Several thin quad hat packages (TQFPs) are analyzed for the evaluation of G, the energy release rate based on the interfacial fracture mechanics. The purposes of this study are; (i) to obtain the values of G as a function of the length of delamination (interfacial crack) and various geometric dimensions, (ii) to obtain the values of G for the package crack, which is believed to be the subsequent stage to the full delamination, and consequently (iii) to investigate quantitatively the effect of the various geometric dimensions on the popcorn cracking during vapor-phase soldering (VPS). Also the method to compute the phase angle (or mode mixity) for the interfacial crack is presented for several TQFPs. Due to the insufficient data for G(int)(c), the question of whether the partial delamination for given alpha (delamination length or crack length) grows or not cannot be addressed. However, this can be used for the prediction of the popcorn cracking in the future study. (C) 1998 Elsevier Science B.V. All rights reserved.