Evaluation of electromigration (EM) life of ENEPIG and CuSOP surface finishes with various solder bump materials

Eutectic PbSn solder joints assembled with CuSOP and ENEPIG surface finished substrates were tested at three different temperatures and input currents to predict EM life. Estimated EM life of CuSOP with PbSn solder system is 3 to 4 X longer than that of ENEPIG surface finish. Both EM failures in solder bumps are caused by void formation at the current crowding area and propagation mechanism, however, they exhibited two distinct failure modes. The Eutectic bump with ENEPIG finish failed due to the UBM over-consumption as a result of reaction of Ni with Sn while the EM failure of bumps assembled with CuSOP was caused by the crack created between bulk and Ni₃Sn₄ IMC. Nevertheless, the UBM was kept intact in the latter case. The difference of UBM consumption rate induced by Cu influx from the substrate is identified as the main reason of this performance difference. The failure mechanisms of both EM failures were proposed and discussed in details.