Wetting of Al pads by Sn-8.8Zn and Sn-8.7Zn-1.5(Ag, In) alloys

Wetting of Al (99 %) pads by Sn-8.8Zn, Sn-8.7Zn-1.5In, and Sn-8.7Zn-1.5Ag (wt%) alloys was studied by means of the sessile drop method. The tests were carried out at 250 A degrees C, using Amasan ALU33A (R) flux, for up to 60 min. We used a setup that allows fast transfer of a sample to and from the hot zone of the furnace. Solidified alloy-substrate couples were cross sectioned and examined by scanning electron microscopy and energy dispersive X-ray analysis. The studied alloys wet Al pads as the wetting angles, determined after cleaning the flux residue from solidified alloy-substrate couples, are lower than 35 degrees. Wetting angles of Sn-8.7Zn-1.5Ag (wt%) alloy are higher and wetting areas are smaller than those of the remaining alloys, possibly due to a small gap between the melting and test temperatures. There is no clear dependence of the wetting angle and spreading area on time. Microstructures of cross-sectioned samples indicate that alloys dissolve the substrate and penetrate along grain boundaries. Since there is no significant difference between the same solder/Al interfaces at different wetting times, it is assumed that the microstructure evolves in < 5 min.