OSP

What it is

OSP (Organic Solderability Preservative) is a surface finish that protects the exposed copper pads on a PCB with a very thin organic film. Unlike HASL it doesn't add any metal to the pad — the copper stays underneath a passivation layer that keeps it wettable long enough to be soldered.

How it's made

After the board is etched and the solder mask applied, the exposed copper is chemically cleaned and then dipped in an OSP bath. Molecules in the bath bond to the copper and build up a film that is typically only a few tenths of a micron thick (around 0.2 – 0.5 μm for common formulations; exact thickness depends on the formulation and process). Thin enough to be invisible to the eye except for a slight colour tint.

The film protects the copper from oxidation and other environmental reactions during storage. When the pad is heated during soldering, the OSP decomposes or is displaced by the soldering process, exposing the underlying copper for the solder to wet.

What it looks like

Flat and pinkish. OSP adds essentially negligible thickness, so the copper pads remain very flat compared with finishes like HASL — one of the reasons OSP is popular for fine-pitch surface-mount work.

How it solders

Very well — iron soldering, reflow, and rework all work as expected. Its flat surface is particularly well suited to fine-pitch surface-mount assembly.

The main practical limitation is shelf life. OSP has a more limited storage life than finishes like ENIG, and its solderability gradually deteriorates with time and with exposure to heat, humidity and contamination. The actual shelf life depends strongly on the particular OSP formulation, packaging, and storage conditions.

Unlike ENIG, OSP doesn't provide a durable metallic surface. The coating is intended primarily to preserve solderability during storage — it's not suitable for applications requiring abrasion resistance, repeated mechanical contact, or exposed contact surfaces.

Why boards sometimes ship HASL instead

We use more than one fab, and different production batches sometimes come off the line with HASL rather than OSP. Both can be used as lead-free, RoHS-compliant finishes, 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 retains the copper's reddish/pink appearance because the protective coating is extremely thin, while HASL looks silvery.