Re: short-run PCB fabs

> With all this talk about fab-ing PCBs, I'm surprised that
>no one mentioned doing it at home. The guy who sits next to me
>here downloaded a PS file with a PCB layout on it, xeroxed it on
>a sheet of that iron-on resist stuff, and etched it with etchant from
>Radio Shack, I believe. It turned out better than we both expected.

I have a bunch of this stuff and haven't tried it yet. I hear different
stories from different people. The mix is more negative than positive from
the guys I know...

> The only limit is on the minimum size of the traces. I think
>it has to be relatively large (like the size of power/ground traces
>on video game PCBs.)

There are some more gotcha's--

1) You need a good dark toner print on the sheet. Photocopiers don't do
that too well. New laserprinter cartridges work well though. ;-)

2) The result is only as good as you can get the toner-transfer to stick to
the PCB. Some people can do it with an iron, or you can get one of those
modified desktop laminators (for ~$150).

3) Trace widths vary with your skill at the process (chemicals used, temp
of etchant, etc.). I guy I knew in college did 8 mil traces pretty
regularly at home, but just using the photo-resist stuff in a
home-darkroom.

4) Holes. Drill your own. You have to balance "size of pad vs. size of
drill bit" and have sharp drill bits or you'll rip the traces off the
fiberglass. Alignment is annoying as all hell too-- it REALLY helps if you
leave the center of the IC pad with a "hole" in the pattern. Most (modern)
PCB routing software doesn't do that by default, but having the hole gives
the drill bit a place to center.

5) Holes. Not plated through-- you'll need to run wire through for vias,
and make sure chips are soldered topside and bottomside if the nets connect
on both pads.

6) No plating. You'll have to buy something like TINIT to tin the copper
on the PCB or it'll oxidize and be really tough to solder on (and look like
hell).

> BUT, looking at Clay's ESB layout, for instance, I think
>it would be possible to do that at home -- it would just take a
>bigger board. Not having seen a Quad-Pokey board, I can't even
>guess if it's possible, but I'm thinking of making my Cine.
>Exorcisor board this way.

Yeah, maybe... I run a lot of traces in-between pads with 10 mil spacing
and that "freeway" of traces from the 22V10 are 10mils wide with 10mils of
space between them. I think the quad-pokey would be easier. It actually
looked like 1/8" trace widths (like you'd make with a Radio-Shack
dry-transfer kit) to me.

For quick-n-dirty a "Sharpie" permanent-ink pen works as an etchant-resist
pen. Just draw on the PCB. If you don't care if the traces and parts are
on the same side of the PCB you can just "draw" the circuit on.

-Clay

Clayton N. Cowgill Engineering Manager
_______________________________________________________________________
/\ Diamond Multimedia Systems, Inc. clay@supra.com
\/ Communications Division http://www.supra.com/
Received on Wed Nov 12 13:57:09 1997