Investigation of Stress in MEMS Sensor Device Due to Hygroscopic and Viscoelastic Behavior of Molding Compound

The stresses due to moisture saturation on microelectromechanical systems (MEMS) sensor devices after exposure to temperature cycling have been addressed. Moisture-, temperature-, and time-dependent material property of molding compounds for the MEMS devices were characterized. To determine the coefficient of hygroscopic swelling of a molding compound and diffusivity (D) of water in the molding compound, dimensional change and weight loss of moisture saturated samples at various temperatures were monitored by the digital image correlation method combined with a weight scale. To obtain the viscoelastic property of the molding compound, a series of stress relaxation tests was performed using dynamic mechanical analysis (DMA). To explain the moisture-induced viscoelastic behavior, a simple assumption was introduced based on the temperature of glass transition point (T_g) shift from the DMA result. The experimental data were utilized in numerical simulations to estimate the temperature- and moisture-induced stress on MEMS sensor devices subjected to temperature cycles.