On 26/03/2011 12:37, Olin Lathrop wrote:
> Oli Glaser wrote:
>> longer etch times mean (as you probably know)
>> more undercut and less accuracy
> Why?  The etch time needs to be longer when the process is slower, but
> you're implying that lateral etching under the mask then somehow slows do=
wn
> less than etching the exposed copper from the top.  Why would that be?

Good question, though I'm not implying quite what you say above. I have=20
wondered about this too and found it difficult to source satisfactory=20
answers, I think it is to do with uneven etching - this is what I have=20
read in a few places:
http://www.electricstuff.co.uk/pcbs.html
http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=3Dguide_pcb
(look under "Etching" in the first link and the end of "Ferric Chloride"=20
in the second)
These sources may not be that reliable, and I'm sure there are a few=20
factors involved - I have seen mention of a slower process that produces=20
fine line widths also:
http://www.p2pays.org/ref/10/09585.htm
(under Cupric Chloride Regeneration)
My thoughts were that as the etching speed is not completely consistent,=20
longer times would allow bigger differences between areas to accumulate,=20
or something along those lines. However, some chemicals probably etch=20
more evenly anyway, so may produce similar results over different time=20
spans, but I *think* the general view is whatever you use, the faster=20
the better (hence the use of heated fluid and spray tanks)
 From my own experience, faster times have definitely produced sharper=20
edges, and made finer resolution possible.


--=20
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