ENIG Finish And Black Pad Defects
An electroless nickel immersion gold finish is popular because it presents a flat surface for fine pitch components, survives storage, and can be used for both soldering and wire bonding. It is also the finish with the most discussed defect in the industry, a condition known as black pad, where the nickel layer under the gold is corroded and the joint that forms on it is brittle.
This article explains what the finish deposits, how the defect develops, why it is difficult to detect, and what can be controlled at the fabricator.
What The Finish Provides
The process deposits two layers by chemical reduction rather than by electroplating, so no current path is needed and the finish can be applied to an isolated pad. The nickel layer provides the barrier and the solderable surface, and it is typically a few micrometres thick. The gold layer is very thin, a fraction of a micrometre, and its function is to protect the nickel from oxidation during storage.
Because the gold is thin, it dissolves into the solder during reflow and exposes the nickel beneath. The joint that forms is therefore a nickel tin intermetallic rather than a copper tin one, and the reliability of the joint depends on the quality of the nickel surface that the gold was protecting.
The Chemistry Of The Deposit
The electroless nickel bath contains a reducing agent, commonly a hypophosphite, which reduces the nickel onto a catalysed copper surface. The reaction incorporates phosphorus into the deposit, and the phosphorus content, usually between seven and ten percent, determines whether the layer is crystalline or amorphous and how it behaves when it is soldered.
The gold step is an immersion reaction, which means the gold displaces nickel from the surface rather than being reduced onto it. That mechanism is self limiting, which is why the gold stays thin, but it also means the nickel surface is being attacked by the gold solution. If the nickel is not uniformly passive, the attack continues in the weak regions and produces the corrosion that defines black pad.
What Black Pad Actually Is
The defect is a localised corrosion of the nickel layer, often along the grain boundaries, with a corresponding enrichment of phosphorus at the surface. The corroded nickel does not wet properly, and the joint that forms is a weak one with a dark, brittle fracture surface. The name comes from the appearance of the pad after the gold has been removed or the joint has been pulled apart.
The corrosion is associated with an aggressive gold bath and with a nickel deposit that is not uniform. A bath that is operated at the wrong pH, temperature or gold concentration attacks the nickel more than it should, and a nickel layer with a low phosphorus content is more susceptible. The two factors interact, which is why the defect can appear on one production lot and not on the next, and why it is difficult to explain after the fact.
Why It Is Difficult To Detect
A black pad joint often passes the visual inspection and the functional test. The solder wets the pad and forms a fillet, because the gold is still present in the areas where the nickel is intact, and the joint conducts. The failure appears later, when the assembly is thermally cycled or mechanically loaded, because the brittle interface cracks.
Destructive testing is the reliable method. A ball shear or a pull test on a sample shows a low strength and a characteristic fracture surface, and a cross section shows the corrosion at the interface when it is examined under a microscope. A solderability test on the bare board, such as a wetting balance, can detect the problem before assembly but only if it is applied to a sample from the lot.
Process Controls That Reduce The Risk
The controls sit at the fabricator. The nickel bath is maintained within its phosphorus range, the gold bath is kept at the specified concentration and temperature and is replaced on a schedule rather than on demand, and the time between the nickel and the gold step is controlled so that the nickel does not oxidise in between. The gold thickness is verified rather than assumed, because a thick gold layer accelerates the corrosion of the nickel.
The board design can help as well. A pad that is electrically isolated takes the full immersion attack, while a pad connected to a large copper area behaves differently, and the difference can show up as a variation across the panel. Reducing the number of very small isolated pads, or connecting them with a thin trace that can be removed later, is a technique that some designs use to even out the process.
Alternatives And When To Use Them
Where the risk of black pad is unacceptable, other finishes are available. An immersion silver finish is flat, solderable and free of the nickel corrosion mechanism, at the cost of a shorter shelf life and a sensitivity to sulfur. A hot air solder levelling finish is robust and cheap but not flat, so it is unsuitable for fine pitch. An electroless nickel electroless palladium immersion gold finish adds a palladium layer that separates the gold from the nickel and reduces the attack.
The choice depends on the assembly requirement. For a fine pitch surface mount board with a long shelf life, ENIG or the palladium variant is usually the answer. For a board that is assembled immediately and has no fine pitch, a simpler finish is adequate. Where the finish is critical, the assembly house and the fabricator should agree on the specification and on the incoming inspection, because the defect is much easier to prevent than to diagnose. The plating process itself is described in the discussion of electroplating additives, and the general sequence is covered under PCB design and fabrication. The joints that form on the finish are accepted against the criteria in quality characteristics.
Process Control and Verification
On a design of this kind, ENIG is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
FAQ
Does black pad always cause a visible defect? No. The joint often looks acceptable and passes test, and the failure appears later as a brittle crack after thermal cycling or mechanical load.
Is a thicker gold layer better for solderability? No. A thick immersion gold layer means the nickel beneath it has been attacked for longer during deposition, which raises the risk of the corrosion that causes the defect.
Can the problem be detected on the bare board? A wetting balance test on a sample can reveal the poor wetting, but the corrosion is local, so a sample may pass while other areas of the panel do not. Cross sectioning a sample is the more reliable check.
