Hi all,
> There is a book entitled, "Photoresist" by W.S.
DeForest (McGraw-Hill, 1975)
I was able to find 5 used copies of this title at abebook.com for about $1.00 each and shipping about $3.00 or $4.00
my used copy is on it's way!
Thanks!
Robert
--- In Homebrew_PCBs@yahoogroups.com, "bebx2000" <bebx2000@...> wrote:
>
>
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> --- In Homebrew_PCBs@yahoogroups.com, Athar Kaludi <ceo_premium@> wrote:
> >
> > Hi All
> >
> > I come across a guy making a printing block of Printing press on Zinc Plate by exposing in sunlight by negative film. Then after developing he etch to remove unexposed area of Zinc.
> >
> > I suppose the same can be applied for PCB. I asked and learned the process there.
> >
>
> Bichromated (dichromated) gelatin has a long history in the area of alternative photographic procrsses. There is a book entitled, "Photoresist" by W.S. DeForest (McGraw-Hill, 1975) that outlines the evolution of photoresist as three periods,
>
> the 1800s natural colloids, gelatin albumen, shellac, gum arabic, glue
> the 1930s polyvinyl alcohol
> the 1950s (onward) polyvinyl cinnamate, positive Shipley resists, dry film
>
> He notes that dichromated fish glue forms a highly etch resistant coating after a post bake of 350-500 deg F which is good for plates, but bad for PCBs. Also, dichromated shellac is mentioned as one of the most successful of the naturally occurring materials for electronic applications with the ability to form fine detail and a comparatively low post bake of 200 deg F.
>
> I think a better emulsion for PCB work can be found in polyvinyl acetate/polyvinyl alcohol formulations by virtue of its water based developing. I have been working on a DIY solder mask following the guidelines of this patent,
>
> IMAGE-FORMING PROCESS 5556735 (Google Patents)
>
> Examples 1 and 2 of this patent give straight forward recipes and processes for making your own solder mask. All of the ingredients are readily available. It is noted in the patent that polyvinyl acetate is only necessary to improve the toughness of the coating
>
> My first attempt failed because I purchased polyvinyl acetate beads and I couldn't get them to dissolve in water. They will dissolve in a number of solvents like acetone, but I didn't do this because I wanted to stick to the patent recipe. The correct product is a polyvinyl acetate dispersion like CM Bond M-2. Polyvinyl alcohol is hydrolyzed polyvinyl acetate and water soluble with a little work. The thing is, however, that both of these ingredients are the main components of white glue (aka Elmer's Glue-all and School Glue, among others). You can find references to the use of sensitized PVA glue as DIY screen printing emulsions.
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> I put aside the exact recipes of the cited patent and have been working with Elmer's Glue-All sensitized with ammonium dichromate together with a green pigment. This is a real trial and error process to determine the percentage of dichromate , thickness of coating and exposure time. The glue applies very easily to the circuit board and seems to self level for a uniform coating. I am pretty certain that it includes the flow aid and surfactant ingredients mentioned in the patent. Working with ammonium dichromate introduces the so called "dark reaction" so that you cannot store your sensitized boards for more than a day before exposure. Diazo may be better.
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> There is another patent,
>
> Solid Photoresist and method of Making Photoresist 4447519
>
> that outlines dipping in a 1% solution of ammonium dichromate before exposure which alleviates the storage problem. But any way you cut it, ammonium dichromate is a nasty substance that just loves to crosslink collagen; of which there is plenty in the human body. So be careful with this stuff.
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> I have had some success with this process, but not in a uniform manner. I do not have a lot of time to devote to this. I am convinced, however, that it can lead to a DIY solder mask once the kinks are ironed out.
>
> Baxter
>