Homebrew PCBs

> Robert Wrote:
> 
> You're mixing 2 techniques; photo-sensitive
> and toner transfer.  The transparencies are
> used to project an image of a circuit onto
> a special photo-sensitive PCB using
> fluorescent light.
> 
> Toner transfer uses special paper, much like
> what they use to transfer iron-on decals
> onto t-shirts.

The funny thing about this statement is that
transperancies work very well for Toner-
Transfer but are quite hopeless for UV-Photo
etching with most laser printers. In UV photo
etching people generaly use vellum if they
can't make a real photo-tool.  The vellum does
not leave pin holes for the UV light to get
through as badly.

Over Head Transperancies work very well for
toner transfer with the right set up.  In fact
I was using OHTs for toner transfer long
before Press-n-Peel blue paper was around.
(I also kept using them after press-n-peel
became available as I think they worked better
in some respects)

I first heard of the concept in a "news"
article saying someone was doing toner transfer
at PARC.  I thought it sounded Fabulous 'cause
at the time I hated the negative photo-resist
process I was using.

I tried paper, the fibers stuck.
I tried scrubbing the fibers.  Stuffed toner.
I tried Clay paper - clay stuck to bare copper
I tried all sorts of different papers.

(nb - this was the days before real colour ink
jets - glossy "photo" paper didn't exist.  Clay
paper for B&W inkjets was new and fandangled)

I was getting better results with "glossy"
papers.  I tried some magazine covers that
I beleive are coated with a thin film of
mylar.  It worked fabulously.  Only problem
now was I had a limited supply of penthouse
magazines to rip the covers off :P

Search continued and I eventually after many
weeks of experimenting found Faber-Castle
OHTs worked fabulously with my HP laserjet.

Unfortunatly They seemed to stop making
those brand trannys on me and I had to switch.
I tried 3m GC3300 for a while - but they
where not as good.  Then I found another
brand called "Highland" which I think are
only in OZ.  They worked almost as good as
the faber-castle ones.

Some people with different laser printers say
3m GC3300 trannys are great.  Maybe with
another laser printer faber-castle might
suck. So your mileage may vary there.

To answer the question from the original
poster.  _YES_ the tranny will shrink with
too much heat.  You have to experiment
with just the right amount of heat.  It
should be JUST below the threshold of where
shrinkage happens.  This is the disadvantage
of using OHTs over the press-n-peel or
photo paper.

Also - when using trannys rather than TTS
paper - there is a knack to removing the
tranny and still leaving the toner on the
copper.  It's about pulling either side of
the tranny to put it under "tension" as it
lifts off.  If you find that large areas of
toner are sticking to the tranny - I will
take some photos and draw some diagrams to
explain what to do (too hard with words).

Good luck - but I will suggest - if you
are serious about PCBs with small* features
or you are going to be doing a lot of them -
have a look at photographic methods.



*small = leadless packages like MLFs - 0.5mm
pins that are 0.23mm wide and 0.5mm from
center to center. YES I know that is 9 thou
track and 10 thou spacing which you can 
produce with TTS - but it's just not that
much fun touching up voids and errors by
hand on stuff that small.