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...
Reexamination Certificate
2000-03-23
2001-06-26
Yoon, Tae H. (Department: 1714)
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...
C524S430000, C524S437000, C524S439000, C524S440000, C524S495000, C524S496000, C252S511000
Reexamination Certificate
active
06251978
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to an improved composite material. More specifically, the present invention relates to a thermally and electrically conductive composite material that is net-shape moldable and easily moldable or castable.
In the heat sink industries, it has been well known to employ metallic materials for thermal and electrical conductivity applications, such as heat dissipation for cooling semiconductor device packages, grounding and electrical signal transmission. For these applications, such as thermally conductive heat sinks, the metallic material typically is tooled or machined from bulk metals into the desired configuration. However, such metallic conductive articles are typically very heavy, costly to machine and are susceptible to corrosion. Further, the geometries of machined metallic heat dissipating articles are very limited to the inherent limitations associated with the machining or tooling process. As a result, the requirement of use of metallic materials which are machined into the desired form, place severe limitations on heat sink design particular when it is known that certain geometries, simply by virtue of their design, would realize better efficiency but are not attainable due to the limitations in machining metallic articles. In addition, electrically conductive members suffer from the same disadvantages in that they are also difficult to form into the desired configuration.
It is widely known in the prior art that improving the overall geometry of a heat dissipating or electrical transmitting article, can greatly enhance the overall performance of the article even if the material is the same. Therefore, the need for improved article geometries necessitated an alternative to the machining of bulk metallic materials to provide thermal and electrical transfer. To meet this need, attempts have been made in the prior art to provide molded compositions that include conductive filler material therein to provide the necessary thermal conductivity. The ability to mold a conductive composite enabled the design of more complex part geometries to realize improved thermal and electrical performance of the part.
The attempts in the prior art included the employment of a polymer base matrix loaded with a granular material, such as boron nitride grains. Also, attempts have been made to provide a polymer base matrix loaded with flake-like filler material. These attempts are, indeed, moldable into complex geometries but still do not approach the desired performance levels found in metallic machined parts. In addition, known conductive plastic materials are undesirable because they are typically very expensive to manufacture because they employ very expensive filler materials. Still further, these conductive composite materials must be molded with extreme precision due to concerns of filler alignment during the molding process. Even with precision molding and design, inherent problems of fluid turbulence, collisions with the mold due to complex product geometries make it impossible to position the filler ideally thus causing the composition to perform far less than desirable.
Moreover, the entire matrix of the composition must be satisfactory because heat transfer is a bulk property rather than a direct path property such as the transfer of electricity. Heat is transferred in bulk where the entire volume of the body is employed for the transfer while a direct path is needed to conduct electricity. Therefore, even if a highly conductive narrow conduit is provided through a much lower conductive body, the heat transfer would not be as good as a body which is consistently marginally conductive throughout the entire body. Therefore, consistency of the thermal conductivity of the entire matrix of the composite body is essential for overall high thermal conductivity. Also, direct paths are required to achieve efficient electrical conductivity.
In view of the foregoing, there is a demand for a composite material that is highly thermally conductive and/or electrically conductive. In addition, there is a demand for a composite material that can be molded or cast into complex product geometries. There is also a demand for such a moldable article that exhibits thermal and/or electrical conductivity as close as possible to purely metallic conductive materials while being relatively low in cost to manufacture.
SUMMARY OF THE INVENTION
The present invention preserves the advantages of prior art thermally and electrically conductive plastic compositions. In addition, it provides new advantages not found in currently available compositions and overcomes many disadvantages of such currently available compositions.
The invention is generally directed to the novel and unique thermally and electrically conductive plastic composite material with particular application in heat sink applications where heat must be moved from one region to another to avoid device failure. According the application, the present invention may be employed for efficient electrical transmission with high electrical conductivity. The composite material of the present invention enables a highly thermally and electrically conductive composite material to be manufactured at relatively low cost. The conductive molding composition of the present invention has a thermal conductivity of approximately 22 W/m°K or more and a volume resistivity of approximately 0.1 ohm-cm or lower and a surface resistivity of approximately 1.0 ohm or lower. The resistivity tests were done using standardized test IEC 93. In accordance with the preferred embodiment of the present invention, the volume resistivity is approximately 0.1 ohm-cm and the surface resistivity is approximately 1.0 ohm. The conductive composition includes a polymer base matrix of, by volume, between 30 and 60 percent. A first thermally and electrically conductive filler, by volume, between 25 and 60 percent is provided in the composition that has a relatively high aspect ratio of at least 10:1. Also in the composition mixture is a second thermally and electrically conductive filler, by volume, between 10 and 25 percent that has a relatively low aspect ratio of 5:1 or less.
During the molding process of the composition of the present invention, the mixture is introduced into a mold cavity and flows into the various part geometries. The high aspect ratio filler generally aligns with the flow of the mixture in the mold but inherently leaves small random voids therebetween. The low aspect ratio filler in the mixture fills the voids between the high aspect ratio filler in the mixture. As a result, the number of interfaces and base matrix thickness between filler members is greatly reduced thus resulting in thermal and electrical conductivity and performance superior to that found in prior art conductive composite materials.
It is therefore an object of the present invention to provide a conductive composite material that has an electrical conductivity much greater than found in prior art composites and an electrical resistivity much lower than prior art composites.
It is an object of the present invention to provide a conductive composite material that is moldable.
It is a further object of the present invention to provide a low cost conductive composite material.
Another object of the present invention is to provide a conductive composite material that enables the molding of complex part geometries.
It is a further object of the present invention to provide a conductive composite material that is significantly lighter in weight than metallic materials.
It is yet a further object of the present invention to provide a conductive composite material that has a thermal and electrical conductivity close to pure or composite metallic materials.
REFERENCES:
patent: T904012 (1972-11-01), Staniland
patent: 3398233 (1968-08-01), Lizasoain et al.
patent: 3673121 (1972-06-01), Meyer
patent: 3708387 (1973-01-01), Turner et al.
patent: 4098945 (1978-07-01), Oehmke
patent: 4307147 (1981-12-01), Ohishi et al.
patent: 4367745 (1983-01-01), W
Barlow Josephs & Holmes, Ltd.
Chip Coolers, Inc.
Yoon Tae H.
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