The cross-interaction between Sn/Cu and Sn/Au interfacial reactions in an Au/Sn/Cu sandwich structure was studied. Field-emission electron probe microanalysis (FE-EPMA) revealed that the Cu content in the three Au-Sn phases (AuSn, AuSn2, and AuSn4) was very low, less than 1 at.%. This meansa that pound Cu from the opposite Cu foil did not participate in the interfacial reaction at the Sn/Au interface. On the opposite Sn/Cu side, Au-substituted (Cu,Au)(6)Sn-5 formed within the initial 1 min of reflow. With prolonged reflow, the Au content in the Au-substituted (Cu,Au)(6)Sn-5 increased and it transformed into a Cu-substituted (Au,Cu)Sn phase with 25 at.% Cu after 1 min of reflow at 250A degrees C. The x-ray diffraction (XRD) pattern confirmed the phase transformation of Au-substituted (Cu,Au)(6)Sn-5 to Cu-substituted (Au,Cu)Sn phase. In addition, there was greater Au consumption in the Au/Sn/Cu sandwich joint structure than in the single Au/Sn reaction case, due to some of the Au participating in the opposite Sn/Cu interfacial reaction.