When the temperature of the package assembly is changed, non‐uniform deformation and local stress may occur owing to the different thermal expansion coefficients of the constituent materials, while concentrated thermal stress may cause significant failure. In this study, we carry out an experiment and analysis of the thermal deformation of a flip chip plastic ball grid array package with respect to temperature change. Interference fringe patterns representing the displacement distribution of each temperature stage are obtained using a Moiré interferometry and the bending deformation behavior and the strain of the solder ball are analyzed thereafter. We compare the deformation behavior of a single‐sided package assembly and a double‐sided package assembly. The effective strain, which significantly affects the failure of the solder ball, is found to be the largest in the solder ball just outside the edge of the chip in the case of the single‐sided package assembly. In the case of the double‐sided package assembly, the maximum effective strain is found in the solder ball just inside the edge of the chip, with a value approximately 50 % larger than in the case of the single‐sided package assembly.