Mechanism investigation of Bi layer formation at anode interface in Cu/Sn-58Bi/Cu solder joint induced by electromigration

Mechanism of Bi layer formation at the anode interface in Cu/Sn-58Bi/Cu solder joint was investigated under current density of 5×10³ to 1.2×10⁴ A/cm².During electromigration process,Bi was the main diffusing species which migrated from the cathode side to the anode side under electromigration force.Bi was the first species to reach anode interface because the diffusion velocity of Bi is faster than that of Sn.Therefore,compressive stress was formed at the anode interface when much more Bi atoms accumulated there and compelled Sn to migrate towards the cathode side,and the continuous Bi layer was formed at the anode interface.In the mean time,tensile stress was formed at the cathode interface due to the departure of metal atoms,which led to voids and cracks formation.The morphology of the Bi layer included planar-type layer and groove-type layer.The diffusing channels of the Bi atoms were Bi grain boundaries,Sn grain boundaries and Sn/Bi interfaces.With the current density and stressing time increasing,the Bi layer thickness increased.The electromigration force and Joule heating took on the main driving force for Bi diffusion and migration.