Resistance spot welding (RSW) is a sheet metal welding process with broad applications, known to be more suitable for low-conductive materials, such as steels, due to concentrated and steady-state heat generation and retention at the metal interface. However, for high conductive metals such as copper, conventional welding processes in resistance spot welding has not been successful. This paper provides a comparative study of resistance welding among steel, aluminum and copper through mechanistic analyses, i.e., analytical solutions calibrated by finite element analyses. It is found when lower conductivity metals, such as steels, are welded, the applied energy can be more concentrated on the interfaces, and the heat dissipation is relatively slow, so that a close to steady-state welding condition can be reached that provides a wide and robust operation window. For welding highly conductive metals having similar melting temperature as that of electrode, the process window becomes much narrower or does not always exist without additional conditioning of materials, design or the welding processes. The physics of RSW process is analyzed based on energy equilibrium, and a new concept of pulse welding process is proposed as a required operation mode for welding copper during temperature ramping up period and prior to electrode melting. A new type of welding limit diagram (WLD) is constructed that contains three welding limit curves (WLC) for nugget formation, and the transient region. The newly constructed WLD allows a clear distinction between welding low- and high-conductive metals, and provides new understanding and a theoretical guidance for widening the weldability window.