ENIG Surface Finish: Process and Reliability

Electroless nickel immersion gold is the finish most often chosen when a board needs a flat, solderable and durable surface. It is also the finish most often blamed when a joint fails. The two facts are related, because ENIG is a two layer coating produced by a chemical process that must be tightly controlled, and its failure modes are subtle.

What the Coating Consists Of

ENIG deposits two metals in sequence. First, a layer of nickel is deposited by an electroless chemical reaction, which requires no electrical connection and therefore coats every exposed copper surface evenly, including the inside of holes. Then a thin layer of gold is deposited by immersion displacement, in which the gold ions in solution replace nickel atoms at the surface.

The nickel provides the barrier and the solderable surface. The gold protects the nickel from oxidation during storage and gives the pad its characteristic appearance. The gold layer is very thin, measured in fractions of a micron, and it is not intended to contribute to the joint. Our OSP surface finish article describes the contrasting approach that adds no metal at all.

Why the Gold Must Be Thin

Gold dissolves readily in molten solder, and a joint that contains too much gold becomes brittle. That is the entire reason the gold layer is kept thin: during reflow the gold dissolves into the alloy and the solder then meets the nickel beneath, which is the surface that actually forms the intermetallic bond.

A finish with a thick gold layer is therefore not better, it is worse. Where a connector requires a thick gold surface for wear resistance, that is a different requirement met by a different coating, usually electroplated hard gold over nickel. Our electrical test notes describe the related contact surface considerations.

The Flatness Advantage

Electroless deposition follows the surface it coats, so the finished pad is as flat as the copper beneath it. That property is why ENIG is chosen for fine pitch components and for ball grid arrays, where a levelled finish such as hot air solder levelling would leave an uneven surface.

Flatness also helps the printing process, because the paste deposit is determined by the aperture and the gap between stencil and board. A finish that varies in height changes that gap locally and therefore changes the deposit. Our pad and hole size article covers the dimensions that interact with it.

Corrosion Resistance and Storage

The gold layer prevents the nickel from oxidising, which gives ENIG a long shelf life compared with an organic coating. Boards can be stored for extended periods and still solder reliably, which matters where production is scheduled far in advance of fabrication.

The coating also resists the handling that occurs during assembly, and it tolerates being probed without losing its solderability, which is why test points on an ENIG board do not need special treatment. That combination of properties makes it the practical default for complex assemblies.

Black Pad and Its Mechanism

Black pad is the failure mode that ENIG is known for. It appears as a dark, brittle layer at the nickel surface, and joints formed on it fail under mechanical or thermal stress even though they look acceptable at inspection.

The mechanism involves excessive corrosion of the nickel during the immersion gold step. Because the gold deposits by displacing nickel, an over active bath or an extended dwell time attacks the nickel grain boundaries and leaves a phosphorus rich surface that does not form a sound intermetallic bond. The result is a joint that fails at the interface rather than in the solder.

Process Control That Prevents It

Prevention is entirely a matter of bath control. Nickel thickness, gold thickness, the pH and temperature of the immersion bath, and the dwell time are all monitored, and the process window is narrower than most people expect.

Where a fabricator measures the gold thickness and the nickel thickness on coupons from every panel and records the bath parameters, black pad is effectively eliminated. Where those measurements are absent, the finish may look correct and still perform badly. Our PCB quality article describes the records worth requesting.

When ENIG Is the Right Choice

ENIG suits boards with fine pitch components, ball grid arrays and long storage requirements, and it suits assemblies that will be probed during test. Its cost is higher than OSP or hot air levelling, and the additional cost buys flatness, storage life and durability.

It is the wrong choice where the budget is the dominant constraint and the board has generous pad sizes, and it is usually the wrong choice for a connector surface that will see repeated mating, where hard gold is required instead. Our notes on thermal boards describe a construction where the finish interacts with the assembly process in a different way.

ENIG Process Records Worth Requesting

The finish is only as good as the bath control behind it, and the evidence for that control is a record rather than a certificate. The following items show that the process was monitored rather than merely run:

• Nickel thickness measured on a coupon from the panel that shipped.
• Gold thickness measured on the same coupon, with the method stated.
• Bath pH, temperature and dwell time logged for the day the panel was processed.
• Nickel and gold thickness uniformity across the panel, not only at one point.
• Whether the nickel bath was at the end of its life during the run.
• A cross section of a plated pad showing the nickel and gold layers.
• Any rework of the finish recorded, since re-plating a pad is not always possible.
• The activation and cleaning steps preceding the nickel deposit.
• The packaging used, since the finish relies on a sealed bag for its storage life.
• The date of plating, so that the storage window can be tracked.

None of these is unreasonable to ask for on a production order, and the answers separate a controlled process from one that happens to be within specification on the day. Our PCB quality article places these records in the wider picture.

Where the finish will be used on a product with a long service life, the thickness measurements are worth keeping with the build record rather than discarding them after the boards ship. A query about a joint failure years later is answered by the record from the panel, and by nothing else.

FAQ

Does ENIG contain lead? No, and that is one of the reasons it became popular as restrictions on lead were introduced. It is also compatible with lead free soldering.

Why is the gold layer so thin? Because gold embrittles solder. The layer only needs to protect the nickel during storage; it dissolves during reflow and the joint forms to the nickel beneath.

Is black pad a defect of the material or the process? It is a process defect. Correct bath control prevents it, which is why the fabricator process record is the relevant evidence.

Can an ENIG board be stored for years? The finish resists oxidation far better than an organic coating, so long storage is realistic provided the packaging remains sealed.

Which finish is best for a ball grid array? A flat finish is required, and ENIG or a similar chemically deposited finish is normally specified for that reason.