Question marks to the extrapolation to lower temperatures in high temperature storage life (HTSL) testing in plastic encapsulated IC'S

A model has been constructed to describe ball-bond corrosion in HTSL stress testing. In this model ion-mobility has been believed to be the rate determining step and has been found to be non-linear for the anti-popcorn plastic. In HTSL testing an Arrhenius type extrapolation of the mean-time to failure (MTTF) at high temperature has been used, to predict the value at lower temperatures. This method proves to be correct for a low-stress plastic. However possibly it underestimates the value for an anti-popcorn plastic based on ion conductivity data. Strikingly, below the Tg of the anti-popcorn plastic, the reaction rates estimated from the ion conductivity data are the same for both the low-stress and the anti-popcorn plastic encapsulants.