Effect of the viscoelastic behavior of molding compounds on crack propagation in IC packages

The lead-free alloys require solder reflow temperatures to be much higher than the previous requirements associated with SnPb solders. Moreover, the reflow temperature is often well beyond the glass transition temperature of molding compounds and/or other polymeric packaging materials. Thus the effect of viscoelasticity of mold compound must have an important effect on IC package reliability. This work presents an analytical investigation of the propagation of the interfacial crack, i.e., interfacial delamination between mold compound and the substrate by considering the viscoelastic behavior of the molding compound using Kelvin-Voigt model. It is shown that the viscoelasticity of mold compound has a significantly influence the energy release rate and the fracture threshold of the interfacial delamination. Regardless of the fracture energy being time-dependent or not, the viscous property of the mold material postpones the appearance of the propagation of the interfacial delamination. The admissive loading time, i.e., admissive reflow time, is connected closely to the retardation time of the mold material. A significant improvement of IC package reliability can be expected increasing the viscosity, i.e., the retardation time of mold compound.