Mechanism of Void Formation in Cu Post Solder Joint under Electromigration

This paper reports experimental observations made on the mechanism of electromigration (EM) voiding and its kinetics active in low-profile solder joint consisting of long Cu-post (∼300×110µm), thin solder layer (15µm), and Ni coated Cu-lead frame assembly. When EM testing on the samples was conducted at accelerated conditions it revealed that although they are immune to EM failure, they contained a small amount of residual void as a result of EM. Kinetic analysis of EM voiding was done based on the analysis of resistance change and post EM microstructural characterization. Our analysis suggests that the origin of the residual voids is EM in Sn and it occurred prior to conversion of Sn to Cu 6 Sn 5 phase. It further suggests that the extent of voiding is determined by the two competing kinetic factors that are producing opposite effect on the voiding. The first is the kinetics of Sn EM because it determines the amount of vacancy responsible for voiding while the second is the Cu EM as it promotes the growth of Cu 6 Sn 5 phase that interrupts voids growth by removing Sn phase in the joint. The kinetic interplay between these two factors makes EM voiding kinetics of the low-profile solder joint to be different from the more conventional solder joint where Cu EM has a rather insignificant influence.