After a long hiatus for various reasons, I finally had the chance to try my hand at loading a TeleMetrum board today!

Sadly, I wasn't able to completely load any boards, because I somehow ended up with the wrong Digi-Key part numbers for 4 capacitors, one of which is a critical value. I placed an order for the missing parts and a few other bits we'll need eventually, hopefully they'll be here in a couple days and I can try to load a board with all the parts.

Instead, what I did today was a a partial load of a board with the goal of testing the various power supplies. Seemed like a good idea, since I've had my share of odd problems with power supplies in the past. This also gave me a chance to try out my solder paste stencil, get some experience hand-placing the tiny 0402 passive parts, and an excuse to see if I could hit the solder reflow temperature profile adequately with my electric cooking skillet and IR non-contact thermometer.

The good news is that it all worked right the first time! We're successfully charging a LiPo battery from USB power, and successfully making 3.3 volts for the electronics from that. The resistor divider designed to allow the LiPo battery voltage to be sampled by the CPU's analog to digital converter is also working fine.

A few observations are in order, however.

Getting the right amount of solder paste down on the board requires some finesse with the spreading blade and the stencil. I think the paste was a bit heavy around the CC1111 footprint, as I ended up with some bridged pads. Since I didn't load the actual CPU part, it could just be that not having something real there to wick up some solder meant there was enough to form the bridges. Don't know. The amount of solder on the ground pad in the middle of the chip looks good, though!

The 0603 sized LED in the power supply circuit was the only part that didn't self-align correctly. I may have had it a bit too far to one side. The pads on the part have notches in the end, and it looks like one side of each is more or less centered, so I think surface tension did the best it could with what I gave it to work with. The LED soldered ok, isn't shorting to anything else, and clearly works fine... but I'd be happier if it were aligned better.

Found and fixed a solder bridge between two pins on the surface-mount USB connector before first application of power. Could be further data that the paste layer was a bit too thick. On the other hand, I had to wiggle the connector around a bit to get it aligned correctly, so I may have smudged the paste into a bridge while doing that.

I also note that there's some visible flux left on the surface of the solder, particularly on big paste areas like the feet of the USB connector. I suspect this means I didn't dwell long enough at either or both of the preheat or ultimate reflow temperatures. Since all the parts clearly soldered adequately to function, I'm still pretty happy... but since the rockets we fly experience violent accelerations during boost, this is something I'll play with on future reflows.

All in all, a great way to spend an afternoon, and a big step forward for the TeleMetrum project!