Joints Soldered on Electroless Ni–Au Surfaces Using Cu-Containing Flux: Strength, Microstructure and Mechanism of Improvement

抄録

The critical issue for soldering on electroless Ni–Au surface finishes is fragile solder joints. Previous studies have indicated that these weak joints are a result of the formation of a P-rich layer at the joint interface. In this study, the new flux, which contains a Cu compound, demonstrates improved solder joint strength for solder ball attachment on electroless Ni–Au surface finishes. Cross-sectional analysis revealed that this new flux gives a thinner P-rich layer at the joint interface than that seen with a conventional flux. Moreover, the formation of a uniformly thin Cu–Sn intermetallic layer on the Ni–P plated surface is observed after reflow soldering. We conclude that this Cu–Sn layer formation during reflow impedes extra diffusion of Ni into solder from the plating surface so that the growth of a P-rich layer can be effectively inhibited and, thus, the joint strength is improved.