JPH09283955A - Heat radiation sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat radiation sheet of high heat conductivity. SOLUTION: A graphite carbon fiber 1 whose average aspect ratio is below 3 is dispersed in a matrix resin 2 to form a heat radiation sheet A. The graphite carbon fiber 1 can be dispersed in the matrix resin 2 so that its fiber longitudinal direction gets in random direction. As a result, though there is anisotropy in heat conductivity in the graphite carbon fiber 1, the heat radiation sheet A whose heat conductivity in its thickness direction is equal to that in its surface direction perpendicular to the thickness direction is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat radiating sheet which is arranged between an electronic component and the like and a heat radiator and which is used for transmitting heat generated from the electronic component and the like to the heat radiator and radiating the heat. .

[0002]

2. Description of the Related Art In recent years, with the remarkable evolution and popularization of personal computers (PCs) and workstations, microprocessors (MP
U) is improving its performance. However, while the performance of MPUs is increasing due to the increase in transistor integration and the clock frequency, how to deal with the increase in the amount of heat generated by the MPUs is becoming a serious problem. As measures against this, various measures such as reduction of operating voltage and cutting of power supply to unused parts of MPU are being considered by chip makers, but nevertheless, the increase in heat generation due to higher performance is advancing year by year. The reality is that

On the other hand, radiators used for semiconductor packaging and the like have been devised in various ways. For example, in order to improve heat radiation efficiency, aluminum heat radiation fins having a large surface area while taking ventilation efficiency into consideration, and this aluminum heat radiation are used. A combination of fins and a small fan motor has been developed and put into practical use. Even in the MPU that does not require a radiator in the past, due to the increase in the amount of heat generated as described above, heat cannot be completely radiated without using the radiator. That is, a radiator is provided on the surface of the MPU so that the heat generated by the MPU is radiated from the radiator.

When a radiator is provided on an electronic component such as an MPU for heat dissipation as described above, a radiator sheet is usually arranged between the electronic component and the radiator. When the electronic component and the radiator are directly joined, a gap is generated between the electronic component and the radiator due to a slight warp or undulation of the joint surface of the radiator, and this void becomes a large resistance for heat conduction. For this reason, a heat dissipation sheet is placed between the electronic parts and the heat sink to prevent the formation of voids between the electronic parts and the heat sink by following the slight warpage or undulation of the joint surface of the heat sink. I am doing it.

As the heat dissipation sheet, there are a flexible rubber-like one and a tape-like one having both sides coated with an adhesive. In order to increase the heat conductivity of the heat dissipation sheet, Japanese Patent Laid-Open No. As disclosed in Japanese Patent No. 151658, etc., an inorganic filler such as boron nitride (BN) or alumina is mixed and dispersed in a matrix resin. Particularly, a high-performance MPU that generates a large amount of heat has been incorporated with an inorganic filler to attach a radiator via a heat-dissipating sheet having a high thermal conductivity.

[0006]

However, as mentioned above, the calorific value of the MPU tends to increase more and more,
In addition, the miniaturization of electronic devices such as notebook PCs and sub-notebook PCs is progressing, and the need to store electronic parts such as MPU in a very narrow space is also increasing. In many cases, it is not possible to attach a container and take a large space for heat dissipation.

Therefore, it is necessary to dissipate the heat generated from the electronic components such as MPU from the radiator as efficiently as possible. For this purpose, the heat radiation interposed between the electronic components and the radiator is required. It is necessary to reduce the thermal resistance of the sheet as much as possible to improve the efficiency of heat conduction from the electronic components to the radiator. That is, it is necessary to increase the thermal conductivity of the heat dissipation sheet as much as possible.

However, the heat conductivity of the heat dissipation sheet provided with Japanese Patent Application Laid-Open No. 3-151658, in which boron nitride is mixed as an inorganic filler, is about 4 W / mK,
At this point it is no longer sufficient. The present invention has been made in view of the above points, and an object thereof is to provide a heat dissipation sheet having high thermal conductivity.

[0009]

The heat dissipation sheet according to the present invention is characterized in that it is formed by dispersing graphite carbon fibers having an average aspect ratio (fiber length / fiber diameter) of less than 3 in a matrix resin. To do. The invention according to claim 2 has a thermal conductivity of 100 W in the fiber length direction of the graphitic carbon fiber.
/ MK or more.

[0010]

Embodiments of the present invention will be described below. In the present invention, graphitic carbon fiber having a higher thermal conductivity than boron nitride, alumina, etc. is used as the inorganic filler. Graphite carbon fiber is made from anisotropic pitch of petroleum pitch or coal pitch as a raw material, and undergoes processes such as melt spinning, infusibilization, and carbonization, and finally 1600 ° C to 3 ° C.
It is manufactured by graphitizing by heat treatment at a high temperature of about 000 ° C. It is known that the heat treatment at this high temperature narrows the layer spacing of the graphite crystals and at the same time increases the size of the crystallites, thereby increasing the thermal conductivity and electrical conductivity of the graphite.

In addition to the method of using pitch as a starting material, there is also a method of synthesizing graphitic carbon fiber by vapor phase growth, such as vapor phase grown carbon fiber (VGCF: Vaper-G).
It is known as a row carbon-fiber). This is to pyrolyze a hydrocarbon gas using a transition metal such as iron as a catalyst and grow it as fibrous carbon, which is graphitized by heat treatment similar to the above pitch-based graphitic carbon fiber. Also in this case, the higher the heat treatment temperature, the higher the thermal conductivity tends to be.

As described above, the thermal conductivity of the graphitic carbon fiber varies depending on the starting material, the preparation method, and the heat treatment temperature. Graphitic carbon fibers having various thermal conductivities are commercially available. . Further, the graphitic carbon fiber has a characteristic that the thermal conductivity is anisotropic. As shown in Fig. 2, the basic structure of a graphite crystal is that a large number of regular hexagonal rings of carbon atoms are connected in a row to form a huge mesh plane.
It has a layered structure in which the mesh planes are stacked in parallel. The thermal conductivity of the graphitic carbon fiber is high in the direction of the mesh plane of the regular hexagonal ring (the ab direction in FIG. 2) and in the direction perpendicular to the mesh plane, that is, the direction in which the layers are stacked (the c direction in FIG. 2). ) Is low. It is said that the heat conductivity in the direction perpendicular to the mesh plane is about 1/200 to 1/400 of the heat conductivity in the direction of the mesh plane. For example, when the graphite crystal is close to an ideal state, the thermal conductivity in the direction of the mesh plane is 1950 W / mK at 300K, whereas the thermal conductivity in the direction perpendicular to the mesh plane is 5.
There is a report that it is 7 W / mK.

On the other hand, in the case of the graphitic carbon fiber, the longitudinal direction of the fiber (fiber length direction) coincides with the direction of the mesh plane of the regular hexagonal ring of the graphite crystal (direction ab in FIG. 2), The radial direction of the fiber coincides with the direction perpendicular to the mesh plane (direction c in FIG. 2). Therefore, in the graphitic carbon fiber, the thermal conductivity in the fiber length direction is high and the thermal conductivity in the fiber diameter direction is low. For example, even if the thermal conductivity in the fiber length direction is 1000 W / mK, the fiber diameter direction is 5 W / mK. In some cases, only thermal conductivity of about mK or less can be obtained.

Although the graphitic carbon fiber has anisotropy as described above, it is also characterized in that the thermal conductivity is extremely high. That is, the currently commercially available boron nitride is 62 W / mK
Degree of thermal conductivity. On the other hand, the thermal conductivity of the graphitic carbon fiber changes depending on various conditions such as the heat treatment temperature as described above.
When the heat treatment is performed at 00 ° C., the thermal conductivity in the fiber length direction is as high as about 1000 W / mK.

Therefore, a heat dissipation sheet having a high thermal conductivity can be obtained by mixing a milled material obtained by crushing such a graphite carbon fiber having a high thermal conductivity with a matrix resin. Since the thermal conductivity is anisotropic, the orientation of the graphitic carbon fiber in the matrix resin is important. That is, the heat dissipation sheet is usually
Since it is used by sandwiching it between a heat generator such as an electronic component and a radiator, the heat conductivity of the heat dissipation sheet is more important in the thickness direction than in the surface direction, and the heat conductivity in the surface direction of the heat dissipation sheet is low. However, if the heat dissipation sheet has a high thermal conductivity in the thickness direction, the heat from the heating element such as an electronic component can be smoothly transferred to the radiator without being largely subjected to alienation, and the heat can be efficiently radiated from the radiator. From this, it can be seen that it is preferable to orient the graphite carbon fibers so that the direction in which the thermal conductivity is high, that is, the fiber length direction is aligned parallel to the thickness direction of the heat dissipation sheet.

However, in reality, it is very difficult to arrange the graphite carbon fibers in the fiber direction parallel to the thickness direction of the heat dissipation sheet. That is, when the milled crushed graphitic carbon fiber is kneaded with the matrix resin and the heat dissipation sheet A as shown in FIG. 1 (a) is molded by the doctor blade method or the like, the graphitic carbon fiber 1 is molded into the matrix resin 2. 1B, most of the graphite carbon fibers 1 have a fiber length direction perpendicular to the thickness direction of the heat dissipation sheet A (z direction in FIG. 1A), as shown in FIG. 1B. Heat dissipation sheet A
Will be oriented so as to face the surface direction (x-y direction in FIG. 1A). This is because the graphitic carbon fiber 1 that is resistant to the flow of the matrix resin 2 tends to move in the direction of the least resistance, and this orientation tendency is due to the aspect ratio of the graphitic carbon fiber 1 (fiber length / fiber length / The larger the fiber diameter, the larger. Therefore, in the thickness direction of the heat dissipation sheet A, the fiber diameter direction in which the thermal conductivity of the graphite carbon fiber 1 is low is oriented, and in the surface direction of the heat dissipation sheet A, the direction of the fiber length in which the thermal conductivity of the graphite carbon fiber 1 is high. Are oriented, and as a result, for example, the fiber diameter is 10 μm, the fiber length is 50 μm or more,
That is, when the graphitic carbon fiber 1 having an aspect ratio of 5 or more is used, the heat conductivity of the heat dissipation sheet A becomes 1/10 or less in the thickness direction and the heat dissipation sheet A having high heat conductivity in the thickness direction. Hard to get.

With respect to such a problem, the above-mentioned Japanese Patent Laid-Open No. Hei 3
No. 151,658 proposes a method of orienting the longitudinal direction of the filler in the thickness direction of the heat dissipation sheet. The first method is a method of extruding a kneaded product of a matrix resin and a filler into a sheet shape by an extruder, slicing the extruded sheet in parallel with the thickness direction, and pressing the sliced surface to form a heat dissipation sheet. And
In the second method, 200 parts by weight or more of a filler is mixed with 100 parts by weight of a matrix resin and kneaded to prepare a powdery rubber-like molding material in which the outer periphery of the filler is coated with a matrix resin, and the powder rubber-like molding material is used in a mold It is a method of putting in and molding in a direction perpendicular to the thickness direction. By these methods, the longitudinal direction of the filler can be oriented in the thickness direction of the heat dissipation sheet. However, the first method requires several steps such as extrusion molding, slicing, and pressing, resulting in poor production efficiency, and the second method involves applying pressure in a direction perpendicular to the thickness direction, so that, for example, the thickness is It is very difficult to mold a thin heat dissipation sheet of about 0.2 mm, and neither is practical as a method for manufacturing the heat dissipation sheet.

Therefore, the inventor of the present invention does not adopt an impractical method of orienting the graphite carbon fiber 1 having anisotropy in thermal conductivity in the fiber length direction in the thickness direction of the heat dissipation sheet A. The heat dissipation sheet A having a high thermal conductivity in the thickness direction was obtained by making the fiber length directions randomly oriented. The heat dissipation sheet A of FIG. 1 (c), in which the graphite carbon fibers 1 are mixed in the matrix resin 2 so that the fiber length directions thereof are randomly oriented, is the thickness direction (z direction of FIG. 1 (a)). The thermal conductivity of is equal to the thermal conductivity in the plane direction perpendicular to the thickness direction (xy direction in FIG. 1A), but the fiber length direction of the graphitic carbon fiber 1 is in the direction perpendicular to the thickness direction. The heat conductivity in the thickness direction is much higher than that of the oriented heat dissipation sheet A of FIG. 1B, and the graphite carbon fiber 1 has a higher heat conductivity than boron nitride or the like. Thus, the heat dissipation sheet A having a higher thermal conductivity in the thickness direction than the conventional one can be obtained.

As described above, the larger the aspect ratio of the graphite carbon fiber, the higher the tendency of the graphite carbon fiber to be oriented in the flow direction during the molding of the matrix resin. Therefore, the present inventor has adjusted the graphite carbon fiber so that the average aspect ratio is less than 3, and kneads the graphite carbon fiber with the matrix resin to use it when molding the heat dissipation sheet A. The present invention has been completed by experimentally confirming that the direction of the graphitic carbon fibers is not affected by the flow of the matrix resin and the fiber length directions of the graphitic carbon fibers are randomly oriented.

As the graphitic carbon fibers having an average aspect ratio of less than 3, those obtained by crushing strands of the graphitic carbon fibers can be used, but mesophase obtained from mesophase pitch of petroleum or coal. It is also possible to use a graphitized product of small spheres, which has a spherical shape and thus has an aspect ratio of 1. Needless to say, a graphitic carbon fiber having a fiber length shorter than the fiber diameter and an aspect ratio of less than 1 can also be used. Therefore, the lower limit of the average aspect ratio is not particularly set in the present invention, but it is practically preferable to set the average aspect ratio to about 0.5 as the lower limit.

As described above, the thermal conductivity of the graphitic carbon fiber varies depending on the starting material, the preparation method and the heat treatment temperature, but in the present invention, the thermal conductivity in the fiber length direction is 100 W.
It is preferable to use a graphitic carbon fiber of / mK or more. When a graphitic carbon fiber having a thermal conductivity in the fiber length direction of less than 100 W / mK is used, it becomes difficult to obtain a significant difference from the one using boron nitride as the inorganic filler. The higher the thermal conductivity of the graphitic carbon fiber, the higher the thermal conductivity of the heat dissipation sheet A, so the upper limit of the thermal conductivity of the graphitic carbon fiber is not particularly set,
The thermal conductivity of graphite carbon fiber in the fiber length direction is 2000 W /
It is difficult to manufacture those having a mK of more than mK, and this is the upper limit for practical use.

On the other hand, it is necessary to use a flexible matrix resin. By using the heat dissipation sheet by sandwiching it between an electronic component such as MPU and the radiator, a space is not generated between the electronic component and the radiator, and the heat dissipation sheet is generated between the electronic component and the radiator. It plays an important role in absorbing and buffering thermal stress and vibration from the radiator to the electronic components, and is flexible as a matrix resin to deform the radiator sheet A against external force. The one that has the property is used.

The matrix resin is not particularly limited, but silicone gel or silicone rubber is preferable because of its excellent flexibility, and other polybutadiene, butadiene styrene, butadiene acrylonitrile, poly, etc. Chloroprene type, polyisoprene type, chlorosulfonated polyethylene type, polyisobutylene type, isobutylene isoprene type,
Acrylic, polysulfide, urethane, and fluorine-based synthetic rubbers can be exemplified.

Regarding the amount of the graphitic carbon fiber mixed in the matrix resin, the larger the amount of the graphitic carbon fiber is, the higher the thermal conductivity is. However, when the amount of the graphitic carbon fiber is too large, the formability of the heat dissipation sheet is improved. And strength problems occur. Therefore, although not intended to be limited, it is preferable to mix the graphitic carbon fiber in the range of 20 to 70 vol% with respect to the matrix resin in consideration of the balance between the thermal conductivity and the formability and strength, and 40 to 60 vol%. Is more preferable.

Then, the heat dissipation sheet can be obtained by mixing the matrix resin with the graphite carbon fiber, kneading the mixture, and forming the sheet into a sheet. As the sheet forming method, various construction methods can be adopted and are not particularly limited, but a doctor blade method and a calendar forming method are suitable for a thin heat dissipation sheet, and an extrusion forming method is used for a thick heat dissipation sheet. Is suitable. The doctor blade method is a method of forming a sheet as shown in FIG. 3, for example.
That is, glass plates 6 are laid side by side on a conveyor belt 5, a casting head 7 and a blade 8 are arranged above this, and a matrix resin is mixed with a graphitic carbon fiber between the casting head 7 and the blade 8. The prepared slurry 9 is supplied. Then, a release film 10 such as a PET film which has been subjected to a release treatment is supplied onto the conveyor belt 5 via a glass plate 6, and is sent together with the conveyor belt 5 as shown by an arrow in FIG. 3 to cast the release film 7. Forming the heat dissipation sheet A by passing the head 7 and the lower side of the blade 8 and applying the slurry 9 with a predetermined thickness on the release film 7 while adjusting the thickness with the blade 8 and curing the same. You can In any of the above-mentioned molding methods, the matrix resin is flown in the surface direction of the sheet at the time of molding, so to produce a heat-dissipating sheet in which graphite carbon fibers are dispersed in random directions, the average aspect ratio is It is necessary to use less than 3 graphitic carbon fibers.

[0026]

The present invention will now be illustrated by examples. (Example 1) The thermal conductivity in the fiber length direction is 1000 W / m.
K, thermal conductivity in the fiber diameter direction is 5 W / mK, fiber diameter is 10
Graphite carbon fiber having an average fiber length of 10 μm and an average aspect ratio of 1 μm was used, and 40 vol% of this was added to a two-component addition reaction type silicone gel (“SE-1885” manufactured by Toray Dow Corning Silicone Co., Ltd.). The amounts were mixed and kneaded. By pouring this kneading slurry onto a PET film having a release treatment on its surface to form a thickness of 0.5 mm by the doctor blade method shown in FIG. 3, and subjecting this to a curing treatment at 80 ° C. for 30 minutes. , Got the heat dissipation sheet.

Example 2 The thermal conductivity in the fiber length direction is 10
Example 1 except that a graphite carbon fiber having an average thermal fiber length of 00 W / mK, a thermal conductivity of 5 W / mK in the fiber radial direction, a fiber diameter of 10 μm, and an average fiber length of 25 μm (average aspect ratio 2.5) was used. A heat dissipation sheet was obtained in the same manner as.

(Example 3) The thermal conductivity in the fiber length direction was 11
Same as Example 1 except that 0 W / mK, thermal conductivity in the fiber diameter direction of 1 W / mK, fiber diameter of 10 μm and average fiber length of 10 μm were used for the graphitic carbon fiber (average aspect ratio 1). A heat dissipation sheet was obtained. Example 4 Graphite carbon fiber having a thermal conductivity of 80 W / mK in the fiber length direction, a thermal conductivity of 1 W / mK in the fiber diameter direction, a fiber diameter of 10 μm and an average fiber length of 10 μm (average aspect ratio 1). A heat dissipation sheet was obtained in the same manner as in Example 1 except that was used.

Comparative Example 1 The thermal conductivity in the fiber length direction is 10
The same as Example 1 except that the graphite carbon fiber (average aspect ratio 10) of 00 W / mK, thermal conductivity of 5 W / mK in the fiber diameter direction, fiber diameter of 10 μm and average fiber length of 100 μm was used. A heat dissipation sheet was obtained.

Comparative Example 2 Boron nitride powder having a thermal conductivity of 62 W / mK and an average particle size of 10 μm was used. The same two-component addition reaction type silicone gel as in Example 1 was mixed with 40 vol.
% And mixed and kneaded. This kneaded material was molded into a thickness of 0.5 mm by a calender molding method using a PET film having a surface subjected to a mold release treatment,
A heat dissipation sheet was obtained by performing a curing treatment at 0 ° C. for 30 minutes.

As described above, Examples 1 to 4 and Comparative Example 1,
Regarding No. 2, the thermal conductivity in the thickness direction (z direction of FIG. 1A) and the plane direction (xy direction of FIG. 1A) of the heat dissipation sheet was compared with the flat plate comparison method defined in JIS A 1412. It measured based on. The results are shown in Table 1.

[0032]

[Table 1]

As can be seen from Table 1, in Comparative Example 1 using the graphitic carbon fibers having an average aspect ratio of 10, the thermal conductivity in the thickness direction of the heat dissipation sheet is due to the orientation of the graphitic carbon fibers. The heat conductivity in the thickness direction of the heat dissipation sheet is close to the heat conductivity in the thickness direction of the heat dissipation sheet in each of the examples using the graphite carbon fibers having a low average aspect ratio of less than 3. The rate was sufficient. Further, when Examples 3 and 4 and Comparative Example 2 are compared, it is confirmed that it is preferable to use a graphite carbon fiber having a thermal conductivity of 100 W / mK or more in the fiber length direction.

[0034]

As described above, the heat dissipation sheet according to the present invention is formed by dispersing graphite carbon fibers having an average aspect ratio of less than 3 in a matrix resin.
The direction of the graphitic carbon fiber is not affected by the flow of the matrix resin during molding, and the graphitic carbon fiber can be dispersed in the matrix resin so that the fiber length direction is randomly oriented. However, even though the graphitic carbon fiber has anisotropy in thermal conductivity, it is possible to obtain a heat dissipation sheet whose thermal conductivity in the thickness direction is close to that in the plane direction perpendicular to the thickness direction. Therefore, the heat dissipation sheet having a higher thermal conductivity in the thickness direction than that of the conventional one can be obtained by the graphite carbon fiber having a high thermal conductivity, and the heat dissipation sheet can be obtained.

By using a graphite carbon fiber having a thermal conductivity of 100 W / mK or more in the fiber length direction, it is possible to reliably obtain a heat dissipation sheet having a higher thermal conductivity in the thickness direction than the conventional one. It is a thing.

[Brief description of drawings]

1A is a perspective view of a heat-dissipating sheet, FIG. 1B is a cross-sectional view of a heat-dissipating sheet in which the fiber length direction of the graphitic carbon fiber is oriented in the plane direction, and FIG. 1C is a fiber length direction of the graphitic carbon fiber. FIG. 3 is a cross-sectional view of a heat dissipation sheet having random orientations.

FIG. 2 is a diagram showing a crystal structure of graphite.

FIG. 3 is a schematic view showing a doctor blade method.

[Explanation of symbols]

 1 Graphitic carbon fiber 2 Matrix resin A Heat dissipation sheet

Claims (2)

[Claims] 1. A heat dissipation sheet formed by dispersing graphite carbon fibers having an average aspect ratio of less than 3 in a matrix resin. 2. The thermal conductivity in the fiber length direction of the graphitic carbon fiber is 100 W / mK or more.
Heat dissipation sheet described in.