Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2002-07-17
2004-04-20
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C523S458000, C257S793000, C257S795000
Reexamination Certificate
active
06723452
ABSTRACT:
This invention relates to an environment-friendly epoxy resin composition for semiconductor encapsulation which cures into a product having satisfactory solder crack resistance on use of lead-free solder and improved flame retardance despite the absence of halogenated epoxy resins and antimony compounds. It also relates to a semiconductor device encapsulated with a cured product of the composition.
BACKGROUND OF THE INVENTION
The current mainstream in the semiconductor industry resides in diodes, transistors, ICs, LSIs and VLSIs of the resin encapsulation type. Epoxy resins have superior moldability, adhesion, electrical properties, mechanical properties, and moisture resistance to other thermosetting resins. It is thus a common practice to encapsulate semiconductor devices with epoxy resin compositions. As the environmental protection becomes important over the latest years, industrial materials are desired to be halogen- and lead-free. This is true to encapsulating agents. More severe requirements are imposed on the encapsulating materials. For example, in the manufacture of semiconductor devices, solder plating is often carried out upon mounting on boards and the plated solder films contain a substantial amount of lead. The environment protection calls for elimination of lead. However, the use of lead-free solder requires an elevated temperature for reflow, which compromises the reliability of epoxy resin encapsulants in terms of reflow resistance.
For semiconductor encapsulating epoxy resin compositions, good adhesion and low moisture absorption must be achieved before the reflow resistance can be improved. An increased loading of the inorganic filler can provide a lower moisture absorption, from which an improvement in reflow resistance is expectable, but results in a higher viscosity, detracting from the flow of the epoxy resin composition upon molding. The epoxy resin compositions are thus required to have a low melt viscosity as well. Such compositions are semi-solid or liquid and thus interfere with efficient operation.
In prior art epoxy resin compositions, halogenated epoxy resins combined with antimony trioxide (Sb
2
O
3
) are often blended in order to enhance flame retardance. This combination of a halogenated epoxy resin with antimony trioxide has great radical-trapping and air-shielding effects in the vapor phase, thus conferring a high fire-retarding effect.
However, the halogenated epoxy resins generate noxious gases during combustion, and antimony trioxide has powder toxicity. Given their negative impact on human health and the environment, it is desirable to entirely exclude these fire retardants from epoxy resin compositions.
In view of the above demand, studies have been conducted on the use of hydroxides such as Al(OH)
3
and Mg(OH)
2
or phosphorus-containing fire retardants such as red phosphorus and phosphates in place of halogenated epoxy resins and antimony trioxide. Unfortunately, various problems arise from the use of these alternative compounds. The hydroxides such as Al(OH)
3
and Mg(OH)
2
have less flame retardant effects and must be added in larger amounts in order that epoxy resin compositions be flame retardant. Then the viscosity of these compositions increases to a deleterious level to molding, causing molding defects such as voids and wire flow. The phosphorus-containing fire retardants such as red phosphorus and phosphates added to epoxy resin compositions can be hydrolyzed to generate phosphoric acid when the semiconductor devices are exposed to hot humid conditions. The phosphoric acid generated causes aluminum conductors to be corroded, detracting from reliability.
SUMMARY OF THE INVENTION
An object of the invention is to provide an epoxy resin composition for semiconductor encapsulation which cures into a product having satisfactory solder crack resistance on use of lead-free solder and improved flame retardance despite the absence of halogenated epoxy resins and antimony compounds. Another object is to provide a semiconductor device encapsulated with the composition in the cured state.
It has been found that when a specific epoxy resin is combined with a specific phenolic resin, and preferably further with a specific flame retardant, a molybdenum ingredient having zinc molybdate supported on an inorganic carrier, there is obtained an epoxy resin composition which cures into a product having satisfactory solder crack resistance on use of lead-free solder and improved flame retardance despite the absence of halogenated epoxy resins and antimony compounds such as antimony trioxide and is thus suitable for semiconductor encapsulation.
In a first aspect, the present invention provides a semiconductor encapsulating epoxy resin composition comprising (A) an epoxy resin having an epoxy equivalent of at least 185 and possessing a skeleton having at least one structure in which two benzene rings can be directly conjugated, carbon atoms having an sp
2
type atomic orbital accounting for at least 50% of all the carbon atoms, (B) a phenolic resin of the general formula (1) and preferably, a mixture of a phenolic resin of the general formula (1) and a phenolic resin of the general formula (2), shown below, (C) a curing accelerator, (D) an inorganic filler, and optionally, (E) a molybdenum ingredient having zinc molybdate supported on an inorganic carrier. A semiconductor device encapsulated with a cured product of the epoxy resin composition is also contemplated.
In a second aspect, the present invention provides a semiconductor encapsulating epoxy resin composition comprising (F) a modified epoxy resin obtained by converting a mixture of 4,4′-biphenol and a phenolic resin of the general formula (2) into a glycidyl product, (G) a phenolic resin of the general formula (1) and/or a phenolic resin of the general formula (2), (C) a curing accelerator, (D) an inorganic filler, and optionally, (E) a molybdenum ingredient having zinc molybdate supported on an inorganic carrier. The second epoxy resin composition is more improved in workability and fluidity. A semiconductor device encapsulated with a cured product of the epoxy resin composition is also contemplated.
In formula (1), R
1
which may be the same or different is hydrogen or an alkyl group having 1 to 4 carbon atoms, and n is a natural number of 1 to 15.
In formula (2), R
2
which may be the same or different is hydrogen or an alkyl group having 1 to 4 carbon atoms, and m is a natural number of 1 to 15.
DETAILED DESCRIPTION OF THE INVENTION
In the epoxy resin composition for semiconductor encapsulation according to the first embodiment of the present invention, component (A) is an epoxy resin having at least two epoxy groups per molecule and an epoxy equivalent of at least 185 and possessing a skeleton having at least one structure in which two benzene rings can be directly conjugated, in the molecular structure. Those carbon atoms having an sp
2
type atomic orbital account for at least 50% of all the carbon atoms. Any desired epoxy resin may be used as long as these requirements are met.
The structures in which benzene rings to construct the epoxy resin skeleton can be directly conjugated with each other typically include a biphenyl structure and a coupling of a benzene ring with a naphthalene ring or the like. When such a conjugated structure is introduced into an epoxy resin, the epoxy resin in the cured state becomes more resistant to oxidation during combustion. Those carbon atoms having an sp
2
type atomic orbital account for at least 50%, often 50 to 99%, preferably 54 to 95%, and more preferably 57 to 90% of all the carbon atoms.
The epoxy resin which contains no conjugated benzene rings in its skeleton and in which those carbon atoms having an sp
2
type atomic orbital account for less than 50% of all the carbon atoms is readily decomposable at elevated temperatures, generating gases and continuing combustion.
It has empirically been found that the epoxy resin which contains conjugated benzene rings and in which those carbon atoms having an sp
2
type atomic orbital account
Aoki Takayuki
Asano Eiichi
Kimura Yasuo
Shimoda Tarou
Shiobara Toshio
Aylward D.
Birch Stewart Kolasch & Birch, LLP.
Dawson Robert
Shin-Etsu Chemical Co. , Ltd.
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