Effects of electromigration on the Kirkendall void and intermetallic compounds growths in lead-free solder joints

Abstract

Kirkendall voids are formed at the interface between and solder and electroplated Cu under the isothermal aging and current stressing condition. Void evolution at the solder and copper joints is critical reliability issue in the solder interconnect under electromigration. Effects of the electric current flow on the Kirkendall void formation at solder joints were investigated using Sn-3.5Ag/Cu joints specially designed to have localized nucleation of Kirkendall voids at the Cu3Sn/Cu interface by S segregation. Under the current density of 1×104 A/cm2, kinetics of Kirkendall void growth and intermetallic compound (IMC) thickening were affected by the electromigration (EM), and both showed the polarity effect. Cu6Sn5 showed a strong susceptibility to the polarity effect, while Cu3Sn did not. The electromigration force induced additional tensile (or compressive) stress at the cathode (or anode), which accelerated (or decelerated) the void growth. From the measurements of the fraction of void at the Cu3Sn/Cu interface on SEM micrographs and analysis of the kinetics of void growth, the magnitude of the local stress induced by EM was estimated to be 9 MPa at the anode and -7 Mpa at the cathode, respectively. And the tensile stress imposed by EM was estimated to be 6 MPa at the anode side, and compressive stress was estimated to be -2 MPa at the cathode side from the flux continuity. Finally, an elastic/plastic analysis of the IMC/solder joint was provided to estimate the tensile stress generation at the Cu3Sn/Cu interface by vacancy annihilation. Furthermore, interfacial reactions between Sn-3.5Ag solder and the electroplated Cu using PEG additive and chlorine anion in the bath were investigated under high current condition. During subsequent isothermal aging at 150℃, there are no voids at Cu3Sn/Cu interface when Cu is electroplated with PEG, chlorine ion alone. In the case of adding chlorine alone in the bath, Kirkendall voids were formed in the Cu3Sn IMC layer. ...