The dependence of the Sn grain structure of Pb-free solder joints on composition and geometry

The effects of solder composition, under bump metallurgy and solder joint geometry on Sn grain morphology in Pb free solder joints were examined. SnAgCu solder joints were examined for compositions ranging from Sn, near eutectic SnCu, near eutectic SnAg, to near eutectic SnAgCu. The geometries of solder joints were varied: diameters from 10 to 150 microns, and heights between ten and sixty microns were selected. Solidification temperatures of the samples after various thermal histories were monitored by means of differential scanning calorimetry. Sn grain morphologies were examined by optical microscopy and by scanning electron microscopy and electron backscattered diffraction in order to determine the amount of Sn interlacing. Correlations between the composition, or geometry, and the amount of Sn interlacing are reported. The nucleation rates of Sn from the melt in these Pb solder joints at a number of different temperatures were also measured. An expression for the nucleation rate of Sn in a controlled collapse chip connection solder joint as a function of temperature was thus formulated, providing a predictive capability for Sn solidification temperatures. This study has shown that variations in the temperature of solidification of Sn from the SnAgCu melt result in significant differences in the Sn grain morphology.