Effect of Bi Segregation on the Asymmetrical Growth of Cu-Sn Intermetallic Compounds in Cu/Sn-58Bi/Cu Sandwich Solder Joints During Isothermal Aging

An asymmetrical interfacial microstructure was observed at both top and bottom interfaces of Cu/Sn-58Bi/Cu solder joints after isothermal aging at 120°C for different times. The asymmetrical interfacial microstructure resulted from asymmetrical Bi segregation, which was attributed to the density difference between Bi and Sn atoms. Bi atoms were driven to the bottom solder/Cu interface by gravity during the liquid soldering procedure since Bi atoms are more massive than Sn atoms. With increasing aging time, Bi accumulated at the bottom Cu3Sn/Cu interface and the Bi segregation enhanced Cu6Sn5 intermetallic compound growth, blocked Sn transport to the Cu3Sn intermetallic compound, and facilitated growth of the Cu6Sn5, based on the measured thicknesses of intermetallic compounds (including Cu6Sn5 and Cu3Sn) at both bottom and top interfaces for Cu/Sn-58Bi/Cu sandwich joints under the same aging process.