On 22/03/2010 19:17, Philip Pemberton wrote:
> 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,
I orginally used a laser printer with Laserstar film, but get much
better results with an inkjet printer. No pin holes, for starters.
Leon
--
Leon Heller
G1HSM