Homebrew PCBs

Leon Heller wrote:
> I don't have any problems using Fotoboard 2 with my home-made UV 
> exposure unit (two tubes about 15 cm from the glass.

So a fairly low powered unit then?

The AZ210 I'm using has four 15W Sylvania F15W/350BLB-T8 tubes on each 
side, spaced about 2" apart, with a rather odd metal reflector that 
looks like it's been hit repeatedly with a very small hammer. Distance 
from board to bulb is maybe 1.5" or so, through a sheet of what appears 
to be glass (though is probably quartz-glass or something similar that 
doesn't block UV).

 > I found the optimum
> exposure level (11 minutes) by using the old strip technique that used 
> to be common in photography.

You mean the "test strip" method? Cover 10%, expose for 30 seconds, 
cover another 10%, expose for another 30 seconds, repeat?

That would seem to be the best way to do it -- though the attraction of 
the step wedge (if you can get it working!) is that it gives you an 
exact adjustment and with 1 or 2 adjustments (one small offcut from a 
double-sided board) you can get an exact figure.

In this case it looks like the issue was a PEBKAC Fault on my part :)

 > Transparencies are printed on Mega's
> Jetstar Premium film using an HP 5940 printer (1200 dpi). Over exposure 
> doesn't seem to matter too much, I've missed the alarm before now and 
> probably left it for 20 minutes, on one or two occasions.

With a base exposure time of 11 minutes, you're still only talking about 
a 2x over-exposure. In my case I had nearly a 5x over-exposure...

What surprises me even more is that the CIF laminate worked under these 
conditions... I've pulled out my lab notebook and it's there in black 
and white: 340 seconds, AZ210 UV box, developed with Seno developer roller.

> I develop in NaOH (12 gm/litre). It takes about 20 seconds.

I'm not touching NaOH with a barge-pole... or at least a full PPE kit.

> I've found that the edges of the material sometimes don't work properly, 
> probably because it's been exposed to fluourecent lighting, which has 
> leaked under the protective coating.

It looks to me like the coating isn't going all the way to the edges. 
I've seen instances where the edge ~1cm is a shade lighter than the 
centre (which does appear to be light exposure) but the major defect 
with my boards is that the coating doesn't actually go all the way to 
the edge.

I haven't had any similar issues with the CIF board (though that was a 
0.8mm RF laminate) and it turns out what I thought was Microtrak 
actually wasn't... so I haven't tried that either. Hey-ho...

> It's important not to use outdated material, the resist seems to go off 
> after about six months.

Is that all? Huh.
Though I did have some Maplin photoresist board here that has utterly 
lost any function whatsoever. I ended up running the timer on the AZ210 
at 999 seconds (the maximum it'll go to), and even step 1 on the test 
wedge was only just going grey. That was with no artwork film...

I ended up stripping the photoresist and using it for toner transfer. 
I'm toying with the idea of buying some spray-on (or liquid) photoresist 
to re-coat a few boards, but it's not a high priority at the moment. I 
think I'd strap the board to a 120mm cooling fan, put the contraption in 
a shoebox, then set it spinning and spray it. In theory, that should 
produce a fairly even coating of photoresist. But again, it's not high 
on my agenda -- wet-laminating sheets of Riston onto blank boards seems 
like a more profitable venture (in terms of likelihood of success).

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