Category Archives: Screaming Circuits Blog

EMS provider Screaming Circuits offers tips and thoughts on design and manufacturing

Microcontroller in the Middle

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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 pretty 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 halfway point between something like the Atmel-based Arduino (or the PIC microcontrollers that I use) and the Beagleboard that 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 …

http://blog.screamingcircuits.com/

Oxidized PCB Finish

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I’ve written a few times about potential and actual problems with different PCB finishes. Sometimes, the results are tragic and sometimes, just annoying. I’m not sure where this one falls on the continuum between the two extremes, or maybe even outside. I suppose it depends on who’s paying the bill and how close these need to be to a final production product.

The surface on the pads had been exposed to the elements a little too long such that, even with good flux, the solder balled up instead of adhering as it was supposed to.

PCB finishes are more robust then they were even a few years ago, (although, a few years before that when we were mostly using leaded HASL, things weren’t so bad) but still, your raw boards should be treated more like food then like shoes. Avoid fingerprints and other contaminants. Try to use them promptly. Store them properly. If your air quality is poor, be even more cautious in your handling and storage.

Duane Benson
Let not the sands of time get in your lunch

http://blog.screamingcircuits.com/

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/

LED Lighting Assembly

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When I first attached a 280 ohm resistor in series with a 5 mm red LED, the word on the street was that LEDs were low power, forever-lasting devices that would just about completely replace incandescent bulbs for simple binary indicators. LEDs spent a brief period as the numerical display device of choice too, until supplanted by the LCD. Regardless, the bottom line was that LEDs were really easy to work with. Just put that resistor in series — usually, you didn’t even need to do the Ohms Law calculation — rules of thumb were good enough.


Well, for simple binary indicators, that still holds true, but the big noise in LEDs these days has little to do with binary indicators. It’s in illumination, and in illumination, all the rules are different.


High-brightness LED illuminations devices are some pretty seriously engineered systems. Most have current regulated power supplies. Portable applications often have buck/boost supplies allowing for constant brightness over the life of the battery. And most have serious thermal design work put into them as well. LED lighting designers not only need to worry about all those power supply issues, but also about heat sinking and exotic design techniques such as metal core PCBs and heavy copper. Though it’s just an LED, the layout and assembly issues are far from trivial.

Duane Benson
Wear shades ’cause when you’re cool, the sun always shines.
Or maybe someone’s just trying to blind you with a bright LED flashlight because your ego got too big

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/

Via Near Pad

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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 below, the pin 1 lead is at risk of having the solder 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?

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/