Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-04-22
2001-09-11
Sellers, Robert E. L. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S065000
Reexamination Certificate
active
06288169
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an epoxy resin composition for semiconductor encapsulation, which has low stress properties and high reliability against moisture, a process for producing the epoxy resin composition, and a semiconductor device encapsulated with the epoxy resin composition.
BACKGROUND OF THE INVENTION
Semiconductor elements, such as transistors and IC chips, are encapsulated into ceramic or plastic packages to be supplied as semiconductor devices protected from the outer environment and easy to handle. Ceramic packages have excellent moisture resistance because of the character of the ceramic material itself and impose little stress to the semiconductor element because of their hollow structure. Ceramic packages therefore achieve highly reliable encapsulation. However, the ceramic materials are expensive, and the ceramic packages are less practical for mass production than plastic packages.
Therefore, plastic packages using an epoxy resin composition have been leading recently. Plastic packages, while suitable for mass production and less expensive, allow moisture to permeate and have a greater linear expansion coefficient as compared with a semiconductor element encapsulated. Therefore, it has been a weighty subject in the art to improve moisture resistance and low stress properties.
The stress problem of encapsulating resins has been coped with by dispersing rubber particles, such as butadiene rubber particles, in the resin matrix. However, since rubber particles exhibit high cohesion, commercially available butadiene rubber particles usually have an average secondary particle diameter of about 100 to 500 &mgr;m. Such rubber particles fail to be dispersed uniformly in the encapsulating resin, which has imposed a limitation on the improvement in low stress properties required of a semiconductor encapsulation material.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an epoxy resin composition for encapsulating semiconductors in which rubber particles are uniformly dispersed, being prevented from agglomerating.
Another object of the invention is to provide a process for producing such an epoxy resin composition without involving complicated steps.
Still another object of the invention is to provide a semiconductor device which is free from the problems of stress and moisture penetration.
The inventors of the present invention have conducted extensive study for the purpose of obtaining an encapsulating resin that achieves reduction in stress and improvement in reliability against moisture attack without affecting the appearance. The inventors study has been focused on a means for uniformly dispersing butadiene rubber particles in an epoxy resin composition while preventing the particles from forming large secondary particles without involving complicated steps. Noting that butadiene rubber particles are liable to agglomerate, the inventors have conducted investigation chiefly into the average particle size of secondary particles and the proportions of secondary particles falling in specific size ranges. As a result, they have succeeded in uniformly dispersing butadiene rubber particles in an epoxy resin composition by using butadiene rubber particles having a specific average particle size and a specific particle size distribution of secondary particles.
The inventors have also found that reliability against moisture is further ensured by adding silicone oil having at least one amino group per molecule together with the specific butadiene rubber particles. The present invention has been completed based on these findings.
That is, the above objects of the invention are accomplished by;
(1) An epoxy resin composition comprising
(A) an epoxy resin,
(B) a phenolic resin, and
(C) butadiene rubber particles having an average particle size of secondary particles of 100 &mgr;m or smaller and having such a size distribution that the proportion of secondary particles having a particle size of 250 &mgr;m or smaller is 97% by weight or more, and the proportion of secondary particles having a particle size of 150 &mgr;m or smaller is 80% by weight or more;
(2) A semiconductor device comprising a semiconductor element encapsulated in the above-described epoxy resin composition.
(3) A process for producing the above-described epoxy resin composition, comprising mixing at least components A, B and C described above; and
(4) A semiconductor device comprising a semiconductor element encapsulated in an epoxy resin composition produced by the above-described process.
Having the above-described specific size and size distribution, the butadiene rubber particles used in the invention are uniformly dispersed in the matrix resin without forming coarse agglomerates, which can be achieved without involving complicated steps, thereby providing an epoxy resin composition for semiconductor encapsulation with ease and at low cost. The semiconductor device sealed in the epoxy resin composition of the invention exhibits excellent low stress properties owing to the uniformly dispersed specific butadiene rubber particles. The epoxy resin composition additionally comprising silicone oil having at least one amino group per molecule exhibits further improved reliability against moisture.
DETAILED DESCRIPTION OF THE INVENTION
The epoxy resin composition according to the present invention comprises (A) an epoxy resin, (B) a phenolic resin, and (C) specific butadiene rubber particles. It is usually supplied in the form of powder or tablets.
The epoxy resin as component (A) is not particularly limited and includes various types of epoxy resins, such as a dicyclopentadiene type, a cresol novolak type, a phenol novolak type, bisphenol type, and a biphenyl type. These epoxy resins can be used either individually or a mixture of two or more thereof. Of these epoxy resins preferred are those having a melting point or softening point that is higher than room temperature. Generally useful epoxy resins include novolak epoxy resins having an epoxy equivalent of 150 to 250 and a softening point of 50 to 130° C. and cresol novolak epoxy resins having an epoxy equivalent of 180 to 210 and a softening point of 60 to 110° C.
The phenolic resin as component (B) acts as a curing agent for the epoxy resin. It is not particularly limited and includes dicyclopentadiene type phenolic resins, phenol novolak resins, cresol novolak resins, and phenol aralkyl resins. They can be used either individually or as a mixture of two or more thereof. Phenolic resins having a hydroxyl equivalent of 70 to 250 and a softening point of 50 to 110° C. are preferably used.
The phenolic resin as component (B) is preferably used in such an amount that the hydroxyl equivalent of the phenolic resin is 0.5 to 2.0, particularly 0.8 to 1.2, per epoxy equivalent of the epoxy resin as component (A).
The butadiene rubber particles as component (C) are usually obtained by copolymerization of butadiene and comonomers such as alkyl methacrylates, alkyl acrylates, and styrene. Examples of typical butadiene rubbers are a methyl acrylate-butadiene-styrene copolymer, a methyl acrylate-butadiene-vinyltoluene copolymer, a butadiene-styrene copolymer, a methyl methacrylate-butadiene-styrene copolymer, methyl methacrylate-butadiene-vinyltoluene copolymer, a methyl methacrylate-ethyl acrylate-butadiene-styrene copolymer, a butadiene-vinyltoluene copolymer, and an acrylonitrile-butadiene copolymer. Preferred of them is a methyl methacrylate-butadiene-styrene copolymer, particularly one having a butadiene content of 70% by weight or less and a methyl methacrylate content of 15% by weight or more. An especially preferred is a methyl methacrylate-butadiene-styrene copolymer having a butadiene content of 40 to 70% by weight and a total content of methyl methacrylate and styrene of 30 to 60% by weight. The weight ratio of styrene to methyl methacrylate is preferably 0.5 to 2.0. A methyl methacrylate-butadiene-styrene copolymer prepared by pulverizing a commercially available Metaburene (a powdered product of Mitsubishi Rayon C
Nakao Minoru
Okuda Satoshi
Usui Hideyuki
Nitto Denko Corporation
Sellers Robert E. L.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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