Homebrew PCBs

Re direct application of silk screen emulsions to a PCB . I don't 
generally expect this to work very well. I previously commented on this. 
Please see:
http://tech.groups.yahoo.com/group/Homebrew_PCBs/message/29416
But, to iterate. Each step used the polar/non-polar property for 
immunity from the films dissolving when stability was desired. For 
instance, if the PCB was to be etched in water based ferric chloride, 
then just about any non-polar solvent based resist worked on the PCB. 
This ranged from silk-screened asphalt ink to sensitized shellac-like 
films, UV cured to provide differential resistance to dissolving in the 
the developing organic (non-polar) solvent. Thus the non-polar resist 
was impervious to the water based etchant. The silk screen used this 
polar/non-polar process in reverse. The emulsion on the screen was polar 
in nature -- polyvinyl alcohol, egg albumin, or some such. Either UV 
cured sensitized emulsions or knife cut films that were then moistened 
and attached to the the screen -- typically wooden frames using 
polyester fabric, as silk had long gone out of use. If extreme precision 
was necessary such as thick film hybrids, then stainless steel fabric on 
aluminum frames was used. But then the organic paint was squeegeed onto 
the PCB and being organic, essentially impervious to the water based 
etchant and as the paint was organic, so also the screen emulsion was 
essentially impervious. In the case of thick film hybrids, the solvent 
based inks were screened onto the ceramic substrates, then fired in a 
kiln or belt furnace. The silk screen emulsions I worked with were as 
described below and sensitized with potassium dichromate. Even the UV 
cured ones, were still soft when wet, just a bit more resistant to 
dissolving as they were "developed." thus they would remain behind while 
the rest of the un-exposed image dissolved away. And as mentioned below, 
the potassium dichromate is also a nasty chemical and presents severe 
disposal problems. Don't put it down the drain either (Chrome is a 
severe carcinogenic--remember the movie "Erin Brockovich.")

The two exceptions I'm aware of were discussed in my previous message.
1)Originally the UV sensitized egg based emulsion used on zinc printing 
plates was developed then dipped in chromic acid (which among other 
things is a also a nasty poison and well avoided) that would harden the 
emulsion. But the real hardener was flame. Just as when you burn food 
onto a skillet, the heat process would bake on the emulsion and I'll 
tell you, it was very, very robust. It took sandpaper to remove it. 
However the amount of heat necessary to bake it would also de-laminate a 
standard PCB, so not exactly a PCB compatible process.
2) The newer dry film resists that develop with water based carbonates 
apparently are totally resistant to dissolving when in acidic solutions 
such as the ferric chloride solutions. (Somebody with actual experience 
with these would have to confirm this for me. I.e., what does the 
developed film feel like in the carbonate solution vs in an acidic 
solution?)
Regards,
Charles Patton


On 4/15/2012 12:26 PM, David wrote:
>
> I think the general concensus is it will probably work, using the 
> spinning method to achieve a thin uniform layer and baking it for a 
> short while, then expose as normal, one less chemical to use as 
> unexposed resist just washes off, not sure how well the exposed area 
> will stick to the copper clad though, a few experiments should wrap it 
> up though!
>
> Another interesting alternative to the various PCB and Silk Screen 
> chemicals available is to make it yourself!
>
> A mixture of Potassium Dichromate [or bichromate] and Egg yoke, or 
> dairy cream, will make a UV sensitive emulsion.[how this stands up to 
> etching though........] not sure of the mixing ratios, but, it is 
> toxic!! a carcinogenic, and chromium dermatitis are two of the more 
> unpleasant results of getting in contact with the stuff!! it is sold 
> on ebay amongst other places;
>
> http://www.ebay.co.uk/itm/Potassio-bicromato-Kaliumdichromat-potassium-dichromate-/180861035286?pt=Laboratorio_e_Scienze_Mediche&hash=item2a1c286316 
> <http://www.ebay.co.uk/itm/Potassio-bicromato-Kaliumdichromat-potassium-dichromate-/180861035286?pt=Laboratorio_e_Scienze_Mediche&hash=item2a1c286316>
>
> if anyone would like to experiment?..........
>
> David
>
> --- In Homebrew_PCBs@yahoogroups.com 
> <mailto:Homebrew_PCBs%40yahoogroups.com>, "mark" <STRIPES97_USA@...> 
> wrote:
> >
> > That's a good question as I was thinking about the same thing when I 
> got a screen printing setup a few months ago, but haven't had time to 
> even do more then open the box to make sure everything was in the kit...
> >
> > --- In Homebrew_PCBs@yahoogroups.com 
> <mailto:Homebrew_PCBs%40yahoogroups.com>, "David" <dgh2259@> wrote:
> > >
> > > Hi All,
> > > After looking at Craig's fasinating PCB spinning methods, an idea 
> came to mind, seeing as the regular chemicals seem to be rare and 
> expensive, has anyone tried silk screen printing emulsions? this in 
> not the inks but the chemical used to make the pattern in a very fine 
> mesh, the emulsion is spread over the mesh with a squeegy and after 
> exposure to UV through the pattern[design] blocking transparency, the 
> unexposed area is washed out with running water, the exposed area 
> remains and is really pretty tough, is this resistant to etching 
> chemicals? would this method work? has anyone tried?
> > > There would be no need to "develop" the PCB, just wash off with 
> water!!
> > > I may be way off track with this but any comments and/or suggestions?
> > >
> > > David
> > >
> >
>
>