Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

Ran out of aluminum foil, decided to try brass etching again (for a
solder paste stencil). Came out pretty good, so I figured I'd share
some thoughts.

I used only rounded-corner holes. Although all my SMT pads are
rectangles, I've always had bad luck with those corners undercutting
while making stencils so I converted all the pads to round before
printing the paste artwork.

I used wet lamination of the UV film, followed by passing it through
the hot laminator at 205 F. I had tried dry lamination of UV film
onto alumimum foil before, but the foil isn't stiff enough to always
stay flat so I got wrinkles. By wet laminating first, I hoped to
avoid those problems, although brass is stiffer than foil it's a lot
more flexible than PCB. I was using 2.5 mil brass, equivalent to 2 oz
copper.

I printed two copies of the artwork, mirror images, and taped them
together ink-side-in. I slid the brass between them and taped the
brass to one of the sheets, so it was all registered and wouldn't
move.

I over-exposed a little. I worried more about too-big holes leading
to weak metal bridges falling off. I could have gone with even
smaller holes though, but I haven't tried applying paste with it yet.

NO AIR. I unplugged the bubbler and used a gentle side-to-side motion
(i.e. motion perpendicular to the brass's surface). I figured, once
the etch breaks through, the current through the hole will clean up
the edges of the hole with minimal undercutting. That seemed to work
really well too.

Although the holes are bigger than the artwork - I'll measure and
correct for that for the next time - the result is nearly "perfect"
and I'm quite happy with it :-)