Thermal conductive sheet

Abstract

To provide a thermal conductive sheet with excellent reworkability and thermal conductivity. [Solution] A thermal conductive sheet comprising a first thermal conductive layer containing first thermal conductive particles and a binder resin, and a second thermal conductive layer disposed on at least one main surface of the first thermal conductive layer and mainly composed of second thermal conductive particles.

Description

[Technical Field]

[0001] This invention relates to a thermal conductive sheet. [Background technology]

[0002] In recent years, electronic components such as power semiconductors (IGBT modules, etc.) and integrated circuit (IC) chips have been generating increasing amounts of heat as their performance has improved. As a result, electronic devices using these components require measures to prevent functional failures caused by temperature increases in the electronic components.

[0003] To prevent malfunctions caused by temperature rise in electronic components, a common method is to promote heat dissipation by attaching heat sinks, heat dissipation plates, or heat fins made of metal to the heat-generating elements of electronic components. When using heat sinks, a sheet-like material with high thermal conductivity (hereinafter sometimes referred to as a "thermal conductive sheet") is used between the heat-generating element and the heat sink to efficiently transfer heat from the heat-generating element to the heat sink.

[0004] Thermal conductive sheets are required to have excellent adhesion to heat-generating and heat-dissipating elements, as well as excellent flexibility to adequately follow the expansion and contraction of heat-generating elements, from the viewpoint of enhancing thermal conductivity. However, thermal conductive sheets with such excellent adhesion and flexibility tend to be sticky. As a result, when storing thermal conductive sheets, if multiple sheets are stacked, the sheets will stick to each other, resulting in poor handling. Therefore, for example, Patent Document 1 proposes a technique to suppress the phenomenon of sheets sticking to each other by attaching powder to at least one side of the base sheet. Furthermore, for example, Patent Document 2 discloses a heat dissipation sheet in which boron nitride powder is present on the surface of a silicone rubber sheet filled with electrically insulating and thermally conductive inorganic powder. Such a heat dissipation sheet can also prevent sheets from sticking to each other. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2007-246664 [Patent Document 2] Japanese Patent Application Publication No. 06-096617 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] In this context, thermal conductive sheets may need to be removed from the substrate, such as heat-generating and heat-sinking elements, during maintenance of the equipment to which they are attached. At this time, it is necessary that no residue of the thermal conductive sheet is left on the surface of the substrate; in other words, excellent reworkability is required. Furthermore, thermal conductive sheets are naturally required to have excellent thermal conductivity. The conventional sheets described above had room for improvement in achieving a high level of both reworkability and thermal conductivity simultaneously.

[0007] Therefore, the present invention aims to provide a thermal conductive sheet that is excellent in terms of reworkability and thermal conductivity. [Means for solving the problem]

[0008] The inventors diligently conducted studies to achieve the above objectives. They then discovered that a thermal conductive sheet comprising a first thermal conductive layer containing first thermal conductive particles and a binder resin, and a second thermal conductive layer disposed on at least one main surface of the first thermal conductive layer and mainly composed of second thermal conductive particles, can achieve high levels of both reworkability and thermal conductivity when the first thermal conductive particles in the first thermal conductive layer are oriented in the thickness direction, thus completing the present invention.

[0009] In other words, the present invention aims to advantageously solve the above problems, and [1] the present invention is a thermal conductive sheet comprising: a first thermal conductive layer containing first thermal conductive particles and a binder resin; and a second thermal conductive layer disposed on at least one main surface of the first thermal conductive layer and mainly composed of second thermal conductive particles, wherein in the first thermal conductive layer, the first thermal conductive particles are oriented in the thickness direction of the first thermal conductive layer, and the first thermal conductive particles and the second thermal conductive particles may be of the same type or different. The thermal conductive sheets described above offer excellent reworkability and thermal conductivity. In this specification, the "main surface" of a thermal conductive sheet means the surface having the largest area on the thermal conductive sheet. The orientation of the first thermal conductive particles in the thickness direction of the first thermal conductive layer means that they are oriented at an angle of 60° to 90° with respect to the main surface of the first thermal conductive layer, and this can be verified by the method described in the examples. Furthermore, in this specification, when the second heat conduction layer is said to "mainly consist of second heat conduction particles," it means that, based on the total volume of the second heat conduction layer, more than 50% by volume consists of second heat conduction particles.

[0010] [2] The thermal conductive sheet described in [1] above preferably has a first thermal conductive particle which is selected from the group including boron nitride, graphite, molybdenum disulfide, and tungsten disulfide, and which has a flaky shape. Such a thermal conductive sheet exhibits even greater thermal conductivity. In this specification, the term "flaky" for thermal conductive particles means that the thermal conductive particles are flaky primary particles, or aggregates composed of such primary particles.

[0011] [3] In the thermal conductive sheet described in [1] or [2] above, it is preferable that the shape of the second thermal conductive particles is flaky. Such thermal conductive sheets offer even greater reworkability and thermal conductivity.

[0012] [4] In any of the heat conduction sheets of [1] to [3] above, it is preferable that the first heat conductive particles and the second heat conductive particles are of the same kind. Such a heat conduction sheet is further excellent in reworkability and heat conductivity.

[0013] [5] In any of the heat conduction sheets of [1] to [4] above, when the first heat conduction layer includes the second heat conduction layer on one main surface, the tack force of the other main surface of the first heat conduction layer without the second heat conduction layer is 10 N / cm 2 or more and 100 N / cm 2 or less. Such a conductive sheet is further excellent in heat conductivity. Note that the tack force of the heat conduction layer can be measured according to the method described in the examples of this specification.

[0014] [6] In any of the heat conduction sheets of [1] to [5] above, the adhesion amount of the second heat conduction layer is 1.5 g / m 2 or more and 20.0 g / m 2 or less. Such a conductive sheet is further excellent in reworkability and heat conductivity. Note that the adhesion amount of the second heat conduction layer can be calculated by subtracting the mass (g) of the first heat conduction layer from the total mass (g) after the formation of the second heat conduction layer and dividing by the area (m 2 ) of the second heat conduction layer.

[0015] [7] In any of the heat conduction sheets of [1] to [6] above, it is preferable that the volume average particle diameter D50 of the second heat conductive particles is 4 μm or more. Such a conductive sheet is further excellent in reworkability and heat conductivity. Note that the volume average particle diameter D50 of the second heat conductive particles means the median diameter based on volume, can be measured in accordance with JIS Z8825, and represents the particle diameter at which the cumulative volume calculated from the small diameter side is 50% in the particle size distribution (volume basis) measured by the laser diffraction method.

Advantages of the Invention

[0016] According to the present invention, a thermal conductive sheet with excellent reworkability and thermal conductivity can be provided. [Brief explanation of the drawing]

[0017] [Figure 1] This diagram illustrates the use of the thermal conductive sheet according to the present invention. [Modes for carrying out the invention]

[0018] Embodiments of the present invention will be described in detail below. Here, the thermal conductive sheet of the present invention can be used by being sandwiched between a heat-generating element and a heat-sinking element. That is, the thermal conductive sheet of the present invention can function as a heat-dissipating member and can constitute a heat dissipation device together with heat-sinking elements such as heat sinks, heat sinks, and heat sinking fins.

[0019] (Thermal conductive sheet) The thermal conductive sheet of the present invention is a laminate in which a second thermal conductive layer is disposed on at least one main surface of a first thermal conductive layer. The first thermal conductive layer contains first thermal conductive particles and a binder resin. The first thermal conductive particles are oriented in the thickness direction of the first thermal conductive layer. The second thermal conductive layer mainly consists of second thermal conductive particles. The first thermal conductive particles and the second thermal conductive particles may be of the same type or different types. The second thermal conductive layer may be disposed on one main surface of the first thermal conductive layer or on both main surfaces.

[0020] The thermal conductive sheet of the present invention has a configuration in which a second thermal conductive layer mainly composed of second thermal conductive particles is disposed on at least one main surface of a first thermal conductive layer containing first thermal conductive particles and a binder resin. Therefore, when peeling from an adherend that adheres to the sheet via the second thermal conductive layer, the residue of components of the thermal conductive sheet on the adherend surface can be effectively suppressed, resulting in excellent reworkability. Furthermore, in the thermal conductive sheet of the present invention, the first thermal conductive particles in the first thermal conductive layer are oriented in the thickness direction, thus providing excellent thermal conductivity.

[0021] Figure 1 shows an illustrative diagram of the use of the thermal conductive sheet according to the present invention. The thermal conductive sheet 10 shown in Figure 1 has a structure in which a second thermal conductive layer 12 is arranged on one main surface of a first thermal conductive layer 11. When using the equipment 30, which includes the thermal conductive sheet 10, a heat sink 21, and a heat generating element 22, as shown in the upper part of Figure 1, the main surface of the second thermal conductive layer 12 is in close contact with the heat sink 21, which is the adherend. In the embodiment shown in Figure 1, the first thermal conductive layer 11 does not have a second thermal conductive layer on the other main surface and is in direct contact with the heat generating element 22, which is the adherend. When it becomes necessary to replace the thermal conductive sheet, such as during maintenance of the equipment 30, the second thermal conductive layer, which mainly consists of second thermal conductive particles, can be peeled off the surface of the heat sink 21 without leaving any residue, as shown in the lower part of Figure 1. The thermal conductive sheet 10, which leaves minimal residue on the surface of the heat sink 21 (the adherend), saves labor costs for removing residue and cleaning the surface of the adherend, resulting in excellent reworkability. Unlike the embodiment shown in Figure 1, when the thermal conductive sheet has a second thermal conductive layer on both main surfaces, it becomes easier to peel it from both adherends (e.g., the heat generating element and the heat sink), resulting in excellent reworkability.

[0022] (First thermal conductive layer) The first thermal conductive layer of the thermal conductive sheet contains first thermal conductive particles oriented in the thickness direction and a binder resin. Because the first thermal conductive layer contains first thermal conductive particles oriented in the thickness direction, it has excellent thermal conductivity. The first thermal conductive layer may also contain other components besides the first thermal conductive particles and the binder resin. Such other components are not particularly limited and known components can be used.

[0023] <First thermally conductive particle> Here, the first thermally conductive particles are preferably flaky in shape. If the first thermally conductive particles are flaky, their thermal conductivity can be further enhanced. The first thermally conductive particles may also have a layered structure, similar to the second thermally conductive particles described later. Furthermore, the first thermally conductive particles, similar to the second thermally conductive particles described later, have a true specific gravity of 3.5 g / cm³. 3 The following is also acceptable.

[0024] Here, the first thermally conductive particle is preferably at least one selected from the group including boron nitride, graphite, molybdenum disulfide, and tungsten disulfide, and among these, boron nitride is more preferable from the viewpoint of imparting insulating properties to the thermally conductive sheet.

[0025] Furthermore, the arrangement of the first thermal conductive particles is not particularly limited, as long as they are oriented in the thickness direction of the first thermal conductive layer, that is, oriented at an angle of 60° to 90° with respect to the main surface of the first thermal conductive layer. There are no particular limitations on the method for creating the orientation of the first thermal conductive particles, but known methods include orientation operations by magnetic field orientation, as described later in the section on "Method for Manufacturing Thermal Conductive Sheets," or orientation operations by laminating thin sheets oriented in the in-plane direction by pressing and slicing them in the lamination direction.

[0026] <<Boron Nitride>> Boron nitride can be classified according to its crystal structure into, for example, hexagonal boron nitride (h-BN), cubic boron nitride (c-BN), wurtzite boron nitride (w-BN), rhombohedral boron nitride (r-BN), and randomly layered boron nitride (t-BN). These boron nitrides may be used individually or in combination of two or more. Among these, hexagonal boron nitride (h-BN) is preferred from the viewpoint of improving the thermal conductivity and compressibility of the thermal conductive sheet. Hexagonal boron nitride is usually in the form of flake-like (plate-like) particles.

[0027] <<Volume-average particle diameter D50 of the first thermally conductive particle>> The volume-average particle diameter D50 of the first thermal conductive particles is preferably 4 μm or more, more preferably 8 μm or more, even more preferably 20 μm or more, even more preferably 30 μm or more, even more preferably 40 μm or more, preferably 300 μm or less, more preferably 250 μm or less, even more preferably 200 μm or less, and even more preferably 150 μm or less. If the volume-average particle diameter D50 of the first thermal conductive particles is above the lower limit, it is possible to reduce the contact resistance between the first thermal conductive particles, and as a result, the thermal conductivity of the thermal conductive sheet can be improved. On the other hand, if the volume-average particle diameter D50 of the first thermal conductive particles is below the upper limit, it is possible to adequately fill the first thermal conductive layer with the first thermal conductive particles, and the thermal conductivity of the thermal conductive sheet can be improved.

[0028] <<Content ratio of the first thermal conductive particles in the first thermal conductive layer>> The content of the first thermal conductive particles in the first thermal conductive layer is preferably 40% by volume or more, more preferably 45% by volume or more, even more preferably 50% by volume or more, preferably 85% by volume or less, more preferably 80% by volume or less, and even more preferably 75% by volume or less. If the content of the first thermal conductive particles in the first thermal conductive layer is above the lower limit, the thermal conductivity of the thermal conductive sheet can be further enhanced. On the other hand, if the content of the first thermal conductive particles in the first thermal conductive layer is below the upper limit, the tackiness of the first thermal conductive layer can be effectively enhanced.

[0029] <Binder resin> The binder resin is not particularly limited, and any thermoplastic resin can be used. Specifically, examples of thermoplastic resins include polybutene, butyl rubber, nitrile butadiene rubber (nitrile rubber), styrene butadiene rubber, acrylic rubber, isoprene rubber, ethylene propylene rubber, fluororubber, and silicone rubber. Among these, it is particularly preferable to use at least one thermoplastic resin selected from the group consisting of acrylic rubber, polybutene, nitrile butadiene rubber, styrene butadiene rubber, fluororubber, and silicone rubber as the binder resin, and it is more preferable to use at least one of fluororubber and acrylic rubber, and from the viewpoint of increasing the tack strength of the first heat conductive layer, it is preferable to contain at least acrylic rubber. These may be used individually or in combination of two or more.

[0030] Furthermore, either a liquid thermoplastic resin or a solid thermoplastic resin can be used as the binder resin. These thermoplastic resins may be used individually or in combination of two or more. However, from the viewpoint of improving the tackiness of the first thermal conductive layer, it is preferable that the binder resin contains a liquid thermoplastic resin.

[0031] <<Content ratio of binder resin in the first thermal conductive layer>> The content of the binder resin in the first heat conductive layer is preferably 15% by volume or more, more preferably 20% by volume or more, even more preferably 35% by volume or more, preferably 60% by volume or less, more preferably 55% by volume or less, and even more preferably 50% by volume or less. If the proportion of the first thermal conductive particles in the first thermal conductive layer is above the lower limit, the tackiness of the first thermal conductive layer can be effectively enhanced. On the other hand, if the proportion of the first thermal conductive particles in the first thermal conductive layer is below the upper limit, the thermal conductivity of the thermal conductive sheet can be further enhanced.

[0032] <Tack force of the first thermal conductive layer> When the first heat conduction layer has the second heat conduction layer only on one main surface, the tack force of the main surface on the side without the second heat conduction layer is 10 N / cm 2 or more, preferably 20 N / cm 2 or more, more preferably 30 N / cm 2 or more, still more preferably 40 N / cm 2 or more, particularly preferably 100 N / cm 2 or less, and may be 80 N / cm 2 or less. If the tack force of the main surface on the side without the second heat conduction layer in the first heat conduction layer is not less than the above lower limit value, the adhesion to the adherend is excellent and excellent heat conductivity can be exhibited. Further, if the tack force of the main surface on the side without the second heat conduction layer in the first heat conduction layer is not more than the above upper limit value, the handling of the heat conduction sheet can be facilitated.

[0033] <Structure of the first heat conduction layer> The structure of the first heat conduction layer is not particularly limited as long as the first heat conductive particles are oriented in the thickness direction. The first heat conduction layer may have a structure in which strips containing a binder resin and the first heat conductive particles are joined in parallel within the main surface of the heat conduction sheet. Alternatively, the first heat conduction layer may have a structure in which the first heat conductive particles are dispersed and oriented in the thickness direction in a uniform matrix made of a binder resin. In any structure of the first heat conduction layer, the heat conduction sheet of the present invention has a structure in which the first heat conductive particles in the first heat conduction layer are oriented in the thickness direction, and further, as long as it includes a second heat conduction layer mainly composed of the second heat conductive particles disposed on at least one main surface of such a first heat conduction layer, it is possible to achieve both high heat conductivity and reworkability at a high level.

[0034] (Second heat conduction layer) The second thermal conductive layer is disposed on at least one main surface of the first thermal conductive layer described above and is a layer mainly composed of the second thermal conductive particles. Since the second thermal conductive layer is mainly composed of the second thermal conductive particles, it is less likely to leave residue on the adherend surface when peeled off, and the reworkability of the thermal conductive sheet can be improved. Here, the content of the second thermal conductive particles in the second thermal conductive layer is more than 50 volume%, preferably 60 volume% or more, more preferably 75 volume% or more, even more preferably 80 volume% or more, particularly preferably 90 volume% or more, even more preferably 94 volume% or more, most preferably 97 volume% or more, and may be 100 volume%, i.e., a layer consisting only of the second thermal conductive particles. If the content of the second thermal conductive particles in the second thermal conductive layer is above the above lower limit, the reworkability of the thermal conductive sheet can be further improved. The second thermal conductive layer may optionally contain other components other than the second thermal conductive particles. Other components include, but are not limited to, the binder resin described above. If the second thermal conductive layer contains other components, the content of such other components must be less than 50% by volume, preferably 40% by volume or less, more preferably 35% by volume or less, even more preferably 20% by volume or less, even more preferably 10% by volume or less, particularly preferably 6% by volume or less, even more preferably 3% by volume or less, and particularly preferably 0% by volume, i.e., the second thermal conductive layer does not contain any other components such as binder resin.

[0035] <Second type of thermally conductive particle> As the second thermal conductive particle, the same type of particle as described above as the first thermal conductive particle can be used. That is, the second thermal conductive particle is preferably at least one selected from the group including boron nitride, graphite, molybdenum disulfide, and tungsten disulfide, and from the viewpoint of reworkability, either boron nitride or graphite is preferred, with boron nitride being more preferred.

[0036] <<Volume-average particle diameter D50 of the second thermally conductive particle>> The volume-average particle diameter D50 of the second thermal conductive particles is preferably 4 μm or more, more preferably 8 μm or more, even more preferably 10 μm or more, even more preferably 20 μm or more, particularly preferably 30 μm or more, even more particularly preferably 40 μm or more, preferably 300 μm or less, more preferably 250 μm or less, even more preferably 200 μm or less, and particularly preferably 150 μm or less. If the volume-average particle diameter of the first thermal conductive particles is above the lower limit above, the reworkability can be further improved. On the other hand, if the volume-average particle diameter of the second thermal conductive particles is below the upper limit above, the thermal conductivity of the thermal conductive sheet can be improved, and the reworkability can be further improved.

[0037] Furthermore, when the second thermal conductive particles have a layered structure, their high flatness results in low friction and high slipperiness. For this reason, the use of second thermal conductive particles with a layered structure is preferable from the viewpoint of improving reworkability. The true specific gravity of the second thermal conductive particles is 3.5 g / cm³ from the viewpoint of improving slipperiness and thus reworkability. 3 Preferably, it is 3.0 g / cm³. 3 The following is more preferable:

[0038] Furthermore, it is preferable that the second thermal conductive particles, like the first thermal conductive particles, have a flaky shape. If the second thermal conductive particles are flaky, the thermal conductive sheet will have excellent reworkability and thermal conductivity. The second thermal conductive particles may also be aggregates made up of flaky primary particles. In addition, it is preferable that the second thermal conductive particles are of the same type as the first and second thermal conductive layers, in order to improve adhesion between them and to ensure good integrity as a thermal conductive sheet. Here, "same type" means the same kind of thermal conductive particles, and the shape and size of the first and second thermal conductive particles may differ. In particular, from the viewpoint of improving the properties of the thermal conductive sheet, it is preferable that the first and second thermal conductive particles are of the same type and both are flaky boron nitride.

[0039] <Amount of adhesion of the second heat conductive layer> The amount of the second heat conductive layer deposited is 1.5 g / m². 2 Preferably, it should be 1.8 g / m 2 It is more preferable that the amount be greater than or equal to 2.2 g / m 2 It is even more preferable that the amount be greater than or equal to 5.0 g / m 2 It is particularly preferable that the amount be greater than or equal to 20.0 g / m². 2 Preferably, it is 18.0 g / m². 2 It is more preferable that the following is the case: 15.0 g / m 2 The following is even more preferable: If the amount of the second heat conductive layer attached is greater than or equal to the lower limit value above, even after being pressurized and heated while interposed between the heating element and the heat sink, it becomes possible to remove the heat conductive sheet without leaving any residue on the adherend surface, thereby further improving reworkability. Also, if the amount of the second heat conductive layer attached is less than or equal to the upper limit value above, the thermal conductivity of the heat conductive sheet can be further improved.

[0040] (Thickness of the thermal conductive sheet) The thickness of the thermal conductive sheet is not particularly limited. The total thickness of the thermal conductive sheet is the sum of the thickness of the first thermal conductive layer and the thickness of the second thermal conductive layer. The first thermal conductive layer is preferably 0.05 mm or more, more preferably 0.1 mm or more, preferably 10 mm or less, and more preferably 1 mm or less. The thickness of the second thermal conductive layer is preferably 0.0001 mm or more, more preferably 0.001 mm or more, preferably 0.01 mm or less, and more preferably 0.005 mm or less.

[0041] (Method of manufacturing a thermal conductive sheet) Furthermore, the method for manufacturing the thermal conductive sheet is not particularly limited. Here, the first thermal conductive layer can be manufactured, for example, by (A) a method involving the orientation of the first thermal conductive particles. Specifically, according to the method described in Japanese Patent No. 6846641, a composition containing at least a binder resin and the first thermal conductive particles can be molded into a sheet by mold molding or the like, an orientation operation using a magnetic field can be performed, and then the resin can be cured to obtain the first thermal conductive layer. Alternatively, (B) a composition containing the above-mentioned binder resin and the first thermal conductive particles can be pressed and molded into a sheet, the resulting sheet-like molded body can be used as a pre-thermal conductive sheet, and a laminate of multiple pre-thermal conductive sheets can be stacked and sliced ​​in the direction of the stacking to obtain the first thermal conductive layer. Then, the second thermal conductive particles can be applied to at least one main surface of the first thermal conductive layer obtained as described above to form a second thermal conductive layer mainly composed of the second thermal conductive particles. The method for manufacturing the first thermal conductive layer will be described below as an example of method (B) above, but the present invention is not limited thereto.

[0042] <Fabrication of the first thermal conductive layer> First, components such as a binder resin and first thermally conductive particles are mixed under arbitrary conditions using a known mixing device such as a kneader, roll, Henschel mixer, or Hobart mixer to prepare a composition for the first thermally conductive layer.

[0043] Next, the obtained first thermal conductive layer composition is formed into a sheet using known molding methods such as press molding, rolling molding, or extrusion molding to form a pre-thermal conductive sheet. In this pre-thermal conductive sheet, the first thermal conductive particles are mainly arranged in the in-plane direction, and it is presumed that the thermal conductivity of the pre-thermal conductive sheet in the in-plane direction is particularly improved.

[0044] Next, the obtained pre-thermal conductive sheets are stacked in multiple layers in the thickness direction using any method, or the pre-thermal conductive sheets are folded or rolled up to obtain a laminate. Here, it is presumed that in the obtained laminate, the first thermal conductive particles are mainly arranged in a direction approximately perpendicular to the stacking direction.

[0045] Then, the obtained laminate is sliced ​​at an angle of 45° or less relative to the lamination direction, using known methods such as the multi-blade method, laser processing method, water jet method, or knife processing method, to obtain a first heat conductive layer made of slices of the laminate. Here, from the viewpoint of improving the heat conductivity of the first heat conductive layer, the slicing angle of the laminate is preferably 30° or less relative to the lamination direction, more preferably 15° or less, and preferably approximately 0° relative to the lamination direction (i.e., in the direction along the lamination direction). It is presumed that the first heat conductive particles are arranged in the thickness direction within the first heat conductive layer obtained in this way. Therefore, it is presumed that the first heat conductive layer obtained by the above method has high heat conductivity in the thickness direction.

[0046] <Fabrication of the second thermal conductive layer> The method for producing the second heat conductive layer is also not particularly limited. When producing the second heat conductive layer, first, the second heat conductive particles described above, and other components such as binder resin as needed, are prepared. Then, the first heat conductive layer produced according to the above is placed on a peelable substrate such as release paper or a slightly adhesive sheet. A predetermined amount of the second heat conductive particles is sprinkled on the main surface of the first heat conductive layer opposite to the main surface in contact with the release paper or slightly adhesive sheet, and the surface is uniformly smoothed, for example, with a squeegee. Then, the second heat conductive layer can be produced by rolling and pressing a layer mainly composed of the second heat conductive particles onto the main surface of the first heat conductive layer at least once using a known pressing method such as a roll press. The second heat conductive layer obtained in this way may be within the range of adhesion described above. The adhesion amount of the second heat conductive layer can be increased by increasing the rolling pressure and decreased by decreasing the rolling pressure. Alternatively, the amount of the second heat conductive layer can be controlled by adjusting the amount of the second heat conductive particles applied. Furthermore, the amount of the second heat conductive layer can also be controlled by changing the number of repetitions of the process, which involves scattering, leveling, and rolling the second heat conductive particles onto the main surface of the first heat conductive layer, as described above.

[0047] By following the above procedure, the heat conductive sheet of the present invention can be manufactured efficiently. [Examples]

[0048] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples. In the following description, "%" and "parts" representing quantities refer to mass unless otherwise specified. Furthermore, when calculating volume fractions, the volume of each component was obtained by dividing the mass of each component by its theoretical specific gravity. Various measurements and evaluations in the examples were performed according to the following methods.

[0049] <Orientation state of the first thermally conductive particle> The orientation angle of the first thermal conductive particles in the first thermal conductive layer in the thermal conductive sheets obtained in each example and comparative example was determined by observing a cross-section of the thermal conductive sheet, cut into a regular octagon, using a scanning electron microscope (SEM, Hitachi High-Technologies, "SU-3500") at a magnification that captured the entire sheet from top to bottom. The magnification at this time was 700x. Specifically, 50 lines were drawn along the major axis of the first thermal conductive particles in the cross-section, and the average angle of the major axis with respect to the surface of the first thermal conductive layer was calculated. If the angle was 90° or greater, a supplementary angle was used. This was performed for eight faces, and the value with the largest value among the eight faces was taken as the orientation angle of the first thermal conductive particles in the first thermal conductive layer. In all examples, it was confirmed that the first thermal conductive particles were oriented at an angle of 60° to 90° with respect to the main surface of the first thermal conductive layer.

[0050] <Thermal resistance value> The thermal resistance of the thermal conductive sheet was measured for test structures equipped with the thermal conductive sheets manufactured in the examples and comparative examples. These test structures have a configuration in which a thermal conductive sheet is interposed between an aluminum plate, which simulates a heat sink as the substrate, and a nickel-plated copper plate, which simulates a heat-generating element as the substrate. The surface of the thermal conductive sheet facing the aluminum plate is the main surface of the second thermal conductive layer. The surface of the thermal conductive sheet facing the copper plate (Ni-plated) is the main surface of the first thermal conductive layer and has tack properties. For the preparation of the test structures, the thermal conductive sheet was interposed between the aluminum plate and the copper plate (Ni-plated) and stored for 80 hours under conditions of a pressure of 0.5 MPa and a temperature of 120°C. The obtained test structures were measured as follows using a transient thermal measurement device (Mentor Graphics, product name "T3Ster") and a pressurizing jig (Keenus Design). An IGBT-equipped power module was used as the heat-generating element placed on the copper plate (Ni-plated) side of the test structure. Furthermore, a heat sink (manufactured by Keynas Design) cooled by a Peltier element was used as the heat sink placed on the aluminum plate side of the test structure. First, the initial temperature of the heat sink in the obtained test structure was set to 30°C (approximately 303K). Next, the heating element of the test structure was heated by passing a current through it at a heating current of 6A for a heating time of 300 seconds. During heating, a pressure of 0.50MPa was applied to the thermal conductive sheet interposed between the heating element and the heat sink. Subsequently, a current of 20mA for a measurement time of 800 seconds was passed between the heated heating element and the heat sink, and the temperature T1(K) of the heating element and the temperature T2(K) of the heat sink were measured. Then, using the obtained T1, T2, and the power W(W) applied to the heating element, the following equation (I) was calculated: X = (T1 - T2) / W ... (I) The thermal resistance value X (K / W) between the heat-generating element and the heat sink via a thermal conductive sheet in the heat dissipation device was determined. A smaller thermal resistance value indicates that heat is more easily transferred from the heat-generating element to the heat sink and dissipated.

[0051] <Tack force of the first thermal conductive layer> The tack force of the first thermal conductive layer against the metal surface was measured using a probe tack tester (manufactured by Resca Co., Ltd., product name "TAC1000") at a temperature of 23°C, with a thermal conductive sheet punched to a diameter of 4 mm as the test specimen. First, the test specimen, attached with double-sided tape to the tip of a φ5 mm flat circular probe, was pressed against the surface of a 5 cm square, 3 mm thick copper plate placed on the stage of the probe tack tester with a load of 3.8 N (385 gf) for 10 seconds. The force required to pull the probe with the test specimen attached away from the copper plate was measured as the tack force of the first thermal conductive layer. The measured value of the tack force against the metal surface is an indicator of the adhesion of the adherend to the metal surface. Therefore, the larger the measured value of the tack force obtained, the better the adhesion of the first thermal conductive layer to the metal.

[0052] <Evaluation of adhesion to the substrate> The first thermal conductive layer of the thermal conductive sheet manufactured in the examples and comparative examples was placed on one of the main surfaces of a nickel-plated copper plate, which was the adherend, so as to be in contact with it. Furthermore, a release sheet was placed over the main surface of the second thermal conductive layer of the thermal conductive sheet, and after rolling it back and forth twice with a 1 kgf roller, the adhesion was evaluated according to the following criteria. A: The heat conductive sheet adheres tightly to the object to be attached, and does not shift even when the object is shaken horizontally. B: If the object to be attached is shaken horizontally, it will shift.

[0053] <Retention rate during rework> The residual rate of the thermal conductive sheet during rework was measured for test structures equipped with thermal conductive sheets manufactured in the examples and comparative examples. These test structures have a configuration in which a thermal conductive sheet is interposed between an aluminum plate, which simulates a heat sink as the adherend, and a copper plate (Ni-plated), which simulates a heat generating element as the adherend. The surface of the thermal conductive sheet facing the aluminum plate is the main surface of the second thermal conductive layer. The surface of the thermal conductive sheet facing the copper plate (Ni-plated) is the main surface of the first thermal conductive layer and has tack properties. For the preparation of the test structures, the thermal conductive sheet was interposed between the aluminum plate and the copper plate (Ni-plated) and stored for 80 hours under conditions of a pressure of 0.5 MPa and a temperature of 120°C. The aluminum plate, which simulates a heat sink, was peeled off the test structure, and the proportion of components derived from the thermal conductive sheet remaining on the aluminum plate surface (based on the area of ​​the main surface of the thermal conductive sheet) was evaluated according to the following criteria. A: No residue of the thermal conductive sheet remains on the surface of the aluminum plate, or if any remains, it is less than 1%. B: Residue of the thermal conductive sheet remains on the surface of the aluminum plate at a rate of 1% to 40%. C: More than 40% of the thermal conductive sheet residue remains on the surface of the aluminum plate.

[0054] (Example 1) <Preparation of the composition for the first thermal conductive layer> As binder resins, 50 parts of solid fluororubber (thermoplastic resin, manufactured by 3M, product name "Dynion FC2211", specific gravity: 1.8) and 50 parts of liquid acrylic resin (thermoplastic resin, manufactured by Toagosei, product name "ARUFON UH2190", specific gravity: 1.1) were prepared at room temperature and pressure. The prepared binder resins and 200 parts of hexagonal boron nitride (manufactured by Dandong Chemical Engineering, product name "HSPD50", volume average particle diameter: 52 μm, specific gravity: 2.3), which has a layered crystalline structure and a flaky shape, were mixed for 5 minutes using a Hobart mixer (manufactured by Kodaira Seisakusho Co., Ltd., product name "ACM-5LVT type") in the presence of 300 parts of ethyl acetate as a solvent. Next, the resulting stirred mixture was vacuum-degassed for 30 minutes, and ethyl acetate was removed simultaneously with degassing to obtain a thermal conductive sheet composition containing a thermoplastic fluororubber that is solid at room temperature, an acrylic resin that is liquid at room temperature, and hexagonal boron nitride. The obtained composition was then placed in a crusher and crushed for 10 seconds.

[0055] <Preheat conductive sheet molding process> Next, 5 g of the crushed composition was sandwiched between 50 μm thick PET film (protective film) that had been sandblasted, and rolled under the conditions of a roll gap of 350 μm, roll temperature of 100°C, roll linear pressure of 50 kg / cm, and roll speed of 1 m / min to obtain a pre-heat-conductive sheet with a thickness of 0.30 mm.

[0056] <Formation of laminates> Next, the obtained pre-heat-conductive sheets were cut to 60 mm in length, 60 mm in width, and 0.30 mm in thickness. 200 sheets were then laminated in the thickness direction using double-sided tape to obtain a laminate with a thickness of approximately 60 mm.

[0057] <Fabrication of the first thermal conductive layer> Subsequently, the laminated cross-section of the obtained pre-heat conductive sheet laminate was sliced ​​at a 0-degree angle with respect to the lamination direction (in other words, sliced ​​in the direction normal to the main surface of the laminated pre-heat conductive sheet) using a woodworking slicer (manufactured by Marunaka Iron Works Co., Ltd., product name "Super Mecha S Super Finishing Planer") while pressing it under a pressure of 0.3 MPa, thereby obtaining a raw material of heat conductive sheet measuring 60 mm in length, 60 mm in width, and 0.2 mm in thickness. Furthermore, the size of the obtained raw material was adjusted to 20 mm in length, 20 mm in width, and 0.2 mm in thickness to obtain the first heat conductive layer.

[0058] <Fabrication of the second thermal conductive layer> The same particles as the first thermal conductive particles (hexagonal boron nitride with a layered crystal structure and a flaky shape (manufactured by Dandong Chemical Engineering, trade name "HSPD50", volume-average particle diameter: 52 μm, specific gravity: 2.3)) were used as the second thermal conductive particles. The amount of the second thermal conductive particles used was 1.0 mg relative to the main surface of the first thermal conductive layer, which measured 20 mm x 20 mm as described above. The particles were evenly spread and pressed down with a spatula to obtain the amount of adhesion shown in Table 1, thereby obtaining a thermal conductive sheet. Various measurements and evaluations were performed on the obtained thermal conductive sheets according to the above procedure. The results are shown in Table 1.

[0059] (Example 2) In the <Preparation of the Second Thermal Conductive Layer>, various operations, measurements, and evaluations were carried out in the same manner as in Example 1, except that the amount of the second thermal conductive particles used and the pressing conditions with the spatula were changed so that the amount of adhesion was as shown in Table 1. The results are shown in Table 1.

[0060] (Example 3) In the <Fabrication of the Second Thermal Conductive Layer>, various operations, measurements, and evaluations were carried out in the same manner as in Example 1, except that boron nitride particles (layered structure, manufactured by Shandong Pengcheng Ceramic New Materials Technology Co., Ltd., trade name "BN-SA", volume average particle diameter 64 μm, specific gravity: 2.3), which are "aggregates" of flake-shaped primary particles, were used as the second thermal conductive particles. The results are shown in Table 1.

[0061] (Example 4) In the <Fabrication of the Second Thermal Conductive Layer>, hexagonal boron nitride (layered structure, flaky, manufactured by Shandong Pengcheng Ceramic New Materials Technology Co., Ltd., volume-average particle size 29 μm, trade name "BN-AY", specific gravity: 2.3) with a volume-average particle diameter of 30 μm was used as the second thermal conductive particle, in the amount shown in Table 1. Except for this, various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0062] (Example 5) In the <Preparation of the Second Thermal Conductive Layer>, graphite (flaky, layered structure, manufactured by Ito Graphite Co., Ltd., trade name "EC300", volume average particle size: 50 μm, specific gravity: 2.2) was used as the second thermal conductive particle instead of boron nitride, in the amount shown in Table 1. Otherwise, various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0063] (Example 6) In the <Preparation of the Second Thermal Conductive Layer>, graphite (flaky, layered structure, manufactured by Ito Graphite Co., Ltd., trade name "EC50", volume average particle size: 250 μm, specific gravity: 2.2) was used as the second thermal conductive particle instead of boron nitride, in the amount attached as shown in Table 1. Otherwise, various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0064] (Example 7) Except for changing the binder resin used in <Preparation of the composition for the first thermal conductive layer> to 70 parts of solid acrylic resin (thermoplastic resin, manufactured by Nippon Zeon Co., Ltd., trade name "H570", specific gravity: 1.1) and 30 parts of liquid acrylic resin (thermoplastic resin, manufactured by Toagosei Co., Ltd., trade name "ARUFON UH2190", specific gravity: 1.1) at room temperature and atmospheric pressure, various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0065] (Example 8) In the preparation of the second thermal conductive layer, various operations, measurements, and evaluations were carried out in the same manner as in Example 1, except that a mixture was used, which consisted of 97.6 parts (by mass) of the second thermal conductive particles and 2.4 parts (by mass) of liquid acrylic resin (thermoplastic resin, manufactured by Toagosei, trade name "ARUFON UH2190", specific gravity: 1.1) mixed at room temperature and atmospheric pressure. The results are shown in Table 1.

[0066] (Example 9) In the preparation of the second thermal conductive layer, various operations, measurements, and evaluations were carried out in the same manner as in Example 1, except that a mixture was used, which consisted of 95.2 parts (parts by mass) of the second thermal conductive particles and 4.8 parts (parts by mass) of liquid acrylic resin (thermoplastic resin, manufactured by Toagosei, trade name "ARUFON UH2190", specific gravity: 1.1) mixed at room temperature and atmospheric pressure. The results are shown in Table 1.

[0067] (Example 10) Except for changing the binder resin used in <Preparation of the composition for the first heat conductive layer> to 30 parts of liquid thermoplastic fluororesin (manufactured by Daikin Industries, Ltd., trade name "Daiel G-101", specific gravity: 1.8) and 70 parts of solid thermoplastic fluororubber (thermoplastic resin, manufactured by 3M, trade name "Dinion FC2211", specific gravity: 1.8) at room temperature and pressure, various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0068] (Comparative Example 1) Except for not forming a second thermal conductive layer, various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0069] (Comparative Example 2) Except for not forming a second thermal conductive layer, various operations, measurements, and evaluations were carried out in the same manner as in Example 10. The results are shown in Table 1.

[0070] (Comparative Example 3) Except for replacing the second thermal conductive layer with aluminum foil (Aluminum Foil Alloy Foil A8079-0, manufactured by Takeuchi Metal Foil & Powder Industry Co., Ltd., with a thickness of 0.006 mm), various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0071] [Table 1]

[0072] As is clear from Table 1, the thermal conductive sheet comprising a first thermal conductive layer in which the first thermal conductive particles are oriented in the thickness direction, and a second thermal conductive layer mainly composed of second thermal conductive particles arranged on the surface of the thermal conductive layer, exhibited excellent thermal conductivity and reworkability. Furthermore, comparative examples 1 and 2, which lacked the second thermal conductive layer, showed inferior reworkability. In comparative example 3, in which aluminum foil was placed in place of the second thermal conductive layer, it showed excellent reworkability, but had a high thermal resistance value and inferior thermal conductivity. [Industrial applicability]

[0073] According to the present invention, a thermal conductive sheet with excellent reworkability and thermal conductivity can be provided. [Explanation of Symbols]

[0074] 10: Thermal conductive sheet 11: First thermal conductive layer 12: Second thermal conductive layer 21: Heat sink 22: Heating element 30:Equipment

Claims

1. A first thermal conductive layer containing first thermal conductive particles and a binder resin, The first thermal conductive layer comprises a second thermal conductive layer disposed on at least one main surface of the first thermal conductive layer and having second thermal conductive particles as its main component, In the first heat conduction layer, the first heat conduction particles are oriented in the thickness direction of the first heat conduction layer. The first thermally conductive particle and the second thermally conductive particle may be of the same type or different types. Thermal conductive sheet.

2. The thermal conductive sheet according to claim 1, wherein the first thermal conductive particles are at least one selected from the group including boron nitride, graphite, molybdenum disulfide, and tungsten disulfide, and have a flaky shape.

3. The thermal conductive sheet according to claim 1, wherein the shape of the second thermal conductive particles is flaky.

4. The thermal conductive sheet according to claim 1, wherein the first thermal conductive particle and the second thermal conductive particle are of the same type.

5. When the first thermal conductive layer has the second thermal conductive layer on one of its main surfaces, the tack force of the other main surface of the first thermal conductive layer that does not have the second thermal conductive layer is 10 N / cm. ２ More than 100N / cm ２ The thermal conductive sheet according to claim 1, which is as follows:

6. The amount of the second heat conductive layer deposited is 1.5 g / m². ２ Above, 20.0g / m ２ The thermal conductive sheet according to claim 1, which is as follows:

7. The thermal conductive sheet according to any one of claims 1 to 6, wherein the volume average particle diameter D50 of the second thermal conductive particle is 4 μm or more.

Images