Silicon Photolithography The PCB Way

[ProjectsInFlight] has been doing some fantastic work documenting his DIY semiconductor fab lately. Next up: exploring down-and-dirty photolithography methods.

If you’ve been following along with this series — and why wouldn’t you? — you’ll recall [ProjectsInFlight]’s earlier experiments, like creating oxide layers on silicon chips with a homebrew tube furnace and exploring etchants that can selectively remove them. But just blasting away the oxide layer indiscriminately isn’t really something you need to do when etching the fine features needed to fabricate a working circuit. The trouble is, most of the common photoresist solutions used by commercial fabs are unobtainium for hobbyists, leading to a search for a suitable substitute.

Surprisingly, PCB photoresist film seemed to work quite well, but not without a lot of optimization by [ProjectsInFlight] to stick it to the silicon using a regular laminator. Also in need of a lot of tweaking was the use of a laser printer to create masks for the photolithography process on ordinary transparency film, including the surprisingly effective method of improving the opacity of prints with acetone vapor. There were also extensive experiments to determine the best exposure conditions, a workable development process, and the right etchants to use. Watch the video below for a deep dive into all those topics as well as the results, which are pretty good.

There’s a lot to be said for the methodical approach that [ProjectsInFlight] is taking here. Every process is explored exhaustively, with a variety of conditions tested before settling on what works best. It’s also nice to see that pretty much all of this has been accomplished with the most basic of materials, all of which are easily sourced and pretty cheap to boot. We’re looking forward to more of the same here, as well as to see what others do with this valuable groundwork.

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Testing Oxide Etchants For The Home Semiconductor Fab

Building circuits on a silicon chip is a bit like a game of Tetris — you have to lay down layer after layer of different materials while lining up holes in the existing layers with blocks of the correct shape on new layers. Of course, Tetris generally doesn’t require you to use insanely high temperatures and spectacularly toxic chemicals to play. Or maybe it does; we haven’t played the game in a while, so they might have nerfed things.

Luckily, [ProjectsInFlight] doesn’t treat his efforts to build semiconductors at home like a game — in fact, the first half of his video on etching oxide layers on silicon chips is devoted to the dangers of hydrofluoric acid. As it turns out, despite the fact that HF can dissolve your skin, sear your lungs, and stop your heart, as long as you use a dilute solution of the stuff and take proper precautions, you should be pretty safe around it. This makes sense, since HF is present in small amounts in all manner of consumer products, many of which are methodically tested in search of a practical way to remove oxides from silicon, which [ProjectsInFlight] has spent so much effort recently to learn how to deposit. But such is the ironic lot of a chip maker.

Three products were tested — rust remover, glass etching cream, and a dental porcelain etching gel — against a 300 nm silicon dioxide layer. Etch speed varied widely, from rust remover’s 10 nm/min to glass etching cream’s blazing 240 nm/min — we wonder if that could be moderated by thinning the cream out with a bit of water. Each solution had pros and cons; the liquid rust remover was cheap easy to handle and clean up, while the dental etching gel was extremely easy to deposit but pretty expensive.

The good news was that everything worked, and each performed differently enough that [ProjectsInFlight] now has a range of tools to choose from. We’re looking forward to seeing what’s next — looks like it’ll be masking techniques.

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Different Etching Strokes For Different PCBs, Folks

[Sebastian] probably didn’t think he was wading into controversial waters when he posted on his experimental method for etching PCBs (in German). It’s not like etching with hydrochloric acid and peroxide is anything new, really; it was just something new to him. But is it even possible these days to post something and not find out just how wrong you are about it?

Sadly, no, or at least so it appears from a scan of [Sebastian]’s tweet on the subject (Nitter). There are a bunch of ways to etch copper off boards, including the messy old standby etchant ferric chloride, or even [Sebastian]’s preferred sodium persulfate method. Being out of that etchant, he decided to give the acid-peroxide method a go and was much pleased by the results. The traces were nice and sharp, the total etching time was low, and the etchant seemed pretty gentle when it accidentally got on his skin. Sounds like a win all around.

But Twitter wouldn’t stand for this chemical heresy, with comments suggesting that the etching process would release chlorine gas, or that ferric chloride is far safer and cleaner. It seems to us that most of the naysayers are somewhat overwrought in their criticism, especially since [Sebastian]’s method used very dilute solutions: a 30% hydrochloric acid solution added to water — like you oughta — to bring it down to 8%, and a 12% peroxide solution. Yes, that’s four times more concentrated than the drug store stuff, but it’s not likely to get you put on a terrorism watch list, as some wag suggested — a hair stylist watchlist, perhaps. And 8% HCl is about the same concentration as vinegar; true, HCl dissociates almost completely, which makes it a strong acid compared to acetic acid, but at that dilution it seems unlikely that World War I-levels of chlorine gas will be sweeping across your bench.

As with all things, one must employ caution and common sense. PPE is essential, good chemical hygiene is a must, and safe disposal of spent solutions is critical. But taking someone to task for using what he had on hand to etch a quick PCB seems foolish — we all have our ways, but that doesn’t mean everyone else is wrong if they don’t do the same.

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Three Ways To Etch Snazzy Brass Nameplates

It’s the little touches that make a project, and a nice nameplate can really tie a retro build together. Such badges are easy enough to make with a CNC machine, but if you don’t have access to machine tools you can put chemistry to work for you with these acid-etched brass nameplates.

The etching method that [Switch and Lever] uses to get down to brass plaques will be intimately familiar to anyone who has etched a PCB before. Ferric chloride works as well on brass as it does on copper, and [Switch and Lever] does a good job explaining the chemistry of the etching process and offers some tips on making up etching solution from powdered ferric chloride. But the meat of the video below is the head-to-head test of three different masking methods.

The first method uses a laser printer and glossy paper ripped from a magazine to create a mask. The toner is transferred to the brass using an office laminator, and the paper removed with gentle rubbing before etching. For the other two candidates he uses a laser engraver to remove a mask of plain black spray paint in one case, or to convert special laser marking paint to a mask in the other.

We won’t spoil the surprise as to which gave the best results, but we think you’ll be pleased with how easy making classy nameplates can be. You can also use electrolytic methods for a deeper etch, but we think acid etching is a little more approachable for occasional use.

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Ask Hackaday: What’s Your Etchant?

Although the typical cliché for a mad scientist usually involves Bunsen burners, beakers, and retorts, most of us (with some exceptions, of course) aren’t really chemists. However, there are some electronic endeavors that require a bit of knowledge about chemistry or related fields like metallurgy. No place is this more apparent than producing your own PCBs. Unless you use a mill, you are probably using a chemical bath of some sort to strip copper from your boards.

The standard go-to solution is ferric chloride. It isn’t too tricky to use, but it does work better hot and with aeration, although neither are absolutely necessary. However, it does tend to stain just about everything it touches. In liquid form, it is more expensive to ship, although you can get it in dry form. Another common etchant is ammonium or sodium persulphate.

pcbyThere’s also a variety of homemade etchants using things like muriatic acid and vinegar. Most of these use peroxide as an oxidizer. There’s lots of information about things like this on the Internet. However, like everything on the Internet, you can find good information and bad information.

When [w_k_fay] ran out of PCB etchant, he decided to make his own to replace it and wrote a great guide on how this is done. He found a lot of vague and conflicting information on the Internet. He read that the vinegar solution was too slow and the cupric acid needs a heated tank, a way to oxygenate the solution, and strict pH controls. However, he did have successful experiments with the hydrochloric acid and peroxide. He also used the same materials (along with some others) to make ferric chloride successfully.

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Testing The Efficiency Of PCB Etchants

etchIn the interest of the scientific method [Feynmaniac] (great name, btw) over on Instructables has posted a little experiment on something we all, no doubt, care about: putting PCB traces in copper clad boards with the most common etchants out there.

The experiment used the ‘ol standard, ferric chloride, and the safe, inexpensive newcomer, vinegar, hydrogen peroxide, and table salt. Finding the most efficient mixture of ferric chloride is easy: just use what’s in the bottle. The vinegar and H2O2 requires some stoichiometry, though, and [Feynmaniac] calculated that with an 8% acetic acid solution and the most commonly available 3% peroxide solution, a 2:3 ratio of peroxide to vinegar is the best. Salt to taste, or until everything turns green.

Four copper clad boards were used for the test, masked off in a ‘barcode’ pattern. Two methods of applying the etchant were used: either rubbing the etchant on with a sponge, or immersing the boards in a bath of the etchant being tested.

In terms of speed, ferric chloride was by far the fastest, with 3 minutes until the board was etched using the rubbing method, or 10 minutes when simply immersed. Vinegar/peroxide took longer with 11 minutes rubbed, and 20 minutes immersed. No differences in the quality of the etch were noticed.

While ferric chloride was by far the fastest etchant, it does have the downside of being environmentally unfriendly and fairly expensive. The vinegar and peroxide etchant is safe, cheap, and can be found in any grocery store on the planet.

This experiment didn’t test other common etchants like HCl and H202, or cupric chloride (which is is the byproduct of HCl and H202). Still, it’s a good confirmation that the vinegar and peroxide method actually works, in case you were wondering.

PCB Agitator From A Broken CD-ROM Drive


Etching PCBs goes a lot better if you agitate the solution in order to carry away the dissolved copper and get fresh etchant to the area. With that in mind [Rohit Gupta] designed a mechanism in Sketch Up before realizing he was going about it the hard way. He ended up basing his agitator on a broken CD-ROM drive instead of starting from scratch.

He uses the CD sled from the drive, ditching the lens and its support structure. To get direct access to the motor that drives the tray he uses an L293D H-bridge chip. This is controlled by an MSP430G2231 microcontroller. The driver board seen in the upper right includes a voltage regulator, three status LEDs, and one user input switch. Once triggered, the sled will move back and forth, contacting an old mouse microswitch which acts as the limiting switch. We find it entertaining that [Rohit] prototyped the circuit on a breadboard, then used that success to etch the final circuit board (shown in the video below).

If you’ve been following the hacker creed and never getting rid of any junk you’ll have no problem finding a donor drive to make one of your own. But just in case you can’t get a hold of this hardware a similar agitator can be built using a hobby servo.

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