WO2024252904A1 - Graphite sheet and electronic device - Google Patents

Abstract

Provided are: a graphite sheet capable of reducing contact thermal resistance on a surface and improving heat radiation performance; and an electronic device comprising the graphite sheet. A graphite sheet (1) has a first main surface (S1) and a second main surface (S2), which is the surface opposite from the first main surface (S1). In the graphite sheet (1), the density of a surface layer part (second surface layer part (T2)) on the second main surface (S2) side is less than the density of a surface layer part (first surface layer part (T1)) on the first main surface (S1) side.

Description

Graphite sheet and electronic device

This disclosure relates to a graphite sheet and an electronic device, and more particularly to a graphite sheet having two surfaces, and an electronic device including the graphite sheet.

Power modules such as Insulated Gate Bipolar Transistors (IGBTs) are attached to heat dissipation components with screws or other means to dissipate the heat generated during operation. In order to transfer heat smoothly from the power module to the heat dissipation components, a solid heat dissipation sheet is sandwiched between the power module and the heat dissipation components instead of the conventional semi-solid grease, which has low thermal conductivity.

Patent Document 1 discloses that in a power module with a heat dissipation component, the power module includes a base plate, a ceramic insulating substrate bonded to one surface of the base plate, a semiconductor element bonded to the ceramic insulating substrate, and a heat dissipation component attached to the base plate side of the power module via a heat dissipation sheet, and that the flatness of the surface of the base plate opposite the surface to which the ceramic insulating substrate is bonded is 20 μm or less.

JP 2019-067801 A

Graphite sheets are used as heat dissipation sheets placed between the power modules and heat dissipation components as described above. However, typical graphite sheets, obtained by pyrolyzing and graphitizing polymer films, have high thermal conductivity in the plane direction but poor surface compressibility. Furthermore, many heat generating elements such as power modules have uneven surfaces. If such graphite sheets are used, the contact thermal resistance between the surface of the heat generating element increases, and there is a possibility that the heat dissipation performance from the heat generating element to the heat sink may decrease.

The objective of this disclosure is to provide a graphite sheet that can reduce the contact thermal resistance on the surface and improve heat dissipation performance, and an electronic device equipped with this graphite sheet.

The graphite sheet according to one embodiment of the present disclosure is a graphite sheet having a first main surface and a second main surface opposite the first main surface, and the density of the surface layer on the second main surface side of the graphite sheet is smaller than the density of the surface layer on the first main surface side.

An electronic device according to another aspect of the present disclosure includes the graphite sheet, a heating element, and a heat sink.

The graphite sheet and electronic device disclosed herein can provide a graphite sheet that can reduce the contact thermal resistance on the surface and improve heat dissipation performance, as well as an electronic device equipped with this graphite sheet.

FIG. 1 is a schematic cross-sectional view showing a graphite sheet according to an embodiment. FIG. 2 is a schematic cross-sectional view illustrating an electronic device according to an embodiment. FIG. 3 is a schematic diagram illustrating a method for measuring the assumed shear strength of a graphite sheet in an embodiment.

1. Overview Hereinafter, a graphite sheet according to an embodiment of the present disclosure will be described. Note that the following embodiment is merely one of various embodiments of the present disclosure. The following embodiment can be modified in various ways depending on the design as long as the object of the present disclosure can be achieved.

Graphite sheets are used, for example, as a thermally conductive sheet that is sandwiched between a heating element and a heat sink and is in close contact with both the heating element and the heat sink to smoothly transfer heat generated by the heating element to the heat sink.

General graphite sheets, obtained by pyrolyzing and graphitizing polymer films, have high thermal conductivity in the plane direction but low surface compressibility. There are also graphite sheets with high surface compressibility, but their thermal conductivity in the plane direction is low. Even if these graphite sheets are stacked, contact thermal resistance occurs between the sheets, so the heat dissipation performance remains low.

The inventors conducted extensive research to solve the above-mentioned problems and discovered that heat dissipation performance can be controlled by creating a specific relationship between specific physical property values on both surfaces of the graphite sheet, leading to the completion of the graphite sheet and electronic device disclosed herein.

An example of a graphite sheet 1 according to an embodiment of the present disclosure is shown in FIG. 1. FIG. 1 is a schematic cross-sectional view showing the graphite sheet 1 according to this embodiment. The graphite sheet 1 in FIG. 1 has a first main surface S1 and a second main surface S2, which is the surface opposite to the first main surface S1. The density (hereinafter also referred to as density (II)) of the surface layer portion on the second main surface S2 side of the graphite sheet 1 (hereinafter also referred to as second surface layer T2) is smaller than the density (hereinafter also referred to as density (I)) of the surface layer portion on the first main surface S1 side (hereinafter also referred to as first surface layer T1). The "first main surface S1 side" means a position closer to the first main surface S1 than the second main surface S2 when viewed from a plane passing through the center of the thickness of the graphite sheet 1. Also, the "second main surface S2 side" means a position closer to the second main surface S2 than the first main surface S1 when viewed from a plane passing through the center of the thickness of the graphite sheet 1.

The graphite sheet 1 of this embodiment can reduce the contact thermal resistance on the surface and improve the heat dissipation performance. The reason why the graphite sheet 1 has the above-mentioned configuration and thus produces the above-mentioned effect is not necessarily clear, but can be inferred as follows, for example. The graphite sheet 1 of this embodiment has a density in the surface layer portion that is smaller in the surface layer portion T1 on the second main surface S2 side than in the surface layer portion T2 on the first main surface S1 side. In the second surface layer portion T2 on the second main surface S2 side, the compressibility is greater due to the smaller density. As a result, even if there are irregularities on the surface of the heating element 20 or the like that is placed in contact with the second main surface S2, the graphite sheet 1 can fully absorb the irregularities. As a result, the contact thermal resistance on the second main surface S2 side can be reduced. On the other hand, in the first surface layer portion T1 on the first main surface S1 side, the density is greater, and therefore the thermal conductivity in the surface direction of the graphite sheet is maintained high. As a result, it is believed that the graphite sheet 1 can improve the heat dissipation performance from the heat generating body 20 etc. to the heat sink 30 etc.

2. Details <Graphite sheet>
The graphite sheet 1 is a sheet containing graphite as a main component. The term "main component" refers to a component having the highest content, for example, a content of 50% by mass or more, preferably 80% by mass or more, and more preferably 99% by mass or more. Graphite is a type of carbon allotrope, and is also called graphite. Graphite has a structure in which layers (graphene layers) in which carbon atoms are arranged in a hexagonal honeycomb lattice shape by sp2 bonds are bonded in multiple layers in the thickness direction by van der Waals forces. Examples of graphite include layers of graphite (graphite layers) stacked together.

The average thickness of the graphite sheet 1 is, for example, 25 μm or more and 1000 μm or less, preferably 50 μm or more and 500 μm or less, and more preferably 100 μm or more and 400 μm or less.

As shown in FIG. 1, the graphite sheet 1 has a first surface layer T1 on the first main surface S1 side, and a second surface layer T2 on the second main surface S2 side. The "surface layer" of the graphite sheet 1 refers to the region from the surface (the first main surface S1 or the second main surface S2) to a depth of 20 μm.

In graphite sheet 1, it is important that density (II) of second surface layer portion T2 is smaller than density (I) of first surface layer portion T1. "Density (II) is smaller than density (I)" means that density (II) is 0.95 times or less of density (I). The "density" of the surface layer portion in graphite sheet 1 refers to the mass value (kg/ ^m3 ) per certain volume in the surface layer portion, and means the arithmetic average value of multiple density measurements obtained by cutting out multiple (e.g., 10) portions of a certain volume from the surface layer portion and measuring each of them.

The ratio of density (II) to density (I) (density (II)/density (I)) is preferably 0.9 or less, more preferably 0.7 or less, and even more preferably 0.5 or less. Density (II)/density (I) is, for example, 0.1 or more, and preferably 0.2 or more.

Graphite sheet 1, as shown in FIG. 1, usually has a dense layer 2 in the first surface portion T1 on the first main surface S1 side. Dense layer 2 is a layer (region) where graphite crystallizes and hardens. Dense layer 2 has higher thermal conductivity in the surface direction. This makes it possible to further increase the thermal conductivity in the surface direction on the first main surface S1 side of graphite sheet 1, thereby further improving the heat dissipation performance of graphite sheet 1. Dense layer 2 can be formed, for example, by pyrolyzing a polyimide film or the like to obtain a carbonized film, which is then graphitized by baking at about 2600°C. The "surface direction" refers to the direction along the surface or the direction parallel to the surface.

When the surface layer of the graphite sheet 1 is measured by the SAICAS method, the deemed shear strength is usually smaller in the second surface layer T2 on the second main surface S2 side than in the first surface layer T1 on the first main surface S1 side. In other words, when the deemed shear strength of the first surface layer T1 is strength (I) and the deemed shear strength of the second surface layer T2 is strength (II), strength (I) > strength (II). "Strength (II) is smaller than strength (I)" means that strength (I) is 1.05 times or more stronger than strength (II). In this case, the second surface layer T2 on the second main surface S2 side of the graphite sheet 1 can be made softer, and the contact thermal resistance on the surface of the second main surface S2 can be further reduced. As a result, the heat dissipation performance of the graphite sheet 1 can be further improved. This deemed shear strength is a value measured at a depth of 0.5 μm to 19 μm from the surface. A small assumed shear strength indicates a soft state.

The SAICAS method, known as the Surface and Interfacial Cutting Analysis System method, is an evaluation method in which a material is cut from the surface layer at a low speed with a sharp cutting blade. Figure 3 is a schematic diagram explaining a method for measuring the deemed shear strength of the surface layer of a graphite sheet. The arrow D in Figure 3 indicates oblique cutting, and the arrow d indicates displacement. By using the SAICAS method, the horizontal force (Fh) and vertical force (Fw) applied to the cutting blade when cutting the surface of graphite sheet 11 can be measured, and the deemed shear strength of the surface layer can be calculated from the horizontal force (Fh) applied to cutting blade 12, the cutting angle of cutting blade 12, and the cross-sectional area. Specifically, the graphite sheet 11 is fixed to a SAICAS DN-20 (manufactured by Daipla Wintes Co., Ltd.), the cutting edge 12 is made of boron nitride and has a width of 2 mm, a rake angle of 20°, and a clearance angle of 10°, and the cutting speed is set to 0.5 μm/sec in the horizontal direction and 0.05 μm/sec in the vertical direction in constant speed mode. The point at which the horizontal load reaches 0.002 N or more is taken as the point at which the cutting edge 12 comes into contact with the graphite sheet 11, and measurements are taken from that point in the vertical direction up to 19 μm, and the deemed shear strength from 0.5 μm to a depth of 19 μm is calculated. The following formula is used for the calculation:

t=Fh×(2A×Cot(φ))
(t: assumed shear strength, Fh: horizontal force, A: cross-sectional area of cutting edge, φ: shear angle)
The ratio of the strength (I) to the strength (II) (strength (I)/strength (II)) is preferably 1.1 or more, and more preferably 1.2 or more. The ratio to the intensity (II) is, for example, 2 or less, and preferably 1.7 or less.

Graphite sheet 1 can be obtained, for example, by pyrolyzing a polyimide film to obtain a carbonized film, which is then graphitized by baking at approximately 2600°C to obtain a graphite sheet having dense layers on both surfaces, and then cutting the graphite sheet into two pieces in a direction parallel to the main surface of the carbonized film.

<Electronic devices>
Fig. 2 is a cross-sectional view showing an example of electronic device 100 of the present embodiment. Electronic device 100 of Fig. 2 includes graphite sheet 1, heating element 20, heat sink 30, and substrate 40. Graphite sheet 1 is sandwiched between heating element 20 and heat sink 30. Since electronic device 100 includes graphite sheet 1 of the present embodiment described above, it is possible to improve heat dissipation performance.

The heating element 20 is a member that generates heat, such as a semiconductor component. Examples of semiconductor components include, but are not limited to, transistors, CPUs (Central Processing Units), MPUs (Micro Processing Units), driver ICs (Integrated Circuits), and memories. The heating element 20 may be composed of, for example, a heat spreader and a chip portion fixed onto the heat spreader. The heat spreader is a plate-like member made of metal or the like, and the chip portion is, for example, a semiconductor package. In this case, the chip portion is disposed on a portion of the heat spreader excluding the outer edge portion, and the outer edge portion may have a plurality of screw holes or the like formed therein that penetrate the heat spreader.

The heat sink 30 is a member to which the heat generated by the heat generating body 20 is transferred. Heat can be released from the heat sink 30. The heat sink 30 is, for example, a heat sink. When the heat sink 30 is a plate-shaped heat sink, the heat sink 30 may further include heat dissipation fins. The heat sink 30 may have a plurality of screw holes, etc., formed at positions corresponding to the plurality of screw holes, etc., in the heat generating body 20 described above.

In electronic device 100, the orientation of first main surface S1 and second main surface S2 of graphite sheet 1 sandwiched between heating element 20 and heat sink 30 is not particularly limited. In electronic device 100 of FIG. 2, first main surface S1 of graphite sheet 1 is in contact with heat sink 30, and second main surface S2 of graphite sheet 1 is in contact with heating element 20. By orienting both sides of the graphite sheet in this way in electronic device 100, it is possible to better absorb the unevenness of heating element 20 and further reduce the contact thermal resistance on the surface, thereby further improving the heat dissipation performance of electronic device 100.

As is apparent from the above embodiment, the present disclosure includes the following aspects. In the following, reference symbols are given in parentheses only to clarify the correspondence with the embodiment.

The graphite sheet (1) according to the first embodiment has a first main surface (S1) and a second main surface (S2) that is the surface opposite to the first main surface (S1). The density of the surface layer (T2) on the second main surface (S2) side of the graphite sheet (1) is smaller than the density of the surface layer (T1) on the first main surface (S1) side.

According to the first aspect, the contact thermal resistance on the surface of the graphite sheet (1) can be reduced, and the heat dissipation performance can be improved.

In the graphite sheet (1) according to the second embodiment, in the first embodiment, the graphite sheet (1) has a dense layer (2) in the surface layer portion (T1) on the first main surface (S1) side.

According to the second aspect, the thermal conductivity in the planar direction on the first main surface (S1) side of the graphite sheet (1) can be further increased, and as a result, the heat dissipation performance of the graphite sheet (1) can be further improved.

In the graphite sheet (1) according to the third embodiment, in the first or second embodiment, the assumed shear strength of the surface layer of the graphite sheet (1) measured by the SAICAS method is smaller on the second main surface (S2) side than on the first main surface (S1) side.

According to the third aspect, the second main surface (S2) of the graphite sheet (1) can be made softer, which can further reduce the contact thermal resistance on the surface of the second main surface, thereby further improving the heat dissipation performance of the graphite sheet (1).

The electronic device (100) of the fourth aspect includes a graphite sheet (1) of any one of the first to third aspects, a heating element (20), and a heat sink (30).

According to the fourth aspect, the electronic device (100) can improve heat dissipation performance by using a graphite sheet (1) that can reduce the contact thermal resistance on the surface.

In the electronic device (100) according to the fifth aspect, in the fourth aspect, the first main surface (S1) of the graphite sheet (1) is in contact with the heat sink (30), and the second main surface (S2) of the graphite sheet (1) is in contact with the heat generating element (20).

According to the fifth aspect, the graphite sheet (1) can absorb the unevenness of the heating element (20) and further reduce the contact thermal resistance on the surface of the graphite sheet (1) that contacts the heating element (20), thereby further improving the heat dissipation performance of the electronic device (100).

The graphite sheet and electronic device disclosed herein can reduce the contact thermal resistance on the surface and improve heat dissipation performance, and can realize an electronic device equipped with this graphite sheet. Therefore, the graphite sheet and electronic device disclosed herein are industrially useful.

Reference Signs List 1, 11 Graphite sheet S1 First main surface S2 Second main surface T1 First surface portion T2 Second surface portion 2 Dense layer 20 Heat generating body 30 Heat dissipating body 100 Electronic device

Claims (5)

A graphite sheet having a first main surface and a second main surface opposite to the first main surface,
a density of a surface layer portion on the second main surface side of the graphite sheet is lower than a density of a surface layer portion on the first main surface side;
Graphite sheet. the graphite sheet has a dense layer on the surface layer portion on the first main surface side,
2. The graphite sheet of claim 1. a deemed shear strength of the surface layer portion on the first main surface side and the surface layer portion on the second main surface side of the graphite sheet measured by a SAICAS method is smaller than that of the surface layer portion on the first main surface side;
2. The graphite sheet of claim 1. The graphite sheet according to any one of claims 1 to 3;
A heating element;
and an electronic device comprising: the first main surface of the graphite sheet is in contact with the heat sink;
The second main surface of the graphite sheet is in contact with the heating element.
5. The electronic device according to claim 4.

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