DJ Delorie wrote:
> Step 8 is a good exposure point for negative films, I don't know about
> positives. Based on the photo, I'd shoot for an exposure time of around
> 85 seconds - your step 1 (340 seconds) is overexposed, but by step 8 (21
> sec) you're underexposed.
I just did a test run at 75 seconds based on some times I found posted
to a forum. That worked pretty well -- the board is in the etch tank at
the moment. It does seem to need a shade more exposure, though -- there
was a little bit of partly-developed coating left in places. Basically,
it's turned blue but not detached from the PCB.
> Note that "step held" is the highest step that's still getting barely
> enough light to do something, so in your picture you're holding
> approximately step 12 - the highest step that still shows any trace of
> your inkjet pattern. Correcting for that also results in an exposure of
> around 85 seconds.
>
> When using a step wedge, you don't always go by which step has the best
> ∗pattern∗, but usually by which step has ∗anything∗.
So let me see if I've got this right... On positive-acting resists, the
step held is the first step that isn't completely black; on a
negative-acting resist, it's the first one that isn't completely clear?
Well that explains a lot. It also implies that my exposure times for the
0.8mm CIF laminate are way off in the weeds somewhere. Although if that
stuff can handle 4x the required exposure and still produce an
almost-perfect image, it has a VERY impressive exposure latitude.
I've got a couple of half-inch wide FB2 offcuts in the bin... I might
have another go at the step-wedge adjustments with the LaserStar film.
Who knows, maybe my laser printer is putting enough toner down, although
I bet I'll still need to use my Isograph pen (a Rapidograph with a
refillable ink tank instead of a cartridge) to fix the pinholes in the
ground planes...
Thanks again,
--
Phil.
ygroups@...http://www.philpem.me.uk/