Homebrew PCBs

Some follow-up notes from today's experiments...

It occurred to me that I've got negative acting film, so I need ink
where there will be NO copper.  This changes the rules a bit :-)

Specifically - the printer can print dark fine lines, even if I say "1
mil" it can print dark enough (I hope) to properly expose the
photofilm.  The problem with the positive print was that bleeding
would cause shorts, but I needed to make the pads as big as they
needed to be, which put lots of ink there.

So what I tried tonight was something different.  I printed a
negative, which is a lot less ink because I normally flood copper onto
the board to reduce etchant usage Thus, rather than print a big pad,
I'm printing a thin outline around it.

I also told PCB to bloat all the copper by 1.5 mil to compensate for
the blurring caused by dot misalignment.  With a 5 mil space, this
leaves 2 mil of ink, plenty for the printer to print with.  It also
moves the ink away by more than the misalignment amount.  If this
works out, I'll just change my design rules to use larger traces and
smaller spaces, like 8/4 instead of 6/6, or 7/3 instead of 5/5.  That
will handle the copper floods properly.

I also tried printing on an old laser transparency film.  Note that
inkjet ink pretty much doesn't dry on this, as it is not absorbed at
all.  I haven't experimented with a heat gun on this; I assume that
something like JetStar will resolve the ink issues.  Anyway...

I'll have to experiment with the ink density.  What happens is that in
large ink areas, there's so much ink that the surface tension pulls
the ink away from the edges.  So for this to be useful, either a more
absorbant transparency will be needed (as noted :), or software to
find these areas and checkerboard their interiors (much like we have
to break up big solder paste areas under thermal pads).  If ink lines
are kept to a few mil, they seem to hold their shape and density
nicely - they're pretty much solid black.  Since they don't dry
though, they will eventually bleed enough to corrupt the plot.

I tried printing on heavy cardstock (8 mil thick vs 3 for paper) with
no change in ink drop misalignment.  Of course, I have no idea how far
the print head is from the paper, or how to change it with my
software.  If I get too close, it smears the ink all over the paper.