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

I am a little surprised at the long discussions about selecting exposure for
just the photo resist. To me that is artificial and not real life. To make
PCBs, there needs to be an artwork and a hold down vacuum frame or glass.
Both will absorb some UV so that will affect the result and the "opaque"
sections of the artwork are not really perfect, they will let through
unintentional UV that also will affect the outcome.



Further, there is undercutting by light creeping around the edges and
development and etching issues. In an ideal situation, the variables should
independently be measured but most hobbyists do not have all the equipment
to do accurate measurements of the individual variables. Also affecting the
result is how the light source is colliminated. So to repeat my previous
suggestion:



Make a parallel test pattern of the skinniest lines and spacing required and
long enough to match the step gauge. Run an exposure test as the real
boards will be exposed, develop just the resist, eyeball the result,
readjust to be in the ball park, run the test again, develop and etch with
the real etching process to be used. Evaluate the result and measure line
width and spacing. From there pick a safe middle exposure value. If the
artwork is too poor it might not even work at all. In that case wider
traces might have to be used and or the artwork improved.



Also for the tests above I leave a little piece of PCB not covered by the
artwork (punch a hole) and I also place a small piece of aluminum foil near
the test pattern. That will give you two references: A perfect opaque
pattern and a perfect transparent pattern.



Since we are on this subject, I see little discussed about the artwork
except different toners, transparency material and so on. Since the only
way to have a perfect artwork would be a metal mask with holes (not
practical) we really need to discuss the ratio between the clear and the
dark areas. It is that ratio that is critical, not just how dark is the
toner.



As an extreme example, using transparencies they have good clear performance
but so-so dark areas. Printing on paper gives much better toner coverage
but much worse "clear" areas. Which material has the best ratio? You might
be surprised by taking the paper artwork and placing the toner downwards and
run a quick test. Exposure will be longer than the clear artwork but the
black is much blacker so you might like the result better than some other
processes.

Happy Etching,

Bertho



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