Homebrew PCBs

DJ Delorie wrote:
> 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.

I'm still confused :)
Ok. so when you exposed the resist for 2 minutes with the step gauge in 
place, then what you observed at step 5 was slightly unexposed 
photoresist. Regions under steps 1 to 4 therefore must of been fully 
exposed. Correct ?

The light at step 5 was attenuated by factor of 2^(-5/2) = 0.176. So 
would I be correct saying to achieve the same exposure energy without 
the step gauge then exposure time will be 2 minutes * 0.176 = 21 seconds ?
But this exposure energy doesn't quite achieve full exposure, so the 
final exposure time use for PCBs will be somewhat longer.


  >> 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).

The data sheet for some resist I once had said 10 to 15 minutes hold time.
  One old book I read said you can check post exposure hold time 
sensitivity to by running a side by side comparison test. this will show 
you if a delay is needed between exposure and development. I've tested 
this myself. It would be good to know the actual minimum time rather 
than just wait around for 15~30 minutes. I guess it would be like most 
chemical reactions where polymerisation reaction continues exponentially.

Also try exposing with the polyester film removed. It will take at least 
5x longer !