Development of Wettability Evaluation Technique

A new wettability evaluating system was developed by combining a contact angle measurement instrument, which optically and insitu measures the contact angle of molten solder, with a conventional wettability tester (meniscograph). Using this new evaluation system, a well-known experiential knowledge that the wettability for Sn-3.5Ag, a typical Pbfree solder, is inferior to that for Sn-37Pb is quantitatively confirmed by the result that the contact angle for Sn-3.5Ag is twice of that for Sn37Pb. The cause for this wettability difference is attributed to the difference in the surface tensions of the solders (the solder-flux interfacial tension). The influences of alloying elements (1%Cu, 1%Zn, 5%Bi, 5%In), fluxes and Au coating on a Cu substrate on the wettability for the Sn-3.5Ag are discussed. In the discussion, three interfacial tensions among the solder, flux and substrate are separately estimated to analyze the mechanisms which make differences in wettability.