Nickel Flash Gold PCB: A Thin Gold Finish Explained

What the Finish Actually Is

Nickel flash gold is a plated finish of nickel with a very thin layer of gold on top, applied by the same electrochemical sequence used for electroless nickel immersion gold but with a much shorter gold deposition. The nickel is the functional layer: it provides a flat, hard, solderable and diffusion resistant surface. The gold is a flash, typically a few hundredths of a micrometre, whose only jobs are to protect the nickel from oxidation before assembly and to make the surface wettable.

The name is used loosely in the industry, and the ambiguity causes trouble. In some plants, flash gold means the gold layer of a conventional electroless nickel immersion gold process, which is measured in the same range. In others it means a thicker electrolytic deposit. Whenever the finish matters, the specification should state the gold thickness and the nickel thickness in numbers rather than using the word flash.

Nickel Flash Gold Against the Other Gold Finishes

• Electroless nickel immersion gold. A nickel layer of three to six micrometres with a gold layer of 0.05 to 0.1 micrometres applied by an immersion displacement reaction. The gold is thin, pure and dense. It is the standard finish for fine pitch assembly and for a long shelf life.
• Nickel flash gold, electrolytically plated. Nickel with a thin electrolytic gold. Harder than the immersion deposit, and it needs a plating connection to every pad, which makes it impractical on an isolated net unless a plating bar is provided and cut afterwards.
• Hard gold. A gold alloy with cobalt or nickel for wear resistance, typically 0.5 to 1.5 micrometres thick over nickel. Used on edge fingers, card connectors and switch contacts, and unsuitable for bonding.
• Soft gold, thick. Pure gold deposited to 0.3 to 1.0 micrometres or more for gold wire bonding. A different specification from a flash.

The finish chosen decides three things: whether the pads solder reliably through the required number of reflow cycles, how long the board can be stored, and whether anything can be bonded to it. Our notes on PCB manufacturing describe how each finish is applied.

Why the Thin Gold Is Enough

Gold does not oxidise, and it dissolves rapidly into molten solder. Those two properties mean a very thin layer is sufficient to preserve solderability: it protects the nickel while the board is in storage and in the assembly process, and it disappears into the joint the moment the solder melts.

The nickel underneath is what provides the solderable surface. Nickel wets well with an activated flux and forms a reliable intermetallic with tin, so the joint is a tin-nickel and tin-copper metallurgy rather than a gold one. That is also why a very thick gold layer is undesirable on a solderable pad: above roughly one percent gold by weight in the joint the solder becomes brittle, which is a real concern where a thick soft gold has been applied for bonding and the same pad has to be soldered.

The thin gold also has a practical limit. It is porous at very low thicknesses, and a porous flash allows the nickel beneath to oxidise in storage, which reduces the solderability rather than preserving it. That is the reason a minimum thickness is specified rather than a maximum only.

Solderability and Shelf Life

The finish holds its solderability better than any organic coating, and it is the reason it is chosen for boards that are stored, shipped, assembled in stages or subjected to more than one reflow. Typical values quoted for a gold finished board are twelve months of shelf life in controlled storage, against six months for an organic solderability preservative.

Three cautions apply. The board should still be stored in a sealed bag with a desiccant rather than open to the air, because the laminate absorbs moisture even if the finish does not oxidise. The gold surface attracts organic contamination, and a board handled without gloves loses solderability faster than the specification suggests. And reflow under a poor atmosphere, or a profile with a long soak, consumes the gold early and exposes the nickel to oxidation during the process rather than before it.

Assembly Consequences

Fine pitch. The flat surface of a plated finish is what allows a fine pitch stencil to print a consistent volume of paste and a thin mask dam to survive. Hot air levelling produces an uneven surface that is unsuitable below a certain pitch; nickel gold does not, which is the main reason it is used on dense assemblies.

Via in pad. Where a via sits inside a pad it must be filled and plated over, and the plated-over surface is normally a copper finish that is then covered by the board finish. Nickel gold is compatible with that sequence, and it also provides a hard surface that resists the paste being drawn into a dished via.

Press fit connectors. A press fit pin relies on a controlled interference in the plated hole. The finish affects the friction and the contact resistance, and nickel gold is one of the finishes used for that application, alongside a pure tin or a silver finish.

Connector contact. A pad that is mated repeatedly needs hard gold, not a flash. The two are often combined on one board with a selective plating process: hard gold on the edge fingers and nickel gold elsewhere. Our notes on PCB assembly describe how the assembly process is set up for each finish.

Where It Should Not Be Used

Wire bonding. Aluminium wedge bonding can work on a thin immersion gold because the aluminium displaces the gold and reaches the nickel, but the process window is narrow. Gold ball bonding needs a thicker pure gold, and a flash finish is not a bonding specification.

High frequency conductor surfaces. The nickel layer is lossier than copper, and on a microwave transmission line the current concentrates near the surface, so a nickel gold finish adds measurable loss. Where the loss matters, a finish without nickel, such as immersion silver or a thin immersion gold over copper, is used on the radio frequency sections.

Repeated mating. A flash of pure gold is soft and wears through quickly. Anything that is plugged and unplugged needs hard gold.

Very high current pads. The joint is fine, but where a heavy current is carried through a pressed contact rather than a soldered joint the finish and the contact area have to be designed for it.

Cost and Specification

Nickel gold is more expensive than hot air levelling and comparable with the other plated finishes. The cost is driven by the thickness of the nickel and the gold, the area to be plated, whether the plating is selective and the process yield. On a dense board with fine pitch pads the premium over hot air levelling is small compared with the improvement in assembly yield, which is why it is the default for fine pitch work.

A specification should state the nickel thickness with its tolerance, the gold thickness with its minimum, the areas to be plated and, where a selective finish is used, which areas receive which finish. It should also state that a measurement is required, because the difference between a compliant and a non compliant thickness cannot be seen. Our notes on quality management describe the measurement methods, and our notes on PCBA testing cover the verification of the finished assembly. For the layout considerations that go with a fine pitch board, our notes on PCB design and layout are the starting point.

FAQ

Is nickel flash gold the same as ENIG? They use the same nickel and gold layers, but the term flash usually indicates a thinner gold. Always specify the thicknesses rather than the name.

Is a thin gold finish good enough for soldering? Yes. The gold dissolves into the solder and the joint forms to the nickel beneath, which is why a thin flash works reliably on a solderable pad.

Can I wire bond to it? Aluminium wedge bonding can sometimes be qualified on a thin immersion gold, but a flash is not a bonding specification. Use a defined soft gold thickness for bonding.

Why does my board need gold instead of hot air levelling? For a flat surface at fine pitch, for a long shelf life, and for a board that will be reflowed more than once.

Conclusion

Nickel flash gold is a thin gold layer over a functional nickel layer: the nickel provides the solderable surface and the diffusion barrier, and the gold protects it until the moment of soldering. It is the right finish for fine pitch assembly and for boards that are stored or reflowed more than once, it is the wrong finish for repeated mating, for wire bonding and for a microwave conductor. Specify the thicknesses, require the measurement, and the name matters less than the numbers.