In Situ Observation of Electromigration-Induced Anomalous Precipitation of Ag 3 Sn Phase in Ag-Containing Solder Joints

The anomalous precipitation behavior of Ag3Sn phase in Sn-3.5Ag lead-free solder joints during liquid–solid electromigration at 1.0 × 104 A/cm2 has been observed using a real-time imaging method with synchrotron radiation. Large Ag3Sn phases precipitated at the anode instead of cathode side in the Ag-containing solder joints during cooling, due to directional diffusion of Ag atoms towards the anode under current stressing. The electromigration-induced directional diffusion behavior of Ag atoms can mainly be attributed to their negative effective charge number (Z Ag * ) of −0.48. Electromigration induced a localized Ag-rich area and thereby a large number of Ag3Sn-based clusters at the anode. This resulted in preferential nucleation and anomalous precipitation of Ag3Sn phases at the anode side in Ag-containing solder joints. The rapid growth of the Ag3Sn plates can be explained in terms of the net atomic flux driven by the electromigration (Jem) and chemical potential (Jchem) effects.