Size and substrate effects upon undercooling of Pb-free solders

Undercooling is the phenomenon that solidification does not occur when the liquid passes its equilibrium phase transformation temperature. It is well known that Sn has significant undercooling. Undercooling influences both micro structural development and morphological evolution of solder joints. The solidified phases and their microstructure are crucial to the solder joint properties. Therefore, knowledge of the undercooling of Sn-based alloys is valuable for their applications as solders. As for the soldering applications, the solder alloys wet and react with the joining substrates. In addition to the undercooling natures of alloys, the undercooling of the solders in the joints might be affected by the substrates as well. Only a few studies are available regarding the undercooling of Pb-free solders, and the substrate effect is not very well understood. The undercooling phenomena of various Pb-free solders are determined in this study, and the effects of size, composition and substrate upon undercooling are examined.