Sn Corrosion and Its Influence on Whisker Growth

The microstructure and crystal structure of condensation-induced corrosion products, vapor phase induced oxidation products, Cu-Sn intermetallics, and Sn whiskers that formed on electroplated matte Sn on Cu-alloy after exposure 2500 h in a 60 degC/93%RH ambient were characterized with scanning electron microscopy, (SEM), focused ion beam (FIB) microscopy, energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and selected area electron diffraction (SAD). The corrosion product was identified as crystalline SnO₂. The oxidation of Sn in condensed water was at least four orders of magnitude larger than that in moist vapor at 60 degC. All Sn whiskers were found to be within 125 mum of the corrosion product. Based on these observations, a theory was developed. The theory assumes that oxidation leads to the displacement of Sn atoms within the film. Because the grain boundaries and free surfaces of the film are pinned, the oxidation-induced excess Sn atoms are constrained within the original volume of the Sn-film. The trapped excess Sn atoms create localized stress, excess strain energy, in the Sn-film. If and when the pinning constraint is relaxed, as for example would occur when the surface oxide on the film cracks, then the Sn atoms can diffuse to lower energy configurations. When this occurs, whisker nucleation and growth begins. The theory was tested by detailed measurements and comparison of the corrosion volume and the whisker volume in two different samples. The volume comparisons were consistent with the theory