This paper is to study the combined behavior of plastic encapsulated MEMS package under humid and elevated temperature environments. Although thermal-induced stress for the materials used in the MEMS package is already fully understood, moisture-induced stress is still not well studied. To cope with humidity effects on the MEMS package, the moisture-induced properties of mold compound are obtained from the proposed experiments. From the weight gain/loss measurement, saturated moisture content and diffusivity were obtained by curve fitting following Fick’s law. In addition, using digital image correlation (DIC) together with precision weight scale, coefficient of hygroscopic swelling (CHS) could also be obtained at different temperatures. The experimental results show that the diffusion coefficient of mold compound material follows Arrhenius’s equation well. Also, it is shown that the CHS of mold compound increases as temperature increases. Finally, using these experimentally obtained moisture properties, conjugated behavior (thermo-hygro-mechanical) of MEMS package under high temperature and humid environment could be estimated through numerical analysis by adopting classical analogy method. The numerical result was also validated by comparing with experimentally obtained warpage of MEMS package at elevated temperatures.

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