Diffusion and electromigration of silver and nickel in lead-tin alloys

Abstract

Measurements of diffusion and electromigration have been carried out for $^{110m}\mathrm{Ag}$ and $^{63}\mathrm{Ni}$ in lead-tin solders in the $\ensuremath{\alpha}$-solubility range which runs from 0 to 12 at.% tin, $150\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}<T<300\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. For silver the diffusivity is somewhat enhanced by the addition of the tin, the effect being more marked at the lower temperatures. The increase amounts to a factor of 2.25 for a 12 at.% tin alloy at $T=150\ifmmode^\circ\else\textdegree\fi{}$C. The electromigration ${Z}^{*}$ decreases in magnitude with the addition of the tin and actually changes sign from plus to minus around 11 at.% tin. On the other hand, the behavior of the nickel is marked by very strong trapping, as the addition of only 500 ppm of tin can greatly decrease the nickel mobility. The application of the standard trapping analysis leads to a binding energy of the order of 1 eV and a surprisingly large negative entropy. Insofar as it has been possible to observe, the presence of the tin does not seem to alter the electromigration ${Z}^{*}$ for the nickel.