| The classical theory of nucleation provides a fairly robust description of the nucleation behavior of liquids below their melting point. This model is based upon the assumption that the nucleus that forms in the undercooled liquid has the equilibrium structure, but recent experimental and molecular dynamic simulation results indicate that this view is too simplistic. Thus, a systematic experimental study of the nucleation and growth of Sn nanostructures in undercooled Sn and Sn alloy samples was conducted.;Nucleation rates in undercooled Sn were found to monotonically increase with the concentration of impurity atoms (Co, Ni, Cu, or Ag). Results of these studies were applied to improve the reliability of Sn-Ag-Cu solder joints in microelectronic packages. Typical Pb-free solder alloys are composed of over 95% Sn, thus the nucleation and growth of Sn plays a large role in determining the reliability of Pb-free solder joints. The effects of alloy composition, volume, and pad metallization on the isothermal nucleation rate, solidification temperature and, Sn grain morphology of near eutectic Sn-Ag C4 solder bumps were examined. It was found that Sn-2.4Ag alloys on Ni pads have relatively low solidification temperatures. The appearance of a particular Sn grain morphology (interlaced Sn, shown elsewhere to have much higher resistance to electromigration degradation) was strongly correlated with solidification temperature. |
