Abstract: Finite element method was employed to analyze the reliability of soldered joint in a CSP device.Anand model was used to establish the constitutive equation of Sn3.0Ag0.5Cu solder, the stress behavior of soldered joint was studied.The results indicate that the maximal stress is located at the upper surface of the soldered joint which is under the outermost of chip.The phenomenon of stress relaxation and accumulated enhancement could be observed obviously from the curves of stress and temperature with time cycle.Reliability of soldered joints with three usually-used heights are compared, and the results shows that the 0.35 mm×0.18 mm one has the best reliability.Moreover, the influence of chip thickness on the reliability of soldered joints are investigated in the last part, the simulation result indicates that the influence is little.