Effects of third element and surface finish on interfacial reactions of Sn–Ag–xCu (or Ni)/(Cu or ENIG) solder joints

In this study, we investigated the effect of the Cu content on the interfacial reactions between the Sn–Ag–xCu solders and Cu substrate. In addition, we evaluated the effects of adding Cu or Ni to Sn–Ag solder on the interfacial reactions of the Sn–Ag–Cu (or Ni)/ENIG joints. The formation and growth of interfacial intermetallic compounds (IMCs) between the Ni-containing Sn–Ag–Ni solder and ENIG substrate were studied and the results were compared to the Sn–Ag–Cu/ENIG system. Increasing the amount of Cu added to the Sn–Ag solder significantly reduced the thickness of the Cu3Sn IMC, while increasing that of the total Cu–Sn and Cu6Sn5 IMCs. (Ni,Cu)3Sn4 and Ni3Sn4 IMCs formed at the Sn–3.0Ag–0.5Cu/ENIG and Sn–3.0Ag–0.5Ni/ENIG interfaces, respectively. These two IMCs grew during isothermal aging and similar interfacial microstructures were found in the Sn–3.0Ag–0.5Cu (or Ni)/ENIG joints. Comparing the Sn–3.0Ag–0.5Cu/Cu and Sn–3.0Ag–0.5Cu/ENIG joints, the growth rate of the IMC layer for the Cu substrate was about 3.3 times faster than that for the ENIG substrate.