Measuring PCB Copper Thickness: Methods Compared

Why Copper Thickness Needs Measuring

Copper thickness appears on a drawing as a weight, such as one ounce per square foot, which corresponds to about thirty five micrometres of foil before processing. What the finished board carries is a different number, because the plating, the etching and the surface finish all change it. The thickness on a trace sets its resistance, its current capacity and its impedance, and the thickness in a hole sets the reliability of the barrel. Where a design depends on any of those, the thickness has to be measured rather than assumed, and the method chosen determines what the number actually means.

Microsection

The cross section is the reference method. A coupon is encapsulated, ground back to the feature of interest and examined at magnification, and the copper thickness is read directly from the image. Its strength is that it shows everything at once: the thickness of the surface copper, the thickness in the hole, the plating distribution across a trace, and the condition of the interfaces. Its weaknesses are that it is destructive, that it measures only the plane that was sectioned, and that the measurement depends on the polishing being perpendicular to the feature and on the magnification being calibrated. It is the method used for arbitration because both parties can look at the same image.

Coulometric and Other Non Destructive Methods

The coulometric method measures the copper in a defined area by anodic dissolution: a small cell with a known aperture is placed on the surface, the copper inside it is dissolved electrochemically and the amount is calculated from the current and time. It is accurate on surface copper and fast, and it can be used on a production coupon. Its limitation is that it measures one small area and gives no information about the hole, and it is affected by the surface finish and by any organic residue. Beta backscatter and X ray fluorescence methods measure thickness by interaction with the material and are fast and non destructive, but they are calibrated against standards that may differ from the alloy and surface condition of the sample, and their accuracy on thin or plated surfaces varies between instruments.

Measuring the Hole Rather Than the Surface

Hole copper is where the reliability question lives, and it is the hardest measurement. The section is the standard method and the only one that shows the distribution around the barrel, the minimum at the middle and the condition of the interfaces. Some shops use resistance measurements on coupons to estimate the average barrel copper, which is quick but insensitive to a local thin spot. Others use eddy current or specialised probes on test coupons. The practical approach is to rely on the section for qualification and on process control, coupons and periodic sections for production, because plating thickness in a barrel is a process property rather than a board property.

What the Numbers Mean

Foil weight on the drawing refers to the starting laminate, and the finished copper on a trace is thinner after etching, particularly on the side walls where the etch undercuts the resist. A trace designed at one ounce may finish between twenty five and thirty five micrometres depending on the process, which is why impedance calculations use the finished value and why the etch factor is applied to the artwork. In a hole, the plating adds to the foil, and the specification is usually a minimum thickness at the thinnest point of the barrel. Surface finish, whether it is an organic coating or a metal such as nickel and gold, sits on top of the copper and is normally excluded from the copper measurement, although an X ray fluorescence instrument will happily measure the whole stack unless it is set up to separate the layers.

Pacific Applying It to a Specification

State copper weight per layer for the laminate, state the minimum plated thickness for the holes, and state which surface the measurement applies to and at which point of the barrel. Require a coupon on every panel for impedance and plating critical designs, and define the measurement method so that a supplier and a customer measuring the same board get the same answer. Where the design depends on trace resistance, specify the finished thickness rather than the foil weight, because that is the number the circuit actually sees. Where the board is for a high reliability application, specify the sampling and the record keeping, since the measurement is only useful if it can be produced later.

FAQ

What does one ounce of copper mean? One ounce of copper per square foot of area, which corresponds to about thirty five micrometres of nominal foil thickness on the starting laminate.

Which method is most accurate? The microsection, because it measures the feature directly and shows the distribution, but it is destructive and only samples the plane that was cut.

Can copper thickness be measured without cutting the board? Yes, with coulometric dissolution or X ray methods, which are fast and non destructive but measure a small area and say nothing about the barrel.

Why is finished copper thinner than the foil weight? Etching removes copper and undercuts the resist, so the finished trace is thinner than the laminate that was ordered, especially on the side walls.

How is hole copper measured? By cross section, with the acceptance decision based on the thinnest point of the barrel rather than on the surface measurement.

Conclusion

Copper thickness is a design parameter that changes during processing, so it should be specified where it matters and measured with a method that answers the right question. Use the section for arbitration and for barrels, use coulometric or X ray methods for fast surface checks, and state the finished requirement rather than only the foil weight. The plating and copper limits available from a supplier are part of PCB capabilities, the current and impedance consequences belong in PCB design and layout, and the plating process itself is described in PCB manufacturing. A prototype PCB assembly build with coupon cross sections confirms the thickness before volume in 2026.