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The effects of Bi on the microstructure of Sn-3.5Ag eutectic solder ribbon prepared by rapid solidification was investigated by optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. Bi refines the primary β-Sn phase and eutectic structure. The mechanism was investigated using differential scanning calorimetry and classical nucleation theory, respectively. The results show that the primary β-Sn phase and eutectic structure can be refined by adding a small amount of Bi. The refinement of the primary β-Sn phase is attributed to high compositional undercooling by Bi addition. The refinement of eutectic structure is caused by the reduction of critical nucleation radius and critical nucleation energy due to Bi segregation. With the addition of 0.3 wt.% Bi, the shear strength of solder joints increases by 12.15%.

Two kinds of Sn-3.5Ag eutectic solder ribbons of 0.13 mm thickness were prepared by a casting–rolling process and a rapid solidification process. The microstructure, phase constitution, melting characteristics, wetting behavior and soldering strength were compared using optical microscopy, scanning electron microscopy, x-ray diffraction, energy dispersive spectroscopy, differential scanning calorimetry and a MTS ceramic testing system. The results show that the microstructure of rapidly solidified solder is finer and more uniform, and the eutectic structure has a higher solid solubility and more homogeneous distribution of Ag in a Sn matrix. The solidus and liquidus temperature decreased, resulting in a 3.3% reduction of pasty range. In addition, the wettability and shear strength of the solder joints increased by 13.2% and 7.9%, respectively.