Refinement Mechanism of Bi Addition on the Microstructure of Rapidly Solidified Sn-3.5Ag Eutectic Solder for Microelectronic Packaging

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%.