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The Chemical Connection: More Solutions to Common PCB Etching Questions

This month’s subject is VeCS (vertical conductive structure), about which I must confess that I know absolutely nothing, since my career has mostly been focused on removing copper from the substrate rather than putting it on. To be honest, I had never even heard of VeCS until I received an email about this month’s theme.

So, rather than pontificating at great length on a subject I know nothing about, although I have been known to do this on occasion (the scientific term for this is “baffling them with bull****”), I decided to comment on some basic questions and problems that keep popping up in spite of them having been around forever and having known solutions.

Most of the inquiries come from the new process engineers who are being promoted as the older, more experienced process engineers retire. They’re so basic that the older people don’t even think about them anymore, let alone pass the knowledge on, so when these questions or problems arise, the new people are unaware that there are explanations and/or solutions.

A Sticky Situation
Here’s a recent one I hadn’t heard of in several years: a black, tar-like substance that builds up in the sump of a cupric chloride etcher. It coats the walls of the sump and plumbing, leaving an oily surface in the etchant sump, and eventually plugging the spray nozzles. Neither acid nor alkaline washes would get rid of this stuff.

Many years ago, this was diagnosed as organic residue leaching from the dry film etch resist as it passed through the etcher. However, the problem diminished over the years as dry film manufacturers addressed the problem and improved the product such that leaching rarely occurred. I assumed this was no longer a problem, but apparently it can still happen.

Fortunately, the solution is a simple carbon filter that removes unwanted organics and can be retrofitted as a closed-loop system on most etchers. It’s a good example of an old problem and an easy solution that has been forgotten because it rarely happens anymore. The main problem has been that people tend to forget the activated charcoal in the filter needs to be changed occasionally, or the problem will re-emerge. Keeping the organics out of the etch bath is simple. Cleaning it off the walls and plumbing in the etch sump after it forms is not.

Fan Fiction
One question that continually arises, especially as line and space tolerances get tighter, is “Can I get better etch uniformity and quality if I switch the etcher from cone nozzles to fan nozzles (or vice versa)?” Sadly, probably not. Nozzle dynamics is a lot more complicated than it appears on the surface.

For instance, one common misconception is that fan nozzles have better undercut performance (less undercut) than cone nozzles and therefore should give better etch quality as circuit traces and spaces get smaller because the spray is coming straight down rather than being spread out as with a cone nozzle.

However, a massive design of experiment done in conjunction with a large computer company (not IBM) showed that there was no difference under the right conditions. The experiment showed that the undercut performance of fan nozzles improved as the nozzle was moved further from the surface, and that cone nozzles performed better as they were moved closer to the surface. When each nozzle was at its optimum distance from the surface to be etched there was no difference in undercut.

The misconception probably comes from putting cone nozzles in an etcher optimized for fan nozzles and observing an increase in undercut. Another misconception is that if you have a smaller space to etch out, a nozzle with a smaller droplet size would do a better job of penetrating it. The truth is that, even with the smallest practical nozzle, the droplet diameters are still in the 500 µm range, which is still huge compared to a 50 µm space.

Also, reducing the droplet diameter by half reduces the droplet volume and therefore mass by a factor of eight. Since impact force is mass x acceleration, decreasing the droplet size reduces the impact of the droplet on the surface and results in a decrease in etch performance.

Perhaps the next time we have a monthly theme that I know nothing about (something that seems to be happening more frequently as I get older and the PCB industry gets more complicated and sophisticated) I’ll go a little deeper into nozzle dynamics. By all means, feel free to experiment with spray nozzles, but don’t get your hopes up too high.

That Etchy Feeling
Here’s another popular and recurring question, especially for newcomers to the etch process: “Can I get better etch uniformity and etch quality by changing which etchant I use (most commonly from cupric chloride to alkaline etch)?” For etch uniformity, the answer is an unequivocal, absolutely not.

Etch chemistry has nothing to do with why a panel etches faster or slower on different areas of the board, to the point where you can no longer meet etched line width uniformity specifications for that product. These things are usually due to misadjusted etching equipment and could go as far back in the process as imaging or even panel preparation. (Plugged spray nozzles are popular, and you’d be surprised at how many people never check their nozzles.) Check these things first before wasting time on trying different etch chemistries.

As far as improvement in etch quality is concerned, the answer is a qualified maybe. Alkaline etch will definitely give you a faster etch rate and, under the proper operating parameters, will improve undercut performance, giving straighter sidewalls on the etched line traces. This comes at the expense of more difficult etchant control and environmental discomfort (ammonia in the air is unpleasant, and the ammonia/copper complex is much harder to waste-treat). There is also a lingering misconception that ferric chloride etch gives better etch factor performance than cupric. This is not true, as discovered in the massive design of experiment mentioned above. Again, don’t waste your time.

Talk about pontificating at great length. Once again, I am approaching my word count limit (I start these things thinking I can’t possibly fill the space with the word count I am given and always end up having to edit for length). Anyway, I hope this has given you something to think about and maybe save you some time.

This column originally appeared in the August 2026 issue of I-Connect007 Magazine.