Intermetallic compound formation in the interface between SAC305 solder and Cu-xZn-yNi substrates

In this study, the interfacial reaction between Sn-3.0Ag-0.5Cu solder on Cu-35wt%Zn, Cu-20wt%Zn-15wt%Ni and Cu-25wt%Zn-10wt%Ni substrates were studied after reflow at 250°C for 90s and isothermal aging at 150°C for 50, 100, 250, and 500h. Addition of Zn or Ni into Cu wetting layer was effective to suppress the excessive growth of IMC and avoid the microvoids formation after thermal aging. In the initial stage of our experiment, there were not significant differences of IMC thickness between using Cu and Cu-xZn-yNi substrate just after reflow. However, IMC thickness after aging at 150°C can be distinctly reduce by adding of Zn and Ni into Cu substrate. The thickness of the combined Cu₆Sn₅ and Cu₃Sn IMC in the joint of the SAC/Cu substrate aged at 150°C for 500h was approximately 5.8um respectively. In contrast to the SAC/Cu system, the total IMC thickness of the SAC/Cu-xZn and SAC/Cu-xZn-yNi was distinctly reduced to approximately 2.0um for SAC/Cu-35Zn, 2.7um for SAC/Cu-25Zn-10Ni and 3.3um for SAC/Cu-20Zn-15Ni interfaces. Even though significant suppression of IMC growth during aging was observed by Ni addition to Cu-xZn layer compared to Cu substrate, the IMC thickness of SAC/Cu-25Zn-10Ni and SAC/Cu-20Zn-15N samples after reflow and aging test was much thicker than SAC/Cu-35Zn sample. Because the rapid growth of Cu₆Sn₅ at the SAC/Cu-xZn-yNi interface was occurred since the Ni in solder accelerate the precipitation and the growth of the Cu₆Sn₅. That is the reason why the IMC thickness of SAC/Cu-xZn-yNi system is thicker than SAC/Cu-35Zn. Finally it can be concluded that the more Zn contents in Cu-xZn-yNi substrate regardless of Ni contents, the less IMC growth was observed which indicated that the IMC growth rate during aging strongly depends on Zn concentration rather than Ni.