Fletchtronics

Day 2 is all about getting the paste onto the board.

I mention that you can use the syringe to put paste on without spending money on a stencil. Some people also swear by X-Acto knives to get the paste on.

When using this method, it's relatively easy to get paste on to the resistor and capacitor pads which are bigger and separated. The USB connector and the AVR though have fine pitched legs. What you can do in this case is spread a bit of paste across the row of pads.

When cooked, most of it will condense and sit on the pads where it's supposed to go. Any extra solder making bridges can be cleaned up with solder wick.

I don't recommend that method though, except for making prototypes. Getting just the right amount of solder paste on the IC and USB header pins reduces the amount of time spent cleaning up, since exactly enough paste sits on each pad there is far fewer bridges.

If you're using Eagle for designing, I recommend using the sparkfun.com Eagle cam job processor to create Gerber files. It makes a nice "stencil" layer that is exactly what's needed. Surface mount pads are cut and thru-holes are covered.

If you don't have easy access to a laser cutter, the service I used to make the one in the video smtstencil.com.

Alternatively, the service at ohararp.com also looks interesting and I plan on trying this service next time. It's a hobby business and uses a material called kapton which might work a bit better than the mylar sheet from smtstencil.com since kapton doesn't melt from the laser.

You might want to send a "4-up" Gerber with your design repeated several times and get 4 stencils in one. I haven't tried this, if anyone knows of good software for manipulating Gerber files like this please let us know in the comments.

UPDATE: As a couple of readers have pointed out, I have trouble in some spots in this video with the alignment. This is mainly because of the setup in the video. We attempted to pin it down to a black background, and I was trying to keep my head out of the frame. Normally I tape the thing to my metal desk and shine a bright desk lamp down on it, which makes the alignment a lot easier.

UPDATE 2: Ryan from ohararp.com sent me the brilliant idea of arranging extra PCBs in a frame to make alignment easier. So I'll take 4 or 8 boards that have quality issues (some boards were a bit burned during the cutting) and glue them down on cardboard. Voila, instant jig!

UPDATE 3: Here's the new jig. I'm going to test it on a batch shortly and we'll see how it does. I opted for a C-like shape so that it's easier to slip the board out the side after putting paste on. I used a bit of crazy glue between the boards to make the C shape sort-of stable, and then I wood-glued the whole thing to cardstock. It wasn't quite sturdy enough so I glued the back again to another piece of the cardstock. Now it feels solid. The stencil is wrapped around the top edge of the jig and taped down with black electricians tape.

Bumble-B Jig

UPDATE 4: Hey it works great! It's a tiny bit harder to keep it clean but I'm sure I'll find better ways to wipe it. The solder paste step took about 8-10 minutes for a batch of 10 versus the original 15. This time savings can really add up. Alignment is now a non-issue.