Electromigration behavior of lead-free solder flip chip bumps on NiP/Cu metallization

Abstract

The electromigration behavior of Sn–2.5Ag and Sn–0.7Cu (in wt %) flip chip solder joints on electroless NiP/Cu metallization at a current density of 1.3×104 A/cm2 was studied. For Sn-2.5 Ag solder, electromigration at 115 °C for 250 h showed a selective dissolution of Ni from the electroless NiP layer forming crystallized Ni3P. At 140 °C, the damage to the NiP layer was accelerated and instability of the NiP/Cu interface was observed. For eutectic Sn–0.7Cu solder, the electromigration behavior at a higher temperature was evaluated. At 180 °C, the NiP/Cu under bump metallurgy (UBM) started to show damage after 50 h. At 200 °C, the entire NiP/Cu layer was damaged, and P in the NiP layer moved to the edge of the anode much faster than the other species forming CuP2 intermetallics. NiP/Cu UBM experiences selective dissolution of Ni at lower temperatures, and the damage of the entire UBM occurred abruptly at the higher temperature.