Electronic components such as chips are generally encapsulated with an epoxy resin composition. For example, the packaging of diodes, transistors and integrated circuits is constructed by such composition. This kind of composition usually consists of epoxy resins, hardeners, curing promoters, fillers and other additives. Conventionally, a composition of this kind is made of o-cresol phenolic novolac epoxy resin and phenolic novolac phenol resin which bear excellent heat resistance and moisture absorption resistance after curing. However, when density of integrated circuits has continuously increased, the traditional packaging assembly units of dual in-line package (DIP) are gradually replaced by quad flat package (QFP), thin QFP (TQFP), small outline package (SOP), thin SOP (TSOP), plastic leadless chip carrier (PLCC) package or ball grid array (BGA) package of big chips and surface adhesion technology.
Packaging encapsulants may undergo temperatures higher than 260.degree. C. when these small and thin packages are surface-adhered on circuit boards. Stress-rupture caused by the mismatch of thermal expansion and by moisture absorption may break the circuit of integrated circuits A more reliable resin composition is therefore demanded to encapsulate big chips by the package of smaller size and thinner thickness.
Efforts have generally been made for packaging encapsulants with directions: (1) to decrease the modulus of elasticity, (2) to decrease the coefficient of thermal expansion, (3) to increase the impact strength, (4) to decrease the moisture absorption rate, and (5) to increase the glass transition temperature, in order to solve the above mentioned problems. Consequently, the resin is demanded to have lower viscosity, good moldability, lower moisture absorption and high glass transition temperature after curing.
A conventional packaging encapsulant for semiconductors in the past was made mostly from raw materials of o-cresol phenolic novolac epoxy resin, phenolic novolac resin, filer, etc. These raw materials are ground, mixed, annealed, cooled, pulverired and granulated. However, most traditional resins can no longer meet the requirements of thin packaging of semiconductors.
At present, an epoxy resin composition for thin packaging has been available on the market. Generally, hardeners of biphenyl epoxy, naphthalol epoxy and phenolic resins with lower moisture absorption are primarily ed, for example, as those disclosed in Chinese Patent Publication No.272981; in Japanese Patent Unexamined Publication Sho-64-65116; and in Japanese Patent Unexamined Publication Hei-6-204360. The amount of the filler can be increased so as to reduce the inner stress caused by the mismatch of thermal expansion when these materials are used. However, drawbacks such as increased elastic moduli and reduced moldabilites of these materials may result from the increased amount of filler in the composition to reduce the moisture abortion.
In view of the aforementioned present drawbacks of related materials, the inventors have intensively studied the material for the epoxy resin composition, which comprises polyalkyl phenol resins or polyalkyl phenol epoxy resins or both, with improved properties for use in a thin packaging of semiconductors. As a result of these studies, they have completed this invention.