Homebrew PCBs

Cristian wrote:
> http://www.crystalaser.com/  has QUV 355-25 Crystal Laser of 25mW at 349 or 
> 351 or 355nm, just inside the photoresist window.
> I don't know how much it is, for the moment, but the head is not so bulky. 
> One can mount it on a CNC Machine easily.
> Of course, some simple optics is necessary, because the beam is 0.9mm diameter.
> 
> Calculations:
> Photoresist needs maximum 100mJ/cmp, that means 4 seconds/cmp with 25mW laser.
> That means 240 seconds for 100x60mm. Four minutes of XY Plotter work.
> If my calculations are correct, that is not too much.
> Cristian


Where did you get 4 seconds from ? Shouldn't the time to expose 1cm^2 be 
equal to:  100mJ/cm^2 / 25mW * (1cm / (0.09cm * 0.09cm)) = 493 secons = 
8 minutes. A typical 100 x 60mm single sided PCB with 50% track usage 
will take at least 4.1 hours !