Resolution of contamination on controller IC bond pads for high Pb multi-stack MCM Clip QFN

Clip QFN is a multi-stack MCM package which composes of multiple dies and multiple clips vertically stacked to achieve optimum device performance and minimize footprint. As the number of devices grows, new challenges arise to meet quality and productivity requirements of the product. Different bond pads for the controller die were introduced which added complexity in making these products manufacturable. Given that the technology for each bond pad is different, it was expected that each will have different response on the current bill of materials (BOM) and process. Contamination on the bond pads was observed during wire bonding process causing low assembly yield and affected mean time between assist (MTBA). To address the issue, current BOM and process at prebond were checked. Root cause of discoloration was determined and found out that the current solder paste and the peak temperature during reflow were causing the discoloration. Process optimization alone was not able to completely eliminate the contamination thus the need to modify the existing BOM and process flow became necessary. Through the modification of existing Clip QFN BOM and process flow, contamination on bond pads was eliminated.