Fiber Protrusion on the Hole Wall: Cause, Effect and Detection

Fiber protrusion is the condition where glass fibers stand out from the resin at the wall of a drilled hole. The fibers are not removed by drilling or by desmear, and the copper that is plated over them follows their contour rather than forming a smooth barrel. The result is a wall with local thin spots, poor adhesion and a texture that can trap chemistry.

The defect is a normal consequence of drilling a glass reinforced laminate, and the question is how much of it is acceptable. A small amount of exposed fiber is present on almost every hole, and it becomes a problem when the fibers are long enough, or the resin recession deep enough, that the plating is interrupted or the wall cannot be cleaned.

What Fiber Protrusion Is

The laminate is a composite of glass fabric and resin. Drilling cuts both, but the two materials behave differently: the resin is softer and is cut and smeared, while the glass is harder and resists the cut more. The result is a wall where some fibers are broken cleanly, some are pulled loose and some stand proud after the resin around them has been removed by the desmear step.

Protrusion is therefore partly created by the drill and partly revealed by the desmear. A hole that looks acceptable after drilling can show pronounced fibers after a permanganate step, because the desmear removes resin and leaves the glass. That is why the acceptance inspection is done after desmear rather than before it.

Drilling Parameters and Fiber Damage

The drill determines how cleanly the glass is cut. A sharp tool with an appropriate chip load shears the fibers, while a worn tool with a low chip load rubs and pulls them. Spindle speed, feed and tool condition are therefore the first parameters to review when protrusion appears, and they are reviewed together rather than one at a time.

The laminate itself contributes. A fabric with a coarse weave presents larger bundles to the drill, and one with a high resin content supports the fibers better. Where a product changes from one laminate to another, the hole wall condition should be re-verified on a section rather than assumed to be unchanged, because the same parameters produce a different result.

Resin Recession and Its Relationship

Resin recession is the loss of resin at the wall, and it is the partner of fiber protrusion. Where the resin is removed, the fibers are exposed; where the fibers are cut back, the resin may stand proud. Both are measured on the same section, and a wall with heavy recession will also show protruding fibers because there is less resin to support them.

Recession is driven by the drilling heat and by the chemistry of the desmear. A hot cut, a dull tool and an aggressive desmear all increase it. The measurement and the significance of the recession are described in the notes on hole wall condition and plating adhesion.

Effect on Plating Adhesion

Plating adhesion depends on the surface it grows on. Copper adheres well to a properly prepared resin and less well to a smooth glass fiber that has not been treated, and a fiber that protrudes presents a surface with a different chemistry from the surrounding resin. The copper follows the fiber, and the interface between the two is a weak path along which the plating can lift.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/assembly-parts-e1599389540536.jpg" alt="Microsection of a hole wall showing protruding glass fibers” />

Adhesion is tested by thermal stress, and the failure that follows from poor adhesion is a pullaway of the plating from the wall. Where a section shows fibers projecting through the plating, the barrel has a built-in stress concentration at every fiber, and the life of the barrel under thermal cycling is shorter than the thickness alone would suggest.

Effect on Plating Thickness

A protruding fiber creates a shadow in the plating solution. The copper deposits on the fiber and less of it reaches the wall behind it, so the thickness measured at the wall can be lower than the thickness measured over the fiber. That is why thickness measurements taken at a single point on a rough wall can be misleading, and why the minimum is taken over several points that include the depressions.

The measurement consequence is important for the acceptance decision. A barrel that reports an acceptable minimum on a smooth area of the section may have a thinner deposit in the recess beside a fiber, and the point where the plating is thinnest is exactly where the adhesion is weakest. The measurement practice is set out in the notes on barrel microsection.

Detection and Measurement

Detection is by microsection, and the wall is examined at several points around the circumference and at more than one depth. The section shows the fibers, the resin between them and the copper that has been plated over both. Magnification has to be high enough to resolve individual fibers, which is beyond the magnification used for a thickness measurement.

The results are recorded as a description with an image rather than as a single number. Terms such as light, moderate or heavy are more useful than a measurement, because the defect is a distribution rather than a dimension, and because the acceptance decision depends on the continuity of the plating rather than on the length of a fiber.

Process Window Adjustments

When protrusion is unacceptable, the first response is to review the drill: a new tool, a higher feed per revolution and a matched spindle speed. The second is to review the desmear, because a shorter or a gentler cycle removes less resin and exposes fewer fibers, at the cost of leaving more smear behind. The two steps have to be balanced rather than optimised separately.

The mechanical preparation before plating is part of the same balance. An aggressive scrub leaves fibers standing proud before the plating line, and a gentle one leaves the surface less receptive. The methods and their effect on the surface are described in the notes on scrubbing and surface preparation.

Detection in Production

In production, the check is a section from a drilled and desmeared coupon taken at the same interval as the plating coupon. The coupon carries the smallest hole and the thickest stack, because those are the holes where the drill generates the most heat and the wall condition is worst. The result is recorded with the drill parameters and the desmear condition.

A faster check that needs no section is to examine a drilled hole under magnification with a light at a low angle, where protruding fibers appear as a rough, sparkling wall. That check detects gross cases and says nothing about the plating that follows. Where the two checks disagree, the section decides, and its preparation is described alongside the other hole wall features in the notes on entry and exit quality.

Process Control and Records

The controls are the drill parameters and the tool condition, the laminate lot, the desmear cycle and the mechanical preparation. Each is recorded per lot, and the section image is attached to the record so that the trend can be read from images rather than from a memory of what the last section looked like.

Where a change of laminate or a change of drill supplier is made, the verification is a section before production rather than after a complaint. One coupon, one section and ten minutes of examination is the cost of knowing whether the wall condition has changed, and it is far less than the cost of stripping plated panels that fail a thermal stress test.

FAQ

Is fiber protrusion always visible after drilling? Not always. The desmear removes resin and reveals fibers that were previously covered, so the condition is assessed after desmear.

Does protrusion cause a plating void? It can. The fiber shadows the wall behind it and the copper can be discontinuous there, particularly in a deep, high aspect ratio hole.

Can the desmear be shortened to reduce protrusion? It can, and the change has to be balanced against the smear that the step exists to remove. Both are measured on the same section.