Effect of Au thin films thickness on the wetting characteristics of eutectic Pb-Sn solder on Ni/Ag solder bath

Wetting of Pb-Sn solder on Au/Ni surface is experimentally studied as a function of the thickness of Au coating on Ni buffer layer. While Au coating is known to increase the wettability of Pb-Sn solder on Ni surface, the effect of Au layer thickness was not examined systematically. This study shows that i) an increase in Au layer thickness from 0.15 to 0.3 µm decreases the surface coverage (spread) of the solder and ii) there is a compositional change in Pb-Sn solder on Au/Ni surface due to formation of a Sn rich region at the solder - Ni interface during a reflow process of the solder. The thickness of Sn rich region near the interface increases with increasing the Au layer thickness. A driving force for the formation of the Sn rich phase is explained using the negative mixing enthalpy of Au-Sn. Since an increase in the Sn content of Pb-Sn solder increases the surface energy and viscosity of the solder melt, an increase in the thickness of the Sn rich region at the solder-Ni interface suppressed the wetting of the solder.