Evaluation and Characterization of Reliable Non-Hermetic Conformal Coatings for Microelectromechanical System (MEMS) Device Encapsulation

Abstract—The thrust of this project was to evaluate commercial conformal encapsulation candidates for low cost aerospace appli-cations. 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 Re-sistance (TTR) and die shear strength were used for the corre-sponding electrical and physical property characterizations. Pary-lene F displayed excellent properties for environmental protection. Silicone elastomers displayed less resistance to the harsh environ-ment 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 elas-tomers 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. 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 con-formal coating combination investigated in this study showed sig-nificant promise for encapsulation of the microelectromechanical system (MEMS) devices. Index Terms—Conformal coatings, leakage current, mobile ion permeation, moisture ingress, surface insulation resistance. I