How the mold compound thermal expansion overrules the solder composition choice in board level reliability performance

IC packages using mold compounds with low coefficient of thermal expansion (CTE) have been introduced in the last decade packages with basically no attention to its impact on board level reliability. In this study, the impact is shown for a large size QFN package. In this parameter sensitivity study, also the solder composition and the flank wettability of the QFN leads are varied. The QFN's soldered to a thick Printed Circuit Board (PCB) are tested under thermal cycling and the outcome is a Weibull failure distribution. Comparing the different results reveals which of the three parameters (mold CTE, solder composition, flank wettability) is the one dominating most the board level reliability. The work is supported by optical inspection on failed samples.