Memos From the Cube

This afternoon I tried my hand at building an AVROpendous board from the kit sent to me by the very generous Matt from the company of the same name. The “kit” contained the bare silkscreened PCB, plus all the components I needed to build it up – essentially, it’s the same finished board that everyone else can buy, except the parts came in a bag rather than attached to the board .

Before today, I’ve only tried surface mount soldering once, on a half-dozen resistors needed for a microcontroller development board last semester (MSP430, containing a TI based microcontroller) in a reasonably large SM size.

This was certainly a learning experience. I got out my magnifying glass, my soldering iron, fine pitch solder, tweezers, solder wick and my components, and set to work. My verdict; SM soldering is fun, but at times quite frustrating. Sneezing and literally losing a component isn’t fun, so it was a good thing I had extras of many of the components. While I don’t know what the exact packge is for the components on the AVROpendous board, I do know that they’re DAMN TINY with the resistors measuring about 1mm lengthwise. It’s a good thing I had a quick lesson with my microprocessors lecturer before I started.

After making a reasonable mess of it but enjoying myself thoroughly, I had a finished board. While I ended up with far too much solder on most of the components, the end result wasn’t too shabby, for a first timer. Holding my breath, I plugged it in, and promptly got nothing.

Ever the optimist, I started running some tests. Stupidly I put in a LED directly to the 3.3V external rails to confirm the board was getting power, which it turned out it wasn’t. Bridging the external VCC to the VBUS line of the micro-USB connector finally resulted in a lit LED, and – what’s that?! – a working board.

Once I verified that bridging the supply worked, I went to grab the LED from the rails to put it away (its purpose as a quick go/no-go tester complete) and ended up almost succeeding in welding the plastic to my finger. Lesson to the wise; 5V + LED + Timer + 200mA supply + Finger != a happy Dean. Nevertheless, I poured over the schematic once more.

I found a likely source of the problem; the only point the board and VBUS meet is via L3, a small ferrite bead inductor. Knowing that a single point of failure usually means that it is the reason for a given failure, I bridged the inductor (this is safe, the inductor is for noise suppression purposes) and head Windows happily enumerate my board.

Once I re-soldered L3, the board was up and running sans a working power LED. That turned out to be a case of reverse polarity, a silly mistake which I do not feel at all silly about, since I haven’t a clue how to tell the polarity of a surface mount LED. As far as I can see, the darn thing’s completely symmetrical, which meant that good ol’ Moore caused me to mount it backwards.

That fixed, I now have my first working surface mount board, and a new addition to my AVROpendous family!

And so ends another enlightening experience. I think I’ll grovel to my lecturer solder the MEGA2561 onto the board John so kindly sent me (more on that in a later post) however, as I really don’t want to kill such an expensive chip.