Gold Plating Thickness Specification for PCB Contacts

Gold is used on circuit boards for two reasons: it does not oxidise, and it survives repeated mating. Both properties depend on how thick the gold is and on what lies beneath it, and the gold plating thickness that suits a low cost consumer product will fail quickly on a connector that is plugged in every day. This article explains how to choose a figure and how to write it on a drawing.

Why Thickness Is the Critical Variable

Gold is expensive, so the instinct is to use as little as possible. The problem is that a thin layer is not continuous over a rough substrate. Electroless nickel, which is the usual underlayer, has a granular surface, and a gold layer of a few hundredths of a micron simply follows those grains and leaves pores through to the nickel. Once a pore exists, the exposed nickel corrodes and the corrosion product migrates across the gold surface, raising the contact resistance of an otherwise sound interface.

The thickness required for continuity therefore depends on the roughness of the layer below. A polished surface needs less gold than a rough one, and a thicker nickel deposit with a finer grain structure reduces the gold needed for the same protection. This is why plating specifications normally state the nickel thickness and the gold thickness together rather than treating gold in isolation.

Hard Gold versus Soft Gold

Hard gold is an alloy of gold with a small amount of cobalt or nickel, deposited from an acidic bath. The alloy raises the hardness and the wear resistance, which is what makes it suitable for edge connectors and for contacts that mate repeatedly. A typical specification calls for 0.5 to 2 microns over 2.5 to 5 microns of nickel, with thicker gold where the mating cycle count is high.

Soft gold, which is nearly pure, is used where a wire bond or a soldered joint must be made. It is too soft for repeated mechanical contact, because it deforms and wears through. Flash gold, typically less than 0.1 micron, is a cosmetic or solderability layer rather than a contact finish, and it should never be specified for a connector that mates more than a handful of times.

The Nickel Underlayer

Nickel does three jobs. It provides a diffusion barrier that stops copper from migrating into the gold, it supplies the hardness that a thin gold layer cannot, and it gives the gold a uniform surface to plate onto. Thicknesses below about 2 microns are usually too thin to be an effective barrier, and thicknesses above 6 microns build up stress and can crack at a bend.

The nickel must also be ductile enough to survive the plating and forming of an edge connector. Electroless nickel with a phosphorus content in the mid range is the usual choice, and the plating bath chemistry determines the grain structure and the internal stress. Attention to the plating bath control pays off here, because a stressed nickel layer will crack under thermal cycling no matter how good the gold looks.

Thickness for Wear Life

Wear life scales with gold thickness but not linearly. A contact with 0.5 micron of hard gold will survive perhaps a few hundred mating cycles, while one with 1.25 microns will survive several thousand under the same conditions. The load, the contact geometry, and whether the mating motion includes sliding all change the figure, so the number should come from a test that reproduces the actual mating rather than from a general table.

Where the contact must survive a very high cycle count, the gold thickness alone will not be enough. A lubricant applied to the contact surface reduces wear dramatically and is standard practice in some connector specifications. Where lubrication is unacceptable, a thicker hard gold and a smoother substrate finish are the two remaining levers.

Fretting corrosion deserves mention at this point, because it dominates many real connector failures. When two contact surfaces are held together but subject to small relative movement from vibration or thermal expansion, the wear debris oxidises and builds up as a resistive film. A hard gold surface resists the motion better than a soft one, and a thin gold layer reaches the nickel underneath much sooner, which is another reason the thickness figure is tied to the mechanical environment rather than to cost alone.

Thickness for Wire Bonding and Soldering

Wire bonding and soldering have different requirements from mating contacts. Bonding needs a soft, clean gold surface with a controlled thickness, because a thick layer of soft gold makes the bond unreliable and a very thin one allows the underlying nickel to oxidise. Where gold is specified for bonding, the thickness is usually tightly controlled within a narrow band rather than given as a minimum.

Soldering to gold brings a different risk. Gold dissolves into molten solder and can embrittle the joint if the gold volume is large relative to the solder volume, which is why a thick gold finish on a solder pad is undesirable. Where a board must both mate and solder, the two areas are normally plated differently, with hard gold on the connector and a solderable finish elsewhere.

Measuring Thickness

Thickness is measured by X-ray fluorescence, which is non-destructive, fast, and able to report the gold and nickel layers separately. The measurement is affected by the surface finish and by the alloy, so the instrument should be calibrated with standards that match the production deposit. Several points on the same feature should be measured, because plating thickness varies with current density and with position on the panel.

Destructive methods are used for qualification rather than for routine control. A microsection through a plated finger shows the gold, the nickel, and the copper in one image and reveals defects such as a porous nickel layer or a bond line that has been etched back. Where a plating specification has been disputed, a microsection settles the question more convincingly than a thickness reading alone.

Writing the Specification

The specification should state the finish by name, the nickel thickness, the gold thickness, and the substrate it is applied to. It should also state where the finish is required, since a gold callout over an entire board is expensive and unnecessary; the drawing should mark the connector area explicitly. A maximum thickness is as important as a minimum where soldering or bonding is involved.

The drawing should also say how the finish is verified, whether by X-ray fluorescence on samples or by microsection at first article, and what the acceptance criteria are. A brief note that quantities the requirement in measurable terms prevents a supplier from delivering a nominal gold thickness that is technically correct and functionally inadequate.

FAQ

How much gold does a connector really need? For occasional mating, 0.5 micron of hard gold over 2.5 microns of nickel is usually enough. For frequent mating, 1.25 microns or more, verified by a wear test that reproduces the real motion.

Is flash gold acceptable on a contact? Only for a very small number of mating cycles, because it is porous and wears through almost immediately. It is best treated as a solderability or appearance finish rather than a contact finish.

Why does the nickel thickness matter so much? It acts as a diffusion barrier and provides hardness. If it is too thin, copper migrates into the gold and the contact degrades even though the gold thickness is correct.