PCB Etch Process Control in Outer Layer Fabrication

Etching is the step that turns a plated image into copper conductors, and it is the last chance the process has to hold the line width that the design assumed. Controlling the etch process means controlling chemistry, equipment and the steps that came before it.

What Etching Has to Remove

The board arrives at the etch stage with copper everywhere and a resist image on top of it. The etch has to remove every part of that copper which is not protected, and nothing else, without damaging the conductor underneath the resist.

The requirement sounds simple and is not. The same chemistry attacks the copper sideways as well as downwards, so the finished conductor is always narrower than the resist image that defined it.

Pattern Plating and Panel Plating

In the pattern plating process, copper is deposited only where the conductor will be, and a metal resist of tin or tin lead is plated over it. The etch then has to remove a single layer of copper from the spaces between the conductors.

In the panel plating process the whole surface is plated first, and the resist defines the conductors. The advantage is simpler plating; the disadvantage is that the etch has to remove copper from the spaces and from the surface everywhere, which makes fine lines much harder to hold.

Etch Factor and Sidewall Shape

The ratio between the depth of the etch and the sideways attack is the etch factor. Values from roughly one to one up to five to one are seen in production, and a higher figure describes a straighter sidewall and a more predictable line width.

Additives in the chemistry are used to influence this behaviour, and their composition is usually proprietary. From a process point of view what matters is that the chemistry, the temperature and the spray pattern are held constant, because the etch factor moves when any of them drifts.

Resist Residue and Copper Roots

Plated copper and tin are normally thicker than the photo resist that defines them. The overhang that forms at the top of the resist traps a small amount of resist during stripping, and because the metal resist shields it, the etch cannot reach the copper beneath.

The result is a thin copper root along the edge of the conductor. It narrows the gap between adjacent lines, changes the impedance and is a common reason for a panel that is rejected at final inspection.

Chemistry Control

Alkaline ammonia chemistry is the standard for outer layer etching because it does not attack the tin resist. The etch rate depends on the free ammonia in solution more than on any other single variable.

Spray etching constantly brings fresh chemistry to the surface, which both carries away the dissolved copper and supplies the ammonia that the reaction consumes. Air sparging converts cuprous ions back to the cupric state, because a high cuprous content slows the reaction markedly.

pH and Ammonia Balance

Adding air also strips ammonia from the bath, and losing ammonia lowers the pH and slows the etch. The two effects have to be balanced, which is why a pH measurement and an automatic addition system are part of a stable line.

Log the pH, the specific gravity and the copper concentration together. A change in one of them is normally compensated by the others, so a single reading taken at the end of a shift explains very little on its own.

Equipment Condition

The etch chamber depends on the pumps and the nozzles that deliver the chemistry. A partly blocked nozzle leaves a stripe of under etched copper, and a worn pump reduces the impingement everywhere without any obvious alarm.

Once the line is running well it should keep running. Frequent stops and starts change the temperature and the chemistry in the chamber, and the first panels after a restart are the ones most likely to be out of specification.

Top and Bottom, Leading and Trailing Edges

The two sides of a panel are not etched identically, and neither are the leading and trailing edges of each board. Spray that has already reacted reaches the lower surface, and the edges see a different flow pattern from the centre.

The variation is managed rather than eliminated. Measure line width at several positions across the panel, compensate the artwork for the known difference and keep the coupon in a position that represents the worst case rather than the average.

Upstream Influence

Many defects attributed to etching were created earlier. Resist thickness variation, plating thickness variation and residues from the imaging step all appear as etch problems, because the etch is only the final step of a long sequence.

Plating uniformity is the most common of these. Where the plated copper is thicker than the resist over its whole height, the copper root appears everywhere on the panel, and no adjustment of the etch chemistry will remove it.

Measurement and Records

Line width and spacing are measured on a coupon that travels with the panel, at defined positions, at a defined magnification. The drum or the camera used, and the sample preparation, have to be the same from batch to batch for the numbers to mean anything.

Records from the etch line belong with the plating data and with the artwork revision, because the three together are what allow a shift to be corrected at its source. The controls used upstream are described in our notes on CAM preparation and on designing for a standard process.

Design Choices That Help the Etch

Copper thickness follows current, but it also decides what the etch can resolve. A layer that carries heavy current and fine geometry at the same time is a compromise that costs yield, and splitting the two functions between layers is usually cheaper.

Uniform copper distribution across the panel helps as well. Isolated fine lines surrounded by large bare areas etch differently from the same lines in a dense region, and a design that keeps the distribution even is easier to reproduce across different copper weights.

Rework and Touch Up

When a panel leaves the etcher with an under etched region, the correction is made by hand, and the limit of that correction is set by the geometry. Removing a short between two fine traces with a blade risks damaging the solder mask and the copper beneath it, and the result is a repair that is difficult to inspect and impossible to repeat.

Where the defect rate is low the practice is tolerable, and where it is not, the process rather than the panel is the problem. A trend of increasing touch up work is a signal that the bath or the spray system has moved outside its window, and the response should be to correct the line rather than to add inspectors.

Etch Compensation in the Artwork

The artwork is normally compensated for the copper that the etch will remove, so the finished conductor matches the design width rather than the drawn width. The compensation factor comes from measurement on previous runs, not from a rule of thumb.

Compensation is applied per layer, because the copper weight, the plating thickness and the density of the image all differ. A single global factor is a reasonable starting point, but it should be replaced by measured values as soon as the line has enough data.

Cleanliness and Bath Life

The bath has a working life defined by the copper it has dissolved and by the contamination it has accumulated. Filtration, rinsing between stages and the condition of the rinse water all feed back into the etch result.

Spent chemistry has to be treated, and the copper recovered from it is part of the economics of the line. A treatment plant that cannot keep up with the etch line is a constraint on the whole factory, not just on one process step.

FAQ

Why is the finished trace narrower than the artwork? Because the etch removes copper sideways as well as downwards. The artwork is normally compensated for the expected loss, and the compensation is verified on the coupon rather than assumed.

What causes a copper root along a trace? Plated metal that is thicker than the photo resist forms an overhang, resist is trapped beneath it during stripping, and the trapped resist shields the copper from the etch. Reducing the plating thickness relative to the resist is the usual fix.

Should inner and outer layers be etched the same way? No. Inner layers have wider spaces and can use a short develop, etch and strip sequence, while outer layers need the plated copper protected by a metal resist before the etch.