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 point of indium is 156.6° C., even lower than that of the eutectic Sn—Pb which is at 183° C., so the soldering process can be carried out 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° 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 substrate, while a In gradient is formed in the In deposited Pb-free solder.