Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
2000-07-06
2002-04-30
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of silicon containing
C428S200000, C428S202000, C428S206000, C428S208000, C428S451000, C252S511000, C252S512000, C252S513000, C252S516000, C524S424000, C524S430000, C524S433000
Reexamination Certificate
active
06379806
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a heat-resistant composite sheet having thermal conductivity and, more particularly, to a thermally conductive heat-resistant silicone rubber composite sheet suitable as a thermally conductive cushion sheet used when a hot pressing process is adopted for forming laminates or flexible boards and for connecting electrodes, such as electrodes of a liquid crystal display, by the use of an electrically conductive anisotropic film. In addition, the invention is concerned with a method of producing the aforesaid silicone rubber composite sheet.
BACKGROUND OF THE INVENTION
Hitherto known thermally conductive electrical insulating materials are, e.g., silicone rubber in which beryllium oxide powder, aluminum oxide powder, aluminum hydroxide powder, magnesium oxide powder or zinc oxide powder is mixed (as disclosed in Japanese Tokkai Sho 47-32400, wherein the term “Tokkai” means an “unexamined published patent application”), and boron nitride-containing silicone rubber which is reinforced by an insulator net (as disclosed in Japanese Jikkai Sho 54-184074, wherein the term “Jikkai” means an “unexamined published utility model application”). These silicone rubbers are already used as heat-dissipating insulators for exothermal components, such as power transistors, thyristors, rectifiers, transformers, power MOS and FET. However, those silicone rubbers have a drawback of deteriorating under the influences of impurities in the thermal conductivity imparting agents and their pH values when used under a high temperature condition, specifically 200° C. or above.
On the other hand, the foregoing thermally conductive electrical insulating sheets have been employed as cushion sheets in the cases of forming laminates or flexible printed circuit boards by means of a press forming machine, or in the case where an electrically conductive anisotropic film is subjected to hot pressing by means of a pressing machine in order to connect electrode terminals of liquid crystal display and an operation circuit-mounted flexible printed circuit board. For such uses, the glass cloth-reinforced silicone rubber sheet in which boron nitride is mixed is disclosed in Japanese Tokkai Hei 5-198344, and the glass cloth-reinforced antistatic silicone rubber sheet in which both boron nitride and an electrically conductive substance are mixed is disclosed in Japanese Tokkai Hei 6-36853. These cases also have the same deterioration problem of silicone rubber under a high temperature condition as mentioned above.
In recent years, the material properties of flexible printed circuit boards and those of electrically conductive anisotropic films have been changed so as to have suitability for high-temperature forming, while the forming temperature has also been raised for increasing productivity by reduction in a pressing cycle time. Under these circumstances, the heat resistance and the thermal conductivity of a silicone rubber sheet as thermally conductive electrical insulator are of increasing importance. Therefore, Japanese Tokkai Hei 7-11010 proposes the silicone rubber simple sheet in which the carbon black containing volatile components, exclusive of water, in a proportion of at most 0.5 weight % is used as a thermal conductivity providing agent to ensure high heat resistance enough to withstand exposure to temperatures of 300° C. or higher and satisfactory thermal conductivity.
However, such an electrical insulating sheet having thermal conductivity is short of strength because it is a simple sheet of silicone rubber, and so there is a fear of breaks by repeated use. In addition, the tackiness of silicone rubber sheet causes another problem that the sheet sticks to the pressure applying tool and the pressed material after hot pressing, and thereby the workability is considerably worsened. Further, when it is used at a temperature of 300° C. or above as in the case where electrode terminals of liquid crystal display and an operation circuit-mounted flexible printed circuit board are connected via an electrically conductive anisotropic film in accordance with a hot pressing process using a pressing machine, the silicone rubber sheet causes a problem such that the volatile components present therein pollute the electrode terminals and the operation circuit. In order to avoid such detrimental effects of the silicone rubber sheet, the heating treatment of the sheet at a high temperature and the resin film lamination on the sheet are generally carried out. However, the heating of the laminated sheet up to a high temperature of 150° C. or above causes a warp in the sheet because the contraction of the rubber occurs, thereby spoiling the workability.
So our intensive studies have been made to look for thermally conductive heat-resistant silicone rubber composite sheets having not only satisfactory thermal conductivity but also sufficient strength, and further causing no firm adhesion to pressurizing tools by rubber tackiness and ensuring good workability. As a result, it has been found that satisfactory results can be obtained when the silicone rubber sheet in which carbon black containing volatile impurities except water in a content of at most 0.5 weight % and at least one ingredient selected from the group consisting of metals, metal oxides, metal nitrides and metal carbides are mixed in specified proportions is reinforced by a heat-resistant resin film, thereby achieving the present invention.
SUMMARY OF THE INVENTION
Therefore, one object of the invention is to provide a thermally conductive heat-resistant silicone rubber composite sheet that has not only high thermal conductivity but also sufficient strength, causes no firm adhesion to pressurizing tools by rubber tackiness and ensures good workability.
Another object of the invention is to provide a method of producing a silicone rubber composite sheet having the aforementioned characteristics.
The foregoing objects of the invention are attained with a composite sheet comprising a silicone rubber sheet and a heat-resistant resin film layer provided on at least one side of the silicone rubber sheet, wherein the silicone rubber sheet is a sheet molded of a silicone rubber composition which comprises (A) 100 parts by weight of organopolysiloxanes having an average polymerization degree of at least 200, (B) 0 to 150 parts by weight of carbon black containing volatile impurities except water in a content of at most 0.5 weight %, (C) 0 to 1,600 parts by weight of at least one thermally conductive substance selected from the group consisting of metals, metal oxides, metal nitrides and metal carbides and (D) a curing agent, provided that the total proportion of the components (B) and (C) is from 10 to 1,600 parts by weight, and having a volatile impurity content of at most 0.2 weight % when it is heated at 150° C. for 3 hours; and with a method of producing the aforesaid composite sheet.
In accordance with the invention, a heat-resistant resin film is stuck on the silicone rubber sheet. So the present thermally conductive heat-resistant silicone rubber composite sheet is free of rubber tackiness troubles, has sufficient strength and can ensure good workability. Further, the present composite sheet does not pollute electrode terminals and operation circuits upon hot pressing because the volatile impurity content therein can be reduced to 0.2 weight % or below. In addition, the carbon black incorporated in the present composite sheet prevents electrostatic buildup to avoid the adhesion of dust to the composite sheet in use and further inhibit electronic components loaded on circuits from causing a breakdown by electric discharge.
DETAILED DESCRIPTION OF THE INVENTION
The organopolysiloxanes as Component (A) of the present silicone rubber composition have an average polymerization degree of at least 200, preferably from 3,000 to 20,000, and are represented by the average composition formula R
n
SiO
(4-n)/2
, wherein n is a positive number of from 1.95 to 2.05 and R represents a substituted or unsubstituted monovalent hydrocar
Nakano Akio
Takamura Toru
Dawson Robert
Millen White Zelano & Branigan P.C.
Robertson Jeffrey B.
Shin-Etsu Chemical Co. , Ltd.
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