OSP Surface Finish In PCB Assembly

An organic solderability preservative, usually shortened to OSP, is not a metal. It is a thin organic film that is applied to the copper surface and that protects it from oxidation until the board is soldered. Because there is no metal layer, the pad geometry is unchanged and the surface remains flat, and the finish is the cheapest of the common options.

This article explains how the film works, what its limits are in storage and through multiple reflows, and where it is the appropriate choice.

What The Finish Is

The film is formed by dipping the board in a solution that contains a molecule with a head group that bonds to copper and a tail that repels water and oxygen. The molecules arrange themselves into a thin, dense layer on the copper, typically a few tens of nanometres thick, and the layer is what keeps the copper from oxidising.

Because the layer is organic, it is consumed during the soldering process. The flux dissolves it and the copper is exposed to the solder at the moment when the solder is molten, which is the ideal point in the process. There is no metal to dissolve into the solder and no intermetallic to grow, which is why the joint chemistry is simply copper and tin.

How It Protects The Copper

The protection depends on the film being continuous and on the copper beneath it being clean and uniform. A copper surface that has been roughened by an aggressive etch, or that is contaminated with an organic residue from the plating, produces a film that is less effective. The preparation before the OSP step is therefore as important as the step itself.

The film is thin enough that it does not affect the appearance of the pad or the profile of the surface, which is one of its advantages for fine pitch work. It is also thin enough that it is easily damaged, and a board that has been handled without gloves or that has been rubbed carries fingermarks that partially remove the film and leave the copper exposed.

Shelf Life And Handling

The shelf life is shorter than that of a metal finish, because the organic layer slowly degrades and because copper can oxidise through a damaged area. The period quoted by the fabricator is typically measured in months under a defined storage condition, and it depends on the packing and the humidity as much as on the film itself.

Handling is the other variable. The film is removed by abrasion, so the board should be handled by the edges and the pads should not be touched. Where a board must be probed during test, the probe marks break the film, and those areas may need to be reworked or accepted as a reduced solderability area. The practices are similar to those that apply to any finish, and the general handling and protection measures are described under board level protection.

<img src="https://www.gopcba.com/wp-content/uploads/2025/08/16-1.png" alt="Board surface after reflow with the finish consumed at the pads” />

Compatibility With Flux And Paste

The film is designed to be removed by the flux that is used in the assembly. A flux that is not active enough may fail to remove it completely, and the residue between the copper and the solder interferes with the wetting. A flux that is too aggressive removes the film but also leaves more residue on the board.

The compatibility is therefore a matter of matching the flux to the finish rather than of choosing the best flux. A no clean paste designed for an OSP finish is formulated to dissolve the film at the preheat temperature, and using a paste intended for a metal finish may give a narrower process window. The paste supplier should be told which finish the boards carry, because the formulation differs.

Assembly Behaviour And Multiple Reflows

The film is consumed at the first reflow, so a board that is processed twice has bare copper at the pads for the second pass. Copper oxidises during the first reflow, and the oxide is harder for the flux to remove than the original film. The second reflow therefore has a narrower window, and the wetting is usually slightly worse than on the first.

For a board that must go through two reflow cycles, the OSP finish is generally acceptable if the process is controlled and the flux is suitable, but it offers less margin than a metal finish. Where a second reflow is followed by a wave or a selective soldering step for through hole parts, the number of thermal excursions accumulates and the finish should be reviewed against the process sequence. The alloy and profile context is described in the comparison of leaded and lead free processes.

Where It Fits

The finish suits a board that will be assembled promptly, that has a single reflow cycle, and that has no requirement for a long shelf life or for repeated probing. That covers a large part of high volume consumer production, where the boards are built and assembled in a continuous flow.

It is less suitable for a product with a long storage period, for a board that must be probed repeatedly, and for an assembly with several reflow cycles. In those cases a metal finish provides more margin, and the choice is a trade between cost and process window. The plating options are described under electroplating additives, and the general fabrication sequence in PCB design and fabrication.

Additional Considerations for This Build

Practical attention to flux compatibility pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating flux compatibility explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, organic solderability preservative is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

FAQ

Is OSP a metal finish? No. It is an organic film a few tens of nanometres thick that protects the copper and is consumed by the flux during soldering.

Can a board with OSP be reflowed twice? It can, but the film is gone after the first pass and the copper oxidises, so the second pass has a narrower process window and the wetting is generally worse.

Why does handling matter more than with a gold finish? Because the film is soft and thin and is removed by abrasion or by oils from the skin, which leaves the copper exposed before assembly.