Popcorning-a fracture mechanics approach

Moisture-induced cracking during solder reflow is a critical reliability problem with plastic-encapsulated microcircuits (PEMs). Such cracking, referred to as "popcorning" occurs from the evaporation and expansion of moisture absorbed by the molding compound. This is the third in a series of papers from a DoD-funded project to provide an expert system for design of PEMs. This study is aimed at establishing a rule-based system to address reliability problems related to popcorning such as interfacial delamination, mold compound moisture sensitivity, and mold compound fracture toughness. This paper addresses the fracture mechanics aspects of popcoming, and more specifically, the propensity of some packages to popcorn than others. The physics of why a TSOP-32 lead package is more susceptible to moisture cracking than a PQFP-52 lead package of the same packaging materials is explained in this paper.