HASL process control

Your explanation should ideally start with an acknowledgement of how difficult the HASL process is to control! The general thickness and visual quality of the deposit will depend on the type of equipment used (vertical or horizontal) and on the settings of solder pot temperature, air knife temperature and pressure, and withdrawal speed. All of these can be controlled, but may need to be set differently for different designs.

Another factor affecting the evenness of deposit is the preheat setting – both solder pickup and drainage are adversely affected if the board arrives at the solder pot insufficiently heated.
There is a balance between having too much solder and having too little, which is difficult to control, with the result that HASL holes will be reduced in diameter (acceptable up to 75µm) but may be plugged (not acceptable).

Things for your consumer inspector to look out for include:

signs of delamination, particularly prevalent with thicker boards
evidence of residues – the fluxes used for HASL are relatively aggressive
exposed copper, due to non-wetting, is an indicator of poor pre-clean procedures
the solder should look shiny – a grainy appearance may be due to copper contamination in the solder pot.