Influence of crystallographic orientation of Sn–Ag–Cu on electromigration in flip-chip joint

The influence of the crystallographic orientation of Sn–3.0 wt%Ag–0.5 wt%Cu flip-chip joints and underfill on electromigration was investigated. The current density applied in our tests was 15 kA/cm2 at 160 °C. Various times to failure of the test samples show a clear dependence of the electromigration behavior on the Sn grain orientations. Different microstructural evolutions were observed in all solder bumps in correlation with the crystallographic orientations of the Sn grains after an electromigration test. The primary failure of the solder joints was caused by dissolution of the Cu electrode at the cathode interface. Rapid dissolution of the Cu electrode occurred when the c-axis of the Sn grains was parallel to the direction of electron flow. On the other hand, slight dissolution of the Cu electrode was observed when the c-axis of the Sn grains was perpendicular to the direction of electron flow. Some grain boundaries interrupt the migration of Cu and the trapped Cu atoms form new grains of intermetallic compounds at the grain boundaries. In addition, underfill inhibits serious deformation of solder bumps during current stressing.