Effect of Sn Grain c-Axis on Cu Atomic Motion in Cu Reinforced Composite Solder Joints Under Electromigration

摘要

Because of its tetragonal structure, beta-Sn exhibits anisotropy in electromigration behavior. The diffusivity of Cu atoms along the c-axis of Sn is much faster than that along the a-axis; therefore, the orientation of the c-axis exerts a strong influence on Cu atomic motion. In this work, the effect of Sn grain c-axis on Cu atomic motion was investigated by using Cu reinforced Sn3.5Ag composite solder joints. The microstructure and morphology of the solder joints were characterized by scanning electron microscopy (SEM) equipped with a backscattered electron (BSE) detector. It was found that intermetallic compounds (IMCs) in the solder matrix were formed due to the migration of Cu atoms to the surface of the solder joint under current stressing. There was a large amount of IMCs on the solder matrix which protruded from the solder matrix. By changing the direction of electron flow, i.e. changing the anode and the cathode side, the IMCs which formed after current stressing in the solder matrix disappeared and the protruding Sn remained unchanged. The results revealed that the growth of IMCs was closely related to the migration of Cu atoms, while Cu atomic motion depended on the orientation of the Sn grain c-axis.