PCB Hole Wall Quality: How to Read a Cross Section

Why the Hole Wall Is the Critical Surface

Every plated through hole is a connection that depends on a thin layer of copper adhering to a wall of drilled resin and glass. The copper must be continuous from the top pad to the bottom pad, thick enough to carry the current and to survive thermal cycling, and bonded well enough that it does not lift or crack. None of that can be judged from the outside. An electrical test proves continuity at the moment of test; a microsection, also called a cross section, shows what the barrel actually looks like, and it is the only direct evidence that the drilling and plating process produced what the drawing specified.

What a Microsection Shows

A coupon or a sample of the panel is encapsulated in resin, ground and polished back until the holes are cut through their centre, and examined at magnification. The image reveals the copper thickness at every point of the barrel, the shape of the plating at the surface pad and at the inner layer connections, the condition of the hole wall beneath the copper, the amount of resin smear or etchback, the presence of voids, and the geometry of the drilled hole including any nailheading or burr. When the sample is also thermally stressed before sectioning, the same image reveals cracks that would otherwise be invisible.

Measuring Plating Thickness

Thickness is measured at several points and the acceptance decision is based on the thinnest point, not the average. The deposit is normally thicker at the surface and thinner in the middle of the barrel because current density and chemical exchange fall off with depth, which is why a deep hole in a thick board is the hardest case. The measured value is compared with the minimum for the applicable class and for the process: around twenty micrometres for standard work, higher for high reliability classes, and higher again for press-fit holes. A measurement taken at the surface and reported as the barrel thickness is a common error, and it can make a marginal board look compliant.

The Defects the Section Reveals

Barrel cracks. Usually at the mid point of the barrel or at the corner where the barrel meets an inner layer pad, caused by thermal expansion mismatch and aggravated by thin plating. A partial crack may still pass electrical test.

Voids and pits. Gaps in the copper deposit caused by trapped air, contaminated chemistry or a rough wall. They reduce the effective thickness and act as crack starters.

Resin smear and inadequate desmear. Resin spread across the inner layer copper by the heat of drilling blocks the connection between the barrel and the inner layer. The section shows a discontinuity that the electrical test may still pass, because the drill has exposed some copper elsewhere on the interface.

Nailheading. The inner layer copper is deformed into a nail shape by the drill, thinning the dielectric and leaving a barrel corner that is more likely to crack.

Etchback or wedge voids. The opposite problem: the resin has been removed beyond the inner layer copper, leaving a gap that plating may not fill, which produces a wedge shaped void.

Delamination and measling. Separation between the resin and the glass weave, often caused by moisture or by excessive drilling heat.

What Causes Them

Drilling conditions set the wall quality: a worn drill, excessive feed, inadequate entry or exit support or poor chip removal all roughen the wall and generate heat. Desmear chemistry and control determine whether the smear is removed without going too far. Plating depends on the current distribution, the agitation, the bath chemistry and the hole geometry, and the aspect ratio sets how hard it is to plate evenly. Laminate moisture content, press parameters and the glass transition temperature determine how the material responds to the drilling and the thermal excursions that follow. A section therefore reads as a record of the whole process chain, not only of the plating line.

How Often to Cut One

A first article should always include a section, and in high reliability work every lot. In production, a sampling plan that includes the deepest and the smallest holes is more informative than a random sample, because aspect ratio drives the failure modes. Where the process has just changed, whether the drill bit supplier, the desmear chemistry or the plating bath, sections at the start and middle of the run will catch a drift before the yield does. The section is destructive, so it must be planned on a coupon that belongs to the panel rather than on a deliverable board.

FAQ

What is a microsection? A polished cross section through the centre of the plated holes, used to measure plating thickness and to inspect the barrel, the hole wall and the interfaces with the inner layers.

Where is plating thickness measured? At several points, with the acceptance decision based on the thinnest point, which is normally in the middle of the barrel rather than at the surface.

Can a board with a cracked barrel pass electrical test? Yes. A partial crack leaves continuity intact until it propagates, which is why a section, and sometimes resistance monitoring, is used.

What is resin smear? Resin spread across the inner layer copper by drilling heat. The barrel appears connected but the interface has little or no adhesion, so the joint fails under stress.

How often should sections be cut? On a first article always, on a sampling basis in production, and whenever the drilling, desmear or plating process has changed.

Conclusion

A microsection is the only direct evidence that a plated hole is what the drawing required, and it should be read as a record of the entire process: drilling, desmear, plating and laminate. Measure the plating at its thinnest point, look for cracks, voids, smear and nailheading, sample the holes with the worst aspect ratio, and cut sections on coupons that belong to the panel. The plating and aspect ratio limits are part of PCB capabilities, the via and stackup planning belongs in PCB design and layout, and the process sequence that the section records is described in PCB manufacturing, with the inspection system behind it in quality management. A prototype PCB assembly build with section data settles the barrel question before volume in 2026.