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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.

Various external load conditions affecting components on electronic devices and modules are constant factors, which need to be considered for the component long-term reliability. Recently, to enhance the high stress component thermo-mechanical cycling performance, various types and configuration using edgebond and edgefill technology are introduced and tested. These applications induce a multi-axis loading condition, which alter the degradation mechanism and failure location during thermal cycling, which need closer investigation. In this study, high stress 12x12mm2 wafer level chip scale packages (WLCSP) were selected and subject to thermal cycling with full-edgebond, dot-edgebond and edgefill adhesive, which improves the characteristic lifecycle numbers base on the configurations, but altered the failure location due to different stress conditions. The -40 to 125oC thermal cycling profile revealed localized degradation per configuration during thermal cycling, showed a shift of the crack propagation path, based on full-edgebond, dot-edgebond and edgefill adhesive sample conditions. Through these series of observation, the interconnect thermal cycling degradation mechanisms are able to be explained. The correlation between the stress condition and microstructure are  presented and discussed based on Electron backscattered diffraction (EBSD) analysis.

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