Bdale Garbee: TeleMetrum Power Supplies
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!