Homebrew PCBs

Adam Seychell wrote:
> I've been experimenting with some UV LEDs for building an exposure unit. 
> I'm using the negative dry film photoresist and discovered to my 
> surprise that there is a limitation on minimum light intensity. If the 
> intensity its too low then no amount of time will cause photo 
> polymerisation of the resist.
> For example, one LED I tested at 15mA, 120mm distance did not cause any 
> polymerisation in 40 minutes. The same LED at 30mm distance, 20mA, took 
> only 30 second for polymerisation.
> 
> I found if the light cannot expose the resist within about 2 or 3 
> minutes then the photoresist do not seem to react at all. At boarder 
> line UV intensity, it partly polymerises leaving behind a soft matted 
> looking resist after development.
> 
> This imposes a design constraint on anyone who wants to make a LED 
> exposure unit must because it there will be a minimum density of LEDs 
> needed in the array. So far I've only experimented with single LED 
> exposures. I'll make a 9 x 9 array for testing.

Have you tried using the same higher current and just increasing the
distance? Maybe the wavelength is increased at lower current, which
would reduce the photon energy.