Effect of Chemistry on Viscoelastic Properties of Moulding Compounds

The present state-of-the-art in simulation of stresses in electronic packages is to model the moulding compound as a viscoelastic material. That means that stresses during packaging and subsequent thermomechanical loading are allowed to relax partially. Such an approach was seen to improve thermomechanical predictions considerably compared to the previously used temperature dependent elastic models. Therefore viscoelastic simulations are now becoming the standard for the modelling of packaging stresses and viscoelastic characterization of new moulding compound materials is now routinely being performed by most of the companies involved with packaging of electronic components. A problem is, however, that the viscoelastic data of moulding compounds turns out to be quite sensitive to variations in the compound chemistry and that this chemistry may vary slightly from batch to batch. Since these variations do not have a large effect on the room temperature properties (like the modulus) they often remain undetected by the standard quality tests. In order to anticipate to these problems it is desirable to develop a fundamental understanding of what parameters influence the viscoelastic properties of thermosets. A quantitative model, such as will be presented in this paper, has the additional advantage that it can be used for the selection of moulding compounds with optimized viscoelastic properties