Barrel plating is a processing method that applies solder plating to a large number of small parts within a rotating barrel (a barrel-shaped container). By agitating the parts and immersing them in the plating solution, a solder layer is formed efficiently and economically on minute parts that are difficult to handle individually. This method is indispensable for the mass production of parts requiring electrical connection, such as lead frames, connector pins, and terminals of electronic components. The main objectives are to prevent oxidation of parts, improve solderability, and ensure long-term electrical connection reliability. In recent years, due to compliance with the RoHS directive, the application of lead-free solder plating has become mainstream, and the optimization of plating solution composition and process control has become a critical issue.