Philip Pemberton wrote:
> 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.
Image here:
http://img403.imageshack.us/img403/3346/xyzbrdtl.jpgThe photoresist seems to get stained by the FeCl - it starts out dark
green/blue, then goes a shade lighter when exposed to UV. The UV-exposed
bits go blue/black in the developer before dissolving. Finally, when it
hits the FeCl3, it goes dark green again... The board seems fine. Not
quite clear like fresh uncoated FR4 (or -- as I've been told -- FR4
that's been developed with HCl+H2O2), but not badly stained either.
Exposure was 75 seconds in a Mega AZ210. Image was printed on Stabilo
inkjet OHP film using a Canon Pixma iP4600. Settings are on the Wiki
(
http://pcbwiki.philpem.me.uk/). Development was CPD5 developer at half
strength (1 part dev to 9 parts water: 50ml dev, then add water to make
up to 500ml). I'm not arguing about the reduced strength: it makes the
developer even cheaper! :)
Even though Mega say using CPD5 with the Fotoboard isn't a good idea, it
works pretty well. YMMV. From the MSDS, it looks like the Fotoboard
developer is... well, a half-strength version of CPD5. Sodium
metasilicate and water, essentially. I still have no idea why the Seno
developer didn't work, though I might raise the issue with Mega at some
point this week.
Also, CPD5 doesn't like being left out in the open air -- mix it as soon
as possible before use, and discard it afterwards. It'll keep for about
an hour if you're doing a couple of boards (say 3 or 4) but starts to
lose its potency once you get past an hour or two after mixing. The
concentrate doesn't seem to suffer from this effect (a bit like ID11 if
you're into photography: stock solution keeps for a few months, mixed
solution is 1-shot).
I should have mentioned how I adjusted the exposure... Here goes.
- Expose at 340 seconds and develop.
- Scan on scanner.
(result: <
http://img413.imageshack.us/img413/8161/xyzvm.jpg>)
- Find first patch that shows evidence of background not being 100%
copper. In this case, that's patch 4 or 5.
- Calculate adjustment factor: sqrt(2)^numSteps. That is, 1.414 ^ 5,
(where ^ = raise to the power of) or 1.414 ^ 4.
- Apply adjustment factor: 340 / adjFactor. Exposure time is between
60 and 85 seconds (the decimal is rounded as normal).
- Because the "normal" exposure seems to be in between two steps,
Exposure time = ((85-60)/2) + 60 = 72.5 seconds
Rounded up = 73 seconds.]
I made a wild assumption that 75 seconds would probably be fine, and in
any case it's a nice round number :)
Essentially you're adjusting to use the minimum exposure that makes all
the exposed photoresist dissolve in the developer.
--
Phil.
ygroups@...http://www.philpem.me.uk/