I need to dig up the data sheet for a better explanation, and I'll post it
later.
I am howver interested in your findings with ferric chloride. I've got some
around here as well, and as you stated its shelf life is a real advantage. I
however have never been able to acheive decent etching with it. I've tried
room temperature, 50 deg C (122 deg F), agitation and aeration, and have
always had problems with non-uniform etching. Part of the board gets over
etched, and yet another section 10mm away will be under etched.
A typical board for me is about 50mm x 50mm (2.5inX2.5in) with around 40-150
SMD components with 0.012 to 0.025 traces.
Thanks
Ron
----- Original Message -----
From: "Adam Seychell" <adam_seychell@...>
To: <Homebrew_PCBs@yahoogroups.com>
Sent: Monday, January 20, 2003 11:12 PM
Subject: Re: [Homebrew_PCBs] sodium persulfate PH stabilization
> I use ammonium persulfate which I think behaves almost identical to
> sodium persulfate, I have read there are some subtle differences with
> etching between the two. I have never heard of needing to "shock" the
> solution. What is the purpose of "shocking" ?
>
> I've read that iron ions and some other metal contaminants can
> accelerate the decomposition of persulfate. The pH of persulfate
> etchants are always acidic (very low pH) because of the decomposition
> products ,
>
> 2H20 + S208(2-) -> 2H(+) + 2S04(2-) + H2O2
>
> You basically get sulfuric acid and peroxide which then further
> decomposes to release oxygen. Heat also speeds decomposition. I've got
> some solution for 2 years that I occasionally use for very light
> etching. The fact I've never heating the solution could be why its
> still showing some strength.
> A common method of completely destroy persulfate is to boil them in
> solution for ten minutes or so.
> You can switch to ferric chloride if you want an indefinite shelf
> life. Just put it in a etching tank so you never have to let the stuff
> touch anything else but the PCB.
> Recommended startup concentration for persulfate is 200g/liter of the
> salt.
>
> Ron Amundson wrote:
> > For some reason this didn't get posted last week, my apologies if it
shows
> > up twice, but I'm making boards on Wednesday, and would like some input.
> >
> > In my Kepro datasheet, it says I can shock the solution by adding some
spent
> > Sodium Persulfate so I don't have to wait for it to stabilize.
> >
> > I had some used solution which was 3 months old that I used to shock the
> > bath. I know the shelf life for etching is 3 weeks, but can it still be
used
> > to add in adjusting the pH? The experience as shown below seemed to
indicate
> > its not possible, but I want to know if I need to look at some other
reason
> > for the slow etch rate.
> >
> > I went ahead and etched, but it took forever. I was successful at 12/12,
but
> > the etching seem to take 15 - 20 minutes. When I had my techs making
proto
> > boards, it seemed we could etch in 2-4 minutes. The solution was at
105-110
> > deg F. The board was quite small, and as I use huge ground planes and
leave
> > dead copper on the board to reduce waste, It seems it should have etched
> > close to the 2 minute time frame rather than 20 minutes.
> >
> > I doing tray etching with manual agitation.
> >
> > Thanks
> > Ron
> >
> >
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