Make a proper PCB exposure unit out of a cheap UV nail curing lamp

What do PCB production and fake fingernails have in common? They both use UV light sources of high intensity and, as luck would have it, those light sources have exactly the same wavelength. Only the ones for PCB production are usually quite costly and the ones for fake fingernails are a bit more competitively priced.

This instructable is about how to use such a device to build a low cost light source, suitable for exposing the various UV sensitive materials encountered in printed circuit board production, like dry film photoresist and UV curable soldermask.

As well as being very low cost (around $20 for all required materials), this build addresses a few issues I’ve seen on other devices on the intertubes:

• Collimation: To simply expose a board with fairly coarse features, you wouldn’t need to do any of this. You could just use the nail dryer as is and call it a day. But to be able to expose small features (down to 5mil, according to this site), you have to make sure all your UV rays come from the same direction, which is exactly perpendicular to the board you are exposing.
• Uniformity of illumination across the whole exposure plane. Imagine you want to expose a really big board, e.g. A4 or letter sized. You’d want the same amount of energy over the whole board, without hot or dark spots. For this, the energy source has to have a certain distance from the exposure plane and you need either a very tightly packed array of UV sources (like UV-LEDs, which can be rather pricey), or an effective reflector design for the UV sources you have at hand, which is what I came up with.
• Exposure time: I have no idea how fast this source is with pre-sensitized positive copper clad material, as I’ve never used that stuff, but with dry film photoresist it feels really fast. Like under two minutes fast. The thing is, I’m not really qualified to properly interpret the results, so I have to gather a few more opinions on this one.

So, while being very low cost, this build will enable you to achieve results that match, or (in some cases) even surpass those of devices that are up to 10 times more expensive.