Microwave heating, as a fast, efficient and clean heating method, has been widely used in materials processing. In this work, the heating characteristics of solder paste in microwave electric field and magnetic field are analyzed according to the microwave dissipation mechanism of metal powder, and the heating effect of solder paste circuit in microwave electric field and microwave magnetic field is studied. The experimental results show that both microwave electric field and microwave magnetic field can quickly heat the solder paste, but the high intensity microwave electric field is prone to excite the plasma and burn the substrate. Contrarily, microwave magnetic field selectively heats the solder paste to achieve rapid heating and melting of the solder joint while keeping the substrate at a low temperature. By comparing the microstructure of the solder joints melted by rapid microwave field with those melted by conventional heating, it was found that the solder joints melted by rapid microwave field heating have extremely thin intermetallic compound thickness, which is conducive to improving the strength of the solder joints. This study provides an excellent solution for the further development of solder paste welding of flex circuits.