Etch Factor and Undercut Control in PCB Etching

Every wet-etched trace ends up slightly narrower at the top than the artwork that defined it, and slightly narrower again at the base. That geometry is described by the etch factor, and it sets the limit on how fine a line a given copper weight can produce. This article explains how undercut forms, why copper thickness dominates the calculation, and which process levers actually move the result.

Defining Etch Factor and Undercut

Etch factor is the ratio of the depth of copper removed to the lateral undercut on one side. A 35 micron thick copper layer that loses 17 microns of width on each side has an etch factor of about 1, which is poor. A 17 micron layer with 5 microns of undercut has a factor near 3.5, which is typical of a well-controlled alkaline ammonia line. The number describes how anisotropic the process is, and a higher value always means straighter sidewalls.

Undercut is the absolute lateral loss, and it is what the board designer sees as a tolerance problem. A trace drawn at 100 microns that finishes at 80 microns and a trace drawn at 60 microns that finishes at 40 microns have both lost 20 microns, but the second circuit has lost a third of its width. That is why fine-line capability is normally quoted as a percentage loss rather than as an absolute number.

Why Copper Thickness Sets the Limit

Wet etching proceeds sideways as soon as the surface opens up, so the total undercut grows with the time the copper spends in the etchant. Thick copper needs a longer dwell time, and the lateral attack continues throughout. A 1 oz layer might finish with 15 to 20 microns of undercut, while 3 oz of copper commonly shows 40 microns or more on the same line, which rules out traces below roughly 150 microns.

The practical consequence is that line width, copper weight, and feature density cannot be chosen independently. A design that needs 100 micron traces at 1 oz must accept the undercut and compensate for it, or move to a thinner base copper with plating up to the required ampacity afterwards. Heavier copper and fine lines together belong to a pattern plating process rather than to a print-and-etch flow.

Chemistry: Acidic versus Alkaline Etching

Acidic cupric chloride and ferric chloride baths are used for inner layers and for heavy copper. They etch quickly and handle thick metal well, but they are relatively isotropic, with etch factors usually between 1.5 and 2.5. Their advantage is cost and process robustness on large panels, and their disadvantage is the wide undercut that limits minimum line width.

Alkaline ammonia etching is the standard choice for outer layers carrying fine lines. The chemistry passivates the copper sidewall as it forms, which slows lateral attack and raises the etch factor to between 2.5 and 4. The bath is sensitive to pH, copper concentration, and chloride level, all of which are monitored continuously, and it must be followed by a thorough rinse because ammonia residues interfere with subsequent plating and solder mask adhesion.

Dry Film, Imaging, and Etch Loss Budget

The resist pattern defines where the etchant may work, so its adhesion and resolution set the starting point. A dry film photoresist that lifts at the edge of a fine line lets the etchant creep underneath and doubles the apparent undercut. Exposure energy, developing speed, and the age of the resist all affect that adhesion, and a line width that drifts through a production lot often traces back to a developing bath rather than to the etcher.

Designers should build the loss into the artwork. If a process shows 20 microns of total width loss, a 100 micron finished trace is drawn at 120 microns in the CAM data. When the intended loss is expressed as a compensation value, the factory and the designer share a single number that can be verified on a coupon, instead of an assumption that the finished width will match the CAD file.

Measuring Etch Factor on a Coupon

Measurement is destructive and requires a cross section. A coupon is potted, ground, polished, and examined under a microscope with a calibrated scale, and both the top width and the base width of the trace are recorded. The difference gives the undercut on each side, and dividing copper thickness by that value gives the etch factor. The same section also reveals whether the sidewall is straight or bowed, which matters more than the number for impedance control.

Because the measurement is destructive, it is normally done on a process control coupon included in each panel rather than on product features. Coupons placed near the panel centre and near the edge show how uniform the etching is, since spray pressure and fresh chemistry reach those regions differently. A single coupon at the panel corner can flatter a line whose centre traces are noticeably narrower.

CAM Compensation for Etch Loss

Compensation belongs in the CAM flow rather than in the operator’s judgement. The measured etch factor is converted into a width offset, applied to all traces on the relevant layers, and recorded with the job. Impedance-controlled traces need the same treatment, but their compensation interacts with the dielectric thickness, so the impedance calculation must use the finished width rather than the drawn width, as described in trace width calculations.

Compensation is layer specific and copper thickness specific. Inner layers at 0.5 oz and outer layers at 1 oz will not lose the same amount, and a design that uses one global offset will be out of tolerance on one of them. Where a board mixes copper weights, the compensation table should be reviewed at the start of the job and confirmed with a coupon before the lot is released.

Process Control Levers

Etching is a chemical process with a small number of dominant variables. Keep the etchant within its specified temperature band, typically 45 to 55 degrees Celsius for alkaline ammonia, and hold the specific gravity and copper concentration inside their windows. Replace or bleed the bath on a schedule rather than on appearance, and check the pH every shift, because a drift of a few tenths will change the etch rate noticeably.

Mechanical variables matter just as much. Spray pressure, nozzle pattern, conveyor speed, and the gap between the spray bars determine how evenly the chemistry reaches the surface. A pump that is losing pressure produces a slow, uneven etch with more undercut at the panel centre. Finally, the resist strip and the rinse must be verified, because residue left on a sidewall will be interpreted later as an undercut problem during inspection.

Design rules for manufacturability tie all of this together. The manufacturable design guidelines that apply to fine-line work are written around the etch factor a factory can hold, and a design that respects them will yield consistently without extra process tuning.

FAQ

What etch factor should a design assume? Ask the fabricator for the measured value on the relevant copper weight and chemistry. For planning, 2.5 to 3.5 is a reasonable expectation for alkaline ammonia on 1 oz outer layers and 1.5 to 2.5 for acidic etching on heavy copper.

Does compensation change impedance? Yes. Impedance depends on the finished geometry, so the calculation must use the compensated width together with the actual dielectric thickness and the measured etch profile.

Can undercut be eliminated? Not with wet etching. It can be reduced by using thinner copper, better resist adhesion, and tighter chemistry control, or avoided entirely by moving to a semi-additive or laser-based process.