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Replenishing HCL + H2O2 PCB etchant?

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Steve Ward

Sat Jul 28 2007, 07:54PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Thanks for the advice Hazmatt, its given me some other options to think about. In any case im building the bubble tank (which should likely work just fine for any etchant i would ever use). So i will stick with the HCl and H2O2 until ive used it up, and in the mean time i will see about some FeCl3 (last time i bought any it was like 5-10 bucks for 16oz (aq)... which is too much in my opinion). According to that page linked above, the bubbling action alone should help regenerate the Cu+2 ions (as the oxygen bubbled through converts Cu+1 to Cu+2) which in turn allow for the etching of Cu. If it works, great... if not i still have a bubble tank to increase my etching speed.

Hazmatt_(The Underdog)

Sat Jul 28 2007, 08:29PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Yea, once you have your tank set up its really no big deal.

If you eventually set up another tank for FeCl3, then you will need the fish tank heater to get it up to about 150*F. It etches cold but that takes a looonnnggg time, and by the time most of the copper etches off, you've gone and over-etched under your artwork, so take the time to heat it up.

ArcLight

Tue Jan 22 2008, 07:00AM

Registered Member #341 Joined: Thu Mar 23 2006, 07:41PM
Location: Northern Illinois, USA
Posts: 69

This is the first time that I've wandered into the chemistry forum (I generally hangout in the Tesla forum.) I wish I had done so before dad died. This thread and the precious metals reclamation threads triggered memories from back when I was in high school (I graduated in 1971.) I don't know if my memories will be of any help, but now that I've consumed a few adult beverages, if I'm a bit nostalgic, I hope that you all will understand.

Dad died in 2006 after suffering from Alzheimers for several years, but in the 1960's, 70's, and early 80's his business was copper etch, and his hobby was reclaiming gold. My knowledge of chemistry is limited, and my memories have dimmed considerably over time. But, I remember that ferric chloride was sold to the smaller shops, while an ammonia based etch was sold to the larger shops, and that dad used concentrated HCL and 35% H2O2 to reclaim gold salvaged from the electrical contacts of reject circuit boards that he obtained from his etch customers.

I remember that the ferric chloride was made by dissolving color TV picture tube masks in HCL until a certain specific gravity was archived. I don't remember if I ever knew what chemicals were used to make the ammonia based etch, but I do remember a 2000 gallon (or larger) tank of anhydrous ammonia beside the main building. Even before dad died I inherited his chemical stash. Most of the really dangerous chemicals had long since been gotten rid of (the lb. of picric acid didn't make the top three on the list of chemicals that scared the hell out of the local fire department,) but there were many less reactive ones left, most of which I have no idea what they were used for. However, in the stash were several carboys of ammonia hydroxide that I'm sure were part of some of his etch experiments. Over the years, I have found good homes for the vast majority of chemicals, but the ammonia hydroxide has remained a difficult orphan.

I remember that dad worked on many different formulations of etch. The problem was that the really good ones i.e. didn't compromise the photo resist, held lots of copper, and worked very quickly, didn't work well in an industrial setting because of the difficulty in keeping them from jumping out of the etch tank. I know that only because on occasion, they jumped out of the test tanks onto the floor (and in at least one case the walls and ceiling) of the garage, causing mom to not speak to either of us for several days.

Part of the deal (at least at that time) in selling etch, was to cart off the used etch when delivering new. Unfortunately, I can't remember what was done with the used ferric chloride. But I remember large tanks of caustic, only because somebody inadvertently routed sulfuric into a caustic tank once, causing a vivid/violent memory. I do remember however, that the used ammonia etch was quite alkaline. So it was neutralized with a concentrated sulfuric drip while in a 10,000 gallon fiberglass tank. The drip rate was regulated by temperature, while being agitated with compressed air, and using HCL scrubbers to capture the ammonia, which when interrupted (the HCL) irritated (literally) the downwind neighbors no end.

When done correctly, the end result was black copper oxide which was sold to the lumber companies, while the resulting neutralized liquid was pumped into a concrete pond, sprayed into the air to evaporate, and the remaining salts sucked out and taken to an EPA dump.

Even after being neutralized and decanted, you only needed to climb into one of the fiberglass tanks of black copper oxide once with a shovel and respirator to remember the experience for a lifetime (every inch of exposed skin burning and the respirator getting so hot that you couldn't touch it.)

Oh the good old days. Clearly, they weren't that damned good.

Regards, ArcLight

babui_crazy_08

Wed Nov 12 2008, 03:01PM

Registered Member #1809 Joined: Wed Nov 12 2008, 12:24PM
Location:
Posts: 3

I have got a way to recycle and reuse persulfate-type copper etchant infinitely. Its an electochemical setup that will cost you a lot when you set it up for the first time. But it does not require any kind of maintainence. You dont have to add any chemicals later. You can reuse the same solution forever.

THEORY : First, I will discuss the theoritical principles involved.

sodium/ammonium persulfate (Na2S2O8 / (NH3)2S2O8) etches copper in the following way :-

Na2S2O8(aq) + Cu(s) ----> Na2SO4(aq) + CuSO4(aq)

The electrochemical mechanism reverses this in the following way :-

CuSO4 ---> (Cu)2+ + (SO4)2-

At cathode (-ve electrode) :

(Cu)2+ + 2e ---> Cu (copper gets deposited on the electrode itself, thus removing copper ions from the solution)

At anode (+ve electrode) :

2 (SO4)2- ---> (S2O8)2- + 2e

Thus the persulfate ions ,ie. the primary oxidiser, is regenerated from the sulfate ions.

REQUIREMENTS : You will need the following components to set up your etching apparatus.

1) a platinum electrode (1 pcs) for the anode : This will be the most expensive component in the whole apparatus. Also note, there are two varieties of Pt electrodes. A) The platinized platium electrode , which is black or grey in colour because it is coated with Platinum Black (an allotrope of platinum), and, B) The shiny platinum electrode, without the coating, which has a shiny silver-like appearance. You are supposed to use the shiny-type Pt electrode. The black one won't work.
2) a copper electrode for the cathode : you may use a length of a copper wire (after striping off the insulation) or a spare bit of copperclad pcb. Make sure this electrode has a large surface area.
3) a glass/plastic container to hold the solution and act as your etching tray
4) sodium persulfate (Na2S2O8) : Alternatively, sodium sulfate (Na2SO4) may be used to begin with and sodium persulfate can be generated from that by electrolysis.
5) a little bit of conc. sulfuric acid (H2SO4) to lower the pH a bit : an acidified solution helps to speed up the etchant-regeneration process.
6) water : ordinary tap water would do but distilled water is better.
7) Power Supply - a mechanism to supply a lot of current (about 5-10 Amps) at controlled voltages to the electrodes : A simple 9-volt battery with a potentiometer might do. But a 9-volt battery can't supply sufficient amount of current. So the regeneration process may be too slow to be acceptable. A better way is to draw the power from the house-electric supply. Use a 12-volt DC wall transformer/adapter to get a 12V DC source of current. Then prepare a custom voltage-regulation circuit that uses the 12V DC output and can provide 5-10 Amps at a voltage that you can control precisely.
(Note : the required output voltages are typically between 0.5V to 4.5V DC with a control precision of atleast 0.1V)
8) You can also add an agitator mechanism (aquarium pump) and a heater mechanism (immersion heater / aquarium thermostat) to the apparatus to speed up the etching process.

SETUP THE APPARATUS :-

Prepare a sodium persulfate solution at a concentration of 230 gram/litre. If you are starting with sodium sulfate instead of sodium persulfate, mix at a concentration of 460 gram/litre. Add to it about 10 ml of conc. sulfuric acid per litre of the solution. Your etching solution is ready.

Now pour your etching solution in your etching tray and drop the electrodes in it. (the copper electrode is supposed to be the cathode (-ve) and the platinum electrode is to be anode(+ve). Make sure you connect them correctly.)

Now, if you mixed your solution with sodium persulfate, you are ready to go. But if you used sodium sulfate, you need to run a current through the solution so that sodium persulfate can be generated within the solution. Calibrate the voltage of the regulator and let the current run for some time (about one hour). (The voltage calibration process is being given below, as a part of the etching process.)

THE ETCHING PROCESS :-

Drop the pcb, which is to be etched, in the solution and start a current in the circuit.

Then you need to adjust the potentiometer or the v-reg to supply the correct voltage.
The goal would be to set the voltage as large as possible but just short of when a vigourous bubbling starts at the platinum anode.

(Note, if you are are using an aquarium air-pump to agitate the solution, you must turn it off during this voltage calibration process. Otherwise you wont be able to distinguish the bubbling at the anode. After calibration, you can turn on the pump and leave the apparatus to etch your pcb.)

Ideally, You must begin with a voltage as low as possible and increase the voltage until you can see a lot of bubbling at the anode. Once you find the threshold point, you must set the voltage to slightly less that.

POST-PROCESSING OF ETCHANT SOLUTION :-

After etching is completed, remove the pcb from the solution and let the current run for some more time. This will allow all copper ions dissolved in the solution to get deposited on the cathode and clean-up the solution. (It is easy to detect presence of copper ions in the solution. Copper ions impart a blue colour to the solution. So wait till the blue colour fades away)

After this process completes, you must remove both the electrodes from the solution before you remove power from the electrodes. If you remove power from the electrodes while they are in the solution, the cathode which is made of copper, turns into an ordinary copper rod, and the etchant immediately starts eating away copper from the electrode. So if you leave the cathode in the solution without power supply, the etchant will essentially get used up and you will need to regenerate it again before you can use it.
Note that, although the solution doesn't react with the platinum electrode, but if left in it for a long time, it can corrode the suface. And you don't want the most expensive part of your etching apparatus destroyed.

Best of Luck with your new etching system
------------------------------------------- ---------------------

Bored Chemist

Wed Nov 12 2008, 05:55PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Interesting .
The efficiency of oxidation of sulphate to persulphate is generally rather low. At the point where all the copper has been stripped from the solution you will have a liquid that contains water, sulphuric acid and sodium (and/ or ammonium) sulphate without much persulphate. I'd like to see some evidence that this has been tested before I went to the trouble of setting it up myself. In particular, I'm sure that the electrolytic production of persulphate only works well in rather strongly acid conditions.

aonomus

Wed Nov 12 2008, 09:29PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

I really hate to derail ever so slightly, but recapping the disposal methods for HCl/peroxide etchant: the only proper option is to boil to dryness ? I live in an apartment and I have at least 1-2L of the spent stuff with CuCl2 precipitating out if I try to etch any more...

Is electrolysis any option? I don't know of any metal that in solution is significantly less toxic than Cu2+ (or at least anything from the transition metal series).

I ask this because I might *move* in 1-2 months and while a jug of acid is ok to move, semi-open containers of etchant are not....

babui_crazy_08

Mon Nov 17 2008, 03:11AM

Registered Member #1809 Joined: Wed Nov 12 2008, 12:24PM
Location:
Posts: 3

Bored Chemist wrote ...

Interesting .
The efficiency of oxidation of sulphate to persulphate is generally rather low. At the point where all the copper has been stripped from the solution you will have a liquid that contains water, sulphuric acid and sodium (and/ or ammonium) sulphate without much persulphate. I'd like to see some evidence that this has been tested before I went to the trouble of setting it up myself. In particular, I'm sure that the electrolytic production of persulphate only works well in rather strongly acid conditions.

You talk of efficiency !!! Until recently, this method have been industrially used for manufacture of H2O2. The process involved electrolytic oxidation of sulfate to peroxydisulfate (ie. persulfate) which was then hydrolysed with water to yield H2O2 and sulfate. The H2O2 was extracted and concentrated and the remaining sulfate solution recycled into the electrolysis chamber.
Search the internet you will surely find something about the efficiency of electrolytic oxidation of sulfate. (Note: This process was used before the AO process was discovered)

Regarding the pH, you are correct in saying that the solution must be strongly acidic. If you read my previous post thoroughly, you'll find I have suggested adding conc. sulfuric acid to the solution to acidify it.

Bored Chemist

Mon Nov 17 2008, 07:09PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

"You talk of efficiency !!! Until recently, this method have been industrially used for manufacture of H2O2."
I know. It was abandoned in favour of a more efficient process.
The reaction is done in very strongly acid conditions as shown here.

The bisulphate is used rather than the sulphate.
I did this synthesis years ago as a student demonstration. The yield wasn't very good. I realise that the commercial version is probably more efficient but I think the home version is probably more similar to the student version
I saw that you had sugested the addition of a little acid (1%). I wasn't kidding when I talked of strongly acid conditions.

Have you actually done this reaction with spent etchant?

babui_crazy_08

Thu Nov 20 2008, 04:38AM

Registered Member #1809 Joined: Wed Nov 12 2008, 12:24PM
Location:
Posts: 3

Bored Chemist wrote ...

........
The bisulphate is used rather than the sulphate.
I did this synthesis years ago as a student demonstration. The yield wasn't very good. I realise that the commercial version is probably more efficient but I think the home version is probably more similar to the student version
I saw that you had sugested the addition of a little acid (1%). I wasn't kidding when I talked of strongly acid conditions.

Have you actually done this reaction with spent etchant?

Well I have not done it in the exact way I suggested it here. I had used stainless steel electrodes instead of platinum and copper. And it had given my almost used up etchant a powerful boost ; the etching speed had nearly doubled with only about 2 Amps of current at 3 volts (approx). I hope the efficiency would be better with a platinum anode.

Now, regarding the article you gave a link to in your previous post, I read it up. It suggested three side reactions that adversely affect the efficiency of the process.

1st) electrolysis of the water which leads to the formation of oxygen at the anode and hydrogen at the cathode;

2nd) acid hydrolysis of the peroxydisulfate (persulfate) ion into peroxymonosulfate ion (ie. Caro's acid)

3rd) reduction on the cathode of the peroxydisulfate ion : (S2O8)-- +2 e- âž 2 SO4--

Now, I don't think we need to worry about the first reaction as long as we don't drive the v-reg beyond the oxygen overpotential (which, as I know, happens to be pretty high for platinum anode).

For the second reaction, this would not be the cause of inefficiency in our case. Caro's acid itself is as good an oxidiser as persulfate ion and hence it would not matter we've got a mixture of peroxymonosulfate & peroxydisulfate. They both will etch copper well enough.

Regarding the third reaction, well I don't understand how it works. persulfate ion is negetively charged. How can it reach the cathode (which is the negetive electrode) and get itself reduced while we have so many positive copper ions in the solution. Don't the positive copper ions seem like a better candidate to win the race and get reduced ??

Well, even if I am worng, we can use a simple fix to solve the problem. If we just place our PCB halfway between the anode and the cathode, then the persulfate ions, which is formed at the anode, will reach the PCB before it can reach the cathode. This way we won't have too many persulfate ions getting reduced at the cathode to affect the efficiency significantly.

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