Hot Air Solder Level (HASL) is a surface finish process involving the application of a thin layer of solder onto the exposed copper pads of a printed circuit board. HASL begins by applying a solder mask to the board, which is then immersed into a bath of molten solder. The molten solder adheres to the available copper surfaces on the board. After removing the board from the solder bath, any excess solder is removed by blasts of hot air knives, resulting in a thin, uniform layer of solder bonded to the unmasked copper areas.