Best Practice: HCl + H2O2 PCB etching (Hydrogen peroxide & hydrochloric acid)

[pedake]

Hi all

I tried my first etch today. Only two strips. It is getting dark and I did not want to do it inside. 

Attached my VERY FIRST 2 tries ever. Just some permanent marker with lines in different thickness and different number of "coatings" (multiple passes with permanent marker).
Summary:

REALLY fast etching
STRONG exothermic reaction
FAST temperature rise (even with very small surface of the strips, see attached)

I am not an experienced etcher (again, very first time). It is hard for me to say whether something is "over etched" or "under etched".
Also, my improvised microscope (a hybrid of a really good logitech webcam with a cheapo USB microscope) stopped working (see attached for your entertainment). So I cannot check the results with pretty close-ups, unfortunately.

I have not tried FeCl. It should be a lot safer (no fumes, not so exothermic reaction). But FeCl is actually quite bad stuff for the environment [correction: mostly nasty on your clothes]. Also pretty nasty if you get it on your clothes (or anything else). My old lab coat is basically all FeCl - with some whitish spots. Also when I see the videos online - it's pretty slow.

"LAB JOURNAL"

Safety

VERY WELL VENTILATED environment!! I did it on my balcony. Do not do this in closed space. Chlorine gas is very nasty and partial pressure of Chlorine is SURPRISINGLY high! 24 % HCl fumes is on the brink of nasty. DO take this seriously. If you don't care about your lungs, eyes and other exposed body parts: consider all metal fixtures and equipment around you. It WILL corrode from the fumes. And very quickly.
USE YOUR SAFETY GLASSES!
USE GLOVES!! Both are really cheap.
Start with small patches of copper.
MARK the containers. ALL of them. With permanent markers. So that you cannot oversee it. Even if only water is in them. Especially since you are (I am) doing that at home - your noodles might taste funny when you accidentally use that food container again....
DO NOT do it in your kitchen, near food, your son's / daughter's bedroom and so on. Best in a workshop or outside. Not your dinner table, not your couch. Chemistry can be bad sometimes.
READ the damn Materials Safety Datasheets (MSDS). Print them out, read them 10 times. Try and remember what it says. It is important. Really is. I mean... REALLY. And read them every time you start doing your thing. Especially when you do something new.


MSDS
HCl:
http://www.sciencelab.com/msds.php?msdsId=9924285

H₂O₂:
http://www.sciencelab.com/msds.php?msdsId=9924301

Acetone:
http://www.sciencelab.com/msds.php?msdsId=9927062

CuCk₂:
http://www.ch.ntu.edu.tw/~genchem99/msds/exp22/CuCl2.pdf

H₂O
Just to be safe: Don't try and drink the deionized water. You might die of hyposalinity if you drink enough of it 

The chemistry
(please, correct if I am wrong - I did not have the time to look everything up right today):

H₂O₂ (aq)+ 2 HCL (aq) + Cu -> 2 H₂O + Cu²⁺(aq) + 2 Cl^- (aq)

While the H₂O₂ serves as oxidant, I guess.
As far as I remember, Cu is almost inert to low(er) HCl-concentrations (<30 %).
So I'll be bold and guess that H₂O₂ could work as a pure initiator (ergo, really small amounts may suffice?).

Materials
See attached picture for some of the materials I used.

1x 10 L PP container with screw cap
1x PP funnel
2x PP beakers, 20 ml, with 2 ml markings
2x PP 0.5 L food containers
1x Polymer tweezers (I used steel, but it'll dessolve :-) )
1x safety goggles (DO use these! Cheap to buy, very valuable when you want to scratch your itchy eye....)
1x big box of cheap disposable vinyl gloves (Nitrile are more versatile. Vinyl will dissolve in Acetone, but are good for Oxidizing agents like HCl. Since I don't mind the acetone, I chose vinyl. Also I like the feel better. Don't buy Latex. They rip and puncture easily, plus they are expensive)
1x 7.8 M HCl (^= 24 %)
1x 8.8 M H₂O₂ (^=40 %)
1x pack of NaOH (I just used baking soda)
1x Acetone
1x Distilled / deionized water (really, tap water is just fine. But I am a sucker for best practice)
1x roll of packing tape (I just used it to mask one side of the dual sided copper clad board)
1x tin shears (for your copper clad boards)
1x steel wool
Xx permanent markers (Just for the try-out)
Xx strips of your copper clad board, FR4 (don't start with an entire board right away. I tried a strip first to get a feeling)
Xx Paper wipes


Steps

cut small strips of copper clad PCB board with large tin shears.
abrasively cleaned surface with steel wool
thoroughly cleaned with acetone and paper wipes
made markings with permanent markers: different widths / areas, geometric forms and layer thicknesses
masked / covered areas (backside) of PCB strip that were not to be etched
Added cold tap water into 1st PP container

added into 2nd PP container as follows:

1st try:

• Room temperature (~20 °C)
• HCl, 24 %, 10 ml
• H2O2, 40 % ~2ml

2nd try:

• Room temperature (~20 °C)
• deionized water, 15 ml
• HCl, 7.8 M 24 %, 10 ml
• H2O2, 8.8 M 40 % ~2ml

Added cold tap water into 1st PP food container
Added following chemicals into 2nd PP food container

After each try, I neutralized the solution with NaOH (baking soda in that case) and diluted it a little with some tap water and put it into a small PET bottle. I marked the bottle with a label and will get rid of it at a local chemical waste disposal service some time soon.



Results

1st try
Some 3 second etching.
Very fast. Very reactive.
Some over-etching apparent.
transparent green solution remains

2nd try:
Maybe 10 - 15 seconds of etching. But not as exothermic as the first try.
Reaction easier to control.
transparent green solution remains
over etching not apparent (at least from what I can tell... )
One "layer" of permanent marker seemed to be enough if no blank spots were left.
Thin lines were apparently not over-etched

TODO

1: reaction control
Control reaction speed. I will try:

• controlling H₂O₂ addition
• diluting (although I guess controlling reduction agent may be better - HCl is slow with Cu anyway)
• pure water and control reactants (dripping in oxidant and reduction agent equally)

2.: Waste management
Find a way to easily neutralize the solution so that I can just safely pour it down the drain.
I'll check on that. If anyone is interested, let me know

3.: Alternatives
I really do not want to bother with FeCl. It's a mess.

I had in mind to try and do it galvanically. BUT: What do I do when a part of the board is "etched" away and there is a gap between two parts? I still need a physical, conducting connection between parts that are to be removed. Maybe there is some solution to this out there. Cannot think of anything, though. probably stupid idea. But I like the idea of an electro-chemical approach to the problem - no nasty chemicals, quick, controllable, broad error margin, safe, clean, etc....

I don't have the space for milling. So not an option. Also high CAPEX and inefficient (not that I'll be producing 10'000 boards a day. Probably I'll make 10 my entire life :-)

4.: learn to actually design a PCB 
Never did it. Let's see what I do first. I ordered a LOT of SMD-stuff from China. So far I only played with through-hole and DIP on prototyping boards & breadboards.
My first project is an Atmega238 fast auto-ranging and not all too precise ohmmeter (I called the project SpeedyOHMalez here in the forum, hehe) for sorting resistors.

5.: Solder mask & silk screening
I ordered UV paint for the solder mask (nice red). I've read a little. I am curious to see how I manage to coat the board evenly.
Need a UV light (anyone in Switzerland has an unused one lying around?) and to study curing time, temperature resistance, etc.

Also I want to find out the best way to apply the "silk screen" on top of the solder mask. I saw a lot of actual silk screening.
Since I'll only make one-offs for starters, I really don't want to make an actual silk screen just for one design. I was thinking: Same process as the solder mask, just inverted, no? so: covering the entire board in (in this case white) UV curing paint, put laser printed mask over it (inverted) and wash the other stuff off. Not very material efficient... but it should work, no?


postscriptum:
I will try and make a complete tutorial out of this as I progress (and when I find the time). I want it to be a little more than the usual "pour this into this" you can find everywhere. Any help, comment, addition, constructive criticism ..... etc.... is much appreciated. Especially if I got things wrong. I'd generally like to stick to HCl as oxidizing agent. It is readily available and cheap.
There are safety issues with this, sure. But I think ignorance is a great tragedy and people should be educated - that should also involve properly handling hazardous chemicals.


 

[Andreas]

1st try:

• Room temperature (~20 °C)
• HCl, 24 %, 10 ml
• H2O2, 40 % ~2ml

2nd try:

• Room temperature (~20 °C)
• deionized water, 15 ml
• HCl, 7.8 M 24 %, 10 ml
• H2O2, 8.8 M 40 % ~2ml

Sorry concentration of both solutions is much too high.
(in this case you will get too much CL2 gas.
In a old book (Dieter Nürmann Werkbuch) I read following concentration:

770ml water
200ml ~35% HCL
30 ml  30% H2O2

etching time around 10 minutes

With best regards

Andreas

[helius]

Electroplating works well for things like PTHs and gold fingers, but it can't really be used for a subtractive process on insulating substrate.
The subtraction of metal by electrolytic bath is called electrochemical machining (ECM), and it only works on conductive substrates. So the only options for subtracting metal patterns on an insulator are chemical (etching with photoresist), physical phase change (laser ablation), or mechanical (milling).

[pedake]

1st try:

• Room temperature (~20 °C)
• HCl, 24 %, 10 ml
• H2O2, 40 % ~2ml

2nd try:

• Room temperature (~20 °C)
• deionized water, 15 ml
• HCl, 7.8 M 24 %, 10 ml
• H2O2, 8.8 M 40 % ~2ml

Sorry concentration of both solutions is much too high.
(in this case you will get too much CL2 gas.
In a old book (Dieter Nürmann Werkbuch) I read following concentration:

770ml water
200ml ~35% HCL
30 ml  30% H2O2

etching time around 10 minutes

With best regards

Andreas

1st try: I agree...

but 2nd try... Don't know.... Why leave it for 10 minutes if it works fine in 20 seconds?
I will try playing (reducing concentration) with the oxigen-source... (H2O2) But why dilute the HCl any further? I already diluted to some 3.5 M or so.
And copper does not react well with HCl anyway.

As to Cl2-gas: Stochiometry is still the same, no? so amound of Cl2-gas should be the same just quicker?

[pedake]

Electroplating works well for things like PTHs and gold fingers, but it can't really be used for a subtractive process on insulating substrate.
The subtraction of metal by electrolytic bath is called electrochemical machining (ECM), and it only works on conductive substrates. So the only options for subtracting metal patterns on an insulator are chemical (etching with photoresist), physical phase change (laser ablation), or mechanical (milling).

well... obviously. :-)
I was trying to think outside the box. I don't know... Two chambers separated by a semi-permeable material (i.e. Cu²⁺-ion-exchange-membrane) and low resistance conductive fluid on the subtracting side... something like that. 

If it is of interest to you, there are papers out there on ion exchange membranes for metallic ions (in solution). Polysulfone I think is one type of membrane they used to sue for that. I used to have problems with quick saturation, sorption rate, etc. when I still did interesting things. But maybe there is something more exciting out there now Also I would have no idea for the conductive fluid...

[Andreas]

Why leave it for 10 minutes if it works fine in 20 seconds?
But why dilute the HCl any further?
As to Cl2-gas: Stochiometry is still the same, no? so amound of Cl2-gas should be the same just quicker?

1. you can get finer structures without under-etching.
2. the concentration of CuCl2 in water is limited.
3. the reaction should be O2 oxidizes copper (CuCl, if available, reacts as catalyst). CuO is diluted within HCL. CL2 gas should not occur. (but is created especially with higher temperatures).

With best regards

Andreas

[KL27x]

But FeCl is actually quite bad stuff for the environment.

Source?

FeCl is an industrial chemical. Tons are made every year, specifically to dump into our drinking water.

[pedake]

But FeCl is actually quite bad stuff for the environment.

Source?

FeCl is an industrial chemical. Tons are made every year, specifically to dump into our drinking water.

I was told at school when I first used FeCl3 - water treatment plants have difficulties when external FeCl3 sources get into their system. They do add FeCl3 in low concentrations themselves but it has a narrow window of operations. It may cause a chain of failures in the water treatment process that can be expensive.
I really do not know details - never checked the facts. I just had that in my head.

Disclaimer: I have a very non-technical job nowadays. My lab days are over, unfortunately. Please, be kind and patient with my mistakes. Trying to keep my mind fresh And I have forgotten much

[janoc]

Re best practices - the best practice is to not use HCl + peroxide, period. Toxic gases are produced, concentrated aggressive chemicals needed, problems with overetching if you aren't really careful, toxic and corrosive waste that isn't trivial to dispose of afterwards. The only advantage of this method is that the chemicals are readily available (if you use diluted 3% peroxide and not concentrated one - that one is getting difficult to get) and cheap.

The industry standard solution for etching boards is ammonium persulfate - that is pretty clean and has only few drawbacks - such as that the solution needs to be heated to some 60C or it won't etch and the prepared etchant is not longterm stable and decomposes, releasing oxygen gas (so don't store it in a gas-tight bottle or it will burst).

The second best is the FeCl3 that you have called "a mess". You don't have a problem dealing with concentrated hydrochloric acid and peroxide, chlorine gas and the acid fumes, but a fairly tame FeCl3 is a mess?  Something doesn't compute here.

The only real issue with a FeCl3 solution is that it stains everything and attacks metal, so don't etch in your stainless steel kitchen sink. However, it is much less aggressive than a concentrated acid.  Also, the FeCL3 solution will last you a long time, it is easy to reuse it for multiple boards and even regenerate it using a bit of HCl. Thus there is much less problem with waste disposal. It is certainly less of an issue than pouring HCl + peroxide + copper compound mixes down the drain after every board as some people do.

I did etch boards using all three methods and I have always gone back to the ferric chloride. It is the least hassle out of the three. The persulfate works great but needs a heated etching tank which I don't have and had to improvise. The acid + peroxide combo - tried it and no thanks.

[sleemanj]

hwga, cue the double headed religious war about firstly which etchant is best, and secondly environmental concerns or lack thereof.

I think i'll just leave the Seychell document link here and back out slowly
http://techref.massmind.org/techref/pcb/etch/CuCl2.htm

me = cucl2, homebrewed by eye not measure, not disposed just dosed with a slug of  h202 and hcl before a job to freshen it up

[con-f-use]

I was going to post that link. Its really the best method in my opinion. The link makes it sound like you need a chemistry lab, but just a container is enough. Before you etch you add a little bit of concentrated peroxide and stir three times. Every once in a while, when etching gets slow, you add a bit of HCl. There is not more to it. No bad fumes, no disposal problem, reasonably fast etching at room temperature. You can probably even use the rest of your solution. Just water it down a tat, keep using it until it goes slow, then add peroxide and watch it turn bright green again. Done.

[KL27x]

+1 on CuCl2.

I, too, keep peroxide on hand, but only for cleaning up any unintentional stains on concrete. To get out the green copper stains, need to use HCl and peroxide, and it comes right out. For my tank, I bubble the bejeezus out of it, and it'll keep on etching even when the acid content gets so low a half inch of white precipitate (copper I chloride?) forms on the bottom. I have never gotten my hands on high concentration peroxide. The compressed air setup I use will regenerate the tank to bright green in a matter of minutes.

This etchant works well without any heating. The amount of under-cutting is the same (pretty low), regardless of temp.

Letting excess etchant evaporate leaves green copper II chloride crystals. Anyone need any dry CuCl2 for making etchant, send me a PM. Just add water and then a little HCl.

[Fred27]

+1 on FeCl3.

I don't understand all those poorly written articles like "Don't use nasty ferric chloride, use concentrated hydrochloridic acid and peroxide instead." They're far nastier both for you and the equipment in your workshop. I'd far rather the odd orange stain than holes, rust and damaged lungs.

Sent from my Nexus 6 using Tapatalk

[KL27x]

Stupid story
One of the most popular electronics-related "Instructables" on Instructables.com is titled Stop Using Ferric Chloride! The author instructs on how to use hydrogen peroxide and HCl, instead. But he misguidedly states that it is indefinitely reusable, because it turns into CuCl2. Which of course it doesnt, unless you immediately dissolve a fair amount of copper in it with aeration. As soon as you mix it up, the peroxide begins to spontaneously decompose. But this false information turns into gospel.

Some months later someone makes a spray etching machine that uses 5 gallons of etchant. So what does he use? Of course he mixes up 5 gallons of muriatic and peroxide, and he cites this other great Instructable. Fires up the machine. Etches a test board in 2 minutes, puts his 5 gallons of "etchant" away and declares win.

Anyhow I thought that was pretty funny. And ironic of the title because ferric is indefinitely reusable.

[con-f-use]

Yes, you have to etch one or two boards right after you mixed peroxide and HCl or add peroxide right before you etch something, but that is not a problem. I can't imagine someone making the etchant and then letting it sit there for days without etching something. As to the instructable, at least it implies, that you need to etch something. For once it is one of the steps that are supposed to be taken in one go. That peroxide gases out, and eventually becomes water is high school knowledge and sometimes stated on the very bottles it comes in. For a reader not to get it, they'd really have to have a slow day, seeing he all but says it here:

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.

http://www.instructables.com/id/Stop-using-Ferric-Chloride-etchant!--A-better-etc/?ALLSTEPS

The constant contact with oxygen from air in a spray etching machine is (effectively) equal to adding a bit of peroxide constantly. So it does work in both cases, as long as people know this little peace of info.

Personally, I don't like to bubble air into my solution (or agitate it) to regenerate. It is loud, needs electricity and is slower and more expensive than concentrated peroxide, which I have to have at hand for or stuff any ways. Also usually the tank are lager, and I don't want that much of the stuff. But other people are in a different situation, so by all means...

They're far nastier both for you and the equipment in your workshop. I'd far rather the odd orange stain than holes, rust and damaged lungs.

Actually they are not. Once the CuCl2 has formed, the whole mixture doesn't gas out chlorine any more and doesn't corrode nearby metals nearly as much as FeCl3 does. It becomes a totally different reaction.

[janoc]

Actually they are not. Once the CuCl2 has formed, the whole mixture doesn't gas out chlorine any more and doesn't corrode nearby metals nearly as much as FeCl3 does. It becomes a totally different reaction.

Chlorine is not the problem when storing this, the HCl fumes from the acid are. Those will reliably escape even well closed bottles with glass stoppers that are supposed to be gas-tight. It takes only a little contamination on the surface (and who wipes a stopper from a concentrated acid?). You don't have to be able to smell it for it to be escaping and it will corrode everything metal in sight over time. Really great thing to have in a shop.

FeCl3 doesn't corrode nearby metals - unless you spill it or mix hydrochloric acid in it. But then it is HCl fumes doing the corrosion, not the FeCl3.

[KL27x]

The constant contact with oxygen from air in a spray etching machine is (effectively) equal to adding a bit of peroxide constantly. So it does work in both cases, as long as people know this little peace of info.

Confuse: You cannot aerate HCL to make it work worth a darn. The reason  you can aerate CuCl2 is because the oxygen drives CuCl + HCL back into CuCl2 and water. Bubbling 99% acid with a tiny bit of copper from one board doesn't generate any H2O2. The multiple oxidation states of copper in cupric (and iron in ferric) are what "store" the oxidative force in the reaction. Acid peroxide has no "battery" for storage. It depends on H202, which is unstable and does not regenerate.

If you put copper in HCl with a bubbler, it will eventually etch, because the air will slowly oxidize the surface of the copper (this is the part that H2O2 does very quickly), and then the acid will etch the oxidized copper. But it will take a long, long time to get started. As the copper content increases, the rate will increase in proportion to the concentration of copper in the solution, until you have actual CuCl2 etchant, etching at CuCl2 speeds.

Once the CuCl2 has formed, the whole mixture doesn't gas out chlorine any more and doesn't corrode nearby metals nearly as much as FeCl3 does. It becomes a totally different reaction.

As far as corrosion ferric and cupric are essentially identical. You want to have a slight amount of HCl in both etchants, optimally. Compared to pure acid +peroxide, the amount of corrosive fumes is very small.

Personally, I don't like to bubble air into my solution (or agitate it) to regenerate. It is loud, needs electricity and is slower and more expensive than concentrated peroxide, which I have to have at hand for or stuff any ways.

To clarify, you cannot use a bubbler with your HCl + peroxide etchant, anyway. Just because you etched a board or two doesn't turn it into CuCl2. The concentration of copper has to be sufficient for it to etch at a reasonable rate. Until it does, you are etching with H2O2 and acid, and a bubbler will not be sufficient to achieve reasonable etching rate. Maybe that is the reason you find it to be "slower." To be sure, cupric is slower than HCl + peroxide. And even if using (proper) cupric, peroxide will increase the etch rate some. But it will also increase the undercutting by utilizing the HCl in the etchant to do the HCl + peroxide trick. Sufficient aeration will achieve good etch rates without peroxide, if your concentration of CuCl2 is appropriate.

There are thousands of people who have a poor understanding of how this stuff works, due to misinformation spreading like wildfire. After that Instructable took off, I have discovered otherwise very smart people who think CuCl2 and HCL + peroxide is essentially the same thing, and that CuCl2 is just "watered down" or "partly used up" HCl + peroxide.

If you have access to 30 or 40% peroxide cheaper than air, than acid peroxide would be feasible to use indefinitely and could eventually reach high cupric level. That stuff is not easily available for everyone. In the US, the average joe will pay upwards of $100.00 a liter. The instability and oxygen make it a hazardous material. Of course a laser stronger than 5mW is also a restricted item over here.

[westfw]

you cannot use a bubbler with your HCl + peroxide etchant

It would still serve to provide mechanical agitation...

I started with HCl and 3% peroxide, and kept dissolving copper (scrap wire) till it started precipitating...
But my resulting etchant is pretty blue-green rather than the "bright green" I've read it should be.   Anyone have any ideas what's wrong?

[sleemanj]

But my resulting etchant is pretty blue-green rather than the "bright green" I've read it should be.   Anyone have any ideas what's wrong?

Add H202.

Here is a PDF copy of seychell without the broken images....
http://sparks.gogo.co.nz/seychell.pdf

The appearance of the solution is probably the best way of determining if Cu1+ concentrations is too high.



 Figure 5 shows color comparisons at various concentrations of dissolved Cu1+ ions. You can clearly see that Cu1+ in solution makes it extremely dark, and even small concentration in the 1 g/Liter range will make the solutions darker and more diffuse than normal. When Cu1+ is completely removed or exists only in trace amounts then the etching solution is a deep green transparent color due to Cu2+. The concentration of Cu2+ is normally so high that you cannot see through more than about 10 mm of solution depth under normal light. Therefore to check if Cu1+ concentration is acceptable, you can look at drops of solution sitting on a white surface.

(...)

Cu+1 Control

To decrease Cu1+ concentration the bath must be aerated {or add H202, accepting that it will also dilute} (...) until the color is a completely transparent bright green, i.e. Cu1+ concentration  << 1 g/L.

You should also read the chemistry section of that document

[KL27x]

If bluish you probably have a super high pH. The tank of water I use to rinse my boards turns bluish.

And FYI unless you used a bubbler you probably didn't get too high a concentration of copper using 3% peroxide only. The 3% is fine to make the starting solution but you have to add the bubbler to get the desired end result.  I have never used the 30% stuff. I suppose that would do the trick to some extent too. But you really want to end up with high copper and low acid. So there should be a transition to where aeration becomes more and more effective and peroxide becomes effective more for recharging the cupric rathER than for its original pupose. I imagine any additional high concentration peroxide will bubble off rather vigorously and simply be wasted. And 3% will ensure that you dilute the solution so much u never get there.

[KL27x]

In a typical bottle/container it is going to look a lot darker. It only looks light green in little drops like in the pic. Or in a thin tank. The picture in the aforementioned Instructable is weak sauce. Kinda expected since he adds peroxide instead of using aeration.

But if it is light blue, you have gone wrong, somehow. Not enough copper and too high pH.

Add some concentrated acid til it turns green. Then add a good dose more for the additional copper you will add. Put in a lot more copper wire. Bubble overnight. It'll look the color of pureed spinach when it is done.

If your solution is really light it is probably best to start over. Or let your solution evaporate in a shallow container in the sun until it is more  concentrated. Then add the acid before continuing.

Fyi, trying to etch with cupric without good aeration will be disappointing. The oxidative  "battery bank" isn't very big, even in concentrated solution. It would take a pretty large volume of charged cupric to etch a board without aeration, else the etch rate would drop off very fast.

When I add acid I also add some water. I have tried adding peroxide instead. Adding 3% peroxide to my CuCl2 does about nothing to my etch rate. I suppose if you dripped it g4adually while etching the board and stirring could be effective. But just mixing some in is not effective. It is not a substitute for aeration. If it works for you, I suspect your acid is high and copper low, and you are etching with acid as much as with cupric.

[LukeW]

you cannot use a bubbler with your HCl + peroxide etchant

It would still serve to provide mechanical agitation...

I started with HCl and 3% peroxide, and kept dissolving copper (scrap wire) till it started precipitating...
But my resulting etchant is pretty blue-green rather than the "bright green" I've read it should be.   Anyone have any ideas what's wrong?

It depends on the ligands, the chloride concentration mainly.

Cu(II) with lots of HCl present gives you green [CuCl₄]^2-.
Cu(II) with plenty of water present relative to the Cl^- concentration gives you blue [Cu(H₂O)6]²⁺.

If you have a solution of Cu(II) in excess HCl it appears green, as is commonly the case with H₂O₂/HCl etch chemistries.

But if you take that same solution and add more H₂O to it, it will change from green to blue, because H₂O will be the dominant ligand that surrounds the copper ions.

[westfw]

unless you used a bubbler you probably didn't get too high a concentration of copper

I did have a bubbler.
It's quite a deep blue-green, not dilute-looking at all, and not brownish like the photo of the expended etchant.
It was precipitating white precipitate with additional copper and air; I thought that was a pretty good sign of decent concentration.

You should also read the chemistry section of that document

I did; But the analysis requires more equipment and nastier chemicals than creating the etchant :-(
Perhaps I should invest in a hygrometer and dust off the chemistry/see if titration with baking soda will work.  (it should, right?)

[KL27x]

My guess still the same. More acid and more copper. When u get white precipitate and your solution actually does look dark brown, then you have it right. It'll be quite opaque and dark when there's any excess of elemental copper in it that is not dissolving. You should have only copper 1 in solution at that point. Darker than a ripe avocado. Almost ink black.

If u get there, you have to add a little acid and aerate after removing the excess copper and it will turn green.

What was your starting acid ratio?

[Ian.M]

Titration with sodium hydrogen carbonate (baking soda) for acid content is likely to be problematic due to copper carbonate precipitating.  You really need a readily soluble white hydroxide of a metal that also has a readily soluble white chloride.  That pretty much rules out everything except alkali and alkali earth metals.  While Sodium Hydroxide is fairly nasty stuff, 1M Sodium Hydroxide solution is safe enough for school chem labs.  Wear disposable nitrile, gloves and normal safety goggles, and have a plentiful supply of cold clean water handy for emergency washdown of any spills.

You don't need a hygrometer if you weigh 100mL of the solution on a 1g or better resolution digital balance.   You don't even need an accurate 100mL measure - just divide by the weight of the same volume of water.  Hint: put your digital balance in a thin clear plastic bag with the end folded underneath to protect it against spills in use.