Archive of the former Yahoo!Groups mailing list: 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