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Old 23rd Jan 2017, 6:44 pm   #8
David G4EBT
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Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,737
Default Re: UV Photo-Resist Dry Film PCB Process

Well I've stuck at this, and it's not proved as simple as depicted in the youtube video, but after several attempts, I've managed to expose and develop a board which I'm happy with, though I've yet to etch it. The UV film has a protective film either side of it and it matters not which side is peeled off to apply the middle UV film to the board. Peeling off the protective layer is an acquired skill which takes some mastering. By attaching a small piece of Selloptape at a corner and another on the other side close by, then gently pulling, one side or the other will peel off. After exposure but before developing, remember to peel off the other side, which detaches easily from the corners of the exposed board.

In the video - and in others in which the use of UV negative dry film is used - there appears to be no difficulties if getting the film to lay perfectly flat to the board. Little attempt seems to be made to smooth the film down, yet it emerges from the laminator blemish free. That hasn't been my experience. The film I'm using appears to be identical to that used in the (excellent) youtube video and at the 3 minute stage in the video when he unrolls the film, ripples can clearly be seen in the film as there are in mine. In my case, when the PCB with film attached is passed through the laminator it leaves small bubbles in the film and from that point there's no use in exposing and developing the board as those bubbles turn into holes. I've tried using an electric iron on the coolest setting with several sheets of paper so as not to melt the film, but again, that produced no better a result.

At about the fifth attempt, I did ultimately manage to get a bubble-free finish by carefully selecting a piece of film with few ripples and gently smoothing it down from one end.

I've experimented with various exposure times and developer concentrations. In the youtube video he used an exposure time of 40 seconds with a four tube box. I'm currently using a 2 x 8 Watt box and the optimum time is 80 seconds. I discovered that by masking off a board so that half was exposed for 40 seconds and the other half for 80 seconds. At 40 seconds, the board was under-developed.

As to the developer concentration, the video suggested between 1 & 2 grams of soda ash (isinglass) per 100 ml of water. That's quite a wide variation, and 100 ml of water isn't very much anyway. 200 ml is about a cupful so I used 3 grams to a cupful of warm water as the powder doesn't dissolve well in cold water. (1 gram is a level teaspoonful).

I've attached a couple of pics, the first of which shows a board with film which has been passed through the laminator a couple of times, showing the ensuing blemishes. With a result like that, there's no point going any further so the protective film needs to be peeled off and the board immersed in acetone for a couple of minutes when the UV film will float off. Then the board has to be cleaned, washed, dried and a new piece of film attached in the hope of better luck next time. The trouble is of course, that if you always do what you always did, you always get what you always got. Or as Einstein rather worryingly put it:

"Insanity is doing the same thing over and over again and expecting different results".

The second pic shows the end result by taking great care to select an unblemished piece of film and very carefully smoothing it down. Of course, the smaller the board, the great the chance of success. I'd estimate the board in the youtube video is about 3cms x 7.5 cms max. My board is about 7.5 cms square.

It's important to ensure that all the the unexposed resist where the black areas of the mask are, which will be dissolved in the developer, is fully removed. It turns grey in the developer and can be rubbed off with the fingers under a running tap and you need to dip it in the developer and under the tap rubbing with fingers till you're happy that the resist has been fully removed. Still on a learning curve with this - I could have produced a PCB much quicker than this using positive UV lacquer, but ultimately, I think this negative film process does have merit and at £5.97 + £1.59 from a UK supplier for 2M x 30 cms, it's a fraction of the price of UV lacquer.

The resist which remains on the board is quite thick and the edges can be felt by fingers.

I hope these notes on my experiences to date are of interest to someone out there, and if anyone else has tried this process, I'd be pleased to hear of their experiences.
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