Grant,
There is some interesting reading inside the technical section on
http://www.oxfordvue.com/http://www.oxfordvue.com/TechInfo/copper_and_acid_normal.htmlthey suggest the correct procedure for acid titrations. It so
happens that you DO NOT use an indicator because its more
accurate to rely on the initiation of tubidity change rather than
color change on indicator. The CuCl2 + NaOH will form insoluble
Cu(OH)2 once all the free HCl acid is consumed. Well that news
makes it easier and cheaper for us.
There is a lot to learning about this etchant and how to properly
use it and maintain it. I've been talking to Phil Culpovich via
email who I think is one of the main people at Oxford V.U.E.,
Inc. Below is a copy of his reply.
--------------------------------
Adam,
Since the production shops that use cupric must have a faster
regeneration speed, they use a controller and oxidizer to speed
and control the process. In your case, the oxygen method is
sufficient and tends to be safer since you virtually can't overdo
the oxidizer additions resulting in the release of chlorine gas.
As to the differences in free acid levels, the range boils down
to a relationship between the need for quality and speed vs. the
wide ranging results coming from different etcher designs. As a
rule of thumb for conveyorized spray etchers, higher acid results
in lower quality. Zero acid works best for quality since there is
no acid undercut. The cuprous that forms during etching under the
nozzles is soluble in HCl so uncontrolled etching occurs between
the nozzles on the top surface of panels in a converized etcher
where free acid is present.
The drawback for zero acid can be a loss of speed but this
doesn't have to be the case. If you have a sufficient quantity of
nozzles with higher flow rates at higher spray pressures, the
acid speed increase disappears while still maintaining the
highest quality and consistency. I will forward a copy of a
comparison of speed between etchers illustrating this point that
was done by Circuit Research Corp. separately from the TechNet
since no attachments are allowed through the listserver.
In any case, in a spray etcher, a higher free acid than 2N has no
appreciable benefits and just causes an environmental, safety and
quality mess.
In the case of your dip tank, etch speed will tend to be slower
anyway unless there is some agitation of the solution or the part
etched. Free acid will help here on the speed, as you mentioned,
because of the solubility of cuprous formed from etching while in
HCl. There would be no discernable quality advantage between zero
acid and any other acid Normality chosen since you are not
controlling the direction of the etch in any way. Keeping the
free acid between 0.1N-0.5N, as you say, should give you the best
compromise between speed, cost, environmental concern and quality.
If the dip tank provides you with sufficient quality and quantity
for what you are etching, then I would say you have progressed as
far as you need to to keep your project under control and costs down.
As to using the chemical oxidizers for regeneration (chlorine
gas, hydrogen peroxide and sodium chlorate), this reduces the
regeneration time required from your current several hours down
to less than 10 minutes for all the oxidizer chemistries and
automated control methods.
Feel free to e-mail me any time if you have more questions.
Best regards,
Phil Culpovich
pculpovich@... www.oxfordvue.com
-----Original Message----- From: TechNet
[mailto:
TechNet@...]On Behalf Of Adam Seychell Sent:
Wednesday, April 09, 2003 2:00 AM To:
TechNet@... Subject:
Re: [TN] cupric chloride parameters
Phil,
You response tells me I am more or less on track with the
chemistry. It amazes me the fact that free acid concentration can
vary across such high range. It wouldn't be surprising then to
hear conflicting arguments on what is the best level of acid to
use in CuCl2 etching.
What I hope to achieve is a 5 gallon immersion etching tank, for
occasional etching of single sided prototype PCB's. What you
describe about the slow regeneration with air bubbles is spot on
to my observations. Yes, the regeneration rate via air bubbles
alone is many times slower than the rate of copper being
dissolved. Fortunately the tank will have limited use and so it
may continue being regenerated for a long time (e.g several
hours) after the PCB has finished etching. I believe the volume
of solution should be sufficient to hold the limited amount of
copper(I) produced during the relativly short etching period.
To test when regeneration is complete, I take a few drops of
etchant and add to 5 ml of water, if a cloudy precipitate forms
then it indicates a significant amount of copper(I) is present
and regeneration must continue. I have tested this on small
scale, and it seems to give a good indiation, the time for
regeneration can be up to 3 or 4 hours. I guess the industry have
much cleaver more automated methods of keeping the solution
regenerated with chemical oxidizers.
Adam
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