Homebrew PCBs

On 24/03/12 21:24, Philip Pemberton wrote:
> On the 170-second side, step 2 became more prominent (but was only
> partly visible). Step 1 was clear (aside from the "Stouffer T2115" text).
> 
> Is this roughly what I should be aiming for?

And the answer is.... no!

I exposed a board at 170 seconds. It had a thin film of photoresist
coating the entire board -- almost invisible, except when it was dunked
in the etchant and the copper didn't change colour. No matter what I
did, this film wouldn't go away.

So I figured I was about half a stop low on the exposure:

  170 * 1.414 = 240
  (240-170) / 2 = 35
  170 + 35 = 205 seconds.

Exposing at 205sec produced a good image (the developer turned a VERY
deep blue this time around -- something to watch out for, at least). The
"clear" areas really are clear (down to copper), and the non-clear areas
are covered with photoresist.

Steps 2 and 3 have now merged (3 was just about visible, but washed off
in the etchant; 4 is ~50% covered). So it seems the rules for Microtrak
are quite simple:

  1) If step 2 is visible, you're underexposing.

  2) If you can see the outline of the step wedge, you're DEFINITELY
underexposing.

  3) If in doubt, add half a stop. You most likely won't overexpose the
photoresist, but you will improve your safety margin.

Lord only knows how this applies to negative photoresist. I'll have to
order some from the previously mentioned Greed-bay sellers and try it.

One of these days I'll figure out the secret to this... and maybe write
a little shareware app to calculate exposure figures based on density
values.

-- 
Phil.
ygroups@...
http://www.philpem.me.uk/