Contact angle measurements of Sn-Ag and Sn-Cu lead-free solders on copper substrates

In this study, the contact angles of four lead-free solders, namely, Sn-3.5Ag, Sn-3.5Ag-4.8Bi, Sn-3.8Ag-0.7Cu, and Sn-0.7Cu (wt.%), were measured on copper substrates at different temperatures. Measurements were performed using the sessile-drop method. Contact angles ranging from 30° to 40° after wetting under vacuum with no fluxes and between 10° and 30° with rosin mildly activated (RMA) and rosin activated (RA) fluxes were obtained. The Sn-3.5Ag-4.8Bi exhibited the lowest contact angles, indicating improved wettability with the addition of bismuth. For all soldering alloys, lower contact angles were observed using RMA flux. Intermetallics formed at the solder/Cu interface were identified as Cu^sub 6^Sn^sub 5^ adjacent to the solder and Cu^sub 3^Sn adjacent to the copper substrate. The Cu^sub 3^Sn intermetallic phase was generally not observed when RMA flux was used. The effect of temperature on contact angle was dependent on the type of flux used. [PUBLICATION ABSTRACT] Key words: Pb-free solder, contact angle, copper substrate, Cu-Sn intermetallics