A Study of the Interaction of Pb-Sn Solder with O 2,H 2 O,and NO 2 by X-Ray Photoelectron (ESCA) and Auger (AES) Spectroscopy

An ESCA and AES study of the corrosion of 1:1 Pb-Sn solder is reported. The solder was reacted at room temperature with O₂ (1 Torr), H₂O (10 Torr), 100 ppm of NO₂ in N₂ (1 atm), and the latter at 50% relative humidity. Pb and Sn foils were examined under similar conditions. The Pb/Sn ratios observed by ESCA and AES for unmelted solder are close to those expected from bulk composition. After melting, a 50% decrease in Pb/Sn was obtained by ESCA; no AES signal for Pb was observed. Before melting, 90% of the Pb and 100% of the Sn were oxidized; after melting 18% Pb and 70% Sn were oxidized. Exposure of solder to O₂ (1 Torr) resulted in a Pb/Sn ratio which decreased as a function of time to a minimum value, rapidly increased through a maximum around 60 min and then decreased to an equilibrium value. The O/(Pb + Sn) ratio increased rapidly to an equilibrium value. The interaction of solder with 10 Torr of H₂O vapor for 5 min resulted in surface enrichment of Sn with the formation of Sn and Pb oxides. The results obtained with dry NO₂ were similar to those with O₂; Pb/Sn decreased rapidly to a minimum and went through a small maximum to an equilibrium value. The distribution of species determined by ESCA before and after the reaction were essentially the same as for O₂ except the oxidation of Pb was suppressed. No nitrogen species were observed. Solder exposed to at 100 ppm of NO₂ in N₂ 50% relative humidity gave similar results to O₂ and dry NO₂. The Pb/Sn maximum occurred at a reaction time approximately 2 to 3 times that of O₂ and dry NO₂ (40 to 60 min). Two nitrogen species were observed for solder treated with 100 ppm of NO₂ in N₂ at 50% relative humidity; binding energies were ∼404 and 400 eV. These have been attributed to NO₂⁻ and NO⁻, respectively. Mechanisms are proposed for reaction of solder with O₂, dry NO₂, and NO₂ at 50% relative humidity.