I found that higher copper gives faster etch. Currently I get
around 15 minutes etch time at 20°C for 35 um copper, bubble
agitation with a solution composed of 160 grams/liter copper and
about 0.2 moles/liter of HCl acid, the specific gravity at 20°C
is 1.280.
I've found three variations of cupric chloride operating
parameters online.
#1
http://users.rcn.com/rexa/Projects/CuCl_ech.html and
#2
http://www.pcbfab.com/iepart3.html#3
http://www.oxfordvue.com/TechInfo/how_does_it_work.html#1#2#3
Copper75 180230grams/liter
HCl1.0 0.20.04moles/liter
S.G1.171.18?1.4g/cm^3
temp.214949°C
I'm think that #1 does not have enough copper. The acid seems to
have little effect on the etch rate. It only needs a bit on there
to keep the copper dissolved and to convert the copper(I) to
copper(II) with dissolved oxygen. Interestingly it even etches ok
with zero free acid in solution. All that happens is you see some
insoluble powder float around on the surface, which is blue
copper(II) hydroxide. In fact some commercial PCB etchers run
cupric chloride tanks around the zero acid level. This is suppose
to help the "etch factor".
One thing I found very important is the copper(I) chloride causes
the solution to go dark brown and it will have huge effect on
slowing down etch rate if it gets too high. You can actually see
copper (I) chloride as a white film on the copper and my theory
is this film makes it physically difficult for fresh copper(II)
chloride to react with the copper metal.
So how do you stop copper(I) chloride build up ?
The copper(I) chloride will go to copper(II) chloride with HCl
and dissolved oxygen. The time it takes to remove most of the
copper(I) can be over 4 hours of bubbling. So we can't expect
this reaction to take place at the same speed as our copper
reaction on the PCB. The way around the problem is to start off
with a solution not containing any copper(I) chloride and rely on
its bulk as a buffer for holding freshly generated copper(I).
With sufficient solution volume then copper(I) build up shouldn't
became an issue during etching. After etching, continue the air
sparging until all (or nearly all) the copper(I) is removed. I
don't think its necessary to remove every last trace of copper(I)
so that you see the solution turn clear green, its takes about
two days of bubbling to get to that level.
An excellent test to see if you have removed enough copper(I)
chloride is to take a test tube filled with few cm^3 of water and
add a few drop etchant, so its diluted ( approx. 5~10%). If any
copper(I) chloride is present it will precipitate out and show up
as a cloudy mixture. The more copper(I) the more cloudy.
I know that the etchant can still look an almost opaque dark
green, indicating there is still some copper(I) chloride in
solution, but the concentration must be very small because it
will not show any signs of visible precipitation when doing the
water dilution test. So, if you see a clear pale green or pale
blue when you do the dilution test then you can be assured that
your etchant is ready to rock and roll again.
Since it appears the free HCl is not important then you probably
could survive with just a few graduated pipettes for a quick and
dirty acid-base titration (no expensive burette). The HCl is
needed in order for the removal of copper(I) chloride during the
air sparging. As a guess I'd say you shoudl aim for a HCl
concentration anywhere from 0.1 to 0.5 Moles/liter (0.4% to 1.6%
w/v HCl).
A good quality hydrometer however will be essential for measuring
the copper content.
Adam
grantfair2001 wrote:
> Hi Adam- the HCl I have is 20 deg. Baume or about 31.5% HCl, how
> should I modify your recipe (in message 1367 - Preparation of cupric
> chloride etchant)?
>
> Grant
>
>
>
> Be sure to visit the group home and check for new Bookmarks and files:
> http://groups.yahoo.com/group/Homebrew_PCBs
>
> To unsubscribe from this group, send an email to:
> Homebrew_PCBs-unsubscribe@yahoogroups.com
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>