By studying reactions between Au foils and Sn(Cu) alloys, we found that the Au consumption rate depended on the Cu-content of the Sn(Cu) solders. The higher Cu-content alloys had faster Au consumption rates. When the Au foil was pre-coated with a Ni layer and then reacted with Sn(Cu) alloys having a Cu-content of more then 1.5 wt%, a stable ternary (Cu,Ni)6Sn5 compound layer was observed on the Au foil. This ternary compound layer then served as a barrier layer that effectively prevented the Au foil from reacting with the molten solder. This result enabled the implementation of a flip-chip assembly process for the fabrication of optoelectronic devices.