Abstract
This invention forms an indium or indium alloy layer on top of a Sn based lead-free solder. The indium or indium alloy layer can be formed by various methods, such as plating., deposition, printing, dipping, etc. The indium containing layer melts during the soldering process, wets the substrate, and forms a sound solder joint. Since the melting temperature of indium is 156.6 DEG C, even lower than that of the eutectic Sn-Pb which is at 183 DEG C, so the soldering process can be carried at a temperature lower than the conventional soldering process. During the soldering process, the indium reacts with the Sn based Pb-free solder alloy. Since the eutectic temperature of Sn-In is at 120 DEG C, during the short time of the soldering process, the surface of the In deposited Pb-free solder remains as the liquid phase and have a good wetting with the substrates, while a In gradient is formed in the In deposited Pb-free solder.