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During the solder reflow process, popcorn cracking will occur in integrated circuit (IC) package under vapor pressure and thermal load. Based on the elasticity theory, the singular stress field around the apex of the delaminated interface between die-pad and resin is obtained numerically. The generalized stress intensity factors are computed to evaluate the residual strength of the structure. Also, the structural stability is discussed by using the strain energy density theory. Two factors that are concerned in this paper include the relative stiffness ratio Edie-pad/Eresin and the relative thickness h~. The results show that lower stiffness ratio gives weaker stress singularity. In addition, larger h~ will increase generalized stress intensity factors and structural stability. As a conclusion for our case, moderate value of h~ (say h~=0) is recommended for design.