Homebrew PCBs

On 28/03/12 07:00, DJ Delorie wrote:
> Assuming your film wants a "hold at 8 on a 21" (riston films do), you
> want the results of your test to show that step 8 had film, but step 9
> didn't.  More exposure = higher steps.  So if you're holding step 4 at
> 340 seconds, and you want to hold step 8, you have to expose for 4x the
> time, or 1360 seconds (~22 minutes).

That's fair enough for a negative resist... the thing is, Fotoboard and
Microtrak are positive-resist boards (they're coated with a liquid
photoresist in various ways; Fotoboard is dip-coated, Microtrak is
roller-coated). Basically, it starts out "hardened" and UV exposure
softens the resist enough to wash away in the developer (typically
sodium metasilicate).

Riston is, IIRC, a negative dry-film resist which hardens when exposed
to UV, and is developed in a sodium carbonate based developer.

Exposing for as long as that will undercut the artwork (if you have a
VERY good phototool master) or just plain fog the whole thing (so all
the resist washes away). It seems with positive resists you want enough
exposure to clear the photoresist from the background, plus about half a
stop.

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