A Study of the Interaction of Pb-Sn Solder with O2, H2O, and NO2 by X-Ray Photoelectron (ESCA) and Auger (AES) Spectroscopy

Abstract

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.