Microstructural evolution of the Au–20 wt.% Sn solder on the Cu substrate during reflow

Reflow reactions of the Au–20 wt.% Sn (Au20Sn) solder on the Cu substrate at 330 °C were conducted and the microstructure of as-solidified solder joint was examined. After reflow for 1 min, the as-solidified solder matrix presented a typical eutectic ζ-(Au,Cu)5Sn + δ-(Au,Cu)Sn lamellar microstructure and two phases, ζ-(Au,Cu)5Sn and AuCu, were formed at the solder/Cu interface. The ζ-(Au,Cu)5Sn phase grew very irregularly at the interface. Upon increasing the reflow time, part of the fine lamellar microstructure coarsened in the as-solidified solder matrix, and therefore the solder matrix displayed a composite microstructure of fine lamellar eutectic and coarse eutectic. In addition, dendritic growth of the ζ-(Au,Cu)5Sn phase was observed and the ζ dendrites were found to grow at a faster rate in the solder matrix of smaller volume. The microstructural evolution at the Au20Sn/Cu solder joint during reflow, including irregular growth of ζ-(Au,Cu)5Sn at the interface, dendritic growth of ζ-(Au,Cu)5Sn in the solder matrix, and development of eutectic microstructure, was explained using related phase diagram and vertical section.