Heat conductive sheet and manufacturing method thereof

Abstract

To provide a heat conductive sheet capable of reducing a decrease in thermal conductivity when a load is increased.SOLUTION: A heat conductive sheet 1 includes a binder resin, a scaly first heat conductive filler, and a non-scaly second heat conductive filler, and the first heat conductive filler and the second heat conductive filler are dispersed in the binder resin, the first heat conductive filler is boron nitride, and the average particle size of the second heat conductive filler is 5 μm or less, the long axis of the first heat conductive filler is oriented in the thickness direction of the heat conductive sheet, and the short axis of the first heat conductive filler is randomly oriented in the in-plane direction of the heat conductive sheet.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present technology relates to a thermally conductive sheet and a method for manufacturing the thermally conductive sheet. [Background technology]

[0002] As electronic devices become more powerful, semiconductor elements are becoming more dense and more highly mounted. Accordingly, it is important to more efficiently dissipate heat generated by electronic components that constitute electronic devices. For example, in a semiconductor device, in order to efficiently dissipate heat, electronic components are attached to a heat sink such as a heat dissipation fan or a heat sink via a thermally conductive sheet. As a thermally conductive sheet, for example, one in which a filler such as an inorganic filler is contained (dispersed) in a silicone resin is widely used. Heat dissipation members such as this thermally conductive sheet are required to have a further improved thermal conductivity. For example, in order to increase the thermal conductivity of the thermally conductive sheet, it has been considered to increase the filling rate of the inorganic filler that is mixed in the matrix such as a binder resin. However, if the filling rate of the inorganic filler is increased, the flexibility of the thermally conductive sheet is impaired and powder fall occurs, so there is a limit to increasing the filling rate of the inorganic filler.

[0003] Examples of inorganic fillers include alumina, aluminum nitride, and aluminum hydroxide. In addition, for the purpose of high thermal conductivity, boron nitride, scaly particles such as graphite, carbon fibers, and the like may be filled into the matrix. This is due to the anisotropy of the thermal conductivity of the scaly particles. For example, carbon fibers are known to have a thermal conductivity of about 600 to 1200 W / m·K in the fiber direction. In addition, boron nitride is known to have a thermal conductivity of about 110 W / m·K in the plane direction and a thermal conductivity of about 2 W / m·K in the direction perpendicular to the plane direction. In this way, it is expected that the thermal conductivity will be dramatically improved by making the fiber direction of the carbon fibers and the plane direction of the scaly particles the same as the thickness direction of the sheet, which is the direction of heat transfer, that is, by orienting the carbon fibers and scaly particles in the thickness direction of the sheet.

[0004] Here, the thermally conductive sheet is manufactured by creating a resin molded body by curing a thermally conductive resin composition, which is a binder resin containing a filler such as an inorganic filler, into a block shape, and slicing this molded body into sheets. However, if the resin molded body cannot be sliced ​​to a uniform thickness when it is sliced, the unevenness of the sheet surface becomes large, and air is trapped in the unevenness during mounting, which causes a problem that the excellent thermal conductivity is not utilized. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2012-201106 A Summary of the Invention [Problem to be solved by the invention]

[0006] To solve this problem, for example, Patent Document 1 proposes a thermally conductive sheet containing a predetermined proportion of boron nitride as a thermally conductive filler, and the thermally conductive filler is oriented in the thickness direction. In Patent Document 1, a silicone resin composition containing a predetermined proportion of boron nitride is extruded into a sheet to form a green sheet, and the green sheets are laminated to form a silicone laminate, and the silicone laminate is cut in the lamination direction to obtain a thermally conductive sheet in which boron nitride is oriented in the thickness direction. However, in this manufacturing method, boron nitride is oriented by extruding the silicone resin composition with a coater or the like, and is oriented in the thickness direction of the thermally conductive sheet, but the orientation angle of boron nitride is thought to be parallel to the extrusion direction of the coater. Therefore, as the load on the thermally conductive sheet is increased, there is a problem that the boron nitride falls at a certain point, causing a sudden decrease in thermal conductivity. In addition, this manufacturing method requires a process of laminating green sheets multiple times, which is a concern for cost increase.

[0007] The present technology has been proposed in consideration of the above-described conventional situation, and provides a thermally conductive sheet and a method for manufacturing the thermally conductive sheet that can reduce the degree of decrease in thermal conductivity even when the load increases. [Means for solving the problem]

[0008] The thermally conductive sheet according to the present technology contains a binder resin, a first thermally conductive filler having a scale shape, and a second thermally conductive filler having a non-scale shape, and in the thermally conductive sheet in which the first thermally conductive filler and the second thermally conductive filler are dispersed in the binder resin, the first thermally conductive filler is boron nitride, the second thermally conductive filler has an average particle size of 5 μm or less, the major axis of the first thermally conductive filler is oriented in the thickness direction of the thermally conductive sheet, and the minor axis of the first thermally conductive filler is randomly oriented in the in-plane direction of the thermally conductive sheet.

[0009] The method for producing a thermally conductive sheet according to the present technology includes the steps of: preparing a resin composition for forming a thermally conductive sheet by dispersing a first thermally conductive filler having a scale-like shape and a second thermally conductive filler having a non-scale-like shape in a curable resin composition; forming a molded body block from the resin composition for forming a thermally conductive sheet; and slicing the molded body block into sheets to obtain a thermally conductive sheet, wherein the first thermally conductive filler is boron nitride, the second thermally conductive filler has an average particle size of 5 μm or less, and the thermally conductive sheet has long axes of the first thermally conductive filler oriented in the thickness direction and short axes of the first thermally conductive filler oriented randomly in the in-plane direction. Effect of the Invention

[0010] According to the present technology, it is possible to provide a thermally conductive sheet that can reduce the degree of decrease in thermal conductivity even when the load increases. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a cross-sectional view showing an example of a thermally conductive sheet according to the present technology. [Diagram 2] FIG. 2 is a perspective view that typically shows flaky boron nitride having a hexagonal crystal shape. [Diagram 3] FIG. 3 is a diagram for explaining an XRD measurement for examining the orientation state of the first thermally conductive filler. [Figure 4] FIG. 4 is a diagram for explaining the XRD measurement surface. [Diagram 5] FIG. 5 is a front view showing an opening having a structure in which a plurality of cells having sides that are not perpendicular to each other are connected together. [Figure 6] FIG. 6 is a front view showing an opening according to a comparative example, where (a) shows an opening with a parallel slit structure, and (b) shows an opening with a mesh structure. [Figure 7] FIG. 7 is a cross-sectional view showing an example of a semiconductor device to which the thermally conductive sheet according to the present technology is applied. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] In this specification, the average particle size (D50) of the thermally conductive filler is the cumulative 50% area length (μm) from the small particle size side of the particle size distribution of the thermally conductive filler, and refers to the area length at which the cumulative value is 50% when the cumulative curve is calculated from the small particle size side of the particle size distribution of the thermally conductive filler, with the total area of ​​the group of thermally conductive fillers being 100%. Note that the particle size distribution (particle diameter distribution) in this specification is calculated based on the volume. For example, a method of measuring the particle size distribution can be mentioned, which uses a laser diffraction type particle size distribution measuring device.

[0013] <Thermal conductive sheet> FIG. 1 is a cross-sectional view showing an example of a thermally conductive sheet 1 according to the present technology. The thermally conductive sheet 1 contains a binder resin 2, a scaly first thermally conductive filler 3, and a non-scaly second thermally conductive filler 4, and the first thermally conductive filler 3 and the second thermally conductive filler 4 are dispersed in the binder resin 2. In the thermally conductive sheet 1, the major axes of the first thermally conductive filler 3 are oriented in the thickness direction of the sheet, and the minor axes of the first thermally conductive filler 3 are randomly oriented in the in-plane direction of the thermally conductive sheet. In such a thermally conductive sheet 1, the decrease in thermal conductivity with an increase in load is small. For example, the thermally conductive sheet 1 has a load of 0.5 to 3 kgf / cm in the thickness direction B. 2 When a load of 1.5 W / m K is applied, the difference between the maximum and minimum effective thermal conductivity values ​​can be kept to 1.5 W / m K or less.

[0014] The components of the thermally conductive sheet 1 will be described below.

[0015] <Binder resin> The binder resin 2 is for holding the first thermally conductive filler 3 and the second thermally conductive filler 4 within the thermally conductive sheet 1. The binder resin 2 is selected according to the properties required for the thermally conductive sheet 1, such as mechanical strength, heat resistance, and electrical properties. The binder resin 2 can be selected from among thermoplastic resins, thermoplastic elastomers, and thermosetting resins.

[0016] Examples of thermoplastic resins include polyethylene, polypropylene, ethylene-α-olefin copolymers such as ethylene-propylene copolymers, polymethylpentene, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, ethylene-vinyl acetate copolymers, polyvinyl alcohol, polyvinyl acetal, fluorine-based polymers such as polyvinylidene fluoride and polytetrafluoroethylene, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polystyrene, polyacrylonitrile, styrene-acrylonitrile copolymers, acrylonitrile-butadiene-styrene copolymer (ABS) resins, polyphenylene-ether copolymers (PPE) resins, modified PPE resins, aliphatic polyamides, aromatic polyamides, polyimides, polyamideimides, polymethacrylic acid, polymethacrylic acid esters such as polymethacrylic acid methyl ester, polyacrylic acids, polycarbonates, polyphenylene sulfide, polysulfones, polyethersulfones, polyethernitriles, polyetherketones, polyketones, liquid crystal polymers, silicone resins, and ionomers.

[0017] Examples of the thermoplastic elastomer include a styrene-butadiene block copolymer or a hydrogenated product thereof, a styrene-isoprene block copolymer or a hydrogenated product thereof, a styrene-based thermoplastic elastomer, an olefin-based thermoplastic elastomer, a vinyl chloride-based thermoplastic elastomer, a polyester-based thermoplastic elastomer, a polyurethane-based thermoplastic elastomer, and a polyamide-based thermoplastic elastomer.

[0018] Examples of the thermosetting resin include crosslinked rubber, epoxy resin, phenol resin, polyimide resin, unsaturated polyester resin, diallyl phthalate resin, etc. Specific examples of the crosslinked rubber include natural rubber, acrylic rubber, butadiene rubber, isoprene rubber, styrene-butadiene copolymer rubber, nitrile rubber, hydrogenated nitrile rubber, chloroprene rubber, ethylene-propylene copolymer rubber, chlorinated polyethylene rubber, chlorosulfonated polyethylene rubber, butyl rubber, halogenated butyl rubber, fluororubber, urethane rubber, and silicone rubber.

[0019] As the binder resin 2, for example, a silicone resin is preferable in consideration of the adhesion between the heat generating surface of the electronic component and the heat sink surface. As the silicone resin, for example, a two-liquid addition reaction type silicone resin can be used, which is composed of a main component containing a silicone having an alkenyl group, a curing catalyst, and a curing agent having a hydrosilyl group (Si-H group). As the silicone having an alkenyl group, for example, a polyorganosiloxane having a vinyl group can be used. The curing catalyst is a catalyst for promoting the addition reaction between the alkenyl group in the silicone having an alkenyl group and the hydrosilyl group in the curing agent having a hydrosilyl group. As the curing catalyst, a catalyst well known as a catalyst used for a hydrosilylation reaction can be mentioned, for example, a platinum group-based curing catalyst, for example, a platinum group metal such as platinum, rhodium, or palladium, or platinum chloride can be used. As the curing agent having a hydrosilyl group, for example, a polyorganosiloxane having a hydrosilyl group can be used. The binder resin 2 may be used alone or in combination of two or more types.

[0020] When a two-component addition reaction type liquid silicone resin consisting of a silicone base and a curing agent is used as the binder resin 2, the compressibility of the thermally conductive sheet 1 can be increased by setting the mass ratio of the silicone base to the curing agent (silicone base:curing agent) to 5:5 to 7:3.

[0021] The content of the binder resin 2 in the thermal conductive sheet 1 is not particularly limited and can be appropriately selected depending on the purpose. For example, the lower limit of the content of the binder resin 2 in the thermal conductive sheet 1 can be 20 volume % or more, may be 25 volume % or more, or may be 30 volume % or more. The upper limit of the content of the binder resin 2 in the thermal conductive sheet 1 can be 70 volume % or less, may be 60 volume % or less, or may be 50 volume % or less. From the viewpoint of improving the compressibility of the thermal conductive sheet 1 and reducing the load dependency of the effective thermal conductivity, the content of the binder resin 2 in the thermal conductive sheet 1 is preferably 30 to 40 volume %, and can also be 30 to 37 volume %.

[0022] <First thermally conductive filler> The first thermally conductive filler 3 is a scaly thermally conductive filler. The scaly thermally conductive filler has a high aspect ratio and isotropic thermal conductivity in the planar direction. The scaly thermally conductive filler is not particularly limited as long as it is scaly, but is preferably a material that can ensure the insulation of the thermally conductive sheet 1. For example, the scaly thermally conductive filler may be boron nitride (BN), mica, alumina, aluminum nitride, silicon carbide, silica, zinc oxide, molybdenum disulfide, or the like.

[0023] Here, the scaly thermally conductive filler is a thermally conductive filler having a long axis, a short axis, and a thickness, a high aspect ratio (long axis / thickness), and isotropic thermal conductivity in the plane direction including the long axis. The short axis refers to the length of the longest part of the scaly thermally conductive filler in the direction perpendicular to the long axis of the scaly thermally conductive filler on the plane including the long axis of the scaly thermally conductive filler. The thickness refers to the average value obtained by measuring the thickness of 10 points on the plane including the long axis of the scaly thermally conductive filler.

[0024] FIG. 2 is a perspective view that shows a scale-like boron nitride 3A having a hexagonal crystal shape, which is an example of a scale-like thermally conductive filler. In FIG. 2, a represents the long axis of the scale-like boron nitride 3A, b represents the thickness of the scale-like boron nitride 3A, and c represents the short axis of the scale-like boron nitride 3A. As the scale-like thermally conductive filler, it is preferable to use a scale-like boron nitride 3A having a hexagonal crystal shape as shown in FIG. 2 from the viewpoint of the thermal conductivity of the thermally conductive sheet 1. The scale-like thermally conductive filler may be used alone or in combination of two or more kinds. The thermally conductive sheet 1 according to the present technology can achieve both excellent thermal properties and low cost by using a scale-like thermally conductive filler (e.g., a scale-like boron nitride 3A) that is less expensive than a spherical thermally conductive filler (e.g., a spherical boron nitride) as the first thermally conductive filler 3.

[0025] The average particle size (D50) of the scaly thermally conductive filler is not particularly limited and can be appropriately selected depending on the purpose. For example, the lower limit of the average particle size of the scaly thermally conductive filler can be 10 μm or more, may be 20 μm or more, may be 30 μm or more, or may be 35 μm or more. The upper limit of the average particle size of the scaly thermally conductive filler can be 150 μm or less, may be 100 μm or less, may be 90 μm or less, may be 80 μm or less, may be 70 μm or less, may be 50 μm or less, or may be 45 μm or less. From the viewpoint of improving the compressibility of the thermally conductive sheet 1 and reducing the load dependency of the effective thermal conductivity, the average particle size of the scaly thermally conductive filler is preferably 20 to 100 μm, and may be 20 to 50 μm.

[0026] The aspect ratio (long axis / short axis) of the scaly thermally conductive filler is not particularly limited and can be appropriately selected according to the purpose. For example, the aspect ratio of the scaly thermally conductive filler can be in the range of 10 to 100. The long axis and short axis of the scaly thermally conductive filler can be measured, for example, by a microscope, a scanning electron microscope (SEM), a particle size distribution meter, or the like. As an example, when scaly boron nitride 3A having a hexagonal crystal shape as shown in FIG. 2 is used as the scaly thermally conductive filler, 200 or more pieces of boron nitride 3A are arbitrarily selected from an image taken by SEM, and the ratio (a / c) of each long axis a to short axis c is calculated to calculate the average value.

[0027] The content of the first thermally conductive filler 3 in the thermally conductive sheet 1 is not particularly limited and can be appropriately selected depending on the purpose. For example, the lower limit of the content of the first thermally conductive filler 3 in the thermally conductive sheet 1 can be 15 volume % or more, may be 20 volume % or more, or may be 25 volume % or more. The upper limit of the content of the first thermally conductive filler 3 in the thermally conductive sheet 1 can be 45 volume % or less, may be 40 volume % or less, may be 35 volume % or less, or may be 30 volume % or less. From the viewpoint of improving the compressibility of the thermally conductive sheet 1 and reducing the load dependency of the effective thermal conductivity, the content of the first thermally conductive filler 3 in the thermally conductive sheet 1 is preferably 20 to 28 volume %, and can also be 23 to 27 volume %.

[0028] <Second thermally conductive filler> The second thermally conductive filler 4 is a thermally conductive filler other than the above-mentioned first thermally conductive filler 3. The second thermally conductive filler 4 is non-scale-like, and examples thereof include spherical, powdery, granular, and flat thermally conductive fillers. In consideration of the effects of the present technology, the material of the second thermally conductive filler 4 is preferably a material that can ensure the insulation of the thermally conductive sheet 1, and examples thereof include aluminum oxide (alumina, sapphire), aluminum nitride, boron nitride, zirconia, and silicon carbide. The second thermally conductive filler 4 may be used alone or in combination of two or more types.

[0029] In particular, from the viewpoint of improving the compressibility of the thermal conductive sheet 1 and reducing the load dependency of the effective thermal conductivity, it is preferable to use aluminum nitride particles and alumina particles in combination as the second thermal conductive filler 4. From the viewpoint of reducing the viscosity of the thermal conductive sheet 1 before thermal curing, the average particle size of the aluminum nitride particles is preferably 1 to 5 μm, may be 1 to 3 μm, or may be 1 to 2 μm. Moreover, from the viewpoint of reducing the viscosity of the thermal conductive sheet 1 before thermal curing, the average particle size of the alumina particles is preferably 1 to 3 μm, or may be 1.5 to 2.5 μm.

[0030] The content of the second thermally conductive filler 4 in the thermally conductive sheet 1 is not particularly limited and can be appropriately selected depending on the purpose. The lower limit of the content of the second thermally conductive filler 4 in the thermally conductive sheet 1 can be 10 volume % or more, may be 15 volume % or more, or may be 20 volume % or more. The upper limit of the content of the second thermally conductive filler 4 in the thermally conductive sheet 1 can be 50 volume % or less, may be 40 volume % or less, may be 30 volume % or less, or may be 25 volume % or less. The total content of the second thermally conductive filler 4 in the thermally conductive sheet 1 can be, for example, 30 to 60 volume %.

[0031] When alumina particles are used alone as the second thermally conductive filler 4, the content of alumina particles in the thermally conductive sheet 1 is preferably 10 to 45 volume % from the viewpoint of reducing the viscosity of the thermally conductive sheet 1 before thermal curing. Furthermore, as described above, when aluminum nitride particles and alumina particles are used in combination as the second thermally conductive filler 4, the content of alumina particles in the thermally conductive sheet 1 is preferably 10 to 25 volume %, and the content of aluminum nitride particles is preferably 10 to 25 volume %, from the viewpoint of reducing the viscosity of the thermally conductive sheet 1 before thermal curing.

[0032] From the viewpoint of improving the compressibility of the thermal conductive sheet 1 and reducing the load dependency of the effective thermal conductivity, the total content of the first thermal conductive filler 3 and the second thermal conductive filler 4 in the thermal conductive sheet 1 is preferably less than 70 volume % and can be 67 volume % or less. From the viewpoint of improving the compressibility of the thermal conductive sheet 1 and reducing the load dependency of the effective thermal conductivity, the lower limit of the total content of the first thermal conductive filler 3 and the second thermal conductive filler 4 in the thermal conductive sheet 1 is preferably 60 volume % or more and can be 63 volume % or more.

[0033] The thermally conductive sheet 1 may further contain other components in addition to the above-mentioned components, as long as the effects of the present technology are not impaired. Examples of the other components include dispersants, curing accelerators, retarders, tackifiers, plasticizers, flame retardants, antioxidants, stabilizers, and colorants.

[0034] [Orientation] As described above, in the thermally conductive sheet 1 in which the first thermally conductive filler 3 and the second thermally conductive filler 4 are dispersed in the binder resin 2, the major axes of the first thermally conductive filler 3 are oriented in the thickness direction B of the thermally conductive sheet 1 shown in Fig. 1, and the minor axes of the first thermally conductive filler 3 are randomly oriented in the in-plane direction A of the thermally conductive sheet 1. This allows the thermally conductive sheet 1 to reduce the decrease in thermal conductivity even when the load is increased.

[0035] The phrase "the long axes of the first thermally conductive fillers 3 are oriented in the thickness direction B of the thermally conductive sheet 1" means, for example, that the proportion of the first thermally conductive fillers whose long axes are oriented in the thickness direction B of the thermally conductive sheet 1 is 50% or more of all the first thermally conductive fillers in the thermally conductive sheet 1. In addition, the phrase "the short axes of the first thermally conductive fillers 3 are randomly oriented in the in-plane direction A of the thermally conductive sheet 1" means that the orientation of the short axes of the first thermally conductive fillers 3 is irregular in the in-plane direction A of the thermally conductive sheet 1.

[0036] In other words, in the thermally conductive sheet 1, when viewed in longitudinal section, each first thermally conductive filler 3 is dispersed with its long axis oriented in the sheet thickness direction B, and when viewed in transverse section, the direction of the short axis of each first thermally conductive filler 3 is irregular in the in-plane direction A of the sheet.

[0037] The orientation state of the first thermally conductive filler 3 can be observed by signal intensity measurement by XRD (X-ray diffraction method). For example, the signal intensity measured from the thickness direction of the sheet is the signal intensity A (corresponding to the 002 plane in FIG. 3) when the sheet surface is irradiated with X-rays when viewed from the thickness direction of the sheet, and the signal intensity B (corresponding to the 110 plane in FIG. 3) when the sheet surface is irradiated with X-rays in the direction corresponding to the diagonal line of the sheet, and the intensity ratio A / B is used as an index of the orientation state. Similarly, the intensity ratio A / B of the XRD signal measured from the front direction and the side direction of the sheet is obtained, and when each signal intensity ratio is compared, the orientation is higher in the direction where A / B is relatively smaller, that is, it can be said that the long axis of the first thermally conductive filler is oriented.

[0038] For the thermally conductive sheet 1 to which this technology is applied, the signal intensity ratios were obtained in the sheet thickness direction, the front direction, and the side direction. Specifically, as shown in FIG. 4, one surface of the sheet in the thickness direction b is the top surface 1a, one side surface of the sheet is the front surface 1b, and the other side surface adjacent to the front surface 1b is the side surface 1c. Then, the signal intensity A (corresponding to the 002 surface in FIG. 3) when the top surface 1a of the thermally conductive sheet 1 is irradiated with X-rays facing upward, and the signal intensity B (corresponding to the 110 surface in FIG. 3) when the X-rays are irradiated in a direction corresponding to the diagonal line of the sheet were measured, and the intensity ratio A / B was obtained. Next, the intensity ratio A / B was obtained in the same manner with the front surface 1b facing upward. Next, the intensity ratio A / B was obtained in the same manner with the side surface 1c facing upward.

[0039] When the XRD signal intensity ratios of the top surface 1a, front surface 1b, and side surface 1c were compared, the XRD signal intensity ratio measured from the top surface 1a, i.e., the thickness direction of the sheet, of the thermally conductive sheet 1 to which the present technology was applied was much smaller than the XRD signal intensity ratios measured from the front surface 1b and side surface 1c. This revealed that the major axis of the first thermally conductive filler 3 was oriented in the thickness direction of the sheet.

[0040] In addition, the difference between the XRD signal intensity ratio measured from the front surface 1b and the XRD signal intensity ratio measured from the side surface 1c was large, which suggests that the minor axes of the first thermally conductive filler 3 are randomly oriented in the sheet surface direction.

[0041] In addition, by using a combination of a first thermally conductive filler 3 and a second thermally conductive filler 4 in the thermally conductive sheet 1, the second thermally conductive filler 4 can support the above-mentioned orientation state of the first thermally conductive filler 3.

[0042] In such a thermally conductive sheet 1, the long axis of the first thermally conductive filler 3 is oriented in the thickness direction B of the thermally conductive sheet 1 (see Figure 1), so that the thermal conductivity in the orientation direction of the long axis of the first thermally conductive filler 3 (thickness direction B of the thermally conductive sheet 1) can be made to be more than twice the thermal conductivity in the non-orientation direction of the long axis of the first thermally conductive filler 3 (for example, the surface direction A of the thermally conductive sheet 1).

[0043] Here, the thermally conductive sheet 1 in which the minor axes of the first thermally conductive filler 3 are randomly oriented in the in-plane direction A of the sheet has a visible pattern on the sheet surface. The shape of the pattern is considered to reflect the random orientation of the minor axes of the first thermally conductive filler 3, and is a pattern having sides that are not perpendicular to each other. The pattern on the sheet surface is, for example, a geometric pattern such as a polygonal shape having sides that are not perpendicular to each other, a pattern of multiple circles or ellipses in succession, or a pattern in which such geometric patterns are mixed with circles and ellipses.

[0044] The thermally conductive sheet 1 has a pattern on the sheet surface having sides that are not perpendicular to each other due to the random orientation of the short axes of the first thermally conductive filler 3, and the random orientation can reduce the decrease in thermal conductivity even when the load is increased. That is, the thermally conductive sheet 1 has a pattern on the sheet surface showing the random orientation of the short axes of the first thermally conductive filler 3, and therefore has high thermal conductivity in the plane direction due to the short axes of the first thermally conductive filler 3 randomly oriented in the plane direction on the sheet surface. This high thermal conductivity is maintained even when the sheet is placed between a heat generating body and a heat dissipating body and a load is applied, causing the long axes of the first thermally conductive filler to tilt. As a result, the thermal conductivity in the sheet thickness direction is maintained due to the short axes randomly oriented in the in-plane direction, even if the long axes of the scale-like first thermally conductive fillers, whose long axes are oriented in the thickness direction, tilt due to an increase in load, and the fluctuation range of the thermal conductivity due to an increase in load can be reduced.

[0045] On the other hand, when the pattern appearing on the surface of the thermally conductive sheet is composed only of sides that are perpendicular to each other, it is considered that the minor axis of the scaly filler contained in the thermally conductive sheet is oriented in a predetermined direction. Since the minor axis of the scaly filler is oriented in a predetermined direction relative to the surface direction of the sheet, the thermal conductivity of such a thermally conductive sheet is also low in the surface direction. Even if the major axis of the first scaly thermally conductive filler, whose major axis is oriented in the thickness direction, falls due to an increase in load, the minor axis is uniformly tilted while maintaining its orientation, so that the thermal conductivity decreases with an increase in load.

[0046] The average thickness of the thermally conductive sheet 1 is not particularly limited and can be appropriately selected depending on the purpose. For example, the lower limit of the average thickness of the thermally conductive sheet can be 0.05 mm or more, and can be 0.1 mm or more. The upper limit of the average thickness of the thermally conductive sheet can be 5 mm or less, and can be 4 mm or less, or can be 3 mm or less. From the viewpoint of the handleability of the thermally conductive sheet 1, the average thickness of the thermally conductive sheet 1 is preferably 0.1 to 4 mm, can be 0.5 to 3 mm, or can be 1 to 2 mm. The average thickness of the thermally conductive sheet 1 can be determined, for example, by measuring the thickness of the thermally conductive sheet at any five points and taking the arithmetic average value.

[0047] In the thermally conductive sheet 1, the long axes of the scale-like first thermally conductive filler 3 are oriented in the thickness direction, and the short axes of the scale-like first thermally conductive filler 3 are randomly oriented in the in-plane direction. Therefore, even if the load increases and the scale-like thermally conductive filler falls over, thermal conductivity is maintained due to the randomly oriented short axes in the in-plane direction, and the decrease in thermal conductivity due to an increase in load can be reduced.

[0048] <Method of manufacturing thermally conductive sheet> A method for producing a thermally conductive sheet according to the present technology includes, for example, the following steps A, B, and C.

[0049] <Process A> In step A, a composition for forming a thermally conductive sheet is prepared by dispersing a scaly first thermally conductive filler 3 and a non-scaly second thermally conductive filler 4 in a binder resin 2. The composition for forming a thermally conductive sheet can be prepared by uniformly mixing the first thermally conductive filler 3, the second thermally conductive filler 4, the binder resin 2, and, if necessary, various additives and a volatile solvent by a known method.

[0050] <Process B> In step B, a molded block is formed from the prepared composition for forming a thermally conductive sheet. Examples of the method for forming the molded block include extrusion molding and die molding. The extrusion molding and die molding methods are not particularly limited, and can be appropriately selected from various known extrusion molding and die molding methods according to the viscosity of the composition for forming a thermally conductive sheet and the properties required for the thermally conductive sheet.

[0051] For example, in an extrusion molding method, when the composition for forming a thermally conductive sheet is extruded through a die, or in a mold molding method, when the composition for forming a thermally conductive sheet is pressed into a mold, the binder resin flows, and the long axis of the scaly thermally conductive filler 3 is oriented along the flow direction.

[0052] The size and shape of the molded block can be determined according to the desired size of the thermally conductive sheet 1. For example, it may be a rectangular parallelepiped with a cross-sectional vertical dimension of 0.5 to 15 cm and a horizontal dimension of 0.5 to 15 cm. The length of the rectangular parallelepiped may be determined as necessary. In the extrusion molding method, a columnar molded block made of a cured product of the resin composition for forming the thermally conductive sheet can be formed.

[0053] Opening Here, in the forming process of the molded body block, the composition for forming a thermally conductive sheet is passed through an opening 5 having one or more cells having sides that are not perpendicular to each other when viewed from the flow direction. For example, such an opening 5 can be a structure in which a plurality of regular pentagonal cells are continuous as shown in FIG. 5(a). In addition, a structure in which a plurality of hexagonal honeycomb cells are continuous (FIG. 5(b)) or a structure in which a plurality of approximately circular cells are continuous (FIG. 5(c)) may be used. In other words, the opening 5 according to the present technology does not include those consisting of only mutually perpendicular sides such as a square or rectangle, for example, a parallel slit structure as shown in FIG. 6(a) or a rectangular mesh structure in which a plurality of mutually perpendicular rectangular cells are continuous as shown in FIG. 6(b).

[0054] The opening 5 may have a cell that is composed of only sides that are perpendicular to each other. The opening shapes of the multiple cells may all be the same, or may be composed of a combination of cells with different opening shapes. The opening shapes of the multiple cells may or may not be regular polygons. The opening 5 may be composed of multiple consecutive cells that have sides that are not perpendicular to each other, or may be composed of a single opening whose opening shape has sides that are not perpendicular to each other.

[0055] By passing through such an opening 5, the major axis of the first thermally conductive filler 3 contained in the composition for forming a thermally conductive sheet can be oriented in the flow direction, and the minor axis of the first thermally conductive filler 3 can be randomly oriented in a direction perpendicular to the flow direction (i.e., non-oriented). This is because, when the composition for forming a thermally conductive sheet is caused to flow in a substantially rectangular parallelepiped mold in the process of forming a molded body block, if the composition is passed through an opening consisting only of mutually perpendicular sides, the minor axes of the first thermally conductive filler 3 are oriented in mutually perpendicular directions, whereas if the composition is passed through an opening 5 having sides that are not mutually perpendicular, the orientation of the minor axes of the first thermally conductive filler 3 is disturbed.

[0056] The openings 5 ​​may have any shape as long as they have sides that are not perpendicular to each other, but are preferably polygonal with five or more sides, which can improve the random orientation of the minor axes.

[0057] In a molded block formed from a composition for forming a thermally conductive sheet that has passed through such an opening 5, the long axes of the first thermally conductive filler 3 are oriented in the flow direction, and in a cross section perpendicular to the flow direction, the short axes of the first thermally conductive filler 3 are randomly oriented.

[0058] <Process C> In step C, the molded body block is sliced ​​into a sheet to obtain a thermally conductive sheet 1. The first thermally conductive filler 3 in a scale-like shape is exposed on the surface (sliced ​​surface) of the sheet obtained by slicing. The slicing method is not particularly limited, and can be appropriately selected from among known slicing devices depending on the size and mechanical strength of the molded body block. The slicing direction of the molded body block is preferably 60 to 120 degrees relative to the flow direction, more preferably 70 to 100 degrees, and even more preferably 90 degrees (vertical) because the long axis of the first thermally conductive filler 3 is oriented in the flow direction. When a columnar molded body block is formed in step B and the long axis of the first thermally conductive filler 3 is oriented in the length direction of the molded body block, it is preferable to slice the molded body block in a direction approximately perpendicular to the length direction in step C.

[0059] In this manner, according to the method for producing a thermally conductive sheet including steps A, B, and C, the thermally conductive sheet 1 described above can be obtained.

[0060] The method for producing a thermally conductive sheet according to the present technology is not limited to the above-mentioned examples, and may further include a step D of pressing the sliced ​​surface after the step C. By including the pressing step D in the method for producing a thermally conductive sheet, the surface of the sheet obtained in the step C can be made smoother, and the adhesion with other members can be improved. As a pressing method, a pair of pressing devices consisting of a flat plate and a press head with a flat surface can be used. Pressing may also be performed with a pinch roll. The pressure during pressing may be, for example, 0.1 to 100 MPa. In order to further increase the effect of pressing and shorten the pressing time, pressing is preferably performed at a temperature equal to or higher than the glass transition temperature (Tg) of the binder resin 2. For example, the pressing temperature may be 0 to 180°C, and may be within a temperature range of room temperature (for example, 25°C) to 100°C, or may be 30 to 100°C.

[0061] <Electronic equipment> The thermally conductive sheet 1 according to the present technology can be disposed between a heat generating body and a heat sink to form an electronic device having a structure in which the sheet is disposed between the heat generating body and the heat sink to dissipate heat generated by the heat generating body to the heat sink. The electronic device has at least the heat generating body, the heat sink, and the thermally conductive sheet 1, and may further have other members as necessary.

[0062] The heat generating body is not particularly limited, and examples thereof include electronic components that generate heat in an electric circuit, such as integrated circuit elements such as a CPU, a GPU (Graphics Processing Unit), a DRAM (Dynamic Random Access Memory), and a flash memory, as well as transistors, resistors, etc. Heat generating bodies also include components that receive optical signals, such as optical transceivers in communication devices.

[0063] The heat dissipator is not particularly limited, and examples thereof include a heat sink, a heat spreader, and the like that are used in combination with an integrated circuit element, a transistor, an optical transceiver housing, etc. In addition to a heat spreader or a heat sink, the heat dissipator may be anything that conducts heat generated from a heat source and dissipates it to the outside, and examples thereof include a heat sink, a cooler, a die pad, a printed circuit board, a cooling fan, a Peltier element, a heat pipe, a metal cover, a housing, and the like.

[0064] FIG. 7 is a cross-sectional view showing an example of a semiconductor device 50 to which the thermally conductive sheet 1 according to the present technology is applied. For example, as shown in FIG. 7, the thermally conductive sheet 1 is mounted on a semiconductor device 50 built into various electronic devices and sandwiched between a heat generating body and a heat dissipating body. The semiconductor device 50 shown in FIG. 7 includes an electronic component 51, a heat spreader 52, and a thermally conductive sheet 1, and the thermally conductive sheet 1 is sandwiched between the heat spreader 52 and the electronic component 51. The thermally conductive sheet 1 is sandwiched between the heat spreader 52 and the heat sink 53, and together with the heat spreader 52, constitutes a heat dissipation member that dissipates heat from the electronic component 51. The mounting location of the thermally conductive sheet 1 is not limited to between the heat spreader 52 and the electronic component 51 or between the heat spreader 52 and the heat sink 53, and can be appropriately selected according to the configuration of the electronic device or the semiconductor device. EXAMPLES

[0065] Examples of the present technology will be described below. In the examples, a thermally conductive sheet was produced and the effective thermal conductivity and compressibility were measured. Note that the present technology is not limited to these examples.

[0066] <Example 1> A resin composition for forming a thermally conductive sheet was prepared by uniformly mixing 34% by volume of silicone resin, 26% by volume of flaky boron nitride (D50: 40 μm) with a hexagonal crystal shape, 20% by volume of aluminum nitride (D50: 1.2 μm), and 20% by volume of spherical alumina particles (D50: 2 μm). The resin composition for forming a thermally conductive sheet was passed through an opening (see FIG. 5) in which a plurality of cells having sides that are not perpendicular to each other were connected, by extrusion molding, and poured into a mold (opening diameter: 50 mm x 50 mm) having a rectangular parallelepiped internal space, and heated in an oven at 60°C for 4 hours to form a columnar molded body block. A peelable polyethylene terephthalate film was attached to the inner surface of the mold so that the release-treated surface was on the inside. The obtained molded body block was sliced ​​into sheets using an ultrasonic cutter to obtain a 1.0 mm thick thermally conductive sheet in which flaky boron nitride was oriented in the thickness direction of the sheet.

[0067] <Example 2> A 1.0 mm thick thermally conductive sheet was obtained in the same manner as in Example 1, except that a resin composition for forming a thermally conductive sheet was prepared by uniformly mixing 37 vol. % silicone resin, 23 vol. % flaky boron nitride (D50: 40 μm) with a hexagonal crystal shape, 20 vol. % aluminum nitride (D50: 1.2 μm), and 20 vol. % spherical alumina particles (D50: 2 μm).

[0068] <Example 3> In the same manner as in Example 1, a thermally conductive sheet having a thickness of 2.0 mm was obtained.

[0069] <Example 4> In the same manner as in Example 2, a thermally conductive sheet having a thickness of 2.0 mm was obtained.

[0070] <Comparative Example 1> A resin composition for forming a thermally conductive sheet was prepared by uniformly mixing 34% by volume of silicone resin, 26% by volume of flaky boron nitride (D50: 40 μm) with a hexagonal crystal shape, 20% by volume of aluminum nitride (D50: 1.2 μm), and 20% by volume of spherical alumina particles (D50: 2 μm). A thermally conductive sheet having a thickness of 1.0 mm was obtained in the same manner as in Example 1, except that the resin composition for forming a thermally conductive sheet was passed through a 6 mm-wide parallel slit (see FIG. 6(a)) by extrusion molding and poured into a mold to form a molded block.

[0071] <Comparative Example 2> A resin composition for forming a thermally conductive sheet was prepared by uniformly mixing 34% by volume of silicone resin, 26% by volume of flaky boron nitride (D50: 40 μm) with a hexagonal crystal shape, 20% by volume of aluminum nitride (D50: 1.2 μm), and 20% by volume of spherical alumina particles (D50: 2 μm). A thermally conductive sheet having a thickness of 1.0 mm was obtained in the same manner as in Example 1, except that the resin composition for forming a thermally conductive sheet was passed through a rectangular mesh-shaped opening (see FIG. 6(b)) by extrusion molding and poured into a mold to form a molded block.

[0072] <Comparative Example 3> In the same manner as in Comparative Example 1, a thermally conductive sheet having a thickness of 2.0 mm was obtained.

[0073] <Comparative Example 4> In the same manner as in Comparative Example 2, a thermally conductive sheet having a thickness of 2.0 mm was obtained.

[0074] <Effective thermal conductivity / difference between maximum and minimum effective thermal conductivity> The effective thermal conductivity (W / m K) of the thermal conductive sheet was measured with a thermal resistance measuring device conforming to ASTM-D5470 under a specified load (0.5 kgf / cm 2 , 1.0kgf / cm 2 , 2.0kgf / cm 2 or 3.0kgf / cm 2 ) was applied to the thermal conductive sheet, and the measurement was performed in the thickness direction.

[0075] In addition, the difference between the maximum and minimum effective thermal conductivity values ​​when a certain load was applied was determined. The smaller this difference is, the lower the load dependency of the effective thermal conductivity is, and it can be said that a stable effective thermal conductivity is achieved even if the load is increased. For example, in Example 1, the maximum effective thermal conductivity is 8.3 W / m K (load 1.0 kgf / cm 2 , and load 2.0kgf / cm 2 ), the minimum value is 7.6W / m K (load 0.5kgf / cm 2 ), the difference is 0.7 W / m K.

[0076] <Compression ratio> The compression rate (%) of the thermal conductive sheet is determined by applying a specified load (0.5 kgf / cm 2 , 1.0kgf / cm 2 , 2.0kgf / cm 2 or 3.0kgf / cm 2 ) was applied and the thickness of the thermally conductive sheet was measured after it had stabilized, and calculations were made from the thicknesses of the thermally conductive sheet before and after the load was applied.

[0077] <Evaluation> The thermal conductive sheets of the examples and comparative examples were evaluated according to the following criteria. A thermal conductive sheet was rated as OK when the difference between the maximum and minimum effective thermal conductivity values ​​was 1.5 W / m K or less, and rated as NG otherwise. The results are shown in Table 1.

[0078] [Table 1]

[0079] As shown in Table 1, the thermally conductive sheets of Examples 1 to 4 have a thermal conductivity of 0.5 to 3 kgf / cm in the thickness direction. 2 It was found that when a load of 1.5 W / m·K was applied, the difference between the maximum and minimum effective thermal conductivity was 1.5 W / m·K or less, and the load dependency of the effective thermal conductivity was low, and the effective thermal conductivity was stable. This is because the thermally conductive sheets of Examples 1 to 4 contain a curable resin composition, a scaly thermally conductive filler, and a non-scaly thermally conductive filler, and the long axes of the scaly thermally conductive filler are oriented in the thickness direction and the short axes of the scaly thermally conductive filler are randomly oriented in the in-plane direction. Therefore, even if the scaly thermally conductive filler with its long axis oriented in the thickness direction falls over as the load increases, the thermal conductivity is maintained by the short axes randomly oriented in the in-plane direction, and the fluctuation range of the thermal conductivity with an increase in load is reduced.

[0080] The thermally conductive sheets of Comparative Examples 1 to 4 had a thermal conductivity of 0.5 to 3 kgf / cm in the thickness direction. 2 It was found that when a load of 1.5 W / m K was applied, the difference between the maximum and minimum effective thermal conductivity was more than 1.5 W / m K, the effective thermal conductivity was highly load-dependent, and it was difficult to reduce the decrease in thermal conductivity with increasing load. This is thought to be because, in the thermal conductive sheets of Comparative Examples 1 and 2, the resin composition for forming the thermal conductive sheet passes through the openings of the parallel slit structure or mesh structure, so that the short axes of the scale-like thermal conductive fillers are also oriented in a predetermined direction in the in-plane direction, and therefore, when the load increased and the scale-like thermal conductive fillers with their long axes oriented in the thickness direction collapsed, the thermal conductivity could not be maintained by the short axes. [Explanation of symbols]

[0081] 1 Thermally conductive sheet, 2 Binder resin, 3 Flake-like first thermally conductive filler, 3A Flake-like boron nitride, 4 Non-flake-like second thermally conductive filler, 50 Semiconductor device, 51 Electronic component, 52 Heat spreader, 53 Heat sink

Claims

1. A thermally conductive sheet containing a binder resin, a flaky first thermally conductive filler, and a non-flaky second thermally conductive filler, wherein the first thermally conductive filler and the second thermally conductive filler are dispersed in the binder resin. In the thermally conductive sheet, the first thermally conductive filler is boron nitride, the content of the first thermally conductive filler is 20% by volume or more, the average particle size of the second thermally conductive filler is 5 μm or less, the long axis of the first thermally conductive filler is oriented in the thickness direction of the thermally conductive sheet, and the short axis of the first thermally conductive filler is randomly oriented in the in-plane direction of the thermally conductive sheet, a thermally conductive sheet having an effective thermal conductivity of 7.5 W / m·K or more when a load of 3 kgf / cm 2 is applied in the thickness direction.

2. The thermally conductive sheet according to claim 1, wherein the content of the second thermally conductive filler is 30 to 60% by volume.

3. The thermally conductive sheet according to claim 1 or 2, wherein the second thermally conductive filler is a mixture of two kinds of thermally conductive fillers, and the content of each thermally conductive filler is 10 to 25% by volume.

4. The thermally conductive sheet according to any one of claims 1 to 3, wherein the total content of the first thermally conductive filler and the second thermally conductive filler is less than 70% by volume.

5. 0.5 to 3 kgf / cm in the thickness direction ２ The thermally conductive sheet according to any one of claims 1 to 4, wherein when a load of is applied, the difference between the maximum value and the minimum value of the effective thermal conductivity is 1.5 W / m·K or less.

6. A step A of preparing a resin composition for forming a thermally conductive sheet by dispersing a flaky first thermally conductive filler and a non-flaky second thermally conductive filler in a curable resin composition; a step B of forming a molded body block from the resin composition for forming a thermally conductive sheet; and a step C of slicing the molded body block into a sheet shape to obtain a thermally conductive sheet, wherein the first thermally conductive filler is boron nitride, the content of the first thermally conductive filler is 20% by volume or more, the average particle size of the second thermally conductive filler is 5 μm or less, in the thermally conductive sheet, the long axis of the first thermally conductive filler is oriented in the thickness direction, and the short axis of the first thermally conductive filler is randomly oriented in the in-plane direction, A method for manufacturing a thermally conductive sheet having an effective thermal conductivity of 7.5 W / m·K or more when a load of 3 kgf / cm 2 is applied in the thickness direction.

7. The method for manufacturing a thermally conductive sheet according to claim 6, wherein in the step B, the resin composition for forming a thermally conductive sheet is passed through an opening formed in a die or a mold and having sides that are not perpendicular to each other.

8. In the above step C, the method for manufacturing a thermally conductive sheet according to claim 6 or 7, wherein the molded body block is sliced in a direction substantially perpendicular to the extrusion direction of the resin composition for forming the thermally conductive sheet to obtain a thermally conductive sheet.

9. A heating element, A heat sink, An electronic device comprising the heating element, the heat sink, and the thermally conductive sheet according to any one of claims 1 to 5 disposed between the heating element and the heat sink.

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