Thermally conductive sheet

Abstract

A heat dissipation sheet comprises a reinforcing material, and resin layers that are laminated on the obverse and reverse surfaces of the reinforcing material. The resin layers contain a resin and agglomerated boron nitride. The D50 aspect ratio of the agglomerated boron nitride, for which the area circle equivalent diameter is within the range of 20-100 μm, is 1.40-1.49.

Description

Thermal conductive sheet

[0001] This invention relates to a thermally conductive sheet.

[0002] With the miniaturization and increased power output of heat-generating electronic components such as the CPU (Central Processing Unit) in personal computers, the amount of heat generated per unit area from these components has become extremely large. This amount of heat can reach about 20 times that of an iron. To prevent these heat-generating electronic components from failing over the long term, cooling of the heat-generating components is necessary. Metal heat sinks and enclosures are used for cooling, but if the heat-generating electronic component and the heat sink are in direct contact, microscopic air may be present at the interface, which can hinder heat conduction. Therefore, to efficiently transfer heat, the heat-generating electronic component and the heat sink are sometimes placed with a thermally conductive material in between.

[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 sheets that are thick and highly flexible, and have excellent conformability to the uneven shape of the mating material. Thermally conductive sheets are relatively thin sheets with excellent handling properties, and can enhance the thermal conductivity between heat-generating electronic components and heat sinks while ensuring insulation. Furthermore, thermally conductive greases are greases made by filling a fluid resin with a thermally conductive filler, offering a high degree of freedom in application shape, and can also reduce thermal resistance by applying them thinly.

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

[0005] Japanese Patent Publication No. 2022-126642

[0006] With the rapid miniaturization, integration, and increased power output of electronic components in recent years, the operating temperatures of heat-generating electronic components have risen, and high voltages are now applied between these components and heat sinks. Therefore, there is a need for further improvements in thermal conductivity and insulation, especially for thinly molded thermal conductive sheets.

[0007] This invention has been made in view of the above-mentioned problems, and aims to provide a thermally conductive sheet with excellent thermal conductivity and insulating properties.

[0008] In other words, the present invention is as follows: [1] A heat dissipation sheet comprising a reinforcing material (glass cloth) and a resin layer laminated on the front and back surfaces of the reinforcing material, wherein the resin layer contains resin and agglomerated boron nitride, and the number-based D50 aspect ratio of the agglomerated boron nitride in the area circle equivalent diameter range of 20 to 100 μm is 1.10 to 1.49. [2] The tap density of the agglomerated boron nitride is 0.30 to 1.50 g / cm³. ３ The heat dissipation sheet described in [1]. [3] The heat dissipation sheet described in [1] or [2], wherein the crush strength of the aggregated boron nitride is 1.0 to 6.0 MPa. [4] The heat dissipation sheet described in any one of [1] to [3], wherein the D50 particle size of the aggregated boron nitride is 10 to 50 μm. [5] The heat dissipation sheet described in any one of [1] to [4], wherein the content of the aggregated boron nitride is 50 to 80 volume% of the total amount of the resin layer.

[0009] According to the present invention, it is possible to provide a thermally conductive sheet with excellent thermal conductivity and insulating properties.

[0010] This is a schematic cross-sectional view showing an example of the thermally conductive sheet of this embodiment. This is a schematic conceptual diagram showing the aspect ratio in the range of area circle equivalent diameter 20 to 100 μm. This shows the particle size distribution of aggregated boron nitride used in the thermally conductive sheet of this embodiment.

[0011] The following describes in detail embodiments of the present invention (hereinafter referred to as "these embodiments"), but the present invention is not limited thereto, and various modifications are possible without departing from its essence. In the drawings, the same elements are denoted by the same reference numerals, and redundant explanations are omitted. Furthermore, unless otherwise specified, positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings. Moreover, the dimensional ratios in the drawings are not limited to those shown.

[0012] 1. Thermally conductive sheet The thermally conductive sheet of this embodiment has a reinforcing material and a resin layer laminated on the front and back surfaces of the reinforcing material, wherein the resin layer contains resin and aggregated boron nitride, and the D50 of the aspect ratio based on the number of aggregated boron nitride particles in the area circle equivalent diameter range of 20 to 100 μm is 1.10 to 1.49.

[0013] Figure 1 shows a schematic cross-sectional view illustrating an example of the thermal conductive sheet of this embodiment. As shown in Figure 1, the thermal conductive sheet 1 of this embodiment has a reinforcing material 20 and a resin layer 10 laminated on the front and back surfaces of the reinforcing material 20.

[0014] Conventionally, aggregated boron nitride has been used as a useful thermally conductive filler from the perspective of improving thermal conductivity. However, aggregated boron nitride does not unconditionally have excellent thermal conductivity, and improvements in thermal conductivity have been made by using multiple types of aggregated boron nitride with different particle sizes from the perspective of close packing.

[0015] In this embodiment, the D50, which is the number-based aspect ratio of aggregated boron nitride particles in the area circle equivalent diameter range of 20 to 100 μm, is defined. That is, the particle group of aggregated boron nitride particles in a specific particle size range is considered, and its shape (aspect ratio) is defined. This aims to further improve thermal conductivity and insulation.

[0016] 1.1. Reinforcement The presence of a reinforcing material in the thermal conductive sheet improves the mechanical strength of the thermal conductive sheet, making it possible to construct a thinner thermal conductive sheet and also tending to improve handling. The position of the reinforcing material 20 is not particularly limited, but for example, it is preferably located at the center in the thickness direction of the thermal conductive sheet, and when the thickness of the thermal conductive sheet is d, it is preferably located at least within a range of ±0.2d from the center in the cross-section in the thickness direction. In other words, it is preferable that the reinforcing material 20 is not located within a range of 0.3d from both surfaces in the cross-section in the thickness direction. As a result, resin layers are arranged on the front and back surfaces of the thermal conductive sheet, and the resin layers come into contact with the heat-generating electronic components and the heat sink, which tends to improve thermal conductivity and insulation.

[0017] While not particularly limited, examples of reinforcing materials include glass cloth; organic fiber cloths such as cotton, hemp, aramid fibers, cellulose fibers, nylon fibers, polyolefin fibers; inorganic fiber cloths such as stainless steel, copper, and aluminum; nonwoven fabrics; resin films; and metal foils such as copper foil, nickel foil, and aluminum foil. Among these, glass cloth is preferred. Using such reinforcing materials tends to further improve insulation and thermal conductivity.

[0018] The thickness of the reinforcing material is preferably 10 to 150 μm, 20 to 100 μm, or 30 to 75 μm. By having the thickness of the reinforcing material within the above range, the thermal conductive sheet can be made thinner while maintaining mechanical strength, which tends to improve thermal conductivity.

[0019] 1.2. Resin Layer The resin layer 10 comprises a resin and agglomerated boron nitride, and may optionally contain silicone oil and a silane coupling agent. The resin layer may also further contain thermally conductive fillers other than agglomerated boron nitride.

[0020] The thickness of the resin layer is preferably 50 to 250 μm, 75 to 200 μm, or 100 to 150 μm. A thinner resin layer tends to improve thermal conductivity, while a thicker resin layer tends to improve insulation. Furthermore, the presence of a reinforcing material maintains the mechanical strength of the thermally conductive sheet, allowing the resin layer to be made even thinner. Note that, as described later, the thickness of the resin layer refers to the thickness of each individual resin layer when a reinforcing material is used and the resin layer exists on both sides of the reinforcing material.

[0021] 1.2.1. Resins The resins are not particularly limited, but examples include silicone resins, silicone rubbers, epoxy resins, acrylic resins, phenolic resins, melamine resins, unsaturated polyesters, fluororesins, polyimide resins, polyamide-imide resins, polyetherimide resins, polyester resins, polyphenylene ether resins, etc. These resins may be used individually or in combination of two or more.

[0022] Among these, silicone resins and silicone rubbers are preferred. These silicone resins and silicone rubbers may be peroxide-cured, condensation-cured, addition-cured, or UV-cured, and may also be in their cured state. Using such resins tends to improve the mechanical strength and flexibility of the thermally conductive sheet, enhance handling, and further improve thermal conductivity and insulation. A curing agent corresponding to each curing type may also be used.

[0023] The resin content is preferably 20 to 55% by volume, 25 to 50% by volume, or 30 to 45% by volume relative to the total volume of the thermal conductive sheet minus the volume of the reinforcing material, i.e., 100% by volume of the resin layer. When the resin content is within the above range, the thermal conductivity and insulation properties tend to be further improved.

[0024] 1.2.2. Aggregated Boron Nitride In this embodiment, aggregated boron nitride having a predetermined particle shape and particle size is used. "Aggregation" refers to the state of secondary particles formed by the aggregation of primary particles. Primary boron nitride particles have a flattened shape, and tend to have poor thermal conductivity in the thickness direction and excellent thermal conductivity in the plane direction. Primary boron nitride particles with such a flattened shape tend to be oriented in the plane direction. By using boron nitride, which has anisotropy in terms of thermal conductivity, as aggregated particles, thermal conductivity is improved by contact between boron nitride particles, and it becomes easier to form heat conduction paths in any direction, thus tending to further improve thermal conductivity. In addition, by using aggregated particles, the proportion of primary particles lying flat in the plane direction of the thermal conductive sheet is reduced, and the thermal resistance in the thickness direction decreases. The secondary particles may be spherical or irregularly shaped lumps.

[0025] Figure 2 shows a schematic diagram representing the aspect ratio of aggregated boron nitride in the area circle equivalent diameter range of 20 to 100 μm. By measuring the area circle equivalent diameter and aspect ratio for each individual aggregated boron nitride particle, plot data like that shown in Figure 2 (left) can be obtained. From this data, the dispersion distribution and cumulative distribution of the aspect ratio based on the number of particles can be determined for the group of aggregated boron nitride particles contained in the area circle equivalent diameter range of 20 to 100 μm (Figure 2 (right)). This allows us to determine the D50 aspect ratio based on the number of particles. The D50 aspect ratio based on the number of particles is the aspect ratio value that corresponds to 50% in the cumulative distribution shown in Figure 2. Similarly, the D10 aspect ratio and D90 aspect ratio based on the number of particles represent the values ​​at which the cumulative distribution reaches 10% and 90%, respectively, in the cumulative distribution curve based on the number of particles.

[0026] In relation to the thickness of the resin layer, particles in the range of 20 to 100 μm in area circle diameter are classified as considerably large. Because these particles are large, they can efficiently transfer heat in the thickness direction, thus contributing to improved thermal conductivity. However, if the aspect ratio is high and the shape is distorted, the length in the long axis direction becomes close to the thickness of the resin layer, resulting in a particle size that reduces insulation.

[0027] From the above viewpoint, in this embodiment, the D50 aspect ratio of the aggregated boron nitride in the area circle equivalent diameter range of 20 to 100 μm is 1.10 to 1.49, preferably 1.15 to 1.48, 1.20 to 1.47, 1.25 to 1.46, and 1.30 to 1.45. When the D50 aspect ratio in the area circle equivalent diameter range of 20 to 100 μm is within the above range, the thermal conductivity and insulating properties tend to be further improved.

[0028] The D10 aspect ratio of aggregated boron nitride in the area circle equivalent diameter range of 20 to 100 μm is preferably 1.05 to 1.35, 1.10 to 1.25, and 1.15 to 1.19. When the D10 aspect ratio is within the above range, the thermal conductivity and insulating properties tend to be further improved.

[0029] The D90 aspect ratio of aggregated boron nitride in the area circle equivalent diameter range of 20 to 100 μm is preferably 1.50 to 2.10, 1.65 to 2.05, and 1.80 to 2.00. When the D90 aspect ratio is within the above range, the thermal conductivity and insulating properties tend to be further improved.

[0030] The D90 aspect ratio / D10 aspect ratio is preferably 1.55 to 1.75, 1.60 to 1.80, or 1.65 to 1.85. When the D90 aspect ratio / D50 aspect ratio is within the above range, thermal conductivity and insulation tend to be further improved.

[0031] The D90 aspect ratio / D50 aspect ratio is preferably 1.20 to 1.50, 1.25 to 1.45, and 1.30 to 1.40. When the D90 aspect ratio / D50 aspect ratio is within the above range, thermal conductivity and insulation tend to be further improved.

[0032] The D10 particle size of the aggregated boron nitride is preferably 1.0 to 20 μm, 2.5 to 17.5 μm, 5.0 to 15 μm, or 7.5 to 12.5 μm. When the D10 particle size is within the above range, the thermal conductivity and insulating properties tend to be further improved.

[0033] The D50 particle size of aggregated boron nitride is preferably 10 to 50 μm, 12.5 to 40 μm, 15 to 30 μm, or 17.5 to 25 μm. When the D50 particle size is within the above range, the thermal conductivity and insulating properties tend to be further improved.

[0034] The D90 particle size of aggregated boron nitride is preferably 30 to 150 μm, 35 to 125 μm, 40 to 100 μm, or 45 to 75 μm. When the D90 particle size is within the above range, the thermal conductivity and insulating properties tend to be further improved.

[0035] The D90 particle diameter / D10 particle diameter of aggregated boron nitride is 2.0 to 12.5, preferably 3.0 to 10, 4.0 to 7.5, and 4.5 to 7.0. When the D90 particle diameter / D10 particle diameter is within the above range, the thermal conductivity and insulating properties tend to be further improved.

[0036] In this embodiment, the D50 particle diameter refers to the median diameter, which is the value at which the cumulative distribution reaches 50% in the volume-based cumulative distribution curve. Similarly, the D10 particle diameter and D90 particle diameter refer to the values ​​at which the cumulative distribution reaches 10% and 90%, respectively, in the volume-based cumulative distribution curve.

[0037] The tap density is preferably 0.30 to 1.50 g / cm³. ３ The concentration is 0.40 to 1.25 g / cm³. ３ The concentration is 0.45 to 1.00 g / cm³. ３ The concentration is 0.50 to 0.90 g / cm³. ３ The concentration is 0.55 to 0.80 g / cm³. ３ Therefore, when the tap density is within the above range, the packing efficiency of the aggregated boron nitride is further improved, and the thermal conductivity and insulating properties tend to be further enhanced.

[0038] Tap density is calculated by placing a certain weight of powder in a container, tapping the container to fill the gaps between particles, and dividing the result by the volume of that container. It can be measured in accordance with JIS R 1628:1997.

[0039] The D10 aspect ratio, D50 aspect ratio, D90 aspect ratio, D10 particle size, D50 particle size, D90 particle size, and tap density can be arbitrarily adjusted by, for example, classifying the aggregated boron nitride. When synthesizing aggregated boron nitride using boron carbide as a raw material, the shape of the boron carbide used becomes the shape of the boron nitride, and the particle size becomes approximately twice that of the raw material. Therefore, by selecting the shape and particle size of the boron carbide and then synthesizing the aggregated boron nitride, the D10 aspect ratio, D50 aspect ratio, D90 aspect ratio, D10 particle size, D50 particle size, D90 particle size, and tap density can also be adjusted. Regarding the shape of the aggregated boron nitride, by hot pressing boron nitride with low crystallinity, a sintered body with a certain degree of alignment of crystal orientations can be obtained, and by pulverizing it, aggregated boron nitride with a high aspect ratio can also be obtained.

[0040] The crushing strength is preferably 1.0 to 6.0 MPa, 1.2 to 5.0 MPa, 1.5 to 4.0 MPa, or 1.7 to 2.8 MPa. When the crushing strength is within the above range, in the pressing step of the method for manufacturing a thermal conductivity sheet described below, the aggregated boron nitride is deformed along with the flow of the resin, and voids are less likely to occur. Therefore, the thermal resistance in the thickness direction is further reduced, no voids occur, and the insulation property tends to be further improved.

[0041] The crushing strength in the present embodiment means a value measured in accordance with the description of JIS R 1639-5:2007 "Fine Ceramics - Measurement Methods for Particle Characteristics - Part 5: Single Particle Crushing Strength". The crushing strength σ (unit: MPa) of one aggregated particle is calculated using the formula σ = α × P / (π × d2) from the values of the dimensionless number α (α = 2.48), which varies depending on the position within the aggregated particle, the crushing test force P (unit: N), and the particle size d (unit: μm). The measurement is performed on 20 or more aggregated particles, and the value at the time of a cumulative failure rate of 63.2% is calculated. For the measurement, a micro compression tester can be used. As the micro compression tester, for example, "MCT-210" (trade name) manufactured by Shimadzu Corporation can be used.

[0042] The content of aggregated boron nitride is preferably 50 to 80% by volume, 52 to 75% by volume, 55 to 70% by volume, and 57 to 65% by volume with respect to the volume obtained by subtracting the volume of the reinforcing material from the total volume of the heat conductive sheet, that is, 100% by volume of the resin layer. When the content of aggregated boron nitride is within the above range, the thermal resistance is further reduced, voids are less likely to occur, and the insulation property tends to be further improved.

[0043] 1.3.3. Other heat conductive fillers The heat conductive sheet of the present embodiment may have other heat conductive fillers other than aggregated boron nitride. The other heat conductive fillers are not particularly limited, and examples thereof include aluminum nitride, aluminum oxide, silicon nitride, silicon oxide, zinc oxide, silica, aluminum hydroxide, magnesium oxide, carbon, and the like. Among these, aluminum oxide is preferable. By using such heat conductive fillers, the thermal conductivity and the insulation property tend to be further improved.

[0044] The other heat conductive fillers may be aggregated particles in which primary particles are aggregated. For example, by using a heat conductive filler having anisotropy in thermal conductivity as the aggregated particles, in addition to improving the thermal conductivity due to the contact between the heat conductive fillers, it becomes easier to form a heat conduction path in an arbitrary direction, and thus the thermal conductivity tends to be further improved.

[0045] The content of the other heat conductive fillers is preferably 0 to 20% by volume, preferably 0 to 15% by volume, preferably 0 to 10% by volume, preferably 0 to 5% by volume, and preferably 0 to 3.0% by volume with respect to the volume obtained by subtracting the volume of the reinforcing material from the total volume of the heat conductive sheet, that is, 100% by volume of the resin layer. Also, the other heat conductive fillers may not be included.

[0046] 1.3.4. Silane Coupling Agent The thermally conductive sheet of this embodiment may contain a silane coupling agent. This further improves the adhesion between the resin and the agglomerated boron nitride, prevents void formation, provides excellent thermal resistance, and improves the dielectric breakdown voltage. In addition, it suppresses the occurrence of delamination and voids on the surface of the resin and agglomerated boron nitride due to the difference in thermal expansion coefficients between the resin and agglomerated boron nitride at high temperatures and during heat cycles, which tends to further improve long-term reliability.

[0047] Silane coupling agents are not particularly limited, but examples include alkyl silane coupling agents such as decyltrimethoxysilane; epoxy silane coupling agents such as 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; amino silane coupling agents such as 3-aminopropyltriethoxysilane, 3-(2-aminoethyl)aminopropyltrimethoxysilane, 3-(2-aminoethyl)aminopropyltrimethoxysilane, and 3-aminopropyltrimethoxysilane; mercapto silane coupling agents such as 3-mercaptopropyltrimethoxysilane and 3-mercaptotriethoxysilane; ureido silane coupling agents such as 3-ureidopropyltriethoxysilane; and siloxane silane coupling agents containing a polyorganosiloxane skeleton. Silane coupling agents may be used individually or in combination of two or more types.

[0048] Among these, coupling agents containing a polyorganosiloxane skeleton are preferred. By using such a silane coupling agent, the adhesion between the resin and the aggregated boron nitride is further improved, and the tensile shear fracture strength can be further improved. As a result, voids are not formed, thermal resistance is excellent, and the dielectric breakdown voltage tends to be further improved.

[0049] The silane coupling agent content is preferably 0.001 to 1.0% by mass, 0.003 to 0.5% by mass, and 0.005 to 0.1% by mass, relative to 100% by mass of aggregated boron nitride. When the silane coupling agent content is within the above range, voids are not formed, thermal resistance is excellent, and the dielectric breakdown voltage tends to improve.

[0050] 1.4. Thickness The thickness of the thermal conductive sheet in this embodiment is preferably 100 to 800 μm, 125 to 600 μm, or 150 to 400 μm. The thinner the thermal conductive sheet, the better the thermal conductivity tends to be, and the thicker the thermal conductive sheet, the better the insulation tends to be. Furthermore, by having a reinforcing material, the mechanical strength of the thermal conductive sheet can be maintained, so the thickness of the thermal conductive sheet can be made even thinner.

[0051] 1.5. Thermal Resistance The thermal resistance of the thermal conductive sheet of this embodiment is preferably 0.01 to 0.90°C / W·mm, 0.02 to 0.8°C / W·mm, and 0.03 to 0.70°C / W·mm.

[0052] 2. Method for Manufacturing a Thermally Conductive Sheet The method for manufacturing a thermally conductive sheet according to this embodiment is not particularly limited, but may include, for example, a resin layer molding step in which a resin layer is formed using a resin composition containing a resin and a thermally conductive filler, and a pressurizing step in which a reinforcing material is sandwiched between the two resin layers formed in the resin layer molding step, and a thermally conductive sheet is formed by heating and pressurizing.

[0053] 2.1. Resin Layer Molding Process The resin layer molding process is a process of molding a resin layer using a resin composition containing a resin and a thermally conductive filler. The resin composition contains a resin and a thermally conductive filler, and may optionally contain a solvent such as toluene, a dispersant for the thermally conductive filler, etc.

[0054] The method for forming the resin layer is not particularly limited, but one example is to apply a resin composition onto a release film such as a PET film and dry it at room temperature or under heating.

[0055] The coating method is not particularly limited, but known coating methods such as the doctor blade method, comma coater method, screen printing method, and roll coater method can be used. Among these, the doctor blade method and comma coater method are preferred from the viewpoint of accuracy of the resin layer thickness.

[0056] 2.2. Pressurization Process The pressurization process involves sandwiching a reinforcing material between two resin layers obtained in the resin layer molding process, heating, and pressurizing to form a heat-conductive sheet with resin layers on both sides of the reinforcing material. During this process, at least a portion of the resin components of the resin layers melt or deform and penetrate into the fibers of the reinforcing material, but voids may inevitably be created during this process. Alternatively, a heat-conductive sheet with a resin layer on one side of the reinforcing material may be formed by pressurizing and heating one resin layer against one side of the reinforcing material.

[0057] The heating and pressurizing conditions are not particularly limited. For example, the heating temperature is preferably 100 to 200°C and 125 to 175°C. The pressurizing pressure is preferably 7.5 to 12.5 MPa and 5 to 15 MPa. Furthermore, the heating and pressurizing time is preferably 30 to 90 minutes and 45 to 75 minutes.

[0058] The present invention will be described in more detail below using examples and comparative examples. The present invention is not limited in any way by the following examples. Unless otherwise specified, each operation was carried out under conditions of 23°C and 50% humidity.

[0059] (Preparation Example 1) Boric acid, melamine, and calcium carbonate (all reagent grade) were mixed in a mass ratio of 70:50:5. The mixture was heated in a nitrogen gas atmosphere from room temperature to 1400°C in 1 hour, held at 1400°C for 3 hours, then heated to 1900°C in 4 hours, held at 1900°C for 2 hours, and then cooled to room temperature to produce hexagonal boron nitride. This mixture was crushed, pulverized, and sieved to obtain aggregated boron nitride BN1 having the particle sizes listed in Table 1 below.

[0060] (Preparation Examples 2-4) Aggregated boron nitride BN2-4 having the particle sizes listed in Table 1 below were obtained in the same manner as in Preparation Example 1, except that the crushing and classification conditions were changed.

[0061] The D10, D50, and D90 aspect ratios of agglomerated boron nitride in the area circle equivalent diameter range of 20 to 100 μm were determined by simultaneously measuring the particle size distribution by laser diffraction scattering and the area circle equivalent diameter and aspect ratio using dynamic image analysis, using a Microtrac-Bell product, "SYNC". For the measurement, 2 mL of sodium hexametaphosphate aqueous solution and 200 mL of water were placed in a 300 mL beaker, 60 mg of agglomerated boron nitride was added, and the mixture was stirred at 100 rpm for 1 minute using a stirrer without homogenization, and the measurement was performed while agglomerated particles were present. Water was used as the dispersant, with a refractive index of 1.33. From the obtained particle image data, data with an area circle equivalent diameter of 20 to 100 μm was extracted, and the aspect ratios D10, D50, and D90 were determined from the cumulative distribution curve based on the number of particles.

[0062] The particle size of agglomerated boron nitride was measured in accordance with the method described in JIS Z 8825:2013 "Particle Size Analysis - Laser Diffraction and Scattering Method". A laser diffraction scattering particle size analyzer (Microtrac-Bell, product name: "SYNC") was used for the measurement. For the measurement, 2 mL of sodium hexametaphosphate aqueous solution and 200 mL of water were placed in a 300 mL beaker, 60 mg of agglomerated boron nitride was added, and the mixture was stirred at 100 rpm for 1 minute without homogenization, and the measurement was performed while agglomerated particles were present. Water was used as the dispersant, with a refractive index of 1.33. The channel divisions were 129 divisions, ranging from 1408 μm to 0.021 μm. From the obtained volume-based cumulative distribution curve, the D50 particle size, D10 particle size, and D90 particle size were determined. Figure 3 shows the particle size distribution of agglomerated boron nitride.

[0063] The tap density of aggregated boron nitride was determined in accordance with the method described in JIS R 1628:1997 "Method for Measuring the Bulk Density of Fine Ceramic Powders". Specifically, aggregated boron nitride powder was subjected to a 100 cm³ test. ３The material was filled into a dedicated container, and after tapping under the conditions of a tapping time of 180 seconds, 180 taps, and a tap lift of 18 mm, the bulk density was measured, and the obtained value was defined as the tap density. A "powder tester" (product name) manufactured by Hosokawa Micron was used for the measurement.

[0064] The crushing strength of aggregated boron nitride was measured in accordance with JIS R 1639-5:2007 "Fine ceramics - Method for measuring grain properties - Part 5: Single grain crushing strength". A microcompression tester (manufactured by Shimadzu Corporation, product name "MCT-210") was used for the measurement. The measurement was performed on 20 or more aggregated particles, and the value was calculated at a cumulative fracture rate of 63.2%.

[0065]

[0066] (Example 1) A resin composition was obtained by mixing aggregated boron nitride BN1, silicone resin (peroxide-curable silicone, manufactured by Toray Dow Corning Silicone Co., Ltd., trade name "CF3110"), silane coupling agent (decyltrimethoxysilane, manufactured by Dow Toray Corporation, trade name "Z-6210"), curing agent (2,5-dimethyl-2,5-di(t-butylperoxy)hexane, manufactured by Kayaku Nurion Co., Ltd., trade name "Trigonox 101"), and toluene.

[0067] The obtained resin composition was coated onto a PET film using the doctor blade method, and left to stand at room temperature for 1 hour to allow toluene to evaporate, forming a resin layer with a thickness of 100 μm.

[0068] A glass cloth (product name "H25" manufactured by Unitika Corporation), which serves as a reinforcing material, was sandwiched between the two resulting resin layers. The mixture was then heated and pressurized at 150°C, 10 MPa, and for 1 hour to form a thermally conductive sheet with resin layers on both sides of the reinforcing material. The thickness of the resulting thermally conductive sheet was 200 μm.

[0069] Table 2 shows the components used and their respective contents. The contents of components other than the silane coupling agent are shown as the volume obtained by subtracting the volume of the reinforcing material from the total volume of the silane coupling agent thermal conductive sheet, i.e., as a percentage of 100% of the resin layer volume. The amount of silane coupling agent used was 0.01% by mass relative to 100% by mass of the total amount of agglomerated boron nitride. The amount of curing agent was 1% by mass relative to 100% by mass of the total amount of silicone resin.

[0070] (Examples 2-3, Comparative Example 1) Except for using aggregated boron nitride BN2-4 instead of aggregated boron nitride BN1, the thermally conductive sheets of Examples 2-3 and Comparative Example 1 were prepared by the same procedure as in Example 1.

[0071] (Evaluation) (Thermal Resistance) A thermal conductive sheet was placed between the TO-3 model heater and the copper plate. The thermal conductive sheet was then tightened with a screw of nominal diameter M3 mm to a tightening torque of 0.5 N·m. The contact area between one TO-3 model heater and the heat dissipation sheet was approximately 6 cm². Subsequently, 15 W of power was applied to the TO-3 model heater using a power supply (manufactured by Kikusui Electronics Co., Ltd., model number: PMC35-3). The temperature of the TO-3 model heater (T1) and the temperature of the copper plate (T2) were measured 10 minutes after the application of 15 W of power, and the thermal resistance was calculated using the following formula: Thermal resistance (°C / W) = (T1 - T2) / 15

[0072] The thermal resistance per unit thickness (°C / W·mm) was calculated by dividing the obtained thermal resistance by the sheet thickness.

[0073] (Dielectric Breakdown Voltage) The dielectric breakdown voltage of the thermal conductive sheet was measured in accordance with JIS C 2110. Specifically, the thermal conductive sheet was processed to a size of 5 cm x 5 cm.

[0074] A φ25 mm columnar electrode was positioned 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 0 V at a rate (500 V / s) such that dielectric breakdown occurred an average of 10 to 20 seconds after the start of voltage application. The voltage at which dielectric breakdown occurred was measured for five test samples, and the average value V was calculated. ５(kV) was obtained. Then, the voltage V ５ (kV) was divided by the thickness (mm) of the test sample to calculate the breakdown voltage (kV / mm).

[0075] ※ In the table, [vol%] indicates the value of each component when the total other than the silane coupling agent is 100% by volume.

[0076] Except that the thickness of the resin layer was set to 150 μm, in the same manner as in Examples 1 to 3 and Comparative Example 1, heat conductive sheets of Examples 1' to 3' and Comparative Example 1' with a thickness of 300 μm were produced, respectively. Then, the thermal resistance and breakdown voltage were measured in the same manner as above. The results are shown in Table 3 below. As the thickness of the heat conductive sheet increases to 0.3 mm, aggregated boron nitride with a large particle size becomes advantageous in terms of thermal resistance and tends to have higher insulation. The applied limit during the breakdown voltage measurement was 10 kV, and those for which breakdown did not occur even at 10 kV were evaluated as "exceeding the measurement limit".

[0077]

[0078] The present invention has industrial applicability as a heat conductive sheet excellent in heat conductivity and insulation.

[0079] 1... Heat conductive sheet, 10... Resin layer, 20... Reinforcing material

Claims

1. A heat dissipation sheet comprising a reinforcing material and a resin layer laminated on the front and back surfaces of the reinforcing material, wherein the resin layer contains resin and agglomerated boron nitride, and the number-based D50 aspect ratio of the agglomerated boron nitride in the area circle equivalent diameter range of 20 to 100 μm is 1.10 to 1.

49.

2. The tap density of the aggregated boron nitride is 0.30 to 1.50 g / cm³. ３ The heat dissipation sheet according to claim 1.

3. The heat dissipation sheet according to claim 1, wherein the crushing strength of the aggregated boron nitride is 1.0 to 6.0 MPa.

4. The heat dissipation sheet according to claim 1, wherein the D50 particle size of the aggregated boron nitride is 10 to 50 μm.

5. The heat dissipation sheet according to claim 1, wherein the content of the aggregated boron nitride is 50 to 80% by volume relative to the total amount of the resin layer.

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