Etch Factor Control and Sidewall Angle in PCB Etching

Etching removes copper, but it does not remove it in a straight line. The etchant attacks sideways as well as downward, so a trace emerges with a trapezoidal cross-section narrower at the top than at the base. The relationship between that sideways attack and the depth of copper removed is called the etch factor, and it determines how much of the designed line width actually survives the process.

What the Etch Factor Describes

The etch factor is the ratio of the copper thickness removed to the amount of lateral undercut produced during the same operation. A high etch factor means the sidewall is close to vertical and the trace retains most of its width. A low etch factor means significant undercut and a trace that is narrower than the artwork intended.

For a given chemistry and thickness, the etch factor is roughly constant, which makes it a useful design parameter. It allows a fabricator to predict how much compensation the artwork needs and to compare one process against another. A value quoted without stating the copper thickness and the chemistry is only half of the information.

Why Sidewalls Are Not Vertical

Etching proceeds in all directions once the copper is exposed to the chemistry. Vertical attack is what the process wants; lateral attack is an unavoidable consequence. Fresh etchant reaches the top of the trace more readily than the base, so the top erodes faster, producing the characteristic trapezoid and a slightly rounded profile at the base.

The transport of fresh etchant and the removal of spent etchant control the shape. Where flow is poor, the spent chemistry sits against the copper and slows the reaction unevenly, producing a rough or irregular sidewall. Where flow is very aggressive, the pattern etches quickly but the resist can be lifted at the edges, producing a different defect entirely.

Etch Factor and Line Width Tolerance

Undercut directly erodes line width tolerance. If the artwork is drawn to the nominal width and the process undercuts by a significant amount, the finished trace is below the minimum, and the current capacity calculation built on the nominal dimension is no longer valid. This is why artwork is compensated rather than drawn at nominal.

The compensation amount is derived from the etch factor and confirmed by measuring production boards. Where the process is stable, a single compensation value per copper thickness works well. Where several thicknesses run on the same line, each requires its own value, because the etch factor depends on how much copper has to be removed. Etching chemistry and its control are described in this guide to the PCB etching process.

Copper Thickness and Etching Behaviour

Thicker copper is harder to etch cleanly. The etchant must remove more material vertically, which means it has proportionally more time to attack laterally, and the resulting sidewall angle becomes shallower. Heavy copper constructions therefore accept wider minimum features and less precise width control than thin foil.

The uniformity of the copper also matters. Rolled annealed copper foil etches differently from electrodeposited copper foil because the grain structure differs, and the same chemistry can produce different sidewall angles on each. Where a design specifies tight line width tolerance on a heavy layer, the copper type and the etch process have to be considered together rather than separately.

Chemistry, Spray and Conveyor Speed

Most production etching uses an alkaline ammonia or cupric chloride chemistry sprayed onto the panel from oscillating nozzles. Spray pressure, nozzle condition and oscillation amplitude determine whether fresh etchant reaches every part of the panel consistently. A blocked nozzle produces a localised etch rate change that appears as a width variation across the board.

Conveyor speed sets the dwell time in the chamber. Faster travel reduces both vertical and lateral etching and can leave copper unremoved in dense areas, while slower travel completes the etch at the cost of more undercut. Finding the speed that clears the tightest area without over-etching the rest of the panel is the central process compromise.

Compensation in Artwork

Compensation is applied to the artwork so that the finished feature matches the design intent. The value is positive for widths, meaning the drawn line is wider than the finished target, and negative for gaps, meaning the drawn clearance is smaller than the finished space. Getting the sign wrong produces a board that fails in an entirely predictable and entirely avoidable way.

Compensation should be validated on each new stackup, because laminate thickness, copper weight and pattern density all affect the result. A coupon with the same feature sizes as the product, measured after etching, confirms whether the applied value is correct. The resulting data is worth retaining, because it becomes the baseline for the next build of the same construction. Uneven copper distribution across a panel also changes etch behaviour, which is why copper balance and thieving are used to even out the pattern.

Measuring the Sidewall

Sidewall geometry is measured by cross sectioning a coupon and examining it under a microscope. The top width, the base width and the copper thickness together define the profile, and the etch factor can be calculated from those three numbers. Because the measurement is destructive, it is performed on samples rather than on every panel.

Non-destructive alternatives give less information. An optical line width measurement reports the top width, which is the most critical dimension for electrical behaviour, but it says nothing about the profile below. Where the design is sensitive to cross-sectional area, a periodic cross section remains necessary to confirm that the optical reading is representative.

Heavy Copper and Fine Line Limits

Heavy copper and fine lines are fundamentally in conflict. Removing three ounces of copper to create a narrow space requires a long etch, and the extended dwell time produces substantial undercut on the adjacent traces. The practical consequence is a minimum feature size that grows with the copper weight, and design rules that must reflect it.

Some fabricators use a two-step etch or a differential etch process to improve the result on heavy copper, and some use a subtractive-then-additive approach for very fine features. These methods add cost and process steps, so they should be reserved for designs where the electrical requirement genuinely justifies them. Choosing a lighter copper with a wider trace is often the more economical solution.

Specification and Process Control

A specification should state the finished line width and tolerance, the copper thickness, and whether the width is measured at the top of the trace or as an average cross-section. Those two definitions can differ by several micrometres on heavy copper, and a supplier and customer using different definitions will never agree on whether a lot is acceptable.

Ongoing control relies on a coupon measured at each etch run, with the results trended over time. A drift in line width usually reflects a change in chemistry concentration, a partially blocked nozzle or a conveyor speed variation, all of which are correctable once identified. Recording the etch parameters with the coupon result turns a scattered set of measurements into a diagnosable process record. Those records belong with the rest of the build documentation described in this fabrication notes checklist.

FAQ

What is a good etch factor? It depends on the copper thickness and the feature size, but a higher value is always better because it means less undercut and a wider finished trace. Fabricators normally establish their own baseline for each copper weight and monitor against it rather than quoting a universal target.

Can undercut be eliminated? No, because the etchant necessarily attacks in all directions once the copper is exposed. What can be controlled is how much lateral attack occurs, through chemistry, spray uniformity and dwell time, and how well the resulting loss is anticipated through artwork compensation.

Why does thick copper make fine lines harder? Because the etchant must remove more copper vertically, which takes longer, and the longer dwell time allows proportionally more lateral attack. The result is a shallower sidewall angle and a wider minimum feature size, which is why heavy copper designs use more generous design rules.