Effects of substrate metallization on the degradation of flip chip interconnects under electromigration

Effects of substrate metallization, i.e. Ni/Au vs. Cu-OSP, are studied to characterize the electromigration resistance of under-bump-metallization (UBM) and solder bumps of a flip-chip package under high temperature operation life test (HTOL). UBM is a thin film Al/Ni (V)/Cu metal stack of 1.5 μm; while bump material consists of eutectic Sn/Pb solder. Current densities of 5,000 A/cm² and ambient temperatures of 125 to 150 C are applied as conditions of current and thermal stressing. It is observed that flip-chip packages with Cu-OSP substrates are having 13-fold improvement in Mean-Time-To-Failure (MTTF) than that of packages using Ni/Au as substrate surface finishes. Both Ni and Au influence the consumption and electromigration of UBM; therefore accelerate solder bump damage. 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 substrate metallization, current polarity, and crowding are key factors to electromigration behavior of flip-chip interconnect.