Homebrew PCBs

Philip Pemberton <ygroups@...> writes:
> My understanding is that my goal is to have a "step held" (the first
> step that's <50% exposed) at step 8. In my case, with positive
> photoresist, that means the first step that's less than 50% clear
> copper should be step 9.

The "last step held" is the last step that has *any* resist remaining,
at least with negative resist.  I've found that you get one step
completely cured, one partial, and one completely missing.  So if you
want to hold step 8, then step 8 should the last one that has any
resist remaining.  If step 9 has any remaining, you've held step 9
instead.  I suppose you could fine-tune your exposure by aiming for
50% held on the last step, but I don't think PCB work needs that kind
of accuracy.

Note that the step you should hold depends on the film.  Riston wants
8, but others may want other steps.  If you don't know what your film
wants, see how many steps your artwork blocks, and aim for a step in
the middle.  Example: expose a stripe pattern artwork through a step
gauge for 8x or 16x your expected normal time.  This should give you
something like this: http://www.delorie.com/pcb/dryfilm2.html

From that, I can see that steps 6 through 13 "work", the midpoing
being step 9 or so.  So whatever that exposure was, I'd use 1/16 of
that time (step 9 down to step 1 = 8 steps, or 2**(8/2)).  This gives
a "normal" exposure that's the same exposure as what that test strip
saw at step 9.

If I decided that step 7 was the best result, I'd use 1/8 the exposure
time (6 steps = 2**(6/2)).  Etc.