Failure Analysis of QFN Surface Discoloration by Copper Redeposition

Tapeless technology of QFN-mr has been a breakthrough in back-end manufacturing. However, back-etching process has some tradeoffs. Surface discoloration at this station has been a chronic defect. In this study, process simulations and material characterizations were performed to determine the root cause. Presence of copper and sulfur was correlated with the high concentration of sulfuric acid through a phenomenon called as copper redeposition.