Screaming Circuits: November 2009


Microcontroller In The Middle

I've written about Open Source hardware before, such as the beagleboard and Arduino. Those are both great options for folks needing to get moving on embedded microcontroller development. The Arduino is Mbed-microcontroller-angledpretty low-end and the Beagleboard is pretty high-end. I think I've run across a good step right in the middle.

A while back at the ESC show, we spoke with a gentleman from ARM about a project that would include an online IDE, and now, here it is. It's not exactly the same as open source, but it solves many of the same problems that open source solves. Mainly, it's a quick and easy way to get up and running with an ARM processor. Well, it looks easy, anyway. I haven't tried it yet. I think I'll see if I can get one and give it a shot.

By the way, we did not build this board. We have built some Beagleboards, but not this particular product. It certainly wouldn't be a problem for us, but that's not why I'm writing about it. It just looks like a great half-way point between something like the Atmel-based Arduino (or the PIC microcontrollers that I use) and the Beaglboard which uses the super-speedy ARM Cortex-A8.

If I can run one down and find the time to fiddle with it, I'll let you know what I think of it in actual use.

Duane Benson
Stay tuned. Bulletins as event warrant.
Maybe...

Speaking of Art in the Process

I really like this picture. It's nothing particularly special. Just a BGA-type attachment point of load (POL) power module in the middle of a big PCB, but the contrasting colors, the angle, the range of focus, the component positioning...  It just looks cool to me.

PTH05060 on boardI know the function is supposed to be the most important thing, but I've always felt that there is a lot of art in PCB design. It's been said that an airplane that looks good will fly good and I think there's something in that phrase for electronics too.

No. I'm not advocating putting the visual appeal ahead of clock rise times, trace impedance and current capacity. It is first and foremost, an electronic device with an important function to deliver. But, I think a visually attractive, while still superbly functional, product makes a statement about the designer's overall attention to detail.

Duane Benson
Or, it might just be making a statement about how tight the development schedule is...

Via-In-Pad - Let It Slide???

No.

Sometimes, you can get by with vias in your pads. Sometimes, but not very often. I wrote about this a while back here. The thing is, I was talking about big pads - like QFN or QFP thermal pads and stuff like that. We never like to see it and it's always a manufacturing risk at some level, but as described in the earlier post, sometimes you can just roll with it.

9x13 via in pad BGA landPretty much never with a BGA pad though. The pic on the left shows just about aTiny vias in qfp pad worst-case scenario. Very big. Very bad. Relatively very big holes anyway. This is for a .5mm pitch Bluetooth module BGA.

The vias in the image on the right worked okay with a QFP because they're really small - practically closed up - and it was lead-free solder. We still wouldn't want to see a via, even that small, in a BGA pad though. Process variations leave enough opportunity for a few of the vias to be open all the way through and even if one BGA ball gets sucked off the BGA, you're out of luck. Even if it's just partially sucked off and still connected, it's much more susceptible to cracking and things like that. (By the way, we did find a way to build the board on the left and make it work. We won't guarantee that we can make something like this work though.)

A lot of fab houses will epoxy fill your vias these days. Even micro-vias. And, yes, you should even have your micro vias filled and plated. Especially with small BGAs. It's just not worth all the risks that come along with it.

Duane Benson
We need little moles to fill those holes

And, Another Thing...

I got a couple of pretty thorough comments on my copper pour post over in the Circuits Assembly blog where it's also posted.

David le Comte wrote:

"...On two layer boards (with 5V CMOS logic in particular) it is very difficult to pass CISP-22 EMC tests without a well grounded flood plane.

In the 1980s it became a requirement for more and more categories of electronic equipment to be tested for EMC compliance.

From first hand experience, I have seen how just adding a flood plane to two-layer boards can reduce EMC by 20dB. (We had to revise existing boards to pass EMC tests during the late 80s and 90s)..." (See his entire comment on the Circuits Assembly blog link above)

That brings up a very good point. EMI. In the prototype/experimental world and the hobby world, there are so many cases where EMI isn't too much of an issue. We don't always think about it. If your project is going into a consumer or business consumer product, no questions, though, EMI is a big consideration. As David indicates, a grounded pour or plane can go a long ways toward keeping stray interference down.

I wonder if an Arduino could pass any EMI standards. Has anyone run it through a lab? Maybe with some good shielding. I bet the "Knight Rider" teeth I made for my pumpkin this Halloween (with a point to point wired PIC16F819) just radiates all over the place.

Duane Benson
Pore, pour, pitiful me

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