HAL stands for Hot Air (Solder) Leveling (German: Heißluftverzinnung). Within printed circuit board manufacturing, HAL is the name of the procedure as well as the resulting surface.
During the HAL procedure, the cleaned boards are immersed in a fluxer and afterwards immersed vertically into liquid solder / tin. After a defined period, the circuit boards are pulled out while simultaneously blown clear of excess solder; the holes are blown clear in the most part.
The resulting HAL surface (tin coating) protects the exposed copper (not covered by solder-stop) from oxidation during storage and processing. The surface is very suitable for soldering but not for fine pitch: The liquid tin forms "drops" on the surface and drifts on edges of holes and pads. The surface on the edges of the circuit boards is very thin, low pad spacing may result in solder bridges, which can lead to soldering problems.
The PCB surfaces immersion gold (ENIG) and ENEPIG which are very flat and thus very suitable for SMD surfaces, have established as an alternative for (fine pitch) SMD (see also HAL vs. immersion gold).
Most HAL facilities use lead-free HAL today, because of the RoHS directive. Previously, and in some industries lead-tin (SnPb) is still in use.