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PCB's etching VERY slowly, muriatic acid and hydrogen peroxide

I recently started using the muriatic acid and hydrogen peroxide solution to etch PCBs.  All of the boards I do at home are simple, single sided, and range in size from 1-4 sq. inches.  The first board etched in a few minutes, the second 15 or better and now it is taking hours (after 5 or 6 small boards).  I have the fish tank pump set up to oxygenate the mixture.  The mixture is a nice green color, which from what I have read is correct.  Any ideas on why it is so slow now or suggestions as to what I need to do differently?

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bdk6's picture

Just a couple more notes.  First, I only recently started using this etchant, so I don't have a lot of experience.  Chemistry is not my stongest point, but I have looked into it a bit.  It seems there are two different etching methods going on.  The first uses mainly the oxygen when you first make the etchant (before it turns green).  Then after using it some it contains and uses mostly cupric chloride which then does the etching. That's the green color.  Those may have very different etching rates by nature.  Also, heat almost always speeds any chemical reaction, to a point.  It also makes more vapor, so be careful.  The cupric chloride needs fresh oxygen to "regenerate" and stay usable.  Pumping air in works, but is slow.  Adding hydrogen peroxide is a lot quicker, but a bottle of it is either 94 or 97% water, so you are diluting your etchant every time you add it.  That is fine if you aren't planning on using it "forever", but one of the main advantages to this etchant is that it will last "forever".  I think you only need a very small amount of hydrogen peroxide, maybe a capful or less.  Don't know, haven't tried that yet.

This guy has done a lot or research and experimentation.  I recommend taking a look at his site.

http://members.optusnet.com.au/eseychell/PCB/etching_CuCl/index.html

One last thing.  I've noticed this stuff gets saturated pretty quick, so maybe if you use a larger quantity in a bigger container that might help.  I use a 1 liter container, about 3/4 full.  Good luck and I would be interested to hear what works and doesn't work for you.

These are all great suggestions.  The mixture has been getting pretty dark lately so it sounds like more hydrogen peroxide is called for.  I use a small ceramic bowl with about 8oz. of fluid, so I'll get out the coffee cup warmer I used when I was using ferric chloride and try that also.  Thanks.

kariloy's picture

Well, the MOST important thing you need to do is adding more hydrogen peroxide everytime you need to use it to keep the solution "fresh".

Also heating up  a bit like bdk6 said should help (although I actually never checked the thermodynamics myself) I never had to had to or bothered to do such a thing.

I prefer adding a bit of (kitchen) salt (also helps drive the reaction in the desired way) and it works for me.

Also I find that careful sideways agitation of the ecthing vat helps (it washes off the forming copper salts exposing more of the PCB's "fresh" copper to be attacked by the etching solution).

bdk6's picture

The temperature will make a big difference.  If you set your tank into a bucket or tank of hot water and let the etchant heat up it should help a lot.  Also, the etchant should be a bright, not dark green.  It will turn darker and darker as you use it.  The re oxygenating seems to be pretty slow.  If you have a safe place to do that you could run the pump continuously.  But I think warming the etchant will make the biggest difference for you.