Materials
HASL
In plain English
A cheap way to coat the copper pads by dipping the board in melted solder and blowing off the extra with hot air — leaving a slightly bumpy but ready-to-solder surface.
The technical version
HASL (Hot Air Solder Leveling) coats exposed copper by dipping the board in molten solder and leveling it with hot air. It is inexpensive and forgiving but produces an uneven surface unsuitable for fine-pitch parts. Lead-free HASL is the RoHS-compliant variant using tin-based alloys.