The best DIY formula for both photoresists and solder mask is contained in this patent,
http://www.google.com/patents/US5556735See Example 2 for an explicit recipe and an outline of the process together with the necessary exposure in mJ/cm2. I bought all of the ingredients, but couldn't carry it through to a satisfactory emulsion because I lacked a high speed mixing mill to incorporate the polyvinyl acetate.
The thing is, however, that Elmer's School Glue is basically the same polyvinyl acetate/polyvinyl alcohol mixture as in Example 2. It also incorporates some flow aid and surfactants. I experimented with it using both diazo and ammonium dichromate sensitizers with a metal halide exposure unit and it does indeed crosslink very nicely. You need to give a coated board a gentle pre-bake to dry it, then apply the patterned transparency and expose it. I used 800 mJ/cm2. The results were satisfactory. The unexposed areas were removable with a little rubbing under water. I had to move on to other things so this is about as far as I went. With some further experimentation, I do believe that this is a viable homebrew process. I was mostly interested in the solder mask aspects.
Baxter
--- In Homebrew_PCBs@yahoogroups.com, "designer_craig" <cs6061@...> wrote:
>
> I did a little research on the silkscreen emulsions by looking at the chemical consultants web site and the MSDS for some of their products. http://www.ccidom.com/