XRF Measurement Of Plating Thickness

X-ray fluorescence is the standard method for measuring the thickness of a thin plated layer without damaging the board. It works by exciting the atoms in the coating and measuring the X-rays they emit in response, and it resolves layers that are too thin to be measured reliably by a cross section.

This article covers how the measurement works, what it can resolve, and how it is calibrated and applied.

How The Measurement Works

The instrument directs X-rays at the surface, and the atoms in the coating absorb the energy and re-emit X-rays at wavelengths characteristic of the element. The intensity of the emission depends on the number of atoms present, which for a layer of known area means its thickness and density. The instrument measures the intensity at the wavelengths of interest and converts them into thicknesses for each layer.

Because each element emits at its own characteristic wavelength, the technique can measure several layers at once, provided their emission lines can be separated. A stack of gold over nickel over copper produces three distinguishable signals, and the instrument solves for the thickness of each layer from the spectrum. That is why the method is used for contact finishes, where the gold and nickel layers are both thin and both matter. The plating processes that produce such stacks are described under electroplating additives for PCB.

What It Can Measure

The technique is at its best on thin layers of a few tenths of a micrometre over a known substrate, which is exactly the range of a gold flash, an immersion silver or a thin nickel barrier. It measures the mass per unit area, so it reports an average over the measurement spot rather than a point value, and the spot size is typically a fraction of a millimetre to a few millimetres.

A larger spot gives a more repeatable measurement and averages over more of the surface, which is an advantage for a rough or a porous deposit and a disadvantage when the feature being measured is small compared with the spot. On a fine pitch pad the spot may cover a neighbouring region with a different deposit, and the reading is then a mixture. The measurement must therefore be taken on a feature large enough for the spot, which is one reason the finish is measured on a coupon pad rather than on the product.

Limits And Interferences

The technique has three practical limits. The first is the substrate: it needs to know what is underneath, because the underlying layers also emit and their signal contributes to the spectrum. Measuring a thin gold on a thick nickel over copper requires the instrument to be set up for that specific stack, and a stack that is not in its library gives a wrong answer.

The second is the roughness and porosity of the deposit. A porous gold layer is measured as an average over the pores, so the instrument can read the intended thickness while the actual coverage is incomplete. That is the failure mode behind a contact that fails despite a passing thickness measurement, and it is the reason a porosity test is used alongside the measurement. The third is the element range, since very light elements emit X-rays that are absorbed before they escape and cannot be measured reliably. The defects that a thickness measurement can miss are described under copper plating defects prevention.

Calibration And Standards

The instrument is calibrated with reference standards of known thickness and composition, and the calibration is verified at the start of each session with a standard that is kept for that purpose. The standards have to match the element and the approximate thickness being measured, because the relationship between intensity and thickness is not linear over a wide range.

Calibration drifts with the condition of the X-ray tube and the detector, so the check is a daily routine rather than an annual one. A reading that is taken without a valid calibration check is a number without a meaning, and a shipment that is accepted on the basis of such a reading has no evidence behind it. The measurement procedure, including the calibration interval, should be part of the process documentation rather than a matter of habit.

Applying It In Production

In production the measurement is normally taken on a coupon or on a designated pad on each panel, at a defined location, and the result is recorded against the lot. The measurement is quick, it is non destructive, and it can be made on a finished board, which makes it suitable for an incoming inspection as well as for a process check.

The spot has to be positioned consistently, and the position matters because plating thickness varies across a panel. The edge of a panel is typically thinner than the centre, so a measurement taken only at the centre reports the best case. Measuring at both positions, or at least recording where the measurement was taken, allows the variation to be seen rather than hidden.

Where Sectioning Is Still Needed

X-ray fluorescence measures thickness on a flat, accessible area. It cannot measure the plating inside a hole, because the beam cannot reach the barrel wall at a known angle. Plating thickness in a hole is therefore still measured by microsection, which is destructive and slow, or by a resistance based method on a coupon.

The two methods answer different questions and both are needed. The fluorescence measurement controls the surface finish, which is what the solder and the contact see, while the section controls the barrel plating, which is what the reliability of the hole depends on. A thickness specification should state which method applies to which feature, because a single figure applied to both is ambiguous. The interface between the measurement and the fabrication process is described under PCB design and fabrication.

Additional Considerations for This Build

Practical attention to XRF measurement 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 XRF measurement 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, nickel is the item that decides how the rest of the board is arranged. 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.

FAQ

Can the measurement damage the board? No. The beam is low power and the technique is non destructive and non contact, so a finished board can be measured and then shipped.

Why does a thickness reading differ between two instruments? Because the calibration, the spot size and the assumed underlying stack differ. Comparing results requires the same instrument setup and the same standards.

Does a passing thickness guarantee a good contact? No. A porous or contaminated deposit can satisfy a thickness measurement while performing poorly. A porosity test and a contact resistance measurement address the properties the thickness does not describe.