Constitutive Modeling of Moulding Compounds

The virtual prototyping of electronic components requires reliable material models for all constituent materials. One of these materials is the moulding compound. This is an epoxy resin filled with inorganic (silica) particles, carbon black and processing aids. It shows a clear viscoelastic behaviour which is not only temperature but also cure dependent. This paper deals with the thermal and mechanical characterization and modeling of such moulding compounds with a focus on the effects of the degree of cure and the filler concentration. 1 ntroduction Epoxy resins are used in electronic packaging processes as adhesives, underfills and as moulding compounds. The epoxy resins shrink during cure which causes a build-up of residual stresses. Due to differences in thermal contraction of the die and the moulding compound, additional thermal stresses are created during subsequent heating or cooling steps (solder reflow) and thermal cycling. These stresses are the major cause of all thermomechanical failure and are directly related to the product reliability. In order to be able to accurately predict the processinduced stresses in an IC package, and to provide a base for optimizing the process parameters, cure-dependent viscoelastic constitutive relations for thermosetting polymer systems are required. Many efforts have been made to predict process-induced stresses in packages. In most of these studies the complex polymer behaviour was simplified by assuming the material to be stress free (i.e. infinitely fast stress relaxation) until the end of the moulding stage and either elastic or viscoelastic during the subsequent cooling and post moulding stages. Since these simplified approaches are unable to accurately predict complex phenomena like warpage of thin packages, a more complete description of the thermomechanical behaviour of the moulding compound is required. In previous papers we proposed a cure-dependent viscoelastic model to describe the evolution of the viscoelastic behavior of an underfill resin in a Flip Chip package during