Homebrew PCBs

I found that higher copper gives faster etch. Currently I get 
around 15 minutes etch time at 20\ufffdC 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\ufffdC 
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
Copper	75 	180	230	grams/liter
HCl	1.0 	0.2	0.04	moles/liter
S.G	1.17	1.18?	1.4	g/cm^3	
temp.	21	49	49	\ufffdC


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
> 
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