Category Archives: Meetings

Proof of Concepts with SMD Parts

SMD Breakout Board Collection
Aggregation of 12 PCB boards for stencil production

As mentioned at the June 12th meeting, I find myself frequently wanting to find the sweet spot for quickly throwing a circuit together to see that gross behavior is as expected and confirm my assumptions correct and the devices perform as advertised before going to the trouble of making a PCB that is likely to be wrong if I’m in too much of a hurry.

But I sometimes find use of SMDs painful with solderless breadboards. First is the headache of having a parallel set of through-hole parts for bread boarding convenience. And of course this is an illusion in many cases: there are no through-hole equivalents for a growing number of components these days. Some shortfalls are obvious, like the current crop of tiny DC conversion chips that seem to be exploring the outer limits of how small a flat pack, no lead package can get. But others are chronic and might be surprising to some readers. Just try finding through-hole versions of some of the specific thermistor types specified for use with things like battery charger chips.  It also gets old quick to have to buy a resistor with the same precision value in both through-hole and surface mount packages.  And is that through-hole diode slapped on really behaving the same as the specific SMD spec’d for the “real implementation?”

I’ve been accumulating little breakout boards as carriers for various SMDs to make this easier, but that often involves a compromise, such as putting a tiny diode between two SOT-23 pads on a breakout that eats up six pins on a breadboard vs two. Other parts are more challenging and lead to semi-monstrosities like this one for a 22uH inductor:

This is what a typical ad hoc collection of parts looks like:

Collection of mostly SMDs on a breadboard

Notice the MSOP12 device is kludged onto a TSSOP14 breakout board. Also notice the (boost) capacitor soldered to the upper left pin of the MSOP12 package in the middle. It’s the barely noticeable bump on pin 12 of the IC (coincidentally below “12”, but electrically connected to header pin “14”, thus the error-prone kludge).

So, although my current breakout collection handles more than 30 devices directly, there are many gaps. This current project will bring the “package coverage” up around the 50 mark, but I estimate I’m only about half done before my hankering for this kind of support mostly dies down. For instance, there would be a lot to gain from handling the common form factors of small switches and connectors, small aluminum electrolytic caps, inductors, transistors, etc. Heck, there’s even real value in making an adaptor that saves me severely abusing a breadboard by cramming TO220 devices into the holes!

Adafruit and SparkFun and others provide some excellent breakout boards at affordable prices (in contrast to Schmartboard’s stuff, which seem to be both too much and too little for my needs). Notice all five of the boards above are Adafruit ones. I have several TI breakout types as well as others. But the actual coverage of package types for the available boards out there is too sparse.

The aggregate PCB rendering above shows the approach I’m taking to build out my collection. This is a collection of 12 PCBs supporting 21 package types that I pulled together for the purpose of making stainless steel stencils.  Some of the boards for larger parts have nearby pads for a few passives to make it less painful to handle bypass caps, boost caps, etc, where short connection paths are important.

The 12 separate designs (and two others) were sent off to OSH Park to get three copies of each design as a small PCB so I can prove out the breakout boards .

Here is what the stencils from OSH Stencils look like for the top and bottom of the “aggregate PCB”:

In theory, the work flow is to put the stencil over a particular breakout (e.g. the one for the SOIC14 package in the upper left corner of the top stencil). The one breakout PCB for the package is captured by a holder made of two acrylic “L” pieces vinyl-taped to a dead flat surface.  A small squeegee (small piece of plastic credit card) is used to paste over the site(s) for a particular board, then parts are placed and hot air or reflow oven soldering is used.

A quick side note about frames. In many commercial environments the stencil is mounted in a surrounding frame that in turn fits into a jig allowing for rapid handling of boards while maintaining precise registration. I was confused Monday: OSH Stencils is only beta testing frame support with two sizes (relative to these boards those sizes are “wow”, and “my lawn is smaller than that”). Contact them for details if you’re interested.

I should also point out that I didn’t spend a lot of time routing these boards. For example, I’ve already had second thoughts about the sense line routes for the two shunt resistor boards that have ‘kelvin connections’ to special middle pads under the resistor ends. Also, as I mentioned Monday, I didn’t spend a lot of time checking things like length to width ratios for some stencil apertures. So the very narrow pad for the 1206 shunt board is technically smaller than the minimum supported size listed for the paste being used (Kester EP256), and the result may be that I can’t actually get paste into this spot properly. The footprint is blown up in the image below. The middle of each set of three pads is .28 millimeters wide. The resistor that sits on these pads is about an eighth of an inch long (yes, king size in relation to how these things are trending).

OK, but how much do those stencils cost?  The minimum is $10, but the incremental cost beyond this is less than a dollar a square inch. The two stencils above came to around $22.  First class postage with tracking adds $2.75 to an order.  My order was completed on a Thursday and I had the stencils in hand the following Monday.  With minimal (.75in) borders, everything you see in the first picture above added less than a dollar to each stencil cost. Note that in most cases you’ll only have a stencil for SMDs on one side.

(Update June 22nd)

The first batch of PCBs came back from OSH Park and are shown in the picture at the top of this posting. The tabs sticking out are cut off and squared off with sand paper before the boards are used.

Finally, it’s important to note that some circuits will not cooperate with solderless breadboards and any arrangement of surface mount parts involving long connection paths. Apart from huge amounts of stray inductive and capacitive  reactance in the circuit paths, the current carrying limits of breadboards are very real, as is the ability of a simple breakout board to shed heat compared to a PCB having copper pours and other design features to properly handle it.

May 8th meeting: well pumps, shade trees, and hot solar

There are a few things to share from this month’s meeting :

  • Folks compared notes about participation at RARSFest last month. The “non-commerical” aspect of Triembed folk’s participation at the next RARSFest is expected to be free. Whoever takes the lead on preparing for the next one can arrange this and get “just the right spot” for tables if they contact the organizers early in the process (4-6 weeks ahead).
  • Shane Trent passed a heavy duty patent law exam and is now a seriously useful resource for area developers wanting to invest in IP via the patent process. Congrats, Shane!
  • Paul MacDougal shared his setup for remotely detecting potential freezups of his well pump.
  • Chip McClelland showed an air temperature logging addition to his repertoire of park service tools being embedded at Umstead. This will allow the park staff to measure the cooling effect of newly planted shade trees.  Chip also passed around his full custom battery management board for folks to admire (built at Pete’s shop).
  • Pete described tests of the Silvertel AG103 Maximum Power Point Tracking solar powered battery charger board and a ten watt Ecoworthy solar panel being prepared for the “Little Library” Jeff Crews (of Splatspace) built and installed at the Durham Scrap Exchange.

December 12th Meeting: Better than Blinky

blinkinglightsfromgoogleimagesPaul MacDougal will talk about alternatives to the classic Arduino beginning program and shares this summary:
“Blink is a great first example for Arduino programming, but a really bad example of embedded programming. With 99.9% of its time spent in delay(), nothing else can happen. This talk will show how to rewrite blink in several different ways to allow it to play nicely with other functions.”

Join Paul and others at NCSU for the regular meeting. More details on the “at NCSU” meeting page.

New in the Meeting Archive

Traffic Intersection Lights

Traffic Intersection Lights

August’s meeting included a project retrospective and demo of Brian Grawburg’s Raspberry Pi/Python driven traffic light simulation. This is part of a set of teaching tools Brian has developed for teaching Python to area youths at the Imagination Station in Wilson, NC Brian’s project is described in his article in the December, 2014 MagPi magazine (page 4) as well as on his project page.

For a handy set of links to this and other TriEmbed presentations visit the meeting archive page.

June 13th Meeting: Open Mike

The monthly meeting of the Triangle Embedded Interest Group will be this Monday starting at 7pm in room 2201 of NCSU Engineering Building Three, 911 Oval Drive, Raleigh. This will be the bldg and room through August. This is straight across the green from building 1 (down the hill from building 2).

This month will be completely “open mike”. Bring your projects, short show and tells, questions and project challenges to share. If you have something suitable for a video recording on YouTube please let us know ahead of time. Ditto if you would like to present at a future meeting.

Maps and other details.

Archive of past meetings.