Characterization and modeling of molding compound properties during cure

During the encapsulation of electronic components stresses are generated due to curing effects and the difference in thermal shrinkage between molding compound and die. These residual stresses add up to the stresses generated during thermal cycling and mechanical loading and may eventually lead to product failure. In this paper we focus on three commercial molding compounds and and analyze in detail the increase in elastic modulus and the change in viscoelastic behaviour during cure. This was done with a special shear tool which allows to measure mechanical properties with sufficient accuracy in the liquid as well as in the solid state. The cure dependent viscoelastic material behaviour was modeled using a cure dependent shift factor and rubber modulus. The viscoelastic behaviour of the molding compounds is also shown not to be stable. During postcure the materials slowly continue to crosslink thereby systematically changing their viscoelastic behaviour. The material models presented here therefore only account for the initial curing stage and do not include postcure.