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 O 2 (1 Torr), H 2 O (10 Torr), 100 ppm of NO 2 in N 2 (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 O 2 (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 H 2 O vapor for 5 min resulted in surface enrichment of Sn with the formation of Sn and Pb oxides. The results obtained with dry NO 2 were similar to those with O 2 ; 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 O 2 except the oxidation of Pb was suppressed. No nitrogen species were observed. Solder exposed to at 100 ppm of NO 2 in N 2 50% relative humidity gave similar results to O 2 and dry NO 2 . The Pb/Sn maximum occurred at a reaction time approximately 2 to 3 times that of O 2 and dry NO 2 (40 to 60 min). Two nitrogen species were observed for solder treated with 100 ppm of NO 2 in N 2 at 50% relative humidity; binding energies were ∼404 and 400 eV. These have been attributed to NO 2 − and NO − , respectively. Mechanisms are proposed for reaction of solder with O 2 , dry NO 2 , and NO 2 at 50% relative humidity.