Controlling the Sn grain morphology of SnAg C4 solder bumps

The effects of solder composition, under bump metallurgy (UBM), and solder joint geometry on Sn grain morphology in Pb free, C4 solder joints were examined. The solder joints examined had compositions of either Sn-1%Ag or Sn-2.4%Ag, and were on either Cu or Ni pads. The geometries of the solder joints were varied: diameters ranged from 11 to 100 microns and heights from 11 to 30 microns. To study the effects of aspect ratio on the Sn grain morphology, pairs of microbumps with similar pad diameters, solder height and composition were bonded together. Samples were reflowed by means of differential scanning calorimetry (DSC); thus the solidification temperatures were carefully monitored. Sn grain morphologies were characterized by optical microscopy with crossed polarizers (XP) and by scanning electron microscopy with secondary electrons (SE) and an electron backscattered diffraction detector (EBSD). Sn-2.4%Ag solder bumps exhibited smaller Sn grains than Sn-1%Ag alloys. The nucleation rates of Sn from the melt in Pb free solder joints at a number of different temperatures were also measured. An expression for the nucleation of Sn in Sn-2.4%Ag and Sn-1%Ag solder joints on Cu or Ni pads was formulated, providing a predictive capability for Sn solidification temperatures. This study has shown that variation in the temperature of solidification of Sn from the melt results in significant differences in Sn grain morphology.