>
> ....Also for double sided boards, with these think gloss papers, how
> to you register the two sides as you can't see the fiducials through
> the paper?
>
I can get within a few milliinches with the following process.
1) Laser print one side (preferably the component side) on clear plastic
(presentation foils for overhead projector or some such.) My plastic is
4 mills thick and will later become the solder mask, if you want one. I
also add a couple of target patterns outside of the active area on the
ends of a diagonal encompassing the active area plus one at another
corner just to help keep front/back correct through the next steps
2) Tape this plastic along one edge to the future PCB.
3) Drill three holes at the ends of a diagonal spanning the active area
plus the odd one for front/back. Drill through the plastic and PCB, but
don't obscure the targets -- preferably just beside the targets. I use
a 0.0385" carbide drill. This is one good choice because #2 sewing
needles are slightly bigger in diameter and will jam into the hole.
(This is very useful as you'll see.)
4) Print your front and back sides on thin glossy magazine pages.
(Thanks to the Homebrew list for this wonderful tip. It's been more
successful than any other paper I tried.)
5) Cut out the active area slightly smaller than the plastic printout
and include the targets. Slip the "back" side under the plastic and
line it up with the plastic print. Remember that the paper patterns are
"backwards" so the "back" will be aligned on the front side of the PCB
and the "front" will be aligned on the back of the PCB. Take a sewing
needle and puncture the paper through the two drilled holes. The
plastic and PCB act like a precision paper punch putting the hole in
position as accurate as you line up the patterns, which is very easy to
do within a few mills.
6) Now un-tape the plastic, turn it over, and place on the back side of
the PCB using two sewing needles through the drilled holes to line up
its position. Tape on one side. take the "front" side paper pattern.
Slip it under the plastic and line it up and again puncture the two
alignment holes it with a needle.
7) Remove the plastic. And taking the "front" paper, place it on the
"front" side of the PCB using two needles to align it to the PCB.through
the holes in the paper. Tape it down. Repeat for the "back" side with
the "back" paper pattern.
8) Now laminate to your board using your favorite method. So far I've
been unsuccessful using a laminator, so I use an iron set to about 345
degrees F.
9) You now have patterns that are aligned with total error of a few
milli-inches. I do 10 mill holes in 30 mill pads with no problem with
hand drilling. You can now take the plastic and drill out the pads
where you want solder and so turn it into a solder stencil.
Side note: The drill I use is a very old ( greater than 35 years old)
precision drill press about 9" high. The problem was accurately hitting
holes as you're looking at the board from about a 45 degree angle. My
solution was to put a cheap ($10) USB web cam under the table looking up
through a hole in the table. Between the camera lens and the table I
placed a piece of plastic on which I mounted a couple of SMT white LEDs
to illuminate the PCB on the table above the camera. I tried to
physically adjust the camera to "bore sight" it to the drill. but it was
just not satisfactory. The ultra simple, precision solution was to save
the clear plastic pieces cut out of vacuum formed plastic packaging lids
and bottoms such as used for lettuce, grapes, cherries, etc. Tape a
piece of that to the table. Then drill the plastic with the 0.038"
drill. The hole is very visible on the webcam, but you can still have
a large field view of the PCB pattern above. So after the board is
etched, I drill the board by aligning the PCB in the webcam view.
0.010" holes are very easy to do this way by hand. ( The only bad thing
is you may not notice where your fingers are as you slide the PCB around
to drill it. So once while concentrating on the monitor screen, lining
up the PCB, I started to drill and put the drill through my thumbnail
into flesh. Ouch. So in this case, don't do like I did. Look down at
your hands also and keep your fingers out of the drill path.)
Regards,
Charles R. Patton