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.