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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.

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  • \$\begingroup\$ It's going to be difficult to give a universal answer because there are many different types and formulations of photo-mask. Try to be more specific about what you're using, what your procedure is, and what exactly is going wrong. \$\endgroup\$ – W5VO Aug 26 '16 at 17:40
  • \$\begingroup\$ Can you post a picture of one of the failed boards? Maybe some one here can tell by the appearance of the finished board what is going wrong. \$\endgroup\$ – JRE Aug 26 '16 at 17:40
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    \$\begingroup\$ An hour under uv sounds awful long. Unless you have a terribly weak lamp, exposure time ought to be closer to 10 minutes. \$\endgroup\$ – JRE Aug 26 '16 at 17:44
  • \$\begingroup\$ If you are exposing the boards for an hour, then working with them in normal light for a few minutes, and they aren't working out, the I'd bet on over exposure to the UV lamp. \$\endgroup\$ – JRE Aug 26 '16 at 17:49
  • \$\begingroup\$ Also, it's not UV "Blacklight". That's too much of an erradic field and wide range of relatively low energy UV wavelengths to really do something with. UVResist commonly has a sensitivity below 390nm, where anything below 240nm will transfer very efficiently through black inks. \$\endgroup\$ – Asmyldof Aug 26 '16 at 17:55
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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.

  1. Uncover 5mm of the PCB (hence the reason we are using the metal ruler), and expose to the 'UV Blacklight' for 5 minutes.
  2. Repeat, i.e. uncover a further 5mm (now 10mm are exposed including the 5mm in step 1) of the PCB, and expose to the UV Blacklight for 5 more minutes.
  3. Repeat for 15mm and 15 minutes, 20mm and 20 minutes, etc, until their is only 5mm remaining unexposed.

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.

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    \$\begingroup\$ It's like photographic darkroom test strips, but with film on copper on fiberglass. The darkroom's fortunately not a lost art! \$\endgroup\$ – user2943160 Aug 27 '16 at 1:08
  • \$\begingroup\$ @user2943160 - Exactly! However, I didn't think many people would remember that, and even fewer how to do it. Also, in my limited experience (and I always use the same PCB material because it works well), a few seconds here and there under 'office lighting' doesn't seem to effect that photoresist. \$\endgroup\$ – gbulmer Aug 27 '16 at 1:32
  • \$\begingroup\$ Rather unlike most usual photographic materials. Between frequency response and fairly low sensitivity, the photoresist is probably easier to handle casually. 5 of 6 semesters of a photo minor doing traditional black and white process will certainly keep that process and its caveats in mind. \$\endgroup\$ – user2943160 Aug 27 '16 at 1:36

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