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.

Dripping Wet Is Too Much

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The headline is obvious. But what’s not so obvious is that some components may look perfectly fine but act like Orville Redenbacher when in the reflow oven.

Well, they won’t actually act like Orville, but rather, like his popcorn.

Sort of. With popcorn, you can tell when it’s popped. With a popped chip, you can’t always tell right away.

Moisture sensitivity is a bigger issue with RoHS-compliant components, but can be an issue in leaded components as well. Even though the parts look like water-tight plastic, they really aren’t. They do absorb 
 moisture and after improper storage, moisture-sensitive chips may popcorn or crack subtly underneath. This can create hard to find or intermittent defects. It is often more of an issue with prototypes because components are quite frequently consigned in partial lots. This may result in impaired moisture-sensitive packaging or storage beyond recommended shelf life.

The message here is that if your parts are labeled moisture-sensitive, don’t open the moisture barrier packaging before sending them to your EMS provider. Or, if you have to open the package, please let them know, so they can bake them at the proper temperature.

Duane
Easy-Bake Oven: $25.99 from Hasbro
No. You can’t use an Easy-Bake oven for your parts

http://blog.screamingcircuits.com

Follow the BGA

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At the Embedded Systems Conference in September, I had a number of folks ask me about mixing leaded and lead-free components on a PCB. It’s a difficult situation for some people — especially when using old and very new BGA form-factor components.

We generally tell people to follow the BGA. Since the BGA has those little solder balls on it, it’s the most sensitive to temperature as far as soldering is concerned. Reflow a leaded BGA at no-lead temperatures and the flux may all burn off and the solder may sag down too far and bridge or dry and crack. Do the reverse and reflow a no-lead BGA at leaded temps and you won’t get a good intermetallic mix and the solder joint will be prone to cracking and other bad stuff.

In most cases no-lead components, other than BGAs can be used on a leaded board. Going the other way isn’t always so easy though, because of the additional 20 C in the no-lead process. Everything’s more sensitive to moisture absorption, so baking parts or keeping them sealed in moisture-free packaging is more important. Some components may melt, especially chip LEDs. And metal can capacitors can pop.

In a prototype world, where you just need to see if something works, you can sometimes get away with a lot more than you can in production, but it’s still not an easy question to answer. Unfortunately, if you’re in the situation of one of the guys that asked about it and have one leaded BGA and one no-lead BGA, you may have to get one of the BGAs reballed or you may just need to redesign on of them out. No easy answer there.

Duane Benson
My 24 hours is almost come
When I to sulphrous and tormenting flames
Must reflow up myself

Random Via-in-Pad Myth #3

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Myth #3: It’s okay to put vias in BGA pads because the extra solder (the solder ball) on a BGA will prevent any problems

9x13 via in pad BGA land Here’s an example of a via in pad situation we don’t like; a large number of vias in the BGA land pads. They’re pretty big via holes too. The primary reason we don’t want to see vias in pads is that when left open, those via holes act like little capillary straws and suck solder off of the pad. A couple of undesirable events can happen depending on the method used during board fab.

If your vias are left open, solder will tend to wick down into the via hole. The larger the diameter, the BGA via in pad worse the wicking problem can be. You might end up without enough solder left to secure the component, or even a solder bump on the bottom side of the board which could interfere with other components or lead to shorts.

The extra solder in the BGA ball can just make a bigger mess on the back side of the board. And the extra space that BGAs have between the component and the PCB make opens even more likely. BGAs are more susceptible to this problem. Not less.

Some people will try to use soldermask to cap the bottom side of the via. However, the solder ball can still be wicked down into the via, leaving an open. Or the soldermask cap map pop off due to thermal expansion or outgassing, leading to a purely open via. Internal air bubbles can migrate up, leading to voids in your solder joint.

Duane Benson
Burma Shave

Funky QFN Land Patterns

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I’ve described the optimal way to create your land and solder paste layer for QFNs a couple of times before. Complex QFN land pattern But that was for a standard square QFN or rectangular DFN. What happens if you look at the bottom of your QFN and it’s all weird like this one?

Does it require a different philosophy for the big pad areas? Should it just be a solid opening because their is more than one thermal pad and they don’t cover the whole area?

Well, the image is an Intersil ISL8200 power module. It’s pretty cool and Intersil was kind enough to actually put the paste layer recommendations right in the data sheet. Unfortunately, not all chip manufacturers do that.

The bad news is that it’s a pretty complex pattern. The good news is that the data sheet gives a diagram with great detail on the required dimensions for the lands and the stencil. And, yes, you treat this just like any other QFN thermal pad. They recommend 50 – 80% paste coverage for the thermal pads just like everyone else. That means that you’ll segment the paste cut-outs in the paste layer for each of the four thermal areas just like you would for the whole pad area on a standard QFN. The data sheet for this part has the specifics.

For similar parts from other manufacturers, you should go to their datasheets and app notes first, but if you don’t find a recommendation, we would suggest doing the segmenting and shooting for somewhere between 50 and 80% coverage. Putting down too much paste is a bad idea for any QFN or DFN, but it’s probably even more critical with a part like this where the solder areas only cover half the part. If there’s too much solder on the underside, it will likely tilt and most likely not solder reliably.

Duane Benson
Don’t eat paste.

http://blog.screamingcircuits.com

ESC Boston 2010

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I just got back from the Embedded Systems Conference in Boston. We go every year, along with the ESC show in San Jose. After the gloom at the show last year, I was pretty apprehensive.

With so many mixed messages relating to the state of our economy, I really had no idea what to expect. Last year, our booth traffic was decent, but the theme of the day was “I’m between jobs”. Not good.

ESC Bosto 2010 day one Looking back at the two days of this year’s show, I’m feeling a lot better about the state of at least the engineering segment of our economy. We started the show with a mob of people at our booth — all five of us blue shirts had people talking with us and a few more in waiting. I don’t think I stopped talking for the first two hours. After that, traffic slowed a bit and I was able to snap a few pictures. We had good crowds throughout both days of the show.

We had our best opening day in the four years I’ve been here. There was no gloom like last year. I only spoke with one between jobs engineer the whole day. The throng of folks at the ESC booth for the backpack giveaway was intense. They were giving away 500 backpacks full of goodies, including a Ti robotics development kit. I’m not sure, but it looked like they got rushed by all 500 people just as the doors opened.
We may not be out of the economic woods yet, but based on my view from the show floor, I’m feeling very upbeat and certainly am confident that our industry is back.

Duane Benson
“I love it when a plan comes together”

http://blog.screamingcircuits.com/


Reruns

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I’ve been pretty occupied with the upcoming Embedded Systems Conference in Boston. The exhibition is next week on Tuesday and Wednesday, Sept. 21-22. Screaming Circuits will be in booth 809. Stop by if you happen to be at the show.

In any case, I’ve been pretty much wrapped up in show preparation so I haven’t had much time for original writing here. That being the case, I’m going to play an old TV sitcom trick and just select some old, but good, content to re-run.

And, there you go.

Duane Benson
Hide Wally Bee. Andre is back and he’s got a fly swatter

http://blog.screamingcircuits.com/

Four Years and Blogging

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The Screaming Circuits blog celebrates its fourth year blogging today (Sept. 14, 2010). In that time, it has had 447 (448, including this one) posts and 250 reader comments (more if you count the spammy ones I deleted).

Popular subjects have been:

A year ago, I noted that the blog had 311 posts, or about 100 per year. That gives 137 posts this last year, so apparently, I’ve picked up the pace a little bit. Some other likely non-interesting statistics: that’s about 140,000 words, a million key presses and 459 images. My fingers must be tired. Maybe bruised on the tips. Only four of them should be sore though. I skipped all the typing classes and came up with my own variety of modified two-finger hunt and peck using just the index and middle fingers of both hands.

For the most part, all of those images, key presses and words have been attempts at passing on useful information and some diversionary blathering with hopefully not too much self-serving drivel. I think I’ve avoided getting into any political discussions during that time. Nothing controversial. That’s probably best given that this blog is the voice of Screaming Circuits, not the voice of me. Maybe one or two of my top 10 lists may have come close though.

Duane Benson
Live long and preserve all raspberries

http://blog.screamingcircuits.com/

Questions on SMD and NSMD

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My prior post discussed SMD (solder mask defined) vs NSMD (non solder mask defined) pads for BGAs. I received a comment on the post asking for a bit of clarification:

My understanding was that SMD pads were preferred from a solderable area standpoint. To use an example from your picture, the pad in the upper-left corner has a larger surface area than any of the others by virtue of the larger exposed trace leading to it. With an SMD pad, this trace would be masked and the solderable area would be identical to the others. Have you found that other maneuverability factors outweigh this concern?

IPC-7905B does recommend NSMD pads for the most reliable solder joint. They do note however, that sometimes SMD pads are used to prevent pad lifting so it’s a bit of a trade-off. Their suggested compromise is to use mostly NSMD pads and SMD pads in the corners. I guess it you have solder joint issues, make sure that your PCB uses NSMD pads. If you have issues with pad lifting, try the SMD corners. If you have a lot of pad lifting, you might have a warping problem that needs a little more attention.

As far as the assembly process goes, we can build them either way without problems – unless there’s an underlying issue causing problems. The difference in pad area isn’t an issue for us – unless you also remove the soldermask from the trace going to the via. Doing that is bad.

Duane Benson
Pop goes the weasel is okay
Pop goes the BGA is not

http://blog.screamingcircuits.com/

SMD vs. NSMD

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In general, we and just about everyone else on the planet recommend NSMD (non solder mask defined) pads for your BGA land. Using copper to define the land gives more precision than using solder mask and, more importantly, gives a more reliable solder joint.

BGA on HASL close with vias between pads There are a few cases where SMD (solder mask defined) pads might be more appropriate. The Beagleboard folks, with their 0.4mm pitch Ti OMAP processor found that at that small a pitch, NSMD pads increased the chances of bridging.

Interestingly, IPC writes that the main advantage of SMD pads is to prevent pad lifting. They further note that since the corner balls are most likely to have lift issues, due to the greater concentration of stresses. If you have concerns about corner pad lifting, you might want to use SMD pads just in the corner for a little extra holding power. (ref IPC-7095B, 6.2.2)

Duane Benson
Danger! 50,000 Ohms

http://blog.screamingcircuits.com/

Modularity and Standards

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Eons ago, (well, it seems like eons) when IBM designed its original PC, it took note of the success of the Apple II with it’s modular expansion system — easily accessible card slots with loads of clear documentation — and added its own variety of modular expansion system. By doing so, the cost of accessories to consumers stayed low, the cost of installing or replacing said accessories stayed low and a whole new industry emerged to create compatible accessories.

I just read a Twitter Tweet (“Tweet” sounds too cutesy to me, so I’m never quite sure what to call those; maybe a “Twoot”?) from Mike Buetow that linked to an article about the latest Toyota recall. It seems that there are a couple of specific solder joints prone to cracking in the ECM (Engine Control Module) of certain models.

The last time I had any real data on the cost to replace an ECM, it was on the order of $1,500. Just scanning around the Internet, I found numbers ranging from $1,000 to $2,000. I’m guessing (I am speaking from near complete ignorance) that maybe two or three hours of that are labor at $90/ hour. That’s a lot of cost in the electronics as well as labor hours that can’t be used for billable hours. With so much of new cars being electronic, this issue is only going to become more extreme.

So, why can’t the auto industry take a cue from the PC industry? Create a standard, easily accessible, electrical bus with standard, easy to manipulate mechanical attributes. Even if they were just standard within each manufacturer, it would still be a big improvement.

Consider this scenario: Buy a Toyota mid-size-car ECM at the local auto parts store. Take it home, plug it into a USB port on your home computer. It auto-runs a link to a specific web site. Enter your car’s VIN number and the site loads firmware that matches the ECM to your car. Take the ECM outside, open your hood, flip a few latches on the water-tight electronics box, pull the old one out and plug the new one in. There you go. Done.

Instead of what is pretty much a massively expensive dealer-only operation, you have half a dozen standard bus ECMs to choose from and about 15 minutes of work that’s not much more difficult than installing a new printer on your PC. And, you’d have less expensive aftermarket options as well. And, a new industry would emerge to design and build those aftermarket options.

Duane Benson
Sadly, not in my lifetime, Batman…

http://blog.screamingcircuits.com/