Effect of UBM thickness on the mean time to failure of flip-chip solder joints under electromigration

Flip-chip solder joints with Cu/Ni/Al underbump metallurgy (UBM) on the chip and an Au/Ni surface finish on the substrate were studied under current stressing at an ambient temperature of 150 degrees C. Three different Ni thicknesses in the Cu/Ni/Al UBM (0.3, 0.5, and 0.8 mu m) were used in order to investigate the effect of the Ni thickness on reliability. The solder used was eutectic Pb-Sn, and the applied current density was 5 x 10(3) A/cm(2). The results show that the combined effect of current crowding and the local Joule heating near the entry points of electrons into the joints induced asymmetric Ni UBM consumption. Once the Ni was exhausted in a certain region, this region became nonconductive and the flow of electrons was diverted to the neighboring region. This neighboring region then became the place where electrons entered the joint, and the Ni UBM there was consumed at an accelerated rate. This process repeated itself, and the Ni-depleted region continued to extend, creating an ever larger nonconductive region. The solder joints eventually failed when the nonconductive region extended across the entire contact window of the joints. This failure model supports the observation that joints with a thicker Ni tend to have a longer average lifetime.