Nucleation and morphology of Cu₆Sn₅ intermetallic at the interface between molten Sn-0.7Cu-0.2Cr solder and Cu substrate

The nucleation kinetics and morphology of Cu₆Sn₅ IMCs at the interface between a Sn- 0.7Cu-0.2Cr solder and Cu substrate were investigated in this study. A Sn-0.7Cu solder was utilized as a reference to elucidate the impact of Cr addition. The mechanical properties of the solder joints were determined via ball-shear tests. Cu coupons were dipped in the molten solders for 1 and 3 s at 240-300 °C, and the morphological analyses were conducted via electron microscopy. Both the solders contained scallop-like Cu₆Sn₅ IMCs. The smallest Cu₆Sn₅ IMCs were observed at 260 °C in both the solders, and the particle size increased at 280 and 300 °C. The IMCs in the Sn-0.7Cu-0.2Cr solder were smaller and thinner than those in the Sn-0.7Cu solder at all the reaction temperatures. The thickness of the IMCs increased as the reaction temperature increased. Inverse C-type nucleation curves were obtained, and the maximum nucleation rate was observed at an intermediate temperature. The shear strengths of the Sn-0.7Cu-0.2Cr solder joints were higher than those of the Sn-0.7Cu solder joints. This study will facilitate the application of lead-free solders, such as Sn-0.7Cu-0.2Cr, in automotive electrical components.