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Q.: I have built a contact printer with two 8 W UV tubes, but after an exposure of three hours nothing happens.
After three hours, actually, all has already happened; the photoresist
has been exposed everywhere in spite of the mask. With UV or Wood
tubes, infact, the needed exposure time is very shorter than with a
solar light bulb. In general, you can try from a few minutes to
ten-fifteen minutes. To determine the correct exposure time you could
try with the method described on the article The
photoetching of printed
circuits - finding the correct exposure time
on this site.
Q.: Even with an exposure time longer than usual, in the development solution tracks become just slightly visible only after several minutes, and after one hour the exposed photoresist hasn't still been removed.
A.: If, when prepared, the concentration of the development solution was suitable (10 g/l or more), the solution could have become ineffective. This can happen after some weeks or months from preparation or from last time it was used. It is also possible that half a litre of solution can be enough just for one breadboard, for example if it is a 20 cm x 20 cm double sided breadboard. Depending on the photoresist type, also a strongly coloured solution may point out it could have become quite ineffective.
Q.: With the same exposure and development times until now successfull, since a bit of time I can't obtain good results anymore. Photoresist is under or overexposed.
A.: If your presensitized boards come from a different supplier, photoresist could be different, and could therefore need a different exposure time. You should carry out again sensitivity tests. You could find the same behaviour when using a spray photoresist different from your previous one.