Etching PCBs with vinegar

When we hear about etching PCBs at home we assume that either Ferric Chloride or Cupric Chloride were used to eat away unmasked copper from the boards. But [Quinn Dunki] just wrote up her PCB etching guide and she doesn’t use either of those. Instead, she combines vinegar, hydrogen peroxide, and salt. It’s easier to find vinegar than muriatic acid (Cupric Chloride is made using this, peroxide, and adding the copper) so this is something to keep in mind if you’re in a pinch (or a Macgyver situation).

The rest of the process is what we’re used to. She’s using photoresistant boards which can be masked with a sheet of transparency instead of using the toner-transfer method. Once they take a bath in the developer solution she puts them in a shallow dish of vinegar and hydrogen peroxide along with a teaspoon of salt. She wipes the surface with a foam brush every minute or so, and inspects them every ten minutes to see if they’re done.

She does discuss disposal. Seems that she throws the solution in the garbage after each use. The liquid will contain copper salts which are bad for wildlife. We’ve heard that you should neutralize the acid and make a block of concrete using the liquid, then throw it in the garbage. Does anyone have a well-researched, ethical, and environmentally friendly way of getting rid of this stuff?

61 thoughts on “Etching PCBs with vinegar

  1. From my understanding, this process results in the formation of copper(II) chloride and sodium acetate. Sodium acetate is a common food additive – see salt and vinegar potato chips. The copper ion is of the same oxidation state as that in copper sulfate, AKA root-be-gone, a chemical designed to be flushed down your drains en masse to kill tree roots intruding in your pipes.

    Considering that copper sulfate is still sold with this purpose in hardware stores all over the US and that this process results in comparatively tiny amounts of copper, flushing it straight-up is in all likelihood not a problem.

      1. DDT is a certain route to a certain, incurable, slow, and painful death. Is THAT supposed to be a bright side vs. malaria?

        The story of DDT is a story of suppressing known scientific dangers deemed (by non-scientists) to be inconvenient truth.

    1. If you live in a sufficiently technologically advanced country, your local wastewater quality control facilities should be designed to deal with chemicals such as these being poured down the drain.

  2. Vinegar works okay but it is really slow and if you have boards with thick copper clad then it doesn’t work much at all. I still prefer muriatic acid and peroxide. It took 5 minutes to etch boards with the muriatic acid that vinegar couldn’t accomplish in 14 hours.

    Don’t throw out used etchant, just keep reusing it, top it off with more acid and peroxide. Once the copper content becomes too high, set it somewhere warm , uncovered, and it will reduce to copper crystals when the liquids evaporate, that you can use for other stuff or dispose of easily.

    1. The process is definitely slower than hydrochloric acid, but I can still etch a normal 1oz copper board of average hobby size in about twenty minutes. For me, the tradeoff of not having to deal with storage, use, and disposal of a strong acid is worth it.

    2. instead of waiting for evaporation to form the crystals… give it some juice, and a piece of something to plate onto.

      Super saturated copper solution + electricity = electroplating.

      You won’t get the reliability(may not fully plate something, if you run out of copper in solution) that you would in normal plating, but it should remove all excess copper from the solution just dandy.

    3. The magic fourth ingredient…..heat. I mix this up in a Pyrex dish on my electric cooktop set to “low” heat (number 2). Once it warms up the reaction becomes very active!

  3. From:

    The sludge method

    I found this to be the only practical method of precipitating cupric chloride etchants for disposal. This method involves adding lime (calcium hydroxide) directly to the cupric chloride etching solution. The copper is converted to hydroxide, just as it would be if sodium hydroxide was added, however the reaction with calcium is much less violent, and the sludge formed is a lot less liquid. When the lime is initially added the acid will first neutralize, followed by the formation of copper hydroxide sludge. As more lime is being added the color of the sludge will start out green because cupric chloride has been made insoluble (not enough water), and eventually becoming a consistent blue color once all copper is takes the form of copper hydroxide. At this point you can either make cement out of it or dry it and put it to land fill in a sealed container.

  4. Something to keep in mind when disposing of stuff: If you try to mix it with another material, all you’re doing is creating a bigger pile of waste. It’s not “environmentally friendly” to do that. (In hazardous waste regulations, mixing any hazardous waste with a non-hazardous waste just makes a larger supply of hazardous waste.)

    cgimark’s comment is perhaps the best way to handle it. Take waste solution, sit it out, and let it crystallize/dry. You’ll end up with a much smaller amount of waste that can be more easily handled. Either send it out with the garbage or save up a few hundred/thousand pounds so that a smelter would have an interest in it…..;^)

    Personally, I’d have no problem dumping small amounts down the sink after properly neutralizing it. Water treatment facilities have to treat/test for copper anyway (most of it comes from piping in your house), and my contribution would be very small in the overall scheme of things. Obviously, if everyone did this at home, it might be a problem. Also, if you’re on a septic system, you’d have to consider the impact of your waste on the health of the microbes. In that case, solid disposal would be best.

    If you don’t want to go the evaporation route because of the volume involved, then I’d suggest neutralizing to a pH of 7 with sodium hydroxide, adding a flocculating agent (you can buy some at a pet store in the fish supply aisle), mixing thoroughly, letting settle, decanting clear liquid off, and then pouring the rest through some small micron filter paper. Let the solids dry and throw in the garbage. You can pour the supernatant liquid down the drain.

    If you’re going to simply pour the liquid down the drain, at least neutralize it to protect your piping.

    1. I think one reason to MIX it with something else is to make it more stable, so it doesn’t leach out into the ground and back into the water supply. If you could get it back into it’s raw state… a chunk of copper, that would be ideal. Then don’t throw it away… recycle it and get paid!

      1. While that’s done for large scale waste applications, I just don’t think it makes much sense for the home hobbyist in terms of time and effort for the benefit achieved. The waste is generally going to a lined landfill anyway.

        I suppose if you had a landscaping project you could just mix it with some portland cement…..

    2. Eeek. As pointed out above, you should neutralize to a higher pH, not the 7 I used. However, keep in mind those are general guidelines. The amount of flocculation you’ll get depends on several variables (copper loading, what you neutralize with, type of floc. agent added, etc.), so YMMV. You’ll have to experiment.

    3. Pouring it down your drain won’t hurt your piping. All the drain plumbing is ABS plastic. The only copper piping is the inlet side of the water system – heater, pipes, faucests, etc. The rest is plastic.

      1. “All the drain plumbing is ABS plastic.”

        That depends on the age of the house. Where I’m currently living there is both cast iron and clay pipe between the drains and the sewer system. Every section that has had work done to it, the existing pipe has been cut out and replaced with plastic, but there hasn’t yet been reason to jackhammer through the basement slab and get at that section, for example.

  5. The best way to get rid of the etchant is to heat it a little and add washing soda. Don’t do it in the kitchen, but outside because it’s not super healthy. First you get CuCO3, but because of the high temperature it will go into CuO (which is black and doesn’t dissolve in water). Than just leave it for 15 minutes, top of most of the water and throw the rest with the black stuff in the garbage.

  6. In the last month I etched 3 boards using vinegar, peroxide, and salt. First 2 boards came out awesome, even with traces at about 6-8 mils (not sure what they were… it was PCB shared to me in PDF format).

    The third board was a Valentines Day card for my wife. I converted her favorite photo of us by lowering the resolution (bigger dots are needed), then convert to monochrome 2-color bitmap, then invert the artwork (because the “black” areas will be the bits that are copper, protected by toner). I’m not too pleased with toner as you have to sharpie any holes, but it’s very accessible to a newbie.

    The photographic effect works best if the substrate is black. Black and toner contrast NICELY. Some fiberglass substrates are tan/brown, which doesn’t yield as much contrast with the copper.

    While I’m happy with vinegar+h202, it does seem more expensive. Eventually I’d like to switch over to stronger acid and photo instead of toner.

    I’m not sure what to do with my copper-infused vinegar either. I was thinking the town dump will take it.

  7. The vinegar/salt solution ultimately produces copper chloride as the etchant. You just end up with sodium competing with copper for the chloride ions, and some sodium and copper acetates from the vinegar.

    It isn’t as chemically pure as a hydrochloric acid mix (with HCl the copper doesn’t have to compete for chloride ions), but that really doesn’t matter at the hobbyist level. It works, and that’s the important part.

    You can reduce metallic copper out of cupric chloride by using a more active metal. If you use iron (steel wool is best.. lots of surface area), you’ll get metallic copper and ferric chloride. Throw in some citric acid and you’ll have what’s known as the ‘Edinburgh Etch’. If you use aluminum, you’ll get metallic copper and aluminum chloride (antiperspirant), which presents fewer disposal problems.

    You don’t need to throw away cupric chloride after etching a board though. Just put it in a jar and let it sit for a day or so. The cuprous ions (which make the solution dark) will oxidize back up to cupric. When the solution is bright green, you’re ready to etch again. It’s called ‘regeneration’.

    You can also regenerate a cupric chloride solution by pouring in a little more hydrogen peroxide, but with the 3% stuff you’ll get at the drugstore, you’ll dilute the solution as well.

    At the hobbyist level, it’s easiest to keep several jars of etchant and use them in rotation. I keep four containers with about 2 cups of etchant in each. I etch directly in the container, and when the solution gets dark enough to be opaque (high cuprous-to-cupric ratio), I put the lid back on that one, move it to the end of the line, and pull out the next fresh one.

    If I get all the way around to a dark solution before I’m done etching, I’ll regenerate with peroxide. Every month or so, I take the jars outside and let them evaporate for a day.

    1. Nice writeup @mstone – thanks!

      How is it that the cupric chloride (formerly vinegar + h202) becomes fresh (more acidic) again?

      You mention the ions oxidizing, and the solution “refreshing”.. but this seems counter-intuitive.

      Don’t all reactive solutions trend towards becoming stable? If so then why would this solution freshen? And lastly, does this have ANYTHING to do with the cloudy powdery precipitate at the bottom of the pool?

      Sorry for so many questions. :)

      1. First of all: never apologize for wanting to know. It’s what we do. ;-)

        Cupric chloride works as an etchant because copper can have two ‘oxidation states’ (numbers of electrons it wants to use in bonds). Copper’s natural state is ‘cupric’ (Cu+2) meaning it can accept two electrons. If you feed it one electron from somewhere, you get a ‘cuprous’ ion (Cu+1).

        Nature likes to balance things, so when you combine a cupric ion with metallic copper (metals have very loosely bound electrons in their outer orbital, so a chunk of metal is basically electron soup with ion dumpings), you get two cuprous ions:

        Cu+2 + Cu0 -> 2Cu+1

        Those ions aren’t soluble, but they bond really well with chloride ions, forming ‘complex ions’ (see below) which *are* soluble.

        Given the choice though, copper wants to be +2, so when it comes in contact with oxygen, it gives up an electron and becomes cupric again:

        2H+1 + Cu+1 + Oaq -> Cu+2 + H2O

        The ‘aq’ subscript means ‘aqueous’, or ‘dissolved in water’. Water is much more complicated than ‘H2O’ at the molecular level, so you’ll have hydrogen and oxygen ions floating around in the mix.

        So.. Cupric ions etches metallic copper because nature wants to balance the charges beween the atoms. Chloride ions make the copper ions water soluble. Oxygen accepts an electron from the cuprous ions, allowing them to become cupric ions again.

        All reactions *do* tend toward their lowest energy state, but regeneration adds fresh oxygen to the mix. Water absorbs free oxygen from the atmosphere, and the atmosphere contains so much oxygen that the supply is infinite for all practical purposes.

        Regeneration *does* consume hydrogen and produce water, and you need extra chlorine to form cupric chloride from the newly dissolved copper ions. You generally keep the etchant acidic though (meaning it has lots of extra H+ ions floating around), and provide plenty of extra chlorine.

        You evaporate the solution every so often to get rid of the excess water.

        The precipitate at the bottom of the pool is cuprous chloride, which is only slightly soluble in water. It’s quite soluble in hydrochloric acid though, so that’s another reason to keep the etchant acidic. If you’re getting precipitate, it means you need more acid.

      1. Citric acid is what’s called a ‘chelant’, ‘ligant’, or ‘complexing agent’. In very general terms, it’s ‘soap that holds metal in solution’.

        A ‘complex ion’ is a cluster of molecules that act like a single ion. The details are Deep Chemistry, with terms like ‘dative covalent bonding’, but the upshot is that metal atoms are really friendly. They have lots of orbitals in their outer shell that can form bonds with other atoms, and enough charge to attract partners.

        If you put a single copper atom in water, it attracts the oxygen end of the water molecules. The water molecules will cluster around as tightly as they can, so you’ll get six of them (north, south, east, west, up, and down). The copper will share electrons with all the water molecules around it, so its +2 charge will get distributed across the whole cluster.. making it a ‘complex ion’. Its official name is ‘hexaquacopper’, and is written [Cu(H2O)6]+2.

        If you want the gory details, start here:

        While the oxygen side of the water molecules points toward the copper atom, the hydrogen ends point outward. The hydrogen ends also have a charge, so they’re loosely attracted to other water molecules in the region. The water basically forms a water-soluble cage around the insoluble metal ion.

        A ‘chelant’ is a molecule that shares multiple bonds with the metal atom in the center. Whenever you want to put metal into solution, it helps to have a chelant.

        One of the best chelants for metals is potassium cyanide. That’s why so many old recipes for plating solutions include it. It has fantastic chemical properties, but does have one tiny drawback: it’s insanely toxic.

        Citric acid is also a chelant. It doesn’t work as well as potassium cyanide, but it also won’t kill you.

        Ferric chloride is a really good etchant because iron can both donate and accept electrons. It will corrode almost anything, including copper, which is ‘noble’ enough that it won’t even react with most acids. Trouble is, the cuprous chloride ions that form on the surface of the copper prevent fresh ferric chloride from reaching the surface, so the surface gunks up and etches slowly.

        Citric acid chelates the copper, allowing the chloride ions to go back to the iron, and keeping the surface gunk-free.

  8. Thinking back to an electroplating experiment I had my scouts run for the chemistry merit badge: if you clipped a 9v battery to an exposed bit of copper on the board and completed the circuit with a nail in the solution, might this not put the petal to the metal and etch the anode relatively quickly?

  9. Another aspect of being green is to reduce. In that respect, make sure to use a poured (solid) copper ground plane. This way, the material to be etched off the board is kept to the bare minimum, reducing the toxic content no matter what method is used.

    1. +1 Couldn’t agree more.

      Besides a ground plane has a lot of benefits as well. Really it’s win-win. The only issue is learning how to do it on your CAD program. On eagle it’s pretty darn simple. pour a polygon, connect it to ground(optional I suppose), and then click rat’s nest. It’ll take care of the routing for you.

  10. What we add to the waste water stream by etching a board or two is a gnat fart in a hurricane. I’ll take it seriously for argument’s sake though. And in general, think green. But have a sense of scale. Unless you do it for religious reasons or something.

    I’m not sure which of the ways to process the material is likely to be forever impervious to breakdown and leaching. Making it into something insoluble and throwing it away makes most sense to me. Recycling it seems unlikely to be worth it- how much copper could you recover? The cladding on boards is pretty thin. Compare this to the amount of aluminum recovered in a drink can. Enough cans, and you can make a buck or two, which you likely spent dragging them to a recycling facility. Even on a bike, you expend calories, right?

    The idea of poured ground planes make a lot of sense, too. Less to etch also means the etchant survives longer.

    What does not make sense is obsessing over every atom of waste material you produce.

    1. Indeed. I referred to this in the article- you probably do more environmental damage driving the stuff to some remote hazardous waste facility than you do putting it in a sealed landfill, or diluting it into the enormous treated water system of a large city.

    2. If we lived in a world where everyone, and every company, was handling their waste in responsible ways, i’d agree with you. But we live in a world, and in a country, where decades of careless “this little bit won’t hurt” has added up to the point where the whole planet is under threat from human carelessness.

      So, you’re right, that little bit of copper some hobbyist generates making a couple circuit boards is not going to have any affect. But as a society, i think it makes a lot of sense for people to stop the “this little bit won’t hurt” mentality, and start obsessing about cleaning up our act.

  11. I might publish on this at some point soon, but industrially etchants which can have their copper reclaimed are typically used. Electroplating copper out of solution is quite viable, and with the right solutions can give you a nice, usable sheet of copper. I’ve done this on a few types of etching solutions, but you need the right electrodes and the right solutions. It becomes increasingly difficult to pull small concentrations out, and at that point you could probably use a more typical neutralization step to pull out whatever little copper is left in solution.

    1. since etching with a weak acid solution like vinegar works best with a thin copper-cladding, then seems like it would be kinda ideal to plate your traces with the same copper you just etched off the board. Maybe the plating could be done in the tank as the etching.

  12. The vinegar is acetic acid and the hydrogen peroxide is the primary catalyst. The acetic acid attacks the copper forming copper acetate. The (NaCl)salt helps transfer ions in solution but is otherwise unaffected because the bond that forms between sodium and chlorine is stronger than the bond that would form from copper chloride. Also, copper acetate is highly toxic to all plants and invertebrates as well as a strong irritant to humans(skin/dust inhalation). The only way I could think of making copper acetate safe is to treat the solution with sodium hydroxide, forming Sodium acetate(hand warmers anyone?) and copper chloride which still isn’t safe but is preferred over the acetate.

    1. Interesting. Those that complain about this being slow makes me wonder if it would go faster using real acetic acid instead of vinegar. Vinegar is relatively weak, if I recall. Besides that acetic acid is/was commonly used in film development, so it shouldn’t be hard to find. Heck, I think I have a bottle around here somewhere I got from a mall photo store a long time ago.

      1. Probably would be faster. People are focusing a lot on the slowness of this, but it is still orders of magnitude faster than a fab house. If you want to go from twenty minutes to two minutes, then muriatic acid (for swimming pools) is also cheap. It’s nasty stuff though, and as a commenter on my article pointed out, it isn’t available in some countries.

  13. Since discovering it, I use this method all the time to do prototype boards. I use single sided, low copper weight boards with a copper fill and find that the salt is key to the reaction. It seems to work best if I use non-iodized salt, dissolve the salt in the vinegar/hydrogen peroxide solution before putting in the board, and don’t skimp on it (within reason).

  14. Simply add a few balls of aluminum foil to the leftover liquid…. The foil will darken and partially dissolve, and the aluminum will reduce the copper ions to copper metal while forming aluminum ions which are known to be MUCH less harmful to the environment.

    Once the green color leaves the solution…. the copper ions have been removed.

    Hackaday, I wish you knew how many extremely interesting and useful hacks are NOT posted to this site or similar forums due to concerns that the information they contain about chemical and other safety hazards won’t be respected by most readers…..

    That isn’t to say that this same factor isn’t a huge problem in industry and academia as well, but there is at least somewhat of a “higher bar” which must be cleared in order to play around.

    When for years and years many people show no respect for simple ideas like not pouring heavy metal ions down the drain, people sometimes choose to refrain from posting about more advanced topics like home semiconductor manufacture etc etc.

    Good luck!

  15. im about to try this today. I would like to throw an idea out there though and that is to use a small-tip hot glue gun VS. a sharpie. it uses the same concept as a sharpie only it wont come off as easy during the wire brush phase but will pop off with minor force… ideas?

  16. Seeing as how there is alot of talk about chemistry, I have a question. The question is off topic but this seems to be the most logical place to ask this type of question. Some years back I was browsing around on the interwebs and I came across this little mixture thats non-toxic and can turn water into snow.. After letting the fake snow sit for some amount of time, it would dissolve and allow you to re-use the original (powder?) to make more. As iv stated, its not toxic and sounds like lots of chemistry fun.. Id really like to know what this product was and where I can purchase it. Im really starting to fall in love with Chemistry again and what better way than to have fun with some snow. Thanks in advance.


  17. Hi everybody!

    Thank you for the ideas. Another question here: Is it possible to separate copper from spent ferric chloride not using electroplating? Like you just want to filter the solution so that copper will be left on the filtering material? Does anybody have an idea on this?

    Thank you very much…

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