Abnormal depletion of Cu metallization pads in line–bump–line solder joints under electron current stressing

•Abnormal electromigration-induced pad's depletion in solder joints•A pronounced Cu depletion in the area away from the current crowding region•Anisotropic Cu electromigration in Sn lattice•β-Sn orientation is a key factor of the electromigration joint reliability

We observed abnormal electromigration-induced Cu depletion in a traditional line–bump–line (Cu/Sn/Cu) solder joint with extremely high Cu depletion away from the current crowding region (CCR). Electron backscatter diffraction analyses showed that the β-Sn crystallographic orientation of the solder alloy is responsible for this abnormal electromigration phenomenon. A pronounced depletion in Cu occurred when the Cu neighbored the Sn grains with c-axes parallel to electron flow, despite the fact that the current density distributed in this type of area was one order-of-magnitude lower than that of the CCR. This observation indicated that the current stress and β-Sn crystallographic orientation played dominant roles in Cu depletion during electron current stressing. Finally, a new failure mechanism is proposed for electromigration in solder joints.