HASL was a process to apply solder to the exposed copper on the boards, especially the pad areas and plated through holes. Prior to HASL, hot air leveling, there was a process called electroplated tin/lead which was used as an etch resist during the board fabrication process.
Once the board was etched the electroplated tin/lead coating was fused, or stated differently was reflowed. It was after this point in the process when the soldermask was applied to the board. This was the standard board process for plated through hole technology. When surface mount came about in the early 1980s and these boards were used, it was the first time that the entire board was subjected to the surface mount reflow processes of the time, which could have been anything from IR ovens, vapor phase systems, Convection ovens, etc..
It was also discovered that the solder expanded under molten conditions and the when the second side of the product was prepared for surface mount component assembly, solder balls were discovered all over the surfaces of the solder mask, especially over any of the conductor traces. It was at this time that the board fabrication process was changed to remove the tin/lead etch resist materials and leave the board simply as a copper board.
The soldermask was then applied over the bare copper, thereby eliminating the problems with solder balls migrating through the soldermask and interfering with the past deposition process on the second side of which was being assembled. After the soldermask was applied the board was then processed through a HOT Air Solder Leveling process coating all the exposed copper with solder, which became known as SMOBC, Soldermask over bard copper.