The HASL process tries to produce a flat thin solder surface by applying flux and liquid solder to the board, and then subjecting the surface to hot air forced through ‘air knives’, so that surplus solder is blown back into the solder pot.
Two types of equipment are available: in the vertical process, the solder on the pad drains to the bottom during the air knife operation; in the horizontal process, the board lies flat, so there is less tendency for the coating to be wedge-shaped in section.
The flatness is affected by the pressure and temperature of the air knives, and by the speed at which the board is drawn passed them. There is a balance between achievable flatness and the thickness of the deposit, overly thin deposits resulting in poor solderability, especially if the boards are stored before use. However, if insufficient solder is removed by the air knives, the profile will be unacceptably domed.
The thickness also depends on geometric factors and the position of the surface. For example, small pads will tend to be more domed than large ones, and large pad areas may have insufficient solder. The thickness of the deposit also depends on the relative thickness and proximity of adjacent solder mask. For through-holes, the amount of solder depends critically on air knife pressure, and too much pressure may expose the intermetallic at the ‘knee’ between hole and wall, which impairs solderability.