ENIG

What it is

ENIG (Electroless Nickel Immersion Gold) is a two-layer chemical plating for the exposed copper pads of a PCB. A layer of nickel is deposited first, acting as a barrier between the copper and gold and providing the surface that solder ultimately bonds to. An even thinner layer of gold is deposited on top to keep the nickel from oxidising during storage. The result is a flat, wettable, gold-coloured pad with a long shelf life.

How it's made

The nickel is deposited electrolessly, without an external electrical current. The gold is then deposited by immersion, using a chemical displacement reaction. Because neither step needs an electrical connection to the pad, ENIG can plate exposed copper surfaces without requiring them to be electrically connected to a plating rail — which makes it well-suited to isolated pads and fine-pitch features.

• Nickel layer — a nickel-phosphorus alloy, typically around 3 to 6 microns in common specifications (exact thickness ranges vary between manufacturers and standards). This is the real workhorse of the finish: it's what solder bonds to, and what keeps the underlying copper isolated from the outside world.
• Gold layer — very thin, roughly 0.05 – 0.1 microns (50 – 100 nm) in common specifications. It's essentially a protective film over the nickel: it prevents oxidation during storage, and is consumed into the solder joint during soldering.

What it looks like

Rich gold-coloured pads with a very flat profile compared with a finish like HASL — no meniscus and no domed top. The finish itself only adds microns of height. (The pad isn't necessarily perfectly flush with the solder mask; fab tolerances, mask thickness, copper thickness and plating thickness all affect the actual profile.) That flatness is one of the main reasons ENIG is used for fine-pitch SMD and BGA assembly.

How it solders

Very well. The very thin gold layer is rapidly consumed into the solder during soldering, allowing the solder to wet the underlying nickel and form a strong joint. Both iron soldering and reflow are straightforward.

The failure mode most associated with ENIG is black pad — a plating defect characterised by excessive corrosion or abnormal phosphorus enrichment at the nickel surface. It can result in poor solderability and brittle solder joints, and is largely invisible before soldering. Black pad is primarily a process-control issue and is uncommon with well-controlled ENIG production, but it's the standard cautionary story around the finish.

How it compares to HASL and OSP

ENIG, HASL and OSP are three common PCB surface finishes. Lead-free HASL is widely used where a low-cost lead-free finish is required; other finishes (immersion silver, immersion tin, hard/electrolytic gold) also exist for specific applications. ENIG sits at the premium end of the three.

• Much flatter than HASL — no meniscus, no domed profile — which makes it well-suited to fine-pitch SMD and BGA assembly.
• Generally longer shelf life than OSP when the board is properly manufactured, packaged and stored. Manufacturers commonly quote shelf lives of 12 months or more, though the real number depends on packaging, humidity, contamination and handling.
• More expensive than either HASL or OSP, though the exact premium varies substantially with fab, board geometry and production volume.
• Gold-coloured cosmetic finish, which some designs specifically want on visible PCB surfaces. (For mating contact surfaces such as edge connectors, ENIG is not the right choice — the gold layer is too thin for repeated mechanical contact. Those specify hard / electrolytic gold over nickel or a dedicated edge-contact finish.)