Homebrew PCBs

Adam Seychell <a_seychell@...> writes:
> Ok. so when you exposed the resist for 2 minutes with the step gauge in 
> place, then what you observed at step 5 was slightly unexposed 
> photoresist. Regions under steps 1 to 4 therefore must of been fully 
> exposed. Correct ?

Yes.  1 to 4 were fully exposed; they got enough UV to cure.  Recall
this is a negative resist, transparent = UV = cure = copper.  Steps 6
through 21 were uncured, and the copper removed when etched.

http://www.thinktink.com/stack/volumes/volvi/filmimag.htm#Calibration

The step gauge is transparent at step 1 and opaque at step 21, so more
UV means more steps 1..N cured.

> The light at step 5 was attenuated by factor of 2^(-5/2) = 0.176. So 
> would I be correct saying to achieve the same exposure energy without 
> the step gauge then exposure time will be 2 minutes * 0.176 = 21 seconds ?

I suppose.  That would mean a 21 second exposure would cure some but
not all of the film, hardly useful for PCB work ;-)

> But this exposure energy doesn't quite achieve full exposure, so the 
> final exposure time use for PCBs will be somewhat longer.

Yes.  There is no point in calculating the 21 seconds result.  That
exposure will not cure the film adequately, and you'll never actually
run your box for that time.  The *only* number that you care about
with the film is the time it takes to get an exposure that holds the
step you need to hold.  So once you figure out the exposure that holds
the step you need, that's how long you run your box to expose your
boards.  It doesn't matter what step '1' works out to be because
you'll never use it.

> The data sheet for some resist I once had said 10 to 15 minutes hold time.

The riston data sheet doesn't say, but thinktink says 15 min to 8
hours.  http://www.thinktink.com/stack/volumes/voli/store/specs/m115spec.htm