Electromigration study in the eutectic SnBi solder joint on the Ni/Au metallization

Microstructural evolution and interfacial reaction in the eutectic SnBi solder joint on the Ni/Au metallization with and without the current stressing of 6.5 × 10³ A/cm² at 70 °C for 5 to 15 days were investigated. Electromigration is found to have significant effects not only on the phase formation at the joint interface but also on the phase coarsening and mass accumulation of Bi in the solder. In the solder joint without the current stressing, only a thin Ni₃Sn₄ phase was formed at the joint interface. For the solder joint with the current stressing, in addition to the Ni₃Sn₄ phase, a thick Au–Ni–Bi–Sn phase was formed at the joint interface after 10 days. However, the Au–Ni–Bi–Sn phase was not observed at the anode-side joint interface after 15 days of current stressing. Coarsening of the Bi-rich grain in the solder joint with the current stressing was much faster than that without the current stressing. Mass accumulation of Bi was observed at the anode side of the solder joint with the current stressing and the thickness of the Bi-rich accumulation layer increased with current stressing time. Based on the accumulation rate of the Bi-rich layer and the growth rate of the Bi-rich grain, the product of diffusivity and effective charge number of Bi in the eutectic SnBi solder is calculated to be 4.72 × 10⁻¹⁰ cm²/s.