Homebrew PCBs

The negative-positive issue seems confusing at times but here's how 
it works. Screen printing requires that the image being printed must 
be opened up or "washed out" from the screen emulsion. 

Exposing the screen emulsion with a film positive consisting of black 
lines representing copper and clear areas representing the areas of 
copper to be etched away will get the job done. 

The black areas of the film prevents UV light from exposing the 
screen emulsion in the areas that need to print. When the screen is 
washed out these areas that didn't get any UV light will wash away 
thus leaving an open screen mesh that represents the desired image.

Just keep in mind that the image you start with with for exposing the 
screen emulsion will be the exact image you will print. Starting with 
a positive film to negative acting screen emulsion will yield a 
postive print or exact copy of the original image on the film you 
started with.

I have used liquid screen emulsions as well as indirect film 
emulsions. Liquid emulsions are applied wet onto a screen and allowed 
to dry. After drying they are ready for exposure and water wash out. 
Indirect film emulsions are supplied in sheets or rolls. They are 
exposed in a contact frame just like making a board with photoresist. 
After exposure the indirect film is developed in a tray, washed out 
and adhered to the screen mesh. After drying the plastic carrier 
sheet is peeled off and the screen is ready for printing. 

Indirect films emulsions are not as rugged as the liquid emulsions 
but have much better resolution for detailed fine line printing. The 
indirect emulsion I use is Autotype Five-Star. I used to use Ulano 
569 liquid emulsion but haven't used it in many years although it is 
an excellent product. My work is very detailed and I use only 
indirect emulsions on stainless steel mesh. I hope this information 
helps those wishing to try screen printing.

Tom