Comparative study of interfacial reactions of high-Sn Pb-free solders on (001) Ni single crystal and on polycrystalline Ni

The interfacial reactions between (001) Ni single crystal and high-Sn solders were compared with those of polycrystalline Ni. The morphology of interfacial intermetallic compound (IMC) grains formed between (001) Ni single crystal and high-Sn solders (pure Sn, Sn-0.7Cu, Sn-0.7Cu-0.1Ni, Sn-1.5Cu and Sn-1.5Cu-0.1Ni, all in wt. %) at 250 °C and 300°C for various durations was investigated. Regular arrangement of interfacial IMC grains aligning along some preferred directions formed on (001) Ni single crystal in Sn-0.7Cu-0.1Ni, Sn-1.5Cu and Sn-1.5Cu-0.1Ni solders, while in pure Sn and Sn-0.7Cu solders, the interfacial IMC grains formed irregularly on (001) Ni single crystal. On polycrystalline Ni, no regular arrangement of the interfacial IMC grains was observed in the five solders. The dramatic morphological change of the interfacial IMC and its regular arrangement on (001) Ni single crystal were explained in terms of (1) the crystallographic orientation relationship between the interfacial IMC and the substrate and (2) the solubility limit of Cu in molten solders based on the Cu-Sn-Ni phase diagram. The morphological change of (Cu,Ni)₆Sn₅ also indicated that at the nucleation stage the Cu₆Sn₅ formed prior to the Ni₃Sn₄ in Sn-0.7Cu-0.1Ni/(001) Ni couple.