Evaluation and characterization of reliable non-hermetic conformal coatings for microelectromechanical system (MEMS) device encapsulation

The thrust of this project was to evaluate commercial conformal encapsulation candidates for low cost aerospace applications. The candidate conformal coatings evaluated in this study included silicone elastomers, epoxies, and Parylenes with bi-layer or tri-layer designs. Properties characterized in this study included mobile ion permeation and moisture ingress resistance, interfacial adhesion variation through thermal shock cycling and 85°C/85% RH aging. Surface Insulation Resistance (SIR), Triple Track Resistance (TTR) and die shear strength were used for the corresponding electrical and physical property characterizations. Parylene F displayed excellent properties for environmental protection. Silicone elastomers displayed less resistance to the harsh environment as compared to the Parylene family (N, C, D types), but it could provide advantages for low residual stress applications. The change in adhesion strength between Parylene C and silicone elastomers after exposure to thermal shock cycling or 85°C/85%RH aging for different time periods were conducted from die shear test in terms of the interfacial failure. SIR values of all the candidate materials after 1000 h exposure to 85°C/85%RH, with 100 V dc for resistance measurement, range from 1×10⁸-1×10⁹ Ω. Leakage current values after 1000 h exposure to 85°C/85%RH, 175 V bias, are in the range of 10^-9 to 10^-11 Amp. The bi- or tri-layer conformal coating combination investigated in this study showed significant promise for encapsulation of the microelectromechanical system (MEMS) devices.