Spreading kinetics of liquid solders over an intermetallic solid surface. Part 2: Lead-free solders

This study is devoted to a phenomenological and quantitative investigation of the physics of spreading of lead-free solder systems (Sn, Sn-0.7Cu, and Sn-3.5Ag) over Cu ₆Sn ₅/Cu ₃Sn/Cu substrates. Wetting kinetics was studied using real-time in situ monitoring of the triple-line kinetics under a controlled atmosphere. Three different intermetallic (IMC) substrates were used for this investigation. It was found that the grain size of the deposited IMC compound coating has a sizable influence on the triple-line movement of the liquid solders considered. More pronounced spreading of solders on IMC substrates was observed for all lead-free solders over an intermetallic with the smallest grain size. Lead-free solder spreading over IMC surfaces exposed to aging at elevated temperatures, hence featuring larger grain size, did not feature a significant improvement of wettability when compared with the behavior of the same solders on virgin copper substrates.