Sn concentration effect on the formation of intermetallic compounds in high-Pb/Ni reactions

The reaction between high-Pb solders and Ni substrate was investigated to understand the Sn concentration effect on the formation of intermetallic compounds. Three PbxSn solders (x = 1, 3, and 5 wt.%) were reacted with Ni substrates at 400 °C for 2-240 min. The reaction products formed at the interface were related to the Sn concentration in the solders. When the Sn concentration was 5 wt.%, Ni3Sn4 formed first and followed by the formation of Ni3Sn2. When the Sn concentration decreased to 3 wt.%, Ni3Sn2 formed first and followed by the formation of Ni3Sn. When the Sn concentration became 1 wt.%, only Ni3Sn2 was. This fascinating concentration dependency was rationalized by using the Pb-Sn-Ni isotherm. In addition, the deep penetration of Pb into the Ni substrate along the grain boundaries was observed. It was likely that the low interfacial energy between high-Pb and Ni caused such an unusual grain boundary penetration.