Friday, January 20, 2012

Encoder Board Evolution

The old encoder board I fabbed for Data Bus has failed. Rather than repairing several lifted traces on the old one, I felt that a more reliable approach was to order a professionally fabbed board via OSH Park.

I took the opportunity to redesign using SMT components so I could shrink the board.

Old board above, new SMT board below
The old design was based on an LM393 dual comparator IC configured as a schmitt trigger. So is the new board, but instead of a single IC, it was easier to route traces using individual, Texas Instruments TL331, SOT-23 comparators. All the passives are 0603 size and the LEDs are 1206 size.

I'm still working on my technique for hand soldering SMT. Here are some things that seem to work so far:
  • Push components out of their tape using the point of a small nail
  • Tin the pads then suck off the solder with solder braid
  • Use a flux pen on all the pads
  • Use a really small tip and a decent iron
  • Use 0.015" solder; I use Radio Shack silver bearing solder
  • Use non-magnetic pliers/tweezers to pick and place parts
  • Use the nail tip to gently nudge them around on the board
  • Hold the parts down with the head of the nail while you solder
  • Tack down one side of the component, add solder if needed
  • Then add solder to the opposite side while holding it
Components tend to get pulled vertical by solder surface tension if you have too much solder on the pads so that's why it seems to work better to remove some solder and to hold the component down with a nail.

It's pretty darned tedious. I ended up reflowing the SOT-23 comparators using my reflow skillet. As I have two encoder boards to populate, I will try reflowing all the parts on one of them and see if that is any easier.

Hopefully the new board will hold up better to the rigors of testing and competition than the home fabricated board. Plus, I'll have spare boards in case something goes awry.

3 comments:

  1. I have been writing software since 1996, so I've had to relearn everything I'd forgotten about electronics in my previous career as a hardware engineer. I didn't get deeply into SMD "Back in the day" so learning to solder them by hand now was quite a learning curve!

    When I ordered the parts for my first board, I ordered the METRIC 0402 size... which are 01005 size in english... they're impossibly small. I was quite frustrated until I realized what the problem was! The leadless chip carrier packages like the 16IS750 SPI UART chip are also quite challenging, I spent DAYS soldering and pulling chips with a hot air rework station to get them to work.

    I'm a big fan of the "Stencil and Skillet Reflow" technique of making boards. it's cheap, simple, and you get really good results. Sparkfun has a bunch of good tutorials on that and SMD rework rechniques. I learned a lot from those.

    One thing I had an issue with, though... I used a Kapton stencil, and some of the pads broke through to their neighbors while applying paste on the SPI UART chip... the pads are just that small. All in all, I wasn't thrilled with the Kapton stencil, the laser warped the plastic where it cut, not so bad for 0603 size parts, but really fine pitch chips were pretty warped.

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  2. I made a similar mistake: I ordered some 0603's only they were metric 0603's so I got these microscopic little 0201 dots. Yikes.

    Thanks for the tips, I do want to try the stencil approach. I've had good enough luck with skillet reflow using pretinned pads. The stencil / solder paste must be a dream.

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  3. Yeah, Stencil and Reflow Skillet is the way to go. The Kepton stencil is great for the bigger parts, down to... say... the ATmega328 or similar chips, but it starts to show it's weakness on the 16IS750.

    Once you stencil on the paste, it's easy to place the parts. Be sure to cook only one at a time in the skillet, as the temperature will be slightly different in each area of the skillet. The great part is the components float up on the solder as it melts and auto align onto the board. You get NICE results.

    The URL for the stencil vendor I used is http://www.applied-electronics.com/product/mylarkapton-laser-cut-smt-stencils/ they cost 25 dollars each, but if you have software that can aggregate your designs on a single sheet, you can put several projects onto one sheet.

    So I'm not the only one that accidentally bought the metric size part! Lemme tell you, those metric 0402 parts are TINY. I mean... if you look away from them, they can wink out of existence "Schrodinger's cat" style. I was like... "Oh come on! I can't solder these! I can't even pick them up with the tweezers!"

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