Effect of High Tg Mold Compound on MEMS Sensor Package Performance

A signal shift following solder reflow or temperature cycle stress testing can severely affect micro-electro-mechanical systems (MEMS) product performance in encapsulated package. This is mainly attributed to nonlinear material property of the mold compound, a key low cost packaging material. Due to viscoelastic characteristic, the mold compound exhibits a temperature and time dependent behavior. Above the temperature of glass transition (Tg) of the mold compound, the viscoelastic behavior is more significant. Thus, when the MEMS package is subjected to high temperature such as solder reflow process, a package induced internal stress on the MEMS sensor is generated depending on the maximum temperature reached and cooling rate. This results in the signal shift. Minimizing this signal shift plays an important role in MEMS product quality and performance. In this study, high Tg mold compounds (Tg: 200°C) were evaluated to alleviate the signal shift. Material property characterization tests were performed and test vehicles were assembled with the selected molding compounds. The MEMS sensor signal was measured before and after the temperature cycle test. Also, the viscoelastic material property of the mold compound was used in finite element analysis to investigate the resulting deformation and stress of the MEMS sensor. Compared to a baseline mold compound (Tg: 110°C), the high Tg molding compound did show about 10% less signal shift in the temperature range of -40°C to 125°C.