I thought that rather than post this in the thread about 'Light Boxes' it would be better to start a new thread specific to this promising method of making printed circuit boards, so here's an update on my progress/lack off, and my experiences to date with the UV dry film technique. I've tried a couple of PCBs - one about 3cms wide x 6cms long (for a solid state magic eye project), the other, about 8 cms square (for an amplified loop aerial project). In each case, the artwork was of a high standard, on micro-porous film printed on an ink jet printer - totally opaque to the eye when held up to a light.
The smaller board (which I've developed but not yet etched), I judge to be a success - the larger board, a failure.
I've attached two pics to illustrate the results. It will be evident that there are blemishes on the larger board, which could, I guess, be touched up, but that's not my style - I don't operate at the 'bodgertronics' end of the spectrum. The reason for the blemishes, is that even though the film came off a roll on a cardboard tube, there were slight wrinkles in it and these didn't disappear when the film was applied to the board. I rubbed it down with my fingers to ensure that the film had no bubbles and passed it through the laminator several times.
I didn't know if the wrinkles were just the film, with the resist firmly adhering to the board, or whether the blemishes were where the resist hadn't adhered to the board. It proved to be the latter. (The board would have been a failure anyway as I must have been asleep at the wheel when I placed the artwork in the exposure box because it was back to front. Huh - after all these years...!). No worries - the resist lifts off in seconds when the board is dipped in acetate, so I cleaned it up and re-applied a new piece of film - four attempts in fact, none of which were perfect - every one had blemishes which are not apparent when the board is put through the laminator.
I'm away for a week from tomorrow, but when I get back - though all the youtube videos on this technique show the boards being put through a laminator to bond the resist to the copper, I may try an electric iron at 100 C - the suggested temperature, to see if that produces a better result.
I've developed a second 'magic eye' PCB with a successful result by carefully selecting a piece of film off the roll which had no wrinkles or creases.
Some tips:
Don't leave the roll of film in daylight. Cut a piece large enough for your needs in subdued light and put the roll back in it's lightproof pacing or over time it will be spoilt and of no use.
Removing the first protective film: There are two clear films - the resist sheet being sandwiched between them. It doesn't matter which side you remove prior to applying the resist sheet to the board. You need to stick two small pieces of Sellotape - one either side, near a corner of the sheet - then gently pull the tape to remove one of the protective films. An acquire skill which I've just about mastered!
Applying the resist sheet to the PCB: Gently smooth the sheet out with light finger pressure to remove all air bubbles. You really only get one got at this - if you try to lift the sheet to re-position it, you risk the resist sticking to the board and coming away from the protective film. (Trust me on that!). When you're happy with it, pass it through a laminator three or four times to firmly stick the film onto the board. If there are any bubbles or wrinkles, it's as well to abort the process, remove the protective film, dip the board in acetone till the resists cleanly lifts off (a couple of minutes or so), then apply a new piece of film. If there are no bubbles or wrinkles, move on to exposure.
Exposure time: I tried one board at I'm minute exposure, and felt that this might be a little too long, so when I did the second, I reduced the duration to 40 seconds (which is the suggested time in the excellent youtube video at the link below.
Removal of protective film after exposure/before developing: Remember to peel off the second protective film to reveal the resist to enable it to be developed.
Developer: The recommended dosage is 1 - 2 grams of soda ash per 100 ml. That's a broad range. I Gram is one level teaspoon (5ml), so I compromised on 1.5 per 100 ml of warm water, but made 200 ml and used 3 level teaspoons. (To put 200 ml into perspective, it's about a cupful).
Developing: It's not very apparent when all the exposed resist has been removed. I dipped the board in the developer for about 30 secs, then removed it from the developer and under a running tap, rubbed the board with some kitchen tissue. The exposed developer turns a murky grey. I did this a few times till no more exposed developer seemed to be coming off, then rinsed the board.
I guess that the smaller the board, the easier it is to achieve success.
No doubt exposure times will vary according to the type of artwork used and the type of exposure box used. In the excellent video at the link below, the artwork was printed on an ink-jet printer onto what is referred to in the film as 'vegetal paper' which is simply what we call 'tracing paper'. I think that, or a laser printer will make the exposure times more critical as the artwork won't be very opaque, though I know some tape two sheets together to try to improve the opacity.
Remember that the artwork must be a negative - not the more usual positive. When developed, all the black areas of the artwork will be washed away - all the clear areas will remain after developing. This is the opposite of what pre-sensitised positive resist boards or UV aerosol lacquer. If you look at the third pic below, all of the white areas will remain - all of the black areas will be washed away when the board is developed and etched.
Despite my mixed results at this early stage, I'm still of the view that this is by far and away the best method of UV PCB production once the technique is mastered and exposure time and developer concentration has been attained according to what equipment is available. I discounted pre-sensitised boards many years ago as they're quite expensive and I always had useless offcuts left over. I settled on self-spray UV lacquer, but it's extremely difficult to spray the board without specs of dust settling on the lacquer and exposure times can vary according to the thickness of the lacquer. I know that some people claim success using the 'iron on' glossy paper print technique but I don't have a laser printer and don't want one.
The Photo-Resist dry film is much cheaper than using UV lacquer or pre-sensitised boards – typically about £2.00 for a roll 2 Metres x 15cms on ebay.
1 KG of sodium carbonate (AKA 'Soda Ash') for the developer is typically £5.50 post free on e-bay:
Here's the link to the excellent youtube video, which makes the whole process appear all very simple!
Note that he used tracing paper (referred to in the film as 'vegital paper') for his mask, and the PCB was produced a test run, with some very narrow tracks, very close together:
https://www.youtube.com/watch?v=2hQfGtSFe_0
The first pic below is of my small board, which - though not etched - appears successful. (After etching it will be cut down to size). The second pic is of the failed larger PCB with the blemishes marked in yellow. Sorry about the poor quality of the photos, but hopefully they give a general idea. A fourth attempt at the larger board is better, but still not to my liking, so I'll be giving that another shot when I get home next week.
Hope that's of interest and might encourage, (rather than deter!), others to give this process a go and to share their experiences.