problem developing photoresist - Page 1
Forum section: electronics / beginners

author: stachu0
author_rank: Newbie
timestamp: January 31, 2025, 06:55:14 pm
content: ahoy, i've been experimenting with thinner and thinner traces on my pcbs, eventually i had to make the big switch from toner transfer to photoresist pcb's. i bought one of these rolls of dry photoresist film. peeled off one side, laminated on a copper board, put my design on top of it, exposed with a nail polish hardener uv lamp for ~17 minutes... the traces were visibly more dark blue than the rest of the board. i took off to develop. i dissolved half a tablespoon of sodium carbonate in 300ml of water, waited 15 minutes (while mixing the water). nothing happened so i added half a tablespoon more. i tried adding a tablespoon more later. nothing really happened to the film. i tried taking it out of the developer and scrubbing it a few times, but if anything came off, it was with the hardened parts too. i also tried heating up the mixture. any ideas? is the film i bought just shit? the sodium carbonate i bought is marketed as a cleaning product, and the only thing mentioned in the ingredients is sodium carbonate. i also checked the ph of the final mixture and it was way over the scale (very alkaline)

author: janoc
author_rank: Super Contributor
timestamp: January 31, 2025, 08:29:20 pm
content: Likely wrong concentration, underexposed film or wrong developer.Eyeballing it when it comes to chemistry is a bad idea when you want good and consistent results. pH is not a substitute for measuring the chemicals, it is only loosely linked to concentration. Same concentrations of different chemicals will have different pH.Are you sure you have used sodium carbonate? If you used the common baking soda, that is sodiumbicarbonate, a very different compound. It will somewhat work as a developer but very slowly.Also, not every resist is made equal, some need different developer.

author: jpanhalt
author_rank: Super Contributor
timestamp: January 31, 2025, 09:35:18 pm
content: The resist I used required NaOH or KOH. What did the manufacturer of the resist you used specify? Have you contacted that manufacturer about the problem?

author: wraper
author_rank: Unknown
timestamp: January 31, 2025, 09:38:57 pm
content: Not reading this large size comic sans, my eyes bleed.

author: janoc
author_rank: Super Contributor
timestamp: January 31, 2025, 11:18:00 pm
content: Quote from: stachu0 on January 31, 2025, 09:20:56 pmEven baking soda is eye irritant.Quote from: stachu0 on January 31, 2025, 09:20:56 pmOk, that makes more sense. But you still shouldn't be eyeballing the concentration by adding spoonfuls of the reagent into water when you feel it is going too slowly. The concentration is usually prescribed, typically around 0.85%-1%. Too low or too high concentrations will both cause problems.Quote from: stachu0 on January 31, 2025, 09:20:56 pmI wouldn't call that resolved - normal development times of dry film resist are a few minutes, not 15+. There is something wrong with your process.

author: janoc
author_rank: Super Contributor
timestamp: January 31, 2025, 11:19:03 pm
content: Quote from: jpanhalt on January 31, 2025, 09:35:18 pmHydroxides are typically used to strip the remaining resist after etching, not for developing. If you use that for developing you can easily strip even what should remain on the board for etching.

author: jmelson
author_rank: Super Contributor
timestamp: February 01, 2025, 12:34:32 am
content: I have been using Think and Tinker 115i dry film resist (which is some version of DuPont Riston DFR). I prepare the blank board with fine sandpaper and then wash with water, put vertical to dry and then wipe with a paper towel and acetone. I laminate the film to the prepared board with a heated roll laminator. I expose the image with 405 nm blacklight lamps for ONE minute only! (I have a box with 4 15 W blacklight tubes spaced a few inches apart and aluminum foil reflector behind them.) Then, WAIT 15 minutes after exposure for the cross-polymerization to complete in the resist before peeling the cover sheet. Finally, the developer should be 1.25 Ounce by weight of Sodium Carbonate (not bicarbonate) to one US gallon of tap water, heated to 100 F (~40C) and developed for just about 1-2 minutes. I generally do double-sided boards, and keep flipping the board over every 15 seconds or so, and gently wiping the surface with my fingertips. After all the undeveloped resist seems to have come off, then wash the board in cool running water with continued fingertip wiping for several minutes until the slimy feeling goes away. Hang the board to drip dry before etching.

author: wraper
author_rank: Unknown
timestamp: February 01, 2025, 12:44:00 am
content: QuoteThis is 1000% overexposed. It's simply impossible to print the mask opaque enough to not overexpose when exposing this long. Most likely you need less than a minute. After developing, you can expose negative photoresist more before etching to make film stronger.

author: wraper
author_rank: Unknown
timestamp: February 01, 2025, 12:47:59 am
content: Quote from: jpanhalt on January 31, 2025, 09:35:18 pmIt was positive photoresist, nearly all films are negative type for which NaOH works as a stripper only. Still those are bad developers and hard to get working right without using very opaque photomask and tight process control. Silicate developer is 100x more forgiving in exposure time, mask quality, developer solution temperature, timing. And overall gives better results.

author: janoc
author_rank: Super Contributor
timestamp: February 01, 2025, 03:27:36 pm
content: Quote from: stachu0 on February 01, 2025, 01:01:04 pmDepends on what is in that lamp. If it has flourescent tubes, those often need exposure time in minutes. However, if it has LEDs, as most modern ones do, there the exposure time is often around only 30 seconds or so.The time will vary based on the light source, how far are your boards from it, resist itself, etc. Do the usual strip test - vary the exposure by e.g. 20 seconds and check which time gives you best results.