High-temperature oxidation of a Sn-Zn-Al solder

The oxidation of 91Sn-9(95Zn-5Al) solder in the liquid state, 250 degrees C, was studied by thermal gravimetric analysis (TGA). The oxidation behavior of 63Sn-37Pb, 91Sn-9Zn, 99.4Sn-0.6Al, and Sn was also investigated for comparison. The weight gains per unit surface 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, while parabolic behavior exists after the linear stage. The rate constants of the oxidation reaction for the two reaction stages were determined. Activation energies for oxidation of the five materials were determined in the range of 250 to 400 degrees C. The activation energies, derived from the linear rate constants for the early stages of oxidation, are 27.7 kJ/mole for 99.4Sn-0.6Al, 23.3 kJ/mole for 91Sn-9Zn, 21.4 kJ/mole for 91Sn-9(95Zn-5Al), 20.5 kJ/mole for 63Sn-37Pb, and 19.8 kJ/mole for Sn. The surface-oxidation behavior was investigated further with electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES). AES profiles showed that oxides of Zn and Al formed on 91Sn-9Zn and 91Sn-9(95Zn-5Al) solders, while tin oxide is formed on 63Sn-37Pb solder.