In a thread now closed, some months ago I mentioned that this technique - which was new to me at the time - seemed to hold much promise, and seemed maybe a little too good to be true! Up to that point, for many years I'd used self-spray positive resist UV lacquer, which can be difficult to apply in terms of excluding dust and pollen floating around, indoors or outdoors. Generally, I held a plastic container above the board to exclude dust while spraying, then lowered the container over the board while it dried, then put it in a desk drawer overnight to harden off. It has to be on a perfectly level surface when spraying, or it runs to one end and leaves an uneven layer of lacquer. (I haven't used pre-sensitised boards for many years as I always ended up with useless offcuts, and with self-sprayed boards, if they're under-exposed, you can always wipe off the lacquer and re-spray the board).
Last Autumn I learned of the negative UV dry film technique and from youtube videos it seemed to hold much promise. The film is cheap and sells in large quantities on e-bay, both with suppliers in the UK and from China. Typical ebay prices are UK suppliers £7.99 post free for 30cm x 5M roll, China, 15cms x 2 M £3.00, 30cms x 1M £2.50 post free.
Initially I tried a 15cms x 2M roll, but it had been squashed flat on it's way to me, so the film was creased and wrinkled and even when the backing film is stripped off, the wrinkles remain. OK for very small boards - say 25mm x 50mm, but no use for anything larger. I then bought a 2M x 30cms roll which came on a still cardboard roll, but nevertheless, it has a few wrinkles as it comes off the roll, and I've found it next to impossible to exclude every single tiny air bubble when applying the film. When passed through the laminator, these air bubbles become more prominent and when the board is developed, it leaves a few blemishes which - if etched - would eat into the tracks.
I recently designed a PCB for the AM/FM converter which featured in Summer 2017 BVWS Bulletin, the original of which built on strip-board, which I dislike with a passion. I've had two attempts today and applying the film and exposing and developing the board. Not happy with the first attempt, so cleaned off the exposed and developed UV lacquer with acetone and had a second go. The second one was more successful. Firstly, though most videos show several passes of the board through the laminator to attach the film to the PCB, only one pass is necessary and any more seems to wrinkle the film and make matters worse. This second attempt left just a few blemishes, which I'll use rub-down transfers to correct prior to etching.
The board is a simple one and only 10.5cms x 4.5cms, but it seems to me that the larger and more complex the board the less the chances are of a successful outcome. However, the upside is that the layer of resist left on the board is quite thick and can be felt by running a finger over the tracks. Exposure time seems uncritical - I used 1 minute in a 2 x 8W box. The sodium carbonate ('soda ash') used as the developer is cheap - typically £5.50 a Kilogram post free on e-bay. Only 5 gms (one level teaspoon) per 100ml of water is needed, so 2 level teaspoons is 200ml of water is ample. I used an empty margarine container and ran a piece of kitchen roll over the board to wipe off the dissolved lacquer, then rinsed in clear water.
I've attached a pic of the board showing the blemishes.
The artwork for the mask has to be a negative but it only takes seconds to change a positive to a negative using MS 'PAINT. You open the file in ‘PAINT’ then use the ‘Select’ option and with the mouse, select the image. Then right click on the image and a drop-down menu will appear. The bottom option on that menu is ‘invert colour’. Select that option and left click, when it will invert the colour to create the negative. Click on ‘save as’ and rename the image ‘PCB negative’ or whatever. I then reverse the image, so that when printed, the ink will be on the side of film next to the PCB in the exposure box to prevent 'undercutting' the tracks when exposed. By adding some lettering to the design, that obviates and confusion as to which way round the artwork needs to be when placed in the box. If you can read the lettering, it's the right way round.
I use 'micro-porous film' and an ink jet printer for the UV mask - totally opaque - far superior to OHP film, and far better than a laser printer. (Most youtube videos using masks printed on laser printers use two or even three layers of film taped to each other to get sufficient opacity). I print a sheet of two positive and two negative layouts to keep my options open. Attached below is the track layout I've devised. Though I've carefully checked it against the circuit, until the converter has been built and tested, I've added it just for interest. The terminations on the little FM module - 'U2' are non-standard, in that unlike DIL ICs, they're not on a 2.5mm grid. I've correctly spaced them to enable the module to be soldered in using veropins.
I know that homebrew PCBs are a minority interest on the forum and my prime reason for this thread isn't to open a discussion about the merits of various PCB techniques or other options - it's simply about the dry film UV technique. I'd be interested to learn of anyone else's experience if they've used the dry film process as a hobbyist, and if they've found a successful way of fixing the film without blemishes. ('Big Clive' who has quite a following on youtube applied his film by wetting the board with water, apparently with success, but I've not tried that).
Has anyone given it a go please?
Previous thread:
http://www.vintage-radio.net/forum/s...=132065&page=2