Thermal conductive sheet

Abstract

The objective is to provide a thermally conductive sheet with excellent thermal conductivity and insulating properties. [Solution] 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 D50 aspect ratio of the agglomerated boron nitride in the area circle equivalent diameter range of 20 to 100 μm is 1.40 to 1.49.

Description

Technical Field

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

Background Art

[0002] With the miniaturization and high output of heat-generating electronic components such as the CPU (Central Processing Unit) of a personal computer, the amount of heat per unit area generated from these electronic components has become extremely large. The amount of heat reaches about 20 times that of an iron. In order to prevent these heat-generating electronic components from malfunctioning over a long period of time, cooling of the heat-generating electronic components is required. For cooling, a metal heat sink or housing is used. However, when the heat-generating electronic component and the heat sink are brought into direct contact, air may exist microscopically at the interface, which may become an obstacle to heat conduction. Therefore, in order to efficiently transfer heat, the heat-generating electronic component and the heat sink may be arranged with a heat conductive material therebetween.

[0003] Examples of heat conductive materials include heat-curable resins filled with heat conductive fillers, heat conductive spacers, heat conductive sheets, and heat conductive greases. A heat conductive spacer is a sheet having a thickness and high flexibility, and is excellent in following the uneven shape of the mating material. Also, a heat conductive sheet is a relatively thin sheet having excellent handling properties, and can ensure insulation while enhancing the heat conductivity between the heat-generating electronic component and the heat sink. Furthermore, a heat conductive grease is a grease obtained by filling a resin having fluidity with a heat conductive filler, has a high degree of freedom in coating shape, and can also reduce the thermal resistance by applying it thinly.

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

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-126642 [Overview of the project] [Problems that the invention aims to solve]

[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. [Means for solving the problem]

[0008] In other words, the present invention is as follows: [1] It comprises a reinforcing material (glass cloth) and a resin layer laminated on the front and back surfaces of the reinforcing material, The resin layer comprises a resin and aggregated boron nitride. The number-based 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. Heat dissipation sheet. [2] The tap density of the aggregated boron nitride is 0.30 to 1.50 g / cm³. 3 That is, The heat dissipation sheet described in [1]. [3] The crushing strength of the aggregated boron nitride is 1.0 to 6.0 MPa. The heat dissipation sheet described in [1] or [2]. [4] The D50 particle size of the aggregated boron nitride is 10 to 50 μm. A heat dissipation sheet as described in any one of items [1] to [3]. [5] The content of the aggregated boron nitride is 50 to 80% by volume with respect to the total amount of the resin layer. The heat dissipation sheet according to any one of [1] to [4].

Effect of the Invention

[0009] According to the present invention, a thermally conductive sheet excellent in thermal conductivity and insulation can be provided.

Brief Description of the Drawings

[0010] [Figure 1] It is a schematic cross-sectional view showing an example of the thermally conductive sheet of the present embodiment. [Figure 2] It is a schematic conceptual diagram showing the aspect ratio in the range of the equivalent circle diameter of 20 to 100 μm of the area. [Figure 3] It shows the particle size distribution of the aggregated boron nitride used in the thermally conductive sheet of the present embodiment.

Modes for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present invention (hereinafter referred to as "the present embodiment") will be described in detail. However, the present invention is not limited to this, and various modifications can be made without departing from the gist thereof. In the drawings, the same elements will be denoted by the same reference numerals, and redundant explanations will be omitted. Also, the 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 in the drawings are not limited to the illustrated ratios.

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

[0013] FIG. 1 shows a schematic cross-sectional view showing an example of the heat-conductive sheet of the present embodiment. As shown in FIG. 1, the heat-conductive sheet 1 of the present embodiment has a reinforcing material 20 and resin layers 10 laminated on the front and back surfaces of the reinforcing material 20.

[0014] Conventionally, aggregated boron nitride has been used as one of the heat-conductive fillers useful from the viewpoint of improving heat conductivity and the like. However, aggregated boron nitride is not unconditionally excellent in heat conductivity, and improvement in heat conductivity has been achieved by using a plurality of types of aggregated boron nitride having different particle diameters from the viewpoint of close packing.

[0015] In this regard, in the present embodiment, the D50 of the number standard of the aspect ratio in the range of the equivalent circle diameter of the aggregated boron nitride of 20 to 100 μm is defined. That is, attention is paid to the particle group of the aggregated boron nitride in a specific particle diameter range, and its shape (aspect ratio) is defined. Thereby, further improvement in heat conductivity and improvement in insulation are achieved.

[0016] 1.1. Reinforcing Material Since the heat-conductive sheet has a reinforcing material, the mechanical strength of the heat-conductive sheet is further improved, so that the heat-conductive sheet can be made thinner, and the handling property also tends to be further improved. The position of the reinforcing material 20 is not particularly limited. For example, it is preferably located at the center in the thickness direction of the heat-conductive sheet, and when the thickness of the heat-conductive sheet is d, it is preferably located at least in the 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 does not exist in the range of 0.3d from both surfaces in the cross section in the thickness direction. Thereby, resin layers are arranged on the front and back surfaces of the heat-conductive sheet, and since the resin layers are in contact with the heat-generating electronic component and the heat sink, the heat conductivity and the insulation tend to be further improved.

[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 keeping 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 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. Resin The resin is not particularly limited, but examples include silicone resin, silicone rubber, epoxy resin, acrylic resin, phenolic resin, melamine resin, unsaturated polyester, fluororesin, polyimide resin, polyamide-imide resin, polyetherimide resin, polyester resin, and polyphenylene ether resin. 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 20-55% by volume, 25-50% by volume, or 30-45% 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. Agglomerated 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. Such flattened primary boron nitride particles 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 is decreased. 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 aggregated boron nitride particle group 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 terms of the thickness of the resin layer, particles in the range of 20 to 100 μm in area circle equivalent 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 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 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 10-50 μm, 12.5-40 μm, 15-30 μm, or 17.5-25 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 30-150 μm, 35-125 μm, 40-100 μm, or 45-75 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 0.40-1.25 g / cm³. 3 The concentration is 0.45-1.00 g / cm³. 3 Therefore, 0.50~0.90 g / cm³ 3 Therefore, it is 0.55~0.80 g / cm³. 3Therefore, 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 classifying the agglomerated boron nitride. Furthermore, when synthesizing agglomerated 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 agglomerated 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. In addition, regarding the shape of agglomerated boron nitride, by hot-pressing boron nitride with low crystallinity, a sintered body with a certain degree of crystal orientation can be obtained, and by crushing it, agglomerated boron nitride with a high aspect ratio can 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, and 1.7 to 2.8 MPa. When the crushing strength is within the above range, in the pressurization step of the manufacturing method of the thermal conductive sheet described later, the aggregated boron nitride deforms along with the flow of the resin, making it less likely for voids to form. As a result, the thermal resistance in the thickness direction is further reduced, voids do not form, and the insulating properties tend to be further improved.

[0041] In this embodiment, the crushing strength refers to the value measured in accordance with JIS R 1639-5:2007 "Fine ceramics - Method for measuring granular properties - Part 5: Single granular crushing strength". The crushing strength σ (unit: MPa) of a single agglomerated particle is calculated using the formula σ = α × P / (π × d²) from the values ​​of the dimensionless number α (α = 2.48), which changes depending on the position within the agglomerated particle, the crushing test force P (unit: N), and the particle size d (unit: μm). The measurement shall be performed on 20 or more agglomerated particles, and the value at the point of cumulative failure rate of 63.2% shall be calculated. A micro-compression tester can be used for the measurement. For example, the "MCT-210" (product name) manufactured by Shimadzu Corporation can be used as a micro-compression tester.

[0042] The content of aggregated boron nitride is preferably 50-80% by volume, 52-75% by volume, 55-70% by volume, and 57-65% 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 content of aggregated boron nitride is within the above range, the thermal resistance tends to decrease further, void formation is less likely, and insulation performance tends to improve further.

[0043] 1.3.3. Other thermally conductive fillers The thermally conductive sheet of this embodiment may contain other thermally conductive fillers besides aggregated boron nitride. These other thermally conductive fillers are not particularly limited, but examples include aluminum nitride, aluminum oxide, silicon nitride, silicon oxide, zinc oxide, silica, aluminum hydroxide, magnesium oxide, and carbon. Among these, aluminum oxide is preferred. Using such thermally conductive fillers tends to further improve thermal conductivity and insulation.

[0044] Other thermally conductive fillers may be aggregated particles formed by the aggregation of primary particles. For example, using a thermally conductive filler with anisotropic thermal conductivity as aggregated particles improves thermal conductivity through contact between the thermally conductive fillers and facilitates the formation of heat conduction paths in any direction, thus tending to further improve thermal conductivity.

[0045] The content of other thermally conductive fillers is preferably 0 to 20% by volume, 0 to 15% by volume, 0 to 10% by volume, 0 to 5% by volume, and 0 to 3.0% 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. Furthermore, other thermally conductive fillers may not be included.

[0046] 1.3.4. Silane coupling agents 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, preventing void formation, resulting in superior thermal resistance and improved dielectric breakdown voltage. Furthermore, it suppresses delamination and void formation 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, or 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, and the thicker the thermal conductive sheet, the better the insulation tends to be. Furthermore, by including a reinforcing material, the mechanical strength of the thermal conductive sheet can be maintained, allowing the thermal conductive sheet to be made even thinner.

[0051] 1.5.Thermal resistance The thermal resistance of the thermally conductive sheet in 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 the thermally conductive sheet of 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 then heated and pressurized to form a thermally conductive sheet.

[0053] 2.1.Resin layer molding process The resin layer molding process is a process of forming 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. [Examples]

[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 then 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 with particle sizes listed in Table 1 below were obtained in the same manner as in Preparation Example 1, except for changes in the crushing and classification conditions.

[0061] The D10, D50, and D90 aspect ratios of agglomerated boron nitride in the area circle equivalent diameter range of 20–100 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 called "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 for area circle equivalent diameters of 20–100 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 division was 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 load. 3 The 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. For the measurement, a "powder tester" (product name) manufactured by Hosokawa Micron was used.

[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] [Table 1]

[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 aggregated 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 using the same procedure as in Example 1.

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

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

[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 φ25mm 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 0V at a rate (500V / s) such that dielectric breakdown occurred an average of 10-20 seconds after the start of voltage application. The voltage at which dielectric breakdown occurred was measured for five test samples, and the average value V5(kV) was obtained. Then, the dielectric breakdown voltage (kV / mm) was calculated by dividing the voltage V5(kV) by the thickness (mm) of the test sample.

[0075] [Table 2] *In the table, [vol%] indicates the value of each component when the total of components other than the silane coupling agent is set to 100 volume%.

[0076] Except for setting the resin layer thickness to 150 μm, thermal conductive sheets for Examples 1' to 3' and Comparative Example 1' were prepared in the same manner as Examples 1 to 3 and Comparative Example 1, with a thickness of 300 μm, respectively. Then, the thermal resistance and dielectric breakdown voltage were measured in the same manner as above. The results are shown in Table 3 below. As the thickness of the thermal conductive sheet increases to 0.3 mm, aggregated boron nitride with larger particle sizes becomes advantageous in terms of thermal resistance, and the insulating properties tend to increase as well. The application limit for dielectric breakdown voltage measurement was 10 kV, and samples that did not exhibit dielectric breakdown even at 10 kV were evaluated as "exceeding the measurement limit."

[0077] [Table 3] [Industrial applicability]

[0078] The present invention has industrial applicability as a thermally conductive sheet with excellent thermal conductivity and insulation properties. [Explanation of symbols]

[0079] 1…Thermal conductive sheet, 10…Resin layer, 20…Reinforcement material

Claims

1. It comprises a reinforcing material and resin layers laminated on the front and back surfaces of the reinforcing material, The resin layer comprises a resin and aggregated boron nitride. The number-based 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. Heat dissipation sheet.

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

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

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

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

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