The effects of grain structure on electromigration failure of the lead-free solder bump

摘要

This paper carries out an electromigration (EM) acceleration test on ball grid array (BGA) samples with Sn96.5/Ag3.0/Cu0.5 solder bumps under constant temperature, and characterizes the structure of β-Sn grains in the lead-free solder bumps. The EM failure modes of the solder bumps of different grain structures were analysed, aiming to disclose the effect of grain structure on the EM failure. Considering the driving forces of the EM (i.e. electron wind force, stress gradient, temperature gradient and atomic density gradient), the atomic density integral (ADI) method was introduced to simulate the void formation and failure lifetime of the EM. The simulation results show that solder bump reliability and failure mode are greatly affected by grain orientation, in that the EM failure occurs rapidly when the c-axis of grain structure of the solder bump is strongly misaligned, or almost perpendicular, to the current direction. The double grain solder bump with grain boundary parallel to current direction boasts a small EM failure and thus a long lifetime.