Homebrew PCBs

Hi Baxter,

 

I am trying the recipe of ammonium dichromate mixed with Elmers glue and I
am finding two things.  First, the Elmers glue does not dissolve in water
without physical agitation.  Take plain, unmixed Elmers strait from the
bottle, spread it on something, and let it dry.  Once dry, immerse it in
water.  The glue does not dissolve in the water unless rubbed lightly with
your finger.  No rubbing, no dissolving.  I left the glue in water over
night and all was still there the next day.  

 

Second, the glue hardened from UV exposure does not stick to the copper well
enough to avoid coming off during the agitation required to remove the
unexposed glue.  If the unexposed glue could dissolve on it's own in the
water, I don't think this would matter.  I would try real PVA polyvinyl
alcohol to see if it dissolves in water without agitation but I don't have
PVA.  I read that Elmers is polyvinyl acetate, not polyvinyl alcohol and
they are two different things. 

 

Jeff

 

  _____  

From: Homebrew_PCBs@yahoogroups.com [mailto:Homebrew_PCBs@yahoogroups.com]
On Behalf Of bebx2000
Sent: Sunday, January 29, 2012 11:39 PM
To: Homebrew_PCBs@yahoogroups.com
Subject: [Homebrew_PCBs] Re: Photo Resist Gelatin Bicromate Process

 

  

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.html

I 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
<mailto:Homebrew_PCBs%40yahoogroups.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
> 
> 





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