Photoresist and electro-etching - Page 1
Forum section: electronics / general

author: bicyclerepairman
author_rank: Newbie
timestamp: May 17, 2025, 01:28:05 pm
content: I'm working on a project to etch a design into metal sheet (ideally stainless steel, but starting off with brass). The etched design will then be filled with paint or clock dial wax. If it works out, the final piece will be stuck to my oven front panel, to replace the worn-out markings.I'm using dry film photoresist. I get good adhesion, and the exposure and development process seems to work fine.To electroetch, I then put the developed metal sheet into a tank of electrolyte, thinly separated from a cathode plate (with strips of bicycle inner tube). I'm using low voltage (< 3v), and the resulting current is quite low (of the order of 100mA).My main problem is that the photoresist flakes off, where there is direct "line of sight" to the cathode. Oddly, where the direct line is blocked by the strips, the resist holds up much better (although the etching is minimal). I suspect my problem is bubble formation, although I did not expect areas under the photoresist to generate any bubbles, as it shouldn't be porous.A second problem is that the etched areas have a very reddish hue, which tells me that some elements in the alloy are being preferrentially removed, which will very likely limit etch depth.I can go into more detail into what I've tried, but wanted to start with some general questions:Has anyone tried this successfully before?Photoresist and acid etching is a common enough process - is there something about the electro process that just won't work? The photoresist shouldn't be porous, but maybe it is slightly conductive?Could I be over-developing (or over-hardening)?Any insight gratefully received!

author: Kleinstein
author_rank: Super Contributor
timestamp: May 17, 2025, 04:47:33 pm
content: For etching brass it is normal to get mainly dissolution of the zink part, leaving copper behind and thus the red color. So that part is normal.With stainless steel one would have a similar problem: dissolve the less noble metals first and leave behind the more noble ones (e.g. nickle).The formation of bubbles (hydrogen) is a plausible cause to the destruction of the resist. It only need tiny holes to get hydrogen formation of the potential difference is large enough. One may be able to reduce the formation of bubbles with lower current and a high PH. This would however also slow down the etching.

author: coppercone2
author_rank: Super Contributor
timestamp: May 18, 2025, 08:14:28 am
content: I think the current density is too high , or maybe something like the solution conductivity being too high?Did you check if the current density is appropriate for the surface are you are processing?

author: cunningfellow
author_rank: Regular Contributor
timestamp: May 18, 2025, 10:26:39 pm
content: The resist shouldn't be porous - but it sounds like it might be.What is your photo tool? Laser printed photo tools are normally not great. Inkjet and appropriate film is better. Sometimes yellow ink is better than black ink.Also what is your developer solution. I have seen reseller of dry film suggest sodium hydroxide when it was not the ideal developer.

author: reboots
author_rank: Regular Contributor
timestamp: May 19, 2025, 04:16:05 pm
content: I've tried electrolytic etching of stainless steel. The etched areas had a grainy, crystalline appearance. Clearly certain elements of the alloy were being preferentially removed. I've also used ferric chloride, which I keep on hand for etching printed circuit boards. That gave a much cleaner, controllable result.I've also done electrolytic etching of aluminum plate, which turned out much better. However I used a toner transfer process for the resist, and found that any slight pinholes or porosity allowed enough conductivity for etching. I subsequently used toner transfer for the general design, and paint fill for any large areas. This was labor-intensive but produced an acceptable result.

author: jmelson
author_rank: Super Contributor
timestamp: May 19, 2025, 11:52:07 pm
content: I used to make solder paste stencils from .003" thick brass shim stock. First, I sanded the shim stock with 600 grit sandpaper, wet. Then,I washed with water, then wiped with a paper towel soaked with acetone. Hang up to dry. Then, laminated the dry film with a heated roll laminator made for PC boards. I had to place a paper shim under the shim stock to get the correct pressure for the lamination. Then expose with a high contrast artwork made from lithographic film. Exposure was with a box of 405 nm blacklight bulbs with aluminum foil behind the bulbs, for only ONE MINUTE! If the exposure is too long, the light gets under the black areas of the film and the resist is hard to develop. Allow the polymerization to complete for 10-15 minutes in the dark after exposure. Too long exposure makes you keep it in the developer too long and weakens it. I develop in Sodium Carbonate solution heated to about 40 C for just 1-2 minutes, while constantly wiping with my fingers, then wash in running water for a couple minutes, then hang to drip-dry.I etch in Ferric Chloride. Also heated to about 50C and sprayed by a PC board etching machine. As soon as it is properly etched, wash with lots of running water to stop the etching process, so it doesn't etch sideways under the resist.Jon