Impact of Ni concentration on the intermetallic compound formation and brittle fracture strength of Sn–Cu–Ni (SCN) lead-free solder joints

• Ni in the SnCu solder can promote the growth of (Cu, Ni)6Sn5 and suppress the growth of Cu3Sn.
• Ni improves the IMC strength, in terms of reducing the percentage of brittle failure and increasing the pull/shear force.
• Cu3Sn layer plays a key role in the brittle failure of solder joint.

Cu6Sn5 and Cu3Sn are common intermetallic compounds (IMCs) found in Sn–Ag–Cu (SAC) lead-free solder joints with OSP pad finish. People typically attributed the brittle failure to excessive growth of IMCs at the interface between the solder joint and the copper pad. However, the respective role of Cu6Sn5 and Cu3Sn played in the interfacial fracture still remains unclear. In the present study, various amounts of Ni were doped in the Sn–Cu based solder. The different effects of Ni concentration on the growth rate of (Cu, Ni)6Sn5/Cu6Sn5 and Cu3Sn were characterized and compared. The results of characterization were used to evaluate different growth rates of (Cu, Ni)6Sn5 and Cu3Sn under thermal aging. The thicknesses of (Cu, Ni)6Sn5/Cu6Sn5 and Cu3Sn after different thermal aging periods were measured. High speed ball pull/shear tests were also performed. The correlation between interfacial fracture strength and IMC layer thicknesses was established.