solder masks! oh my!!

solder masks, oh my! i have started investigating the facility and methods need to make solder masks as part of my research to upgrade my homebrew PCB production. i recently purchased some UV curable ink; the price is right at about $15 per 50ml. it takes very very little. your objective should be to spread as thin a film as possible on the pcb. next you set up a typical exposure setup layering a mask over the parts that you don't want to cure (thus a positive process). you can make this tight by clamping the entire layers or use a vacuum exposure frame. i just lay a heavy piece of glass on the top to smoosh it together. when exposed to direct sunlight for about 3-4 minutes, it is enough to cross-link the polymers in the exposed ink.

i didn't understand how to set up the exposure 'sandwich' at first, the first ink never dried and I made a sticky mess of my screens. i soon learned that the cured material will not stick to polypropylene which can be sourced from your friendly 'zipper' food storage bag (Zip sumthin') or those clear sleeves for three hole binders...
so add a drop or two of ink, lay down a sheet of polypropylene, and squeegee the ink to a thin film. lay down and precisely align your mask, and expose. immediately after this first exposure use a rag with solvent (cotton swabs aren't bad either) to dab away the uncured areas over the pads. then set the board out in the light for a while to fully cure the ink.

the results/lessons learned from my initial tests were that
• i use too much ink; i am too used to painting with a trowel...
• get all the air pockets out from under the poly
• that stuff is hard to get through when fully cured
  • clean the pads thoroughly
  • easier to scrape away errors before the ink is fully cured
  • test all the traces before you got to all this trouble, you are locking yourself out of accessing the traces for repairs
• did I mention the stuff gets hard

all-in-all, i have decided that this is a process i will have to practice many times to get down. the price can't be beat though. i found a dry film called Dynamask 5000, this is a film that i will cut to size and apply with the laminator. it uses a negative process close to the photo-intaglio processes i used to do in school. i will report on this later.