Size and Substrate Effects upon Undercooling of Pb-Free Solders

The degrees of undercooling of various Pb-free solders are determined using differential scanning calorimetry. The effects of size, composition, and substrate upon undercooling are examined. Ni is the most effective element among Cu, Ni, and Ag in reducing the undercooling of Sn solders, both as an alloying addition and as a substrate. The degrees of undercooling and their variations are more significant for smaller-sized solders, but the relative orders of undercooling of various solders remain the same. It is concluded that the primary factors controlling undercooling are the primary solidification phase and the substrate. Different compositions of melts could have different primary solidifications, resulting in different degrees of undercooling. When the primary solidification phase and the substrates are the same, the degrees of undercooling could be different if the compositions of the melts are different. However, this compositional effect is not very significant.