Thermally conductive sheet

Abstract

A thermally conductive sheet that contains boron nitride aggregate particles having an average particle diameter D50 of 100 µm or greater, has a sheet thickness H greater than 200 µm, and has a ratio (D50 / H) of the average particle diameter D50 to the sheet thickness H of 0.25 or greater.

Description

Thermally conductive sheet

[0001] The present invention relates to a thermally conductive sheet.

[0002] As heat-generating electronic components, such as central processing units (CPUs) in personal computers, become smaller and more powerful, the amount of heat generated per unit area by these electronic components has become extremely large. This amount of heat can reach approximately 20 times that of an iron. To prevent these heat-generating electronic components from breaking down over the long term, they must be cooled. Metal heat sinks and housings are used for cooling. However, when a heat-generating electronic component is directly in contact with a heat sink or similar object, microscopic air may exist at the interface, hindering heat conduction. Therefore, to efficiently transfer heat, a thermally conductive material may be placed between the heat-generating electronic component and the heat sink or similar object.

[0003] Thermally conductive materials include thermally conductive spacers, thermally conductive sheets, and thermally conductive greases, which are made by filling thermosetting resins with thermally conductive fillers. Thermally conductive spacers are thick, highly flexible sheets that can conform to the uneven shapes of mating materials. Thermally conductive sheets are relatively thin sheets that are easy to handle and can improve the thermal conductivity between heat-generating electronic components and heat sinks while ensuring insulation. Thermally conductive grease is made by filling a fluid resin with a thermally conductive filler, which allows for a high degree of freedom in application shape and can reduce thermal resistance by applying it thinly.

[0004] For example, a known thermally conductive sheet has a structure in which silicone composition layers containing a thermally conductive filler are laminated on both sides of a reinforcing layer (see, for example, Patent Document 1).

[0005] Japanese Patent Application Laid-Open No. 2022-126642

[0006] With the recent rapid trend toward miniaturization, high integration, and high output of electronic components, the operating temperatures of heat-generating electronic components have risen, and higher voltages are being applied between the heat-generating electronic components and heat sinks. This has led to demand for further improvements in thermal conductivity and insulation, especially for thinly molded thermally conductive sheets.

[0007] The present invention has been made in view of the above problems, and has as its object to provide a thermally conductive sheet having low thermal resistance and high breakdown voltage.

[0008] That is, the present invention is as follows. [1] A thermally conductive sheet comprising boron nitride agglomerated particles having an average particle size D50 of 100 μm or more, wherein the sheet thickness H exceeds 200 μm, and the ratio of the average particle size D50 to the sheet thickness H (D50 / H) is 0.25 or more. [2] The thermally conductive sheet according to [1], having an orientation index of 20 or more and 100 or less. [3] The thermally conductive sheet according to [1] or [2], wherein the boron nitride agglomerated particles have a crushing strength of 5.0 MPa or less. [4] The thermally conductive sheet according to any one of [1] to [3], comprising a non-crosslinked silicone oil. [5] The thermally conductive sheet according to any one of [1] to [4], comprising a silicone resin. [6] The thermally conductive sheet according to any one of [1] to [5], comprising a reinforcing material and resin layers laminated on the front and back surfaces of the reinforcing material, wherein the resin layers contain the boron nitride agglomerated particles and a resin. [7] The thermally conductive sheet according to any one of [1] to [6], wherein the particle size D90 of the boron nitride agglomerated particles is 150 to 400 μm. [8] The thermally conductive sheet according to any one of [1] to [7], wherein the particle size D10 of the boron nitride agglomerated particles is 30 to 50 μm. [9] A method for producing a thermally conductive sheet, comprising a pressing step of hot-pressing a resin composition containing boron nitride agglomerated particles having an average particle size D50 of 100 μm or more with glass cloth to form a thermally conductive sheet having a sheet thickness H of more than 200 μm, wherein the ratio of the average particle size D50 to the sheet thickness (D50 / H) is 0.25 or more.

[0009] According to the present invention, it is possible to provide a thermally conductive sheet having low thermal resistance and high breakdown voltage.

[0010] Fig. 1 is a schematic cross-sectional view showing an example of the thermal conductive sheet of the present embodiment, Fig. 2 is a schematic cross-sectional view showing the state of large-diameter agglomerated particles of boron nitride in the thermal conductive sheet of the present embodiment, Fig. 3 is a schematic cross-sectional view showing the state of boron nitride agglomerated particles of a particle size generally used in a conventional thermal conductive sheet,

[0011] An embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail below, but the present invention is not limited to this embodiment and various modifications are possible without departing from the spirit of the present invention. In the drawings, the same elements are given the same reference numerals and redundant explanations will be omitted. Furthermore, positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings unless otherwise specified. Furthermore, the dimensional ratios of the drawings are not limited to those shown in the drawings.

[0012] 1. Thermally conductive sheet The thermally conductive sheet of this embodiment contains boron nitride agglomerated particles having an average particle size D50 of 100 μm or more, has a sheet thickness H of more than 200 μm, and has a ratio of the average particle size D50 to the sheet thickness H (D50 / H) of 0.25 or more.

[0013] A schematic cross-sectional view showing an example of the thermally conductive sheet of this embodiment is shown in Fig. 1. As shown in Fig. 1, the thermally conductive sheet 1 of this embodiment is not particularly limited as long as it is a sheet containing boron nitride agglomerated particles as a thermally conductive filler, but it may be in an embodiment having a resin layer 10 containing a resin and boron nitride agglomerated particles, or in an embodiment further having a reinforcing material 20 with resin layers 10 laminated on the front and back surfaces of the reinforcing material 20.

[0014] FIG. 2A shows a cross-sectional schematic diagram illustrating the state of large-diameter agglomerated particles of boron nitride in the thermally conductive sheet of this embodiment, and FIG. 2B shows a cross-sectional schematic diagram illustrating the state of small-diameter agglomerated particles of boron nitride in a conventional thermally conductive sheet, as an example of the cross-sectional state of a conventional thermally conductive sheet.

[0015] Conventionally, boron nitride agglomerated particles of a predetermined particle size have been used to pack boron nitride agglomerated particles 11' at a higher density. However, as shown in Figure 2B, when boron nitride agglomerated particles 11' of a commonly used particle size are used, although the packing density can be improved, heat tends to be transferred in a complex manner within the thermally conductive sheet from one surface to the other (see arrow F').

[0016] In contrast, in this embodiment, as shown in Figure 2A, the thermally conductive sheet is constructed using large-diameter boron nitride agglomerated particles 11 with an average particle size D50 of 100 µm or more, so that the ratio (D50 / H) is 0.25 or more, which makes it easier for heat to be transferred more directly and in larger amounts from one surface to the other (see arrow F), further reducing the thermal resistance.

[0017] In general, when coarse particles are used, voids are likely to form in the thermal conductive sheet, which may cause a new problem of a decrease in breakdown voltage, but by making the sheet thickness H exceed 200 μm, the breakdown voltage can be ensured. The configuration of the thermal conductive sheet will be described in detail below.

[0018] 1.1. Thickness The thickness H of the thermally conductive sheet of this embodiment exceeds 200 μm, preferably 220 to 800 μm, 240 to 700 μm, 260 to 600 μm, or 280 to 500 μm. When the thickness H of the thermally conductive sheet exceeds 200 μm, the breakdown voltage tends to be further improved. Furthermore, the thinner the thickness H of the thermally conductive sheet, the more improved the thermal conductivity tends to be. Furthermore, since the presence of a reinforcing material allows the mechanical strength of the thermally conductive sheet to be maintained, the thickness of the thermally conductive sheet can be made thinner.

[0019] 1.2. Orientation Index The orientation index of the thermal conductive sheet of this embodiment is preferably 20 to 100, 25 to 95, 30 to 90, 35 to 85, 40 to 80, 45 to 75, or 50 to 70.

[0020] The primary particles of boron nitride have a flat shape, and tend to have poor thermal conductivity in the thickness direction but excellent thermal conductivity in the plane direction. Furthermore, the flat primary particles of boron nitride tend to be oriented in the plane direction. Therefore, it is generally believed that the higher the orientation index, the higher the proportion of primary particles constituting the boron nitride agglomerated particles that lie flat in the plane direction of the thermal conductive sheet, and that this usually results in a decrease in thermal conductivity. However, in this embodiment, the orientation index being within the above range means that the thermal conductivity tends to be excellent.

[0021] Although the reason for this is not particularly limited, a low orientation index means that the boron nitride agglomerated particles are not deformed during pressing to form the thermally conductive sheet. As described above, the use of coarse particles generally tends to increase the likelihood of void formation in the thermally conductive sheet. However, in this embodiment, by using boron nitride agglomerated particles with an appropriate crushing strength, the particles deform during pressing, reducing voids, thereby achieving both high thermal conductivity and high insulating properties. Therefore, when the orientation index is low and the boron nitride agglomerated particles are not deformed, voids remain in the thermally conductive sheet, and the voids impede the thermal path, resulting in low thermal conductivity. Furthermore, if the boron nitride agglomerated particles are excessively deformed and collapsed, resulting in an overall horizontal orientation, the thermal conductivity will be low. Therefore, in this embodiment, the orientation index is set to the above range, thereby achieving both high thermal conductivity and high insulating reliability.

[0022] In this embodiment, the "orientation index" refers to a value measured in accordance with the following method. An X-ray diffraction spectrum of the boron nitride agglomerated particles in the thermal conductive sheet is obtained by performing X-ray diffraction measurement on a thermal conductive sheet containing boron nitride agglomerated particles, and peak intensities I(002) and I(100) corresponding to the (002) and (100) planes are obtained from the X-ray diffraction spectrum. The obtained peak intensities can be used to calculate the orientation index [I(002) / I(100)] of the boron nitride agglomerated particles. As an X-ray diffraction device, for example, an "ULTIMA-IV" (trade name) manufactured by Rigaku Corporation can be used.

[0023] 1.3 Resin Layer The resin layer contains a resin and boron nitride agglomerated particles, and may contain silicone oil as needed. When the thermally conductive sheet of this embodiment further includes a reinforcing material 20, the resin layer 10 may be disposed on one surface of the reinforcing material 20, or on both surfaces thereof as shown in FIG. 1.

[0024] The thickness of the resin layer is preferably 50 to 300 μm, 75 to 250 μm, or 100 to 200 μm. The thinner the resin layer, the more improved the thermal conductivity, and the thicker the resin layer, the more improved the insulation. Furthermore, the presence of a reinforcing material allows the mechanical strength of the thermally conductive sheet to be maintained, allowing the thickness of the resin layer to be made thinner. Note that the thickness of the resin layer refers to the thickness of each resin layer when a reinforcing material is used and the resin layer is present on both sides of the reinforcing material, as described below.

[0025] 1.3.1 Resin The resin is not particularly limited, but examples thereof include silicone resin, silicone rubber, epoxy resin, acrylic resin, phenolic resin, melamine resin, unsaturated polyester, fluororesin, polyimide resin, polyamideimide resin, polyetherimide resin, polyester resin, polyphenylene ether resin, etc. These resins may be used alone or in combination of two or more.

[0026] Among these, silicone resins and silicone rubbers are preferred. These silicone resins and silicone rubbers may be peroxide-curable, condensation-curable, addition-curable, or ultraviolet-curable, or may be those cured by these curing reactions. By using such resins, the mechanical strength and flexibility of the thermally conductive sheet are further improved, and the handling properties are further improved, and the thermal conductivity and insulating properties tend to be further improved.

[0027] The resin content is preferably 25 to 55 volume %, 30 to 50 volume %, or 35 to 45 volume % relative to the total volume of the thermally conductive sheet minus the volume of the reinforcing material, i.e., 100 volume % of the resin layer. By having the resin content within the above range, thermal conductivity and insulation properties tend to be further improved.

[0028] 1.3.2 Boron Nitride Agglomerated Particles The thermal conductive sheet of this embodiment uses relatively large boron nitride agglomerated particles. The agglomerated particles refer to secondary particles formed by agglomeration of primary particles.

[0029] The average particle size D50 of the boron nitride agglomerated particles is 100 μm or more, preferably 105 μm or more, 110 μm or more, 115 μm or more, 120 μm or more, 125 μm or more, 130 μm or more, or 135 μm or more. The upper limit of the average particle size D50 of the boron nitride agglomerated particles is preferably 250 μm or less, 200 μm or less, 180 μm or less, 160 μm or less, or 140 μm or less. When the average particle size D50 is 100 μm or more, thermal conductivity tends to be further improved. When the average particle size D50 is 250 μm or less, voids are less likely to occur, and breakdown voltage tends to be further improved.

[0030] In this embodiment, the average particle size D50 refers to the median diameter, which is the value at which the cumulative distribution is 50% on a volume-based cumulative distribution curve. Similarly, the particle sizes D10 and D90 described below refer to the values ​​at which the cumulative distribution is 10% and 90% on a volume-based cumulative distribution curve.

[0031] The ratio (D50 / H) of the average particle diameter D50 to the sheet thickness H is 0.25 or greater, preferably 0.27 or greater, 0.29 or greater, 0.31 or greater, 0.33 or greater, or 0.35 or greater. The ratio (D50 / H) is preferably 1.00 or less, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, or 0.45 or less. Furthermore, the range of the ratio (D50 / H) may be determined by combining any one of the multiple lower limit candidate values ​​described above with any one of the multiple upper limit candidate values ​​described above. Specifically, the ratio (D50 / H) may be preferably 0.25 to 1.00, 0.27 to 0.90, 0.29 to 0.80, or 0.31 to 0.70. The ratio (D50 / H) can represent the proportion of one agglomerated particle of boron nitride relative to the thickness. When this ratio (D50 / H) is 0.25 or greater, that is, when the proportion of one agglomerated particle of boron nitride relative to the thickness is high, the heat conduction path becomes thicker as shown in Figure 2A, and thermal conductivity tends to be improved.

[0032] The thermally conductive sheet may contain a plurality of boron nitride agglomerated particles having different average particle sizes D50. In this case, it is sufficient that the average particle size D50 of at least one type of boron nitride agglomerated particle satisfies the above-mentioned ratio (D50 / H), and it is preferable that all of the boron nitride agglomerated particles satisfy the above-mentioned ratio (D50 / H).

[0033] The particle size D90 of the boron nitride agglomerated particles is preferably 100 to 500 μm, 125 to 450 μm, or 150 to 400 μm. When D90 is 100 μm or more, thermal conductivity tends to be further improved. Furthermore, when D90 is 500 μm or less, voids are less likely to be generated, and breakdown voltage tends to be further improved.

[0034] The particle size D10 of the boron nitride agglomerated particles is preferably 20 to 70 μm, 25 to 60 μm, or 30 to 50 μm. When D10 is 20 μm or more, thermal conductivity tends to be further improved. Furthermore, when D10 is 70 μm or less, voids are less likely to be generated, and breakdown voltage tends to be further improved.

[0035] The crushing strength of the boron nitride agglomerated particles is preferably 5.0 MPa or less, 4.5 MPa or less, 4.0 MPa or less, 3.5 MPa or less, 3.0 MPa or less, 2.5 MPa or less, or 2.0 MPa or less. Generally, when coarse particles are used, voids tend to be easily formed in the thermally conductive sheet, and the breakdown voltage tends to be reduced. In this regard, if the crushing strength of the boron nitride agglomerated particles is 5 MPa, the boron nitride agglomerated particles will deform together with the flow of the resin in the pressurizing step of the method for producing a thermally conductive sheet, which will be described later, making it less likely that voids will be formed, and a higher breakdown voltage tends to be achieved.

[0036] The crushing strength of the boron nitride agglomerated particles is preferably 0.1 MPa or more, 0.5 MPa or more, or 1.0 MPa or more. When the crushing strength of the boron nitride agglomerated particles is 0.1 MPa or more, the boron nitride agglomerated particles are less likely to break during the pressurizing step, which tends to ensure the thermal conductivity expected of large-diameter agglomerated particles such as those indicated by arrow F in Figure 2A.

[0037] In this embodiment, the crushing strength refers to a value measured in accordance with the description in JIS R 1639-5:2007 "Fine Ceramics - Measurement of Granule Properties - Part 5: Single Granule Crushing Strength." The crushing strength σ (unit: MPa) of a single agglomerate particle is calculated using the equation σ = α × P / (π × d²) from the dimensionless number α (α = 2.48), which varies depending on the position within the agglomerate, the crushing test force P (unit: N), and the particle size d (unit: μm). Measurements are performed on 20 or more agglomerate particles, and the value at a cumulative failure rate of 63.2% is calculated. A microcompression tester can be used for the measurement. An example of a microcompression tester that can be used is the "MCT-210" (trade name) manufactured by Shimadzu Corporation.

[0038] 1.3.3. Other Thermally Conductive Fillers The thermally conductive sheet of this embodiment may contain other thermally conductive fillers besides the boron nitride agglomerated particles. Examples of other thermally conductive fillers include, but are not limited to, aluminum nitride, aluminum oxide, silicon nitride, silicon oxide, zinc oxide, silica, aluminum hydroxide, magnesium oxide, and carbon. Among these, aluminum oxide is preferred. The use of such thermally conductive fillers tends to further improve thermal conductivity and insulation.

[0039] The other thermally conductive filler may be an agglomerated particle formed by agglomeration of primary particles. For example, by using a thermally conductive filler having anisotropy in thermal conductivity as an agglomerated particle, the thermal conductivity due to contact between the thermally conductive filler particles is improved, and a thermal conduction path in any direction is easily formed, which tends to further improve the thermal conductivity.

[0040] The average particle size of the other thermally conductive fillers is preferably 0.05 to 120 μm, and more preferably 0.1 to 70 μm. When the average particle size of the thermally conductive filler is within the above range, the dispersibility and packing properties of the thermally conductive filler in the thermally conductive sheet tend to be further improved. In this embodiment, the average particle size refers to the median diameter D50. Furthermore, when the thermally conductive filler is in the form of agglomerated particles, the average particle size refers to the particle size of the agglomerated particles (secondary particles).

[0041] 1.3.4. Non-Crosslinkable Silicone Oil The thermally conductive sheet of this embodiment, particularly the resin layer, may further contain a non-crosslinkable silicone oil. Examples of silicone oils include, but are not limited to, peroxide-curable, condensation-curable, addition-curable, and ultraviolet-curable non-crosslinkable compounds without curable functional groups. Examples of such non-crosslinkable silicone oils include, but are not limited to, dimethylpolysiloxane and diphenylpolysiloxane.

[0042] The viscosity of the non-crosslinkable silicone oil at 23°C is preferably 50,000 to 150,000 mm ２ / s, and 75,000 to 125,000 mm ２ When the viscosity of the silicone oil at 23° C. is within the above range, the thermal conductivity and insulating properties tend to be further improved.

[0043] The non-crosslinkable silicone oil may be used alone or in combination of two or more types with different structures, viscosities, etc. In this embodiment, the one that ultimately becomes a solid form is referred to as a silicone resin, and the one that is a liquid form is referred to as a silicone oil, and the two are distinguished from each other.

[0044] The content of the non-crosslinkable silicone oil is preferably 0.5 to 8.0% by volume, and more preferably 1.0 to 5.0% by volume, relative to the total volume of the thermally conductive sheet minus the volume of the reinforcing material, i.e., 100% by volume of the resin layer. By keeping the content of the silicone oil within the above range, thermal conductivity and insulating properties tend to be further improved.

[0045] 1.3.5. Surfactant The surfactant is not particularly limited, but for example, a polymer having a silicone (meth)acrylic monomer unit γ is preferred, and a copolymer having a (meth)acrylic monomer unit α having an anionic group, a (meth)acrylic monomer unit β having a cationic group, and a silicone (meth)acrylic monomer unit γ is more preferred. The use of such a surfactant tends to improve the dispersibility of the thermally conductive filler component and further improve thermal conductivity.

[0046] The content of the surfactant is preferably 1.0 to 8.0 volume %, 2.0 to 6.0 volume %, 3.0 to 6.0 volume %, or 4.0 to 5.0 volume % relative to the total volume of the thermally conductive sheet minus the volume of the reinforcing material, i.e., 100 volume % of the resin layer. By having the content of the surfactant within the above range, the dispersibility of the thermally conductive filler component tends to be improved, and thermal conductivity tends to be further improved.

[0047] The surfactant will be described in more detail below. In this embodiment, a "monomer" refers to a monomer having a polymerizable unsaturated bond before polymerization, and a "monomer unit" refers to a repeating unit that constitutes a part of the surfactant after polymerization and is derived from a specific monomer. Furthermore, (meth)acrylic includes acrylic and methacrylic, and (meth)acrylic monomers include (meth)acrylate and (meth)acrylamide. Furthermore, hereinafter, "(meth)acrylic monomer unit α" and the like will also be simply referred to as "unit α" and the like.

[0048] The (meth)acrylic monomer unit α is a repeating unit having an anionic group. The anionic group is not particularly limited, but examples thereof include a carboxy group, a phosphate group, a phenolic hydroxy group, and a sulfonic acid group. Among these, one or more groups selected from the group consisting of a carboxy group, a phosphate group, and a phenolic hydroxy group are preferred. The presence of such a group tends to further improve the dispersibility of the filler component.

[0049] Such a (meth)acrylic monomer is not particularly limited, but examples thereof include acrylic acid, methacrylic acid, acid phosphooxypropyl methacrylate, acid phosphooxypolyoxyethylene glycol monomethacrylate, acid phosphooxypolyoxypropylene glycol monomethacrylate, phosphoric acid-modified epoxy acrylate, 2-acryloyloxyethyl acid phosphate, 2-methacryloyloxyethyl acid phosphate, 4-hydroxyphenyl acrylate, 4-hydroxyphenyl methacrylate, 2-methacryloyloxyethyl succinic acid, and 2-acrylamido-2-methylpropanesulfonic acid.

[0050] The (meth)acrylic monomer unit β is a repeating unit having a cationic group. The cationic group is not particularly limited, but is preferably, for example, one or more selected from the group consisting of a primary amino group, a secondary amino group, a tertiary amino group, and a quaternary ammonium salt. Among these, a tertiary amino group is more preferred. The presence of such a group tends to further improve the dispersibility of the filler component.

[0051] Such (meth)acrylic monomers are not particularly limited, and examples thereof include 1-aminoethyl acrylate, 1-aminopropyl acrylate, 1-aminoethyl methacrylate, 1-aminopropyl methacrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, t-butylaminoethyl (meth)acrylate, dimethylaminoethyl methacrylate quaternary salt, 1,2,2,6,6-pentamethyl-4-piperidyl acrylate, 1,2,2,6,6-pentamethyl-4-piperidyl methacrylate, 2,2,6,6-tetramethyl-4-piperidyl methacrylate, and dimethylaminoethyl acrylate benzyl chloride quaternary salt.

[0052] The (meth)acrylic monomer unit γ is a silicone (meth)acrylic monomer unit, which is a (meth)acrylic monomer that does not contain a cationic group or an anionic group in the molecule but has a silicone group.

[0053] The (meth)acrylic monomer unit γ preferably has a skeleton that has high affinity or compatibility with other matrix components. The (meth)acrylic monomer unit γ has a silicone skeleton such as dimethylsiloxane, methylphenylsiloxane, or diphenylsiloxane. By having such a skeleton, compatibility with other matrix components is improved, and the dispersibility of the filler component in the thermal grease tends to be improved.

[0054] Such a (meth)acrylic monomer is not particularly limited, but examples thereof include (meth)acrylic monomers having a siloxane skeleton, such as α-butyl-ω-(3-methacryloxypropyl)polydimethylsiloxane, etc. The unit γ may be used alone or in combination of two or more.

[0055] 1.4. Reinforcing Material The presence of a reinforcing material in a thermally conductive sheet tends to further improve the mechanical strength of the thermally conductive sheet and improve its handling. The position of the reinforcing material 20 is not particularly limited, but it is preferably located at the center of the thermally conductive sheet in the thickness direction. In other words, it is preferable that the reinforcing material 20 is not present on either surface of the cross section in the thickness direction. This allows resin layers to be disposed on the front and back surfaces of the thermally conductive sheet, and the resin layers contact the heat-generating electronic components and the heat sink, which tends to further improve thermal conductivity and insulation.

[0056] The reinforcing material is not particularly limited, but examples thereof include glass cloth; organic fiber cloth such as cotton, hemp, aramid fiber, cellulose fiber, nylon fiber, and polyolefin fiber; inorganic fiber cloth such as stainless steel, copper, and aluminum; nonwoven fabric; resin film; and metal foil such as copper foil, nickel foil, and aluminum foil. Among these, glass cloth is preferred. The use of such a reinforcing material tends to further improve insulation and thermal conductivity.

[0057] The thickness of the reinforcing material is preferably 10 to 150 μm, 20 to 100 μm, or 30 to 75 μm. When the thickness of the reinforcing material is within the above range, the thermal conductive sheet can be configured to be thin while maintaining mechanical strength, which tends to further improve thermal conductivity.

[0058] 1.5. Thermal Resistance The thermal resistance of the thermally conductive sheet of this embodiment is preferably 0.01 to 0.50° C. / W·mm 2 , 0.02 to 0.20° C. / W·mm, or 0.03 to 0.10° C. / W·mm.

[0059] 2. Manufacturing Method of Thermally Conductive Sheet The manufacturing method of the thermally conductive sheet of the present embodiment is not particularly limited, but may be any method that includes, for example, a resin layer forming step of forming a resin layer from a resin composition containing boron nitride agglomerated particles having an average particle size D50 of 100 μm or more, and a step of pressurizing the resin layer so that the ratio of the average particle size D50 to the sheet thickness (D50 / H) is 0.25 or more.

[0060] In addition, when the thermally conductive sheet has a reinforcing material, the pressurizing step may include forming two resin layers in the resin layer forming step, sandwiching the reinforcing material between the two resin layers, and then heating and pressurizing the resin layers to form the thermally conductive sheet. Furthermore, the method may include a penetration step of applying silicone oil to the surface of the resin layer obtained and allowing it to penetrate.

[0061] 2.1 Resin Layer Forming Step The resin layer forming step is a step of forming a resin layer from a resin composition containing a resin and boron nitride agglomerated particles. The resin composition contains a resin and boron nitride agglomerated particles, and may also contain a solvent such as toluene, a dispersant for the boron nitride agglomerated particles, and the like, as necessary.

[0062] The method for forming the resin layer is not particularly limited, but examples thereof include a method in which a resin composition is applied to a release film such as a PET film and then dried at room temperature or under heat. During this process, volatile components such as solvents contained in the resin composition volatilize, and voids can inevitably occur during the process.

[0063] The coating method is not particularly limited, and known coating methods such as a doctor blade method, a comma coater method, a screen printing method, a roll coater method, etc. Among these, the doctor blade method and the comma coater method are preferred from the viewpoint of thickness accuracy of the resin layer.

[0064] 2.2. Pressurization Process The pressurization process is a process in which a reinforcing material is sandwiched between two resin layers obtained in the resin layer molding process, and then heated and pressurized to form a thermally conductive sheet having resin layers on both sides of the reinforcing material. During this process, at least a portion of the resin component of the resin layer melts or deforms and penetrates into the fibers of the reinforcing material, and voids may inevitably form during this process. Alternatively, a thermally conductive sheet having a resin layer on one side of the reinforcing material may be formed by pressing and heating one resin layer against one side of the reinforcing material.

[0065] The heating and pressurizing conditions are not particularly limited. For example, the heating temperature is preferably 100 to 200°C, and more preferably 125 to 175°C. The pressing pressure is preferably 7.5 to 12.5 MPa, and more preferably 5 to 15 MPa. The heating and pressurizing time is preferably 30 to 90 minutes, and more preferably 45 to 75 minutes.

[0066] 2.3. Penetration process The penetration process involves applying silicone oil to at least one surface of the resulting thermally conductive sheet. This allows the silicone oil to penetrate into the voids inside the thermally conductive sheet, reducing the void ratio. This tends to further improve the breakdown voltage.

[0067] The present invention will be described in more detail below using examples and comparative examples. The present invention is not limited to the following examples. Unless otherwise specified, all operations were carried out at 23°C and 50% humidity.

[0068] Example 1 Boron nitride agglomerated particles 1 (average particle size D50: 110 μm, crushing strength 9 MPa), silicone resin (peroxide-curable silicone, trade name "CF3110" manufactured by Toray Dow Corning Silicone Co., Ltd.), surfactant, toluene, curing agent (2,5-dimethyl-2,5-bis(t-butylperoxy)hexane, trade name "Trigonox 101" manufactured by Kayaku Nouryon Co., Ltd.), and silicone oil (KF-96H-1,000,000 CS, manufactured by Shin-Etsu Chemical Co., Ltd., viscosity 1,000,000 mm ２The resin composition thus obtained was applied to a PET film by a doctor blade method, and the film was left to stand at room temperature for 1 hour to volatilize the toluene, thereby forming a resin layer.

[0069] The surfactant used above was prepared by the following method: First, 100 parts by mass of a (meth)acrylic monomer consisting of 48.4 mol % acrylic acid (manufactured by Toagosei Co., Ltd.), 1.6 mol % 1,2,2,6,6-pentamethyl-4-piperidyl methacrylate (manufactured by ADEKA Corporation, "ADK STAB LA-82"), and 50.0 mol % α-butyl-ω-(3-methacryloxypropyl)polydimethylsiloxane (weight average molecular weight 5,000, manufactured by JNC Corporation, "Silaplane FM-0721") was added to an autoclave equipped with a stirrer. Next, 0.05 parts by mass of azobisisobutyronitrile (manufactured by Tokyo Chemical Industry Co., Ltd.) as an initiator relative to 100 parts by mass of the total of the (meth)acrylic monomers, and 1,000 parts by mass of a mixed solution of toluene (special reagent grade) and 2-propanol (special reagent grade) in a volume ratio of 7:3 were added as a solvent, and the autoclave was purged with nitrogen. The autoclave was then heated in an oil bath at 65°C for 20 hours to carry out radical polymerization. After completion of polymerization, the mixture was degassed under reduced pressure at 120°C for 1 hour to obtain a surfactant copolymer.

[0070] The two resin layers sandwiched a glass cloth reinforcing material (trade name "H47" manufactured by Unitika Ltd.) and heated and pressed at 150°C, 10 MPa, and pressure for 1 hour to form a thermally conductive sheet with a resin layer on both sides of the reinforcing material. The thickness of the resin layers on both sides of the reinforcing material was approximately half the thickness of the thermally conductive sheet.

[0071] Table 2 shows the contents of each component used and the boron nitride agglomerated particles, silicone resin, and silicone oil in the thermally conductive sheet obtained by the above procedure. The contents are shown as the volume obtained by subtracting the volume of the reinforcing material from the total volume of the thermally conductive sheet, i.e., as a percentage of 100% by volume of the resin layer. Furthermore, the amount of curing agent is negligible, and some of it may volatilize during the reaction, so it is not included in the above volume as it is considered to be essentially absent.

[0072] The orientation index of the boron nitride agglomerated particles in the thermally conductive sheet was measured according to the following method. An X-ray diffractometer (manufactured by Rigaku Corporation, product name: "ULTIMA-IV") was used for the measurement. First, a measurement sample was prepared by filling the recess of a deep glass cell attached to the X-ray diffractometer with a thermally conductive sheet. The measurement sample was irradiated with X-rays, and after baseline correction, the peak intensities of the (002) and (100) planes of the measurement sample were determined, and the ratio [I(002) / I(100)] was taken as the orientation index.

[0073] The crushing strength of the boron nitride agglomerated particles was measured in accordance with the description in JIS R 1639-5:2007 "Fine ceramics -- Determination of granule characteristics -- Part 5: Single granule crushing strength." For the measurement, a microcompression tester (manufactured by Shimadzu Corporation, product name "MCT-210") was used. The measurement was performed on 20 or more agglomerated particles, and the value at the cumulative fracture rate of 63.2% was calculated.

[0074] The particle size of the boron nitride agglomerated particles was measured in accordance with the method described in JIS Z 8825:2013, "Particle size analysis - laser diffraction and scattering method." A laser diffraction and scattering particle size distribution analyzer (manufactured by Microtrac-Bell, product name: "Microtrac MT-3300EXII") was used for the measurement. For the measurement, 2 mL of aqueous sodium hexametaphosphate solution and 200 mL of water were placed in a 300 mL beaker, and 60 mg of boron nitride agglomerated particles were added. The mixture was stirred at 100 rpm with a stirrer for 1 minute without treatment with a homogenizer, and the measurement was performed in the presence of agglomerated particles. Water was used as the dispersant, and the refractive index was 1.33. There were 129 channel divisions ranging from 1408 μm to 0.021 μm. The average particle size D50, particle size D10, and particle size D90 were determined from the obtained volume-based cumulative distribution curve.

[0075] The average particle diameters D50, D10 and D90 of the boron nitride agglomerated particles 1 to 5 used in the examples are as follows:

[0076] ​Example 2 A thermally conductive sheet of Example 2 was produced in the same manner as in Example 1, except that boron nitride agglomerated particles 2 (average particle size D50: 120 μm, crushing strength: 1.5 MPa) were used instead of boron nitride agglomerated particles 1 (average particle size D50: 110 μm, crushing strength: 9 MPa), and the coating thickness of the resin composition was reduced to a thickness of 220 μm.

[0077] Example 3 A thermally conductive sheet of Example 3 was produced in the same manner as in Example 2, except that the coating thickness of the resin composition was increased to a thickness of 450 μm.

[0078] Example 4 A thermally conductive sheet of Example 4 was produced in the same manner as in Example 2, except that the coating thickness of the resin composition was reduced to a thickness of 400 μm.

[0079] Example 5 A thermally conductive sheet of Example 4 was produced in the same manner as in Example 1, except that boron nitride agglomerated particles 3 (average particle size D50: 150 μm, crushing strength: 1 MPa) were used instead of boron nitride agglomerated particles 1 (average particle size D50: 110 μm, crushing strength: 9 MPa), and the coating thickness of the resin composition was adjusted to a thickness of 300 μm.

[0080] (Example 6) The thermal conductive sheet of Example 6 was produced in the same manner as in Example 2, except that silicone oil was not used, the amount of silicone resin used was 40.5 vol%, and the coating thickness of the resin composition was adjusted to a thickness of 400 μm.

[0081] Example 7 A thermally conductive sheet of Example 7 was produced in the same manner as in Example 2, except that the coating thickness of the resin composition was adjusted to a thickness of 300 μm.

[0082] Comparative Example 1 A thermally conductive sheet of Comparative Example 1 was produced in the same manner as in Example 1, except that boron nitride agglomerated particles 4 (average particle size D50: 80 μm, crushing strength: 9 MPa) were used instead of boron nitride agglomerated particles 1 (average particle size D50: 110 μm, crushing strength: 9 MPa), and the coating thickness of the resin composition was reduced to a thickness of 200 μm.

[0083] Comparative Example 2 A thermally conductive sheet of Comparative Example 2 was produced in the same manner as in Comparative Example 1, except that the coating thickness of the resin composition was increased to a thickness of 300 μm.

[0084] Comparative Example 3 A thermally conductive sheet of Comparative Example 3 was produced in the same manner as in Example 1, except that no silicone oil was used, the amount of silicone resin used was 40.5 vol%, boron nitride agglomerated particles 5 (average particle size D50: 70 μm, crushing strength 2 MPa) were used instead of boron nitride agglomerated particles 1 (average particle size D50: 110 μm, crushing strength 9 MPa), and the coating thickness of the resin composition and the thickness of the thermally conductive sheet were 300 μm.

[0085] Comparative Example 4 A thermally conductive sheet of Comparative Example 4 was produced in the same manner as in Example 2, except that a mixture prepared by adding 28 parts by mass of JER Cure 114 (curing agent, manufactured by Mitsubishi Chemical Corporation) to 100 parts by mass of JER 806 (epoxy resin, manufactured by Mitsubishi Chemical Corporation) was used instead of the silicone resin and silicone oil, and the coating thickness of the resin composition and the thickness of the thermally conductive sheet were 200 μm.

[0086] (Evaluation) (Thermal Resistance) A thermally conductive sheet was placed between a TO-3 model heater and a copper plate. The thermally conductive sheet was then fastened using screws with a nominal diameter of M3 mm at tightening torques of 20 cN m, 40 cN m, and 60 cN m. The contact area between one TO-3 model heater and the heat dissipation sheet was approximately 6 cm. ２Thereafter, a power supply device (manufactured by Kikusui Electronics Co., Ltd., model number: PMC35-3) was used to apply 15 W of power to the TO-3 model heater. Then, 10 minutes after the application of 15 W of power, the temperature (T1) of the TO-3 model heater and the temperature (T2) of the copper plate were measured, and the thermal resistance was calculated using the following formula: Thermal resistance (°C / W) = (T1 - T2) / 15

[0087] (Breakdown Voltage) The breakdown voltage of the thermally conductive sheet was measured in accordance with JIS C 2110. Specifically, the thermally conductive sheet was processed into a size of 5 cm x 5 cm.

[0088] 25mm diameter cylindrical electrodes were placed so as to sandwich the test sample, and an AC voltage was applied to the test sample. The voltage applied to the test sample was increased from 0V at a rate (500V / s) such that breakdown occurred on average 10 to 20 seconds after the start of voltage application. The voltage at which breakdown occurred was measured for five test samples, and the average value V ５ (kV). And the voltage V ５ The dielectric breakdown voltage (kV / mm) was calculated by dividing the dielectric strength (kV) by the thickness (mm) of the test sample.

[0089] The upper limit of the dielectric breakdown voltage measurement by the above device is 10 kV, and Examples 3 to 7 exceeded this upper limit.

[0090]

[0091] INDUSTRIAL APPLICABILITY The present invention has industrial applicability as a thermally conductive sheet having excellent thermal conductivity and electrical insulation properties.

[0092] 1...thermal conductive sheet, 10...resin layer, 11...large diameter agglomerated particles of boron nitride, 11'...boron nitride agglomerated particles, 20...reinforcing material​

Claims

1. A thermally conductive sheet comprising boron nitride agglomerated particles having an average particle size D50 of 100 μm or more, a sheet thickness H of more than 200 μm, and a ratio of the average particle size D50 to the sheet thickness H (D50 / H) of 0.25 or more.

2. The thermally conductive sheet according to claim 1, wherein the orientation index is 20 or more and 100 or less.

3. The thermally conductive sheet according to claim 1, wherein the boron nitride agglomerated particles have a crushing strength of 5.0 MPa or less.

4. The thermally conductive sheet according to claim 1, which contains a non-crosslinkable silicone oil.

5. The thermally conductive sheet according to claim 1, which contains a silicone resin.

6. The thermally conductive sheet according to claim 1, comprising a reinforcing material and resin layers laminated on the front and back surfaces of the reinforcing material, the resin layers containing the agglomerated boron nitride particles and a resin.

7. The thermally conductive sheet according to claim 1, wherein the particle size D90 of the boron nitride agglomerated particles is 150 to 400 μm.

8. The thermally conductive sheet according to claim 1, wherein the particle diameter D10 of the boron nitride agglomerated particles is 30 to 50 μm.

9. A method for manufacturing a thermally conductive sheet, comprising a pressing step of hot-pressing a resin composition containing boron nitride agglomerated particles having an average particle size D50 of 100 μm or more with glass cloth to form a thermally conductive sheet having a sheet thickness H of more than 200 μm, wherein the ratio of the average particle size D50 to the sheet thickness (D50 / H) is 0.25 or more.

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