Flip chip plastic ball grid array (FC-PBGA) is unique and has been widely used. During FC-PBGA's practical application, the analysis of its failure mechanisms under high temperature, electricity, water vapor and other comprehensive environmental stress conditions is very important for improving its application reliability. In this paper, with 0.13-μm m/6-level copper-based FPGA with FC-PBGA package, failure mode of the device is exposed under external stress, which is dominated with thermal-mechanical stress generated by high temperature thermal reflow process. And the failure mechanism corresponding to the failure mode is analyzed. Results show that the basic cause of the failure is the thermal-mechanical stress, which is induced by the combination of internal and external reflow temperature difference and high temperature reflow process when assembled. The thermal-mechanical stress makes the solder bumps on the flip chip melt again and bridges the adjacent solder bumps to lead to the device short failure. It also induces crack or delamination in the underfill, crack in solder bump or solder bump fallen off to lead to device open failure. The residual stress on Cu/low-k interconnect structures damages the structure integrity and affects the reliability of FC-PBGA packages.