Homebrew PCBs

Hi Steve and the group,

Most of the dry film photo resist used for printed circuits requires 
lamps to have the peak spectral output at 365 nanometers. They will 
respond to other wavelengths but more power will be required. A 
quartz lamp will probably work because many years ago we accidently 
exposed some dry film with stray light from a quartz tube. The way to 
test this is to laminate some copper panels with photo resist and use 
a Stouffer 21 step wedge. This step wedge is nothing more than a 
calibrated piece of film that starts out clear and progresses to 
opaque black in 21 steps. I think we ran our exposure to retain a 
step 8 during development. This is probably an overkill for homebrew 
circuits but some may want to try this out because it will give you 
the absolute final word on exposure performance. These test strips 
are very inexpensive. The old fashion trial and error method will 
also work.

More info is located here. 

http://www.stouffer.net/Using21step.htm

Best results will be obtained by using a higher power lamp and 
INCREASE the distance from the lamp to the board being exposed. This 
allows the light beam to become collimated or "staightened" out. If 
you have ever had undercutting problems try this out as it may help.

If anyone want to try out the quartz type of lamp start out at least 
12" or more from the board being exposed and adjust the time to get 
the correct exposure. It may take several minutes but I think it 
should work. We never used these in a commercial setting because 
exposure speed for production was important. I wouldn't worry about 
this for hobby circuits...  whats a few extra minutes if it works?
BTW, these quartz lamps were used for years in machines that would 
melt solder plated boards and fuse the solder to the underlying 
copper.  Don't get too close to your film when using this type of 
lamp!

Don't worry about glass types because the commercial exposure units 
use regular window glass for the exposure tray with a mylar plastic 
sheet over the top. The board is between these layers and a vacuum 
pump removes the air for a very tight contact with the top and bottom 
films. This tray slides into the exposure box for the proper amount 
of time and then rolls back out automatically. The vacuum is 
released, board removed and the process starts over. A hobby version 
of this machine could be built at a very low cost. I have thought 
about making these myself and selling them. Quartz glass is the 
proper type to pass UV light but it just isn't required.

The photo resist we used was DuPont 4715 which is their 4700 series 
and 1.5 mils thick. Here is a web site that might be of interest.

http://www.dupont.com/pcm/techinfo/general/index.html

I hope this info helps.

Tom