Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

For initial exposure testing remove the variable of the negative by instead
placing guaranteed opaque objects on the photosensitive layer.

For example place a coin and maybe black electrical tape.

It is easy to be fooled by a poor negative that is partially transparent.



Also do a step exposure. Double the exposure time for each test. You could
have multiple little PCBs or just one and selectively cover areas for the
same result.

Expose the whole board for let's say one minute.

Cover one section of the board with a second PCB or something opaque.

Expose the rest of the board for another minute and then cover up an
additional section.

Expose the rest of the board for 2 minutes, cover up...

Expose for 4 minutes, 8 minutes, 16, .

After development you can see how long time is needed for the exposure.



Afterwards you need to evaluate your negative to see how much light you lose
and how transparent the black it is. Ideally you should not lose anything
and it should block 100% in the black areas.

As DJ suggested, a step wedge is very useful and a big timesaver. When I
used to do PCBs I always included one on the edge of the PCB to verify that
the process is under control.



As many have suggested, there are lots of UV sources that are repeatable and
works 24 hours a day.

Bertho



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