JPH0216698B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an insulating heat dissipating sheet, and particularly to a method for manufacturing an insulating heat dissipating sheet that efficiently transfers heat generated inside a heat-generating electronic component to a heat dissipating fin or a metal heat dissipating plate. Traditionally, power transistors and hybrid ICs
Heat-generating electronic components such as
To avoid deterioration of its characteristics or damage, it was attached to a heat dissipation fin or a metal heat dissipation plate via an insulating heat dissipation sheet. Greased mica plates and silicone rubber sheets are used as insulating heat dissipation sheets, but they do not have very good thermal conductivity.
As the output of heat-generating electronic components increases, there is a drawback that sufficient heat dissipation cannot be achieved. On the other hand, compact heat dissipation fins and metal heat dissipation plates with very high thermal efficiency have appeared, but conventional insulation sheets that do not have sufficient thermal conductivity have been replaced by small and high-performance heat dissipation fins and metal heat dissipation plates. However, a sufficient heat dissipation effect could not be obtained. It has also been considered to use a sheet having an aluminum oxide film formed by anodizing aluminum foil as an insulating heat dissipation sheet. However, since the aluminum oxide film absorbs moisture and the electrical insulation properties deteriorate, an insulating heat dissipating sheet with high electrical reliability cannot be obtained. An invention in which some improvements have been made to this configuration is disclosed in Japanese Patent Application Laid-Open No. 135154/1983. The present invention is a heat-resistant laminate that can be post-processed and is made by anodizing the surface of aluminum foil and then coating the aluminum foil with a solvent-soluble polyimide resin. However, the method for manufacturing the heat-resistant laminate disclosed in the publication as a means of achieving the present invention is to perform a single anodic oxidation treatment on aluminum foil, dry it once, and then coat it with polyimide resin. It is. That is, in this manufacturing method, the aluminum oxide film is formed by just one anodizing treatment, so the withstand voltage is low. In addition, since this manufacturing method requires drying once, the polyimide resin does not completely fill the micropores in the aluminum oxide film, making it impossible to improve the electrical reliability of the aluminum oxide film. The purpose of the present invention is to provide a method for manufacturing an insulating heat dissipating sheet that eliminates the above-mentioned conventional drawbacks.The present invention aims to provide a method for manufacturing an insulating heat dissipating sheet that eliminates the above-mentioned drawbacks of the conventional method. It is characterized by the ability to form a highly thermally conductive and electrically perfect insulating film extremely easily through a series of continuous treatments to achieve complete sealing. Next, the method for manufacturing an insulating heat dissipating sheet of the present invention will be explained in detail based on the drawings. FIG. 1 is a longitudinal cross-sectional view of the aluminum foil and the aluminum oxide film after the aluminum foil has been subjected to the first anodization in the manufacturing method of the present invention. After degreasing and polishing in a system solution, washing with water, then pickling in a nitric acid solution, washing with water,
The aluminum oxide film 2 is then anodized in a solution of at least one of phosphoric acid and chromic acid.
is formed. Further, FIG. 2 is an enlarged vertical cross-sectional view of the aluminum foil and the aluminum oxide film, and the aluminum oxide film 2 has micropores 3 lined up, and the innermost ends of the micropores 3 are located in the aluminum foil. It reaches close to the surface of 1.
Further, as shown in FIG. 3, the aluminum oxide film 2 is dotted with defects such as open defect holes 4 and non-open defect cavities 5 in which aluminum oxide is usually missing. According to the present invention, after the first anodic oxidation, it is washed with water, and then mixed with an aqueous solution of at least one of ammonium borate and ammonium tartrate, methanol, ethanol, and N.
- Anodic oxidation is performed again in a mixed solution with at least one selected from methylpyrrolidone to further form an aluminum oxide film. Fourth
The figure is an enlarged vertical cross-sectional view of the aluminum foil and the aluminum oxide film after the second anodic oxidation and further formation of the aluminum oxide film. The aluminum oxide film formed between the aluminum foil 1 and the foil 1 grows into the aluminum foil below, and the aluminum oxide film becomes thicker in that portion. Reference numeral 6 in the figure indicates a portion where the aluminum oxide film described above has grown and its thickness has increased. After performing the second anodic oxidation, if necessary, after washing in an N-methylpyrrolidone solution, the polyimide resin is immediately immersed in an N-methylpyrrolidone solution, and then the polyimide resin is dried and cured. Then, the fine pores 3 existing in the aluminum oxide film 2 and defective portions such as open defect holes 4 and non-open defect cavities 5 are sealed with the polyimide resin. Note that the polyimide resin also permeates into the non-open defect cavities 5 through the micropores 3. Next, the first anodic oxidation treatment in the manufacturing method of the present invention will be explained. Orthophosphoric acid equivalent concentration in a solution of at least one of phosphoric acid and chromic acid 1 to
800g/, chromic anhydride equivalent concentration 1~400g/,
Liquid temperature 0~70℃, current density 0.1~10A/ ^dm2 , voltage
It is preferable to carry out anodic oxidation under conditions of 10 to 200 V to form an oxide film with a thickness of 1 to 50 μm. Next, the second anodic oxidation treatment will be explained. Ammonium tetraborate concentration in a mixed solution of at least one of ammonium borate and ammonium tartrate and at least one selected from methanol, ethanol, and N-methylpyrrolidone, tartaric acid Ammonium concentration 0.1~100g/, water 10~900g/,
Liquid temperature 0~70℃, current density 0.01~1A/ ^dm2 , voltage
Anodic oxidation is performed under the conditions of 10 to 1000V, but since it is difficult to measure the oxide film thickness, the processing time is 10 to 1000V.
It is preferable to set it as 120min. Next, the immersion treatment of the polyimide resin in the N-methylpyrrolidone solution after the second anodic oxidation treatment will be described. It is preferable that the polyimide resin concentration in the solution is 10 to 700 g/, the liquid temperature is 100° C. or less, and the immersion time is 1 min or more. According to the present invention, it is necessary to perform the first anodizing treatment, the second anodizing treatment, and the immersion treatment in the polyimide resin solution continuously. If the first anodizing treatment and the second anodizing treatment are not performed consecutively, air will be drawn into the micropores created by the first anodizing treatment, and the second anodizing treatment will be performed. The process will not be completed completely. Furthermore, for the same reason, if the aluminum oxide film dries between the second anodizing treatment and the immersion treatment in the polyimide resin solution, some parts of the micropores are not completely filled with polyimide resin, and the aluminum oxide It is not possible to improve the electrical reliability of the film. That is, the present invention specifically forms an electrically perfect insulating film by sequentially performing a process for forming an aluminum oxide film and a process for sealing micropores and defective areas, that is, open defective holes and non-open defective cavities. It is characterized by being extremely easy to form. Note that an aluminum oxide film formed by anodizing in a solution other than the first anodizing solution, such as a sulfuric acid solution or an oxalic acid solution, generates many cracks at about 200°C. However, the aluminum oxide film formed by the first anodic oxidation treatment of the present invention was 300%
No cracks occur even at high temperatures above ℃. The reason for this is that the micropores in the aluminum oxide film formed by the first anodizing treatment are relatively large, which absorbs the difference in thermal expansion coefficient between the aluminum oxide film and aluminum. think. Further, by further forming an aluminum oxide film through the second anodizing treatment, the withstand voltage is improved, but the electrical reliability is still insufficient. After the second anodic oxidation treatment, the defect portions, that is, the open defect holes and the non-open defect cavities are sealed with the polyimide resin by immersion treatment in a polyimide resin solution, thereby improving the withstand voltage. Note that since polyimide resin has relatively high heat resistance and chemical resistance, it is superior in heat resistance and chemical resistance to cases where other resins are used. Furthermore, a thin film of the resin is automatically formed on the surface of the foil sealed with the resin. Next, examples of the present invention and comparative examples thereof will be described. Example 1 At least one side of an aluminum foil was anodized to form an aluminum oxide film with a thickness of 20 μm by anodizing at a concentration of 55 g/orthophosphoric acid in a phosphoric acid solution, a liquid temperature of 25° C., a current density of 1 A/dm ² , and a voltage of 120 V. After washing with water, ammonium tetraborate concentration 10 g/, water 100 g/, liquid temperature 20
°C, current density 0.05A/dm ² , voltage 500V, processing time
It was anodized again under the conditions of 60 min, and then immersed in the polyimide resin solution under the conditions of a polyimide resin concentration of 200 g/N-methylpyrrolidone solution, a solution temperature of 25°C, and an immersion time of 60 min to remove the aluminum oxide film. The micropores and defective areas were sealed with polyimide resin. The insulating film formed by the above series of continuous treatments is heated to 300°C.
However, the withstand voltage after heat treatment at 300℃ is 2.0KV, and the thermal conductivity is 0.42cal/sec・cm・
It was warm at ℃. Example 2 At least one side of an aluminum foil was anodized to form an aluminum oxide film with a thickness of 20 μm by anodizing at a concentration of 55 g/orthophosphoric acid in a phosphoric acid solution, a liquid temperature of 25° C., a current density of 1 A/dm ² , and a voltage of 120 V. After washing with water, ammonium tetraborate concentration in a mixed solution of ammonium borate aqueous solution and ethanol was 7.5 g/, water 200 g/, liquid temperature 20°C, current density 0.05 A/dm ² , voltage 500 V, treatment. After being anodized again for 60 min, and then washed in N-methylpyrrolidone solution, the polyimide resin was immediately anodized using the following conditions: polyimide resin concentration in N-methylpyrrolidone solution was 200g/, liquid temperature was 25°C, and time was 60min. By immersing it in a resin solution, the micropores and defective areas existing in the aluminum oxide film were sealed with polyimide resin. The insulating film formed by the above series of continuous treatments does not generate cracks even at 300℃, has a withstand voltage of 2.0KV after heat treatment at 300℃, and a thermal conductivity of 0.42cal/sec・cm・℃
It was hot. Example 3 At least one side of an aluminum foil was exposed to a chromic acid solution with a concentration of chromic acid anhydride equivalent to 30 g/, a liquid temperature of 40°C,
After forming an aluminum oxide film with a thickness of 20 μm by anodizing at a current density of 0.5 A/dm ² and a voltage of 100 V, it was washed with water, and then the ammonium tartrate concentration in the mixed solution of ammonium tartrate aqueous solution and methanol was 7.5 g/, Water 200g/, liquid temperature 20℃, current density 0.05A/dm ² , voltage 400V, processing time 60min
The polyimide resin was anodized again under the following conditions, and then washed in N-methylpyrrolidone solution. Immediately after the polyimide resin was washed in N-methylpyrrolidone solution, the polyimide resin concentration was 200g/, the solution temperature was 25℃, and the immersion time was 60min. By immersing it in a solution, the micropores and defective areas existing in the aluminum oxide film were sealed with polyimide resin. The insulating film formed by the series of continuous treatments is
No cracks occur even at 300℃, the withstand voltage is 1.8KV after heat treatment at 300℃, and the thermal conductivity is 0.42cal/sec.
It was warm at cm・℃. Comparative Example 1 At least one side of an aluminum foil was anodized to form an aluminum oxide film with a thickness of 20 μm under the conditions of a concentration of orthophosphoric acid in a phosphoric acid solution of 55 g/, a liquid temperature of 25°C, a current density of 1 A/dm ² , and a voltage of 120 V. After that, it was washed with water and dried. Next, the ammonium tetraborate concentration in the mixed solution of ammonium borate aqueous solution and N-methylpyrrolidone was 10 g/100 g/water.
, liquid temperature 20℃, current density 0.05A/dm ² , voltage
It was anodized again under conditions of 500V and treatment time of 60 minutes, washed with water, and dried. Next, the polyimide resin was immersed in a polyimide resin solution in an N-methylpyrrolidone solution to seal the micropores and defective parts existing in the aluminum oxide film with the polyimide resin. The insulating film formed by the discontinuous process does not generate cracks even at 300°C.
The withstand voltage after heat treatment at ℃ was as low as 1.0KV. Comparative Example 2 At least one side of an aluminum foil was anodized under the conditions of sulfuric acid equivalent concentration of 183 g/, liquid temperature of 25°C, current density of 1A/dm ² and voltage of 10V to form an aluminum oxide film thickness of 20μm.The insulation film was heated at 200°C. After heat treatment at 300℃, the withstand voltage is
It was extremely low at 0.2KV. Comparative Example 3 The thermal conductivity of mica plates and silicone rubber sheets conventionally used as insulating heat dissipation sheets is 1.8×10 ^-3 cal/sec・cm・℃, and that of silicone rubber sheets is 6.0×10 ^{- 3} cal/sec・cm・℃,
This is extremely low.

【table】

[Table] According to the above, the manufacturing method of the present invention improves thermal conductivity by sequentially performing the first anodizing treatment, the second anodizing treatment, and the immersion treatment in a polyimide resin solution. A highly electrically perfect insulating heat dissipation sheet can be provided very easily. Therefore, the insulating heat dissipation sheet obtained by the manufacturing method of the present invention has a large heat dissipation effect and high electrical insulation, so it can fully demonstrate the performance of small and high-performance heat dissipation fins and metal heat dissipation plates, and can be used for electronic The benefits to industry are huge.

[Brief explanation of drawings]

FIG. 1 is a longitudinal cross-sectional view of the aluminum foil and the aluminum oxide film after the first anodization has been performed on the aluminum foil, and FIGS. 2 and 3 are enlarged views of the aluminum foil and the aluminum oxide film, respectively. FIG. 4 is an enlarged vertical cross-sectional view of the aluminum foil and the aluminum oxide film after the aluminum foil has been subjected to the second anodic oxidation. DESCRIPTION OF SYMBOLS 1... Aluminum foil, 2... Aluminum oxide film, 3... Fine pore, 4... Open defect hole, 5... Non-open defect cavity, 6... Aluminum oxide film thickness increased portion.

Claims (1)

[Claims] 1 At least one side of the aluminum foil is coated with phosphoric acid,
After forming an aluminum oxide film by anodizing in a solution of at least one of chromic acids,
Washing with water, and then anodizing again in a mixed solution of at least one aqueous solution of ammonium borate or ammonium tartrate and at least one selected from methanol, ethanol, and N-methylpyrrolidone to oxidize the After thickening the aluminum film and washing it in an N-methylpyrrolidone solution if necessary, the polyimide resin is immediately immersed in an N-methylpyrrolidone solution to eliminate the micropores present in the thickened aluminum oxide film. and a method for producing an insulating heat dissipation sheet, characterized by a series of continuous treatments for sealing defective parts with the polyimide resin.