Thermally conductive sheet

Abstract

This thermally conductive sheet comprises a resin composition containing a silicone and a thermally conductive filler, can be reproduced, and maintains a thermal conductivity of 90% or more when regenerated once.

Description

Thermally conductive sheet

[0001] The present invention relates to a thermally conductive sheet. This application claims priority to Japanese Application No. 2023-166156, filed on September 27, 2023, and incorporates by reference all of the contents of the above-mentioned Japanese application.

[0002] A heat-generating component such as an IC chip is usually attached to a heat-dissipating component such as a heat sink via a thermal interface material (TIM), and the heat generated in the heat-generating component is conducted to the heat-dissipating component via the thermal interface material. A known example of the thermal conductive material is a thermally conductive sheet in which a thermally conductive filler is blended in a resin matrix.

[0003] Many conventional thermally conductive sheets are made of resins whose resin matrix is ​​fully cross-linked. In this case, it is difficult to recycle the finished product. In this respect, thermally conductive sheets made of resins whose resin matrix is ​​cross-linked have a large environmental impact.

[0004] Meanwhile, Patent Document 1 proposes a recyclable, highly thermally conductive elastomer composition and a sheet made of the same. This highly thermally conductive elastomer composition uses SB (styrene-butadiene rubber), SBS (styrene-butadiene-styrene rubber), SIS (styrene-isoprene-styrene rubber), or the like as a matrix component.

[0005] JP 2008-37916 A

[0006] The highly thermally conductive elastomer composition described in Patent Document 1 is said to be recyclable, but its performance after recycling is unclear.

[0007] Under these circumstances, the present inventors have conducted extensive research and have come up with a solution to provide a thermally conductive sheet that has high material recyclability and maintains good thermal conductivity even after recycling.

[0008] A thermally conductive sheet according to one embodiment of the present invention is made of a resin composition containing silicone and a thermally conductive filler, is reproducible, and maintains thermal conductivity of 90% or more after a single reproduction.

[0009] The thermally conductive sheet according to one aspect of the present invention can be reproduced (level material recycling) while maintaining good thermal conductivity.

[0010] Fig. 1 is a cross-sectional view schematically showing an IC chip to which a heat sink is attached via a thermally conductive sheet according to an embodiment of the present invention. Fig. 2A is a perspective view schematically showing an example of a thermally conductive sheet according to an embodiment of the present invention, and Fig. 2B is a partially enlarged view of the cross section taken along line A-A in Fig. 2A. Fig. 3 is a cross-sectional schematic diagram showing the tip portion of an extruder and a T-die used in an example of a method for producing a thermally conductive sheet according to an embodiment of the present invention. Figs. 4A to 4D are schematic views showing another example of a method for producing a thermally conductive sheet according to an embodiment of the present invention.

[0011] The following is a summary of the embodiments of the present invention: (1) A thermally conductive sheet made of a resin composition containing silicone and a thermally conductive filler, which is reproducible and maintains a thermal conductivity of 90% or more after a single reproduction.

[0012] (2) The thermally conductive sheet of (1) above preferably has a viscosity of 100 kPa·s or more and 300 kPa·s or less, measured with a capillary rheometer at a shear rate of 10 (1 / s) and a temperature of 30°C.

[0013] (3) In the thermally conductive sheet of (1) or (2) above, the content of the thermally conductive filler is preferably 50% by volume or more and 70% by volume or less.

[0014] (4) In the thermally conductive sheet of any one of (1) to (3) above, the silicone preferably contains 90% by mass or more of polydimethylsiloxane having a mass average molecular weight MW of 60,000 or more and 700,000 or less.

[0015] (5) In the thermally conductive sheet of any one of (1) to (4) above, the silicone is preferably composed solely of uncrosslinked silicone.

[0016] (6) In the thermally conductive sheet of any one of (1) to (5) above, the thermally conductive filler preferably contains an anisotropic thermally conductive filler and a non-anisotropic thermally conductive filler.

[0017] (7) In the thermally conductive sheet of (6) above, it is preferable that the anisotropic thermally conductive filler is one or both of carbon fiber and scaly graphite powder, and the non-anisotropic thermally conductive filler is at least one selected from the group consisting of zinc oxide particles, aluminum nitride particles, aluminum oxide particles, and magnesium hydroxide particles.

[0018] (8) In the thermally conductive sheet of (3) above, it is preferable that the silicone contains 90% by mass or more of polydimethylsiloxane having a mass average molecular weight MW of 60,000 or more and 700,000 or less, the thermally conductive filler is carbon fiber, flake graphite powder, and non-anisotropic thermally conductive filler, the carbon fiber content is 30% by volume or more and 40% by volume or less, and the graphite powder content is 5% by volume or more and 25% by volume or less.

[0019] Hereinafter, embodiments of the present invention will be described. In the invention of this disclosure, the term "thermally conductive sheet" is a concept that includes both a block-shaped product obtained by molding, such as extrusion molding, and cut products (including sliced ​​sheet-shaped products) obtained by appropriately cutting the block-shaped product. Here, an embodiment of the thermally conductive sheet will be described using a sliced ​​sheet-shaped product as an example.

[0020] The thermally conductive sheet according to this embodiment is a component provided between an IC chip and a heat sink. The thermally conductive sheet is made of a resin composition containing silicone and a thermally conductive filler. Here, silicone is a polymeric compound having a main skeleton formed by siloxane bonds. FIG. 1 is a cross-sectional view schematically illustrating an IC chip to which a heat sink is attached via the thermally conductive sheet according to this embodiment. FIG. 2A is a perspective view schematically illustrating an example of a thermally conductive sheet according to an embodiment of the present invention. FIG. 2B is a partially enlarged view of the cross section taken along line A-A in FIG. 2A. Note that in the drawings of this application, FIGS. 1 to 4 are all schematic views and do not accurately reflect the actual dimensions of each component.

[0021] 1, the thermally conductive sheet 1 is disposed between the IC chip 11 and the heat sink 12. The thermally conductive sheet 1 is used with one surface in contact with the IC chip 11 and the other surface in contact with the heat sink 12. In this way, the heat sink 12 is attached to the IC chip 11 via the thermally conductive sheet 1. Therefore, the heat generated by the IC chip 11 is dissipated by the heat sink 12 to the outside of the housing (not shown).

[0022] 1 shows a usage example of the thermally conductive sheet 1 attached to the top surface of one IC chip 11. On the other hand, the thermally conductive sheet according to the embodiment of the present invention may be a thermally conductive sheet attached so as to simultaneously cover heat-generating components such as multiple IC chips.

[0023] The thermally conductive sheet 1 can be reproduced (level material recyclable). Therefore, the thermally conductive sheet 1 is environmentally friendly. The thermal conductivity of the thermally conductive sheet 1 is maintained at 90% or more after one reproduction. In other words, the thermal conductivity decreases with each reproduction by 10% or less of the thermal conductivity before reproduction. The thermally conductive sheet 1 has excellent level material recyclability.

[0024] 2A and 2B, the thermally conductive sheet 1 contains a matrix component 2 and a thermally conductive filler 4. In the thermally conductive sheet 1, all components other than the thermally conductive filler are collectively referred to as the matrix component. The thermally conductive sheet 1 may have weld lines formed in the thickness direction.

[0025] The matrix component 2 contains silicone. Therefore, the thermally conductive sheet 1 has excellent heat resistance. In this embodiment, the silicone is mainly composed of uncrosslinked silicone. Here, "mainly composed of uncrosslinked silicone" means that the silicone may be partially crosslinked, as long as the recyclability is not impaired. It is preferable that the silicone is composed only of uncrosslinked silicone. It is preferable that the silicone composition used to form the thermally conductive sheet 1 does not contain a crosslinking agent.

[0026] The uncrosslinked silicone is preferably polydimethylsiloxane, a silicone whose side chains are all methyl groups and contain no unsaturated groups. The silicone preferably contains 50% by mass or more of the polydimethylsiloxane, more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more. The polydimethylsiloxane is a polymer with low reactivity and excellent stability. Therefore, by increasing the proportion of the polydimethylsiloxane in the silicone, the thermal conductivity retention rate when the thermal conductive sheet 1 is recycled can be improved. In addition, increasing the proportion of the polydimethylsiloxane is suitable for providing a thermal conductive sheet 1 with excellent flexibility. The polydimethylsiloxane may be an oil or a millable type.

[0027] The molecular weight of the polydimethylsiloxane is preferably 60,000 or more and 700,000 or less in terms of mass average molecular weight MW. If the mass average molecular weight MW of the polydimethylsiloxane is less than 60,000, the polydimethylsiloxane tends to bleed out from the thermal conductive sheet 1. On the other hand, if the mass average molecular weight MW of the polydimethylsiloxane exceeds 700,000, the moldability and processability during production of the thermal conductive sheet 1 tend to be poor.

[0028] In the present invention, the weight average molecular weight MW of polydimethylsiloxane is the weight average molecular weight measured using gel permeation chromatography (GPC) using polystyrene as a standard substance in accordance with JIS-K7252-1:2008 "Plastics - Determination of average molecular weight and molecular weight distribution of polymers by size exclusion chromatography - Part 1: General rules."

[0029] The kinematic viscosity of the polydimethylsiloxane is preferably 10,000 cps or more and 100,000 cps or less, as measured with an Ubbelohde viscometer at 25° C. If the kinematic viscosity is less than 10,000 cps, the polydimethylsiloxane will tend to bleed out of the thermally conductive sheet 1. On the other hand, if the kinematic viscosity exceeds 100,000 cps, the hardness of the thermally conductive sheet 1 will increase, and when the thermally conductive sheet 1 is disposed between an IC chip and a heat sink, the sheet may have poor adhesion and conformability to the contact surfaces of the IC chip and the heat sink.

[0030] The silicone may contain silicone having a crosslinkable functional group such as a vinyl group in an uncrosslinked state.

[0031] The matrix component 2 may contain other elastomer components and the like to the extent that the required properties of the thermally conductive sheet 1 are not impaired.

[0032] The matrix component 2 may contain common additives such as flame retardants, reinforcing agents, fillers, softeners, plasticizers, antioxidants, tackifiers, antistatic agents, kneaded adhesives, and coupling agents. Examples of the flame retardants include aluminum hydroxide, platinum-based compounds, triazole-based compounds, iron oxides such as red iron oxide and black iron. These may be used alone or in combination of two or more.

[0033] The matrix component 2 preferably contains a coupling agent such as a silane coupling agent as an additive. In particular, when zinc oxide particles are contained as the thermally conductive filler, it is preferable to contain a silane coupling agent. By using a combination of zinc oxide particles and a silane coupling agent, the flexibility of the thermally conductive sheet 1 can be increased.

[0034] Examples of the silane coupling agent include methyltrimethoxysilane, ethyltrimethoxysilane, propyltrimethoxysilane, butyltrimethoxysilane, pentyltrimethoxysilane, hexyltrimethoxysilane, hexyltriethoxysilane, octyltrimethoxysilane, octyltriethoxysilane, decyltrimethoxysilane, decyltriethoxysilane, dodecyltrimethoxysilane, dodecyltriethoxysilane, hexadecyltrimethoxysilane, hexadecyltriethoxysilane, octadecyltrimethoxysilane, octadecyltriethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropyltriethoxysilane, etc. These may be used alone or in combination of two or more.

[0035] The thermally conductive sheet 1 contains a thermally conductive filler 4. The thermally conductive filler 4 preferably contains an anisotropic thermally conductive filler (hereinafter also referred to as an anisotropic filler) and a non-anisotropic thermally conductive filler (hereinafter also referred to as a non-anisotropic filler). In this case, it is easy to ensure both high thermal conductivity and flexibility, and the moldability during manufacturing is also good. On the other hand, if the thermally conductive sheet 1 contains only an anisotropic filler, cracks are likely to occur in the molded product during the manufacturing process (including the recycling process). Furthermore, if the thermally conductive sheet 1 contains only a non-anisotropic filler, it is difficult to increase the thermal conductivity of the thermally conductive sheet 1.

[0036] When the thermally conductive sheet 1 contains an anisotropic filler and a non-anisotropic filler, it is preferable that the particle size of the anisotropic filler is larger than the particle size of the non-anisotropic filler, which is suitable for ensuring excellent thermal conductivity.

[0037] In the thermally conductive filler, an anisotropic filler refers to a filler with an aspect ratio of 2.0 or more, and a non-anisotropic filler refers to a filler with an aspect ratio of less than 2.0. The aspect ratio of the thermally conductive filler refers to the larger of the ratio of the major axis of the filler to the minor axis of the filler and the ratio of the major axis of the filler to the thickness of the filler. The major axis, minor axis, and thickness correspond to the length, width, and height, respectively, of the circumscribed rectangular parallelepiped of the filler.

[0038] The anisotropic filler includes fibrous fillers, scaly fillers, plate-like fillers, flaky fillers, etc. The aspect ratio of the anisotropic filler is preferably 5.0 or more. The non-anisotropic filler includes spherical fillers, irregular fillers, etc.

[0039] The thermally conductive sheet 1 preferably contains a fibrous filler, a scaly filler, and a non-anisotropic filler as the thermally conductive filler, which makes it easier to increase the thermal conductivity of the thermally conductive sheet 1.

[0040] In this case, the content of the fibrous filler is preferably greater than the content of the scaly filler and the content of the non-anisotropic filler, respectively, which makes it easier to increase the thermal conductivity of the thermally conductive sheet 1.

[0041] The thermally conductive sheet 1 contains, as the thermally conductive filler, for example, carbon fibers 4C, scaly graphite powder 4G, and non-anisotropic filler 4P such as zinc oxide particles, aluminum nitride particles, aluminum oxide particles, or magnesium hydroxide particles. In this case, it is preferable that the carbon fibers 4C and graphite powder 4G are oriented substantially in the thickness direction of the thermally conductive sheet 1, and the non-anisotropic filler 4P is dispersed throughout the thermally conductive sheet 1. A thermally conductive sheet 1 having such a configuration is particularly suitable for increasing thermal conductivity.

[0042] The fiber length of the carbon fiber 4C is preferably 20 μm or more. If the fiber length is less than 20 μm, a heat conduction path is difficult to form, and the thermal conductivity of the thermal conductive sheet 1 may be poor. On the other hand, a preferred upper limit of the fiber length of the carbon fiber 4C is 500 μm from the viewpoint of ease of filling the thermal conductive sheet with the carbon fiber. The fiber length of the carbon fiber 4C is more preferably 50 μm or more and 300 μm or less, and even more preferably 100 μm or more and 300 μm or less.

[0043] The fiber diameter of the carbon fiber 4C is preferably 5 μm or more. If the fiber diameter is less than 5 μm, a heat conduction path is difficult to form, and the thermal conductivity of the thermal conductive sheet 1 may be poor. The preferred upper limit of the fiber diameter of the carbon fiber 4C is 20 μm from the viewpoint of processability when producing the thermal conductive sheet 1 (including when recycling).

[0044] The fiber length of a fibrous filler such as carbon fiber refers to the arithmetic mean value of the length in the fiber direction determined using a microscopic image of the fibrous filler. The fiber diameter of a fibrous filler refers to the arithmetic mean value of the dimension in the radial direction determined using a microscopic image of the fibrous filler. The fiber length and fiber diameter are determined based on the measurement results obtained by acquiring a microscopic image of the fibrous filler, randomly selecting 20 fibrous fillers from the image, and measuring the length in the fiber direction and the dimension in the radial direction of the selected fibrous fillers.

[0045] As the carbon fiber 4C, one type of carbon fiber may be used, or two or more types of carbon fiber may be used. Carbon fibers with different thermal conductivities may be used in combination as the carbon fiber 4C. When carbon fibers with high thermal conductivity are used, the thermal conductivity increases, but flexibility tends to decrease. Therefore, by using carbon fibers with different thermal conductivities in combination, it becomes easier to increase both the thermal conductivity and flexibility of the thermal conductive sheet 1.

[0046] The graphite powder 4G is preferably shaped like a flake. In this case, the graphite powder 4G is oriented in the thickness direction of the thermally conductive sheet 1, which is suitable for increasing the thermal conductivity in the thickness direction. The thermally conductive sheet 1 may contain graphite powder of a shape other than a flake shape as the graphite powder 4G. Scale-shaped graphite powder and graphite powder of a shape other than a flake shape may be used in combination.

[0047] The particle size of the flake graphite powder 4G is preferably 5 μm or more and 50 μm or less. If the particle size of the flake graphite powder 4G is less than 5 μm, it is difficult to form a heat conduction path within the thermal conductive sheet 1. On the other hand, if the particle size of the flake graphite powder 4G exceeds 50 μm, it is difficult to pack the powder densely. The particle size of the flake graphite powder 4G is preferably 10 μm or more and 30 μm or less.

[0048] The particle size of a scaly filler such as scaly graphite powder refers to the D50 (50% median diameter) of the maximum length in the plate surface direction obtained using a microscope image of the scaly filler for particle size measurement. The particle size is determined by obtaining a microscope image for particle size measurement of the scaly filler, randomly selecting 20 scaly fillers from the image, measuring the maximum length in the plate surface direction of the selected scaly fillers, and then based on the measurement results.

[0049] The zinc oxide particles are preferably irregular in shape, since in this case the zinc oxide particles are less likely to damage components of the manufacturing equipment.

[0050] The particle size of the zinc oxide particles is preferably 0.1 μm or more and 10 μm or less. If the particle size of the zinc oxide particles is less than 0.1 μm, it is difficult to form a heat conduction path. On the other hand, if the particle size of the zinc oxide particles exceeds 10 μm, the orientation of the carbon fiber 4C and the scaly graphite powder 4G is likely to be hindered. In addition, the zinc oxide particles are likely to damage the steel members of the manufacturing equipment.

[0051] The aluminum nitride particles are preferably irregular in shape, since in this case the aluminum nitride particles are less likely to damage components of the manufacturing equipment.

[0052] The particle size of the aluminum nitride particles is preferably 0.1 μm or more and 10 μm or less. If the particle size of the aluminum nitride particles is less than 0.1 μm, it is difficult to form a heat conduction path. On the other hand, if the particle size of the aluminum nitride particles exceeds 10 μm, the orientation of the carbon fiber 4C and the scaly graphite powder 4G is likely to be hindered. In addition, the aluminum nitride particles are likely to damage the steel members of the manufacturing equipment.

[0053] The aluminum oxide particles (also called alumina particles) are preferably spherical in shape, since this makes it easier to fill the thermally conductive sheet with alumina particles than with other shapes, and the alumina particles are less likely to damage components of the manufacturing equipment.

[0054] The particle size of the alumina particles is preferably 1 μm or more and 10 μm or less. If the particle size of the alumina particles is less than 1 μm, it is difficult to form a heat conduction path. On the other hand, if the particle size of the alumina particles exceeds 10 μm, the orientation of the carbon fiber 4C and the graphite powder 4G is likely to be hindered. Furthermore, the alumina particles are likely to damage the steel components of the manufacturing equipment.

[0055] The magnesium hydroxide particles are preferably irregular in shape, since in this case the magnesium hydroxide particles are less likely to damage components of the production equipment.

[0056] The particle size of the magnesium hydroxide particles is preferably 1 μm or more and 10 μm or less. If the particle size of the magnesium hydroxide particles is less than 1 μm, it is difficult to form a heat conduction path. On the other hand, if the particle size of the magnesium hydroxide particles exceeds 10 μm, the orientation of the carbon fiber 4C and the graphite powder 4G is likely to be hindered. Furthermore, the magnesium hydroxide particles are likely to damage the steel members of the manufacturing equipment.

[0057] In the embodiment of the present invention, the particle size of the thermally conductive filler other than the fibrous filler and the scaly filler refers to the value of the median diameter (d50) measured by a laser diffraction / scattering method.

[0058] The content of the thermally conductive filler 4 in the thermally conductive sheet 1 is preferably 50% by volume or more and 70% by volume or less. In particular, when the thermally conductive filler 4 contains carbon fibers 4C, flake graphite powder 4G, and non-anisotropic filler 4P, the preferred content of the thermally conductive filler 4 in the thermally conductive sheet 1 is 50% by volume or more and 70% by volume or less, where the total content of the carbon fibers 4C, flake graphite powder 4G, and non-anisotropic filler 4P is less than 50% by volume, sufficient thermal conductivity may not be ensured. On the other hand, if the total content exceeds 70% by volume, the thermally conductive sheet 1 becomes too hard and may not be able to conform to the shape of the component to which the thermally conductive sheet 1 is attached, such as an IC chip, during installation. Furthermore, the thermally conductive sheet 1 is prone to cracking when bent. Furthermore, during the slicing process, which is one step in the manufacturing process of the thermally conductive sheet 1, cracks may occur in the sheet during molding, making it impossible to maintain the sheet shape. In the thermally conductive sheet 1, the total content of the carbon fiber 4C, the scaly graphite powder 4G, and the non-anisotropic filler 4P is preferably 55% by volume or more and 70% by volume or less. This is more suitable for achieving both good thermal conductivity and appropriate hardness.

[0059] The content of carbon fiber 4C in thermally conductive sheet 1 is preferably 30% by volume or more and 40% by volume or less, which is suitable for ensuring good thermal conductivity of thermally conductive sheet 1. When the content of carbon fiber 4C in thermally conductive sheet 1 is 30% by volume or more and 40% by volume or less, it is further preferable that the content of graphite powder 4G is 5% by volume or more and 25% by volume or less. By setting the total content of the thermally conductive filler to 50% by volume or more and 70% by volume or less and keeping the content of carbon fiber 4C and scaly graphite powder 4G within the above ranges, it is suitable for maintaining good thermal conductivity even after recycling.

[0060] The total content of the scaly graphite powder 4G and the non-anisotropic filler 4P in the thermally conductive sheet 1 is preferably 20% by volume or more and 35% by volume or less, from the viewpoint of ensuring good thermal conductivity.

[0061] The thermally conductive sheet 1 preferably has a viscosity measured with a capillary rheometer at a shear rate of 10 (1 / s) and a temperature of 30°C (hereinafter referred to as the viscosity measured with a capillary rheometer) of 100 kPa·s or more and 300 kPa·s or less. This is suitable for maintaining the thermal conductivity of 90% or more when the thermally conductive sheet 1 is reproduced.

[0062] If the viscosity measured by the capillary rheometer is less than 100 kPa·s, the silicone composition has poor moldability, making it difficult to mold the thermally conductive sheet 1. Even if molding is possible, the orientation of the anisotropic filler is poor. If the viscosity measured by the capillary rheometer exceeds 300 kPa·s, the thermal conductivity decreases upon regeneration, making it difficult to maintain a thermal conductivity rate of 90% or more. One reason for the decrease in thermal conductivity each time the composition is recycled is thought to be damage to the thermally conductive filler during the process of re-preparing the silicone composition (kneading process) and the process of extrusion molding the resulting silicone composition (extrusion molding process). In particular, when a fibrous filler such as carbon fiber is used as the thermally conductive filler, the fibrous filler is prone to breakage during the kneading process or extrusion molding process, and breaking of the fibrous filler can significantly decrease the thermal conductivity. In contrast, if the composition of the thermally conductive sheet 1 is such that the viscosity measured by the capillary rheometer is 300 kPa·s or less, the thermally conductive filler is less likely to break during the kneading and extrusion molding processes for reproducing the thermally conductive sheet 1, and the thermal conductivity is less likely to decrease even when the thermally conductive sheet is reproduced. Furthermore, if the viscosity measured by the capillary rheometer exceeds 300 kPa·s, the sheet may be difficult to reproduce due to poor fluidity and the tendency for agglomerates to form during the kneading process during reproduction, and the anisotropic filler may be difficult to orient when the silicone composition is molded into a sheet. Furthermore, if the viscosity measured by the capillary rheometer exceeds 300 kPa·s, the thermally conductive sheet 1 is prone to cracking when bent, which may result in poor handleability. The viscosity measured by the capillary rheometer is more preferably 150 kPa·s or more and 250 kPa·s or less.

[0063] The viscosity measured by the capillary rheometer is measured using a measurement sample made by breaking down the thermal conductive sheet 1. The viscosity measurement by the capillary rheometer is performed in accordance with JIS K 7199 (1999). The shear rate is 10 (1 / s) and the measurement temperature is 30°C.

[0064] The apparent thermal conductivity of the thermal conductive sheet 1 is preferably 10 W / mK or more. The apparent thermal conductivity is the apparent thermal conductivity when the thermal conductive sheet 1 is compressed and deformed by 20% in the thickness direction (hereinafter also simply referred to as apparent thermal conductivity). The apparent thermal conductivity is the thermal resistance value (Kcm) measured in a state where the thermal conductive sheet is compressed to a thickness of "thickness before measurement x 0.8" (20% compression). ２ / W) and the thickness (cm) of the thermally conductive sheet 1 at the time of measurement, according to the following calculation formula (1).

[0065] Apparent thermal conductivity (W / mK) = thickness of thermal conductive sheet at time of measurement (cm) ÷ thermal resistance value when compressed and deformed by 20% (Kcm ２ / W)×100 (1) The apparent thermal conductivity is more preferably 25 W / mK or more, and further preferably 32 W / mK or more.

[0066] The thickness of the thermally conductive sheet 1 is not particularly limited, but is, for example, 0.05 mm or more and 3.0 mm or less. In this case, the thermally conductive sheet 1 can be suitably used as a member that efficiently transfers heat between the IC chip 11 and the heat sink 12. The thickness of the thermally conductive sheet 1 is preferably 0.05 mm or more and 2.5 mm or less. This ensures better heat dissipation performance while ensuring conformability to the contact surfaces of the IC chip 11 and the heat sink 12. On the other hand, if the thickness of the thermally conductive sheet 1 is less than 0.05 mm, it may not be able to fully conform to the contact surfaces of the IC chip 11 and the heat sink 12. Furthermore, if the thickness exceeds 2.5 mm, the thermal resistance of the sheet itself may result in poor heat dissipation performance.

[0067] The planar shape of the thermally conductive sheet 1 is, for example, rectangular. In this case, the vertical and horizontal dimensions of the thermally conductive sheet 1 may be determined taking into consideration the dimensions of the component to which the thermally conductive sheet 1 is attached, such as the IC chip 11, and for example, both the vertical and horizontal dimensions are independently 10 mm to 120 mm. The planar shape of the thermally conductive sheet 1 is not limited to rectangular, and may be a shape other than rectangular, such as circular or elliptical. In the case of a circular shape, the diameter is, for example, 10 mm to 120 mm. In the case of an elliptical shape, the major axis or minor axis is, for example, 10 mm to 120 mm.

[0068] The thermally conductive sheet 1 is manufactured, for example, by a manufacturing method described below, into a large thermally conductive sheet, which is then punched into a predetermined shape and size. In an embodiment of the present invention, the remaining portion of the large thermally conductive sheet after punching out the thermally conductive sheet 1 into a predetermined shape and size can be used as a material for remanufacturing, enabling level material recycling. In this respect, an embodiment of the present invention is environmentally friendly.

[0069] Next, we will explain the method for producing the thermally conductive sheet 1. The thermally conductive sheet 1 can be produced, for example, by a first production method that involves the following steps (a) to (c): (a) preparing a silicone-based composition containing uncrosslinked silicone, a thermally conductive filler, and optional components such as a flame retardant and a coupling agent; (b) molding the prepared silicone-based composition; and (c) slicing the molded silicone-based composition into sheets.

[0070] First, step (a) is carried out to prepare a silicone composition. Here, the silicone composition is prepared by, for example, kneading uncrosslinked silicone, a thermally conductive filler, and various additives as needed using a twin roll mill. At this time, some or all of the components may be supplied as a compound.

[0071] Next, the prepared silicone composition is molded in step (b) and sliced ​​into sheets in step (c). The silicone composition can be molded, for example, using an extruder. Figure 3 is a cross-sectional schematic diagram showing the tip of an extruder and a T-die used in the manufacture of a thermally conductive sheet 1 according to an embodiment of the present invention. The silicone composition introduced into the extruder 30 is stirred and kneaded by a screw 34 and introduced into a first gap 32 of the T-die along a flow path 31.

[0072] The silicone-based composition stirred and kneaded in the extruder 30 is first squeezed in the vertical direction (thickness direction) by the first gap 32 to form a thin strip. At this time, the anisotropic thermally conductive filler mixed in the silicone-based composition is oriented in the flow direction (extrusion direction) of the silicone-based composition. Therefore, in the thin resin sheet 40 formed after passing through the first gap 32, the anisotropic thermally conductive filler is oriented in the plane direction of the resin sheet 40. In this embodiment, for example, carbon fiber or flaky graphite powder corresponds to the anisotropic thermally conductive filler.

[0073] When the thin resin sheet 40 in which the thermally conductive filler is oriented passes completely through the first gap 32, the flow direction of the sheet, which was limited to the extrusion direction, is released and the flow direction changes to a direction approximately perpendicular to the extrusion direction. After completely passing through the first gap 32, the resin sheet 40, whose flow direction has changed to a direction approximately perpendicular to the extrusion direction, is further extruded toward the second gap 33. As a result, the resin sheet 40, now approximately perpendicular to the extrusion direction, is folded and stacked in the second gap 33. At this time, most of the anisotropic thermally conductive filler (carbon fiber and flake graphite powder) is oriented in the plane direction of the resin sheet 40, so that the anisotropic thermally conductive filler in the resin sheet 40 stacked in the second gap 33 is oriented along the thickness direction (the vertical direction in FIG. 3 ).

[0074] In this way, in step (b), the silicone composition is extruded to form a resin sheet 40 in which the anisotropic thermally conductive filler is oriented in the extrusion direction, and then the resin sheet 40 is folded and laminated to form a block. The extrusion of the silicone composition may be carried out in a heated environment of, for example, 30 to 150°C.

[0075] In the T-die, the depths of the first gap 32 and the second gap 33 (i.e., the dimensions of the first gap 32 and the second gap 33 in the direction perpendicular to the paper surface in FIG. 3 ) are substantially uniform throughout the T-die. Furthermore, the depth dimensions of the first gap and the second gap are not particularly limited, and various design changes are possible depending on the product width of the thermally conductive sheet 1 to be manufactured.

[0076] Then, in step (c), the block of thin resin sheets 40 is sliced ​​in a direction perpendicular to the thickness direction. As a result, a thermally conductive sheet 1 having a predetermined thickness and in which the anisotropic thermally conductive filler is substantially oriented in the thickness direction can be obtained. The block of resin sheet 40 produced in step (b) can also be used as a sheet-like thermally conductive sheet according to an embodiment of the present invention. The slicing process can be performed using, for example, an ultrasonic cutter or the like.

[0077] The method for manufacturing the thermally conductive sheet 1 is not limited to the first manufacturing method described above, but may be, for example, a second manufacturing method in which the following steps (d) to (f) are performed. Figures 4A to 4D are diagrams illustrating the second manufacturing method.

[0078] (d) preparing a silicone composition containing uncrosslinked silicone, a thermally conductive filler, and optional components such as a flame retardant and a coupling agent; (e) molding the prepared silicone composition; and (f) slicing the molded silicone composition into sheets.

[0079] First, step (d) of preparing a silicone-based composition is carried out. Here, for example, uncrosslinked silicone, a thermally conductive filler, and various additives added as needed are kneaded with two rolls 51. Then, a sheet is formed to produce a resin sheet 50 (see FIG. 4A). At this time, some or all of the components may be supplied as a compound. Furthermore, kneading may be carried out, for example, by heating the rolls 51 to 30 to 150°C.

[0080] Next, step (e) of molding the silicone-based composition is performed. In this step (e), resin sheets 50 made of the silicone-based composition are folded and stacked so that the resin sheets 50 are in close contact with each other (see FIG. 4A ). For example, by continuously supplying the resin sheets 50 onto a table 53 that periodically reciprocates, a laminate can be obtained in which the resin sheets 50 are stacked while being folded. At this time, the anisotropic thermally conductive filler is oriented in the plane direction of the resin sheets 50.

[0081] Thereafter, the folded portion of the resin sheet 50 is cut and removed using a cutter 54 (see FIG. 4B), resulting in a stack 55 of a plurality of resin sheets 50 that are not connected to one another.

[0082] Next, step (f) is performed in which the obtained laminate 55 is sliced ​​using a cutter 57 in a direction perpendicular to the surface of the resin sheet 50 (see FIG. 4C). This allows the thermally conductive sheet 1 to be obtained (see FIG. 4D). This process also allows the thermally conductive sheet 1 to be manufactured. The same method as in the first manufacturing method can be used for the slicing process.

[0083] Next, a method for reproducing the thermally conductive sheet 1 will be described. First, the finished thermally conductive sheet 1 is cut. Next, the cut thermally conductive sheet is kneaded using two rolls to prepare a silicone-based composition. Thereafter, similar to the first or second manufacturing method, a process for molding the silicone-based composition and a process for slicing the molded product into sheets are performed to reproduce the thermally conductive sheet. At this time, fragments of the thermally conductive sheet generated during the manufacturing process of the thermally conductive sheet 1, such as fragments of the thermally conductive sheet generated when punching out the thermally conductive sheet 1 having the predetermined shape and dimensions described above, as well as silicone-based composition that did not constitute the thermally conductive sheet 1, can also be recovered and used as materials for reproducing the thermally conductive sheet 1.

[0084] Hereinafter, the embodiments of the present invention will be described in more detail with reference to examples.

[0085] The raw materials used in the examples and comparative examples are as follows: (Silicone (matrix component)) Silicone (A): Silicone oil (KF-96-100,000cs, manufactured by Shin-Etsu Chemical Co., Ltd.) Silicone (B): Vinyl group-containing compound (MR-53, manufactured by Dow Toray Industries, Inc.) Peroxide: (RC-4 50P FD, manufactured by Dow Toray Industries, Inc.)

[0086] (Thermal conductive filler) Carbon fiber (A): K223HM (fibrous, fiber length: 200 μm / fiber diameter: 11 μm) manufactured by Mitsubishi Chemical Corporation Carbon fiber (B): K23EHM (fibrous, fiber length: 200 μm / fiber diameter: 11 μm) manufactured by Mitsubishi Chemical Corporation Graphite powder: CPB (flake, particle size: 22 μm) manufactured by Nippon Graphite Industries Co., Ltd. Alumina particles: ASFP-09S (spherical, particle size: 1 μm) manufactured by Denka Company Limited Zinc oxide particles: Zinc oxide type 1 (irregular, particle size: 0.8 μm) manufactured by Sakai Chemical Industry Co., Ltd. Magnesium hydroxide particles: EP-1A (irregular, particle size: 2 μm) manufactured by Konoshima Chemical Co., Ltd. Aluminum nitride particles: HF01Da (irregular, particle size: 1 μm) manufactured by Tokuyama Corporation

[0087] Example 1 In this example, a thermally conductive sheet was produced by Manufacturing Method 1. 100 parts by mass of silicone (A), 118 parts by mass of carbon fiber (A), 60 parts by mass of carbon fiber (B), 60 parts by mass of graphite powder, 309 parts by mass of zinc oxide particles, and 16 parts by mass of magnesium hydroxide particles were kneaded using two rolls 51 and then sheeted out to produce a ribbon-shaped sheet (silicone-based composition) with a thickness of approximately 1.0 to 1.2 mm.

[0088] Next, the ribbon-shaped sheet was extruded in a rubber short-screw extruder 30 using a die having a 1 mm first gap and a 10 mm second gap (see FIG. 3 ) to produce a 10 mm thick sheet (block). The temperature inside the extruder was adjusted to 100°C. The resulting sheet was then sliced ​​perpendicular to the thickness direction to produce a 2 mm thick thermally conductive sheet. The slicing was performed while ultrasonically vibrating a single-edged fixed blade. In this example, the volume fraction of the carbon fiber (A) in the entire thermally conductive sheet was 20 vol%, the volume fraction of the carbon fiber (B) was 10 vol%, the volume fraction of the graphite powder was 10 vol%, the volume fraction of the zinc oxide particles was 20 vol%, and the volume fraction of the magnesium hydroxide particles was 2.5 vol%, resulting in a total volume fraction of the thermally conductive filler of 62.5 vol%.

[0089] Example 2 A thermally conductive sheet was manufactured in the same manner as in Example 1, except that a ribbon-shaped sheet approximately 1.0 to 1.2 mm thick was prepared using the following method. 100 parts by mass of silicone (A), 150 parts by mass of carbon fiber (A), 100 parts by mass of graphite powder, 14 parts by mass of magnesium hydroxide particles, and 18 parts by mass of aluminum nitride particles were kneaded using two rolls 51 and then sheeted out to produce a ribbon-shaped sheet (silicone-based composition) approximately 1.0 to 1.2 mm thick. In this example, the volume fraction of carbon fiber (A) relative to the entire thermally conductive sheet was 30% by volume, the volume fraction of graphite powder was 20% by volume, the volume fraction of magnesium hydroxide particles was 2.5% by volume, and the volume fraction of aluminum nitride particles was 2.5% by volume, resulting in a total volume fraction of the thermally conductive fillers of 55.0% by volume.

[0090] Example 3 In this example, a thermally conductive sheet was manufactured by Manufacturing Method 1. 100 parts by mass of silicone (A), 0.53 parts by mass of silicone (B), 0.23 parts by mass of peroxide, and 225 parts by mass of graphite powder were kneaded using two rolls 51 and then sheeted out to produce a ribbon-shaped sheet (silicone-based composition) with a thickness of approximately 1.0 to 1.2 mm.

[0091] Next, the ribbon-shaped sheet was extruded in a rubber single-screw extruder 30 using a mold with a 1 mm first gap and a 10 mm second gap (see FIG. 3) to produce a 10 mm thick sheet (block). The temperature inside the extruder was adjusted to 100°C. The resulting sheet (block) was then heat-treated at 170°C for 40 minutes to crosslink it. The resulting sheet was then sliced ​​perpendicular to the thickness direction to produce a 2 mm thick thermally conductive sheet. The slicing was performed while ultrasonically vibrating a single-edged fixed blade. In this example, the volume fraction of the graphite powder in the entire thermally conductive sheet was 50.4 vol%, and the total volume fraction of the thermally conductive filler was 50.4 vol%.

[0092] Comparative Example 1 A thermally conductive sheet was produced in the same manner as in Example 1, except that a ribbon-shaped sheet having a thickness of approximately 1.0 to 1.2 mm was produced by the following method. 100 parts by mass of silicone (A), 330 parts by mass of carbon fiber (A), 110 parts by mass of carbon fiber (B), 175 parts by mass of alumina particles, and 525 parts by mass of zinc oxide particles were kneaded using two rolls 51 and then sheeted out to produce a ribbon-shaped sheet (silicone-based composition) having a thickness of approximately 1.0 to 1.2 mm. In this comparative example, the volume fraction of carbon fiber (A) relative to the entire thermally conductive sheet was 33.75% by volume, the volume fraction of carbon fiber (B) was 11.25% by volume, the volume fraction of alumina particles was 10% by volume, and the volume fraction of zinc oxide particles was 20% by volume, resulting in a total volume fraction of the thermally conductive fillers of 75.0% by volume.

[0093] Comparative Example 2 A thermally conductive sheet was manufactured in the same manner as in Example 3, except that a ribbon-shaped sheet approximately 1.0 to 1.2 mm thick was prepared using the following method. 100 parts by mass of silicone (A), 1.6 parts by mass of silicone (B), 0.68 parts by mass of peroxide, and 225 parts by mass of graphite powder were kneaded using two rolls 51 and then sheeted out to prepare a ribbon-shaped sheet (silicone-based composition) approximately 1.0 to 1.2 mm thick. In this comparative example, the volume fraction of the graphite powder relative to the entire thermally conductive sheet was 50% by volume, and the total volume fraction of the thermally conductive filler was 50.0% by volume.

[0094] [Evaluation Test] (1) Apparent Thermal Conductivity at 20% Compression The thermally conductive sheets prepared in the Examples and Comparative Examples were further cut to prepare evaluation samples with a diameter of 33 mm and a thickness of 2 mm. Using a TIMtester 1400 (manufactured by AnalysisTech) as a measuring instrument, the sheet was compressed to a thickness of "thickness before measurement x 0.8" (20% compression), and the thermal resistance was measured in that state. Furthermore, based on the obtained thermal resistance value, the apparent thermal conductivity at 20% compression was calculated. The results are shown in Table 1.

[0095] (2) Viscosity by Capillary Rheometer The thermally conductive sheet was cut into small pieces, and a sufficient amount of pieces to fill a barrel was prepared as a measurement sample. The viscosity of the measurement sample was measured at a shear rate of 10 (1 / s) using a capillary rheometer (RHEOGRAPH 75, manufactured by GOETTFERT). The results are shown in Table 1. Other measurement conditions were as follows: Short die L / D = 0 / 1 (L = 0.2 mm), Barrel diameter: 15 mm, Measurement temperature: 30°C.

[0096] (3) The handling properties of the sheet were evaluated according to the following criteria: Good: The sheet can be handled without any problems. Poor: The sheet cracks when it is bent even slightly during handling.

[0097] (4) Recyclability (thermal conductivity retention rate) The obtained thermally conductive sheet was cut into small pieces, kneaded with two rolls 51, and then sheeted out to produce ribbon-shaped sheets with a thickness of approximately 1.0 to 1.2 mm again. The method employed in each example and comparative example was then repeated to reproduce the thermally conductive sheet. The thermal conductivity (apparent thermal conductivity when compressed by 20%) of the obtained thermally conductive sheet was measured again, and the retention rate (%) relative to the previous apparent thermal conductivity was calculated using the following formula. The results are shown in Table 1. Retention rate (%) = (apparent thermal conductivity at second compression / apparent thermal conductivity at first compression) × 100

[0098]

[0099] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, and it is intended to include all modifications within the meaning and scope of the claims.

[0100] REFERENCE SIGNS LIST 1 Thermally conductive sheet 2 Matrix component 4 Thermally conductive filler 4C Carbon fiber 4G Graphite powder 4P Non-anisotropic filler 11 IC chip 12 Heat sink 30 Extruder 31 Flow path 32 First gap 33 Second gap 34 Screw 40, 50 Resin sheet 51 Roll 53 Table 54, 57 Cutter 55 Laminate

Claims

1. A thermally conductive sheet made of a resin composition containing silicone and a thermally conductive filler, which is reproducible and maintains thermal conductivity of 90% or more after one reproduction.

2. The thermally conductive sheet according to claim 1, having a viscosity of 100 kPa·s or more and 300 kPa·s or less, measured using a capillary rheometer at a shear rate of 10 (1 / s) and a temperature of 30°C.

3. A thermally conductive sheet according to claim 1 or 2, wherein the content of the thermally conductive filler in the thermally conductive sheet is 50 volume % or more and 70 volume % or less.

4. A thermally conductive sheet according to any one of claims 1 to 3, wherein the silicone contains 90 mass% or more of polydimethylsiloxane having a mass average molecular weight MW of 60,000 or more and 700,000 or less.

5. A thermally conductive sheet according to any one of claims 1 to 4, wherein the silicone is composed solely of uncrosslinked silicone.

6. A thermally conductive sheet according to any one of claims 1 to 5, wherein the thermally conductive filler contains anisotropic thermally conductive filler and non-anisotropic thermally conductive filler.

7. The thermally conductive sheet according to claim 6, wherein the anisotropic thermally conductive filler is one or both of carbon fiber and scaly graphite powder, and the non-anisotropic thermally conductive filler is at least one selected from the group consisting of zinc oxide particles, aluminum nitride particles, aluminum oxide particles, and magnesium hydroxide particles.

8. The thermally conductive sheet according to claim 3, wherein the silicone contains 90 mass% or more of polydimethylsiloxane having a mass average molecular weight MW of 60,000 or more and 700,000 or less, the thermally conductive filler is carbon fiber, flake graphite powder, and non-anisotropic thermally conductive filler, the carbon fiber content is 30 volume% or more and 40 volume% or less, and the graphite powder content is 5 volume% or more and 25 volume% or less.

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