Homebrew PCBs

DJ Delorie wrote:
> Adam Seychell <a_seychell@...> writes:
>> 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 ?
> 
> Yes.  1 to 4 were fully exposed; they got enough UV to cure.  Recall
> this is a negative resist, transparent = UV = cure = copper.  Steps 6
> through 21 were uncured, and the copper removed when etched.
> 

thanks, its getting a bit clearer now. So lets say your test expose for 
120 seconds with the step gauge and full cure occurs at step 4. The 
level of UV radiation at step 4 might be your final goal when exposing PCBs.

Exposure time for PCB fabrication will then be:
120 / (1.41 * 1.41 * 1.41 ) = 42 seconds

This is because step 1 is 100% transmission, and step 4 has
2^(-3/2) = 1/(1.41 * 1.41 * 1.41 ) = 35% transmission.

An alternate explanation is in terms of energy. To get equal exposure 
energy for arbitrary time and transmission you have to keep constant 
(time * transmission) product.

120 sec * 35% = t * 100%

t = 120 * 0.35 / 1.00
   = 42 seconds


I cannot understand why you need a 5.5 minute PCB exposure based on your 
results. A 42 second PCB exposure agrees a lot more with my experiences.


>> The data sheet for some resist I once had said 10 to 15 minutes hold time.
> 
> The riston data sheet doesn't say, but thinktink says 15 min to 8
> hours.  http://www.thinktink.com/stack/volumes/voli/store/specs/m115spec.htm

interesting. They say in "note 2"  that it can be developed immediately 
it may compromises adhesion. I found adhesion increases over time too, 
and so that's second reason for leaving it. This may be especially true 
for wet lamination, so water has dime to diffuse.