Screaming Circuits: September 2009

ESC Booth 909

Hi all - especially all of you in the Northeast. Just a reminder that we're in booth 909 at the Embedded Systems Conference next week. Stop by and see the workmanship on the Beagleboards that we built and chat about what we can do to help you with your prototype and short-run production needs. We'll also have a representative from our Design Engineering Group on hand in case you need design help.

Enter to win one of our daily contests - each day we're giving away Up to $2000.00 in assembly labor. All you have to do is stop by our booth and let us zap your card. As long as you don't have a mini Faraday cage around it, that should enter you into the contest. We'll notify the winners by email and announce them here on the blog after I get back to the West Coast.

BB we built powered on cropped I'm not a Linux expert - my embedded programming is pretty much limited to 8-bit micro programming - so we don't have anything cool running on the Beagleboard we built. I did manage to get a distro up and running but all it will do is just show you that it works. I'm using the Angstrom image that the Beagleboard folks have for download. I had purchased an already set up SD card with Angstrom on it, but I messed up the boot parameters or something trying to get WiFi working and couldn't make that SD card boot anymore. I thought it would be cool to show the Screaming Circuits website on the Beagleboard, but I gave up on WiFi after that. The hardware works. That's what counts. Right?

We'll also have one to hand around and look at so you can examine our package on package and .4mm pitch BGA workmanship up close and personal.

See you all (well, some of you) there on Tuesday or Wednesday.

Duane Benson
Can someone bring me a Brat from Jacob Wirth?
My dad says they're really good and I won't have a chance to go there

A Few More Via In Pad No-no's

Move via to the left This first footprint will result in a minimal connection at best on the pad on the left. The via takes up nearly the whole thing so the pad barely has any metal and the open via will suck the solder down in it. There was plenty of spacing below, above and to the left on this board, so the easy solution would simply be to move the via in one of those directions far enough so that it's not in the pad and there is a web of solder mask between it and the pad.

Move via and equalize pads The second image has two problems. It has a similar via in pad issue and it also has unequal pad sizes. Having a small pad on one side and a larger pad on the other side encourages tombstoning.

Unequal pad sizes will likely exhibit different amounts of surface tension in the solder, which is a cause of tombstoning. The solder paste may melt at different times on the different sized pads due to different thermal mass, again a common cause of tombstoning in small parts. Also avoid putting different width connecting traces on the two different pads. Watch for anything that may act as a heat sink on one pad and not the other - large traces, inner layers, big components nearby. Things like that.

Duane Benson
Boot Hill is six blocks that way and to the right sixty paces.

Three Years And Posting!

Hey - I just realized that today is the three-year anniversary of the Screaming Circuits blog. Cool.

This is post number 311, so that's just over 100 yer year. Three years really isn't that long in the grand scheme of things, but I suppose in Internet years, it's pretty okay.

My first post, was, as is the case with many blogs, mostly useless. On the other hand is

int main()
printf("hello, world");
return 0;

mostly useless? Those few lines of code have started countless developers on to writing trillions or maybe hundreds of lines of code. Okay, so I'll call it lame, but not useless. How's that? But, speaking of useless, here's some useless blog trivia. According to Wikipedia (which we all know is 110b% accurate), Justin Hall, back in 1994, was one of the first bloggers as was Jerry Pournelle. Hmmm. I'm not really sure how to come up with the exact first blogger. It may not even be possible to identify the first one.

Back in the day, I used Compuserve to host my website which had some characteristics of a blog. (I discussed the value of a website for businesses) but it wasn't really the same thing. I remember reading Jerry Pournelle in Byte Magazine way, way back. But that was in print so we can't call it a "blog" in the sense of "web-log". On the other, hand, why is the specific technology all that important? He was journaling to a broad audience. He just couldn't do so on his own time-scale and comments had to come back through postal mail. Certainly, I enjoyed his musings and his plog (print log???) was one of the reasons I read the publication. That's got to count. Anyway, I have no idea when the first blog was written.

Back in 2006 when I started this thing, I didn't really know what I would do with it, but I hope it has been of value to some of you folks. It just seemed like a good way to pass on hints and tips and help other folks make better pcbs. And here, 311 posts, about 90,000 words, half a million key presses later, it's September 14, 2009.

Enough of this. I have a tradeshow to prepare for.

Duane Benson

ESC Show is Near

The Embedded Systems Conference is almost here again. We'll be in booth 909 next Tuesday and ESC_Boston 08 Wednesday. Drop by and say "hi." See what we're up to and enter to try and win our contest. You can also take a look at the Beagleboards we've built and talk about how we might be able to help you out with your prototypes or small-lot production runs. 

And, just what is that contest, you might ask? Why, it's a chance to win up to $2,000 in assembly labor from us. At the end of each day of the show, we'll pick someone at random from our lead file and award them the prize. All you have to do is stop by our booth and have us scan your card. Then, if you are chosen to be the winner, you can use that $2,000 of assembly labor to have us build some stuff for you this year.

We'll also have someone in our booth this year to talk about our engineering design services. We've been introducing engineering design services, such as layout, under the Screaming Circuits brand this year. Our parent company, Milwaukee Electronics Companies, has been offering custom design services for many years and will have a representative in the booth to chat with any of you that might need some help with your design load.

The clock is ticking. We'll see you at the show!

Duane Benson

Hi Ho, Hi Ho, It's off to Via I go

Here's just another via in pad story. In the first example (the green one), all those open vias are like 15 miles wide - okay, drop the "e", but they're really wide. You can't quite drive a truck through them, but solder will get sucked down into them. Lead-free solder will even go down something that wide. Without some modifications, this is not a very buildable board.

Big vias in padThe second picture (the red one) also has "open" vias, but they are a whole lot smaller. We never like to see open vias, but in the second case, we'll probably just go ahead and use it as-is. It shouldn't be a problem with lead-free solder and may even be okay with leaded. With something like this, we might just run one, check it out and see how it goes and then decide if it's buildable as is based on that.

My guess is that the vias in the red board are about six mills. Maybe even a little smaller. When I hold it up to the light, I can't even see through it. That means that even if the vias weren't intentionally plugged, they effectively are. You will likely get some voiding under the part, but not much in this case.

If your application needs exact predictability in terms of the amount of voiding under the QFP or QFN, then your only choice is to have the vias filled and plated over at theTiny vias in qfp pad board house. If you can get by with some variability (most component manufacturers say that around 50% is okay for most applications, but check with your part manufacturer to be sure), then a good rule of thumb might be that if you can't see light through it, it will likely build okay. If in doubt, get in touch with us and ask to speak with one of our manufacturing engineers about it.

There is not doubt about the green board. Those vias are too, too big to be left open. But, the vias in the red one might just be small and closed off enough to use.

Duane Benson
When does 01631D = 8ABBA9?

Package on Package Layout

Amkor POP sm Last week in one of my Beagleboard posts, I was musing on the issues related to layout with a package-on-package form factor. I pondered a couple of options, but Occam's Razor held true. Gerald Coley, the designer of the BeagleBoard pointed out that the CAD software simply doesn't care. And that's perfectly logical.

If you're designing with POP like the Ti OMAP processor, just don't worry Beagle-smabout it. There are no layout issues - except for all of the normal ones like escape routing, trace current capacity, crosstalk, components spacing, vias in pads, etc, etc. Just place the bottom part where it's supposed to be, like you would if it isn't POP.

The manufacturing folks, on the other hand, do care. But that's easy. Just put the POP top part at the same XYtheta location in the centroid (pick and place) file and make sure the build instructions are clear that it is a POP part. Simple.

Duane Benson
Curse you Red Baron!

Parts Substitution Woes

I've written a bit about parts substitution before and how it a simple sub can either not matter or cause a board to go up in smoke. Here's a case where the component was 100% electrically okay to be put in the design, but the form factor messed everything up.

Cap parts sub woes

The PCB was laid out with a tantalum cap in mind, but somewhere in the process, a metal can electrolytic was tossed in. Sometimes tantalum vs. standard electrolytic can make an electrical performance difference, but here it just mucked with the mechanics.

The pads are the wrong size and spacing and the base of the cap is too big so it gets in the way of other parts. Just a little cautionary tale. If someone else is doing the parts ordering for you, make sure that either they don't make any subs without contacting you first, or give them an approved subs list.

Duane Benson

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