Hi Jeff,
The book I cited really didn't give any recipes. The reference to shellac was in a chapter devoted to the historical development of resists. Later chapters fell into the nether world of organic chemistry. This patent, 4447519, in examples 4 and 5 gives the proportions. c2h5oc2h4oh is Ethylene Glycol Monoethyl Ether which is a solvent. Union Carbide Corp. trademarked it as Cellosolve. I think it is now marketed as Butyl cellosolve. See,
http://www.chemistrystore.com/Chemicals_A_F-Butyl_Cellosolve.htmlI really don't have any further details about shellac. I assume that the above solvent would remove it and its use with the sensitizer is to soften the shellac for better penetration. The post baking is to continue the crosslinking to further harden the resist. A good source for this sort of thing is under the topic of "alternative photographic processes" Look at the APUG Forum: Alternative processes. I think the PVA/PVac/Elmer's glue approach is better than using shellac, or the other glues like fish or rabbit because of the water "developing" step.
While digging through my files I came across a screen printing post for a glue sensitizer formula,
quote:
"An alternative to photo emulsion(the answer)
Wed, March 28, 2007 - 9:15 PM
1 tsp of Ammonium Dichromate -> 1 oz. Water
Dissolve and mix with 8 oz. Elmer's Glue
see below (dry measure equiv.)
1 tsp = (14.3 gm/3) = 4.76 gm
Fairly flexible open time, decent exposure times
(10-20 minutes under daylight), rinse with water, and a
bulletproof stencil is yours."
end quote
Baxter
--- In Homebrew_PCBs@yahoogroups.com, "Jeff" <jeff.heiss@...> wrote:
>
> Hi Baxter,
>
> I was wondering if I could ask you a few details about your comment from the Photoresist book on ammonium dichromate and shellac. Does the book mention a ratio of dichromate powder to shellac? What is the dichromate dissolved in before mixing into the shellac? After the shellac is UV exposed, how is the unexposed shellac removed? Is there a develop step after exposure like traditional resist? Is the 200 deg F post bake after board coating and before UV exposure?
>
> Jeff
>
>