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 have 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.
----- Original Message -----
From: Adam Seychell
To: Homebrew_PCBs@yahoogroups.com
Sent: Tuesday, January 25, 2005 10:09 PM
Subject: Re: [Homebrew_PCBs] UV lamps?
> ----- Original Message -----
Overall light energy and collumination are completely separate
parameters with respect to photoresists. More photons simply means a
faster photochemical reaction. Point blank range fluorescent tubes can
expose in seconds, however light undercutting is terrible.
Adam
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