Effects of UBM thickness on electromigration in Pb-free solder joints

Pb-free solder interconnect applied in flip chip is a predominant technology for wafer-level chip-scale package (WLCSP). In WLCSP, the die shrinks area for power available, the bump shrinks volume and increase quantity for interconnect efficient. As a result, the bump current density is now approaching 10⁴ A/cm² level, where electromigration (EM) becomes a significant reliability issue. The EM failure depends on many factors, such as the working temperature, the under bump metallization (UBM) thickness and others. In this paper, the effects of the different Ni/Au UBM thickness on electromigration in the Sn/3.5Ag/0.5Cu solder bumps have been studied. Samples with thick Ni/Au diffusion barrier layers have longer MTTF than those with thin ones. More Ni atoms in the UBM layer delay EM failure occurring. Another unique EM behavior is the polarity effect on IMC growth at the anode and IMCs dissolution at the cathode. In the stable stage of EM procedure, electron-flow pushes Ni atoms to move from the cathode to the anode, and a lot of intermetallic compounds (IMCs) accumulate at the anode. Atom migration generates stress gradient at the solder/Ni interface near the cathode, which produces micro-void and cracks there. Voids propagate along the interface in the direction of stress gradient decreasing. At the last stage of EM procedure, large cracks appear at the interface and enhance contact resistance dramatically. Consequently, joule heat crowds at the high resistance area and melts the interconnect interface instantly. The IMC evolutions of EM samples are compared with those of thermal aging samples at 180°C.