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Old 18th Jul 2017, 7:48 pm   #21
Oldcodger
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Default Re: UV Dry Film PCB technique - experiences to date.

Just opened my photoresist to find this little gem ( attached) in CHIN ENGLISH.
Reads like the test question on "who wants to be a millionaire" .Suspect someone put the Foo before the Egg - with respect to the protective layers .
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Old 19th Jul 2017, 6:41 pm   #22
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Default Re: UV Dry Film PCB technique - experiences to date.

Having had a closer look at the resist sheet, I've noticed that one side is a purpley colour, and removing this leaves a clear sheet. I'd have thought from what David said, and the "instructions ( last post), that the resist was the middle of a sandwich.
As the purple layer is outermost on the roll, I'd assume that the clear thin layer is the resist- anyone confirm?

Last edited by Oldcodger; 19th Jul 2017 at 6:48 pm.
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Old 19th Jul 2017, 9:21 pm   #23
David G4EBT
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Default Re: UV Dry Film PCB technique - experiences to date.

No!

There is a clear transparent protective film on each side of the UV film.

When you've cut the film to size, you have to put a small piece of sellotape on each side of the three-layer film and gently pull just one side off. (Doesn't matter which side peels off). Then you carefully apply the UV film to the laminate (with the second protective film stil in place), excluding air bubbles before putting it through the laminator to bond the film to the laminate.

If - when you've put it through the laminator there are any bubbles, you might as well peel off the protective film, clean off the UV film from the copper by asking it in acetone for a minute or two and have another go. The film is so cheap that it's only cost you your time!

After you've exposed the PCB, you then peel off the second sheet of protective film before developing the PCB. The developer dissolves all the areas that were not exposed to UV - all the areas (tracks, pads, ground planes etc) that received UV light will remain in place as the 'etch resist'. This is of course the opposite of positive UV.

It's a bit of a knack to peel the protective film off, but you get the hang of it after a few goes. It doesn't matter which side you peel off first.

(There are videos on YouTube).

Hope that helps.

Good luck with it.
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Old 19th Jul 2017, 9:58 pm   #24
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Default Re: UV Dry Film PCB technique - experiences to date.

For clarity, David, what colour is the UV film. So far ,I've ended up with a clear bit and a UV coloured bit . From what you say, clear bit is the protector.
One side came off with no problem before laminating. Biggest problem was stopping the sellotape "mating", i.e sticking together.
Your explanation helps makes sense of the "instructions " that came with the film. I found it easy enough getting one clear film off the resist. But the rest of the "instructions" that came with the film were more of what I'd expect in an Oriental user manual .
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Old 20th Jul 2017, 12:40 pm   #25
David G4EBT
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Default Re: UV Dry Film PCB technique - experiences to date.

You peel the clear protective film off one side only initially - it doesn't matter which side you peel off.

Then you lay the unprotected side of the purple film to your PCB laminate, taking great care to exclude all air bubbles. The you put it through a laminator to seal the purple UV film to the PCB. From my experience, you need only one pass and if you end up with bubbles in the film, it doesn't solve that however many times you pass it through the heated laminator.

Assuming that there aren't any bubbles, you then put the PCB in the light box with the negative UV mask and expose it. Having done that, you peel off the upper protective film and place the exposed PCB in the developer. Check for any blemishes and touch them up with whatever - modelling paint, rub down transfers, nail varnish etc. If you're in luck, that won't be necessary.

I don't use pan-scrubs etc to clean the laminate before applying the film - I use 0000 grade wire wool, otherwise you risk creating score marks in the copper which can result in tiny cracks across tracks after etching, which are only visible with a magnifier. If you tin the board, that might bridge the cracks but it's best to not create them in the first place.

I've attached a sketch to make things a bit clearer.

Hope that helps.
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Old 20th Jul 2017, 3:37 pm   #26
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Default Re: UV Dry Film PCB technique - experiences to date.

Many thanks ,David - great help. I've managed to get the test piece exposed and strangely enough ,it was the same side of the protector that came off each time. With the board exposed, the other protective layer decided to part company. And that was the end with the highest exposure,( I decided on four slots of approx 35secs ) ,divided into four parts of the board, so perhaps I may increase the exposure time by another 35sec.
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Old 20th Jul 2017, 6:20 pm   #27
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Default Re: UV Dry Film PCB technique - experiences to date.

To add to my last post, and possibly give others food for thought. Two boards I'm in process of making are 2.3x4 in and I found that on one side of the film selotape took nicely, but on the other side it wouldn't . So I moved on .Ive had a roll of double sided tape kicking around for years .Fit two strips the right size to a scrap bit of wood ( I used an offcut of Contiboard) place one side of the cut film on this- use selotape on the other ( the side that took the tape well) and hey presto - film left on wood. As a bonus , the used tape + film is easy to remove from the wood.
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