More mysteries of electrolysis

Electric field strength is affected (enhanced) by sharp corners. Which comes to much the same thing.

Was the cathode a whole cylinder? That could explain it.

Best regards, Spehro Pefhany

You might get a better result with two of these making a full circle when you etch - or using a heavier resist on the ends.

Well, the rate of oxidation (and thus metal removal) is proportional to the electric field (in volts/meter, for example). The metal is all at the same potential, and so is the cathode. However, the shape of the electric field as the potential falls from anode to cathode is determined primarily by the shape of the anode. A sharp corner or spike on the surface will concentrate the electric field, causing a large potential drop in a short distance from the surface, and give a high rate of oxidation (that's why electropolishing works to level a surface). Inside a pit the potential is almost constant so the electric field is very low and oxidation is very slow so high spots get eaten away and low spots left alone. Think how much easier it is to draw a spark from a sharp point, compared to a smooth sphere - the potential drop is the same but it happens over a much shorter distance at the sharp tip. in this case, though, I think the problem is something else or otherwise the corners of the edge of the strip would have been eaten away. Most likely defects or inclusions in the metal, or variations in hardness or composition. If that's the case there isn't anything you can do except get new metal, or maybe try annealing it first. If it is due to the shapes of anode and cathode or the buildup of nonuniform concentration gradients as the etch proceeds, vigorous stirring really will help. Just make up a lance with lots of < 1/32" holes, bend the tube into a loop and put it at the bottom of your tank, and apply 20 to maybe 60 psi of air.

----- Regards, Carl Ijames wrote in message news: snipped-for-privacy@4ax.com...

I have never tried to etch a piece of metal that was not flat.

Here is a face of a polar sundial prepped the usual way, using my go-to resist with an extra coat "to make sure". The main difference was a cylindrical cathode parked in the centre of the semicircle.

The result sucks:

..and the next 4 pics.

What sucks more is the fact that I cannot explain why. Are ions attracted to corners?

Michael Koblic, Campbell River, BC

Correct. Like charges repel and, even in a conductor, will concentrate at the ends of a bar or the corners of a plate. That's why lightning rods are pointed and high voltage generators have spheres at the HV end. In your case could you make the part longer and cut off the ends when you're done etching? Or better yet make a hoop and etch 2 at the same time? Art

The field is greater at the right at the corners, but you have some messed-up areas far from the corners too.

Here are some thoughts, you will have to decide if they make sense. My experience with resist and lithography is on semiconductor wafers, which is different.

If the resist is really in good contact with the metal this should not happen. What I see in the close-ups is that the metal appears to be brushed creating lots of grooves. These might allow the electrolyte to migrate in from the ends if the resist does not fully wet the bottom of the grooves. Although the pattern of etching looks more like a bunch of pinholes on certain areas of the resist. Could the resist have become damaged or contaminated in these areas? Could there have been particles in these areas before the resist was applied? A conductive particle embedded in the resist would be etched away and would then create a pinhole.

Does this resist tend to shrink at all after application? If so, then putting on the second coat might actually tend to peel off the bottom layer as the top layer shrinks.

wrote

Yes.

Like charges repel, and push each other into outside corners where there is no opposing charge beyond to push back. Lightning rod tips are pointed to encourage this, and high voltage generators rounded to minimize it.

Perhaps you could ask a circuit board manufacturer for help. Plating the inside of drilled holes is the reverse problem, ions aren't as attracted to shielded areas. I don't know how they solve it but I had to follow their design rules to get a good yield of complex multilayer circuit boards.

A rule that may apply to you is to have a relatively constant density of plating (etching) area across the board, so the plating solution isn't depleted unevenly in the boundary layer.

Some solid power planes are converted to open grids to average out the copper density. The grid also helps control warping from thermal contraction after multilayer boards are laminated together in a hot press. jsw

Electroless plating is used for deep holes.

Nickel, then gold.

Joe Gwinn

Gee. Why didn't I say that? Art

I didn't see your post.

Right. I sort of suspected that might be the case but I had a difficulty visualizing it under water so to speak. Underwater St. Elmo's fire?

I find that hard to process. Constant density of area?

Again, not sure what that means. having said that, normally I use a stainless steel grid as a cathode for my flat pieces. Interestingly I find that there is a small but definite gradient in the tank: I have the electrodes side by side and if I do a large piece the etching at the top of the tank is slightly shallower than at the bottom. For practical purposes it is not relevant, just interesting.

Michael Koblic, Campbell River, BC

How does that work? It was, as a matter of fact, a stainless steel bolt. In my simple mind I did not expect the "far side" of the cylinder to be relevant. I expected the current to take the path of the least resistance. However, that is not the case as when I observed the process hydrogen bubbles were produced all around the cylinder, not just on the front part. Also there is significant penetration on the "far side' of the work piece showing that the current in fact had to get around the corner.

Michael Koblic, Campbell River, BC

Right. The longer ends are the obvious solution I proposed in the legend to one of the pics. The other suggestion was to make the resist at the ends thicker or at least more resistant (taping them). One has to wonder if doing that will shift the zone of foul biting to another spot on the work piece.

Another thing I have not investigated in this context is the current intensity. I wonder if there is a threshold effect for this and if running the process at a lower current for a longer period of time would be better.

One problem with that I have discovered in the past is that not all resists withstand prolonged immersion in some electrolytes. The one I am using now should be pretty safe, though.

Michael Koblic, Campbell River, BC