PCB Surface Finish Thickness Measurement and Control

A surface finish does its job within a thickness range, and both ends of that range cause problems. Too thin and the finish is consumed before the joint forms; too thick and the joint becomes brittle or the finish stops behaving as designed.

Immersion Gold Over Nickel

The gold protects the nickel during storage and dissolves into the joint during soldering. It has to be thick enough to cover the nickel fully and thin enough to dissolve completely.

The nickel beneath is what forms the joint, so its thickness matters more than the gold in most cases. Our plating notes describe the measurement of both.

How the Measurement Is Made

X-ray fluorescence is the production method, because it is fast and non destructive. It reports a thickness that assumes a density and a composition, so it has to be calibrated against a known sample for each finish.

A destructive microsection is used to confirm the calibration. Our test coupon notes describe the sample.

Where the Measurement Is Taken

The measurement belongs on a coupon that received the same process as the board, and on the board itself where the finish is critical. A reading from a coupon that was plated separately is not evidence.

The location on the coupon should be recorded, because a finish varies across a panel. Our fabrication notes list the points to confirm.

Ranges That Protect Solderability

Each finish has a working range that the supplier publishes and that experience narrows. A specification should state a range rather than a minimum, because the upper end matters too.

The range should be checked against the storage life that is expected. Our solderability notes describe the test that confirms it.

Black Pad and Its Detection

A nickel surface that has been over attacked during the gold step appears dark and does not solder. The condition is a process problem rather than a thickness problem, and a thickness reading can be normal while it is present.

The detection is by solderability test and by inspection of the nickel surface. Our black pad notes describe the causes.

Storage and Shelf Life

The finish changes during storage through oxidation and through the diffusion of the layers beneath. The shelf life should be stated with the storage conditions that were assumed.

A board stored beyond its life should be re tested rather than used. Our storage notes describe the conditions that extend it.

Verification

The verification is a calibrated XRF reading on a coupon from the same process, a microsection to confirm the calibration at intervals, and a solderability test on the same coupon.

Our quality notes describe how the records are kept.

Process Control and Verification

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.

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. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

FAQ

Is thicker gold always better? No. Excess gold dissolves into the joint and makes it brittle, which is why the range has an upper limit as well as a lower one.

Can XRF measure nickel under gold? It measures both when the instrument is configured for it, which is why the calibration sample should contain both layers.

What does gopcb provide for surface finish control? We provide a stated range for each layer rather than a minimum, XRF readings calibrated against a known sample and taken on a coupon from the same process, periodic microsection confirmation, a solderability test on the same coupon, and a shelf life stated with the storage conditions it assumes.