Hole Wall Quality: Drill Burrs, Desmear and Plating Adhesion

Plating only adheres to a surface it can reach, and the surface inside a drilled hole is created by a rotating cutter that tears fibres, smears resin and leaves debris. The drilling and desmear steps decide whether the copper that follows is connected to the barrel or sitting on top of a layer of burnt resin.

What the Drill Leaves Behind

A drill bit cuts by shearing the resin and the glass, and the quality of the cut depends on the speed and the feed as much as on the bit. Too high a feed tears the glass bundles and leaves them projecting into the hole, while too low a feed rubs the laminate and generates heat that melts the resin.

The result is a wall that varies around its circumference: a rough area where the fibres were cut cleanly and a smooth, glossy area where the resin was smeared. The hole wall is therefore never uniform, and the process has to make the worst part of it plateable. A wall that is rough over most of its circumference plates well, because the roughness provides the keying, while a wall that is smooth and glossy plates poorly even when it is clean.

Burrs at Entry and Exit

Where the drill exits the copper it pushes a burr out of the foil, and the burr is a thin lip that can fold back into the hole or stand above the surface. A burr inside the hole shadows the plating and leaves a void, while a burr on the surface interferes with the resist and with the solder mask that follows.

The drill burr is controlled with a backing board on the exit side, with a sharp bit and with a reduced feed at the exit rather than through the whole hole. The burr height is measured on a sample under a microscope, and the acceptance is a maximum height rather than an absence. The burr is also worse on a heavy copper layer, because there is more metal to push out of the way, which is why a heavy copper stack is drilled with a different parameter set from a standard one.

Smear and Why It Has to Be Removed

Resin smear is a film of melted epoxy that the drill drags across the inner layer connections, and it is invisible after drilling because it looks like the rest of the wall. It is insulating, so a plated barrel over a smeared copper plane is connected to the plane thermally but not electrically.

Desmear removes the film with a permanganate chemistry that etches the resin surface as well as dissolving the smear, which is why the process also improves adhesion. The amount of etch is controlled, because too much leaves the glass fibres standing proud and creates a rough surface that plates unevenly. The etch also removes the top layer of resin between the fibres, and the depth of that removal is measured on a section rather than inferred from the chemistry in the tank.

Aspect Ratio and Its Consequences

The aspect ratio is the board thickness divided by the drill diameter, and it describes how difficult the hole is to clean, to plate and to inspect. A ratio of 8 to 1 is routine, and beyond about 12 to 1 the process window narrows quickly and the plating throw becomes the limiting factor.

A high ratio also makes the drill more likely to wander and to leave a barrel that is thinner at the middle than at the ends. Our hole copper notes describe how that variation is measured, and the figure is what decides whether the design can be built at the required class.

Drill Parameters That Matter

The parameters are the spindle speed, the feed per revolution, the retract rate and the number of hits per bit, and they are set together for each drill diameter and stack. A retract that is too fast drags the bit against the wall and produces a rough hole, while one that is too slow adds heat without improving the cut.

Bit wear is counted in hits and the limit comes from the hole wall quality rather than from a calendar. A worn bit produces a hole that is smaller than nominal and a wall that is rougher, so the drill programme is supported by a bit life log that is checked against the measurement. The log is more informative than the count itself, because the same number of hits produces different results on different stacks, and the log shows where the change occurred.

Entry and Backing Materials

An entry material on top of the stack protects the copper from burrs and cools the bit, and a backing board under the stack does the same at the exit. The materials are chosen so that they do not smear themselves, and a backing board that is too soft leaves a worse burr than no backing at all.

The stack itself is part of the same system, because a panel that is not held flat will drill holes with different quality across its length. The drilling machine’s pressure foot and the stack’s own flatness are therefore checked alongside the drill parameters. A stack that is not held flat also lets the bit enter at an angle, which produces an oval hole and a wall that is thicker on one side than the other.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/electronics-manufacturing-pbc-assembly.jpg" alt="Drill burr on a copper foil surface after drilling” />

Inspection of the Hole Wall

The wall is inspected after drilling and after desmear on a sample that is sectioned, and after plating on a sample that is both sectioned and pulled. The inspection looks for smear, for fibre protrusion, for burrs and for the uniformity of the plating that follows.

The inspection is destructive, so it is done on a coupon rather than on the panel, and the coupon is drilled on the same stack with the same parameters. Our plating thickness notes describe the same coupon from the plating side, which is what makes the two measurements comparable.

Effects on Plating Adhesion and Reliability

Plating adhesion depends on the mechanical keying of the copper to the desmeared resin and on the cleanliness of the inner layer connection. Where the smear was not removed, the connection is a physical contact rather than a metallurgical bond, and it fails after a few thermal cycles.

That failure appears as an intermittent open, and it is one of the failure modes that no electrical test at the time of manufacture will detect. The defence is the process control at drilling and desmear, verified by the coupon section, rather than any test of the finished board.

Documentation and Process Control

The record should carry the drill parameters per diameter, the bit life limit, the desmear chemistry and etch amount, and the coupon results for each lot. Where a parameter is changed, the coupon is re-run before the change is released into production.

The drawing should state the hole tolerance, the aspect ratio limit and the plating class, since all three constrain what the shop can do. Our fabrication notes set out how those limits are written so that a design that cannot be built is caught before the order rather than after it. A note that gives the hole size without the aspect ratio leaves the shop to discover the limit by failing to plate the middle of the barrel, which is an expensive way to learn it.

FAQ

Does desmear damage the laminate? It etches the resin deliberately, and the amount is controlled so that the surface is prepared rather than eroded. Over-etching leaves fibres standing proud and produces a plating interface that looks rough in a section.

Can smear be detected without a section? Not reliably, because the film is transparent and looks like the wall. The section after desmear is the standard check, and it is taken from a coupon drilled with the production parameters.

Why does a burr matter if the hole is plated afterwards? Because the plating has to cover the wall, and a burr hides part of it. The resulting thin or missing plating is inside the barrel, where it cannot be seen and where it will fail under thermal cycling.