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 3.5 Ag 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 Cu 6 Sn 5 compounds were observed and occupied nearly the whole surface with clear polarization effect in one sample. In contrast, very few Cu 6 Sn 5 were found on the surface of the solder matrix in the other sample after current stressing, and the Cu 6 Sn 5 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.
Due to the asymmetric structure of Sn unit cell, the electromigration behaviors of Sn based solder joints were significantly depended on the crystal orientation of Sn grain. The present investigation illustrated the effect of Sn grain c -axis on the intermetallic compounds (IMCs) growth under current stressing. The Cu particles reinforced Sn3.5Ag solder joint was selected to obtain various observed surfaces of the sample after 46 days current stressing. The front side, back side, right side and anode side of the sample represented different phenomena, which reflected and proved the importance of c -axis orientation on Cu atomic migration in three-dimension space. Besides, another composite solder joint was polished severe times on the observed surface which was full of IMCs. It showed that the IMCs were covered on the Sn-based solder, and there was crack at the cathode underneath the front surface after 38 days current stressing. The IMC growth behavior would be well understood by systematic study in this work.
危险品化学品经营许可证（带存储）