Earl T. Hackett, Jr. wrote:
> This is getting a bit off topic.
>
> The comments below are close and when I started working with photoresists that was my opinion as well. However, when we examined the results of resolution testing, the data showed there was something other than just columination and exposure at work. Tests at DuPont showed that oxygen migration within the photo resist from under the opaque areas of the phototool had a major effect on resolution of the spaces. Oxygen inhibits free radical polymerization. High intensity light generates so many free radicals in the clear area of the phototool that dissolved oxygen is consumed almost instantly and polymerization proceeds quickly. As oxygen migrates from under the opaque areas of the photo tool those areas loose the only thing that prevents polymerization. Stray light from poor columination or reflection from the copper surface would initiate polymerization in what should have been shadowed areas. If you get the clear areas polymerized quickly the dissolved oxygen doesn't
h
> ave time to migrate and it prevents polymerization in areas of lower light intensity. In other words, high intensity will overcome the effects of poor columinization. The reverse is not necessairly ture because of scattered reflection from the copper surface. This effect was demonstrated in dozens of production shops.
>
> However, this is really getting down into the weeds as most home brew folks don't have (or shouldn't have) the high intensity mercury arc lamps that are needed for fast exposure times. Even though I have one in my shop I'm yet to turn it on because I have not yet installed sufficient protection from the radiation it will produce. I'd rather develop a solid documented toner transfer process which will do everything I need than fuss around with high resolution photoresists.
The black-light blue mercury HID lamps (125W, 250W, and 400W) are sold for stage
lighting effects and don't put out the shorter wave UV. With columation in a decent
lightbox, fine traces and spacing are easy. I just made a 15cm x 15cm pcb with 12mil
tracks running along the length of it with no problem at all. And unlike toner
transfer, it was easy to get perfect registration for both sides. I'm using a
125 watt HID mercury blacklight in a cylindrical reflector, 60cm globe-pcb
spacing, and 90sec exposure time.