Etch Factor and Line Width Control in PCB Production

The etch factor is the number that connects the artwork to the finished trace, and understanding it is what allows a shop to hold a line width tolerance on thick copper. When copper is etched it is attacked sideways as well as downwards, so the finished trace is narrower at its base than at its top, and the amount of that loss has to be predicted rather than discovered after the panel is etched.

What the Etch Factor Measures

The etch factor is the ratio of the etch depth to the sideways undercut, usually measured on a microsection of a test pattern. A high factor means the etchant removed copper mostly in the direction it was aimed, and a low factor means a large sideways loss.

It is not a constant. The factor changes with the etchant chemistry, the copper thickness, the conveyor speed, the spray pressure and the resist profile, so the same artwork produces different line widths in two shops with different lines.

Why the Base Width Matters

The base of the trace is the narrowest part, and it is the dimension that carries the current and the impedance. A trace that measures correctly at the top and is narrower at the base will have a higher resistance and a lower current capacity than the design assumed.

The effect grows with copper thickness, because a thicker layer takes longer to etch and gives the sideways attack more time to act. That is why a heavy copper board needs a different compensation from a one ounce board.

Measuring the Under Etch

The measurement is taken on a polished section of a test pattern that includes isolated traces, dense traces and a solid area. The width at the top, the width at the base and the etch depth are all recorded, and the factor is calculated from them.

A coupon with the same copper thickness and the same pattern density as the product is the only useful sample. Our etching process guide covers the chemistry and the equipment that the measurement is used to control.

Compensating the Artwork

Compensation is applied to the artwork so that the finished base width matches the design. The amount is determined from the measured factor rather than from a table, because the factor is specific to the line and the product.

A shop that records the compensation used for each combination of copper thickness and pattern density builds a library that makes the next similar job predictable. Without it, every heavy copper order starts from a guess.

Dense and Isolated Features

The etch rate is not the same everywhere on a panel. A small isolated trace is surrounded by etchant on three sides and etches quickly, while a trace in a dense area is partly shielded and etches more slowly.

The difference is the reason a design with both dense and isolated features is difficult to etch. A compensation that suits the isolated traces will leave the dense area under etched, and one that suits the dense area will over etch the isolated traces.

Resist Profile and Sidewall

The resist defines the shape that the etchant sees, so its sidewall matters. A resist with a vertical wall gives the etchant a straight path down, while one with a sloped or rounded edge lets the etchant attack the copper under the resist from the start.

Imaging quality, resist thickness and the development step all affect the profile. Our quality documentation describes how these conditions are classified at gopcb.

Etchant Type and Its Effect

Alkaline ammonia etchant gives a relatively high etch factor and is the usual choice for fine lines. Acid etchants behave differently and are often used for heavy copper where a high copper capacity is needed.

The choice affects the compensation, so a change of etchant chemistry is a change of artwork. A shop that transfers a job between two etch lines should re-verify the line width rather than assume the settings transfer.

Line Width Measurement on Production

Production measurement is normally done with an optical system that measures the top width, because the base is hidden under the resist during etching. The relationship between the top and base widths from the coupon is then used to infer the base.

The measurement should be taken at several points across the panel, because the etch rate varies with position as well as with pattern density. A single reading in the centre of the panel is not representative.

Process Control Points

The controls are the etchant chemistry and its copper content, the temperature, the spray pressure and nozzle condition, the conveyor speed, the resist profile and the measured line width on a coupon.

Every one of those is recorded per lot, and the coupon result is the number that proves the artwork compensation is still correct. A drift in the etch factor shows up first as a change in line width on the coupon, long before it becomes a rejected panel.

Process Control and Verification

On a design of this kind, copper thickness is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. 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.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

Process Control and Verification

On a design of this kind, copper thickness is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. 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.

Process Control and Verification

On a design of this kind, copper thickness is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. 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.

FAQ

What is a good etch factor? It depends on the chemistry and the copper thickness, and it is compared against the shop’s own baseline rather than against an industry figure. What matters is that the factor is measured and repeatable.

Why do isolated traces come out thinner than dense ones? Because they are surrounded by etchant on more sides and are etched faster. A design with both feature types needs a compensation compromise and a tolerance that allows it.

How is the base width measured if it is under the resist? The relationship between the top width and the base width is established on a coupon by microsection, and that relationship is then used on production panels.