Homebrew PCBs

jcarlosmor wrote:
>  

> And by the way, as someone stated, UV Leds are not used for UV curing or 
> imaging of PCB processes. If you want to build a real-use UV 
> photoimaging box, do not waste money on UV leds. Use blacklight 
> fluorescent tubes. They are cheaper, and provide real results. In fact, 
> they are used widely in PCB making (However, for fast batch use, there 
> is no substitute for high power mercury arc lamps).
> 

After experience with both 395nm UV LED and BL tube (spectral peak 
365nm) light sources, I would totally agree. The major problem with LEDs 
is light non-uniformity and inadequate intensity for solder mask and 
positive resists. Only the negative dry film photoresists seem to be 
sensitive enough. For some reason, extending the exposure time with LEDs 
does not work on these materials.

One would expect excessive light undercut from the relatively wide angle 
radiation pattern emitted by an array of BL tubes. In my experience, 
0.2mm (0.008") line/spaces are easily achievable. It is critical the ink 
side of transparency makes contact with the photoresist. I found the 
ratio of maximum/minimum exposure time can be high as 1.5 while still 
maintaining an acceptable deviation in line & space widths.

The limit to minimum line & space is not the exposure source, but that 
of inkjet printer resolution.

Adam