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Simulation analysis for interfacial failure of a poymer sealed MEMS device

In this paper, interfacial failures (delamination and crack) of a MEMS device during humidity reliability testing are described. Three failure mechanisms are identified: thermal/mechanical stress due to the mismatch in the CTE (Coefficient of Thermal Expansion) of different materials. hygroscopic stress due to moisture absorption and swelling of polymer materials characterized by the CME (coefficient moisture expansion) and hygroscopic-thermal stress considering steam driven stress are analyzed. The observed regular failure pattern of the device suggests that the roles of thermal stresses and CME mismatch-induced hygrostress in interfacial failures of the device during reflow are minor, and that delamination and crack during reflow are primarily driven by evaporated steam. In order to solve the reliability problems, Physical size, process and material of the device are optimized. Simulation plays a key role in the study of the influence of the physical size of the device. Meanwhile, a new package structure with double seal rings is proposed, which greatly reduces the effect of steam-driven stress. Following the modifications suggested by the simulations, the MEMS device passed the temperature/humidity test.