Author Archives: Duane

About Duane

Duane is the Web Marketing Manager for Screaming Circuits, an EMS company based in Canby, Oregon. He blogs regularly on matters ranging from circuit board design and assembly to general industry observations.

Inverted QFN Land Pattern

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Ever experienced the heartbreak of inverted land pattern? It’s not supposed to happen, but every now and then, it does. Maybe something happened when creating a custom footprint. Maybe, somehow it got
inverted in the CAD software and then placed on the wrong surface layer.

Maybe it was a subliminal attempt to make up for those giant open vias in the thermal pad. Who knows. But, it happened, so now what?

You could re-spin the whole board. Ugh. That’s, like, wasteful and stuff. Certainly, if this is a production build, you’ll have to re-spin. For some prototype applications, like if it’s a high frequency or RF thingy, you may very well have to get a new set of PCBs fabbed, too.

But, sometimes in the prototype world, you may be able to salvage the board run. We used to do stuff like this all the time with through-hole parts — need an extra chip, just dead bug hang it on up there.

Flip the chip over and use some small gauge wire — maybe wire-wrap wire — and hand-wire to the upside down chip. Gluing it down first may be helpful. Just keep in mind that since the thermal pad isn’t soldered to the board, you will lose some thermal performance. Maybe solder a small heat sink on it or something. And don’t forget to wire that pad to ground too (if it’s supposed to be grounded).

Duane Benson
Just put it on the seventh surface of your tesseract and it will fit right.

http://blog.screamingcircuits.com/

More Thoughts on Via Near Pad

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The other day, I wrote about vias near pads. The post got a couple of interesting comments.

In one of the comments, Mitch said, “When I was learning PCB design in the 1980s, I was taught by a mentor that understood assembly very well.” I think that highlights a big component of the problem. I suspect that a lot of folks doing layout today were not taught by anyone but themselves.

CAD packages may have instruction manuals and tutorials, but learning how to use a software package is a lot different than learning how to do the actual process well. It’s possible to be very proficient at using a word processor, but still not know how to write well.

It’s not an uncommon scenario these days, especially after the economic suckiness of last year, to come in to work expecting to hand off a schematic to the layout engineer only to find that “tag you’re it.”

Howard, in another comment, suggested that in his experience, filling and plating over vias in pads typically only adds about 8% to the PCB cost. In smaller prototype quantities, it may be a little more then that, but what’s the cost of a failed assembly? If you have the room to move the vias off the pads, the only cost may be in layout time. If space is critical or if there are signal/noise/thermal issues that force the vias to be in the pads, then you’ll just have to spend the extra to fill and plate.

If you do find yourself suddenly tasked with layout and you’ve never done one before, find a mentor (or maybe a Minotaur), read up online, call up a manufacturing person, study the Screaming Circuits blog. What ever you do, figure out all these little traps like vias in pad, components library foot print issues, spacing issues, thermal issues, etc. Then dive into the layout and learn from each one. Drink some tea too. It can relax you. Just try to stay away from Oreos and ice cream late at night.

Duane Benson
What’s the deal with 1729?

http://blog.screamingcircuits.com/

Want Data

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So, I tried to participate in this SparkFun “free day” this morning. They were giving out $100 worth of goodies free per customer (up to a combined total of $100,000), starting at 9 am MST (8 am PST here in Oregon).

I was pretty excited about it and had decided to get a new PIC programmer and some pre-assembled jumper wires. I hate crimping those little things by hand. I put it on my calendar for the night before and again for that morning. Then, I found out that I had forgotten a dentist appointment at 8 that morning. Bummer.

Just in case it would take more than an hour to burn through that $100,000, I went ahead and got ready. I logged in and put the items in my cart. I left the browser sitting there waiting. All I had to do was click the “Place Order” button when I returned after getting my teeth scraped.

But, alas, when I got back, the site was timed out and not accessible. I refreshed, tried a different browser, refreshed again, etc. I did once get enough of the site to load to see that they had only sold through about $19,000 thus far. Okay, that’s not so bad. I could finish making my latte and get in to the office. Maybe try there.

Then, at the office, I was never able to get anything at all from the site to load. All full up. I had to go to a meeting at 10 and I thought that if they stayed at around $20,000 per hour, I might just have a chance of getting through when the meeting was over. But, it was not to be. When I checked in again at 11:30, all $100,000 was sold through. My guess is that so many people were trying in the first hour that the servers only had enough bandwidth to process $20,000. After that, enough people gave up trying that the hardware could get the final $80,000 through in the next 44 minutes and 50 seconds.

Now here’s where my quest for data comes in. I was never able to get more then one click into the process. If all connections were equal, I would presume that everyone would have had the same results. Even if by random chance, someone found a pause long enough to get one page loaded, the chances of each subsequent step would drop astronomically. So, what is it about the Internet that gives some people priority over others? I’d love to see a geographic overlay of the folks that got an order placed combined with their distance from a backbone. Is it distance from a backbone (in hops or in miles) or is it distance from the SparkFun server?

In any case, good for them. It was a fun idea and great gesture of “thanks” Bummer for the inability to handle the load. Here’s a Twitter quote from Chris Anderson on the subject: “Google’s servers can’t keep up with Nexus demand; Free Day brings down Sparkfun. It’s 2010 — why do we still have these scaling problems?”

Ironically, when I first went to Twitter to copy that quote, Twitter was reporting overcapacity and I had to wait awhile for all the tweets to come back.

Duane Benson

If only my packets were more aggressive.

http://blog.screamingcircuits.com/

Pitch Switching

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I’ve been reading of 0.3 mm pitch BGAs, but those aren’t totally new. I’m not sure if we’ve done any 0.3 mm pitch before, but we’ve been doing 0.5 mm for years and have done plenty of 0.4 mm pitch as well, even in package-on-package (POP) forms.

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 1.27 mm 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 below came from the Aries Electric website.]

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, 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 microvias and tiny trace and space advanced (expensive) PCBs. Could be quite handy and same some money.

Duane Benson
Platform shoes are back!

http://blog.screamingcircuits.com/

Parts Substitution Gone Big

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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.

But 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, 50V. 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…

http://blog.screamingcircuits.com/

Newark Electronics and Eagle CAD — Interesting

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I just read that Newark purchased CadSoft, including Eagle CAD. I guess it’s probably old news to everyone but me. The press release about it that Newark posted on its website, Element 14, has a date of Aug. 13. I find this purchase 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, Digi-Key, NXP, National Instruments) and Sunstone’s PCB123 connects up with Digi-Key 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?

http://blog.screamingcircuits.com/

Ye Hardware Engineer on Quest for Firmware

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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 10 (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

http://blog.screamingcircuits.com/

A Few Hints of the Centroid File

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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 guide on the centriod file. Here are a couple of illustrations from the guide. First, the point of origin needs to be centered in the part.

It should be centered in a box that contains the outline of the pins as well as the body of the part. The chips (above, left) are easy. The connector (above, 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 (below, left) and rotation on the bottom side is simply a mirror image, left to right with clockwise rotation.

Diodes and other passives should have their zero rotation vertical, with pin one (if there is one) facing up. That would place the cathode up for diodes and the positive side up for electrolytics and other polarized two lead parts.

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

http://blog.screamingcircuits.com/