Key parameters for fast Ni dissolution during electromigration of Sn0.7Cu solder joint

Abstract

This paper discusses the key parameters for fast Ni dissolution induced electromigration (EM) damage during reliability EM test of Sn0.7Cu solder joints. The results confirm that Sn grain orientation plays the important role on Ni dissolution, which has been reported in literatures. More important, efforts have been made to understand what the key parameters that can modulate the Sn grain orientation distribution of multiple solder joints during solder joint assembly process are. Statistic Electron backscatter diffraction (EBSD) analyses are used to characterize the Sn grain orientation distribution changes. A critical angle between Sn grain c-axis and substrate normal has been defined based on the statistical analyses. It is found that fast Ni dissolution mainly happens when the angle ranged from 0º to 40º during EM reliability testing. With this understanding, the impacts of solidification cooling rate and Cu column dissolution during assembly reflow have been studied. It is concluded that fast cooling rate will increase the chance of Sn grain c-axis align with substrate normal and promote Ni dissolution during EM. On the other hand, Cu column dissolving during reflow can increase the Cu concentration in solder joint and decrease the possibility of c-axis along with substrate normal. Besides Sn grain orientation, it is interesting to find that solder joint height can also impact Ni dissolution during EM. Longer solder joint height can induce faster Ni dissolution.