HASL

What it is

HASL (Hot Air Solder Levelling) is one of the most common and traditional surface finishes for the exposed copper pads on a PCB. Bare copper oxidises over time, which makes reliable soldering progressively more difficult; HASL protects the exposed copper with a coating of solder that provides good solderability and shelf life when the boards are stored appropriately.

How it's made

After the copper is etched and the solder mask applied, the exposed copper is coated with molten solder — the traditional process dips the whole panel into a solder bath, though modern lines can also spray or curtain- apply the alloy. Hot-air knives then blow the excess solder off, leaving a coating on every exposed pad. That's the “hot air levelling” step. The resulting coating's thickness and profile vary with pad geometry and process conditions.

Our boards use lead-free HASL, typically based on tin/copper or tin/silver/copper alloys rather than the traditional leaded tin/lead. This avoids the restricted lead used in traditional HASL and is suitable for RoHS-compliant products (RoHS itself doesn't mandate any particular finish or alloy; lead-free HASL is one of several compliant options).

What it looks like

A silvery finish that is typically somewhat uneven or rounded compared with very flat finishes like ENIG or OSP. Under magnification you can usually see the meniscus at the edge of each pad. Exact profile depends on pad size and process conditions.

How it solders

Very easily — the pad is already coated with a solderable alloy, which generally makes initial wetting straightforward. Iron soldering, reflow, and rework all work as expected. For through-hole headers the solder-coated pad provides an immediately solderable surface around the plated hole and works particularly well with hand soldering.

Why boards sometimes ship OSP instead

We use more than one fab, and different production batches sometimes come off the line with OSP rather than HASL. Both finishes are lead-free, both are suitable for RoHS-compliant products, and both are functionally equivalent for the breakouts we make — you can hand-solder headers into either finish the same way. The visible difference is largely cosmetic: OSP usually retains the copper's reddish/pink colour because the protective coating is extremely thin, while HASL looks silvery.