PCB Etching: Chemistry, Etch Factor and Width Tolerance

Etching is the step that turns a copper-clad panel into a circuit. A resist pattern is printed or laminated onto the copper, the panel passes through a chemical bath that dissolves the metal not protected by the resist, and what remains is the conductor pattern. Simple in principle, and full of process detail that determines whether the finished trace is the width the designer intended.

This article covers how the process works, what the chemistry does, and how the tolerance on trace width is controlled.

The Sequence Before Etching

Etching is not the first step and cannot be treated in isolation. The panel is cleaned, the resist is applied and exposed, and the pattern is developed so that the resist remains only where copper is wanted. Any variation in that sequence appears in the etched result, which is why most etch defects are actually resist defects.

The resist choice matters. Dry film resist gives a uniform thickness and holds fine features well, while liquid resist can achieve thinner layers and better resolution but requires more careful coating. Screen printing is used for coarse patterns where cost matters more than resolution, and it remains common on single-sided boards and on panels where the smallest feature is above about 0.3 mm.

What the Chemistry Does

Most production PCB etching uses an alkaline etching solution based on ammonia, typically a cupric chloride or an ammonia complex, which dissolves copper at a controlled rate and is regenerated continuously. The chemistry is regenerative: the dissolved copper is recovered and the bath is replenished, so the etch rate stays stable across a long production run.

Ferric chloride is the alternative, common in smaller shops and for prototype work. It is simpler to handle and does not need the same pH control, but it etches less uniformly and has a shorter bath life. The choice affects the achievable tolerance more than it affects the basic capability.

Etch Factor and Sidewall Angle

Etching proceeds sideways as well as downwards, so a trace is narrower at its base than at the top of the resist. The ratio of the etch depth to the sideways undercut is called the etch factor, and it determines the sidewall angle of the finished conductor. A high etch factor gives near-vertical walls and a trace width close to the pattern width.

This is the reason a 0.1 mm feature on 35 micron copper is routine while the same feature on 140 micron copper is difficult. The thicker the copper, the more sideways etching occurs before the metal is cleared, so the trace loses more width and the profile becomes trapezoidal. Budgeting for that loss is part of the design.

Uniformity Across the Panel

Etching is a diffusion-limited process, so the local chemistry changes as copper is dissolved. Areas with a large exposed copper area deplete the etchant and etch more slowly, while sparse areas etch faster. The result is that features of the same nominal width come out different widths depending on where they sit.

The standard remedy is to even out the copper distribution with thieving bars and dummy features, so that every region of the panel exposes a similar area to the etchant. Where the design has a large ground plane next to a dense fine-line area, the difference can be significant, and the fabricator may ask for the pattern to be balanced before the panel is released.

Tolerance on Trace Width

The tolerance quoted on a trace width has to cover the artwork, the resist exposure, the etch factor and the copper thickness variation. A typical specification on a standard process is plus or minus 20 percent on a narrow trace, tightening to a much smaller absolute figure on wider features where the relative contribution of the undercut is smaller.

For impedance-controlled designs this matters directly, because a 20 percent width variation on a narrow trace moves the impedance by several ohms. That is why impedance-controlled boards are specified with a test coupon, and why the fabricator adjusts the artwork so that the finished width lands at the target. The trace width and current calculation should use the minimum finished width rather than the nominal one for the same reason.

Defects and Their Causes

Undercut is not a defect in itself, but excessive undercut is. It produces traces that are narrower than intended and that lift during thermal cycling because the copper-resist interface has been etched away. Over-etching shows up the same way, as does an etch that runs too long while the panel is held in the line.

Residue is the opposite problem. A thin film of copper left between two traces creates a latent short that passes a visual inspection and fails an electrical test, or worse, fails a humidity test months later. It is usually caused by an exhausted bath, by insufficient spray pressure, or by a feature geometry that the spray cannot reach. The plating and copper defect prevention measures describe how the cleaning and plating steps before etching affect the result.

Etching in Heavy Copper and Fine Line

Heavy copper etching is a separate discipline. The bath chemistry, the spray configuration and the conveyor speed are all adjusted, and the trace width tolerance widens accordingly. A fabricator who builds heavy copper regularly will hold a wider tolerance by design and will expect the designer to accept it.

Fine-line etching moves in the other direction, using thinner copper, a thinner resist and tighter process control to hold features below 0.075 mm. This is where the manufacturable design rules become a negotiation rather than a specification, because the achievable geometry depends on the specific line and its recent process history.

Design Choices That Help the Etcher

Keep the copper distribution even, avoid large isolated planes next to dense routing, and add thieving where the fabricator asks for it. Use rounded or chamfered corners rather than sharp internal angles, because an acute angle traps etchant and etches faster than the surrounding copper. Keep the minimum gap between traces consistent, since a single narrow gap determines the etch time for the whole panel.

Where the design has both fine features and heavy copper, consider splitting them onto different layers or onto different panels. A single panel that has to satisfy both requirements will be run to the slower of the two, which costs more than separating them.

FAQ

Does etching remove copper under the resist? Slightly, and that sideways removal is what sets the sidewall angle. Good process control keeps the undercut small relative to the copper thickness, but it cannot be eliminated, which is why the finished width is always less than the pattern width.

Why does the same design etch differently between two fabricators? Because the etch factor depends on the chemistry, the spray configuration, the conveyor speed and the copper thickness, and each line has its own combination. This is why an impedance-controlled design should specify the target and let the fabricator adjust the artwork.

Can etching be done in a home workshop? It can, using ferric chloride or a persulphate solution in a tray, with a printed resist or a photoresist film. The achievable resolution is far lower than a production line and the uniformity depends on agitation, but the process is the same in principle.