JPS6346524B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention mainly relates to a flat or three-dimensional insulating sheet with excellent thermal conductivity used between an electronic component and a heat sink. In recent years, as electronic components such as semiconductor devices have become smaller and more dense, various demands have been made for electrically insulating materials. Among them, electronic components such as power transistors and thyristors are at risk of generating heat during use, resulting in deterioration of their characteristics or damage. Therefore, a heat dissipation fan metal plate or the like is usually attached to these electronic components via an insulating sheet with good thermal conductivity to protect the components. Conventionally, such insulating sheets include mica sheets, polyester films, etc., and these sheets are generally coated with grease to improve adhesion to parts. Although these sheets are cheap, they do not have sufficient thermal conductivity, and the process of applying grease is complicated, and the applied grease may leak out or deteriorate due to heat if used for a long time. This was not desirable due to its drawbacks. Also, sheets are known in which synthetic rubber contains thermally conductive inorganic powder such as alumina or zinc white as a filler. Generally, the thermal conductivity of these sheets can be increased by increasing the volume ratio of thermally conductive inorganic powder in the sheet. However, as the volume ratio of the filler increases, the moldability and mechanical strength of the sheet tend to decrease, causing practical problems such as loss of flexibility and becoming brittle and easily broken when bent. result. Therefore, in the above-mentioned sheet made of synthetic rubber containing inorganic powder as a filler, the volume ratio of the filler cannot be made sufficiently high, and therefore sufficient thermal conductivity cannot be obtained. In addition, it has been proposed to insert woven fabric or non-woven fabric into a sheet of synthetic rubber composition, or to mix single fibers into the sheet, but although improvements in strength can be seen, they lack the effect of increasing the volume ratio of the filler. It wasn't enough. The present inventors conducted intensive studies to overcome these drawbacks of the conventional technology, and as a result, they focused on the property that polytetrafluoroethylene resin (PTFE) easily fibrillates when compressive shear stress is applied. The inventors have discovered that the volume ratio of the filler in the sheet can be made extremely high by gripping the sheet with fibrillated PTFE, leading to the present invention. That is, the present invention
A composition comprising 50 to 95% by volume of an electrically insulating and thermally conductive inorganic powder and 5% by volume or more of a binder, the binder being a polytetrafluoroethylene resin containing 0.2 to 30% by volume. (PTFE) and a synthetic rubber having a volume ratio of 1/4 or more to the PTFE, and the powder is gripped by the binder containing the PTFE which has been fibrillated by compressive shear force. This is an insulating sheet with excellent properties. The thermally conductive inorganic powder used in the present invention includes electrically insulating alumina, zinc white, magnesium oxide, beryllium oxide, silica, mica, boron nitride, alumina nitride, etc., and these may be used alone or in combination of two or more. used in combination. Regarding the particle size and shape of the powder,
Although not particularly limited, it is preferable to have a shape close to a sphere with a size of about 0.2 to 100 μm from the viewpoint of forming the sheet. In the present invention, if the powder is in the composition,
It only needs to contain a high proportion of ~95% by volume, 50
If it is less than 95% by volume, thermal conductivity will be insufficient, and if it exceeds 95% by volume, mechanical strength will decrease, which is not preferred. In particular, if the powder is contained in an amount of 70 to 95% by volume, very good thermal conductivity can be obtained. Various types of PTFE can be used in the present invention, but PTFE obtained by emulsion polymerization is particularly suitable.For example, PTFE obtained by emulsion polymerization is particularly suitable. Fine powder obtained by PTFE is used as a binder for inorganic powder, mixed with synthetic rubber. The synthetic rubber used in the present invention is a rubber-like elastic body having heat resistance and electrical insulation properties, such as silicone rubber, fluorocarbon rubber, acrylic rubber, and chlorosulfonated polyethylene. Silicone rubber is more preferred. This synthetic rubber not only gives the resulting sheet flexibility and adhesion, but also fills the fine voids that remain between the inorganic powders or between the inorganic powder and the PTFE.
It works to increase the thermal conductivity of the sheet. In the present invention, a binder of 5% by volume or more is required, and the binder includes the above-mentioned PTFE and synthetic rubber, and other additives such as a vulcanizing agent, a coloring agent, an anti-aging agent, etc. Included accordingly. PTFE is contained in the composition in an amount of 0.2 to 30% by volume, preferably 1.0 to 30% by volume.
If the PTFE content is 20% by volume and less than 0.2% by volume, sufficient mechanical strength and flexibility will not be obtained, and as the volume ratio of the powder increases, the sheet will become brittle and break easily. If the volume exceeds %, the binding force between PTFE becomes too strong and becomes hard, resulting in a decrease in adhesion as a sheet, which is not preferable. On the other hand, synthetic rubber has a volume ratio of 1/1 to PTFE.
It is necessary to contain 4 or more, and if it is less than 1/4, the amount of PTFE increases too much, making it hard and reducing the adhesion of the sheet, which is not preferable. When such a composition is kneaded, compressive shear force is applied and it is mixed with synthetic rubber.
PTFE easily fibrillates. This fibrillated PTFE then wraps around the powder and grips the powder, creating a kneaded material that does not allow the powder to fall off. Kneading in the present invention can be carried out using a kneading machine such as a mixer, a kneader, a ball mill, a Banbury mixer, a roll mixer, a screw mixer, etc., to disperse the composition and to fibrillate the PTFE. It will be done. Sufficient fibrillation is advantageous because post-processing steps such as pressing and calendering can be simplified. Note that the timing of fibrillation of PTFE in the present invention is not particularly limited;
PTFE may be fibrillated by compressive shearing force in advance and then kneaded with powder, synthetic rubber, etc., or PTFE and powder may be kneaded in advance and fibrillated and then synthetic rubber etc. Of course, you can also mix it with
Compositions such as powder, PTFE, synthetic rubber, etc. may be mixed and kneaded at the same time. During kneading, synthetic rubber vulcanizing agents, vulcanization accelerators, stabilizers, processing improvers, flame retardants, colorants, solvents, etc. can be added as necessary. The kneaded material is made into a flat or three-dimensional sheet by using a calendar roll, an extruder, a knife coating, a press, or the like. Note that the term "sheet" in the present invention includes not only a flat sheet but also a three-dimensional molded sheet such as a pipe. Kneading and sheeting may be performed in one step or in separate steps depending on the equipment used. This type of kneaded sheet has a much higher powder volume ratio than conventional ones, resulting in a flexible sheet, and despite the high powder packing density, it also has high mechanical strength and excellent thermal conductivity. Excellent in sex. Furthermore, in order to obtain a planar insulating sheet with excellent strength, it is necessary to further roll this kneaded sheet with a plurality of calender rolls having different circumferential speeds. In other words, if the sheet-like material is folded or rolled repeatedly by changing the rolling direction, a larger compressive shear force will be applied, so if the powder's packing density is low, it will increase, and the packing density will increase if there are almost no voids or A uniform product with no voids can be obtained, and the mechanical strength is also improved by fibrillating PTFE. The peripheral speed ratio of the roll should be about 1.1 to 3 times,
The temperature of the roll is 20, which makes it easy for PTFE to fibrillate.
The heating may be carried out at a temperature of .degree. C. or higher, preferably 50.degree. C. or higher. However, the temperature must be below the crosslinking temperature of the synthetic rubber. The pressure of the rolls may be approximately 1 to 50 kg/cm ² , but too high a pressure is not preferred because it becomes hard and loses flexibility. It is preferable to further roll the sheet material rolled in one direction in the orthogonal direction, since the powder and the like will be more uniformly dispersed and the mechanical strength in the vertical and horizontal directions will be averaged. It goes without saying that, depending on the case, this sheet-like material may be folded at a certain angle and rolled. The sheet-like material thus repeatedly rolled is vulcanized or cross-linked by heating and pressurizing to obtain an insulating sheet having even better properties. It goes without saying that reinforcing materials such as nonwoven fabric, woven fabric, net, thread, etc. may be inserted into this insulating sheet as necessary. Furthermore, it may be three-dimensionally molded, such as a pipe shape. The physical properties of the insulating sheet thus obtained were measured and evaluated by the following method. (1) Measurement of thermal resistance A test piece with a thickness of 0.40 mm was placed between a power transistor (2SD-428, TO-3 type) and a heat dissipation fin (H type,
6 x 12 cm square), and tighten with a tightening torque of 5
The thermal resistance value (°C/watt) was measured when a current of 2 A and a voltage of 10 V was applied. The smaller the thermal resistance value, the higher the thermal conductivity.
The heat dissipation performance was good for practical use if it was 0.5°C/watt or less. (2) Measurement of tensile strength JIS K6301 using Tensilon type tensile tester
Measured according to. If the tensile strength was 30 Kg/cm ² or more, it could be used practically without any problems. (3) Measurement of folding strength Using a Karl Frank type folding strength tester,
The upper end of a 3 x 20 cm test piece with a thickness of 0.40 mm was attached to a clamp and the lower end was attached to a 1 kg load stand, and the number of times it took for the test piece to break at a bending angle of 180° was measured. A folding strength of 200 folds or more indicates that it is extremely flexible and desirable, while a folding strength of 10 folds or less indicates that it is brittle and easily cracked and is of little practical use. Examples 1 to 10 Fine powder of Teflon 6J (trade name, manufactured by Mitsui Fluorochemical Co., Ltd.) as PTFE, alumina powder with a particle size of 30 μm, and dimethylvinyl silicone rubber were kneaded in various proportions. In addition, as a vulcanizing agent, 1.5% by weight of peroxide based on the weight of the silicone rubber was added. A mixture of these in the proportions shown in Table 1 is kneaded in a kneader, then made into a sheet with two rolls, and then rolled four times at a temperature of 60°C with rolls at a circumferential speed ratio of 1:1.5 to obtain a thickness. A sheet-like material of 0.40 mm was obtained. Next, apply this seal-like material to 150
It was vulcanized at ℃ to obtain an insulating sheet with a thickness of 0.4 mm. Comparative Examples 1 to 6 In contrast to the above examples, those containing no PTFE at all (Comparative Examples 1, 3 and 5), those containing more than 30% by volume of PTFE (Comparative Examples 2 and 4), and those containing PTFE in a silicone rubber of PTFE. Those having a volume ratio of less than 1/4 (Comparative Examples 2, 4, and 6) were used as comparative examples, and sheets with a thickness of 0.4 mm were manufactured using the same manufacturing method as in the examples. These Examples and Comparative Examples are summarized and their formulations and physical properties are shown in Table 1.

[Table] Comparative Examples 7 and 8 Formulation of Example 7 (PTFE 10% by volume, alumina 70
% by volume, silicone rubber 20% by volume), in Comparative Example 7, fluorine fiber with a fiber thickness of 3 denier and fiber length of 15 mm was used instead of PTFE, and in Comparative Example 8, fluorine fiber with a fiber thickness of 3 denier and a fiber length of 15 mm was used.
A 0.4 mm sheet was made using the same manufacturing method as in Example 7, using glass fiber with a diameter of 25 μm and a length of 15 mm instead of PTFE. Example 11 The formulation of Example 7 (PTFE 10% by volume, alumina 70
An insulating sheet with a thickness of 0.4 mm was obtained using the same manufacturing method as in Example 7, using boron nitride powder with a particle size of 25 μm instead of alumina. These Examples and Comparative Examples are summarized and their formulations and physical properties are shown in Table 2.

【table】

[Table] The insulating sheet obtained by the present invention is used for heat dissipation of electronic components and is extremely excellent.
It is also useful as a heat dissipation spacer material for heat pipes.

Claims (1)

[Claims] 1 Consists of a composition containing 50 to 95% by volume of electrically insulating, thermally conductive inorganic powder (hereinafter referred to as "powder") and 5% by volume or more of a binder, where the binder is
The PTFE contains 0.2 to 30% by volume of polytetrafluoroethylene resin (hereinafter referred to as "PTFE") and synthetic rubber of 1/4 or more by volume to the PTFE, and is fibrillated by compressive shearing force.
An insulating sheet with excellent thermal conductivity, which is obtained by holding the powder with the binder containing the powder.