ENIG Black Pad: Causes, Detection and Prevention

An electroless nickel immersion gold finish protects the copper and it presents a flat, solderable surface. It also introduces a corrosion cell at the nickel surface, and the failure it produces is hard to see.

How the Finish Works

The nickel is deposited by a chemical reduction and it covers the copper completely, including the barrel. The gold is then deposited by displacement, which limits its thickness and it also attacks the nickel slightly.

The result is a thin gold layer over a nickel barrier, which is a good solderable surface and a barrier to copper diffusion. Our surface finish notes describe how it compares with the alternatives.

Black Pad

The defect is a corrosion of the nickel at the grain boundaries, which leaves a surface that the solder does not wet. It appears as a dark, brittle interface rather than as a joint.

The joint forms initially and then fails in service or during thermal cycling, because the weak layer is between the nickel and the solder rather than at the surface. Our joint defects notes describe the appearance.

What Causes It

The corrosion is driven by the plating chemistry and by the process control. A bath with the wrong stabiliser level, a pH outside the window, or a gold bath that is too aggressive all contribute.

The condition is a process condition rather than a design condition, which means it is verified on the board rather than specified on the drawing. Our plating notes describe the thickness verification that goes with it.

Detection

The visual check can show a dark pad, and the defect is often not visible. A solderability test on a sample is more reliable, and a cross section shows the corrosion at the interface.

A dye and pry test after assembly is the standard method for a suspect batch, because it shows the failure at the interface rather than at the surface. Our failure analysis notes describe the method.

Prevention

The plating bath must be within its specification window, which means the chemistry is analysed at a defined interval rather than when a defect appears.

The gold thickness matters as well, because a thicker gold means a longer displacement reaction and more attack on the nickel. The thickness should be controlled to a window rather than to a minimum. Our fabrication notes notes list the attributes that should be stated.

Where the Risk Is Highest

The risk is highest on a fine pitch area where a small pad has a large perimeter relative to its area, and on an assembly that will see thermal cycling.

A board with a mix of finishes, or one where the finish was applied without control, should be sampled rather than assumed to be acceptable.

Verification

The verification is the bath analysis, the thickness measurement on a coupon, the solderability test and, where the risk is high, a dye and pry after assembly.

The result should be recorded against the panel, so that a suspect batch can be identified rather than the whole production. Our test coupon notes describe the structure that carries the measurement.

Additional Considerations for This Build

Practical attention to nickel corrosion pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating nickel corrosion explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, gold thickness is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. 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.

Process Control and Verification

On a design of this kind, gold thickness is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

Process Control and Verification

On a design of this kind, gold thickness is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Is black pad always visible? No. The pad can look normal and the interface can be corroded, which is why the dye and pry and the cross section are used.

Does a thicker gold layer reduce the risk? It increases the displacement reaction and it usually makes the corrosion worse. The thickness should be controlled to a window.

What does gopcb provide for ENIG control? We provide bath analysis at a defined interval, thickness control to a window on a coupon, solderability testing of the finish, cross section and dye and pry for suspect batches, and panel level records that allow a suspect batch to be identified.