Electromigration Challenges in FCQFN Packaging: Impact of Solder Thickness and Shape on Reliability

A specially-designed flip-chip quad flat no-lead (FCQFN) package test vehicle featuring Cu pillar interconnects was investigated for the electromigration (EM) study to meet the high-reliability requirement for high-power automobile applications. The microstructure and electrical performance of these Cu pillar interconnects were examined, focusing on variations in plated solder thickness and interconnect geometry under extreme EM conditions. The interconnects were stressed at a current density of 20 kA/cm2 at 180˚C environmental atmosphere, accompanied by in-situ electrical resistance monitoring. Variations in electrical resistance were correlated with the microstructural evolutions. The electrical resistance exhibited a 100% increase compared to the initial value, while the SnAg solder within the Cu pillar interconnects was entirely consumed and subsequently transformed into Sn-based intermetallic compounds (IMCs). Additionally, a polarity effect was observed in the Cu pillar interconnects, influenced by varying electron flow directions. To optimize the EM performance of the Cu pillar interconnects, the effects of plated solder thickness and interconnect geometry on EM behavior were systematically evaluated. It was found that a reduced solder volume contributed to a superior lifetime, attributed to the rapid formation of a complete IMCs structure from microstructural analyses. Furthermore, a specially designed modification to the Cu pillar interconnect structure demonstrated potential for enhancing EM performance based on both microstructural and simulation analyses. The results and findings presented in this study provide valuable design guidelines for FCQFN packages in the future, especially regarding enhancing interconnect reliability.