A study in flip-chip UBM/bump reliability with effects of SnPb solder composition

Abstract

This paper aims to investigate electromigration phenomena of under-bump-metallization (UBM) and solder bumps of a flip-chip package under a high temperature operation life test (HTOL). UBM is a thin film Al/Ni(V)/Cu metal stack of 1.5 /spl mu/m, while the bump material consists of Sn/37Pb, Sn/90Pb, and Sn/95Pb solders. Current densities of 2,500 and 5,000 A/cm/sup 2/ and ambient temperatures of 150 to 160/spl deg/C are applied to study their impact on electromigration. It is observed that bump temperature has a more significant influence than current density on bump failures. Owing to its higher melting point characteristics and lower Sn content, high-lead bumps are observed to have an 8 to 12-fold improvement in Mean-Time-To-Failure (MTTF) compared with eutectic Sn/37Pb. Individual bump resistance history is used to evaluate UBM/bump degradation. The measured resistance increase is from bumps with electrical current flowing upward into the UBM/bump interface (cathode), while bumps with opposite current polarity cause only minor resistance change. The identified failure sites and modes reveal structural damage at the region of UBM and UBM/bump interfaces in the form of solder cracking or delamination. The effects of current polarity and crowding are key factors in the observed electromigration behavior of flip-chip packages.