Electrolytic etching

[d1str0]

Hello!
I'd like to use this post to introduce myself and also ask my first question.
I'm a printmaker and also an electronics enthusiast from Chile. Most of my projects usually involve some sort of automation, temperature control, microcontrollers and refrigeration (don't know why but is what I like). Today I have a project with my wife (also a printmaker) intended to replace the classic etching techniques (similar to etching a pcb with acid) with an electrolytic process. Basically this technique consist of masking a copper plate whit resin and then placing it in a tank filled with of Copper Sulfate and distilled water. Inside this tank there is a stainless steel mesh placed parallel to the copper plate. A low voltage (0,3V to 0,5V) and high current is applied to the circuit, the copper plate being the anode and the mesh as the cathode.


This is the set-up. The mesh is closer to the camera and two copper plates are being etched at the same time in the back.

This electro-chemical process removes copper atoms from the copper plate etching its surface where is not masked with resin, but here we have a problem: Isolated lines and lines closer to the edge of the plate are etched way more aggressively than clustered lines or lines closer to the center, making for an uneven etching. I'm quite certain this is due to those isolated lines having greater current density that the others sharing the same area.


This is a copper plate etched with this method. Darker lines are deeper and closer to the edges of the drawing.

Does anyone knows a way to mitigate this effect, or maybe point me to the right literature?
Do you think there is even a solution to this problem?
Or should I ask somewhere else?
Thanks!

[Brumby]

I believe you are on the right track with your understanding - but I don't have the expertise to say this definitively.

I also see that your setup is basically the same as an electroplating cell, but with an interest in the anode rather than the cathode.  I have seen electroplating where the shape was complex and to avoid the excessive build up of plating in some places and inadequate plating in others, they craft custom shaped anodes - but it is clear evb149 has done a bit more homework on this...

The only idea I have at this time is to suggest masking off portions of the cathode mesh you are using around the areas of over-etching with an insulating material.  I was wondering about something like plasticine or candle wax, but I don't know how well they would work.  Exactly how it might look is an unknown - whether larger contiguous areas would work - or a pattern that looks like the halftone method used to print pictures in newspapers.

I have no idea if this would work - or whether it would work well enough for your needs.  I also don't know how practical it would be, but I offer it to you as a thought - and maybe something you could experiment with...

[d1str0]

Thank you evb194  and Brumby.

The biggest problem with masking is that before setting the copper plate in the tank, all its surface is masked with resin and then scratched with a stainless steel "pencil" with a very fine point. This way you draw directly over the copper plate. There is no way to compensate at this point for the depth of the lines because you are only removing the resin masking and exposing the copper behind.



Here is a copper plate in process of being drawn by a student. The black "paint" is the resin and the yellow lines act as guides. These are erased before setting the plate in the tank.

You could take a picture of the finished plate, process it with a computer and somehow design a mask that evens the depth of the grooves and somehow use it inside the tank, but it seems rather over-complicated. On the other hand, bubbling air or moving the solution seems like worth trying, although I think it might only help speeding up the process.
The moving electrode seems like a good idea, but again adds complexity to the process and the set-up. Maybe is worth trying it as some sort of CNC / electrolytic milling machine.

I thought there may be some way of modulating the current or some obscure pixie wrangling technique that could help reduce this "high density" effect.

[Ian.M]

It might be worth trying an extremely small electrode separation so the tendency of the current to spread laterally and interfere with nearby scratches is minimised.   e.g. a stainless foil electrode, and some sort of felted fibre separator holding the electrolyte.   I'm not sure whether you should set it up to do the whole plate at once, or maybe move a smaller electrode wrapped in the separator around manually like brush plating, dipping it in a beaker of electrolyte at intervals to refresh it.

[T3sl4co1l]

Try a lower current density, and circulation.

There may also be additives to improve material removal, but I'm not sure offhand.  (There are additives to improve deposits, but those act on the cathode.)

Tim

[Brumby]

I'm not sure you have understood my suggestion on "masking", d1str0.

The idea I had was to mask the cathode - the mesh you have in the solution as the other electrode.  Something like this:



To be effective, it will need to be aligned with the target and will probably need to be reasonably close - for the reasons Ian.M mentioned.


I know this is extra work - but it seems to be something, if it works, will improve the outcome as you hoped.

The exact shape of the mask on the cathode I have shown is a wild guess - and is just to give you an idea.

[dorin]

It's obviously a problem of non-uniform current density, but what causes this in turn is not obvious.

1. It could be due to the cathode mesh being too large relative to the area you want to etch such that it "stretches" current away to the sides, not perpendicular to the surface.
2. It could be due to the cathode mesh being either too far or too close, sorry but it's too much effort to think this one through, you can just test different distances.
3. It could be a non-linear effect of local ion accumulation resulting in electrolyte impedance change. I think only strong bubble flow can help with this one.

In order to better identify the source I strongly suggest you test etch a uniform drawing, for example a lattice of 1x1cm (or smaller). Try using a ruler and scratch as evenly as possible. Then we can see whether the edges of the board are still affected by higher current densities or not, and how the line thickness looks close to the lattice joints.

Without knowing the test result, I would suggest anyway you have electrolyte flow (with bubbles or not), a smaller cathode mesh area the size of your copper board, at a distance of at least 3x the steel mesh eye size (this one is just so you don't get the steel mesh pattern bias your drawing - but I assume you already know this because it's not your main problem so far).