Effects of Sn grain c-axis on electromigration in Cu reinforced composite solder joints

Abstract

Sn grain orientation in solder matrix has recently been considered as one of the principal failure contributions in lead-free solder joints. Since β-Sn exhibits high anisotropy, the differences of electromigration behavior of solder joints with various grain orientations are dramatic. With different angle (θ) between the c-axis of Sn grain and the direction of electron flow, the solder joints often demonstrate varying degrees of electromigration-induced damage. The present investigation illustrated significant differences in microstructural features on the surface of Cu reinforced composite solder joints, even with similar angle θ. Two Cu particles reinforced Sn_3.5Ag composite solder joints with the similar angle θ of 35° and 40° were selected to investigate the effects of Sn grain c-axis on electromigration under high current stressing for 528 h. A large number of Cu₆Sn₅ compounds were observed and occupied nearly the whole surface with clear polarization effect in one sample. In contrast, very few Cu₆Sn₅ were found on the surface of the solder matrix in the other sample after current stressing, and the Cu₆Sn₅ which was formed after reflow disappeared with subsequent current stressing. Even though polarization effect was not obvious, and the cathode interfacial dissolution was observed on such sample. Systematic study revealed that the angle θ alone was not a sole factor to determine the electromigration damage. It has to be considered along with the coordinate system containing the orientation of the c-axis of the Sn grain, a more dominative factor for the diffusing species.