Synchrotron radiation in situ study on the solidification of Cu/Sn-58Bi/Cu solder joint under temperature gradient

Synchrotron radiation real-time imaging technology was conducted to in situ investigate the thermomigration and solidification behavior of Cu/Sn-58Bi/Cu solder joint during reflow under temperature gradient. The high concentration of Bi in solder retarded the thermomigration of Cu atoms and the interfacial reaction of the solder joint. Both the growth of the interfacial intermetallic compounds (IMCs) and the dissolution of the Cu substrates in the solder joints were inconspicuous. During the solidification, Bi-rich phase nucleated in the bulk solder and grew fast in the shape of triangular or quadrangular prism. Subsequently, the Sn-rich phase nucleated and dendritically grew in a high speed. Finally, the biphase separation in the bulk solder was clearly observed. The growth mechanism of both Bi-rich and Sn-rich grain under temperature gradient was discussed.