Interfacial Behaviors in Cu/Molten Sn-58Bi/Cu Solder Joints Under Coupling with Thermal and Current Stressing

The interfacial behaviors of Cu/molten Sn-58Bi/Cu solder joints under the coupling effect of a temperature gradient and the current stressing have been investigated. The most obvious change of the interfacial behaviors under the individual electromigration (EM) and thermomigration (TM) in molten solder was the asymmetrical growth of interfacial Cu-Sn intermetallic compounds (IMCs), which grew rapidly as the stressing time prolonged. The growth rates of the interfacial IMCs induced by TM under a temperature gradient of 40 degrees C/cm were slightly faster than EM under the current density of 0.5x10(4)A/cm(2). However, the microstructure evolution and interfacial behaviors changed obviously when the thermal wasdistributed unevenly across the entire solder stripe under current stressing. It was found that there was a Bi-rich layer adhered to the anode side and a distributed Cu6Sn5 phase existed in the solder matrix when the anode and the cold end were at the same end. Additionally, a large number of Bi-blocks and Cu-Sn IMCs were dramatically observed in the solder matrix when the anode and the hot end were on the same side. The main reason for this result may be attributed to the significant change of the diffusion of Bi atoms under the thermo-electric coupling conditions. In the initial melting stage of solder, the Bi atoms in molten Sn-58Bi solder rapidly diffused to the anode and then began tobe reversely dissolved into the solder matrix. The experimental results proved the additional temperature gradient played a positive or negative role on the reverse dissolution of Bi atoms in the heat preservation process. [GRAPHICS] .