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More curious electrochemistry

I was unhappy with the depth of my brass etching. Calculations suggested that I should be getting about 0.020" depth for the given current and time but I was getting about 0.006". I thought that the explanation was that the current was wasted on gas production at the anode (whatever the gas may be).

I was not sure what the problem was. I found a pot of my old electrolyte (bought from a commercial vendor) and tried it. I quickly found two things;

1) At current density over 0.1A/cm2 there was gas production on anode and if one went even higher there was gas production on the cathode as well.

2) At the beginning of the process there are some weird things happening. My power source delivers constant current and to do so the voltage fluctuates wildly (up to 20 V) and at one point there was an oscillatory type of effect where the voltage dropped, gradually increased and dropped again at about 1 of these events every two seconds.

Anyway I managed to etch a piece of brass and separately a piece of copper with an area of 100mm2 to the depth of 0.009" which was consistent with calculated prediction.

Today I tried my new electrolyte (CuSO4, H2SO4 in H2O) on a piece of brass with an area of about 450 mm2 (I had an accident with the resist). At the start of the process I set the current at 0.45A and got it with hardly any voltage at all. At 8 minutes the instability started as above, then it settled with the current steady and voltage of 10V. After 90 minutes the instability reappeared but was less dramatic, the voltage oscillating between 2 and 4 volts keeping the current steady.

There was no detectable gas production throughout.

I went for total 2 hours. The result is here:

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I cannot even begin to guess why the current blew a hole where it did while only removing between 0.0065" and 0.0075" elsewhere.

The cathode, BTW, showed a pretty even deposit of copper throughout.

Michael Koblic, Campbell River, BC

Reply to
mkoblic
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A question occurs to me. Was the area with the hole at the bottom, top, or a side in the etchant?

Also -- was there anything stirring the etchant during the etching? I would suggest that you get a magnetic stirrer (with the magnet in a Teflon coating so it does not get etched), which might help reduce the buildup of either coatings, or perhaps a very fine gas layer, not enough to bubble free, but enough to protect part of the surface part of the time. The oscillation of the voltage might be explained by a buildup of a fine insulating layer of gas bubbles, which eventually pop as the area being etched drops, allowing the voltage to swing low again as more area is being etched. Since you have a constant current supply, I would expect the voltage to be proportional to the inverse of the area being actively etched at a given moment. And the current, while constant at the power supply, will be varying with the exposed area actually being etched.

While I've not used the mag stirrer during electro-etching, I have used it for chemical etching of printed circuit boards, and it makes a big difference there.

Just some thoughts, DoN.

Reply to
DoN. Nichols

What Don said, plus perhaps try air agitation. I etch PCBs and us a length of stainless tube with small

Reply to
Dennis

I etch brass shim stock with ferric chloride to make stencils for solder paste for PC boards. My spray etcher has a heater in it, and I get the etchant up to about 35 - 40 C. The spray system is a typical pump (Little Giant-style with stainless or maybe titanium impeller and shaft) and the wand is PVC tubing with holes drilled in it. A fiberglass window screen is used as a diffuser to break up the jets. I get complete etch-through of .003" shim stock in about 3 minutes. No electrochemistry involved here, just etchant and agitation.

Your results look like the effect of no agitation and gas bubble buildup blocking the electrolyte.

Jon

Reply to
Jon Elson

I used Ferric Chloride for years but gave it up because I could get better results with electricity. I used to heat it to about the same temp and agitate manually.

Anyway, I think I know the answer to my original problem. It has to do with the way I put the resist on. I am hoping to find the time to do a couple of confirmatory tests tomorrow.

Michael Koblic, Campbell River, BC

Reply to
mkoblic

Well, so much for that theory!

I put on the resist differently this time and run the test again with mechanical agitation of the electrolyte. The result is here:

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(and the next picture) - i.e. exactly the same!

Even with magnification one could see no bubbles during the process but there was a lot of black crap that covered the etched areas as the voltage rose to keep the current constant. With agitation this got worse briefly and then it disappeared.

The finished area looks better but there is still a depth gradient from the bottom of the area to the top - 0.009" to 0.008" (the starting thickness was 0.018"). This of course ignores the hole! Actually, this depth of etch is consistent with the calculated results

I do not understand why the current is drawn preferentially to this edge of the area. Why not to all the other boundaries? One can see on the back picture that it is beginning to work on one of the vertical boundaries but nothing like the top one.

I also run another test to see if the stencils I use leave any appreciable glue behind that might hinder etching. I took 0.003" off a

0.005" piece using 3A current for 1/2 hour (the area was appx 1900 mm2). There was no evidence of the glue being a problem. There was much preferential etching at the edges, some of these worn completely away, but that I understand and have seen before.

I have generated more questions than I answered. All I know is that agitation is good and that no gas is produced on anode below the current density of 0.15A/cm2.

Michael Koblic, Campbell River, BC

Reply to
mkoblic

I don't know much about etching, but I have done a lot of black and white film processing. In that case, consistent, repeatable agitation is critical because the solution in immediate contact with the work decomposes rapidly and agitation provides a method for getting fresh solution against the metal. There's also a relationship between the degree of agitation and the amount of undercutting you get at the resist edge.

Reply to
Jim Stewart

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