Copper Plating Thickness in Vias

What the Plating Has to Do

The copper plating in a via is both the electrical connection between the layers and the mechanical structure of the barrel. It has to carry the current, it has to withstand the expansion of the laminate during soldering and thermal cycling, and it has to survive the drilling and the handling. Because the barrel is thin and the laminate around it expands far more than copper does, the plating is the component under the most strain in the whole board. Its thickness and its ductility are therefore the two properties that determine whether the via will survive the product’s life.

How the Barrel Is Plated

After the drilling and the desmear, a thin conductive layer is deposited, usually by electroless copper or by a direct metallisation process, and then the copper is electroplated to build the thickness. The electroplating depends on the current’s ability to reach the inside of the hole, and that ability falls as the hole becomes deeper and narrower. The result is a thickness that varies: the plating is thickest at the surface and at the entry, and thinnest in the middle of the barrel and at the knee. The variation is the reason the specification is written on the minimum rather than the average.

The Aspect Ratio

The aspect ratio, the board’s thickness divided by the hole’s diameter, is the strongest influence on the plating. A low ratio allows the solution to circulate freely and the plating to be uniform; a high ratio restricts the flow and the current’s distribution, and the middle of the barrel receives less copper. The practical consequences are that a small hole in a thick board is harder to plate, that the tolerance on its thickness is wider, and that the probability of a thin spot rises. This is why a high aspect ratio design should be discussed with the fabricator before the stackup is fixed, rather than discovered at the first microsection.

Measuring the Thickness

The thickness is measured on a microsection, at several points around the barrel and at the knee, and the minimum is compared with the specification. Because the measurement varies by position, the standard procedure is to measure at defined locations and to report the minimum, usually together with the average. A non destructive measurement is also possible with a dedicated instrument, but the section remains the reference. Where the minimum is close to the limit, the process is marginal even if the average is comfortable, which is why a single number in a report can hide the risk.

Plating Quality and Ductility

The thickness is not the only property that matters. The copper’s structure determines its ductility, and a bright, brittle deposit with a high impurity content can crack under the strain that a ductile one would accommodate. The plating’s chemistry, the current density and the additives control the structure, and the result is verified by the elongation measured on a test coupon or by the behaviour of the barrel in thermal cycling. A hole that meets the thickness specification but cracks in the cycling test has identified a quality problem rather than a thickness problem, and the cure is in the plating chemistry rather than in the process time.

Thermal Stress and the Barrel

The laminate expands far more than copper when it is heated, so the plating in the barrel is stretched along its axis during soldering and during thermal cycling. The strain concentrates at the knee and at the middle of the barrel, which are also the thinnest locations, and the combination is what causes the classic barrel crack. The countermeasures are a thicker plating, a more ductile copper, a lower expansion laminate and a shorter barrel. Where a product sees severe thermal cycling, the specification of the plating should reflect it rather than rely on the general requirement.

Specifying and Verifying

The specification should state the minimum thickness, the measurement method and the location, and it should be supported by a ductility requirement where the application demands it. The verification uses a coupon that contains the product’s smallest hole and highest aspect ratio, and the section is taken at the end of the process, because the thickness is affected by every step from the metallisation to the plating. The results should be recorded with the panel’s identity and compared across lots, so that a trend is visible before a failure occurs. Where the fabricator’s process is stable and the data is consistent, the inspection frequency can be reduced without losing control.

Plating for a High Current Path

Where a via carries a significant current, the barrel’s resistance matters. A thin plating has a higher resistance per unit length than a thick one, and several vias in parallel divide the current and the heat. The design should use enough vias for the current and should keep them close to the device so that the copper path between them is short. Where the via is used only for thermal conduction, the same logic applies, and the number of vias is often the most effective lever. The plating thickness and the via count together determine whether the path is adequate.

FAQ

Why does the plating vary around the barrel? Because the current and the solution reach the inside of the hole less easily, so the entry and the surface receive more copper.

What is the aspect ratio? The board’s thickness divided by the hole’s diameter, and it determines how difficult the plating is.

Why is the minimum measured rather than the average? Because the reliability is set by the thinnest point, which is usually at the knee or the middle of the barrel.

Can a thicker plating be brittle? Yes. The deposit’s structure and impurities decide its ductility, which is a separate property from the thickness.

Where does a barrel crack start? Usually at the knee or the middle of the barrel, where the plating is thin and the strain is highest.

Conclusion

The plating thickness and its ductility decide whether a via survives thermal cycling, so specify the minimum, control the aspect ratio and verify on a coupon. Copper plating belongs to PCB manufacturing, the stackup and the hole sizes are part of PCB design and layout, and the measurement is described under quality management. Plating requirements for a new stackup are fixed during prototype PCB assembly in 2026.