Molding Compound Trends In A Denser Packaging World. I. Technology Evolution

Abstract

Epoxy molding compounds have mlwd o~r the years to meet the stringent requirements of encapsulating ever w e complex circuitry, larger die sizes, and shrinking features.Traditionally, compound development targets the m m y market first, since the latter incorporates the latest technological advances. As a result, the functional requirements of the compounds are tailored to provide the optimal properties for the form factors of these memory packages. Subsequently, these compounds are introduced to the other product classes such as logtc, analog, and ASIC, and further modified to fulfill any potentially new specific requirement. However, as the configurations become more standardized, the functional specifications of molding compounds rewedfor packaging m y ICs merged with those of the other package fm fnctors. Thus, current mold compound developmental efforts aim at optimizing the compound properties for a gim family of packages such as PQFP or TSOP, rather than a @pen class ofdetcica.This paper discusses the evolution of the mold compound technology, from the generation of standard nmolac-based epoxies to the third generation of ultra-low stress, siZicune-modij?ed, highly-filled compounds. Although each generation has difm'ng compmndingfirmulations, each can be charactenzed by an "effective stress index", which normalizes both modulus and coefficient of thermal expansion with the package fonnjictor into a generic expression that MI be used for benchmarking. The interaction of these material developments with a d m c e s in IC technology, such as reduced die dimensions, shrinking feature sizes, and ever expanding packaging options, is also addressed. The arrent typical mold compound must satisfy functional requirements ofmoldabi2ity, low stress, crack resistance, and high thermal wnductivity. 1.This is the first in a series of papers exploring the technology issues involved with molding compounds used for electronic packaging. Development trends, reliability concerns, material interactions, and optimization of properties for the best package performance will be addressed in the series.This issue examines the trends of compound development, which initially mirrored advances in the memory market, but more recently, attempted to satisfy the functional requirements of classes of packages rather 0-7803-14328/93 $3.00 831953 IEEE I 204 than categories of IC devices. For instance, paper-thin packages present different assembly and reliability challenges than the voluminous standard DIPS, SOS, or PLCCs. Whether memory, analog, or logic ICs are inserted into the packages is no longer the driving force in molding technology. As a result, all the compound requirements of moldability, curing stresses, adhesion, moisture absorption, anti-popcorn toughness, and thermal stability undergo continuous revision. Remarkably, although different epoxy formulations are offered to meet various specific needs, the compounds must all meet the same end hurdle -to pass or surpass the battery of qualification and accelerated reliability tests...