Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
1999-06-07
2001-03-27
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C523S440000, C523S443000
Reexamination Certificate
active
06207296
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a novel inorganic filler which can be highly loaded in resin compositions, typically epoxy resin compositions without increasing viscosity, thus providing resin compositions having a low melt viscosity enough to mold. It also relates to an epoxy resin composition loaded with the inorganic filler and a semiconductor device encapsulated with a cured product of the composition.
2. Prior Art
Prior art epoxy resin compositions used for the encapsulation of semiconductor devices generally contain less than 80% by weight of an inorganic filler. To improve the reflow soldering resistance, it becomes a recent mainstream technique that a low viscosity resin is as a main component and a large amount of an inorganic filler is blended in the resin to reduce a percent water absorption. When such highly loaded resin compositions having a very low content of the resin component are produced by conventional well-known continuous kneaders, the resulting compositions have a high viscosity and poor moldability because the wetting of the filler with the resin is insufficient.
In this regard, we found that a substantial drop of moldability occurs when the inorganic filler used contains a large proportion of a filler component having a specific surface area in excess of 10 m
2
/g and a particle size of less than 2 &mgr;m, especially less than 0.5 &mgr;m. Also, when a filler is produced by a conventional method, the content of submicron particles (having a particle size of less than 1 &mgr;m) somewhat varies between lots. Then moldability depends on a particular lot of filler. Such inconvenient problems can be overcome by redesigning the screw of the continuous kneader or extending the kneading time. Unfortunately, these approaches are low in productivity and add to the cost.
It is thus desired to overcome the problems associated with high loadings of inorganic filler.
SUMMARY OF THE INVENTION
An object of the invention is to provide an inorganic filler which can be highly loaded in resin compositions while maintaining a low melt viscosity enough to mold. Another object of the invention is to provide an epoxy resin composition loaded with the inorganic filler. A further object of the invention is to provide a semiconductor device encapsulated with the epoxy resin composition.
We have found that by removing a fraction of fine particles having a particle size of less than 2 &mgr;m from starting inorganic filler particles as by classification or wet sieving and thereafter adding fine particles having a controlled particle size, particle size distribution and specific surface area, there is obtained a particulate inorganic filler which can be blended with a liquid or solid powder resin, typically epoxy resin to form a resin composition having stable flow properties and improved moldability.
More particularly, the flow properties of an encapsulating resin composition largely affect the molding of modern thin packages and multi-pin large-size packages. For resin compositions serving as an encapsulant, the current approach taken in order to find a compromise between filling improvement and low water absorption is to blend a spherical filler containing a large proportion of a submicron filler component having a mean particle size of less than 1 &mgr;m, desirably less than 0.5 &mgr;m. Such a spherical filler is generally manufactured by pulverizing silica stone into a fine powder and melting the powder in a flame into spherical particles. With this method, it is difficult to control the content of a fine spherical filler fraction having a mean particle size of less than 2 &mgr;m, especially less than 0.5 &mgr;m, and that content often varies between different lots. Also a subtle change of manufacturing conditions results in a change of the particle size distribution of a fine spherical filler fraction of less than 2 &mgr;m. A variation of such a fine filler fraction can largely affect the flow and mold properties of an epoxy resin composition loaded therewith. To achieve an improvement under such circumstances, we have found that a unique inorganic filler is obtained by removing a fraction of fine particles having a particle size of less than 2 &mgr;m and having a greater specific surface area from starting inorganic filler particles and adding thereto controlled fine particles having a mean particle size of 0.1 to 2 &mgr;m and a specific surface area of 3 to 10 m
2
/g. Using this particulate inorganic filler, a resin composition, typically an epoxy resin composition free of the above-mentioned problems is obtained.
According to a first aspect of the invention, there is provided a particulate inorganic filler having a mean particle size of 5 to 40 &mgr;m, which is obtained by removing a fraction of fine particles having a particle size of less than 2 &mgr;m from inorganic filler particles having a mean particle size of 10 to 50 &mgr;m and adding thereto particles having a mean particle size of 0.1 to 2 &mgr;m and a specific surface area of 3 to 10 m
2
/g as measured by a nitrogen adsorption BET method. Preferably, 1 to 30 parts by weight of the particles having a mean particle size of 0.1 to 2 &mgr;m and a specific surface area of 3 to 10 m
2
/g is added to 100 parts by weight of the inorganic filler particles from which the fine particle fraction has been removed.
In a second aspect, the invention provides an epoxy resin composition comprising an epoxy resin, a curing agent, and an inorganic filler which is as defined above.
Also contemplated herein is a semiconductor device encapsulated with a cured product of an epoxy resin composition as defined above.
DETAILED DESCRIPTION OF THE INVENTION
Briefly stated, the particulate inorganic filler according to the invention is obtained by removing fine particles having a particle size of less than 2 &mgr;m from starting inorganic filler particles and adding instead particles having a mean particle size of 0.1 to 2 &mgr;m and a specific surface area of 3 to 10 m
2
/g.
The starting inorganic filler should have a mean particle size of 10 to 50 &mgr;m, preferably 15 to 40 &mgr;m. If the mean particle size is less than 10 &mgr;m, which means that the filler has a too small particle size, an epoxy resin composition loaded with a large proportion of the filler would become too viscous to mold. If the mean particle size is more than 50 &mgr;m, a cured product would become low in strength. The starting inorganic filler contains fine particles having a particle size of less than 2 &mgr;m. The starting inorganic filler containing 0.5 to 25% by weight, especially 0.5 to 15% by weight of fine particles having a particle size of less than 2 &mgr;m is convenient because the removal of the fine particles is easy and inexpensive. Further, the starting inorganic filler should preferably have a specific surface area of 0.5 to 4 m
2
/g, especially 0.5 to 3 m
2
/g as measured by a nitrogen adsorption BET method
In the first step of the invention, a fraction of fine particles having a particle size of less than 2 &mgr;m originally contained in the starting inorganic filler is removed.
The fine particles can be removed by passing the starting filler through a classifier such as a cyclone or a wet screen during preparation of the filler. The particles to be removed are fine particles having a particle size of less than 2 &mgr;m, especially less than 0.1 &mgr;m and a specific surface area of more than 50 m
2
/g. The content of fine particles in the starting inorganic filler varies between different lots of the filler manufacture. A slight variation of the fine particle content can largely affect the molding properties because the fine particles have a greater specific surface area. Therefore, in order to provide stable molding properties, it is very important to control the content of such fine particles.
In the second step of the invention, controlled fine particles are added to the inorganic filler from which the fraction of fine particles having a particle size of less than 2 &mgr;m has been removed, thereb
Asano Eiichi
Fukumoto Takaaki
Higuchi Noriaki
Shiobara Toshio
Tomiyoshi Kazutoshi
Aylward D.
Dawson Robert
Millen White Zelano & Branigan P.C.
Mitsubishi Denki & Kabushiki Kaisha
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