Stop Using Ferric Chloride Etchant! (A Better Etching Solution.)




Ferric chloride is a traditional home-use circuit board etchant. It's easy enough to come by, and the Ferric by itself is no big environmental problem. However, once you've etched a board with it, you're left with a solution with a bunch of copper chloride in it. This dissolved copper is an environmental problem, and you can't just pour it down the drain (legally) -- you're supposed to take it to a hazardous waste facility. (For instance: How to Dispose of Ferric Chloride in this FAQ. )

Wouldn't it be nice if there were an etchant that you could re-use indefinitely so that you don't have to worry about disposing of the copper, and that could be made in lifetime supply for like $10.00 with ingredients bought at hardware and drugstores? (And it's prettier too.)

I got seven words for you: Copper Chloride in Aqueous Hydrochloric Acid Solution! (Exclamation point!)

But how're you going to get CCiAHAS? Conveniently enough, by starting out with a simple two-ingredient starter etchant, and doing a bunch of etching.

Step 1: Ingredients: the Starter Etchant

For the starter etchant itself, you only need two ingredients: hydrochloric acid and hydrogen peroxide.

(OK, actually three. But the third one's copper. See the chemistry section for an explanation.)

Hydrochloric (muriatic acid, "pool acid", etc.) is available at a hardware store. The acid I got is 31.45% (or 10M) and should run around $5 per gallon. Which is more than you'll ever, ever need.

The peroxide is normal 3% for mouthwash or cleaning cuts, and can be bought at a drug store for $2-3 for a big bottle.

You'll also need a non-metallic container that fits your PCB and two standardized measuring cups.

As long as you're in the hardware store, pick up some acetone if you don't already have some. It's useful for removing the etch resist. (That's for another instructable.)

Step 2: Put the Lime in the Coconut...

Measure out two quantities of hydrogen peroxide and pour it into your non-metalic container.

Measure out one quantity of hydrochloric acid and pour it in. ("Do like you oughta, add acid to water" to minimize the chance of an out-of-control exothermic reaction.)

Be careful with the acid. This stuff (at 10 molar) is strong. Mine fumed a bit when I took the cap off. Don't breathe it directly, and be sure you've opened the kitchen window.

The starter etchant you've just made, on the other hand, is not so bad -- around 3M HCl with a medium-strong oxidizer. I find it doesn't fume much at room temperature when I'm re-using a batch.

That said, you've got to be very careful to keep it away from metal -- especially your stainless-steel kitchen sink. It'll eat the stainless coating right off. Keep plenty of water flowing at all times when you've got any of this (even a drop) near the sink.

Step 3: Add PCB and You're Etching.

Toss the PCB into the solution and it'll take off.

If this is the first time you're using this batch of solution (and I presume it is), it'll etch super-fast. This small board took only 2 minutes. Yikes!

Since I use a deep container, I tend to swirl it around as it etches. This stuff is so active, though, that I'm not sure it's necessary.

Keep the window open for ventilation because the starter solution gives off a little chlorine gas. (The end-etchant gives off much, much less.)

Also, note how the etchant gets greener over time as it eats away the copper. This is good news.

What's happening is that you're dissolving the copper from the board and turning it into cupric chloride. In the long-run, the cupric chloride will be doing most of the etching (instead of requiring disposal). For now, just watch your solution turn light green. Next time you use it, the color will deepen.

Step 4: Chemistry Break

(Note: I'm an economist, not a chemist. Please leave a comment if you've got any corrections and/or additions to this stuff!)

I stumbled on this idea when I saw this website: Etching with Air Regenerated Acid Cupric Chloride by Adam Sechelle. Cupric chloride can be re-used indefinately by topping up the acid levels and adding oxygen (bubbled in from the atmosphere). Sounds cheap and environmentally friendly to boot.

The website's got a lot of good chemistry info on cupric chloride etching. His data on etching speeds is great, and his simple titration procedure for maintaining the acidity of the solution is pretty nice.

To make the cupric chloride solution, he dissolves a bunch of copper wire in hydrochloric acid, and mentions maybe using hydrogen peroxide to speed up the oxidation, but doesn't go into detail.

Which got me thinking. You didn't have any cupric chloride yet, but you can make it by dissolving copper. Dissolving copper is the name of the etching game. So we can make one etchant that makes another etchant that's infinitely re-chargeable. Elegant.

Turns out that hydrochloric/peroxide is a common home-brew etchant (and I've re-re-invented the wheel, again) but I guess that people got so used to throwing away their "spent" etchant that they don't think about re-using it. The whole point of this instructable is that you don't throw it away, but use the dissolved copper forevermore as your long-run etchant.

Here's what's going on chemically:

Before there's much copper dissolved in the solution, Cu + 2 HCl + H2O2 -> CuCl2+ 2H2O is the dominant net reaction. That is, the extra oxygen in solution from the peroxide is oxidizing the copper metal, in presence of the acid, to make copper (II) chloride. That's our starter etchant. The resulting CuCl2 shoud be a nice emerald green color.

After you've dissolved a lot of copper into the solution, and used up all the peroxide, the copper chloride does most of the etching for you: CuCl2 + Cu -> 2 CuCl. That's the end etchant.

Eventually you etch so much that you convert all the CuCl2 into CuCl, which doesn't dissolve copper (and is a yucky brown color). As long as you've got enough acid in the solution, you can simply add more oxygen to re-oxidize the copper(I), making more copper(II) chloride and water: 2 CuCl + 2 HCl + O -> 2 CuCl2 + H2O. And then you can etch again.

Bottom Line:

Two things to maintain: CuCl2 levels and acid levels.

CuCl2: After all the peroxide is used up, and the solution starts turning brownish, you'll have to add oxygen to regenerate the solution again: toss in a few more capfuls of peroxide or bubble air through the solution or swirl it around vigorously, or just pour it into an open container and wait. It's easy to tell when you're ready to etch again, because the solution turns green.

It's also impossible to add too much oxygen by adding air, so bubble/swirl to your heart's content. If you're using peroxide to add oxygen, be sparing -- a little goes a long way, and it's mostly water so you're diluting your etchant by adding it.

Acid: Note that HCl is being consumed in the starter etchant and the regeneration reactions. So we're going to have to add a bit more acid as time goes by. If you notice that it's harder to re-green your brown etchant, it's probably time to start thinking acid.

I've tried the titration described on Adam's site a couple times, and it's pretty easy but requires an accurate scale and pure lye (back to the hardware store...). It's easier to just toss in a capful of acid every few batches of boards, which seems to do the trick for me.

Step 5: Save the Etchant for Next Round. You're Done.

Once you're done etching, pour the etchant back into your storage bottle, rinse off the board, flux, drill, populate, and solder.

Some final notes here:

1) You can make quite a bit of this stuff very easily, and since you're re-using it, there's no real reason to skimp; put plenty of etchant in your "tank." When you use too little FeCl etchant, for instance, it can get saturated with copper and slow down which can result in long etching times and pitting or undercutting or worse. When I'm etching a board with copper chloride, I'll pour a couple extra inches of solution into the container. It's reusable anyway, and the extra exposure to oxygen just regenerates it. Live large.

2) Don't make too much. As you keep re-using the solution, you're going to need to add a little more acid and a little more peroxide every once in a while. If you've got a 750 milliliter container, start out with less than 500 milliliters of solution. Give yourself some room to grow over time. After all, the main point is to avoid having to dispose the copper in spent etchant.

3) If you've got too much volume of etchant (it will happen eventually) you can evaporate out the extra water by putting it in a shallow (non-metallic) pan or beaker or whatever and letting it sit for a while. This concentrates the copper in solution, giving you a stronger etchant. It'll also re-oxidize some of the copper for you, a bonus. Remember when you're adding the peroxide that you're actually adding 97% water.

4) The linked website suggests that the acid levels in the etchant are not critical as long as there's some acid in solution to do the CuCl2 regeneration. The amount of CuCl2 (vs CuCl) present is easy to diagnose by the color of the solution. Add oxygen to re-green, and add a bit of acid if that's not working. Worst case is that you may have to wait a few more minutes per etch with a sub-optimal bath. This isn't rocket surgery.

5) I do have an aquarium pump ($6 at fish store) that I've used to re-activate my solution. Sometimes I'd leave it on for a few hours while I'm at work if I've been etching a lot. But lately I've been lazy/impatient and tossed in a couple capfuls of peroxide. Both seem to work just fine.

6) The environmental benefit of etching with copper over ferric lies mostly in not having to dispose of the copper that comes off your boards every few times you etch. When and if you do end up with too much copper etchant, please treat it like the hazardous waste that it is -- look into your local hazardous chemical disposal options. There's no getting around the fact that copper salts are (for instance) poisonous to fish even in very dilute concentrations.

Step 6: Alternative (overly-complex) Method: Make Cupric Chloride Faster.

When I originally started trying to make Cupric Chloride etchant, I hadn't thought of just using the regular procedure of etching to get there. So I deliberately dissolved a bunch of copper from a wire.

I don't think it's a particularly good idea, but here's how I got to the end-stage etchant faster.

I mixed the acid/peroxide 1:1 instead of 1:2. The idea was to have a bunch of acid leftover for later regeneration. I don't think it's a good idea, and I wouldn't do it again. 1:2 is probably better, and results in more copper in solution faster with less fuming.

To control the fumes, I used the patent-pending (just kidding) Two-Pint-Glass Fume-Containment-Apparatus. Pour in the peroxide, add the copper, then put one glass on top of the other. Pour the acid down through a small gap between the two glasses and re-seal. Voila. No fumes. (See video. I think I did it with water as an example.)

I also kick-started the formation of cupric chloride by first making copper oxide, which turns to cupric chloride just in the presence of acid alone. This isn't necessary at all, but it was fun. Heat up a coil of copper wire on the stove to red-hot and you get a flakey coating of copper oxide.

Otherwise, it's basically the previous procedure, so just see the pics for notes. I wouldn't recommend it anyway. The less copper you dissolve, the less copper needs to be (eventually?) disposed of, and the acid/peroxide etchant is plenty easy to use.

The two-cup technique is cute. I still recommend it.

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785 Discussions


10 months ago

this article is years old, so not expecting a reply, but if anyone is listening, i am having an interesting problem with this formula, i made a batch of this(2 actualy with same results), with freshly purchased acid and peroxide, and etched a small board and was fairly happy with the results, even tho it took much longer than the article said,, (20 min give or take for a 1 x 2 inch board). but the real issue is, when i put the second board in just a hour or 2 later.. it sat for hours with no etching, even scrubing the board after would not remove any copper, its almost as if after one use the solution completely neutralized. i even left the board in overnight, with no progress, any idea why such strange behaviour?

3 replies

Reply 17 days ago

Try 12 or 15% Hydrogen Peroxide from a Beauty supply house. (It's the stuff they use to make blonds.) :-)


Reply 9 months ago

hi, just wanted to share my experience after trying this method.

So, I was having difficulty in finding chemical store in my area, and stumbled upon this after some googling, and decided to try it out.

I grabbed myself two cleaning products from groceries store. one is cleaning products for toilets containing 20% HCL. and the other one is some form of liquid soap used to remove stain on your clothes, contain 5% of Hydrogen Peroxide (H2O2).

By mixing two parts of HCL and 1 part of Hydrogen Peroxide (I also added 1 part of water) its ( 120ml HCL+ 60ml H2O2 + 60ml water)

after mixing all the liquid in plastic container, I then put a piece of small PCB in it.

at first i did not see any visible reaction, and thought that i failed (because i add water :D). Then i let the thing do whatever it is doing, and come back a couple of hours later to only to find that all copper on the pcb already wiped out and the liquid solution turned green.

after that, i tried to put another pcb and I can see reaction, I let the pcb submerged in it for a few minutes without doing anything, not even touching anything. and it worked great, wiped all the copper on the pcb in a view minutes.

The Real Elliotcoffeeoutlaw

Reply 10 months ago

How are you agitating the solution? What temperature is it? How did you clean and/or prepare the board's surface beforehand?

20 min for a fresh acid/peroxide mix is _way_ long if you're constantly swirling stuff around, unless it's very cold.

Look at the board as it etches. If you swirl a bit and you see a thin cloud of darker stuff coming off the board -- dissolved copper -- then you're not agitating enough. Swirl more / continuously.


4 months ago

Your idea is great. But allow me to share my experience with ferric chloride: I have been using the same FeCl3 500ml bottle for over 8 (eight) years straight. What I do, after etching my small and eventual PCB production is to return the solution to the bottle, as simple as that.
When I am going to use it again I just avoid to shake the bottle and gently pour the FeCl3 out of it into another container. I have not noticed significant higher corrosion times and to me it is still very usable. Another very interesting technique for corrosion that allows you to use a VERY small amount of FeCl3 (10 or 20ml!) is to wear gloves, get the board and use a small sponge soaked in FeCl3 and gently rub the board. 10 x 10cm boards are fully corroded in less than 4 minutes! I never wasted time again letting my board soaking for lots of minutes in a container and it uses a lot less etchant. Hope this helps. Cheers! Jeffo.


2 years ago

what if i happen to have a spare medical oxygen tank? asking cause i know little about chemistry but want to avoid things that go boom and etch with out the help of fire department.

1 reply

Reply 6 months ago

Of the two, the medical oxygen tank is the dangerous one, it has two major risks:
The cylinder stores gas at around 3000 psi/ 20 Bar, in a workshop unsecured where it could fall is keeping a loose unexploded bomb handy - there's a lot of mechanical energy stored, enough to level a brick garage - if it's damaged it can rupture or shed the valve (less destructive, just a short burst of 10 - 20 G acceleration and it'll stop moving when it fails to pass through its third brick wall...);
Also, a rupture or accidental venting a slow but steady leak, can raise the O2 concentration to dangerous levels, greatly increasing the effects of ignition (to the point, has happened, where granny lights a cig with her oxygen mask around her neck and all the windows closed and the match burns out in a fraction of a second, is dropped and ignites granny's nightie...)

H2O2 and HCl will only burn holes in you, not your house down ☺


6 months ago

i used bathrooms flash liquid and oxygen water and worked pretty nice and fast, but I used it only to wipe out the copper. I didn't have any circuit design.


8 months ago on Step 1

I'm having problems getting the solution to etch. I started with 2 parts hydrogen peroxide to 1 part muriatic acid, and dropped in a piece of copper tubing. So far, so good. Solution turned a pretty aqua blue. I tried an etching (jewelry type) and it etched somewhat. I didn't like the results, but chalked it up to inexperience. It was about a week before I tried again. Since then, working on design.

I tried again today, and it doesn't seem to be etching at all. The solution was dark green. I added a bit more acid and a bit more hydrogen peroxide and tried again, with no etching.

What am I doing wrong?


Question 10 months ago on Introduction

Good day!!! Will this solution etch steel as well as copper?

I have recently been having issues with my ferric chloride etch...


Tip 10 months ago on Step 6

Does this acid eat away at a tin coating or just copper?


1 year ago

Will this solution eat into epoxy or any type of adhesive? I want to use it instead of ferric chloride to etch a knife blade which has epoxy on between the blade and handle scales.

2 replies

Reply 1 year ago

I doubt it will eat into the epoxy or other resin type adhesives. The PCBs substrates can be Epoxy (FE-3,FR-4,FR-5) or polyester or PF Resins and it does them no harm at all


Question 1 year ago

Have you mentioned that we must read the instructable twice to understand every sentence in it? Anyway thanks a lot.


2 years ago

I wonder if you could use the solution for copper plating between etches?

1 reply

1 year ago

Thanks for this instructable !!.. I had a hard time trying to get Ferric Chloride to etch my little boards (1 inch x 2 inches).. It was probably a bad batch I bought.. I used your method.. and like magic I saw the 1 ounce copper PCB dissolving right in front of my eyes.. it is awesome.. You really need to be on top of the process since this combination of Muriatic Acid + Hydrogen Peroxide is aggressive.. it produced very nice clean boards.. and none of the nastiness associated with Ferric Chloride.. thumbs up !!!

one a side note.. I used plastic locking clamps I found at Lowes.. they are perfect to handling small boards of let's say 3 inches wide.. Thanks!


2 years ago

The only problem with this is that the polymers in the ink can be sensitive to acid. You can potentially end up with a soup of random organics many of which can be carcinogenic. Many chlorinated carbon compounds are know to cause cancer. Especially if you reuse the etchant.

The nice thing about the ferric chloride (and why it is commonly used) is that it generally ignores the ink. Every chemist around knows you can use different acids to etch copper, but the reason they went with ferric chloride is because acid doesn't just etch the copper.