Impact of Conformal Coating Induced Stress on Wafer Level Chip Scale Package Thermal Cycling Performance

Abstract

Conformal coating is commonly used for harsh environment to protect electronics from moisture and chemical contaminants. But the stresses imparted by the conformal coating can cause degradation to the package thermal cycle performance. Full coverage of the component with conformal coating material can prevent potential corrosion induced degradation but imply a local compression stress during thermal cycling, resulting a different thermal cycling performance compared to non-coated components. In this study, 8x8mm2 wafer level chip scale packages (WLCSP) were subjected to 5% NaCl aqueous spray test with and without full conformal coating, then thermal cycled from -40ºC to +125ºC. Weibull reliability statistics indicated that fully conformal coated components experience characteristic life cycle number reduction from 404 cycles to 307 cycles, a 24% lifetime reduction, comparing to no conformal coated, no salt spray test applied components. The correlation between crack propagation and localized recrystallization were compared in a series of cross section analyses using polarized imaging and electro-backscattered diffraction, which revealed that the conformal coating induced a z-axis tension and compression strain during thermal cycling, resulting in an accelerated degradation at the solder interconnect. Linear Laser profilometer measurements showed that fully conformal coated samples experienced a higher z-axis height displacement change relative to non-conformal coated samples when exposed to 125 °C with 10 minutes dwell. To prevent this z-axis strain a reworkable edgebond adhesive was applied with full conformal coating configuration, which demonstrate an increase of characteristic lifecycle number to 2783 cycles, suggesting that the mitigation of the z-axis strain can vastly enhance the thermal cycling performance.