Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
1999-04-14
2001-04-24
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C523S466000
Reexamination Certificate
active
06221509
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to epoxy resin compositions suitable for semiconductor encapsulation comprising an epoxy resin, a curing agent, and an inorganic filler. The invention also relates to semiconductor devices encapsulated with these compositions in a cured state.
2. Prior Art
Epoxy resin compositions used as encapsulants for the latest low-profile packages are loaded with greater amounts of spherical fused silica for reducing moisture absorption and preventing crack formation when the package is soldered to a substrate. However, such a high loading of fused silica results in a very small coefficient of thermal expansion. When copper is used as the lead frame material, the difference between the expansion coefficient of the lead frame and the expansion coefficient of the epoxy resin composition becomes large. This results in greater stress during thermal cycling and solder reflow,,and thus a higher tendency for crack formation.
One traditional approach to this problem is to modify the expansion coefficient of the epoxy resin composition by mixing the fused silica with another inorganic filler having a large coefficient of expansion. Many patents describe the formulation of mixtures of fused silica and crystalline silica as the inorganic filler in epoxy resin compositions.
However, because the crystalline silica used in these patents is composed entirely of crushed material, crystalline silica cannot be included in the amount required in recent compositions having high loadings of spherical fused silica. As a consequence, it was impossible to lower the moisture absorption of the cured encapsulant to the desired level. The alternative is to use spherical crystalline silica. Unfortunately, spherical crystalline silica is very difficult to obtain because it cannot be industrially manufactured. While some spherical crystalline silica is naturally available, this is the product of long years of wear in streambeds. As such, its use for encapsulating state-of-the-art semiconductor devices leaves something to be desired in terms of purity. In addition, such material cannot be given a particle size distribution optimal for high loadings.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide semiconductor encapsulating epoxy resin compositions which are capable of high loadings of inorganic filler, allow easy control of the coefficient of thermal expansion, and give cured products having a high thermal conductivity and a low moisture absorption. Another object of the invention is to provide semiconductor devices encapsulated with these compositions in a cured state.
We have found that, by forming cristobalite from spherical fused silica adjusted to the optimal particle diameter, spherical fused silica can be crystallized from the fused state to cristobalite while retaining the original shape and particle size distribution. The spherical cristobalite, thus obtained without alteration of the particle size distribution or shape, allows higher loadings of inorganic filler, higher thermal conductivity, and easy control of the coefficient of expansion to be achieved. By including the resulting spherical cristobalite as a filler in a specific proportion within an epoxy resin composition, the composition can be endowed with higher filler contents and better thermal conductivity, the coefficient of expansion can be adjusted as desired, and the moisture absorption can be reduced.
Accordingly, the present invention provides a semiconductor encapsulating epoxy resin composition comprising an epoxy resin, a curing agent, and an inorganic filler, wherein 1 to 90% by weight of the inorganic filler is spherical cristobalite. The invention also provides a semiconductor device encapsulated within this composition in a cured state.
Owing to the blending of spherical cristobalite as an inorganic filler in an amount of 1 to 90% by weight of the entire filler, the epoxy resin compositions are capable of high loadings of inorganic filler and allow easy control of the coefficient of thermal expansion. The compositions provide cured products having a high thermal conductivity and low moisture absorption, and are highly suitable as encapsulants for state-of-the-art low-profile packages and other applications.
REFERENCES:
patent: 5298548 (1994-03-01), Shiobara et al.
patent: 58-029858 (1983-02-01), None
patent: 58-68955 (1983-04-01), None
patent: 61-85432 (1986-05-01), None
patent: 280428 (1990-03-01), None
patent: 4349146 (1992-12-01), None
patent: 72617 (1995-01-01), None
patent: 9227326 (1997-09-01), None
Chemical Abstracts, vol. 99, No. 12, Abstract No. 89336 (Sep. 9, 1983).
Chemical Abstracts, vol. 126, No. 12, Abstract No. 165458 (Mar. 24, 1997).
Aoki Takayuki
Hirano Tatsurou
Osada Shoichi
Shiobara Toshio
Suzuki Nobukazu
Aylward D.
Dawson Robert
Tatsumori Ltd.
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