Copper Foil Profile After Etching: Etch Factor and Trace Sidewall

Etching does not cut a trace with vertical sides. The chemistry removes copper downwards and sideways at the same time, so the finished conductor has a trapezoidal cross-section whose width at the top is set by the resist image and whose width at the base is set by the etchant. The copper foil profile after etching is therefore the result of two processes competing, and it decides the finished line width, the spacing, and the impedance of every controlled trace on the board.

What the Etched Profile Looks Like

A typical profile shows a resist image at the top of the trace, a slightly wider base where the copper meets the laminate, and a sidewall that slopes inward at an angle that depends on the process. Under the resist there is usually a small overhang where the etchant has worked beneath the resist edge but has not broken through.

Three numbers describe the profile: the top width, the bottom width and the height, which is the copper thickness. A fourth, the amount the trace is wider at the base than at the top, is the practical measure of sidewall slope and is what most process specifications actually control.

Etch Factor and Its Arithmetic

Etch factor is the ratio of the depth of copper removed to the lateral undercut beneath the resist, expressed as a dimensionless number. An etch factor of 3 means that for every 1 unit of copper removed vertically, the trace has narrowed by about 0.33 units on each side. Higher numbers mean straighter sidewalls.

The arithmetic matters because it converts a process property into a design consequence. A 35 micrometre foil etched with a factor of 3 loses roughly 12 micrometres of width on each side; the same foil etched with a factor of 1.5 loses about 23 micrometres, which is a 20 percent narrowing of a 100 micrometre trace.

Foil Thickness and Profile

Thick copper is harder to etch vertically. A 70 micrometre foil holds a lower etch factor than a 35 micrometre foil under the same chemistry and the same conveyor speed, because the etchant must stay in contact longer to remove the full depth. This is why heavy copper boards are usually specified with wider minimum line widths and larger spacings.

The surface of the copper foil also affects the result. A rough foil presents a greater area to the etchant along the trace edge, and the roughness is transferred to the sidewall after etching. It rarely causes a functional problem at moderate frequencies, but it becomes visible in a microsection and it changes the adhesion of the resist in a way that affects undercut.

Etching Uniformity Across the Panel

The profile is not the same everywhere on a panel. Etchant reaches the centre of a large copper area less readily than the edge, and the resulting difference in etch rate produces traces that are slightly narrower in dense regions and slightly wider in open regions. On a panel with mixed density the variation can be 10 percent of line width.

Uniformity is improved by copper balancing, by thieving bars that consume etchant evenly, and by the mechanical design of the etcher itself. Measuring line width at several fixed positions on a test coupon is the way to separate a machine problem from a design problem, and the copper balance of the artwork is the place to start.

Chemistry, Speed and Trace Sidewall

Three process variables dominate the sidewall angle. Etchant concentration sets the rate; temperature raises it; conveyor speed sets the contact time. An acid etchant that is too warm attacks the sidewall faster than the bottom of the trench and produces a tapered trace, while an etchant that is too cold leaves copper at the bottom and produces a wide, poorly defined trace.

Spray pressure and nozzle condition affect the same result from the mechanical side. High pressure drives fresh etchant into the trench and improves the etch factor, but excessive pressure can lift fine resist and cause breaks. The optimum is found on a test coupon and then held, because the same settings are used for every product that passes through the line.

Undercut and Overhang

Undercut is the removal of copper beneath the resist. Overhang is the unsupported resist that is left when the copper beneath it has been eaten away. Both are inherent to wet etching, and both become a problem when the trace is narrow, because a trace whose base is much narrower than its top no longer has the cross-section the design assumed.

The relationship between the two is the practical definition of a well controlled etch. Where undercut is small the resist overhang is small, the sidewall is nearly vertical and the finished line width matches the artwork. Where undercut is large the overhang can fold over and break during rinsing, leaving a ragged edge that later causes a line width tolerance failure or a short.

Impedance Control and the Finished Trace

Impedance control depends on the finished geometry, not the artwork. A single-ended trace over a plane has an impedance set by its width, its thickness, the dielectric height and the dielectric constant, and a change of a few micrometres in width moves the impedance. Because etching removes material from the top of the trace as well as the sides, the effective width for impedance purposes is neither the top nor the bottom but a value between them.

This is why impedance coupons are etched and measured rather than calculated from the artwork alone. The coupon sits on the same panel as the product, so it sees the same chemistry and the same conveyor speed, and its measured impedance includes the etch factor of that panel. Adjusting the artwork width to bring the coupon into specification is the normal correction.

Inspection by Microsection

Profile is verified by sectioning a coupon and measuring top width, bottom width and copper thickness under a microscope, with the resist still in place where possible so that undercut can be seen directly. The standard microsection preparation applies, with the sample cast and ground so that the trace is cut perpendicular to its length.

Measurement before etching is equally important as a reference. Comparing the top width after etching with the resist image before etching separates a resist imaging problem from an etching problem, and that comparison is much cheaper than adjusting the etcher to compensate for a defect that originated at exposure.

Process Window and Records

The process window is described by the etchant concentration and temperature range, the conveyor speed range, the spray pressure and the etch factor measured on the coupon. Each is recorded with the lot, and the etch factor is tracked as a trend rather than as a single value, because it drifts as the bath ages and as nozzles wear.

Corrective action should follow the measurement. A falling etch factor with a stable bath points to nozzles or to resist adhesion; one that falls together with concentration points to chemistry. Recording both together is what makes the distinction possible without experimental lots.

FAQ

What etch factor should a fine-line process achieve? Between 2.5 and 4 is typical for standard acid etching on 35 micrometre foil, with the higher values achieved on lines wider than 100 micrometres. Very fine lines naturally show lower values.

Is a tapered trace a defect? Not in itself, because all wet-etched traces are tapered. It becomes a defect when the finished width falls outside the tolerance the design needs, or when the taper changes the impedance of a controlled trace.

Why does line width vary across a panel? Mainly because the etchant reaches dense and open areas at different rates. Copper balancing and thieving reduce the difference, and measuring at fixed coupon positions shows what remains.