Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Utility Patent
1999-06-17
2001-01-02
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S437000, C524S588000, C525S477000, C525S479000, C528S015000
Utility Patent
active
06169142
ABSTRACT:
This invention relates to thermal conductive silicone rubber compositions yielding low-hardness, heat transfer cured products and a method for preparing the same.
BACKGROUND OF THE INVENTION
Since heat-generating electronic parts such as power transistors and thyristors are deteriorated by the heat generated therein, the prior art often employs a countermeasure of attaching heat sinks to electronic parts for heat dissipation or installing electronic parts in contact with metallic chassises of equipment for heat to find a way out. To improve both electrical insulation and heat transfer, heat dissipating insulative sheets of silicone rubber loaded with thermal conductive fillers are often disposed between heat-generating electronic parts and heat sinks.
As the degree of integration is increasing in electronic equipment such as personal computers, word processors and CD-ROM drives, the amount of heat generated by integrated circuit chips such as LSI and MPU in the equipment is also increasing. Conventional cooling means are unsatisfactory for accommodating some integrated circuit chips. Especially in the case of personal computers of the portable notebook type, a large heat sink or cooling fan cannot be mounted in the equipment because the available interior space is limited. In these equipment, integrated circuit chips are mounted on printed wiring boards which are typically made of glass-reinforced epoxy resins and polyimide resins having poor heat transfer. It is then impossible to relieve heat to the boards through heat-dissipating insulative sheets as in the prior art.
Under the circumstances, one common approach is to dispose heat-dissipating parts of the natural convection or forced convection cooling type in proximity to integrated circuit chips such that the heat generated in the chip may be conveyed to the heat-dissipating part. If the heat-dissipating part is in close contact with the chip, the heat transfer therebetween is poor because of surface irregularities. Even if a heat-dissipating insulative sheet intervenes therebetween, because of somewhat poor flexibility of the heat-dissipating insulative sheet, heat expansion can apply stresses between the chip and the part, leading to a failure. If each circuit chip is provided with a heat-dissipating part, an extra space becomes necessary, compromising the target of equipment size reduction. For this reason, it is a common practice to cool a plurality of chips with a single heat-dissipating part. In particular, since tape carrier package MPUs for use in notebook type personal computers are thinner and produce greater amounts of heat than other chips, full considerations must be taken on their cooling system.
The performance of MPU improves as the drive frequency becomes higher every year. Since the amount of heat generated increases accordingly, there is a demand for a more thermal conductive material.
For several chips having different heights, a low-hardness, high-thermal conductivity material which can fill in gaps between the chips is demanded. To meet such a demand, there were proposed thermal conductive sheets that are highly thermal conductive and flexible and adaptable to gaps of various dimensions.
JP-A 2-196453 discloses a sheet comprising a silicone layer formed of a silicone in admixture with a thermal conductive material such as metal oxide and having a necessary strength to handle, on which a soft flexible silicone layer is laid. JP-A 7-266356 discloses a thermal conductive composite sheet comprising a silicone rubber layer loaded with a thermal conductive filler and having an Asker C hardness of 5 to 50 and a porous reinforcement layer having pores with a diameter of at least 0.3 mm. JP-A 8-238707 discloses a sheet comprising a flexible three-dimensional reticulated or foam body in which skeleton lattices are surface coated with thermal conductive silicone rubber. JP-A 9-1738 discloses a thermal conductive composite sheet having a reinforcing film or cloth incorporated therein, which is tacky on at least one surface and has an Asker C hardness of 5 to 50 and a thickness of up to 0.4 mm. JP-A 9-296114 discloses a heat-dissipating spacer formed of a composition comprising an addition reaction type fluid silicone rubber and a thermal conductive insulating ceramic powder, the composition curing into a part having an Asker C hardness of up to 25 and a thermal resistance of up to 3.0° C./W.
However, if one attempts to further increase the thermal conductivity of such low-hardness thermal conductive sheets by blending a large amount of thermal conductive filler therein, the flow of the composition is drastically exacerbated to such an extent that the molding and working thereof becomes difficult.
As a solution to this problem, JP-A 1-69661 discloses a high thermal conductive rubber/plastic composition filled with an alumina filler consisting of 10 to 30% by weight of alumina particles having a mean particle size of up to 5 &mgr;m and the balance of spherical corundum particles having a mean particle size of at least 10 &mgr;m and configured to be free of cutting edges. With such a combination of alumina particles alone, if one attempts to load the composition with 80% by weight or more of alumina particles, the composition is limited in flow.
SUMMARY OF THE INVENTION
An object of the invention is to provide a thermal conductive silicone rubber composition which is designed so as to suppress the viscosity increase of a liquid silicone rubber composition due to heavy loading of aluminum oxide and which can be molded and cured into a silicone rubber part having a high thermal conductivity and low hardness. Another object of the invention is to provide a method for preparing the thermal conductive silicone rubber composition.
The inventor has found that a silicone rubber composition of the addition reaction curing type comprising an alkenyl-bearing organopolysiloxane, aluminum oxide powder, and an alkoxysilane having a monovalent long-chain hydrocarbon group maintains a certain degree of fluidity even when the composition is heavily loaded with aluminum oxide. Then the composition can be effectively molded and worked, and reduced in hardness. The high loading of aluminum oxide imparts a high thermal conductivity. Without a need for expensive fillers such as boron nitride and aluminum nitride, high thermal conductivity molded parts can be manufactured. It has also been found that by mixing an alkenyl-bearing organopolysiloxane with aluminum oxide powder and an alkoxysilane having a monovalent long-chain hydrocarbon group, milling the blend at a temperature of 40° C. or higher, for thereby reducing the viscosity of the blend, and adding a platinum group metal catalyst and an organohydrogenpolysiloxane to the mixture at a temperature below 40° C., there is obtained a silicone rubber composition which is more advantageous.
Accordingly, the present invention provides a thermal conductive silicone rubber composition comprising
(A) 100 parts by weight of an alkenyl-bearing organopolysiloxane having a viscosity of up to about 500,000 centistokes at 25° C.,
(B) 300 to 1,200 parts by weight of aluminum oxide powder,
(C) 0.05 to 10 parts by weight of an alkoxysilane of the following general formula (1):
R
1
a
Si(OR
2
)
(4−a)
(1)
wherein R
1
is a substituted or unsubstituted monovalent hydrocarbon group of 6 to 20 carbon atoms, R
2
is an alkyl group of 1 to 6 carbon atoms, and letter a is equal to 1, 2 or 3,
(D) a catalytic amount of a platinum group metal catalyst, and
(E) an amount of an organohydrogenpolysiloxane containing at least two hydrogen atoms each attached to a silicon atom in a molecule, the amount of the organohydrogenpolysiloxane being such that the molar ratio of SiH groups in component (E) to alkenyl groups in component (A) may range from 0.05/1 to 3/1.
In another aspect, the invention provides a method for preparing the thermal conductive silicone rubber composition, comprising the steps of mixing components (A), (B) and (C) while heating at a temperature of at least 40° C., and blen
Hashimoto Takeshi
Nakano Akio
Dawson Robert
Millen White Zelano & Branigan P.C.
Shin - Etsu Chemical Co. Ltd.
Zimmer Marc S.
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