Desmear And Hole Wall Quality In PCB Drilling

Drilling a hole in a laminated board generates heat, and the heat softens the resin at the wall of the hole. The softened resin is smeared along the wall by the rotating drill, and a thin film of it is deposited over the copper of the inner layers where the hole passes through them. That film is an insulator, and if it is left in place the plating that follows cannot bond to the copper, which produces an interconnection that passes a continuity test and fails later.

This article explains what drilling does to the hole wall, why the smear matters, how it is removed, and how the result is verified.

What Drilling Does To The Hole Wall

The drill cuts through resin, glass fibre, and copper, and the three materials behave differently. The copper is ductile and forms a small burr, the glass fibre is hard and brittle, and the resin softens with the heat of the cut and deforms plastically. The wall of the finished hole is therefore not clean copper and resin but a mixture, and the resin has been pushed, rather than cut, over the copper targets of the inner layers.

Two other features appear at the same time. At the entry, the copper foil can be lifted into a small lip called a nail head or an entry burr, and at the exit the burr can be larger and can short a neighbouring conductor if it is not removed. The drill itself wears, and a worn drill generates more heat and produces a worse wall, which is why the tool life is limited by the number of holes rather than by the cutting edge alone.

Why Smear Matters

The plating that forms the connection between the hole and an inner layer is deposited on top of whatever is on the wall. If a resin film covers the copper, the plating bonds to the resin, and the resulting structure has no metallic connection to the inner layer at that point. The board passes a continuity test because the barrel is continuous, and the connection to the inner layer is made only where the smear was thin or absent, which means the current path is smaller and less reliable than the drawing suggests.

The failure appears under thermal stress. During reflow and in service, the barrel expands more than the laminate and the connection at the inner layer is strained, and a connection that is partly resin and partly copper separates early. The result is an intermittent open that appears in the field, usually after thermal cycling, and it is one of the classic reliability failures of a plated through hole.

Desmear Methods

The standard process is a permanganate desmear. The board is first treated with a swelling solvent that penetrates the resin and loosens it, then exposed to a permanganate solution that oxidises the resin at the wall, and finally neutralised to remove the manganese residues. The swelling step is what makes the process effective on a resin that would otherwise resist the oxidiser, and the control of its time and temperature is as important as the permanganate itself.

Plasma treatment is used where the material demands it. The board is exposed to a plasma that attacks the resin, which is effective on a polytetrafluoroethylene material that a wet process handles poorly, and it also cleans the wall and improves the adhesion of the plating to the resin. The trade is the cost of the equipment and the batch nature of the process. Some materials are treated with a sulfuric acid process instead, and the choice follows from the resin chemistry rather than from the fabricator preference.

<img src="https://www.gopcba.com/wp-content/uploads/2026/06/DFM-分析-1.jpg" alt="Backlight test performed on a plated through hole” />

Etchback And Its Limits

A related process removes some of the resin deliberately to expose the copper of the inner layer, which increases the contact area between the plating and the target. That removal is called etchback, and it is measured as the depth to which the resin is recessed behind the copper. A small etchback improves the connection, while an excessive one removes the resin that supports the glass fibres and leaves a weak wall where the plating can pull away during thermal stress.

The practical window is therefore narrow. Too little treatment leaves smear and a poor connection, and too much leaves a mechanically weak wall. Both conditions are visible in a microsection, which is why the process is monitored with a cross section on a regular basis rather than with a process parameter alone.

Verification

The classic test is a backlight inspection. A thin section of the hole wall is illuminated from behind, and the amount of light that passes through is a measure of the resin that remains over the copper. A wall with no smear transmits almost nothing, and a smear of increasing thickness transmits more, so the result is expressed as a grade rather than as a measurement. The test is quick and it is applied to a sample from each panel or each lot.

The microsection follows when the backlight result is marginal or when a qualification is being performed. It shows the wall profile, the extent of the etchback, the thickness of the plating at the inner layer connection, and any separation between the plating and the target. The two tests together are the evidence that the desmear is working, and they are repeated whenever the material, the drill, or the chemistry changes. The plating that follows is described under electroplating additives and via filling in HDI, and the sequence in full under PCB design and fabrication.

Process Control and Verification

On a design of this kind, smear is the item that decides how the rest of the board is arranged. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

Process Control and Verification

On a design of this kind, smear is the item that decides how the rest of the board is arranged. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

FAQ

What is smear? A thin film of resin that the drill has softened and dragged across the copper of an inner layer. It insulates the copper from the plating and prevents a metallic connection.

How is desmear verified? With a backlight test, which measures the light transmitted through a thin section of the hole wall, and with a microsection when the result is marginal or when a process is being qualified.

Can a hole be over treated? Yes. Excessive etchback removes the resin that supports the glass fibres and leaves a wall where the plating can separate during thermal stress.