THT parts are usually welded by means of Wave Soldering. This allows all parts to be welded on PCB at one time. First, cut the foot closer to the board and bend slightly so that the parts can be fixed. The PCB is then moved to the water wave of the cosolvent, allowing the bottom to contact the cosolvent, so that the oxide on the bottom metal can be removed. After heating the PCB, this time it is moved to the molten solder and welded to the bottom.
The method of automatic welding of SMT parts is called reflow Soldering (Over Reflow welding). A paste weld containing a solvent and solder, which is first treated once the component is mounted on the PCB, and then processed after PCB heating. After PCB cools, the soldering is done, and then the final test of the PCB is ready.