sheet by drying and / or curing as necessary.
By the way, the presence of voids in the heat conductive sheet causes a decrease in thermal conductivity and dielectric breakdown reliability. In order to prevent the formation of voids (spaces) in the sheet, a pressing step is provided to press the coating film to remove the void spaces from the coating film before the coating film is completely dried and cured. Is also preferable. The pressing method is not limited, but it is preferable to pressurize the coating film evenly. In this case, if a step of semi-curing the coating film by heating, light irradiation, or the like to form a semi-cured sheet is provided prior to the pressing step, the pressing can be easily performed.
Further, when a curable resin or a resin composition is used as the resin contained in the ink, it is preferable to provide a curing step such as heating the coating film or irradiating the coating film with light.

Further, the step of peeling the above-mentioned carrier from the coating film may be included. When the carrier has characteristics that are not suitable for the use of the heat conductive sheet, for example, when the use of the heat conductive sheet is an insulating layer of an electronic circuit, a conductive material such as a metal foil is used as the carrier. In some cases, it is preferable to peel the carrier from the heat conductive sheet. The timing of this peeling step is not limited, but it is preferably after the coating film has hardened. However, if there is no reason that the characteristics of the carrier are not preferable in light of the use of the heat conductive sheet and the heat conductivity is high, the carrier can be used as the heat conductive sheet while remaining.

In the present embodiment, a plurality of the above-mentioned coating films may be formed and laminated for the purpose of obtaining a sheet having a desired thickness.
For example, a laminated sheet can be easily obtained by laminating and curing two coating films before they are completely cured. At the time of laminating, it is preferable to press to prevent the formation of gaps (voids) between the laminated surfaces. As described above, if voids are formed in the heat conductive sheet, problems such as a decrease in dielectric breakdown strength may occur. When pressing is performed during lamination, the space in the coating film can be removed at the same time in addition to the gaps between the laminated surfaces, so that the above-mentioned pressing step does not need to be provided separately.
If a step of semi-curing the coating film by heating, light irradiation, or the like to form a semi-cured sheet is provided prior to the step of laminating the coating film, the laminating can be easily performed.

Further, if an uneven pattern is provided in advance on the surface of the coating film before it is completely cured, and these pattern-forming surfaces are laminated and cured so as to face each other, a sheet having a high degree of vertical orientation of inorganic particles can be obtained. However, it was found by the research of the present inventors.
The reason why a sheet having a high degree of vertical orientation of inorganic particles can be obtained is not clear, but the vertical orientation of the inorganic particles in the coating film once obtained in the step of forming the coating film by stencil printing is subsequently determined by the coating film. Where it may be disturbed by the influence of gravity etc. until it hardens, when the uneven surfaces of the coating film with the uneven surface are laminated so as to face each other, the boundary surface between the two coating films moves inorganic particles. It is presumed that this is to regulate and suppress the disorder of orientation. However, the mechanism does not depend on this.

In order to produce the above-mentioned laminated sheet, it is preferable to further have a step of forming an uneven pattern on the surface of the coating film before it is completely cured.
The method of forming the unevenness pattern on the coating film surface is not limited, and examples thereof include a method of transferring the unevenness to the coating film surface using a transfer plate or a roller having the unevenness pattern. In this case, if a semi-curing sheet forming step is provided in which the coating film is semi-cured by heating, light irradiation, or the like prior to the step of forming the uneven pattern, the uneven pattern can be easily transferred. Further, it is preferable because the transferred uneven pattern is easily retained.

Further, as another aspect of producing the laminated sheet as described above, stencil printing is used as pattern printing to form two discontinuous coating films having a regular pattern, and these are laminated, preferably under pressure. It may be cured with.

In the above embodiment, the shape and size of the concave and convex portions of the concave-convex pattern are not limited, but for example, the height of the convex portion / the depth of the concave portion is about 10 to 200 μm, and the coating film thickness is 1. It can be about / 5 to 1/1.
Further, it is preferable that the concave portion and the convex portion have a shape that can be fitted to each other. From this point of view, the uneven pattern is preferably, for example, a grid shape or a line shape (striped shape) in which square concave portions and convex portions are regularly arranged.

In the present embodiment, the characteristics of the heat conductive sheet to be produced are not limited.
The thickness of the sheet may be appropriately determined according to the intended use, and may be, for example, about 10 to 1000 μm.

Further, the thermal conductivity in the thickness direction of the heat conductive sheet is preferably 4.0 W / m · K or more, more preferably 5.0 W / m · K or more, and 6.0 W / m · K or more. Is more preferable. There is no upper limit to the thermal conductivity, and the larger the thermal conductivity, the more preferable. According to the manufacturing method of the present embodiment, it is possible to manufacture such a heat conductive sheet having a high thermal conductivity.

Next, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these examples.

The method of measuring the characteristics of the heat conductive sheet produced in Examples and Comparative Examples is shown below.
<Evaluation method of each characteristic>
(1) Thermal conductivity A test piece (10 mm × 10 mm × thickness 1 mm) was cut out from the laminated sheets obtained in Examples and Comparative Examples, and tested with a laser flash using a NETZSCH xenon flash analyzer LFA447 type thermal conductivity meter. The thermal conductivity (W / m · K) of the piece was measured.

(2) Orientation strength ratio (I (002) / I (100))
The orientation of the boron nitride primary particles in the laminated sheet is the intensity of the I (002) diffraction line (2θ = 26.5 °) and the I (100) diffraction line (2θ = 41.5 °) by the X-ray diffraction method. ) To the strength (I (002) / I (100)).
Specifically, a test piece (5 mm × 5 mm × thickness 0.2 mm) was cut out from the laminated sheets obtained in Examples and Comparative Examples, and “Fully automatic horizontal multipurpose X-ray diffractometer SmartLab” (manufactured by Rigaku Co., Ltd., Using an X-ray source: CuKα ray, tube voltage: 45 kV, tube current: 360 mA), X-rays are irradiated in the thickness direction of the test piece to form an I (002) diffraction line and an I (100) diffraction line. The intensity was measured.
The thickness direction of the hexagonal boron nitride primary particles coincides with the crystallographic I (002) diffraction line, that is, the c-axis direction, and the in-plane direction coincides with the I (100) diffraction line, that is, the a-axis direction. .. When the boron nitride primary particles constituting the aggregate of the boron nitride particles are completely randomly oriented (non-oriented), (I (002) / I (100)) ≈6.7 ("" JCPDS [Powder X-ray Diffraction Database] "No. 34-0421 [BN] crystal density value [Dx]). The smaller (I (002) / I (100)) is, the more the a-axis direction of the hexagonal boron nitride particles is oriented in the thickness direction.

(Example 1)
100 parts by mass of epoxy resin ("850-S" manufactured by DIC), 56 parts by mass of curing agent (phenonovolac-based curing agent, "DL-92" manufactured by Meiwa Kasei), and curing catalyst (2phenylimidazole, manufactured by Shikoku Kasei) A resin composition was prepared by mixing 0.1 parts by mass.
Next, 1.5 parts by mass of 3-glycoxydpropyltrimethoxylane (manufactured by Tokyo Kasei) was added dropwise as a coupling agent to 100 parts by mass of primary boron nitride particles (“SGP” manufactured by Denki Kagaku Kogyo Co., Ltd.) having an average particle size of 18 μm. And stirred with a mixer.
Subsequently, in 30 parts by mass of cyclopentanone, 40 parts by mass of the resin composition and 60 parts by volume of the boron nitride primary particles to which the coupling agent was added were mixed to prepare an ink.
This ink is applied onto a copper foil by printing using a screen printing plate (manufactured by Nakanuma Art Screen Co., Ltd., 3D80 mesh-80-225 μt NU-30 + 10 μt, mesh hole diameter: 238 μm, space ratio: 54%) (coating thickness: 100 μm). Next, the coating film was dried at 130 ° C. for 15 minutes and peeled off from the copper foil to prepare a semi-cured sheet. 14 semi-cured sheets from which the copper foil was peeled off were prepared, and the coated surfaces (opposite surfaces of the copper foil) of the 14 semi-cured sheets were laminated together and vacuumed at 180 ° C., 10 MPa, and 60 minutes. It was cured while pressing to prepare a 1 mm thick laminated sheet.

(Example 2)
Example 1 except that a bar coater (SA-203 ROD No. 130 manufactured by Tester Sangyo Co., Ltd.) was rolled on the surface of the coating film to transfer a striped uneven pattern (depth: 100 μm, pitch: about 50-200 μm). A laminated sheet was obtained in the same manner.

(Comparative Example 1)
Instead of printing with a screen printing plate, a doctor blade (manufactured by Imoto Seisakusho Co., Ltd.) was used to apply ink smoothly onto the copper foil (coating thickness: 100 μm), but the thickness was 1 mm in the same manner as in Example 1. Laminated sheet was obtained.

(Comparative Example 2)
A 1 mm laminated sheet was obtained in the same manner as in Example 1 except that two sheets obtained by coating ink on a copper foil (coating thickness: 500 μm) by potting using a pipette having a hole diameter of 1 mm were laminated.

The thermal conductivity of the laminated sheets produced in Examples 1 and 2 and Comparative Examples 1 and 2, the orientation strength ratio of the boron nitride primary particles in the laminated sheet (I (002) / I (100)), and the coating efficiency were determined. It is summarized in Table 1.
The coating efficiency was comprehensively evaluated in consideration of the time and effort required to coat the ink on the copper foil in a square shape of 100 mm × 100 mm. In Examples 1 and 2 and Comparative Example 1, the ink could be applied to a square shape of 100 mm × 100 mm in a few seconds with a coating machine, and batch production was easily possible. On the other hand, in Comparative Example 2 using the potting method, since the ink was spotted one by one, it took 30 seconds or more to apply the ink in a square shape of 100 mm × 100 mm. Further, when ejecting ink from the pipette, there is also a problem that the boron nitride particles are clogged at the tip of the pipette and separated from the resin and the solvent.

Since the heat conductive sheet manufactured by the manufacturing method of the present invention has high thermal conductivity and also insulating properties, various applications (for example, circuit substrates for power equipment) that require thermal conductivity and insulating properties are required. It can be suitably used as a material for a heat-dissipating insulating layer or an adhesive layer in a heat-generating electronic component such as a semiconductor power device.

Claims (8)

A method for producing a heat conductive sheet, which comprises a step of stencil printing an ink containing scaly hexagonal boron nitride particles and a resin to form a coating film .
In the step of forming the coating film, a plurality of coating films are formed,
The step of laminating the plurality of coating films is further included.
Between the step of forming the coating film and the step of laminating the coating film, a step of forming an uneven pattern on the surface of the coating film is further included.
A method for producing a heat conductive sheet, in which a plurality of coating films are laminated so that the surfaces on which the uneven pattern is formed face each other in the step of laminating the coating film . The method for producing a heat conductive sheet according to claim 1, further comprising a step of heating the coating film to form a semi-cured sheet. The method for producing a heat conductive sheet according to claim 1 or 2 , wherein the hexagonal boron nitride particles have an average particle size of 0.1 μm to 500 μm. The method for producing a heat conductive sheet according to any one of claims 1 to 3 , wherein the pore diameter of the plate used in the stencil printing is 1.1 times or more the maximum particle size of the hexagonal boron nitride particles . The method for producing a heat conductive sheet according to any one of claims 1 to 4 , wherein the stencil printing is screen printing. The method for producing a heat conductive sheet according to claim 5 , wherein the space ratio of the plate used in the screen printing is 0.1 to 90%. The method for producing a heat conductive sheet according to any one of claims 1 to 6 , wherein the ink further contains a solvent having an evaporation rate of 4.5 times or less that of butyl acetate. The method for producing a heat conductive sheet according to any one of claims 1 to 7 , wherein a discontinuous coating film having a regular pattern is formed in the step of forming the coating film.