Homebrew PCBs

A few bits of anecdotal experience and personal observations about photo 
resists (mostly from experiences 40 years ago.)

1) I always thought of photo resists as primarily polar (water soluble) 
and non-polar (organic solvent such as Toulene.)   You chose the resist 
to match the need application.  I.e., if you were going to make a silk 
screen to apply paint patterns, you chose a polar resist such as gelatin 
or polyvinyl alcohol because it was essentially immune to the petroleum 
solvents in the paint.  If you were etching a PCB (that uses and aqueous 
solution) you chose a non-polar  resist (Dychem aerosol) that required a 
solvent such as Toluene or Tri-chlor so it was completely immune to the 
aqueous etchant.

2) Having said that, it has already been mentioned the process of making 
printing plates.  I knew nothing of how to make printed circuits so I 
thought of a local platemaking shop and went there to learn how to etch 
metal.  It was an education.  They got around the problem of an aqueous 
resist in a rather nasty fashion.
   a) The resist (I believe it was either dichromate sensitized 
polyvinyl alcohol or egg albumin)  was flowed onto the zinc plate and 
essentially air dried at near room temp.
   b) The plate was then exposed with a negative and an arc lamp.
   c) The plate was washed with warm water to remove the unexposed resist.
   d) The plate was dipped in a chromic acid solution.
   e) The plate was placed on a flame burner and the temperature raised 
until the resist turned a dark chocolate brown color.  At this point the 
resist is so totally cross-linked because of the chromic acid and heat 
so that no solvent, polar or non-polar will touch it.   They etched with 
splash/spray of ferric chloride.    An additional step was to powder the 
surface with "dragon's blood"(?) and by allowing most of it to slide 
off, then setting it with heat(?).  That additional process could pretty 
much control undercutting, even when etching as deep as 0.050" or more.  
As previous posters mentioned however, that heat treatment is well 
beyond what a PCB can handle.

3) Later I learned the proper processes.  We primarily used 
silkscreening with an asphalt like ink and bubble etched with ferric 
chloride.  So the screens were make using di-chromate sensitized 
gelatin  per 1) above being an aqueous resist worked very well with a 
solvent based ink.  No high temperatures needed.  The gelatin was coated 
on paper and we would dip it into ammonia dichromate, roll a plastic 
sheet onto the surface, and expose through a positive  with an arc 
lamp.  Basically the carbon tissue photographic process. Soak in water 
and wash off the paper sheet and unexposed gelatin. The pattern remained 
on the plastic sheet.  Then press the pattern into the silkscreen.   The 
other process was to sensitize polyvinyl alcohol and coat the silkscreen 
with it, dry, and expose, then wash it out.  Either method worked.  
Presentized sheets were available but didn't have a good shelf life for 
our consumption rate.

4) The times we needed one-offs or finer lines we used the aerosol 
Dychem resist that we developed with trichloroethylene .(Tri-Chlor)

The solvent based films were bad news as the solvents 
are/**/carcinogenic and bad for the lungs.  The new films with carbonate 
developers I believe work on the principle  that the developer is 
alkaline and the etchant is acidic so the PH controls whether the film 
stays intact.  (someone more knowledgeable about this could certainly 
clarify this point.)

Anyway, some notes from the dinosaur age.

Regards,
Charles R. Patton


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