Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!mailrus!ames!think!mintaka!mit-eddie!uw-beaver!milton!whit
From: whit@milton.acs.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: PCB making ?
Summary: Paper is transparent enough
Keywords: UV exposure, paper negative, printed circuit fabrication
Message-ID: <1134@milton.acs.washington.edu>
Date: 22 Dec 89 02:17:36 GMT
References: <2652@servax0.essex.ac.uk> <317@marvin.moncam.co.uk> <2952@munnari.oz.au>
Reply-To: whit@milton.acs.washington.edu (John Whitmore)
Organization: University of Washington, Seattle
Lines: 46

In article <2952@munnari.oz.au> oby@munnari.oz.au (David O'Brien) writes:
>In article <317@marvin.moncam.co.uk>, emmo@moncam.co.uk (Dave Emmerson) writes:
>> In article <2652@servax0.essex.ac.uk>, zotog@SunLab14.essex.ac.uk	 (Zotos G) writes:
>> > Is it posible to photocopy the piece of paper with the track layout on a
>> > clear plastic film used on overhead projectors and use this ...
>> Very unlikely. Even if you can get enough density in the image area,
>> it will tend to be grainy. Also, you may not get 1:1 copies
>> 
>> You need to use 'Ortho Lith' photographic film, in a lith developer, or
>> at least a VERY high-contrast developer. You can handle this in red
>> safelight and dish develop it quite easily.
>> Dave E.
>
>  I have found these Overhead Projection transparency films produce
>  grainy artwork,and if used develop pourous Photoresist finish.      
>
>  David O'Brian
>  University of Melbourne
>  Australia
>     
	The use of high-contrast negatives when contact printing to
the final (copper) surface is essential to avoid pinholes.  Most people
use high-contrast UV sensitive film (Kodak Kodalith film is what the
U. of WA. prefers), which is not itself a good Xerox-process substrate.
In fact, transparency film is also not a good Xerox-process substrate,
and paper prints almost always come out denser (because of the
toner powder, which is most sticky to a rough paper surface and only
lightly coats a slick smooth plastic film).
	I had good luck with a two step process: photocopy to exact
size on paper, then contact print paper to Kodalith.  This requires
the UV lamp to shine through the paper onto the Kodalith, which
takes about 10 minutes (opposed to 1 minute for Kodalith-to-Kodalith
prints), but that is not a tricky step.  The Kodalith is then
the negative for the photoresist exposure on the copper surface.
Of course, the paper is printed-side down; less obviously, the Kodalith
film goes coated-side down as well, onto a black surface (in my case).
The extra few minutes of exposure time is going to depend on the paper
weight and the lamp character, of course; experiment on some test
strips to determine the exposure before you use full-size sheets.
The Kodalith negatives are developed with xylenol, and are trivial
to deal with in comparison with the copper etching baths; they
use the same UV illumination for exposure that most resists (like KPR)
for PC boards use.

I am known for my brilliance,            John Whitmore
 by those who do not know me well.