Homebrew PCBs

Adam Seychell <a_seychell@...> writes:
>  From what I can gather, the transmission gauges are logarithmic. With 
> 21 step gauge, each successive step has the transmission 1/sqrt(2) 
> factor relative to the previous step.

Yes.  So, my first exposure was step 5 at 2 min and I wanted to get to
step 8 (three steps), so I multiplied 2 min * 1.4 * 1.4 * 1.4 = 5.5
min.

> step 8 means 6.25% transmission. if you exposed 5.5 minutes, then your 
> real exposure should be 5.5*0.0625 = 0.34 minutes (21 seconds).
> Is my math correct ?

Math, perhaps.  Logic, no.  If a 5.5 min exposure gets to step 8, then
my true exposure time (for pcb film) is still 5.5 minutes.  The idea
is that you don't want a "just barely works" exposure, you want an
*ideal* exposure.  The step gauge tells you how far past "just barely"
you went.  For Riston, you want an exposure that will just barely
harden the film under the 8th step.  That gives you an ideal exposure
for film not covered by the step gauge - exposed film gets really
exposed, and covered film is not nearly exposed enough to harden.
Think of it as a halfway point between "barely enough" and "almost too
much".

Of course, my next step is to expose with the step gauge and a striped
pattern from my printer, to see what kind of range the printer offers.
The number of extra steps "held" under the ink indicates how
UV-blocking the ink is.  I can thus test each color separately, and
see which holds the most extra steps.

> Are you using the hot roll laminator to apply film ?

That time, yes.  I also wetted the pcb as somewhere I read to do that,
but it was a bad idea.  I'll keep trying things until I find what
works for me.

> I will update my wet lamination web page sometime soon, since I've 
> worked out some improvements to the process.

I'm looking forward to trying that, especially if it gets better
results :-)

> Also I just discovered the importance of leaving the PCB stand after 
> exposure for > 5 minutes.

I've read that many places.  The curing creates a gas that needs to be
trapped to complete the curing process, or something like that.  I
left my test boards in the UV box (with the leds off, of course) for
15-30 minutes (the box is painted black inside for just this purpose).