etching PCB: sulphuric acid but why peroxide aswell?

[JingleJoe]

I was really looking for hydrochloric acid but I found sulphuric acid which works almost as well (so I'm told) when mixed with peroxide, for etching copper to make printed circuit boards.
But I have a question, why is the peroxide required? Why can't I etch with sulphuric acid alone?
I'm no chemistry boffin so I pose this question to any chemistry experts on electro-music.com

[blue hell]

Hmm ... I do not know, but what I do know is:

be sure to read up on the subject before you experiment ... this stuff may explode if you do not know what you are doing

For a starter http://en.wikipedia.org/wiki/Piranha_solution it has some info on the chemistry involved as well.

The basic idea for the mix seems to be that the etched copper will sink to the bottom of the bath after a while and can be mechanically removed - so it keeps going like forever, unlike ferric chloride. But for that it seems to need some 'stabilizers' as well .. and temperature management.

[JingleJoe]

http://web.princeton.edu/sites/ehs/labsafetymanual/cheminfo/piranha.htm
well shit, can't store it and it eats plastic, but if it eats plastic why is this uggested as a pcb etchant? surely it'll take off the transfered toner resist film aswell?

[mosheen]

Sulfuric acid is some evil stuff. I'd avoid it if possible.

[blue hell]

[PHOBoS]

maybe it works with photosensitive PCB's ?

I'd never heard of it but after reading a bit it doesn't sound like something you want to try anyway.
Allthough maybe it's fitting for a mad scientist to blow up your lab once in a while.

[analog_backlash]

Hi JingleJoe.

My chemistry is a bit rusty at the moment (ha ha), but I remember that for a metal to dissolve in a solution containing abundant H+ ions (i.e. an acid), it needs to have a negative standard electrode potential relative to hydrogen (which by definition has a standard electrode potential of zero volts). If this is so, you get:

Metal + H+ = Metal+ + Hydrogen (gas) - not a balanced equation!

In this process, the metal is being oxidized to the Metal+ ion (or 2+ etc. according to the metal) and the hydrogen is being reduced from H+ in solution to gaseous hydrogen. This is the case for example with iron (-0.44V) or zinc (-0.76V). However, copper has a standard electrode potential of +0.34V, so it does not react (normally).

Things get more complicated if an oxidant is also present. In fact, the oxygen in the air will allow copper to be attacked by sulphuric acid, but very slowly. The reaction is faster in hot concentrated sulphuric acid, but this is not a good way to etch PCBs ! Adding an oxidizing agent (such as hydrogen peroxide) will dramatically increase the corrosion of copper, presumably because it helps to oxidize the metal to its Mx+ ions (this is where I get a little vague - I may need to swat a bit on this ). Nitric acid (even diluted) readily attacks copper as it is also quite a strong oxidizing agent. The non-ferric chloride etching crystals contain persulphates which when dissolved in water produce hydrogen peroxide.

I hope that helps - I may get back to you when I've revised a bit

Gary

[diablojoy]

I dont recommend sulfuric its going to be really really nasty fume wise like you would need to do it outside in a strong breeze kind of way.
the peroxide (H2O3) is added to speed up the process adds a bit more oxygen to the mix i think though i am no chemist.
personally i prefer amonium persulphate , you do need to get the temp up
though, around 50 degee's celcius works ok.
the etching is really good , quick and no bad fumes at all.
I actually use a sink full of hot water with a small plastic parts bin
to etch in . an old icecream container would also work
about a 150 mils of really hot water from the kettle to a couple of heaped teaspoons of amonium persulphate seems to work best for me.
just keep swishing it around. takes 5 -10 minutes depending on temp and strength.

[analog_backlash]

Yep, the ammonium persulphate is the best bet (if you don't like ferric chloride). I think that any very acid solution is likely to attack the etch resist.

ADDITIONAL:

This link goes through the minutiae of the copper + persulphate reaction:

www.chemicalforums.com/forums.html?topic=21772.0

Can't get the f****** link to work! Select this www.chemicalforums.com press the Search button and put in copper persulfate reaction. Then click on Re: Products of a copper + persulfate reaction?

Also, sorry about my spelling of sulPHuric acid. I know it's not the official IUPAC name now (sulFuric is), but it's the way I've always spelt it!

P.S. If anyone knows why my links won't work, please put me out of my misery .

[JingleJoe]

[diablojoy]

[quote]Yep, the ammonium persulphate is the best bet (if you don't like ferric chloride). I think that any very acid solution is likely to attack the etch resist. [/quote
I used ferric chloride for years but its now hard to get here ,
tried hydrochloric and peroxide mix , noxious as hell and not a good etch
and yes it did attack the resist. it was always a toss up as to what would go first the copper or the resist. finally tried amonium persulphate and havent looked back since.

[JingleJoe]

how recyclebale or re-usable is ammonium persulphate?

[analog_backlash]

I think that my internet is screwed up somehow, as your link doesn't work either (I've had other problems apart from this).

Anyway, yes I can confirm that sulphuric acid plus potassium nitrate does work, dissolving copper very easily. The problem is (apart from the etch resist possibly not surviving) is that nitrogen dioxide (a brown toxic gas) is a by-product.

I thought that this might be about disposal problems. I've got a gallon of used ferric chloride solution which I was planning to dispose of by the "sludge" method by adding sodium hydroxide to precipitate out all of the metal (iron and copper hydroxide) and then filter off the sludge. But I was then going to dry the precipitate and keep it, as I have thought of a way of removing the copper and then regenerating the ferric chloride. This is as yet untested, so I'll let you know the results.

My main problem is how to source chemicals. It used to be dead easy, but after 7/7 and all that...

Gary

[Luka]

my issue with fe2 is disposal.

i heard using sulphuric acid you can equalize it out and dispose of it in a more environmentally friendly manner

can anyone confirm?

any good strategies for disposal of other etchants which are not bad for environment?

[diablojoy]

[JingleJoe]

[JingleJoe]

[mrmrshoes]

I found these series of videos, might be useful.

Make 10 Etchants for Copper Printed Circuit Boards
http://www.youtube.com/watch?v=Q4tWEse2rDI

[JingleJoe]

[diablojoy]

[JingleJoe]