Anti-Tarnish Protection: 5 Checks for Bare Copper

Anti-tarnish protection is the thin organic coating applied to bare copper so that the surface is still solderable when the board is finally used. Without it, copper oxidises within days in a warm shop and within hours in a humid one, and the oxide that forms is enough to spoil wetting.

The treatment is a compromise. A coating thick enough to protect the surface for months can interfere with flux and with the soldering reaction, while one that is too thin protects nothing. The useful control is a coating weight that is measured rather than judged by the colour of the panel.

Why Bare Copper Tarnishes

Copper reacts with oxygen and with moisture in the air, and the reaction is accelerated by heat, by humidity and by residues left on the surface. The first product is a thin oxide that is barely visible; given time it thickens and darkens, and the dark colour is the visible end of a process that began hours after the panel left the rinse.

Contamination makes it worse. Salts from a poorly rinsed panel, fingerprints and acid residues all hold moisture against the metal and speed the reaction, which is why a clean surface lasts far longer than a dirty one even with the same protection. A panel stored with acid residue on the surface can tarnish inside a day, so the rinse before the coating matters as much as the coating itself.

Chemistry Options for Anti-Tarnish

Most anti-tarnish treatments are organic azole compounds, applied from a dilute bath at a controlled temperature and pH. They work by forming a thin complex film on the copper that blocks the diffusion of oxygen and water to the metal underneath.

The chemistry is often confused with the solderability preservative used as a final finish. The two are related, but an anti-tarnish coating is usually thinner and is intended for a shorter interval, while a solderability preservative is specified as part of the finish and carries its own thickness requirement. The two are measured by the same analytical methods, but only one is expected to survive a year of storage, so the specification should name the film and the weight instead of describing the surface as protected.

Coating Weight and Coverage

Coating weight is measured in milligrams per square decimetre and is the value that determines how long the protection lasts. Under-coating leaves areas unprotected, while over-coating produces a film that interferes with the soldering reaction and can look wet or streaky.

Coverage is a separate question from weight. A panel with a uniform weight across its area behaves well, while one that carries the same average in a patchy distribution will fail first in the thin areas, which are usually the hole walls and the panel edges.

Rinse and Drag-Out Before the Coating

The anti-tarnish bath is sensitive to what arrives with the panel. Acid carried from a previous tank lowers the pH and changes the film that forms, while alkaline carry-over can precipitate the chemistry and leave a cloudy bath that coats unevenly. A panel that reaches the bath with a wet surface forms the film over a layer of water, and that film adheres poorly and tarnishes first.

Drip time, flow rate and a dedicated pre-dip are the usual controls. Because the coating is thin, the contamination that matters is small, and the rinse discipline that protects this step is the same one that protects any wet process on the line.

Drying, Handling and Packaging

The coating has to be dried without being driven off. A dryer that is too hot removes the film along with the water and leaves patches that tarnish first, while a panel that is left wet picks up contamination from the air and from any surface it touches.

Handling after drying is part of the process. Bare hands transfer salts and oils straight through a thin coating, so gloves and a defined grab area are process controls rather than good habits. Storage and handling practice is described in storage humidity control.

Solderability Testing and Retention

Solderability is the property the coating exists to protect, and it should be measured rather than assumed. A wetting balance test or a dip and look check on a sample from storage shows how much of the protection is still present after weeks on the shelf. Sampling should be planned so that the tested panel has been stored under the same conditions as the stock it represents.

Retention over time is what matters commercially, and it is set by the coating, the packaging and the storage conditions together. Where a customer specifies a shelf life, the shop should be able to show a test result that supports it rather than a promise. Finish selection is described in OSP solderability preservative.

Storage Conditions and Shelf Life

Storage determines how long the protection lasts. Cool, dry conditions slow the reaction, while a humid warehouse with wide temperature swings accelerates it and condenses moisture on the surface every night.

Packaging closes the gap. Sealed bags with desiccant, pallets kept off a concrete floor and stock rotated so that older panels ship first all extend the usable life of a protected surface, and the shelf life practice that governs it is described in bare board shelf life.

Removal Before the Next Process

Anti-tarnish coatings are designed to be removed or displaced during soldering, and most fluxes handle them without difficulty. Where a panel is going through a process that is sensitive to organics, however, the coating has to be removed first, usually with a mild acid or an alkaline cleaner.

Removal has to be complete. A partly removed coating leaves a patchy surface that wets unevenly, which produces solder defects that look like a flux problem. Where the coating is removed, the panel should be processed promptly, because the protection is gone. Removal should be followed by a rinse and a dry, since the cleaner that takes the film off leaves residue of its own.

Records and Troubleshooting

The record should carry the coating weight, the bath analysis, the drying conditions and the packaging used. With those values a tarnish complaint can be traced to the coating, the packing or the storage instead of being attributed to the customer.

Troubleshooting follows the appearance of the defect. Tarnish that follows the panel edge points at drying or at drag-out, while a patchy pattern points at coating weight or at a contaminated bath. Surface cleanliness after storage is confirmed with the same methods used for assembly cleanliness measurement, and reference methods are published by IPC.

FAQ

How much coating weight is enough? The figure comes from the chemistry supplier and from the shelf life the product has to meet, and it is typically low, measured in milligrams per square decimetre. It should be verified by analysis rather than by colour.

Does the coating affect solderability? It can if it is too thick, because the flux has to displace the film before the solder can wet the copper. A weight inside the specified range helps solderability by protecting the surface; one above it hinders it. Where a panel has to be stored for a long period, a heavier coating inside the range is the better answer than a second layer of the same chemistry.

Can a tarnished panel be recovered? Often it can, by removing the oxide with a mild micro-etch and re-applying the protection, provided the oxide has not become thick enough to affect the copper thickness. The decision should be made by measurement.