Reliability of interfaces for lead-free solder bumps

This study aims at a better understanding of the reliability of lead-free solder bumps for flip-chip applications. Without the presence of lead, copper in the under bump metal (UBM) layer dissolves extensively into the molten solder during reflow, which forms a thick intermetallic layer at the interface. It was found that with a annealing step before reflow, the thickness of the intermetallic layer can be reduced. When nickel is used as the UBM and SnAgCu is used as the bump material, (Cu,Ni)/sub 6/Sn/sub 5/ ternary intermetallic is formed at the interface. The ternary intermetallic increased the chance of failure at the interface during ball shear test. After thermal cycling, the probability of failure at the interface further increased.