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.

44 thoughts on “Different Etching Strokes For Different PCBs, Folks

    1. I agree! To my surprise though, the complainants were primarily not German. Actually, to the contrary, the Germans were more on the favorable opinion side. The terrorist watch list part, however, is definitely a German / European aspect thanks to our Ausgangsstoffgesetz.

      1. I was at OBI (Lahr) last week and saw that they sell 30 % hydrochloric acid. Never seen it in Freiburg, though. HCL/H2O2 was what the PCBs fabs were using 20 years ago, why the surprise… some methods are much more known than others. (Next time try microkrotroller.net for some extra vitriol, if you want: it sadly never disappoints on providing discouraging, OT, and clueless comments, unless you already ditched it, what I recommend).

        1. Pretty much no PCB fabs ever used acid/peroxide but ferric chloride instead. Acid/peroxide is terrible from the fumes and corrosiveness of both fumes and the chemicals point of view, there is very little benefit in using that in an industrial setting.

          1. That’s why they invented fume hoods, no? Corrosive fumes are quite common in industrial applications, but in the concentration used here is easily manageable with ventilation and if the fume hoods are not metal (ie polymer based) it doesnt degrade from the fumes.

            The etchant itself is reusable. I’ve made a batch of it years ago and still use it, 50+ times now, just add a small amount of H2O2 when it gets brown.

            It doesn’t burn your skin (while it can be in high concentrations, not at what is required for etching – I’ve had an accident where some spilled on my head (it was being kept up high away from little hands and the lid wasn’t put on correctly, I knocked into the shelves and it fell). It dyed my clothes green. I showered and wouldn’t have known any difference, no red marks on my skin or anything. It stores quite easily for long periods of time (despite my lack of safety) and its easily disposable (https://assist.asta.edu.au/question/4314/copper-sulfate-use-and-disposal#:~:text=Safe%20handling%2C%20recycling%20and%20disposal%20of%20copper%20sulfate&text=Do%20not%20dispose%20solutions%20down,is%20toxic%20to%20aquatic%20life.&text=displace%20copper%20ions%20by%20adding,be%20washed%20down%20the%20sink.). Much easier than ferric chloride. Genuinely not sure why its not more common other than the use of HCL which in the concentration necessary here is commonly available in most hardware stores as brick/outdoor tile cleaner.

      2. When there were the terrorists attacks here in Brussels in 2015, they gave on the radio the list of chemicals to make TATP, and I had all 3 of them to make PCBs in the kitchen, including Hydrogen Peroxyde.

        Soon after, the paint shop where I used to buy Hydrogen Peroxyde was under tight control for the quantities they could sell.

    1. That would be the superficial, incomplete reaction equation that leads to the unnecessary warnings about this method being hazardous.

      Another reaction occuring is: H2O2 + HCl -> H2O + HOCl, which through
      HOCl + HCl -> H2O + Cl2 also yields chlorine.

      What happens with the Cl2? Probably get released and cause a serious hazard to anyone working with it, right? No!

      H2O2 + Cl2 -> 2HCl + O2

      There’s a concentration threshold corresponding to the RELEASE of chlorine gas!

      To quote from a scientific paper titled “Oxidation of hydrogen chloride with hydrogen peroxide in aqueous solution”: “The oxidation of hydrogen chloride with hydrogen peroxide at common temperature with the evolution of chlorine into the gas phase occurs at
      a hydrogen chloride concentration in a solution exceeding the threshold value (5.2 M).” The paper can be found here: https://link.springer.com/article/10.1134/S1070427208010035

        1. In the etching process it’s necessary to convince the copper to go into solution. With low concentrations of HCl, copper doesn’t like to play well. In a later step, solved Copper(I)-Chloride will be oxidized to the +2 state, yielding Copper(II)-Chloride, which itself is a well established etchant.

  1. HCl and H2O2 are perfectly good etchant provided the strength is low, cheap, readily available and reusable just by adding more H2O2. The more copper it takes on the more it is CuCl2 etchant, google for cucl2 seychell to find a pdf all about that.

    Of course mu easier these days to just get pcbs from china.

    1. and once you have CuCl2 as an echant, aerating (I used an aquarium pump and airstone) it will prolong and speedup it’s activity . Something like :
      CuCl2 + Cu > 2 (CuCl)
      2(CuCl) + O2 > 2(CuCl2)
      (chemistry was a long time ago so open to corrections…)

  2. I have been one of the people taking him to the task for his rather ignorant post – and then pooh-pooing and patronizing people instead of addressing the issues raised.

    Safety or not of this etchant aside, acid + peroxide combo isn’t a great etchant for multiple reasons:

    – Concentrated peroxide can be difficult to get, 30% hydrochloric acid can found but it is also getting more tricky to get. Pool cleaners use different things these days.

    – The process tends to be hard to control and thin traces get easily overetched. The boards he shows are pretty trivial in that regard, so there it wasn’t a concern but if someone tries to etch a dense board with this they will have problems. I know that everyone wants to have board etched in 5 minutes but when the thin traces are ruined what good does that that do? Ferric chloride or persulfate have no such issues.

    – The process releases really bad corrosive fumes. Not necessarily chlorine gas (even though that can happen too), esp. if you are stupid and use higher concentrations of the acid and/or heat the solution.

    – Hydrochloric acid is a terrible thing to have in your (non-chemistry) lab. Why? That thing releases fumes, even when in carefully closed original bottle. It will reliably corrode any metal in 1-2m radius, sometimes more over time. Really wonderful thing to have next to your expensive tools and equipment. What you saved on ordering a proper etchant you will spend on repairing/replacing your tools.

    His use was also rather unsafe. Even ignoring the lack of any mention of any PPEs, he also shows etching in tall beaker that is easy to accidentally topple over and spill instead of a flat tray or bowl. And no containment around it whatsovever – bad, because the process produces bubbles (esp. in higher concentrations) that spray the etchant around and one has to agitate the mix from time to time. All ample opportunities to have droplets of the etchant landing where they shouldn’t. Not to mention the possibility of actually overturning the beaker by accident. I know we are all supermen with perfect limb coordination but accidents do happen. Better look dumb but be prepared than have it happen – and then be dumb because you weren’t ready for it.

    And when getting that pointed out, he proceeds to completely miss the point by demonstrating its supposed safety by sticking his fingers in the beaker. That will for sure help a lot when he spills that mix on his desk or carpet …

    So yay. It is great when one has people who replace knowledge and skill with misplaced self-confidence and arrogance publishing stuff on HaD – and potentially giving poor example to follow to newbies who don’t know any better.

    1. Concentrated peroxide is easy to find in many places – it’s used for bleaching hair. I was amazed by US recommendations to use peroxide on wounds etc until I discovered US peroxide is only 3%.

      Every country has a different set of beliefs and laws about which chemicals are so dangerous they kill you on sight, which are only used by terrorists, and which are perfectly normal and healthy to splash around cleaning your home etc. So alternative chemistries can be very helpful.

    2. Yes, indeed you were. And still you haven’t done any research to back up your claims, which are broadly still completely wrong. With 17 minutes etch time, it can hardly be called hard to control. That no chlorine gas can be released at those concentrations is backed up by a cited scientific paper indicating a release threshold of 5.2 M HCl, below which absolutely no chlorine gas will be released. Guess they didn’t teach you the relevance of that in competitive chemistry. Your warnings about the HCl fumes would also only be correct if a solution of high enough concentration would be stored / used in the home lab. If you would have read the article, you would know that a 8 % stock solution is used. Which doesn’t fume, even smell all that badly, nor damage containers like usual conc. HCl does.

      On the beaker issue: Sure, it’s likely a bad idea to use laboratory beakers to contain chemicals. I need to share that with a faculty of a local University which etches all their PCBs in glass beakers. I am sure they’ll give you an award for surpassing the knowledge of the safety guys.

      All in all, it can be said that – just like with your original responses – you neither read the article nor did you actually think about the actual concentrations used. If someone would actually use conc. HCl or at least a high enough concentration, all of your statements would be correct. But they aren’t. And you’ve been repeatedly been made aware of the concentration levels being used.

      It’s funny that you call my scientifically backed article as replacing “knowledge and skill with misplaced self-confidence and arrogance” when in fact that’s what you’ve been doing the entire time. Projecting much? :-)

      1. You are still beating at strawmen and ignoring the important stuff/missing the point.

        I never claimed anything about chlorine (that was others). I am aware you can get chlorine only in high concentrations.

        > If you would have read the article, you would know that a 8 % stock solution is used

        And you actually buy your acid in 8% concentration? I kinda doubt that, when the commonly sold concentration is 30%. Where are you keeping *that* bottle, esp. after it has been opened?

        If you talk about that university lab – ask them to show you their storage cabinet where they store chemicals next time. And notice the amount of crud and rust that collects anywhere near where a 30% a HCl bottle is stored thanks to the fumes that escape even a well closed glass bottle with a sealing rubber or glass stopper in it. Even more so if it is the common plastic bottle with the screw-on cap once the seal has been broken on it and they don’t have ventilation/fume extraction in that cabinet.

        The same about the beaker – the point is not that working with beakers is unsafe per se but that you aren’t taking basic and cheap precautions against easily preventable accidents by using a beaker for something it is not really suited for. And not having a tray or something else under it to catch any accidents either. Have never had a beaker crack/break? Or catch one with the sleeve of your lab coat (you are using PPEs, right?) and spill it? How much does a plastic tray cost?

        However, what you are showing is arrogance and patronizing others who mayhaps have a bit more experience than you do and had these things beaten into them in those university labs. That includes also your “safety demonstration” of sticking your fingers into the beaker.

        1. You can buy 10 % HCl though, at least in Germany.

          I have no desire of underlining my chemistry and professional lab experience. But if you say so, I’ll accept defeat in that I don’t know much about mundane mixtures as HCl + H2O2 and lab safety. I’ll better stick to my area of expertise of perovskite oxide catalysts and lanthanoide group based fluorescent compounds.

          Don’t get me wrong, I understand your concerns. But they really do not reflect any actual knowledge. Anyone listening to enough safety briefings can mirror your objections. In the lab we identify those people as the ones that run to the emergency showers after getting 1 M HCl standard solution on their lab-coats. Coincidentally, it’s the same group of people that never stays for a PhD program. Or are particularly welcome at parties.

          Staying far away from everything that might be dangerous and applying immense safety precautions for everything might be a good general idea. Certainly a safe one. But real expertise includes being able to judge just how dangerous something really is. While it is generally not a good idea to stick your fingers into molten metals, it’s relatively safe to briefly stick your hand inside molten lead. It’s the same here. In your opinion me sticking my fingers in the etchant was a gross display of ignorance, arrogance and lack of knowledge. However, it really is a high degree of expertise. In Germany we have to do a “Gefährdungsbeurteilung” (hazard assessment) for every experiment and every chemical we let students / pupils come in contact with. Even measured on the very strict rules and regulations governing schools, I would be allowed to let 5th graders use this etchant in the manner described.

          1. Or are particularly welcome at parties? HA!

            Unless people knew how much of a substance is allowed in drinking water I think they’d be shocked after they got it tested, because all chemicals are bad of course, Oh my god there’s Arsenic and Dihydrogen Monoxide in my drinking water! HA!

            Anecdotally I knew of a guy who practically washed his face and hands in mercury to show it was safe to handle (albeit momentarily and probably for shock value as well).

            I think the point here is knowing what you’re handling and treating it with the respect it deserves. Everything when looked through a particular lens is bad, training yourself to know what is dangerous (in particular what concentrations and conditions) and using good guidelines like following H&S protocols where they exist in quality controlled environments (if you’re doing this at home).

      2. As someone who has actually tried this method I can say without doubt that the fumes from this method are awful and should not be underestimated. I was initially working inside a room with an open window as ventilation. It was not enough. Had to hold my breath and take the whole lot outside. I’m reasonably sure it was HCl gas rather than Cl. Either way, it’s back to good old messy, staining ferric chloride for me.

    3. Thats the joy of opensource culture, you Are completely free to contribute by posting your own guide instead of berating those who actually do. Is it perfect, no, but it’s a lot more helpful and valuable than what you are doing.

  3. i just figure, regardless of whether it was a good idea to do it in the first place…after the fact, you would know if it had released much chlorine gas because wouldn’t you smell it? if Sebastian already did it and didn’t get fumed out of his workshop then i don’t think there’s any point someone coming along afterwards and telling him that he did.

  4. I use 30% H2O2 + kitchen salt + citric acid. Takes a bit longer, but I don’t have to worry about storing HCl.

    Disposing of the resulting waste is limited by high concentration of Copper, which is toxic and forbidden to just pour out to the drain.

    I am still a little bit worried about storing H2O2, because in can explode at 50°C, but at least you can potentially dispose of it by simply diluting it enough.

  5. Great etching technique. I’ve used it myself, and it’s far less messy and unpleasant than ferric chloride.

    It’s also possible to use vinegar, peroxide, and salt, for an even “safer” process, but I haven’t tried that combination.

    1. I use less than a teaspoon of ferric chloride on a portion of sponge for small boards, and it’s really fast (faster than dipping in solution). Cleanup is also simple, because nothing got dirty, except the disposable sponge.

    2. Vinegar (acetic acid) and salt produce hydrochoric acid, so vinegar, peroxide, and salt is the same as hydrochloric acid and peroxide – you’ll just have a more difficult time getting the proportions correct.

  6. The pearl clutching is nothing new.

    When I was in middle school, ‘they’ refused to sell me glycerin and fuming nitric and sulfuric acids.
    Bloody fascists!

    Fortunately I was soon directed to the reloading department at the local gun store and all was well.
    Also started me on my ID forgery carrier. Fake ID to get powder, not beer till slightly later.

    In my experience, pyromania and explosive pyromania are perfectly normal stages of development for young male future engineers. Strangely, not so much the females. Small sample though, only 40 years of working.

    1. We had a lecture in Cambridge on pyrotechnics in the Chemi department.
      Looking around before it started, was funny, a who’s who of Cambridge pyromaniacs.
      Many faces I recognised from work, too!

  7. If you have 30% HCl, you don’t need anything more than the common 3% Peroxide, because you’d be diluting the HCl and adding peroxide at the same time. In my quest to make a bunch of CuCl etchant, I used about half-and-half Pool acid and 3% peroxide. It worked fine, dissolving a bunch of scrap copper (wire.)
    The fumes are worse than common “consumer” etchants like Ferric Chloride; it’s hard to tell whether there is chlorine, or just HCl fumes. The CuCl etchant sorta requires aeration, and that kicks stuff into the air as well; best done outside away from anything you don’t want corroded (but this is true of aeration of ANY etchant.)
    I don’t find beakers more prone to tipping/spillage than trays, and having something deeper cuts down a bit on fumes and droplets.
    The whole “acid to water” thing is probably moot with 37% (or less) HCl, and I’m not sure I can guess which direction you should pour 30% peroxide into 30% acid…
    The danger comes from people with no chemistry background at all going “It says on twitter I can etch PCBs with pool acid and hair bleach”; SOME level of warning is appropriate.

  8. IDK about pearl clutching or nanny state or whatever. I stand by my response on that twitter thread: I tried it, following instructions I found online. The fumes etched metal objects within the room and I developed a pretty nasty cough that lasted for a week. This was about 20 years ago, so perhaps the available information is better. However, no one can argue that this method contains more risk, either through bad instructions or lack of care in following them.

    In any case, I switched back to ferric chloride without any complaints (or health-affecting incidents) for a decade, and then it became easy to order good PCBs online, so why bother with any of that.

  9. I’ve tried the vinegar + hydrogen perxodie + sodium chloride etchant in the past and it was terrible. It took a long time and produced bad results, so I went straight back to ferric chloride. Of course these days PCBs are so cheap from china, I just order mine from JLCPCB.

  10. This method is perfectly safe as long as the place is well ventilated and you don’t let the mixture warm up. The latter means: use it in a shallow container, and discard promptly after use.

    We used it at university as the “officially sanctioned” method. More than once I picked up the PCB with my bare hands and I still have all my fingers. Of course, we had a fume hood, which was properly corroded after many years of use.

    Only once we had an incident, and it was caused by keeping the mixture in a measuring cylinder for too long before using- totally preventable.

    There’s an alternative: use this method to bootstrap a CuCl3 etchant (2 CuCl3 + Cu -> 3 CuCl2) and then only use the H2O2 to regenerate the etchant (convert the CuCl2 back to CuCl3).

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