Thermally conductive sheet and method for manufacturing the thermally conductive sheet

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 increasingly dense and highly packaged. This has made it important to more efficiently dissipate heat generated by the electronic components that make up the electronic devices. For example, in semiconductor devices, electronic components are attached to heat sinks, such as heat dissipation fans and heat sinks, via thermally conductive sheets to efficiently dissipate heat. Thermally conductive sheets, for example, made of silicone resins containing (dispersed with) fillers such as inorganic fillers, are widely used. Heat dissipation components such as these thermally conductive sheets are required to have even higher thermal conductivity. For example, increasing the fill rate of inorganic fillers blended into matrices such as binder resins has been considered to improve the thermal conductivity of thermally conductive sheets. However, increasing the fill rate of inorganic fillers can impair the flexibility of the thermally conductive sheets and cause powder shedding, limiting the extent to which the fill rate can be increased.

[0003] Examples of inorganic fillers include alumina, aluminum nitride, and aluminum hydroxide. To achieve high thermal conductivity, boron nitride, graphite, and other scale-like particles, as well as carbon fibers, are sometimes filled into the matrix. This is due to the anisotropy of the thermal conductivity of the scale-like particles. For example, carbon fiber is known to have a thermal conductivity of approximately 600 to 1200 W / m·K in the fiber direction. Boron nitride is known to have a thermal conductivity of approximately 110 W / m·K in the plane direction and approximately 2 W / m·K in the direction perpendicular to the plane direction. Thus, by aligning the fiber direction of the carbon fiber and the plane direction of the scale-like particles with the thickness direction of the sheet, which is the direction of heat transfer, i.e., by orienting the carbon fiber and scale-like particles in the thickness direction of the sheet, a dramatic improvement in thermal conductivity can be expected.

[0004] Here, thermally conductive sheets are 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, and then slicing this molded body into sheets. However, if the resin molded body cannot be sliced ​​to a uniform thickness, the unevenness of the sheet surface will become large, and air will be trapped in the unevenness during mounting, which can prevent the excellent thermal conductivity from being utilized. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-201106 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 percentage of boron nitride as a thermally conductive filler, with the thermally conductive filler oriented in the thickness direction. In Patent Document 1, a silicone resin composition containing a predetermined percentage of boron nitride is extruded into a sheet to form a green sheet, which is then stacked to form a silicone laminate. The silicone laminate is then cut in the stacking direction to obtain a thermally conductive sheet with boron nitride oriented in the thickness direction. However, in this manufacturing method, the boron nitride is oriented by extruding the silicone resin composition using a coater or the like, and is oriented in the thickness direction of the thermally conductive sheet. However, the orientation angle of the boron nitride is thought to be parallel to the extrusion direction of the coater. Therefore, as the load on the thermally conductive sheet increases, the boron nitride collapses at some point, resulting in a rapid decrease in thermal conductivity. Furthermore, this manufacturing method requires multiple green sheet stacking processes, which increases costs.

[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 extent of decrease in thermal conductivity even when the load increases. [Means for solving the problem]

[0008] A thermally conductive sheet according to the present technology includes a binder resin, a flaky first thermally conductive filler, and a non-flaky second thermally conductive filler, and the first thermally conductive filler and the second thermally conductive filler are dispersed in the binder resin, wherein the first thermally conductive filler is boron nitride; The content of the first thermally conductive filler is 20% by volume or more, The second thermally conductive filler has an average particle size of 5 μm or less, the major axes of the first thermally conductive filler are oriented in the thickness direction of the thermally conductive sheet, and the minor axes of the first thermally conductive filler are randomly oriented in the in-plane direction of the thermally conductive sheet. 3kgf / cm in the thickness direction 2 When a load of 1000kJ / s is applied, the effective thermal conductivity is 7.5W / m K or more. It is something.

[0009] A method for producing a thermally conductive sheet according to the present technology includes the steps of: Step A of preparing a resin composition for forming a thermally conductive sheet by dispersing a scaly first thermally conductive filler and a non-scaly second thermally conductive filler in a curable resin composition; Step B of forming a molded body block from the resin composition for forming the thermally conductive sheet; and Step C of slicing the molded body block into sheets 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 second thermally conductive filler has an average particle size of 5 μm or less, and the thermally conductive sheet has the major axes of the first thermally conductive filler oriented in the thickness direction and the minor axes of the first thermally conductive filler oriented randomly in the in-plane direction. 3kgf / cm in the thickness direction 2 When a load of 1000kJ / s is applied, the effective thermal conductivity is 7.5W / m K or more. It is something. [Effects of the Invention]

[0010] According to the present technology, it is possible to provide a thermally conductive sheet that can reduce the extent of decrease in thermal conductivity even when the load increases. [Brief explanation 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. [Figure 2] FIG. 2 is a perspective view schematically showing flaky boron nitride having a hexagonal crystal structure. [Figure 3] FIG. 3 is a diagram for explaining 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. [Figure 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 one another are connected. [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 INVENTION

[0012] In this specification, the average particle size (D50) of the thermally conductive filler refers to the area length (μm) of the cumulative 50% 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 reaches 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 on a volume basis. Examples of methods for measuring the particle size distribution include methods using a laser diffraction particle size distribution analyzer.

[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, with the first thermally conductive filler 3 and the second thermally conductive filler 4 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. This thermally conductive sheet 1 exhibits a small decrease in thermal conductivity with increasing load. For example, the thermally conductive sheet 1 exhibits 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 can be 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 used to hold 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 depending on the properties required of the thermally conductive sheet 1, such as mechanical strength, heat resistance, and electrical properties. The binder resin 2 can be selected from 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 copolymer, acrylonitrile-butadiene-styrene copolymer (ABS) resin, polyphenylene-ether copolymer (PPE) resin, modified PPE resin, aliphatic polyamides, aromatic polyamides, polyimide, polyamideimide, polymethacrylic acid, polymethacrylic acid esters such as polymethyl methacrylate ester, polyacrylic acids, polycarbonate, polyphenylene sulfide, polysulfone, polyethersulfone, polyethernitrile, polyetherketone, polyketone, 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 thermosetting resins include crosslinked rubber, epoxy resin, phenolic resin, polyimide resin, unsaturated polyester resin, diallyl phthalate resin, etc. Specific examples of 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] Considering the adhesion between the heat-generating surface of an electronic component and the heat sink surface, a silicone resin is preferable as the binder resin 2. Examples of silicone resins that can be used include two-component addition reaction silicone resins, which are composed of a base material containing an alkenyl group-containing silicone as the main component, a curing catalyst, and a curing agent having a hydrosilyl group (Si-H group). Examples of silicones having alkenyl groups include polyorganosiloxanes having vinyl groups. The curing catalyst promotes 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. Examples of curing catalysts include catalysts well known for use in hydrosilylation reactions, such as platinum-group curing catalysts, such as platinum group metals such as platinum, rhodium, and palladium, and platinum chloride. Examples of curing agents having a hydrosilyl group include polyorganosiloxanes having a hydrosilyl group. Binder resin 2 may be used alone or in combination.

[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 binder resin 2 in 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 binder resin 2 in thermally conductive sheet 1 can be 20% by volume or more, or may be 25% by volume or more, or may be 30% by volume or more. The upper limit of the content of binder resin 2 in thermally conductive sheet 1 can be 70% by volume or less, or may be 60% by volume or less, or may be 50% by volume or less. From the viewpoint of improving the compressibility of thermally conductive sheet 1 and reducing the load dependency of the effective thermal conductivity, the content of binder resin 2 in thermally conductive sheet 1 is preferably 30 to 40% by volume, and can also be 30 to 37% by volume.

[0022] <First thermally conductive filler> The first thermally conductive filler 3 is a flaky thermally conductive filler. The flaky thermally conductive filler has a high aspect ratio and isotropic thermal conductivity in the plane direction. The flaky thermally conductive filler is not particularly limited as long as it is flaky, but a material that can ensure the insulation of the thermally conductive sheet 1 is preferred. For example, the flaky 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 term "scaly thermally conductive filler" refers to a thermally conductive filler having a major axis, a minor axis, and a thickness, a high aspect ratio (major axis / thickness), and isotropic thermal conductivity in the plane containing the major axis. The minor axis refers to the length of the longest part of the scaly thermally conductive filler in the direction perpendicular to the major axis of the scaly thermally conductive filler on the plane containing the major axis of the scaly thermally conductive filler. The thickness refers to the average value obtained by measuring the thickness at 10 points on the plane containing the major axis of the scaly thermally conductive filler.

[0024] FIG. 2 is a perspective view schematically illustrating scaly boron nitride 3A having a hexagonal crystal structure, which is an example of a scaly thermally conductive filler. In FIG. 2, a represents the major axis of the scaly boron nitride 3A, b represents the thickness of the scaly boron nitride 3A, and c represents the minor axis of the scaly boron nitride 3A. From the viewpoint of the thermal conductivity of the thermally conductive sheet 1, it is preferable to use scaly boron nitride 3A having a hexagonal crystal structure as the scaly thermally conductive filler, as shown in FIG. 2. One type of scaly thermally conductive filler may be used alone, or two or more types may be used in combination. The thermally conductive sheet 1 according to the present technology can achieve both excellent thermal properties and low costs by using, as the first thermally conductive filler 3, a scaly thermally conductive filler (e.g., scaly boron nitride 3A) that is less expensive than a spherical thermally conductive filler (e.g., spherical boron nitride).

[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, or 20 μm or more, or 30 μm or more, or 35 μm or more. The upper limit of the average particle size of the scaly thermally conductive filler can be 150 μm or less, or 100 μm or less, or 90 μm or less, or 80 μm or less, or 70 μm or less, or 50 μm or less, or 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 can also be 20 to 50 μm.

[0026] The aspect ratio (major axis / minor axis) of the scaly thermally conductive filler is not particularly limited and can be appropriately selected depending on the purpose. For example, the aspect ratio of the scaly thermally conductive filler can be in the range of 10 to 100. The major axis and minor axis of the scaly thermally conductive filler can be measured using, for example, a microscope, a scanning electron microscope (SEM), a particle size distribution analyzer, 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 boron nitride 3A particles are arbitrarily selected from an image taken with an SEM, and the ratio (a / c) of the major axis a to the minor axis c of each particle is determined and the average value is calculated.

[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 vol% or more, or may be 20 vol% or more, or may be 25 vol% or more. The upper limit of the content of the first thermally conductive filler 3 in the thermally conductive sheet 1 can be 45 vol% or less, or may be 40 vol% or less, or may be 35 vol% or less, or may be 30 vol% 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 vol%, and can also be 23 to 27 vol%.

[0028] <Second thermally conductive filler> The second thermally conductive filler 4 is a thermally conductive filler other than the above-described first thermally conductive filler 3. The second thermally conductive filler 4 is non-scale-shaped, 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 thermally conductive sheet 1 and reducing the load dependency of the effective thermal conductivity, it is preferable to use a combination of aluminum nitride particles and alumina particles as the second thermally conductive filler 4. From the viewpoint of reducing the viscosity of the thermally conductive sheet 1 before thermal curing, the average particle size of the aluminum nitride particles is preferably 1 to 5 μm, and may be 1 to 3 μm or 1 to 2 μm. Furthermore, from the viewpoint of reducing the viscosity of the thermally conductive sheet 1 before thermal curing, the average particle size of the alumina particles is preferably 1 to 3 μm, and 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% by volume or more, or 15% by volume or more, or 20% by 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% by volume or less, or 40% by volume or less, or 30% by volume or less, or 25% by 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% by 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 thermally conductive filler 3 and the second thermally conductive filler 4 in the thermal conductive sheet 1 is preferably less than 70% by volume and can be 67% by 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 thermally conductive filler 3 and the second thermally conductive filler 4 in the thermal conductive sheet 1 is preferably 60% by volume or more and can be 63% by 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 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 minimize the decrease in thermal conductivity even when the load increases.

[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 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. Furthermore, 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] That is, in the thermally conductive sheet 1, when viewed in longitudinal section, each first thermally conductive filler 3 is dispersed with its long axis directed 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 measuring signal intensity using XRD (X-ray diffraction). For example, the signal intensity measured in the thickness direction of the sheet is signal intensity A (corresponding to the 002 plane in Figure 3) when X-rays are irradiated onto the surface of the sheet when viewed from the thickness direction of the sheet, and signal intensity B (corresponding to the 110 plane in Figure 3) when X-rays are irradiated in a direction corresponding to the diagonal of the sheet, and the intensity ratio A / B was used as an indicator of the orientation state. Similarly, the intensity ratio A / B of the XRD signals measured from the front and side directions of the sheet was determined, and when each signal intensity ratio was compared, the direction in which A / B was relatively smaller indicated a higher orientation, i.e., the longer axis of the first thermally conductive filler was more oriented.

[0038] For a thermally conductive sheet 1 to which this technology was applied, signal intensity ratios were calculated in the sheet thickness direction, front direction, and side direction. Specifically, as shown in Figure 4, one surface of the sheet in the thickness direction b is designated as the top surface 1a, one side surface of the sheet is designated as the front surface 1b, and the other side surface adjacent to the front surface 1b is designated as the side surface 1c. Then, signal intensity A (corresponding to the 002 surface in Figure 3) was measured when X-rays were irradiated with the top surface 1a of the thermally conductive sheet 1 facing upward, and signal intensity B (corresponding to the 110 surface in Figure 3) was measured when X-rays were irradiated in a direction corresponding to the diagonal of the sheet, and the intensity ratio A / B was calculated. Next, the intensity ratio A / B was calculated in the same way with the front surface 1b facing upward. Next, the intensity ratio A / B was calculated in the same way 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 this technology was applied was much smaller than the XRD signal intensity ratios measured from the front surface 1b and side surface 1c. This indicates that the major axes of the first thermally conductive filler 3 are oriented in the thickness direction of the sheet.

[0040] Furthermore, when the XRD signal intensity ratio measured from the front surface 1b was compared with the XRD signal intensity ratio measured from the side surface 1c, the difference was large, suggesting that the minor axes of the first thermally conductive filler 3 were randomly oriented in the sheet surface direction.

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

[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 the thermal conductivity in the direction in which the long axis of the first thermally conductive filler 3 is oriented (thickness direction B of the thermally conductive sheet 1) can be more than twice the thermal conductivity in the direction in which the long axis of the first thermally conductive filler 3 is not oriented (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 thought to reflect the random orientation of the minor axes of the first thermally conductive filler 3, and is a pattern with sides that are not orthogonal to each other. The pattern on the sheet surface is, for example, a geometric pattern such as a polygonal shape with sides that are not orthogonal to each other, a pattern of multiple consecutive circles or ellipses, or a pattern in which these geometric patterns are mixed with circles or ellipses.

[0044] The thermally conductive sheet 1 has a pattern on the sheet surface with non-orthogonal edges due to the random orientation of the minor axes of the first thermally conductive filler particles 3. This random orientation minimizes the decrease in thermal conductivity even when the load increases. In other words, the thermally conductive sheet 1 has a pattern on the sheet surface showing the random orientation of the minor axes of the first thermally conductive filler particles 3, resulting in high thermal conductivity in the in-plane direction due to the minor axes of the first thermally conductive filler particles 3 randomly oriented in the in-plane direction on the sheet surface. This high thermal conductivity is maintained even when the sheet is placed between a heat generating element and a heat dissipating element, for example, and a load is applied, causing the major axes of the first thermally conductive filler particles to tilt. As a result, the thermally conductive sheet 1 maintains thermal conductivity in the thickness direction due to the minor axes randomly oriented in the in-plane direction, even when the major axes of the scale-like first thermally conductive filler particles, whose major axes are oriented in the thickness direction, tilt due to increased load, thereby minimizing the fluctuation in thermal conductivity associated with increased load.

[0045] On the other hand, if the pattern appearing on the surface of the thermally conductive sheet is composed only of mutually perpendicular edges, it is considered that the minor axes of the contained scaly filler particles are oriented in a predetermined direction. In such a thermally conductive sheet, the minor axes of the scaly filler particles are oriented in a predetermined direction relative to the in-plane direction of the sheet, and therefore the thermal conductivity in the in-plane direction is also low. Furthermore, even if the major axes of the scaly first thermally conductive filler particles, whose major axes are oriented in the thickness direction, tilt due to an increase in load, the minor axes tilt uniformly while maintaining their orientation, and therefore the thermal conductivity decreases as the load increases.

[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, or 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, or 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, but can also 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 mean value of the measurements.

[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, the thermal conductivity is maintained by the randomly oriented short axes in the in-plane direction, and the decrease in thermal conductivity due to the increase in load can be reduced.

[0048] <Method for manufacturing thermally conductive sheets> The 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 thermally conductive sheet-forming composition. Examples of methods for forming the molded block include extrusion molding and molding. The extrusion molding and molding methods are not particularly limited, and any of various known extrusion molding and molding methods can be appropriately selected depending on the viscosity of the thermally conductive sheet-forming composition, the properties required for the thermally conductive sheet, and the like.

[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 axes of the scaly thermally conductive filler 3 are oriented along the flow direction.

[0052] The size and shape of the molded block can be determined depending on the desired size of the thermally conductive sheet 1. For example, it can 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 can be determined as needed. In the extrusion molding method, a columnar molded block can be formed from a cured product of the resin composition for forming the thermally conductive sheet.

[0053] Opening In the step of forming the molded body block, the thermally conductive sheet-forming composition is passed through an opening 5 having one or more cells with sides that are not perpendicular to each other when viewed from the flow direction. An example of such an opening 5 is a structure in which a plurality of regular pentagonal cells are connected, as shown in FIG. 5(a). Other examples include a structure in which a plurality of hexagonal honeycomb cells are connected (FIG. 5(b)) and a structure in which a plurality of substantially circular cells are connected (FIG. 5(c)). In other words, the opening 5 according to the present technology does not include those having only perpendicular sides, such as a square or rectangle, such as a parallel slit structure shown in FIG. 6(a) and a rectangular mesh structure in which a plurality of rectangular cells that are perpendicular to each other are connected (FIG. 6(b)).

[0054] The opening 5 may have cells partially formed of only sides that are perpendicular to each other. The opening shapes of the multiple cells may all be the same, or may be formed by combining cells of different opening shapes. The opening shapes of the multiple cells may or may not be regular polygons. The opening 5 may be formed by a plurality of consecutive cells having sides that are not perpendicular to each other, or may be formed by 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 axes of the first thermally conductive filler 3 contained in the thermally conductive sheet-forming composition are oriented in the flow direction, and the minor axes of the first thermally conductive filler 3 can be randomly oriented (i.e., non-oriented) in a direction perpendicular to the flow direction. This is thought to be because, when the thermally conductive sheet-forming composition is caused to flow in a substantially rectangular parallelepiped mold in the step 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 disrupted.

[0056] The openings 5 ​​can 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 the molded body block formed from the thermally conductive sheet-forming composition that has passed through such an opening 5, the major 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 minor axes of the first thermally conductive filler 3 are randomly oriented.

[0058] <Process C> In step C, the molded body block is sliced ​​into sheets to obtain a thermally conductive sheet 1. The scaly first thermally conductive filler particles 3 are 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 known slicing devices depending on the size and mechanical strength of the molded body block. Because the long axis of the first thermally conductive filler particles 3 is oriented in the flow direction, 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 (perpendicular) relative to the flow direction. When a columnar molded body block is formed in step B and the long axis of the first thermally conductive filler particles 3 is oriented in the length direction of the molded body block, in step C, the molded body block is preferably sliced ​​in a direction approximately perpendicular to the length direction.

[0059] In this way, 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 disclosure is not limited to the above-described examples. For example, the method 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, thereby improving adhesion to other components. A pair of pressing devices consisting of a flat platen and a flat-surfaced press head can be used for pressing. Pressing may also be performed using a pinch roll. The pressure during pressing may be, for example, 0.1 to 100 MPa. To enhance the pressing effect 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 (e.g., 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 element and a heat sink, for example, to form an electronic device having a structure in which the sheet is disposed between them to dissipate heat generated by the heat generating element to the heat sink. The electronic device has at least the heat generating element, the heat sink, and the thermally conductive sheet 1, and may further have other components as necessary.

[0062] The heat generating element 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 graphics processing unit (GPU), a dynamic random access memory (DRAM), and a flash memory, as well as transistors and resistors. Heat generating elements 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 heat sinks and heat spreaders that are used in combination with integrated circuit elements, transistors, optical transceiver housings, etc. In addition to heat spreaders and heat sinks, the heat dissipator may be anything that conducts heat generated from a heat source and dissipates it to the outside, and examples thereof include radiators, coolers, die pads, printed circuit boards, cooling fans, Peltier elements, heat pipes, metal covers, housings, etc.

[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 element and a heat dissipating element. The semiconductor device 50 shown in FIG. 7 includes an electronic component 51, a heat spreader 52, and the 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 selected appropriately depending on the configuration of the electronic device or semiconductor device. [Example]

[0065] Examples of the present technology will be described below. In the examples, thermally conductive sheets were fabricated and their effective thermal conductivity and compressibility were measured. However, 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 hexagonal crystalline flake-shaped boron nitride (D50: 40 μm), 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 extruded through an opening (see Figure 5) with multiple cells connected together, with non-orthogonal sides, into a rectangular mold (opening diameter: 50 mm x 50 mm) and heated in an oven at 60°C for 4 hours to form a cylindrical molded block. A release polyethylene terephthalate film was attached to the inner surface of the mold, with the release-treated surface facing inward. The resulting molded block was sliced ​​into sheets using an ultrasonic cutter to obtain 1.0 mm-thick thermally conductive sheets in which the flake-shaped boron nitride particles were 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% of silicone resin, 23 vol% of hexagonal crystalline flake boron nitride (D50: 40 μm), 20 vol% of aluminum nitride (D50: 1.2 μm), and 20 vol% of 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 hexagonal scaly boron nitride (D50: 40 μm), 20% by volume of aluminum nitride (D50: 1.2 μm), and 20% by volume of spherical alumina particles (D50: 2 μm). A 1.0 mm-thick thermally conductive sheet was obtained in the same manner as in Example 1, except that the resin composition for forming the thermally conductive sheet was extruded through a 6 mm-wide parallel slit (see FIG. 6(a)) 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 hexagonal scaly boron nitride (D50: 40 μm), 20% by volume of aluminum nitride (D50: 1.2 μm), and 20% by volume of spherical alumina particles (D50: 2 μm). A 1.0 mm-thick thermally conductive sheet was obtained in the same manner as in Example 1, except that the resin composition for forming the thermally conductive sheet was extruded through a rectangular mesh opening (see FIG. 6(b)) 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 using 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 3.0 kgf / cm 2 ) was applied, the measurement was carried out in the thickness direction of the thermal conductive sheet.

[0075] The difference between the maximum and minimum values ​​of the effective thermal conductivity when a predetermined load was applied was also determined. The smaller this difference, the lower the load dependency of the effective thermal conductivity, and the more stable the effective thermal conductivity will be even if the load is increased. For example, in Example 1, the maximum value of the effective thermal conductivity was 8.3 W / m K (at a load of 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 compressibility (%) 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 3.0 kgf / cm 2 ) was applied and the thickness of the thermally conductive sheet was measured after it had stabilized, and the thickness was calculated from the thickness 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 with a difference between the maximum and minimum effective thermal conductivity of 1.5 W / m K or less was rated as "OK," and any other value was rated as "NG." 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 had a thermal conductivity of 0.5 to 3 kgf / cm in the thickness direction. 2 It was found that when a load of 100 kJ / s was applied, the difference between the maximum and minimum values ​​of the effective thermal conductivity was 1.5 W / m·K or less, indicating that the load dependency of the effective thermal conductivity was low and the effective thermal conductivity could be stably exhibited. This is thought to be 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 particles, whose long axes are oriented in the thickness direction, fall over as the load increases, the thermal conductivity is maintained by the randomly oriented short axes in the in-plane direction, and the fluctuation range of the thermal conductivity with increasing load is small.

[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 When a load of 100 kJ / s was applied, the difference between the maximum and minimum effective thermal conductivity values ​​exceeded 1.5 W / m K, indicating that the effective thermal conductivity was highly load-dependent and that it was difficult to minimize the decrease in thermal conductivity with increasing load. This is thought to be because, in the thermally conductive sheets of Comparative Examples 1 and 2, the resin composition used to form the thermally conductive sheet passed through the openings in the parallel slit structure or mesh structure, causing the minor axes of the scaly thermally conductive fillers to orient in a predetermined in-plane direction. Therefore, when the load increased and the scaly thermally conductive fillers, whose major axes were oriented in the thickness direction, collapsed, the minor axes prevented the thermal conductivity from being maintained. [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, the first thermally conductive filler is boron nitride; The content of the first thermally conductive filler is 20% by volume or more, the second thermally conductive filler has an average particle size of 5 μm or less; the major axes of the first thermally conductive filler particles are oriented in the thickness direction of the thermally conductive sheet, and the minor axes of the first thermally conductive filler particles are 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 types 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 5. The thermal conductive sheet according to claim 1, wherein when a load of 1.5 W / m·K is applied, the difference between the maximum and minimum values ​​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 scaly first thermally conductive filler and a non-scaly second thermally conductive filler in a curable resin composition; A step B of forming a molded block from the resin composition for forming a thermally conductive sheet; and a step C of slicing the molded body block into sheets to obtain thermally conductive sheets, the first thermally conductive filler is boron nitride; The content of the first thermally conductive filler is 20% by volume or more, the second thermally conductive filler has an average particle size of 5 μm or less; In the thermally conductive sheet, the major axes of the first thermally conductive filler particles are oriented in a thickness direction, and the minor axes of the first thermally conductive filler particles are randomly oriented in an in-plane direction; A method for producing 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 producing a thermally conductive sheet according to claim 6, wherein in step B, the resin composition for forming the thermally conductive sheet is passed through an opening formed in a die or mold and having sides that are not perpendicular to each other.

8. 8. The method for producing a thermally conductive sheet according to claim 6 or 7, wherein in step C, the molded body block is sliced ​​in a direction approximately 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 thermally conductive sheet according to any one of claims 1 to 5, disposed between a heat generating body and a heat dissipating body.

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