High-Temperature Oxidation of a Sn-Zn-Al Solder

The oxidation of 91Sn-9(95Zn-5Al) solder in theliquid state, 250°C, was studied by thermalgravimetric analysis (TGA). The oxidation behavior of63Sn-37Pb, 91Sn-9Zn, 99.4Sn-0.6Al, and Sn was alsoinvestigated for comparison. The weight gains per unitsurface area descend in the order: 63Sn-37Pb > Sn> 91Sn-9Zn > 91Sn-9(95Zn-5Al) > 99.4Sn-0.6Al.The initial weight gains of the materials investigated increase linearly with reaction time, whileparabolic behavior exists after the linear stage. Therate constants of the oxidation reaction for the tworeaction stages were determined. Activation energies for oxidation of the five materials weredetermined in the range of 250 to 400°C. Theactivation energies, derived from the linear rateconstants for the early stages of oxidation, are 27.7kJ/mole for 99.4Sn-0.6Al, 23.3 kJ/mole for 91Sn-9Zn, 21.4kJ/mole for 91Sn-9(95Zn-5Al), 20.5 kJ/mole for63Sn-37Pb, and 19.8 kJ/mole for Sn. Thesurface-oxidation behavior was investigated further withelectron spectroscopy for chemical analysis (ESCA) and Auger electronspectroscopy (AES). AES profiles showed that oxides ofZn and Al formed on 91Sn-9Zn and 91Sn-9(95Zn-5Al)solders, while tin oxide is formed on 63Sn-37Pbsolder.