Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

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.