Screaming Circuits: November 2009

My mbed Is Up

I wrote about the new mbed development board a while back and mine just arrived over the holiday weekend. I have to say, true to it's promise, It was the easiest piece of development hardware that I've ever brought up:

  1. Take it out of the shipping box
  2. Plug in the USB cable to the board and my computer
  3. Wait a minute for it to be recognized and open up like a USB thumb drive
  4. Double click on the web shortcut in the drive
  5. Register
  6. Click the Compiler link
  7. Pull up a code sample and modify it a bit (I didn't need to modify it, but I did anyway)
  8. Click the compile button
  9. Save it to the mbed as though it were a USB thumb drive
  10. Press the reset button on the mbed board

That's ten steps, but it's only ten steps. There was nothing else to do. Nothing. The longest step was number seven which took me about two minutes. I programmed a "Knight Rider" sweeper with the four on-board LEDs. I made one of those for my Jack-o-lantern back at Halloween, so it was the first test program that popped into my head.

I built the Jack-o-lantern sweeper with eight LEDs and an 8 bit PIC16F819. The PIC I used came in an 18 pin thru-hole DIP package, costing $3.22 at Digi-Key, and I hand soldered it all on an old perf board. It runs at 20MHz, has 16 GPIO, 3.5K program code space, 256 bytes of FLASH and 256 bytes of RAM.

Mbed pinout The 32 bit NXP LPC1764 runs at 100MHz in a 100 pin LQFP and costs $8.70 in quantity of one at Digi-Key. (The dev board, of course, costs more then that) It has 512K of program FLASH and 64K of RAM. The dev board can have up to 25 GPIO (the chip can have up to 70 GPIO with your hardware) along with the standard assortment of peripherals that can be configured, including six hardware PWM channels. The mbed dev board is like a breakout board configured as a 40 pin 0.1" DIP so it will be easy to prototype with.

The processor, being a fine-pitch package really isn't hand-solderable like the PIC except for by the most adventurous of folks, but that's where Screaming Circuits comes in. Why wait for your custom hardware before starting on the software. Get one of these mbed dev boards to work on your software while the EE folks are designing the custom hardware. Then, when they're done, we'll assemble up the prototypes and you can integrate it all together. Take some time out of your development schedule that way.

I've wanted to try out an ARM processor for quite a while, but prior to this, haven't found the right way to do so while keeping within the limits of my time availability and skill set, but this looks like it could very well do the job.

Duane Benson
Robots rule!

Top Ten Electronic Things To Be Thankful For in 2009

Bb input pwr sect w turkey

It's that time of year again when we take stock of what's good in our little worlds. Since I'm writing this on my work blog, I'll keep my top ten items focused on work-related thingys.

Number 10: Allocation!? Well, maybe. Nobody likes parts shortages and allocation, but maybe, just maybe, it means that we're seeing the light at the end of the recession tunnel.

Number 9: The mighty QFN. Yes, I know the package can be a pain to layout properly, but the size reductions we can get with it are pretty cool. It used to take something like a TO-220 or D2Pak to drive an amp of current drain, but some of these new devices can do it in a little QFN (properly laid out, of course) form-factor.

Number 8: 99.47% on-time delivery in the last year. That's less then one job late per month - and remember, if we're one day late, the assembly is half off and if we're two day's late, the assembly is free.

Number 7: The Beagleboard being open source. It's really opened up the world of high-end non-i86 embedded processors to a very large segment of the industry that just couldn't quite get there before. Well done Beagleboard folks!

Number 6: The Internets. Back in the olden days when I was burning my fingers soldering up discrete transistors and plain TTL and such, I had a shelf of data books. I think I may still have an old purple National Semiconductor TTL data book buried in a box somewhere. It was always cool to page through those data books, and, of course, I didn't need to be online in order to find what I needed, but heck, I can find it all now and even more without getting up and walking across the floor to my book shelf. In fact, I pretty much don't have to move at all anymore thanks to the Intertubes.

Number 5: Google translator. Earlier today, I got an email written in German. Before online translators, I wouldn't have been able to do anything with it and I would have missed a very big opportunity. The email was from a barrister in the tiny country of Togo. Apparently, he's been looking for an heir to pass an inheritance to and can't find one. He said that he went to the American embassy and they suggested me. If not for the Google translator, I would have missed out on this wonderful opportunity to get seven million dollars transferred right into my bank account.

Number 4: Level translators. It's still a pain to deal with interfacing signals at different voltage levels; like a 5V I2C device to a 3V I2C bus to a 1.8V GPIO, but it was way more of a pain before easy to use level translator chips became widely available. Especially the bi-directional chips. Much more convenient.

Number 3: Better static protection built into chips. Yes, we still religiously use static ground straps. We have a conductive floor and wear foot straps and anti-static jackets and have anti-static stuff all over the place, but chips are so much more robust then they used to be. I can remember the old 4000 series CMOS chips. It almost seemed like if you breathed wrong, they'd get zapped.

Number 2: The LGA form-factor package. Just kidding. LGAs are annoying. Sure, there are some redeeming qualities: low profile, a RoHS part can go both leaded and unleaded, decent heat transfer. But, they also don't flex as well as a BGA and the pads have the disdvantages of both BGA and QFN packages. Basically, they're just annoying.

Number 1: And the number one electronic thing that I'm thankful for are these little Flash 8-bit microcontrollers like the PICs (that I use) and Atmels (like the Arduino uses). Holy mackerel, they make life a lot easier. All that GPIO, no support chips. And, self programmable flash. Ahhhh... Anybody out there still have a UV EPROM eraser?

Duane Benson
Embedded in my head

What it is...

Dan got it. As for what it has to do with PCBs, well, not a whole lot except that my desk was all messy that day so I was using the carpet as a backdrop to photograph a pc board.

What it is Misc components on boards

Yes. I know. It's a trick question, and not all that interesting in the end, but I was suffering from lack of sleep that day. I thought it just looked cool and so it seemed like a good idea at the time.

Duane Benson

Tented QFN/QFP Via in pad

Tented vias in padHere's a pretty decent example of mask-tented vias in the thermal pad of a QFP. Most manufacturers recommend no more then 100 - 125 um wider than the via to minimize voiding and thermal insulation in cases like this. This is a reasonably inexpensive way to handle vias in the thermal pad. Sometimes though, the tents will pop open allowing solder to wick down through the via.

The mask over the center via on the right looks a little thin, so you'd want to give it an extra look over after reflow to make sure it's okay. (We'd do that here, of course)

We'd rather not see this technique on really small parts because it gets difficult for the fab house to put the mask down with enough precision. With small parts, filling and plating over the vias is the preferred technique. Well, that's always the preferred method. It's just more important with smaller parts and BGAs. This method is acceptable for most QFPs and larger QFNs though.

Duane Benson
All your via are belong to us

What is it?

What Misc components on boardsWhat is this and what does it have to do with PCBs?

As you ponder that question, if you care to ponder that question, keep in mind that I have at my disposal, not just the work of my company, Screaming Circuits, but I have the entire Internets to draw from and the Internets have photos from all of the history of electronics. So, this photo might actually not relate directly to via-in-pad or small component tombstoning.

There will be no prize for the correct answers, but you will likely get at least 3.2 picoseconds of fame if you post your answer as a comment here. And while a picosecond may not seem like much, if you could save picoseconds in a leyden jar, and then do the same over and over again, you could eventually end up with a usable duration.

Duane Benson
To have enough time
to do the things you want to do
you just need a fast enough internal clock speed

Pin BGA Interconnects

My post "Speaking of Art in the Process" used a photo of a point of load power module as an example. The specifics aren't really relevant to this post though, but a commenter by the name of "Me" asked what type of pins those are connecting the module to the main PCB.

"Do you know where to get those pins to attach two boards like that?
I mean do they sell just the pin for example on digikey and give it a name, or is it just wire.
Can't see if they are pins with a lip to lift board to a set height."

Module assembly from side Module assembly several This part came with the pins already on so I don't have a specific part number for the interconnect pins. I have some underside photos here that give a better view. They are basically solder-type terminal pins with a solder washer and BGA ball on one side (to attach to the main PCB) and either a press-fit or solder type side to affix to the module PCB.

I wasn't able to find this exact part on DigiKey or Mouser. Vector sells the solder washers and lots of interconnect pins of this sort, so they may be able to steer you to them with a phone call. This board uses the BGA style, but we've seen other POL modules of a similar type with thru-hole solder pins too. DigiKey has lots like that. Here's one example of some thru-hole terminal pins from Mil-Max. You could use the solder-washers (like a T124 from Vector) to put some space between the module and the PCB.

Duane Benson

QFN Land Pattern Mix-Up

Another QFN oopsie here. The QFN looks good and the stencil cut-outs look good. The mask, though, as shown in the middle, is not good. In general, you shouldn't put mask on the QFN land pad except to cover up open vias.

The QFN center thermal pad for this part should be free of solder mask. The stencil does give a good amount of paste -Mask and stencil issues roughly 50% coverage, maybe a little more. That's the way we like it. But, with the mask on the board, there will be too much solder and too much with nothing to stick to. It will either flow to the small metal areas and raise the QFN up so the sides don't solder or the excess solder may turn into solder balls and short something out.

There are some cases where it's okay to only expose the metal in small areas of the QFN thermal pad (see here), but if you do that, the stencil has to match the pattern. Otherwise, like in yesterday's post, you'll probably end up with a gloppy mess.

Duane Benson
That's the way aha aha your pcbs like it aha aha

Solder Paste Stencil Opening

Stencil openings too bigHere's a case of "close, but no cigar" with the stencil opening. The pads are, in fact, covered by the openings, but as you can see, the openings are too big.

This stencil would end up laying way too much paste down. Some of it would be on the solder mask which might bubble up and turn into solder balls. All in all, the use of this stencil might just lead to something of a gloppy mess.

When you're making your paste layer in the library component (presumably, this was custom made), it's sometimes appropriate to make the paste opening the same size as the pad and it's sometimes appropriate to make the opening smaller, but it's never appropriate to make the opening bigger then the pad size.

After writing this, I for some reason got curious as to the origin of the phrase: "close, but no cigar." I know it's been around a long time, but I couldn't come up with any plausible meaning for it. Then I remembered this thing called the Internet, so I looked it up. According to a couple of different sites, carnival booths, like the big hammer, would give out cigars as prizes so if you almost made it, the Carney, would say "close, but no cigar." Huh. Interesting, but much less interesting then I had expected.

Duane Benson
Sorry. We don't give out cigars if your stencil is good

More On Allocation

Not long ago, I wrote a bit about allocation and parts substitution issues. Once again, I can't stress enough that if you're in a hurry, you need to be prepared with some subs ahead of time. Especially with the don't-think-about-them passives. Right now, that's where the biggest issues seem to be. IC's are starting to show supply problems too, but the passives are the big hitter at the moment.

BOM line items Something else to be wary about in times like this is the prospect of counterfeit parts. Unfortunately, when supply gets tight, that's when the opportunists come out of the woodwork. Legitimate suppliers like DigiKey do a pretty good job of keeping their parts real, but unknown suppliers need to be looked at with caution. We do use some brokers here, but we qualify them carefully and keep watch on how they are doing things.

If you really, really need a specific component that has 12-week lead times everywhere and some unknown Internet supplier can get you some now, be careful. It may be a lifesaver, but be prepared to test the part to make sure it's really what you are expecting. And, if you just can't get that perfect part, maybe a slight design change is the only way to get some of that 12 weeks back.

Duane Benson
Drivin' over Kanan, solderin' on my soul
There's people out there turnin' duds into gold

Friday The 13th

I'm always writing about stuff that goes wrong (and, hopefully doing a decent job of describing how to avoid such problems). But here it is, Friday the 13th - the traditional bad day of bad days. Break a mirror and watch a black cat cross your path while walking under a ladder.

Or not.

Good QFP Here's a QFP part we soldered on. The land-pattern was right. The size and spacing was good. The pad size was good. The fillets all look good. The solder-mask registration is good. No problems. It's an IPC, class II job and everything meets IPC class II. Cool.

Good BGA pads No vias in these BGA pads. No open vias without mask near the pads. Decent registration on the soldermask. Nice, planar surface on the pads. Even the vias between the pads are plated over and masked off.

The via caps do look kind of funky, being bigger then the BGA land pads, but that's okay. It's just a nice decent silver finish PCB. And, no tarnish or surface contamination to be seen anywhere.

Duane Benson
Hmmm. Interesting.

« October 2009 | Main | December 2009 »