Homebrew PCBs

Hi,

Can someone please help me interpret the results I'm getting from a step
wedge test?

I'm working with "Microtrak" pre-coated boards and JetStar Premium
inkjet film. The problem with Microtrak is that when developed... it
doesn't really change colour (it stays a light yellow-green which is
nearly invisible on top of the copper).
To work around this, I scanned the boards into GIMP, then ran these
filters across the image:
  - Colours -> Components -> Channel Mixer; R=+100, G=0, B=-100,
      Monochrome on.
  - Colours -> Auto -> Stretch Contrast
  - Image -> Mode -> Greyscale

The board was inserted in the UV unit, with the step wedge on top of the
photoresist, and a clear piece of JetStar Premium film on top of the
step wedge.


Here's my first test, done at 340 seconds (standard test time for my UV
box):
  http://img860.imageshack.us/img860/7346/201203240001340secgry.jpg


The first visible step appears to be #4, with #3 only faintly visible.
To me, that suggests I need to adjust the exposure down around two stops:
  340 / sqrt(2) / sqrt(2) = 170 seconds


And here's my second try, at 170 seconds:
  http://img535.imageshack.us/img535/2597/201203240002170secgry.jpg

I also tried immersing the board in etchant for a few seconds -- on the
340-second side, step 4 became more prominent, step 3 and below
disappeared almost completely.

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?

Also, based on the step wedge, it looks like I need a HD-LD of around 5
stops (log-density 0.7) in order to get a reliable transfer. JetStar
provides a measured HD-LD of D=1.75, giving a ~3:1 margin. LaserStar
provides a margin of only 1.414:1... talk about working on a knife edge.

I think I might have to write this up for the PCBWiki :)

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