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

Abstract

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 260 °C for 2 min. 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 CoSn 2 and finer Cu 6Sn 5 particles, while only one ternary (Cu, Ni) 6Sn 5 interfacial compound was detected between the solder alloy and the electroless nickel and immersion gold (ENIG) coated substrate. The same trend was also observed for the Sn–Ag–Cu and Sn–Cu solder joints. Compared with the CoSn 2 particles found in the Sn–Co–Cu solder and the Ag 3Sn particles found in the Sn–Ag–Cu solder, the Cu 6Sn 5 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 3.5 μm, 4.3 μm and 4.1 μm, 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.