Atom motions of copper dissolved in lead-tin alloys

The diffusion and electromigration of copper in lead-tin alloys containing up to 12 at.% Sn have been studied for sample temperatures between 100 and 317°C. The diffusion measurements were made by the standard sequential-sectioning technique, adapted to the 12.8-h half-life of the isotope ${}_{}{}^{64}{}_{}{}^{}\mathrm{Cu}$. The rapid decrease of copper diffusivity with increasing concentration of tin is interpreted as evidence for trapping with a binding energy of about 0.3 eV. Purity of the lead solvent proved to be very important in measuring the low-temperature diffusivity. Although our values of the copper diffusivity in "pure" lead are slightly higher than other reported values, our Arrhenius curves tend to fall at the lowest temperatures, showing the effect of trapping by minute amounts of unwanted impurities. The solubility of copper in the lead and in the lead-tin alloys was determined from those diffusion measurements where the surface concentration exceeded the solubility limit. One feature which is still under investigation is that, at low temperatures, the diffusivity as a function of tin content goes through a minimum and then increases perceptibly. The electromigration results were obtained by the steady-state method. The effective charge number for copper was determined to be about unity, independent of tin concentration. In other words, the electromigration force on the mobile atoms is unchanged by the trapping phenomenon, although, of course, the time to reach equilibrium is prolonged.