PCB etchants

Jan 23, 2005 3 Replies
N

I keep reading on various websites that 'Persulphate' is a better etchant than Ferric Chloride from Radio Shack, but I don't know how strong to mix it and cannot seem to find any actual formulae.



I have, as it happens, plenty of both Ammonium and Potassium Persulphate, and have tried mixing what I thought was a strongish solution of each, but it seems very slow compared to Ferric Chloride. I did heat it, and agitated the dish constantly, and the Potassium salt did go a bit blue, but slow...slow...slow! I also tried adding a small amount of Mercuric Chloride, which I understand is supposed to be the approved catalyst, but nothing much happened except that the copper became plated with mercury.



Can anybody offer any advice about this? (And which is better, the Ammonium or Potassium salt?)



Thanks Nigel Eddis NYC



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D

On Sun, 23 Jan 2005, it was written:

Yes, if you are still in here to read this, the ammonium persulphate can be a bit slow, but it is much more gentle on the circuit pattern, including lines drawn by "permanent" markers. Outgassing is also less problematic. Reaction is concentration dependent. For a small double sided board around 4x3 cm, I use around a heaping teaspoon and heat to about 70 C. I just wait it out, as I am not usually in a big hurry and am more concerned about a quality etch, and also environmental/health issues in using some of the alternatives.

Dominic

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B

Hmmm. At the lab where I used to work we tried ammonium persulfate exactly one time... it completely dissolved the "permanent" felt-tip lines, whereas with ferric chloride we rarely had problems. We just used the ammonium persulfate right out of the bottle (from Radio Shack, I think... many years ago), so maybe it needed dilution first?

Amazingly, we did find that most pre-mixed ferric chloride is too

*concentrated* to etch properly, so slow that it appears to be exhausted on its first use. Dilution down to 1.30 specific gravity fixes it right up.

Best regards.

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

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S

I use ammonium persulphate here for my home projects, I use those iron on laser printer masks. It will even etch out surface mount IC pads perfectly. Usually takes about 3-5 minutes to fully etch the board, however i usually make the mix a bit weaker as to save money. The directions on this stuff that i have are to mix 400 grams of ammonium persulphate in one to one-and-a-half litres of water heated to about 60 to 70 degrees C

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