Modeling of viscoelastic effects on interfacial delamination in IC packages

In the analysis of plastic-encapsulated IC packages, the epoxy molding compound is often modeled as an elastic material. However, many polymers show some viscoelastic behaviour which causes its Young's modulus to be not only temperature-dependent but also time-dependent. This paper describes a study of the effect of viscoelasticity on interfacial delamination in IC packages. A finite element fracture mechanics technique was used. A small initial crack was assumed at the edge of the pad-encapsulant interface. Fracture mechanics parameters such as energy release rate and mode mixity were calculated for various processing conditions first assuming that the encapsulant was elastic, and then again assuming that the encapsulant was viscoelastic. For various thermal histories differences in the values of energy release rate and mode mixity were determined and compared. The results show substantial discrepancy between the temperature-dependent elastic model and the viscoelastic model, which indicates that the effects of viscoelasticity of the molding compound may not be negligible.