Comparative study of interfacial reactions of High-Sn lead-free solders on single crystal Cu and on polycrystalline Cu

This work compared the interfacial reactions of single crystal Cu and polycrystalline Cu with the high-Sn solders. The morphology aspects of Cu₆Sn₅ intermetallic compound (IMC) grains formed between (001) single crystal Cu and high-Sn solder bath (pure Sn, Sn-0.7Cu, Sn-3Cu) at 250degC and 300degC for different durations ranging from 10 s to 10 min were investigated. The Cu₆Sn₅ grains formed in all solder baths at 250degC displayed common scallop-type, while those formed in pure Sn and Sn-0.7Cu baths at 300degC displayed rooftop-type in early stage of the interfacial reaction (less than 10 min). Both the top-view morphology and cross-section morphology of the interfacial Cu₆Sn₅ IMC formed on single crystal Cu at 300degC are different from those formed on polycrystalline Cu. From the top-view, the rooftop-type Cu₆Sn₅ grains formed on (001) single crystal Cu are elongated along two perpendicular directions; from the cross-section view, the IMC layers present a relatively planar intermetallic structure. Furthermore, Cu₆Sn₅ grains formed between Sn-0.7Cu/(001)Cu partly changed from rooftop-type to scallop-type when the reaction time is up to 10 min; Cu₆Sn₅ grains formed between pure Sn/(001)Cu remained rooftop-type even after the reaction for up to 10 min. It is considered that reaction temperature and Cu concentration in the solder bath have an effect on the formation of rooftop-type Cu₆Sn₅ grains. The dissolution rate of single crystal Cu in the high-Sn solder is higher than polycrystalline Cu in solder bath at 300degC, which indicated that the morphology of Cu₆Sn₅ affects the dissolution of substrate Cu.