An Electromigration Study of Cu Pillar Interconnects in Flip-chip QFN Packaging under Extreme Conditions for High-power Applications

A specially-designed flip-chip quad flat no-lead (FCQFN) package test vehicle with a Cu pillar/Ni metallization/Sn-1.8Ag solder/Ag/lead frame configuration was investigated for the electromigration (EM) study to meet the high-reliability requirement for high-power automobile applications. The microstructure and electrical performance of the specially-designed Cu pillar interconnects in FCQFN packages were investigated under extreme EM conditions, accompanying an in situ electrical resistance monitoring. The interconnects were stressed at a current density of 10 kA/cm
²
at 150°C, 160°C, and 180°C environmental atmosphere. The electrical resistance variations during the in situ EM experiment could be interpreted from the microstructure evolutions after electric current stressing. The Sn-1.8Ag Pb-free solder in the Cu pillar interconnect has been completely consumed and transformed into Sn-based intermetallic compounds (IMCs) after the EM experiment in a high operating temperature. The polarity effect was also observed in the Cu pillar interconnects with different electron flow directions. Finally, an EM prediction model of the Cu pillar interconnect was built in this study based on a Black-type EM time-to-failure (TTF) equation. The maximum currents of the specially-designed Cu pillar interconnects allowable for different operating temperatures under certain failure criteria were estimated practically using the model as proposed under the actual use conditions and lifetimes. The results and findings presented in this article provided a design guideline for FCQFN packages in the future concerning interconnect reliability.