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Old 10th Jan 2017, 9:04 pm   #1
David G4EBT
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Default UV Photo-Resist Dry Film PCB Process

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.
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Old 11th Jan 2017, 12:49 am   #2
joebog1
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Default Re: UV Photo-Resist Dry Film PCB Process

For what its worth!! I gave up on dry film 35 years ago. Its really designed for "factories" .
By that I mean a dedicated area with "artists" to do the work. I use aerosol spray positive photo resist and have good success. The sun is my light box !! Its a point source with the source 93 million miles away, so VERY accurate. Without " bleedthrough" around the edges of the tracks. I print my positives on matt clear film on a normal B/W laser printer.
I design the PCB,s using an ancient DOS program using Linux and DosBoxEmulator.
The program is called Protel Easytrax. It was freeware many years ago so I "guess" its free now. I still have an origional 3 1/2 inch floppy of it. ( hint)

To prevent the "dry spots" as per your photos wash the PCB stock with a powder cleanser normally used for bathroom use. rinse it with clean water, and dry it well, using a CLEAN soft cloth. If there are kinks in the film, the glue thats used will not adhere. Apply the film immediately the board stock is dry. BE VERY careful of dust!! the smallest dust particle will give a hairline fracture of track/tracks that are the very devil to find later.

I built a very expensive lightbox using Philips UV tubes specifically designed for PCB work, and gave up after many many attempts. The edges of the tracks are furry !! I was never able to get really tight edges to the tracks using a lightbox
The sun has HUGE amounts of UV ( especially in north queensland) and as I said is a point source.
When you print the negative/positive reverse the image as necessary, so that the "ink/toner ALWAYS is in contact with the board.
Gently heating the "developer" to about 70 degrees F will help in getting nice resist edges.
Agitate the PCB in the developer ( just like developing a photograph in days of old) to ensure "clean" developer is always washing across the board.

I still use ferric chloride for etching, and find that warming it up also helps in achieving nice clean edges.

Without boring the mods too much, you can PM me if you wish and I will try and write a how to do for you.
I found the biggest benefit was experience, BUT gave up on dry film as it is SSSOOOOO variable. Changes in temperature, humidity, cleanliness of copper film, all made huge differences.
Spray laquer works everytime.
As does the sun as my UV source.
If you are trying to make PCB,s commercially, I would suggest winning the pools first.

Hope I have been helpful

regards

Joe
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Old 11th Jan 2017, 3:32 pm   #3
Oldcodger
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Default Re: UV Photo-Resist Dry Film PCB Process

David- please keep on with posts on this topic. At the moment, I'm hovering between purchase of a lightbox off E bay and making my own, so I value your input.
Mods- can you let this run, as a source of education. Well worth the time, David has spent on this topic.
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Old 11th Jan 2017, 5:32 pm   #4
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Default Re: UV Photo-Resist Dry Film PCB Process

Of course. It will only be closed if taken off topic.
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Old 11th Jan 2017, 6:38 pm   #5
David G4EBT
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Default Re: UV Photo-Resist Dry Film PCB Process

Thanks for reading the thread Joe, and for your helpful comments. I've been making PCBs using positive UV spray since the late 1980s, but only learned recently of the 'dry film' negative process which I thought might be easier and less time consuming than spray and once the exposure time and developer concentration has been optimised, would give consistent results. It certainly looks that way in the video. For me, it's early days yet. One reason for the wrinkles and bubbles might be that the laminator is too high a temperature.

As to artwork, I've had a go with various programs and haven't got very far with them. Some of the freeware ones have shortcomings for DIY PCBs as the tracks are needlessly thin and the pads needlessly small. In part, that's because the software is intended for the user to use a PCB Pool to get the boards professionally made. If - as occasionally happens - I see a PCB designs created by such programs, ('Easy PCB'?), I set about editing the design by increasing the size of the pads and thickening the tracks to prevent 'undercutting' when the board is etched. I use MS 'PAINT' for that, and for creating artwork to my own requirements. For simple one-off boards, I occasionally use 'rub-down' PCB transfers.

I print the artwork onto 'micro porous film' from a UK firm called 'Crafty Computer Paper' which creates excellent opaque masks, and print them so that the ink side is to the PCB to prevent 'undercutting' as the broad is exposed to UV.

As to etchant, I supposed much depends on what chemicals are availed which might differ from county to country, but I abandoned Ferric Chloride years ago - it's really messy stuff and I'm surprised that it's still quite widely used. Sodium persulphate (outside the UK often spelt 'persulphate') is a far better option. It's fine white crystals and clear when mixed with water, so when the board is etching, you can see the progress. Over time, it turns pale blue, and can be sorted for re-use. No nasty brown sludge. It works quicker if heated to 50C, for which I use an acquarium heater with the thermostat shorted out, and I've made a little tank which has an acquarium pump and 'airstone' to bubble air through the etchant to speed up the process.

I always clean the board scrupulously, rinse it in water then wipe it with IPA. When using the positive resist spray, I do it in the garage as that's as dust free as I can get, and having placed the board on a level surface, I shield in with a large plastic browl hovering above it, and spray the board beneath the bowl, then lower the bowl to cover the board till dry. That way, very few specks of dust fall onto the lacquer.

I generally get good results, but the dry film process seems much less of a faff than spraying the board with lacquer and leaving it overnight to harden off. There are quite a lot of videos on YouTube which indicate very successful results with the negative film, so when I get back home from London next week (counting the days!), I'll see if I can find out which the film getting little bubbles and wrinkles When passed through the laminator. The film is cheap enough to experiment with!

Thanks once more for your input Joe.
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Old 11th Jan 2017, 6:49 pm   #6
David G4EBT
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Default Re: UV Photo-Resist Dry Film PCB Process

Quote:
Originally Posted by Oldcodger View Post
At the moment, I'm hovering between purchase of a lightbox off E bay and making my own, so I value your input.
If you can find a used box on e-bay, fine, but for my money, if the outfit which is selling off the ex Maplin Insect UV zappers referred to in the 'Light Box' thread still have any left, I'd convert one of those, per the YouTube video. Well reasoned, well constructed, and economical. Much easier than gathering all the bits together from here and there, and cheaper even than a pair of UV tubes from. Mega or wherever.

Though I built a timer into miy box, as it was part of the magazine design, I wouldn't bother with one - you can just use the second hand on a watch or stopwatch. The basic (but expensive) boxes sold by the likes of Mega don't have a timer, and those that do are often mechanical and not very accurate.

Good luck in your quest.
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Old 12th Jan 2017, 10:15 am   #7
Terry_VK5TM
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Default Re: UV Photo-Resist Dry Film PCB Process

I use a timer on my light box, as you can bet the phone will ring or some other interruption occur.

Also used a very similar system to Davids in the ~30years of making boards semi-commercially, but changed to using UV imageable ink instead of the dry film (although I still have the equipment for applying dry film).

The ink (TER20 by Taiyo), while a little expensive maybe for home use, is negative acting and a teaspoon would cover around a square foot of pcb material. I used a rubber craft roller to apply it, but it can be screened or even sprayed on.

Commercial outfits using the ink usually curtain coat the boards (curtain coating is passing the board through a vertical stream of the ink).

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