Copper Thickness Measurement on Inner Layers of a PCB

Inner layer copper carries the current, forms the reference planes and sets the impedance of every controlled line on the board, yet its thickness is one of the least visible properties of a finished panel. It is buried under laminate and only a coupon or a section reveals it. Measuring it correctly, and interpreting the result against the right specification, is what closes the loop between the design and the delivered board.

Why Inner Layer Copper Thickness Matters

The finished thickness of an inner layer trace sets its resistance and therefore its voltage drop and its temperature rise under current. It also sets the cross section that determines how much current a plane can carry, and it interacts with the dielectric thickness to define the impedance of a controlled line.

On a plane layer, the copper thickness affects the spreading of heat, the shielding effectiveness and the current distribution across the plane. On a signal layer it affects the etch profile, because a thicker foil needs a longer etch and produces a more sloped sidewall and more undercut. Our current capacity guide explains how the section is used to calculate a temperature rise.

How the Thickness Is Specified

The starting point is the foil weight, quoted in ounces per square foot, and the finished thickness is not the same figure. Lamination, oxide treatment and the plating applied during processing all add to or remove material, and the drilled and plated holes change the local situation further. A one ounce foil starts at roughly thirty five micrometres and the finished trace is usually a little thinner after etching.

The specification should therefore state what is being measured: the base foil, the finished trace on an outer layer, or the finished inner layer after lamination. Ambiguity here is the most common source of a measurement that appears out of specification while the process is actually correct.

<img src="https://www.gopcba.com/wp-content/uploads/2025/08/11.jpg" alt="Microsection of an inner layer trace showing copper thickness” />

Microsection Measurement

A microsection is the reference method. A coupon or a board sample is mounted, ground and polished, and the copper thickness is measured on the polished face under a microscope with a calibrated scale or an image analysis system. It is direct, it can be taken at a defined location and it shows the structure at the same time.

The measurement depends on the quality of the preparation. A section that is not polished flat shows a thickness that is larger than the true value, and one that is over etched or rounded at the edge shows a smaller one. The magnification and the calibration should be recorded with the result, and the same operator technique should be used across a series.

Non Destructive Methods: XRF and Eddy Current

X-ray fluorescence measures the characteristic emission from the copper when the surface is excited, and it gives the thickness of the layer it can see. It is fast and non destructive, which makes it suitable for a production check, but it is a surface method and it is affected by the material underneath if the layer is thin.

An eddy current gauge measures the effect of the copper on a coil, and it works through a coating or a mask. It is useful on a finished outer layer, and it is less suited to an inner layer that is buried under laminate. Both methods should be correlated with a microsection before being used as a production acceptance tool, because the correlation is what makes the number meaningful.

Coupon Based Verification

The most practical way to verify an inner layer is a coupon that is built with the same foil, the same lamination and the same etching as the product. The coupon can be sectioned without destroying a board, and it can also be measured electrically to confirm the resistance that the design expects.

Our test coupon notes describe how the coupon is laid out on the panel and how the measurements are taken. A coupon that is placed in a different area of the panel from the product may see a different etch condition, so its position should be chosen to represent the product.

Where the Measurement Is Taken

A single measurement on a panel is not a statement about the panel. The thickness varies with the position, because the etching rate depends on the local copper density and on the position in the spray, and the plating distribution varies across the panel as well. Measurements on a wide trace, a narrow trace and a plane can give three different answers on the same layer.

The measurement plan should therefore specify the feature, the location and the number of readings. A reading taken on a wide plane is not representative of a fine signal trace, and mixing the two in one specification invites a disagreement that neither side can resolve.

Sources of Variation Across a Panel

The main contributors are the etching process, the plating process and the lamination. Etching is faster where the copper density is low, because the etchant is not depleted locally, so a trace next to a large plane sometimes etches differently from one in open area. Plating thickness falls towards the centre of a panel on a high aspect ratio product.

Lamination can also change the dimension by flowing resin, which affects the dielectric rather than the copper but changes the measured impedance. Our laminate properties guide describes how the resin system behaves under heat and pressure.

Interpreting Results and Communicating Them

A thickness result has to be presented with the method, the location and the uncertainty. A microsection reading to the nearest micrometre and an eddy current reading to the nearest tenth look very different when placed side by side, even when they agree within their own uncertainties.

The acceptance decision should use the specification range for the feature being measured, not a generic figure. Our plating thickness guide covers the measurements taken on plated holes and surfaces, and our quality documentation describes how a thickness result outside the range is classified at gopcb.

Linking Thickness to Performance

The purpose of the measurement is to predict performance, so the result should be tied back to a functional quantity: the resistance of a trace, the impedance of a line or the temperature rise of a plane. A thickness that is inside the specification but at the low end still produces a measurable change in all three, and the design margin should be able to absorb it.

Where the margin is thin, the measurement plan should be tighter rather than the specification looser, because the cost of a reduction in thickness is paid in the field rather than at the etcher.

FAQ

What is the most accurate method for inner layer copper? Microsection is the reference method, because it measures the layer directly and shows the structure. The non destructive methods are useful for production once they have been correlated with a section.

Can an eddy current gauge measure an inner layer? It cannot see through the laminate reliably, so it is used mainly on outer layers and on coated surfaces. Inner layers are verified by section or by a coupon.

Why does the finished thickness differ from the foil weight? Because lamination, treatment and etching all change the dimension, and the plating applied during processing adds material in the holes and on the surface. The specification should state which stage is being measured.