Homebrew PCBs

designer_craig wrote:
> 
> Issues to be resolved:
> 1.Is the exposure of photo resist linear ie. is high power UV for a 
> short time the same as low power UV for a longer time.

No, but I think if the intensity causes polymerisation under 30 seconds 
or so then it is linear. (i.e exposure time = 1/intensity).
I discovered this when experimenting with UV LEDs, and noticed that if 
the intensity was below a certain level then no amount of exposure time 
(exampled > 1 day) would polymerise. It seemed photo polymerisation 
needed to be under 3~5 minutes or it just didn't happen. My guess is a 
slow side reaction happening during exposure that mitigates photo 
sensitivity. Maybe oxygen diffusion through the mylar film.


> 2.Code to make multiple passes for trace and shape widths from the point 
> gerber data.
> 
> 3.How to get a .001" dia round spot. The laser diode output is not 
> round. How to measure the spot dia.

microscope slide and a microscope. Or better is expose point on 
photoresist and develop, then view photoresist under microscope.

> 
> 4.How long will it take to expose a 3'x5" board?
> 

Assuming a 60mj/cm^2 exposure requirement, and 85 mW source then your 
7.5*12.5cm = 93cm^2 PCB will take  60/85*93 = 65 seconds.