From 3be63ca93cf4968d8f0a592de22cecd4f480474b Mon Sep 17 00:00:00 2001 From: Bdale Garbee Date: Sun, 1 Mar 2009 18:25:18 -0700 Subject: [PATCH] how I spent my afternoon today... --- .../blog/posts/TeleMetrum_Power_Supplies.mdwn | 59 +++++++++++++++++++ 1 file changed, 59 insertions(+) create mode 100644 bdale/blog/posts/TeleMetrum_Power_Supplies.mdwn diff --git a/bdale/blog/posts/TeleMetrum_Power_Supplies.mdwn b/bdale/blog/posts/TeleMetrum_Power_Supplies.mdwn new file mode 100644 index 0000000..54fdb85 --- /dev/null +++ b/bdale/blog/posts/TeleMetrum_Power_Supplies.mdwn @@ -0,0 +1,59 @@ +[[!tag tags/rockets]] +After a long hiatus for various reasons, I finally had the chance to try my +hand at hand loading a [TeleMetrum](http://altusmetrum.org/TeleMetrum) +board today! + +Sadly, I wasn't able to completely load any boards, because I somehow ended +up with the wrong [Digi-Key](http://digikey.com/) 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! + -- 2.30.2