Abnormal polarity effect of electromigration on intermetallic compound formation in Sn–9Zn solder interconnect

Interfacial reactions in a eutectic Sn–Zn solder/Cu interconnect were investigated under electric current stressing. After applying a constant direct current at 4.26 × 104 A cm−2 for 166 h, an abnormal “polarity effect” was found where the intermetallic compound layer at the cathode was about 2.3 μm thicker than that at the anode. Upon increasing the temperature to about 185 °C, Cu–Zn compounds were replaced by Cu–Sn compounds at the anode and by a mixture of Cu–Zn and Cu–Sn compounds at the cathode.