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 !