Evaluation of die edge cracking in flip-chip PBGA packages

Abstract

Increasing die size and large CTE (Coefficient of Thermal Expansion) mismatch in FC-PBGA (Flip-Chip Plastic Ball Grid Array) packages have made die fractures a major failure mode during reliability testing. Most die fractures observed before was die backside vertical cracking, which was caused by excessive package bending and backside defects. However, due to die edge defects induced by the singulation process and the choice of underfill material, an increasing number of die cracks were found to initiate from die edge and propagate horizontally across the die. In order to improve package reliability and performance, die edge cracking has to be eliminated. An extensive finite element analysis was completed to investigate die edge cracking and find its solutions. A fracture mechanics approach was used to evaluate the effect of various package parameters on die edge initiated fractures. Strain energy release rate was found to be an effective technique for evaluating die edge initiated fractures from singulation-induced flaws. The impact of initial flaw size and a variety of package parameters was investigated. Unlike in die backside cracking, the dominant parameters causing die edge horizontal fractures are more closely related to local effects.