So I just wanted to drop by and say thanks, and talk a bit about what I’ve been up to with my stencil.
When I made the prototypes of Bumble-B, I put solder paste on the boards by hand. This is the kind of thing you don’t think about very much during initial planning on your first SMD board. So I tried to apply the paste with a sharp awl, and an X-Acto knife. In the end I just ended up squirting the stuff directly onto the pads from the syringe that the paste came in. It’s a messy technique. It works OK for capacitors and resistors and things with fairly big / isolated pads, but for ICs with a fine pitch, there’s just no way to put paste on each pad by hand without driving yourself totally crazy. So for that case, you end up putting a big log of the paste over a whole row of pins, and then use solder wick to clean up any excess bridges at the end.
Taking an hour per board might be fine for prototyping, but for producing Bumble-B for sale, I needed a faster solution. So I ordered a stencil from Pololu. I swipe the paste over the whole board at once now, and it takes about a minute to do an entire board. I don’t have a jig, the stencil is just taped to my desk, so a lot of that time is actually spent just lining it up with the holes. Making a decent jig is on my TODO.
So I’ve been refining my technique for building these and have learned a couple things.
First, I wanted to do this in batches so I get a few per batch. I found out the hard way that managing more than 5 of these on a small hotplate causes problems, they need too much attention. My lovely $16 hotplate from Target has an uneven heating element, so I’m constantly moving the boards around while heating, to try to keep it even.
Another thing I learned the hard way was how to apply the paste. At first I was very cavalier about the whole thing, used a razor blade in a plastic handle. The handle forced me to have a certain angle which was nearly perpendicular to the stencil. A sharp blade pointing down at my thin mylar stencil turned out to be a disaster of course. One slip and I had a nice slice right through the IC part of the template. Thankfully ordering replacement stencils from Pololu was simple, and I didn’t have to go through the approval process again. The replacement shipped very quickly. Now I skip the blade holder and use the blade naked - and hold the thing nearly parallel to the sheet as I work.
So now I have a decent working technique to build boards quickly. A batch of five boards now takes me between an hour and an hour and a half.
Thanks Pololu for the cool service, I have two suggestions. Perhaps you can start a service where you cut me a jig into thicker plastic, the thickness of my PCB. Second, I really wish Pololu offered a “4-up” version or something, where the design is repeated four times. My tiny design sits inside a mile of wasted plastic, it would have been nice to have spares. I would have paid extra for 4-up.