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Old 20th July 2004, 03:03 PM   #11
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Lightbulb Re: PCB Etch Probs

Quote:
Originally posted by richie00boy
I am trying to etch a board that developed beautifully (as always). The board is at least a year old. I'm using sodium persulphate etch (the clean stuff) following the instructions exactly. My tank bubbles a bit, thanks to the fishtank pump tube.

Basically it's not etching. It's attacked the edges slightly and the copper has barely gone pink. It's got a kind of grid pattern across the copper if you look carefully. This is after about an hour in the tank. According to the instructions it should take about 10 mins in a bubble tank, 30-45 mins in a plain tank. At 45 degrees C.

Are my problems due to the solution not being hot enough or is it because the board is old? I thought it was only the developing that was affected by age. The solution was warm, but has obviously cooled down over the hour.

Hi,
You should at least develop the PCB in the alkaline solution for 5 minutes. An underexposed PCB does not develop well, no matter how long and how hot you try to etch it. Make sure you have the right exposure time. This is rather critical. Old material requires somewhat longer exposre. My PCB material darkens the areas where the copper has to remain in the developer.
Hope this helps.
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Old 20th July 2004, 03:11 PM   #12
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Hello JMateus.

It depends on how much copper area there is to etch. Last one I did (some 25x15 cm.) took about 20~30 minutes. If you've got the patience to stir the solution as it etches it will go noticeably faster.

If time of etching is important (which in diy I don't think it is), raising the oxygenated water volume percentage will dramatically accelerate the process. It will also generate more heat and fumes in the process. Again, I DO NOT RECOMEND THIS.



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Old 20th July 2004, 03:25 PM   #13
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IT is definitely too cold.
I put a stainless steel tank on a small burner on my stove set to medium. In the SS dish I put some boiled water, I then put a smaller glass dish into the SS dish floating on the water, I put my etchant into the glass dish and wait for it to come up to temperature (usually about 5 minutes or less) and this keeps the whole thing heated nicely while I am etching a board.
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Old 20th July 2004, 06:07 PM   #14
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It is not too cold I have rubbed a small part of the board with wire wool and put back into the tank. Wthin 10 minutes (the recommended time) the rubbed part was perfectly etched.

Elso, I appreciate what you say but my board also develops so you can see the tracks, too. What is puzzling me is that the developer appeared to clean away the unwanted areas perfectly. Even under close scrutiny I cannot see any tint to the copper.

I will cut some more board and expose for much longer and see what happens.

Many thanks to all who answered.
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Old 20th July 2004, 11:31 PM   #15
cpemma is offline cpemma  United Kingdom
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A few comments that may help from http://www.atmsite.org/contrib/Webb/pcb/
Quote:
Chemicals : A note on chemicals... There are a lot of acids and other chemicals that can be used to etch a copper board. Conversely, it may surprise you that some well-known acids like hydrochloric acid, HCl, do not dissolve copper very well, at least, not without addition of an oxidizer like hydrogen peroxide, H2O2. I thought I would share some of my experiences about the use of some different chemicals. HCl could be used, as I mention, with presence of an oxidizer. This tends to be rather slow process, and unfortunately, many permanent markers chip off of the board if aggitated in this case. So you will likely end up having to re-draw the whole layout several times (at least) before your PCB will be ready. HCl also produces a lot of toxic outgassing. Acetic acid, CH3CO2H, also dissolves copper, but unfortunately is also a very good organic solvent, dissolving ink from virtually any source. Hot sulphuric acid, H2SO4, might seem like a good solution, and indeed, it will dissolve copper, but most inks are not resistent to this acid. H2SO4 reacts with copper to give off some terribly toxic sulphur and copper oxide (SxOy and CuxOy) gases that you really do not want to inhale, and won't do much for your social life either. Nitric acid, HN03, could also be used, but I am told it produces some dangerous outgassing, and it's a really nasty acid that you really would rather avoid. Ferric (FeIII) chloride, FeCl3, is comonly employed in the USA, but in much of Europe its use is illegal for environmental reasons. Thus far, the safest, most effective way I have found to etch a board is with one of the persulphate (S2O82-) salts, like sodium or ammonium persulphate. These do not generate outgassing, do not smell bad, and are not so objectionable to environmentalists. Further, persulphate solutions will not dissolve any permanent ink (that I know of). Unlike most of the other chemicals I mentioned, the persulphates are legal and readily available virtually everywhere in the world. I therefore choose sodium persulphate, which is often more available and about half the price of ammonium persulphate.

Etching : Make up the persulphate in water. About 1 teaspoon/25 ml of water is usually adequate. Heat this solution to near boiling in a glass container slightly larger than the board to be etched. Drop in the board, close the lid and submerge the bottle slightly in hot water to keep it warm. The reaction is faster at warmer temperatures. Something like 50-80 degrees C will work. This normally takes me around 30 minutes with occasional mild aggitation. Afterwards, the board can be rinsed, dried and drilled.
I suspect your old board has a passive surface layer, a thin but tight film that's developed over the storage period that resists corrosion. Several minutes dipped in Coke (dilute phosphoric acid) would probably shift it and allow normal etching to proceed.
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Old 21st July 2004, 08:51 AM   #16
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I had actually read that article you quoted when I was Googling to try and see if there were any reported problems with etching with Sodium Persulphate. One thing that concerned me about it was on the tub of crystals I got, it says under no circumstances allow to go above 50 degrees C or it will render the solution less effective and also massively reduce the storage life.

Thanks for the tip on the coke. I will definitely try it as I have about 4 other boards that are awaiting etching and I don't want to waste them and have the hassle of cutting and developing another lot.

Won't the coke also take off some of the resist though? And if persulphate is an acid, how come that doesn't get rid of the unwanted film? Or is the persulphate just an oxidiser and I need an acid to shift the film?
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Old 21st July 2004, 11:08 AM   #17
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And will clear distilled vinegar be a straight swap for coke?
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Old 21st July 2004, 11:58 AM   #18
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Quote:
Originally posted by richie00boy
One thing that concerned me about it was on the tub of crystals I got, it says under no circumstances allow to go above 50 degrees C or it will render the solution less effective and also massively reduce the storage life.
That does indeed happen, the hot solution gradually breaks down to sodium sulphate and sulphuric acid, with oxygen bubbles coming off.

2Na2S2O8 + 2H2O --> 2Na2So4 +2H2SO4 + O2

If you're making up small quantities & discarding after use that wouldn't be a problem.
Quote:
will clear distilled vinegar be a straight swap for coke?
Vinegar is acetic acid and, according to the link, "Acetic acid, CH3CO2H, also dissolves copper, but unfortunately is also a very good organic solvent, dissolving ink from virtually any source. "
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Old 21st July 2004, 02:18 PM   #19
SteveA is offline SteveA  United States
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Default sodium vs ammonium persulfate

Yes, ammonium persulfate is different from sodium persulfate. The ammonium salt will be slightly acidic in water. I prefer it to ferric chloride for a number or reasons.
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