I'm in a heavy prototyping phase with my software-defined radio projects. Every time DorkbotPDX has a board order, I make sure I've got one or two boards in the order. Of course, once I get the boards, I have to assemble them. And since radio-frequency projects involve high frequencies, small surface-mount components are essential.
How small? It's common to use "0402" resistors, capacitors, and inductors on a circuit board that operates at gigahertz frequencies. The name "0402" comes from the dimensions of the components – 0.04" x 0.02" (1.0mm x 0.5mm). That's really tiny. I don't want to hand-solder those because I drink too much coffee and can't hold still enough… So I investigated reflow soldering, a technique my Dorkbot colleagues have used with great success. It usually involves an cheap hot plate and some solder paste. The solder paste is applied to the board at places where solder needs to join components. Then the components are placed on top of the solder paste points. When the assembly is heated, the solder paste melts, and the surface tension of the molten solder pulls the components into place on their respective solder pads. It's immensely easier than hand-soldering, and produces beautiful results.
The Hard and Messy Way
My first attempt involved applying solder paste in coarse blobs on the board, using a toothpick. That barely worked. The largest problem was that some solder joints had too much solder and flowed together with other solder joints. Other solder joints were too dry and in some cases didn't connect reliably between the board and component.
In the real world of mass-production circuit board assembly, solder stencils are used. The solder paste is applied through a screen (stencil) that ensures the right amount of solder is applied in precisely the right places. As an individual, you can order laser-cut stencils cut from metal or expensive, high-temperature Kapton plastic. I didn't want to wait for a stencil, take on the added cost, or have to procure large amounts of specialized plastic.
A colleague of mine suggested cutting stencils on the laser cutter we have in the office. Our laser cutter can't cut metal, but it can cut paper. Since I'm making prototypes, not mass-producing products, it's OK if the paper stencil is destroyed by using it. So I cut a stencil from paper and tried it.
The result with the paper stencil was OK, but not great. The paste bled underneath the stencil. In those spots, I had too much solder. It was also a bit tricky keeping the paper still as I applied the solder paste with the squeegee. Then, I had a revelation…
Adhesive Mailing Labels – SRSLY?
I have these Avery mailing labels that I use to address Chronulator kit shipments to my customers. They're made of paper, and they stick to stuff. What if I cut my stencil from a mailing label? Let's give it a try…
To start with, I need to prepare a solder paste template. I output a PDF with the paste layers shrunk a bit. I shrink the paste layer because the mailing label is quite thick, and therefore so is the paste when it's applied. So I need to compensate for the thick paste by making the paste areas smaller.
From the paste template file, I laser the mailing label with the Epilog 45W laser at work. My settings are "speed 25" and "power 25". I use raster mode, as it tends to produce more accurate, even results.
Then, I cut out the template. I leave enough border so I have some room to spread the solder paste without getting it on the board. I also need a border to hold on to when I peel off the label.
I clean the circuit board with rubbing alcohol to make sure there's no dirt or grime that will foul up soldering. Then I stick the template to the circuit board. Lining up the stencil is a bit tricky, but the mailing labels are only mildly sticky, and can easily be pulled up and reapplied if the alignment is off.
I've noticed that the mailing label adhesive gets stickier after being on the board a while. If you leave the label on the board for more than a day or two, it's very hard to remove without ripping (and messing up your solder paste).
Like Buttering Toast
I apply the solder paste with a little metal squeegee. At first, I apply a lot of solder paste (several times what I need) and squeeze it against the board to get the paste deep into the template's holes. The layer of paste is very thick.
After I have a thick layer of paste on the board, I scrape off all the excess – until I can see the template paper through the paste. Sometimes, I wind up scraping some of the paste out of a hole in the template, so I have to re-apply the paste and re-squeegy it to be smooth.
Once the solder paste is even, it's time to peel off the template!
The hardest part of the whole process is placing the components on the board without damaging the solder paste. It's essential to have a good eyesight, a good pair of tweezers, a steady hand, and lots of patience.
If you do smudge your solder paste, you can try to reshape it with the tip of a pen knife. And if one of your components doesn't want to move into place, nudging it with the side of a pen knife blade seems to be the easiest way.
Turn On the Heat
After all the components are placed on the board, gently transfer it to the hot plate. Or better yet, put the board on the hotplate before you place the components.
Other people use fancy PID controllers to control their hot plate temperature. Since I'm building very small prototypes, I don't need a lot of temperature precision. I warm up the board to the point where I see the first wisp of smoke (130C or so, I think), then turn off the heat and let it sit for a minute to stabilize the temperature. I turn the heat back on until I see the solder melt on all the components. I immediately turn the heat off. I let the board cool down for a couple of minutes, then move the board off the hot plate to cool the rest of the way. I'm probably stressing the components by shock-cooling them…
Go Forth and Modify
At this point, the board is done and it's time to hook it up and see it work! So far, all the boards I've made with this technique (four) have worked great (except for the solder flux incident). Good luck to all who try it, and I hope to see people discussing improvements…