Homebrew PCBs

On 3/10/2011 5:26 AM, David C. Partridge wrote:
>
> ....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