Effect of Cu concentration on the interfacial reactions between Ni and Sn-Cu solders

The eutectic 99.3Sn-0.7Cu solder (wt%, Sn-0.7Cu) is the most promising lead-free replacement for the eutectic Sn-Pb solder in wave-soldering applications. In this study, the effect of a small perturbation in the Cu concentration on the reaction between the Sn-0.7Cu solder and Ni was investigated. Specifically, four Sn-xCu solders (x = 0.2, 0.4, 0.7, and 1) were reacted with Ni at 250 degreesC. A slight variation in Cu concentration produced completely different reaction products. When the Cu concentration was low (x = 0.2), the reaction product was (Ni1-xCux)(3)Sn-4. At high Cu concentrations (x = 0.7 and 1), the reaction product was (Cu1-yNiy)(6)Sn-5. When the Cu concentration was in-between (x = 0.4), both (Nil-xCux)(3)Sn-4 and (Cu1-yNiy)(6)Sn-5, formed. The above findings were rationalized using the Cu-Ni-Sn isotherm. the results of this study imply that the Cu concentration must be strictly controlled in industrial production to produce the desired intermetallic at the interface.