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The reliability of solder joints is now recognized as critical to the packaging of interconnects. Hence, predicting solder shapes and selecting materials are major concerns in the microelectronics industry. This investigation presents the method for predicting the shape of the solder joints; the selection of the solder material was also studied to assess thermal-mechanical reliability of the solder joints of the Double-Layer Wafer-Level Chip Scale Package (DLWLCSP), developed by Electronics Research & Service Organization/Industrial Technology Research Institute (ERSO/ITRI). The shape of the solder joint was predicted by energy-based program, named Surface Evolver. After the shape of the solder joint was predicted, the temperature dependent nonlinear finite element models of DLWLCSP were carried out. The results revealed that prediction of the solder shape by energy-based method had excellent agreement with experimentally measured result. The nonlinear finite element model revealed that 95.5Sn/3.8Ag/0.7Cu solder joints provided the superior performance comparing with eutectic 63Sn/37Pb and 96.5Sn/3.5Ag solder joints in DLWLCSP.