For the positive photo-resist method of making circuit boards, I use UV blacklight to expose the board (with the artwork on it) for an hour, just so that I can make 100% sure its exposed properly.
Because the photo-resist layer on a circuit board is sensitive to light, what I like to know is, from the time I turn off the UV blacklight (to finish the exposure), how long can room light shine on the board before the board renders useless when it is being developed in a standard PCB developer solution?
or should I be super paranoid and work in a pitch-black environment starting from the moment the board is finished being exposed until I successfully predict the moment that the unwanted photo-resist is removed?
I ask this because I keep failing at making circuit boards from using the standard sodium hydroxide developer, and I'm looking into ordering sodium silicate, but until I get some, I'm trying to understand if working in pitch-dark settings actually makes a difference.
If not, then I'll ditch the sodium hydroxide and wait for sodium silicate.
Best Answer
If you keep failing to make a board, it is probably because you have exposed the board for too long to the 'UV Blacklight', or the UV Blacklight is virtually usless.
I have used a 'professional' UV exposure box, and it takes about 100 seconds. It uses 'proper' UV tubes, which cost about 20GBP for two 15W tubes, which will expose an area bigger than A4. Hence the reason I think your PCB might be overexposed or the 'UV Blacklight' is virtually useless.
I have handled photosensitive PCB material for a couple of minutes in indoor fluorescent light with no ill effects. I keep the material covered in ordinary 'kitchen paper' while moving around a lighted building between exposure box, developer, and etching tank. However, your material may be less forgiving, so use a red light, or bunch of red LEDs to work, so that you don't get too paranoid, but can still see what you are doing.
First try to calibrate how long it takes to fully expose the photoresist. This will not make a usable circuit board, but will save you wasting more PCB material. Edit: it is the same process as making a photographic exposure test strip, or darkroom test strip. You should be able to find explanations by searching the www.
Cut a strip of PCB material, about 1cm wide and the width of your PCB material, let's say 5cm. You are going to expose it in stages so that you can estimate approximately how long it takes to change the photoresist through exposure to your UV Blacklight.
Cover the strip of PCB material with something UV opaque. Use a metal ruler, with some paper between it and the PCB to avoid scratching the photo resist.
NOTE: I assume you are not being exposed to the 'UV Blacklight', and have some way to cover the PCB and UV Blacklight so you are not exposed to it, or you have a timer, and can switch it on while you are well away from it. I am making this point because I really have no idea how much UV is emitted by your UV Blacklight. It may be dangerous, or utterly useless.
Develop the photoresist.
When you etch the PCB you should have some copper and an area of no copper. Measure that, and you should know at what time the board was being overexposed. Of course, it might not be exposed at all, if the UV Blacklight is virtually useless.
You should now have a an idea, within 5 minutes, of how long the PCB photoresist needs to be exposed to your UV Blacklight in order to develop an image, or you now know that you need to buy or make a UV exposure box.
If the board was overexposed in the first 5 minutes (which would indicate you have 'proper' UV tubes), repeat the whole procedure but in 30 second steps starting at 0 seconds. Hence the reason for only using a 1cm wide strip, this may take several experiments, and you don;t need to waste much board material in each experiment.
For fine detail, you might need to do this all again, centred around the time when the board went from copper to clean, but at smaller time steps, to identify a more accurate exposure time. For the UV exposure box I have used, we needed to get the time within 10 seconds for finer detail, i.e. 0.25mm track and space.
Please report back if the board is underexposed after an hour. IIRC that would suggest that the UV blacklight is less effective than exposing the board outside on a bright, but lightly cloudy day.