Electromigration reliability of SnAgxCuy flip chip interconnects

Thermo- and electro-migration reliability of under-bump-metallization (UBM) and lead-free bumps of a flip-chip package is studied under high temperature operation life test (HTOL). UBM is a thin film Al/Ni(V)/Cu metal stack of 1.5 gm; while bump material consists of Sn/37Pb, Sn/95Pb, Sn/4Ag/0.5Cu, and Sn/xAg/yCu solders. Current densities of 5,000 and 20,000 Amps/cm² and ambient temperatures of 115 to 150°C are applied to study their impact on electromigration. Owing to its higher melting point feature and less content of Sri phase, high-lead bumps are observed to have 13-fold improvement in characteristic life (t63) than that of eutectic Sn/37Pb. Also, measured electromigration lives of Sn/4Ag/0.5Cu and Sn/xAg/yCu are 4- and 12-fold improvement with respect to eutectic solder. Resistance history of solder bump chain is employed to evaluate UBM/bump degradation. The measured resistance increase is from bumps with electrical current flowing upward into UBM/bump interface (cathode), while bumps having opposite current polarity cause only minor resistance change. The identified failure sites and modes from aforementioned high resistance bumps reveal structural damages at the region of UBM and UBM/bump interface in forms of solder cracking or delamination. Effects of current polarity and crowding are key factors to observed electromigration behavior of flip-chip interconnects.