Effect of thin gold/nickel coating on the microstructure, wettability and hardness of lead-free tin–bismuth–silver solder

This paper investigates the coating thickness and surface morphology of gold/nickel (Au/Ni) layer on copper (Cu) substrate. A cross-sectioned SEM analysis confirmed that the Au/Ni coating was uniform. The top Au layer with average thickness of 0.7 µm appeared to have a very smooth surface without any defect such as cracks and delamination. However, the thin Au/Ni coating greatly influenced the interfacial structure and material properties of electronic interconnections. In the reference Cu substrate/Sn–Bi–Ag solder system, an island-shaped Cu₆Sn₅ IMC layer at the interface could be clearly observed at the initial reaction stage. After a prolong reaction, a very thin Cu₃Sn IMC layer was formed with excessive growth of the Cu₆Sn₅ IMC layer which can deteriorate the electronic interconnection life-span. However, in the Au/Ni coated substrate/solder system, a very thin scallop-shaped ternary Ni₃Sn₄ IMC layer formed without the Cu₃Sn IMC layer, indicating that the Au/Ni coating hindered the growth of the IMC layer, which consequently changed the activation energies and refined the microstructure. Additionally, the overall micro-hardness of the Au/Ni coated substrate/solder system is higher than that of the reference solder system.