HASL

Hot Air Solder Leveling (HASL) is a PCB surface finish in which copper pads are coated with solder by immersion in a molten solder bath and then leveled with directed hot air to produce a flat, solder-covered surface. The process typically involves cleaning, flux application, immersion in solder (lead-containing or lead-free alloys), followed by hot-air leveling knives that sweep across the board to remove excess solder and improve surface planarity before cooling.

HASL provides a robust, solderable finish with good wetting and low cost. It is compatible with conventional wave soldering and offers decent solder joint reliability for many through-hole and mixed-technology boards. Leaded HASL historically used Sn-Pb solders, while lead-free HASL uses SAC or similar alloys. The resulting copper pads are coated with solder, which serves as a protective barrier against oxidation and as a solderable surface.

Drawbacks include uneven surface planarity, relatively thick solder deposits on pads and around vias, and poorer suitability for very fine-pitch components or high-density interconnections. The finish can introduce warpage in larger boards and may complicate stencil design for solder paste. Lead-containing HASL is restricted in RoHS environments, leading many manufacturers to prefer lead-free HASL or alternative finishes such as ENIG, ENEPIG, or OSP, depending on high-reliability and density requirements.

HASL remains a common option for low-cost, rapid-turnaround boards, prototyping, and applications where its advantages outweigh planar and density limitations.