HASL (Hot air Leveling):

Hot Air Solder Leveling (HASL) is one of the most common types of PCB surface finishes used in the industry today. HASL finishes are composed of solder, with proportions of approximately 63% tin and 37% lead, This finish is applied by first dipping the circuit board into a molten pot of the tin/lead alloy after the Solder Mask has been applied. Next, a Hot Air Leveler (HAL) removes the excess solder, using hot air knives to leave behind only the thinnest possible layer. This remaining layer of solder protects the traces underneath from corrosion, while easing the task of soldering components to the board during the PCB Assembly Process by pre-tinning the pads on the board. HASL is a very cost-effective surface finish compared to other types of finishes, and thus is considered a great choice for general purpose boards.