The purpose of this paper was to investigate the reason why the interface delaminations occurred in the QFP structure. The stress concentration was the key factor responsible for the microcrack occurrence. In order to figure out which factors affect the stress distribution, the material’s attributes and the structure’s dimension were considered. The finite element program ABAQUS6.9 together with the analytical solutions of singular stress fields was used to calculate the stresses near the interface edges. In conclusion, it appeared that strong mismatches of the thermal expansion coefficients and material elastic constants of the chip and PCB would lead to serious singular stresses at the interface edges. Stress intensity factor K and stress singularity order λ were given, based on which failure criterions could be established to predict the potential interface delamination. It is suggested that the good design of packaging structure depends on proper material selection and dimension determination.