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
[log in to unmask]
www.oxfordvue.com

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Adam Seychell
Sent: Wednesday, April 09, 2003 2:00 AM
To: [log in to unmask]
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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