Homebrew PCBs

Hi Grant and the group,

I use two types of silkscreening emulsions. They are Ulano CDF-4 and 
Autotype Five Star. Both of these are film types. Here are the web 
sites:

Ulano   www.ulano.com

Autotype  www.autotype.com

I like the Five Star film emulsion the best. It exposes in 30 seconds 
on my 1500 watt mercury vapor lamp unit. After exposure it requires 
development in a weak hydrogen peroxide solution for one minute. A 
low pressure warm water wash will remove the excess and leave the 
desired image. At this point you lay the wet film on a flat surface 
and put the clean pre-wetted blank screen onto the film. The film 
will adhere to the screen and the excess water can be blotted with 
newsprint paper. A fan can be used to speed up the drying time. When  
dry the plastic carrier sheet easly peels off and the screen is ready 
for printing. This type of film is called INDIRECT because it is 
processed first and then applied to a clean screen.

Another Autotype indirect emusion I have used is called NOVASTAR.
This emulsion is developed using only warm water. The only negative I 
have found with this film is the lack of resolution on very fine 
detail. For hobby circuits this will probably never be a problem.
On the plus side, you don't have to use any special developers.

The Ulano CDF-4 film is similar but is applied to a wetted screen, 
dried, exposed, washed out and dried again. This film is called a 
DIRECT emulsion because it is applied first and then processed. Your 
exposure system must be able to hand the thickness and size of the 
screen frame. The CDF-4 exposure times are much longer than the 
indirect types and can take up to five minutes even on high power 
units.

Cleaning these emulsions from your screen requires bleach or the 
special cleaners from Autotype or Ulano. Be careful when using bleach 
with aluminum screen frames. Bleach will attack and discolor the 
aluminum. I have also noticed that the bleach method works best if 
you apply it to the screen and let it sit for up to an hour on 
stubborn emulsions. Scrubbing with a nylon bristle brush will help to 
remove every trace of emuslsion. Don't worry about stains left behind 
in the fabric. One way to check for complete emulsion removal is to 
hold the screen up to a light source and inspect the fabric with a 
magnifying glass. The slightest bit of emulsion will show up as 
clogged holes in the fabric mesh.

I use a UV cured etch resist that is stripped using a weak, 1% 
caustic solution. Common store bought lye will work fine. Screenable 
resists that will air dry are also available. I have a UV curing 
system and prefer that method because you never have to worry about 
the ink drying in the screen and clogging the pattern. There are 
spray cleaners available to solve that problem so its not really a 
big deal. The only time you will have a problem with drying is when 
you stopped printing for more than five minutes or so and don't clean 
the screen.

I purchase my inks from TAIYO. They have all types of screening inks 
for circuit board production including solder mask and legend inks.

Taiyo America   www.taiyo-america.com

Another good source is Lawson Screen Products. They have almost 
everything needed to screen print and even offer training seminars.

Lawson Screen Products   www.lawsonsp.com

Except for very small quantities, I process all of my boards by 
screen printing. I only have a manual press and it is not difficult 
to print up to 250 impressions per hour using these methods. You can 
also print control panels, boxes and enclosures etc. Screen printing 
is a valuable skill. 

Tom