Board Finish Selection Guide

A board finish is the thin layer on the exposed copper that keeps it solderable until the assembly is done. It is chosen late, it costs very little, and it decides whether the first assembly run goes smoothly or produces a batch of joints that will not wet. Four families cover almost every product, and each has a failure mode that is worth knowing before the choice is made.

What the Finish Has to Do

The finish has to keep the copper free of oxide from the moment the board is made until the moment it is soldered. Copper oxidises within hours in air, and an oxide layer is not wettable by solder.

It also has to survive storage. A product that is built in batches may have boards on the shelf for months, and a finish that degrades in that time will produce a yield loss when the batch is finally run.

It has to be compatible with the assembly process, including the flux and the temperature. A finish that dissolves in the flux or that reacts with the alloy is a finish that will not work whatever its data sheet says.

The <a href="https://www.gopcba.com/solderability-test-guide/” title=”solderability test”>solderability test is the way to check that the finish survived, and it is worth running on a sample from any new supplier.

Hot Air Solder Levelling

HASL covers the copper with a thin layer of solder and levels it with a blast of hot air. It is the cheapest finish and the most forgiving of a poor surface.

The surface is not flat, because the solder forms a meniscus over every pad. That is acceptable for a coarse pitch part and a problem for a fine pitch one, where the uneven surface disturbs the paste deposit.

The finish has a good shelf life and it is easy to inspect visually. The lead free version has a higher melting point and a rougher surface than the tin lead version, which is a consideration for a dense board.

The process is described in the HASL guide, and it remains the right answer for a low cost board with a pitch above about half a millimetre.

ENIG and Its Variants

Electroless nickel immersion gold deposits a nickel barrier and a thin gold layer, which gives a flat surface and a long shelf life. The gold protects the nickel, and the nickel protects the copper.

The flat surface suits a fine pitch part and a ball grid array, where the coplanarity of the pad matters. The finish also resists the multiple reflows that a double sided assembly needs.

The failure mode is the black pad, where the nickel surface is attacked during the gold deposition and the joint becomes brittle. The defect is a process problem, and it is the reason the nickel thickness and the gold thickness both have to be specified.

A variant uses a thicker gold for a wire bond or for a contact, and the thickness should be stated per application rather than as a general note.

Immersion Silver and Tin

Immersion silver gives a flat surface at a moderate cost, and it solders well with most fluxes. Its weakness is the atmosphere, since silver tarnishes in the presence of sulphur and the tarnish is not wettable.

The storage therefore has to be controlled, and the boards are usually packed with a paper that absorbs the sulphur compounds. A board that has been on a shelf in a factory with a rubber floor may be unsolderable without a strong flux.

Immersion tin is a flat finish with a good shelf life and a low cost. It forms an intermetallic layer with the copper over time, and the layer grows until the tin is consumed, which limits the storage life.

Neither finish is as robust as gold, and both are used where the cost matters and the environment is controlled. They should never be chosen for a product that will be stored for a long time in an uncontrolled place.

Organic Preservatives

An organic solderability preservative is a thin organic film that protects the copper and that is displaced by the flux during soldering. It is the cheapest flat finish available.

The surface is flat and coplanar, which suits a fine pitch part, and the process is simple. The weakness is the shelf life, which is measured in months even in a sealed bag, and the sensitivity to handling.

The film is also sensitive to the number of thermal cycles, so a double sided assembly with two reflows may exhaust it. Where the product needs multiple reflows, another finish is safer.

The properties and the storage rules are described in the OSP guide, and the finish is widely used on consumer products that are assembled quickly.

Choosing for the Application

Start with the pitch. A coarse board can use HASL, while a fine pitch or a ball grid array needs a flat finish. That single question removes half of the options.

Then consider the number of reflows and the storage time. A long shelf life and several reflows point to gold, while a short and simple assembly points to an organic finish or silver.

The third consideration is the joint reliability. A gold finish forms a strong joint but a brittle one if the gold is too thick, and the specification has to control that. The alloy choice interacts with the finish as well.

The fourth is cost and availability. HASL and OSP are the cheapest, silver and tin are in the middle, and ENIG is the most expensive. The difference is small per board and large per batch.

Practical Rules

State the finish, the thickness and the storage condition on the drawing, and do not rely on a generic note.

Match the finish to the pitch, the number of reflows and the shelf life, rather than to the price alone.

Run a solderability test on a sample from each batch where the product is critical, since the test is cheap and the failure is expensive.

Keep the finish and the flux choice together, because a finish that needs an active flux will fail with a no clean process.

Process Control and Verification

On a design of this kind, solderability is the item that decides how the rest of the board is arranged. 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.

FAQ

Which finish is best for a fine pitch BGA? A flat finish, which means ENIG, immersion silver or OSP. HASL is unsuitable because the surface is not coplanar.

Why does ENIG sometimes fail? Because of the black pad defect, where the nickel surface is attacked during the gold deposition. The nickel and the gold thicknesses both have to be specified.

How long can a board be stored? It depends on the finish. Gold and HASL last a long time, while an organic preservative is measured in months in a sealed bag.