The Impact of Conformal Coating on WLCSP Thermal Cycle Performance: Degradation Mechanism and Mitigation Method
Authors: Andy Hsiao, Tae-Kyu Lee, Kola Akinade, Cherif Guirgis, Edward Ibe, Karl Loh Company: Portland State University, Cisco Systems Inc., Zymet Date Published: 9/17/2017
Abstract: Conformal coating is commonly used for harsh environments to protect electronics from moisture and chemical contaminants. But the stresses imparted by the conformal coating can cause degradation to the package’s thermal cycle performance. In this study, 8x8mm2 wafer level chip scale packages (WLCSP) were thermal cycled from -40ºC to +125ºC with and without conformal coating applied. The correlation between crack propagation and localized recrystallization were compared in a series of cross section analyses using polarized imaging. Mapping the localized recrystallization was found to be a useful analytical method for characterizing the stress-induced degradation mechanism of the solder interconnects. Partial and full coverage of conformal coating show different thermal cycling performance. A reworkable edgebond adhesive was investigated, to determine if it can mitigate the negative effect of conformal coatings on thermal cycle performance.