Perhaps one of the chemists/physicists here can explain a problem I had making a Printed Circuit Board.

I have made my own PCBs for more years than I care to remember by printing a 'photo-negative' transparency, then laying photo-sensitive board on this, and exposing it to UV light using a UV lightbox. The result is the copper I wish to keep is protected by the photo-negative, and the copper I wish to remove is exposed to UV light. The board is then 'developed' in sodium hydroxide, before etching. I have successfully made hundreds of PCBs by this method.

Today, I wanted to make another board (set of limit switch mounts for a CNC machine). I produced the 'photo-negative' in the same manner I always do – and this is where I was hit by encroaching senility – first I placed the PCB board on the light-box, copper side up, then placed the photo-negative on top of it (In the wrong order, and I know, but it comes to all of us eventually!).

I then exposed it for the usual time, and developed as usual. Then I had the first inklings something was not right – the developed board seemed to be a very slightly fuzzy negative of what was required. At this point I had not realised I had placed the negative/board in the lightbox in reverse order. Eventually I realised what I had done (so the board was scrap), but then began to wonder how this can happen – how did light get through (or around) the board to create a pattern on the top of the board – the side away from the UV light? And why was the resulting pattern 'reversed' – that is, the track areas were exposed (so would later etch away), and the areas between tracks left solid?

I could just imagine light leaking around the board to affect the top side, but this had the photo-negative on it, and when the UV light-box lid is closed, the whole 'sandwich' is pressed down by a layer of black plastic foam, so it is hard to see how this leakage happens. The board is 1.5mm thick fibre-glass, coated on one side with copper, which would also block light, I assume. So why did the pattern appear on the side of the board away from the UV light, and why was it negative?

Once I realised what had happened (but not why), I used the same photo-negative and a board from the same batch to make the PCB correctly, and it came out perfectly, so I don't believe there was a problem with the board.

I know the mistake I made, but am at a loss to explain why it worked as it did, so am interested to see any suggestions.

PGK – The light box is produced specifically for PCB exposure, and when shut, the aluminium box does not seem to provide much chance of extraneous light ingress – it closes tightly, and is clamped shut by the catches. Internally there is about a 1cm thick black foam cushion which presses down on the board, transparency – I usually do process in a 'fairly' dark room, because I need to handle the board for maybe a minute with no protection, to set it up in the box – then I just leave it in the room. I'll take a look and see if I can improve that, but again I have to note that the quality of the 'reverse developed' board is fairly good – much better than I would have expected under low light exposure conditions.

Martin: the 70 seconds exposure time was arrived at experimentally – I exposed a number of sample pieces at differing times, until I arrived at what seemed to provide the best definition – 70 to 90 seconds in my case. Longer times (~ 200 secs) ruin the board – it appears grossly over-exposed, and when etched, small details disappear, so I think you are correct, and I would appear to have a brighter lamp than you. I have also noticed that old sensitized board has similar effects. From what you say, the board itself would be heated – I have to admit I did not check that (something else to check!). I am not sure what the wattage of the UV lamps are (will check), but the box is sold as a professional UV developing box for PCBs, so I just cracked on and used it. It does do the job perfectly if I load it up correctly – this effect we are discussing was due to my error in loading the board and transparency incorrectly.

I have tried the transparency 'turned over' so the ink is not in contact with the board – no difference, but I will try as you suggest and leave the lamp off, to see if it is some other cause (can't imagine what would do it though!)

Incidentally, I use an inkjet printer to produce the transparencies. It can be set to a very dense print, and I much prefer it to the Laser printer. I cannot seem to achieve anything like the same density of print with the laser (big disappointment – I bought the laser for this work – now its just a door-stop) – if I examine the transparency from a laser under a lens, it seems to show very thin lines which transferred to the PCB, whereas the inkjet printer produces a solid dense printout. (its an Epson 3150) – it has access to almost every printing parameter I could wish for, and it is possible to set the density of each colour (i use black) to a very wide range, and also brilliance and contrast.

I do appreciate that the laser printer could also be set up incorrectly, but does not seem to be able to produce as dense printout as the inkjet, and the inkjet is so much quicker.

Thanks for the thoughts, everyone – this has got me puzzled.

Hi, Martin.

It seems you are making PCBs on a professional basis – mine are just for hobby use, although I have been doing them for a long time. I use the traditional etch method with a 'bubble' etch bath, but I now have a small CNC router which I intend to try and use for milled boards – reason being I have a box full of 'rejects' over the years using the photosensitive etch method, and they are scrap unless I use a different method. I find 'spray-on' photolacquer is pretty useless. I was making limit switch boards for this router when I made my mistake and noticed this effect.

I have etched one of the boards – its definitely 'reversed'. I will try to get some photos, but my camera is giving me trouble, so I can't produce good close-ups immediately.

For info, my lightbox has 4 x 14 Watt tubes covering an area about 5mm less than A4 dimensions. It is made by Gie-tech GmbH in Germany. I usually use genuine Bungard photo-sensitive glass-fibre board, usually 1.5 mm thick, both single and double sided.

The effect is reproducible – I tried again this afternoon, and it did the same thing. I also tried the transparency both ways up, to see if anything was transferred to the board, or a chemical effect, but no difference. I also tried, as you suggested, putting a sample in the box without the UV light on, and nothing happened.

I did think that it was possible that if the ink on the transparency was not fully dry, it could transfer to the board, but on second thoughts, the copper which is now 'exposed' is under the ink, so when it is etched, it disappears – if ink was transferred to this area, it would at least hinder etching, if not stop it altogether – so its very puzzling.

I appreciate your interest and efforts, and of your colleagues, to find an answer – I must admit I am thoroughly stumped – its going to keep me thinking for a while yet. If I come up with anything, I'll add it to this thread and 'bump it up'

Just an idea, since it is reproducible, if the 'inverted' effect, however it happens, could be fine-tuned to produce fine detail, it would be a simple way to produce a double-sided board from a single transparency! – I might get rich yet!

Best Regards,