Screaming Circuits: December 2009


Via Near Pad

Passive vias in both pads Vias don't go well in pads, of course. In fact, I think it's fair to say that vias go as well in pads as large igneous rocks go in peanut butter and jelly sandwiches.

But it's not just vias in pads that annoy people. Vias near pads can be pretty much a nuisance too. In the SOIC pictured here on the right, the pin 1 lead is at risk of having the solder Via near pad SOIC wicked off the pad and down into the via. If you've got to have a via right near a pad like that, always make sure there is something between the pad and the via that will keep the solder away. A thin line of solder mask, or even silk screen, like with the pads and vias on pins 12 and 14, will do. Anything to stop the solder from going where you don't want it to go.

Duane Benson
Jersey barriers, perhaps?

MOSFET Temps

How do you keep your MOSFETs running cool?

  • Flyback diodes?
  • Managing the PWM frequency?
  • Big heat sinks?
  • Fans?
  • More MOSFETS in parallel?
MOSFETs

Duane Benson
Remember, no matter where you go, there you are.

My Screaming Favorites from 2009

Years ago, it seemed like the last two weeks in December were just full of retrospectives on the year. It was all over the media all the time. I don't really hear so much of that any more, which might be a good thing, because it kind of made me a little sick at times. Certainly many lists are around, but it just doesn't seem to be such a big deal. Or maybe, I just don't pay attention anymore.

I'm in just that kind of a mood though, so I thought I'd put out my own little retrospective. It's not really a top-ten list, but close enough.

Trade shows: Still got to be the Embedded Systems Conference. I love engineer shows. Years ago, I used to go the Comdex and CES. Way, way back, I went to the West Coast Computer Faire (I was there when the Mac was first shown). Comdex and CES all so glurgy and more about hype then real stuff. At ESC, most of the companies are there showing things that I like and most of the attendees are there to actually learn. It's just cool.

It was kind of sad to see such a sharp decline in companies participating both in San Jose and Boston this year. I think we saw about the same number of folks wandering the show floor as past years, so that at least was good, but I do hope this show remains strong.Ti_beagle_board_top2 (Small)

Embedded dev boards: This is a three-way tie between the Beagleboard, from Ti, the mbed, from NXP /  Arm and a PIC based board that I made myself. 

The Beagleboard really sets a new standard for power and accessibility in the embedded development world. As far as I can see, it's a game changer in those terms. Really fine work and making it affordable and open source has made it accessibly to a huge community that would likely have not jumped were it positioned as a high-cost closed development system.

Mbed-microcontroller-angledThe mbed does for ease of programming and learning what the Beagleboard does for power and features. mBed is truly amazing in terms of how easy it is to get up and running with a 32 bit processor. Again, I don't think I've seen this big of a leap in ease of development ever.

I could list the Arduino here, and it's a viable contender in the 8-bit class, but I'm876-CTRL_rev2.1 001 g more of a PIC guy and I'm a little biased toward mine because, well, it's mine. The Arduino gets enough attention in other places anyway. Mine is of a similar caste as Arduino, was first designed in 2005 and has gone through a number of iterations since. It has IMHO a better I/O structure and a little bus to easily connect to some small motor controllers I've designed.

New chip packaging: Package on Package (POP) has been around for a while, but I think it's just finally starting to come in to its own this year, and we've just started assembling it this year. It's a pretty cool way to chomp some more size out of a small little embedded design. The Ti OMAP (used in the Beagleboard) isn't the only POP that we've assembled here at Screaming Circuits, but it's probably the most visible example.

Consumerish thing: I'd have to say electronic ink, as used in the Kindle and other electronic book readers. I haven't spent a whole lot of time with any of these, so I'm not totally sure it's ready for prime time yet, but I think it's very cool and very promising.

Movement: This is a pretty easy one. The open source hardware movement (I hope). Open source has been serious business in the software world for a long time, but until recently, the hardware community hasn't jumped on the concept. Now we have Beagleboard, Arduino and a gazillion others. There are even a number of web sites pretty much devoted to open source hardware and related subjects like circuit bending.

My only concern is that the hardware folks may get overwhelmed and go back into hiding. Over on the Beagleboard Google group, though it's supposed to cover both HW and SW, the topics are virtually all software related. A few HW exclusive discussion boards (like chiphacker.com) have popped up and may get traction, but there's a lot of catching up to do.

My honorable mention in the movement department would be the closely related "after hours hardware" community. This includes hobbyists, circuit benders and hackers (of the good sort). I think the barriers to entry to starter hardware development are lower then any time since the early 1980's. That's a good thing. The more people involved in electronics as a hobby, the more we will have heading down that career path and the more new small businesses we will have start up. All a very good thing. Certainly a lot of creativity going on in this arena.

That's all I've got for now. So I'm calling the list closed. Maybe more later. Maybe

Duane Benson
Merry Christmas, Yo, Ho, Ho Green Giant and A Bottle of Rum

You got C in my L. No, you got L in my C

Parts too close

Just another tale of a poor little capacitor feeling lonely and trying look up to a big inductor for advice and guidance. Sadly, Henry, the inductor rebuffed the little uf with nary a word and kept his emf to himself.

Duane Benson
Ell Sea can you say the donzer lelight

Pitch Switching

I recently started reading the magazine Chip Scale Review. It's a different take on things than I'm used to. Most of what I read for work is in the engineering and assembly realm, but this one goes back to the component packaging. I think it will be a good one in terms of keeping up on what sorts of things we'll need to be assembling in the future.

So far, I haven't seen anything really scary in it. There is talk of .3mm pitch BGAs, but those aren't totally new. I'm not sure if we've done any .3mm pitch before, but we've been doing .5mm for years and have done plenty of .4mm pitch as well, even in package-on-package (POP) forms.

Pitch switching adapter Speaking of really fine pitch BGAs and CSP type things, one topic I found interesting has to do with pitch switching adapters. It's basically a small PCB platform that has an underside footprint of a 1mm or 1.27mm pitch BGA and a land pattern on top for a fine pitch BGA. It has solder balls on the bottom, so once sandwiched together, it's treated just like a big BGA for assembly purposes. [Credit where credit is due: The image I'm using came from the Aries Electric web site.]

Such a part can negate the need to re-spin the PCB if your big part is updated and replaced in a new fine-pitch form-factor. (Although, personally, I can only imagine that if the chip is rev'ed, there will be some other change that has to be made to go along with it). The theory is, that if you've got a really expensive design, this might be a viable option allowing you to upgrade without a relayout.

Certainly though, at the very least, this could allow you use some newer fancy chips without having to resort to filled micro vias and tiny trace & space advanced (expensive) pcbs. Could be quite handy and same some money.

Duane Benson
Platform shoes are back!

What's Wrong With This Picture?

Vias through stencilopeningsWhat's wrong with this picture?

I forgot to warn you that there'd be a pop-quiz. It's only worth 10% of your final grade though, so not to worry.

We're fully into the rainy season here in Oregon now. It's dumping and the puddles are puddling up. Last weekend, we thought we were in for another big snow and ice mess, like December 2008, but it turned out to be just media hype. We warmed up and it's back to the usual 40 degrees and raining.

Unlike with a QFN thermal pad, we do want open holes in our streets. We want the liquidy stuff to go down underneath and not stay up top, 'cause that would make our feet wet and Johhny doesn't like wet feet.

Duane Benson
If it's bipolar, it's a "collector." If it's a MOSFET, it's a "drain." What is it if it's a street?

Parts Substitution Gone Big

Cap too big tant I've mentioned some cautions with parts substitutions before; wrong V values on barrier or flyback diodes, counterfeit parts and things like that. Here's another example of something to watch out for if your supply is tight and choices are limited.

One of the things that I've run across a couple times, especially when hunting down capacitors, is the package size issue. Say, I need a 16uf, 10V cap on one of my boards. It's not a critical app. I don't particularly care about ESR, temperature or even much about tolerance. I just need a little head room in case of minor spikes or power line ripple. I'm not expecting a lot. I just want that safety margin.

Cap too big electBut when I run over to my parts supplier, the specific cap I picked two weeks ago, when I started the design, is out of stock or jumped in price. I want to get building, so I just look through my parts drawers for something close. There it is, a 22uf, 50volt cap. It'll still work just fine. The problem is, of course, that I neglected to realize that the part  jumped up a notch in size. Bummer days.

I've run across the same problem, not due to a sloppy sub, but also due to picking the wrong footprint in my CAD package. I find that particularly easy to do with SMT electrolytic caps.

The other thing in these examples to watch out for is the open vias next to the pads. Granted, they aren't in the pads, but they are close and without any kind of a break in the metal before the via. In the left pad of the yellow tantalum cap, I added in an example of a little solder mask dam between the pad and the via. That's the way you should do it. Even though the vias are off pad, solder can still wick away and down the via - especially with leaded solder. Bad news if that happens.

Duane Benson
Have no fear, Underdog is here...

Newark Electronics and Eagle CAD - Interesting

So, I just read that Newark purchased Cadsoft Eagle. I guess it's probably old news to everyone but me. The press release about it that Newark posted on their website Element 14, has a date of August 13. I find this purchase to be an interesting development and I don't quite know what it means, or if it means anything.

I guess partnering is becoming a trend. Certainly, we're involved in some good partnerships (Sunstone, Digikey, NXP, National Instruments) and Sunstone's PCB123 connects up with DigiKey parts. It does make sense. The engineer's job has just gotten more difficult with this recession and the ensuing reduction in support staff. That's pretty much what our ECOsystem partnership is all about - taking the disparate tasks involved in getting a prototype built up and reducing the steps and complexity involved in the process.

The Eagle / Newark deal does have me very curious. For one, I hope the CadSoft folks got a good deal. Their product has done a lot toward lowering the barriers to electronics design and they deserve a lot for that. The big questions are for the future. Will Eagle remain as accessible as it is? Will Newark throw a lot of resources into it and keep it moving forward? Will it get good attention or will it be treated as an impulse buy and not be given focus or direction? Hmmm...

Duane Benson
What about Element 32?

Ye Hardware Engineer on Quest for Firmware

I've been spending some time with the mbed here and I'm convinced that there are a lot of good uses for this little thing. One in particular popped in to my head with a lot of vigor.

Back in December of 2008, I listed ten (Octal) things to do to help get through a lousy economy (read #3). If you're pretty much a pure hardware engineer, now might just be a good time to develop some firmware skills.

In my mind, one of the biggest problems going from hardware to firmware isn't the programming itself. That's not really as tough to pick up as you might think. But it's the environment. The tool chains. The configurations. Make files, environment variables, linked libraries, boot loaders, ICSP, flag bits... There's a load of ancillary junk that gets in the way. Some micros require purchased proprietary compilers. Some use open source. Should you use C or C++ or ASM? Too many choices.

Well, here's something that gets rid of all of that extra junk. Plug in an mbed and in minutes you can be experimenting with C programming on an embedded micro controller. Use the onboard LEDs and sample programs to get instant gratification. Plug in some external LEDs or a sensor of some type (maybe from sparkfun) as you get a little more versed in the language. Save the data to the FLASH and graph it in Excel or something.

My personal feeling is that a hardware engineer is much more employable these days with the ability to write firmware. I haven't found a better way to get started then with an mbed. You can worry about all of the other details later, but use this little guy to teach yourself to code.

Duane Benson
It's been a soft day's night, and I've been coding like a frog

A Few Hints of The Centroid File

NOTE: I've recently (8/18/2010) updated this post and the downloadable PDF to match the IPC 7351. We will properly assemble both ways, but this now matched industry standard.

Every now and then, we get questions on the centroid file (AKA Pick and place file or SMT locations file). Most CAD applications will create one for you. If you use Eagle, download our ULP and run it to create a centroid from your board.

If you want to poke around and need some hints on what's what you can download our Understanding the Centroid file r2. Here are a couple of illustrations from the guide. Fist, the point of origin needs to be centered in the part.

Chip originCopy of inline SMT connectorIt should be centered in a box that contains the outline of the pins as well as the body of the part. The chips on the left are easy. The connector, to the right, is a little more ambiguous, but as you can see, it's centered around the imaginary box containing the area.

Top-side rotation goes counter clockwise as shown on the left image and rotation on the bottom side is simply aChip rotation Chip rotation Bottommirror image left to right, with clockwise rotation.

Diodes and other passives should have their zero rotation horizontal, with pin one (if there is one) on the left. Passives orientation r2

That would place the cathode left for diodes and the positive side left for electrolytics and other polarized two lead parts.

Duane Benson
If you get dizzy spinning counter clockwise, go to Australia

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