Exposed Copper on PCB Pads: Finishes and Solderability

Bare copper is the best conductor on the board and the worst surface to leave exposed. It oxidises within hours in a normal room, and the oxide that forms does not wet with solder, does not accept a wire bond and does not present a stable contact resistance. Every surface finish on the market exists to solve that problem in a different way.

The choice between them is therefore a question about the assembly process, the storage time and the number of times the surface has to survive being touched. This article explains what happens to exposed copper, how the common finishes prevent it and how to decide which one a given product actually needs.

Why Exposed Copper Is a Problem

Copper reacts with oxygen, with moisture and with the sulphur compounds present in ordinary air. The resulting layer is thin, but it is chemically different from the metal beneath it, and solder cannot form a reliable intermetallic bond through it. Flux removes some of the oxide, which is why a board that has been stored badly sometimes solders anyway, but the process window shrinks until the assembly house has to widen the flux chemistry or raise the peak temperature.

The problem is uneven. A pad that was touched by a bare hand oxidises differently from one that was not, and the contamination left by a fingerprint is invisible until the joint fails. That variability, rather than the average oxide thickness, is what makes an unprotected surface unreliable in production.

What Happens During Storage

Storage conditions decide how quickly the surface degrades. Humidity accelerates the reaction, sulphur bearing pollution accelerates it further, and temperature controls the rate. A board stored in a sealed bag with desiccant and a humidity indicator behaves very differently from a board left on a shelf in a workshop.

Time is the other variable, and its effect depends on the finish rather than on the copper. An organic coating degrades gradually as it is consumed, while a metallic finish changes more slowly but can still form intermetallics with the copper beneath it, which is why even plated finishes have a stated shelf life and a recommended re-bake procedure.

Organic Coatings and How They Work

An organic solderability preservative is a thin film that bonds to the copper and blocks the oxygen, then decomposes or dissolves during soldering so that the molten solder contacts clean metal. The layer is only a few hundred nanometres thick, which is why the surface stays flat and the process works on fine pitch components.

The advantage is cost and flatness. The disadvantages are the shelf life and the sensitivity to handling, since the film is easily damaged by repeated touching or by an aggressive cleaning step. It is normally the finish of choice for a high volume consumer assembly that is built soon after fabrication.

Metallic Finishes and Their Trade-offs

Metallic finishes replace the copper surface with a different metal or with a stack of metals. Electroless nickel with immersion gold gives a flat, long lived surface that works for soldering and for wire bonding. Immersion tin and immersion silver provide a flat solderable surface at a lower cost but with a shorter shelf life and their own failure modes.

Hot air solder levelling is the traditional answer. It produces a thick, robust coating that survives multiple reflow cycles and long storage, at the cost of a surface that is not flat. The roughness limits it to pitches where the paste deposit can tolerate the variation, which excludes the finest packages.

Mask Defined and Copper Defined Pads

The mask opening around a pad changes the geometry of the finished surface. A copper defined pad has its dimensions set by the etched copper, while a mask defined pad has them set by the opening in the solder mask. The two behave differently in assembly, and the choice interacts with the finish because a mask defined pad exposes less copper and therefore has a smaller wetted area.

Whichever definition is used, the mask should not overlap the pad in a way that traps flux or creates a shadow during reflow. The tolerance between the copper and the mask opening belongs in the fabrication drawing, and on fine pitch it is often the number that decides whether the process yields.

Solderability Testing

Solderability is measured, not assumed. A wetting balance test or a dip and look test on a sample from the production lot shows whether the surface still wets within the specified time, and the result is compared against a standard rather than against a previous batch.

The test is particularly important for a board that has been stored, reworked or shipped between sites. A supplier that provides a solderability report with each lot gives the assembly house a starting point, and a product that has a long interval between fabrication and assembly should have that report as a condition of acceptance.

Handling, Packaging and Shelf Life

Handling rules protect the finish rather than the board. Gloves, a defined grip area and separation sheets between panels prevent the contamination that shows up as a dewetted pad, and vacuum packaging with desiccant preserves the finish during transport.

The shelf life stated by the fabricator assumes those conditions. Where a board exceeds the stated interval, the usual response is a bake and, on some finishes, a re-clean rather than a decision to use it as it is. Baking an organic coating too hot destroys it, so the procedure depends on the finish and should be confirmed rather than improvised.

Design and Documentation Rules

State the finish in the fabrication drawing together with the thickness where a metallic layer is used, and state which surfaces are exempt, since a connector edge or a bonding pad may need a different treatment from the rest of the board. Where a finish is applied selectively, the drawing has to show the boundary rather than describing it in words.

gopcb supplies boards across the common finishes with solderability data and packaging that matches the assembly schedule, so the surface the design assumes is the surface the assembly house receives.

Cost should be weighed against the assembly schedule rather than against the board price alone. The cheapest finish on the quotation is often the most expensive in the factory, because a surface that has to be re-cleaned, re-baked or re-inspected consumes assembly time and yield that the board price never captured. Where the interval between fabrication and assembly is long or unpredictable, paying for a finish that survives storage is usually the cheaper decision in the end.

FAQ

Can a board with bare copper pads be soldered successfully? Sometimes, with an aggressive flux, but the process window is narrow and the result varies across the panel. It is not a production process.

What is the shelf life of an organic coating? Typically six to twelve months in sealed packaging with desiccant, and much less once the bag is opened. The fabricator’s stated interval should be treated as a limit rather than a target.

Which finish is best for wire bonding? A thin gold layer over a nickel barrier, usually the electroless nickel immersion gold type, because it is flat, uniform and stable enough for aluminium or gold wire.

Related reading: PCB manufacturing processes, PCB manufacturing tolerances, lead free versus leaded solder, and PCB design quality characteristics.