The main aim of this paper is to study for the warpage and von Mises stress of PBGA package caused by the variations of mechanical properties of materials in IR-reflow process. At first, taking advantage of the package symmetry only one quarter of the package was modeled by MSC. Marc. The influence of heat transfer was considered by comparing the results of Coupled mechanical-heat transfer analysis and Mechanical analysis. In the second part, the coefficient of thermal expansions and elastic modulus of molding compound and substrate are selected as the four control factors, and the influence of the four control factors to the warpage and von Mises stress were observed. At last, emperical formulas to predicted warpage and von Mises stress values were obtained.