Nailheading in PCB Drilling: Causes and Limits
What Nailheading Is
When the drill passes through an inner layer pad, the copper there is thin and ductile and the laminate around it is comparatively soft. If the drill pushes rather than cuts, the copper is deformed outwards and downwards into the resin, taking on the shape of a nail head around the hole. The result is a local thinning of the dielectric between that inner layer and the next one, and a barrel corner that sits on a deformed rather than a flat pad. Nailheading is measured as the amount by which the copper is displaced beyond its original thickness, usually expressed as a ratio of the deformed thickness to the original, or as a reduction in the dielectric spacing.
Why It Matters
Three problems follow from it. The first is dielectric thinning: if the copper is pushed into the resin, the insulation between that layer and the adjacent one is reduced, which reduces the breakdown voltage of the board locally and changes the impedance at that point. The second is the barrel corner geometry: the plating has to follow the deformed copper, which produces a sharp transition where the barrel meets the inner layer, and that transition is a crack initiation site during thermal cycling. The third is measurement ambiguity: a nailhead can make an annular ring measurement look larger or smaller than it is, and it can hide a marginal connection that would otherwise be visible. In high reliability work the defect is therefore limited explicitly, because a board with severe nailheading can pass an electrical test and still fail in service.
What Causes It
The dominant cause is a drill that is dull or worn past its useful life, because a dull cutting edge rubs and pushes instead of shearing. Excessive feed per revolution has a similar effect, since the drill is asked to remove too much material per edge and the thrust rises. Insufficient or incorrect entry and exit material reduces the support that holds the copper flat. A high spindle speed with a low feed generates heat, which softens the resin and lets the copper deform more easily. Harder laminates with a high glass content increase the thrust needed and therefore the deformation. And a panel with a high layer count, or with an inner layer pad that is thick relative to the surrounding dielectric, is simply more prone to the effect because there is more copper to displace.
How It Is Measured and Limited
Nailheading can only be assessed on a cross section, where the deformed copper is visible against the laminate. The measurement is made on the worst inner layer of the worst hole in the sample, and the acceptance criterion is expressed either as a maximum ratio of deformation or as a minimum remaining dielectric thickness, with the applicable class of the rigid board specification providing the framework. The limits are tighter for high reliability products and for controlled impedance layers, because the dielectric spacing is part of the electrical design there. A coupon that travels with the panel is the right place to measure it, since the defect is destructive to assess.
Preventing It
Prevention is a drilling discipline. Enforce the hit count so that drills are replaced before they dull appreciably, and record it. Set the feed and speed from the laminate manufacturer recommendation rather than from habit, and verify by measuring the result rather than assuming it. Use the correct entry and exit materials for the layer count, and keep them clean: a used entry sheet that has been drilled once provides less support than a fresh one. Control the drill bit geometry and the point angle for the material, and monitor spindle runout, pressure foot condition and back up pressure, because all three affect the thrust and therefore the deformation. Where the laminate is hard or the layer count is high, a sharper point and a slightly lower feed usually reduce the effect more than any change to the plating.
Interaction With the Rest of the Process
Nailheading is often found together with other drilling defects, because the same wear or feed condition produces them: rough walls, resin smear, burrs and a hole that is out of round. When a section shows nailheading, the useful response is to look at the whole drilling result rather than to treat it as a single parameter. It also interacts with the desmear step, because the deformed copper can shadow the resin beneath it and make the smear harder to remove, and with plating, because a deformed corner changes the current distribution in the barrel. Fixing the drilling is therefore the first step in fixing several downstream symptoms at once.
FAQ
What is nailheading? Deformation of inner layer copper into the laminate by the drill, which thins the dielectric locally and changes the barrel corner geometry.
Why is it a problem? It reduces dielectric spacing and breakdown voltage, changes impedance locally, and creates a stress concentration that can initiate a barrel crack during thermal cycling.
How is nailheading measured? On a cross section, as a ratio of the deformed copper thickness or as a reduction in the remaining dielectric thickness, on the worst layer of the sample.
What causes it? A worn or dull drill, excessive feed, inadequate entry and exit support, and hard laminates or high layer counts that raise the drilling thrust.
How is it prevented? Enforce drill hit counts, use the laminate supplier recommended speed and feed, use fresh entry and exit material, and monitor spindle runout and pressure foot condition.
Conclusion
Nailheading is a drilling defect with electrical and reliability consequences, and it is invisible without a cross section. Enforce the drill life, set speed and feed from the laminate data, support the entry and exit properly, and measure the result on a coupon rather than assuming it, especially on high layer counts and impedance critical layers. The drilling and plating limits that apply are part of PCB capabilities, the stackup planning belongs in PCB design and layout, and the drilling process that causes it is described in PCB manufacturing. A prototype PCB assembly build with a coupon section confirms the result before volume in 2026.
