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

摘要

In the present work, Kirkendall void formation and drop impact reliability between Pb-free solder and Cu metallization were investigated. Pb-free solders, Sn-3.5Ag and Sn-3.5Ag-xZn (x=0.1, 0.5, 1, 3, and 7wt%), were reacted with Cu metallization. After thermal aging of Sn-Ag solder on electrodeposited Cu at 150°C, Kirkendall voids were distributed either in the Cu_3Sn layer or at the Cu/Cu_3Sn 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/Cu_3Sn interface with an additive, but distributed in the Cu_3Sn layer without an additive. On the other hand, an addition of Zn to Sn-Ag solder effectively suppressed the formation of Cu_3Sn 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 3wt% Zn completely suppressed Cu_6Sn_5, which were known as a brittle phase, during a reflow. Drop impact tests showed that Sn-3.5Ag solder joint with electroplated Cu using an additive drastically degraded with subsequent aging and fracture occurred at the Cu/Cu_3Sn interface, where a large amount of voids existed. In the case of solder joint with Cu foil, dropreliability slightly decreased with subsequent aging. Also, the number of drops to failure (N_f) of Sn-3.5Ag-3wt%Zn solder joints was almost doubled compared to Sn-Ag solder joint. The beneficial role of Zn was ascribed to suppression of Cu_6Sn_5 and precipitation of Zn-containing IMCs.