In situ study on reverse polarity effect in Cu/Sn–9Zn/Ni interconnect undergoing liquid–solid electromigration

•Abnormal reverse polarity effect in Cu/Sn–9Zn/Ni interconnect during L–S EM was observed.•The reverse polarity effect was resulted from directional diffusion of Zn to cathode.•Positive effective charge number is responsible for directional diffusion of Zn atom.•The effective charge number value of Zn was calculated to be +0.63 based on a model.•This effect is beneficial to EM reliability of micro-bump solder interconnect. Synchrotron radiation real-time imaging technology was used to in situ study the interfacial reactions in Cu/Sn–9Zn/Ni solder interconnects undergoing liquid–solid electromigration (L–S EM). The reverse polarity effect, evidenced by the continuous growth of intermetallic compound (IMC) layer at the cathode and the thinning of the IMC layer at the anode, was resulted from the abnormal directional migration of Zn atoms toward the cathode in electric field. This abnormal migration behavior was induced by the positive effective charge number (Z∗) of Zn atoms, which was calculated to be +0.63 based on the Cu fluxes and the consumption kinetics of the anode Cu. Irrespective of the flowing direction of electrons, the consumption of Cu film was obvious while that of Ni film was limited. The dissolution of anode Cu followed a linear relationship with time while that of cathode Cu followed a parabolic relationship with time. It is more damaging with electrons flowing from the Ni to the Cu than that from the Cu to the Ni. The simulated Zn concentration distributions gave an explanation on the relationship between abnormal migration behavior of Zn atoms and the dissolution of Cu film under electron wind force. The abnormal directional migration of Zn atoms toward the cathode prevented the dissolution of cathode substrate, which is beneficial to improve the EM reliability of micro-bump solder interconnects.