Interfacial reactions and drop reliabilities of lead-free solder joints with electrolytic Cu metallizations.

In the present work, Kirkendall void formation and drop impact reliability between Pb-free solder and Cu metallization were investigated. Pb-free solders, Sn-3.5 Ag and Sn-3.5 Ag-xZn (x=0.1, 0.5, 1, 3, and 7 wt%), were reacted with Cu metallization. After thermal aging of Sn-Ag solder on electrodeposited Cu at 150degC, Kirkendall voids were distributed either in the Cu₃Sn layer or at the Cu/Cu₃Sn interface. Much more voids were existed at the solder joint with electroplated Cu using an additive than without an additive. Also, voids were distributed near Cu/Cu₃Sn interface with an additive, but distributed in the Cu₃Sn layer without an additive. On the other hand, an addition of Zn to Sn-Ag solder effectively suppressed the formation of Cu₃Sn IMC even though 0.1 wt% Zn, thereby eliminating Kirkendall voids at the interface of solder and Cu metallization during thermal aging. Adding more than 3 wt% Zn completely suppressed Cu₆Sn₅, which were known as a brittle phase, during a reflow. Drop impact tests showed that Sn-3.5 Ag solder joint with electroplated Cu using an additive drastically degraded with subsequent aging and fracture occurred at the Cu/Cu₃Sn interface, where a large amount of voids existed. In the case of solder joint with Cu foil, drop reliability slightly decreased with subsequent aging. Also, the number of drops to failure (Nf) of Sn-3.5 Ag-3 wt%Zn solder joints was almost doubled compared to Sn-Ag solder joint. The beneficial role of Zn was ascribed to suppression of Cu₆Sn₅ and precipitation of Zn-containing IMCs.