The heat un-balance and the immiscibility of the Cu and Fe alloy system are the key challenges during Cu/Fe resistance spot welding. Utilising electrodes of different geometries enabled the formation of a mixed Cu-Fe fusion zone. It is identified that despite the immiscibility of Cu and Fe in the solid-state, metastable liquid phase separation enables a metallurgical joining between Cu and Fe. The liquid phase spinodal decomposition led to the formation of an ultra-fine interconnected network of Cu-rich and Fe-rich phases in the fusion zone. The higher hardness of the fusion zone compared to the base metals, the susceptibility to solidification cracking, and the electrode indentation in the Cu sheet govern the mechanical properties of the Cu/Fe resistance spot welds.