Remove copper from phenolic PCB without damaging it

[agos46]

Hi all,

I'm working on a project of making a home made Etherwave Theremin. I'm actually in the phase of fabricating the PCB, and for the process I used a CNC milling machine from my university, and the results were pretty good. The problem is that I can't remove the unused copper with the machine, because it would wear a lot of bits (there is a good amount of unused copper in my board) and the lab operator said i can only mill the traces.

I need to remove every excess of copper because the circuit is extremely sensitive to parasitic capacitance. Is there any way to remove it by hand? I was thinking on using heat to loosen the copper and pliers to pull it out, but im worried on damaging the board. I'm using a soft phenolic copper clad (FR2 I think). What do you think I can do?

[CatalinaWOW]

A.  This may seem a little crazy, but paint the traces with pitch or other etch resist and then etch the copper away as you would in conventional PCB fab.  The theremin is not a complex circuit so it will be practical to do this by hand.  If you miss a few dibs and dabs of the bulk copper it won't horribly affect operation in most cases, and if it does you can scrape it off with a knife (practical for only very small areas).

B.  Chalk the whole thing up to learning and send the design off to one of the cheap board fabs.  They really are economical these days.

C.  Buy some larger facing bits and do the programming to use this bit size to remove most of the copper before returning to the original size and finishing the removal.  It will be painful getting the board lined back up since you have already removed it from the machine, but worst case you can just do another board.  (A variant on this option is to do the analysis to find exactly where parasitic capacitance is most troublesome and do only those regions).

[edavid]

I need to remove every excess of copper because the circuit is extremely sensitive to parasitic capacitance.

It's not *that* sensitive, otherwise it wouldn't work.  Something like 1/4" of isolation routing would be good enough.

[agos46]

Thank you for your response. I think I will go with option A posted by CatalinaWOW. Just painting the traces and applying etchant. Any good etch resist and technique for applying it you would recommend?

Traces are fairly thin (24mil and 40mil) but the grooves made by the CNC should make it a lot easier

[CatalinaWOW]

Best options for resist and etching depend on your local circumstances.  Just google making PCBs and pick a recipe that uses material you can obtain easily.  I have used several of the traditional etchants and one non traditional and they all worked, with their own peculiarities.  Most people recommend avoiding Ferric Chloride, it is messy and hard to legally dispose.

Same story on resists.  Lots of things work.  Some marginally (like petroleum jelly) and others quite well.  Depending on your circumstances you may have access to a near infinite supply of one good option - fingernail polish.  Just ask the women in your life for the colors they no longer like. 

Be sure to clean your boards well (fingerprints work well as a resist when you don't want them to) getting them to a clean bright copper state.

[KL27x]

You can remove large areas of copper pour exactly how you have imagined. The most important tool you need is a specific shaped chisel. If you have access to a belt sander, you're set.

Take a 1/8" to 1/4" steel rod or allen wrench or what not, and grind it into the shape of a regular wood chisel. Roughly 25 to 30 degree bevel. Then very gently round the sole/back of this chisel into a slight arc (side to side, not front to back). You can give the back a little angle, here. It doesn't have to be parallel, I,e you don't have to grind the entire length of the rod, or anything. Just the tip. Then sharpen it.

You work this chisel under the copper with the same orientation as paring with a wood chisel: back down, bevel up. But you don't push it forward. You just constantly twist the chisel on the radiused back, which levers/pries the copper away from the board on either end of the sharp edge. Once you get it started, the copper will slowly but surely peel away with this gently prying. Once you get a large enough tab, you can just peel with pliers, but the chisel could do it alone. And the chisel will take care of odd parts the rip off and get left behind. Aside from the gouges you make at the initial attack point, you can do this leaving virtually no marks on the substrate, at all. Once the peel has started, you do not need any direct contact between the edge and the board... it is contacting only the copper.

Anyhow, this works for me, and it is very neat and easy. If you like to try it, I am pretty sure you will agree.