DIY - UV film starts "exposing" when dipped into the developer solution - Page 1
Forum section: electronics / general

author: bogdan2014
author_rank: Regular Contributor
timestamp: March 08, 2021, 07:58:30 pm
content: I've been making PCBs at home using the UV film method with good results. However, there's one thing that's bothering me and I can't seem to figure out why it happens.I use a calcium carbonate solution to develop the board after exposure. But when I place the board into the solution, the unexposed parts quickly start to harden, making them very difficult to clean. It's like it gets exposed by the solution.I tried different concentrations, different exposure times and the result is pretty much the same. Sometimes it hardens so much that I have to scrape it off.

author: M4trix
author_rank: Frequent Contributor
timestamp: March 08, 2021, 08:04:57 pm
content: Calcium carbonate ? You mean, Sodium carbonate ?

author: bogdan2014
author_rank: Regular Contributor
timestamp: March 08, 2021, 08:32:14 pm
content: Quote from: M4trix on March 08, 2021, 08:04:57 pmYes, that one

author: 2N3055
author_rank: Super Contributor
timestamp: March 08, 2021, 08:33:11 pm
content: Quote from: bogdan2014 on March 08, 2021, 07:58:30 pmIt is UNDEREXPOSED.. You should prolong film exposure.EDIT. I misunderstood Negative/Positive .. It is overexposed.

author: bogdan2014
author_rank: Regular Contributor
timestamp: March 08, 2021, 10:04:47 pm
content: Quote from: 2N3055 on March 08, 2021, 08:33:11 pmBut those parts are not exposed anyway. How is that contributing to the issue?

author: jpanhalt
author_rank: Super Contributor
timestamp: March 08, 2021, 11:32:36 pm
content: Just get a red bulb as used for B&W photography and problem solved. If you have fluorescent lights, there is plenty of near UVA to affect that photoresist.

author: M4trix
author_rank: Frequent Contributor
timestamp: March 09, 2021, 12:59:02 am
content: Well, I do a lot of DIY PCBs with dry film photoresist. A lot of trials and errors before I got it right. In your case, overexposing is probably the culprit. That's why I always do an UV exposure test every time when I have a different dry film batch or if the expiration date has been reached. Btw, according to the most manufacturers specs, the average shelf life of dry film photoresists is 6 months. This is not true. I kept some no name Chinese dry film in the fridge about 3 years and it still had the same adhering properties and the same exposure time ! Therefore, testing the exposure time is essential. Another important thing is to have a good printer that can print really quality dark images, otherwise the UV light will breach through the areas where it shouldn't. That can also be the culprit in your case. Doubling two images one over another can help but again, UV exposure test should be done. Btw, I use Bungard's dry film photoresist and with my 'rig' it takes only 16 seconds !Edit: jpanhalt, mentioned fluorescent lights. I found that this isn't very critical. But, just in case, I do all the work under yellow light.

author: bogdan2014
author_rank: Regular Contributor
timestamp: March 09, 2021, 10:01:32 am
content: QuoteYeah that might be the issue, my printer doesn't really print that dark. I used to double the images when I had a crappier UV unit but then I had a lot of light bleed between the layers, so it was actually worse.Exposure from ambient light isn't an issue since I keep the room I work in minimally lit. I even developed the board in the dark at some point since I also thought that's the issue.As far as exposure time... 30s is the lowest I've done with UV led strip.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 09, 2021, 10:38:27 am
content: I use an inkjet for best results. The transparency I use (Pictorico brand) has a microcrystalline material (ceramic?) on its surface. It is advertised for artists. Otherwise, I use a good quality transparency. The color doesn't have to be black. In fact, yellow works quite well. The advantage is that inkjet colors flow.

author: jmelson
author_rank: Super Contributor
timestamp: March 09, 2021, 05:11:51 pm
content: I don't know what film you are using, I use some Riston film I get from Think and Tinker.I use filtered black light bulbs, with a peak emission of 405 nm. I have an exposing box with a crinkled aluminum foil reflector, and the black light tubes are about an inch apart. The master artwork is made on litho film with a homemade laser photoplotter.The black part of the litho film is pretty dark, but if held up to the light, you can still see some light getting through.I expose the resist for ONLY one minute! If you expose longer, you start to get the issue your report, not all the resist dissolves.Now, with my Riston dry film, the developer does NOT harden it, it dissolves the part that has not been exposed to the UV. The UV, itself, does the hardening. Oh, here's one trick. After exposure, leave the cover film layer on the resist for 10 minutes in dark conditions, this completes the polymerization step. Then peel the cover and develop in 40 - 50 C developer solution for only 2 minutes. Wipe the board continuously with fingertips to help expose the resist to the developer. If it bothers your skin, use a soft sponge.Use yellow "bug lights" for room illumination while laminating, exposing and devloping the resist, to prevent unwanted exposure.Also, I scrub the board with fine sandpaper (600 - 1200 grit), then clean with acetone, before laminating the film. You don't want any oily film or oxidation to interfere with the resist adhesion or etching step.Jon

author: Fred27
author_rank: Unknown
timestamp: March 09, 2021, 06:43:00 pm
content: Quote from: jpanhalt on March 09, 2021, 10:38:27 amI agree. Back when I used UV photo resist I had great results with "T shirt mode" on a Canon MG5250 which put down a lovely think layer of ink. For some reason everyone seems to stick with laser printers even though inkjet seems far better for this. Maybe they're getting confused with toner transfer?

author: 2N3055
author_rank: Super Contributor
timestamp: March 09, 2021, 07:05:38 pm
content: Inkjets with pigment ink work well. On a printer with separate black ink, I got very good result by not insisting on using separate black, but let the it print with CMYK. It blocked UV a bit better, although to naked eye it looked more transparent. Also it worked best when I didn't insist on highest resolution..Short story, I would do test exposure any time I would start using new material. There is actually standardized exposure test pattern...

author: sleemanj
author_rank: Super Contributor
timestamp: March 09, 2021, 08:15:28 pm
content: As with others, your exposure looks long. You can use a surprisingly short exposure, to where the image is only just visible, sufficient enough to develop it, then after developing give it a longer blast of UV to fully cure (and also helps to see any bits that got missed in developing).Download or make a test pattern pcb (here's minehttps://github.com/sleemanj/pcb-test-pattern) and tune to your minimum exposure time, you don't have to etch, just develop and see how long exposure you need to get a strong enough image to stand developing