Measurement of residual stresses in encapsulant of a PQFP

This paper reports a preliminary result of residual stresses measured on a plastic quad flat pack (PQFP) using the hole drilling strain gage (HDSG) method. The experimental results are compared with predictions made previously on the identical package based on finite element analysis (FEA). It was found that although there is a large variation in absolute values of residual stress between tests, all results show essentially the same trend: the residual principal stress decreases as the hole depth increases. This indicates that the stress concentration is the highest on the surface of plastic encapsulant and decreases gradually toward the inside of package. A qualitatively similar trend is predicted from FEA study based on the model with a viscoelastic encapsulant. The experimental results exhibit a much larger variation across the thickness of encapsulant than the FEA prediction. The FEA prediction based on elastic model grossly overestimates the residual stresses by more than an order of magnitude compared to the experimental results.