Intermetallic compound formation in Sn-Co-Cu, Sn-Ag-Cu and eutectic Sn-Cu solder joints on electroless Ni(P) immersion au surface finish after reflow soldering

The interfacial reactions between Sn-0.4Co-0.7Cu eutectic alloy and immersion Au/electroless Ni(P)/Cu substrate were investigated after reflow soldering at 260degC for 2 minutes. Common Sn-4.0Ag-0.5Cu and eutectic Sn-0.7Cu solders were used as reference. Two types of intermetallic compounds (IMC) were found in the solder matrix of the Sn-0.4Co-0.7Cu alloy, namely coarser CoSn₂ and finer Cu₆ Sn₅ particles, while only one ternary (Cu,Ni)₆ Sn₅ interfacial compound was detected between the solder alloy and the ENIG (electroless nickel and immersion gold) coated substrate. The same trend was also observed for the Sn-Ag-Cu and Sn-Cu solder joints. Compared with the CoSn₂ particles found in the Sn-Co-Cu solder and the Ag₃Sn particles found in the Sn-Ag-Cu solder, the Cu₆Sn₅ particles found in both solder systems exhibited finer structure and more uniform distribution. It was noted that the thickness of the interfacial IMCs for the Sn-Co-Cu, Sn-Ag-Cu and Sn-Cu alloys was 3.5mum, 4.3mum and 4.1mum, respectively, as a result of longer reflow time above the alloy´s melting temperature since the Sn-Ag-Cu solder alloy has the lowest melting point