Sunday, September 23, 2012

Adventures in reflow soldering

Late in the afternoon I woke up feeling well enough to do some hardware work. I'd been considering whether to populate and test another small section of the Instruction Pointer board, or just do the rest of the board in one marathon session. Experiments suggested that if I populated much more I wouldn't be able to use the solder stencils for the rest, which sort-of settled the issue.

At about 6pm I used the stencil to put solder paste on as much of the board as I could, then started placing components. At first this was slow, but eventually I developed a fairly efficient rhythm. Still, I had a bit more than half the board to populate, or about 300 parts to place. It took me about four hours, by which the solder paste had mostly dried out, so rather than the components settling into the paste like a foot into soft mud, they sat on top. I was rather worried that even with a low airflow, the hot air soldering wand would blow components away before the solder paste reflowed. Indeed, this happened with a few components, but most stayed in place.

Reflow soldering has to be seen to be appreciated. The dull-gray solder paste suddenly turns bright silver, and misshapen smears suck together like a scene from the Terminator 2 movie. Components that are slightly misaligned rise and twist into alignment, then settle to sit flat on the board. It looks like animation, but it's surface tension in action.

And then there are the failures. Resistors rise on one end until they look like partly-raised drawbridges, a condition aptly named "drawbridging", or until they're completely upright, known as "tombstoning". Transistors rise on one lead, looking like an acrobat balancing on one arm. All these have known causes, mostly related to poor solder paste application or uneven heating, both offenses I'm horribly guilty of committing. A couple components simply flew away, like a speck of dust blowing in a gust of wind and barely perceptible until a careful inspection shows a couple of empty pads where there should be a resistor. There's a reason I bought 100 extra resistors.

I think for the next board I'll just populate the whole thing at once, which will minimize my problems with the stencils. I may also try the hot plate method of reflow, which gives a more even temperature distribution and avoids blowing components away.

With the missing components replaced and the soldering problems fixed, I checked for major problems (like a short between the +5V and Ground buses). Not finding any real problems, I plugged the thing into my test jig. My simple test driver doesn't give a real test, but the few signals I probed looked reasonable.

By this point it was midnight, and I crawled off to bed.

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