Category Archives: Screaming Circuits Blog

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

The ‘A’ Word

Posted on by

Screaming Circuits is seeing more and more components in short supply or on allocation these days. A while back, we took a survey of our customers and found that on average, an engineer would spend about 16 hours sourcing parts for a prototype design.

Schottky top My question is has that changed? There are a few chip companies with a lot of parts in short supply, but what I hear the most about is the passive components. If you’ve designed a very specific power or radio chip, for example, I can see how a twelve week lead time can be a very big issue. But if it’s just a 47pf, 6V cap, a resistor or diode, is it really that difficult to find a sub quickly?

How much of an issue is parts availability today — really? Is it something that has a lot of visibility and little impact? Or is it something where the visibility and the impact are both pretty big? How much of a hassle and time sink is it for you now?

Duane Benson
I’ll trade you a pair of .022 for one .047

http://blog.screamingcircuits.com/

8 bit vs. 32 bit Microcontrollers

Posted on by

There’s a lot of talk these days about the new generation of 32-bit microcontrollers and the demise of the 8-bit controller. I’m a big fan of the Beagleboard and mbed boards (ARM Cortex A8 and ARM Cortex M3). And the Cortex M0 processor looks to be a very promising low-end 32-bit ARM. By the looks of it, ARM could end up ruling the below-X86 world soon.

But, one consideration to the 8 vs. 32 discussion that I haven’t heard much about is the start-up effort required and the barriers to entry for non-experts. The new ARM Cortex-M processors look to be a great move toward addressing the low-cost and low-power end of the microcontroller market, but they don’t really address the buildability issue and the category-entry issue.

At Screaming Circuits, we run into quite a few designs in industries that are just now beginning to automate. In many of these cases, mechanical engineers, not software or electrical engineers, are tasked with putting the brains into the product. These mech folks have to learn, design, layout, build and code. The PIC and Atmel processors, with their through-hole or big SMT packages, easy 5V power, low clock-speeds and huge base of community support make an impossible job possible for the new entrants into the embedded world. If a thru-hole part with a 20MHz clock can do the job, novice designers can greatly increase their chances for a successful design than if they have to deal with fine pitch parts and 100 MHZ clocks.

In a perfect world, this wouldn’t be a concern, but as it is, a lot of companies need these parts that are easy to implement for a new designer. M0’s may be priced in the sub-$1 range, but piece price is not the only component of “cost.”

Duane Benson
“Apple II forever”

http://blog.screamingcircuits.com/

DB-25 With Issues

Posted on by


Take a look at this footprint. We’re looking at the solder-side of a DB-25 connector footprint. Given all the SMT and HDI (high density interconnect) we see these days, this looks primitive; like it could be straight out of 1979 or something. But it is a recent PCB design. Some of those good old-standard parts still have a place in today’s world.

However, old or new, issues can still pop up. So, just what do you see wrong with this one? At least two issues are pretty obvious, and a third may be open to debate.

Duane Benson
What’s wrong with this picture?

http://blog.screamingcircuits.com/

Who’s Responsible for the Footprint?

Posted on by

I recently wrote a bit about CAD library parts for QFNs and a reader posed an interesting question in response to it:

Okay, that makes sense.. But why don’t manufacturers of the part put out their footprints and schematic symbols for their parts in some common format?

That is a very good question. When you purchase or download a CAD package, it will typically come equipped with an established library. Those libraries vary greatly in coverage and quality though, and, of course, only cover parts available at the time of release — and only a subset of the most popular, at that.

Who’s job is it to make workable library components and or new ones? Ultimately, it usually falls to someone in the organization doing the circuit design. Some people will pay a third party to make the parts. NXP has made the complete library for its chips for the PCB123 CAD package. Some independent companies make it their business to create and maintain libraries. But, really, who should be responsible?

A component manufacturer needs to document a new chip anyway, but they’d have to make a library part for a dozen or so CAD packages for each part variation, depending on how much coverage they want. CAD companies would need to create maybe a hundred thousand or a million library parts, depending on how much coverage they want, but the CAD package is useless without the library. The designer would need to create just the library components needed for the specific design, but that could easily double to design time. I guess that’s why we have what we have — a combination of the above.

Still, the big question is: why isn’t their a standard format for the libraries? That would make everyone’s life easier. So, CAD folks, why no standard?

Duane Benson

http://blog.screamingcircuits.com/

Mysteries of Engineering

Posted on by

I (and many, many of us, presumably) have been reading more about all of the Toyota woes and the to-date unanswerable questions. Still, so much of the material written about the issues seems to be coming from the untrained. Certainly, human behavior suggests that some of these problems could be the result of operator error. But, I’m not an expert in human behavior, so I can’t really say. And, certainly, problems do crop up in complex machinery, like cars. I don’t know if that supposition falls within my area of expertise, but a few decades of operating motor vehicles gives me some personal empirical data on that one.

The area that does bother me the most is probably those that speculate that since the problem hasn’t been found, it doesn’t exist. This is an area where I can claim some level of expertise as well as plenty of personal empirical data.

It is possible to spend uncountable hours testing various possible conditions and still never uncover the one scenario that will cause a systems failure in the hands of the general public. Many years ago, I worked for a company that designed, built and sold projectors. In that day, these were big things with short-life, very hot, incandescent lamps. We thought that we had done a very through job of testing under various conditions and had been selling the product for a little while when reports started filing in of bulbs exploding. It wasn’t just a simple break. The bulbs were exploding with such force that the bulb area was filled with a fine grained, razor sharp glass dust. Nasty.

During a weekend burn in session with a couple dozen projectors, including some returned from the field, the engineer monitoring the process thought he heard a gunshot and dove to the floor. It wasn’t a gunshot, but it was the first clue in a long investigative process that did end up finding the problem. It seemed that if a bulb was too deeply seated in the socket by a couple of millimeters, the reflection of the filament in the mirror would exactly line up with the actual filament, causing it to melt and arc. The arc would run in one direction, down the filament leg to the base and stop.

One filament leg had a few coils of small diameter tungsten wire wrapped around it. The other leg did not. Depending on the orientation of the supposedly non-polar bulb, the arc would either run down the leg with no coil or the leg with the coil.

If the arc ran down the leg without the coil, nothing happened other then the bulb needed to be replaced. If it ran down the leg with the coil, that small amount of additional vaporized tungsten increased the internal pressure sufficiently to explode the quartz bulb in a very catastrophic manner. Okay, now that’s weird and obscure. Technically, you could call it operator error. If the customer had just inserted the replacement bulb the exact same way we inserted the bulbs during production, the problem would never have happened. But, realistically, it was a design flaw that set the customers up for a failure.

Duane Benson
Duck and cover

http://blog.screamingcircuits.com/

Thermal Mass Follow Up

Posted on by


RoHS has been with the electronics manufacturing world for quite a while now but there is still a lot of issues and uncertainty associated with it. As I wrote not long ago, even parts that are supposedly compliant can in some cases not cut it.

Taylor asked in the comments section of that post: “Have you noticed any pattern in capacitor manufacturers exhibiting this problem? How can make sure to spec a capacitor that is more robust?”

I can’t say that I’ve seen a real consistent pattern with components from different manufacturers here. It’s a case where the design engineer may have to compare the exact thermal specs from different components’ data sheets and throw in a good measure of intuition and judgment as well.

In some cases, you might be able to replace a couple of capacitors with a single of a larger value, but in general, if you need multiples, combining them won’t do. There are certainly good reasons to parallel up capacitors. You may need a few of different values to cover different frequencies. You may have a clearance issue and not have enough height for a taller cap. Or you may need to keep the ESR (Effective Series Resistance) down. Whatever the reason, if you need a number of caps close together, and they are big SMT electrolytics, you could be setting yourself up for this problem.

Image A illustrates the issue found in that earlier post. The thermal mass of all of those big metal can caps can slow the solder melt. The most vulnerable pads are the two inside pads for C3 and C4. Keep the heat up long enough to fully melt the solder on all pads and you may destroy the caps, or other components.

You could just spread the two rows apart a bit like in illustration B. This might be enough to allow all pads to solder well or, if nothing else, it would give you enough room to touch up with a soldering iron.

Probably the most common solution though is to take the approach used in illustration C. Just put all the caps in a row so none of the pads are vulnerable.

If you need a compact layout like A, you’ll just need to spend some extra time with data sheets to find a specific cap with a bit of extra RoHS temperature margin. Look at the maximum solder temp, the maximum dwell time and the profile curve if available. Don’t forget to check your other components too to make sure that the extra reflow time won’t harm them, either.

Duane Benson

http://blog.screamingcircuits.com/

Beagleboard Innovation

Posted on by

Open source hardware makes me happy. Open source has been around in the software world for a long time, but it’s still fairly new in the hardware dimension. I think 2009 might just be the year that OSHW reached critical mass. Certainly, the Arduino is now everywhere, but there are other great opens source hardware projects getting some coverage too. My favorite is still the Beagleboard — a super powerful ARM Cortext-A8 based computer in an open source 3″ x 3″ form factor.

Gerald Coley of Texas Instruments wrote a great article in EDN Magazine last year. His work is a finalist for the best contributed article category in EDN’s Innovation Awards. Check it out. It’s a great read.

Duane Benson

Curse you, Red Baron!

http://blog.screamingcircuits.com/

More Thermal Mass Issues

Posted on by

Yesterday I wrote about some thermal mass related traps. Here’s another one we see now and then.

The top image shows good flat-topped caps. The bottom and inset has overheated bulged and damaged caps. These caps are RoHS compliant — supposedly. Their data sheet calls them out as RoHS compliant and their temperature specs and recommended reflow profiles indicate RoHS compliance. So what happened?

Well, they are compliant pretty much only in singles. A single of these caps will solder up fine and not be damaged. However, put four in close proximity like this and the solder paste on the inside pads will not melt at the recommended profile. They need a bit more heat because the thermal mass of the four parts close together sinks heat away from the inside solder pads. In the end, we hand soldered these specific parts to solve the problem, but for production, either a more thermally robust part would be needed or the part spacing would need to be changed to compensate for the combined thermal mass.

Duane Benson
“Hot” is a relative term

http://blog.screamingcircuits.com/

Thermal Mass

Posted on by

I mentioned thermal mass in a recent post and was thinking over my oatmeal that its a subject deserving of more attention. That’s more attention to thermal mass, not to oatmeal. Although oatmeal is a pretty healthy food, so it probably deserves more attention then it gets these days.

When most people think of thermal issues, the considerations tend to be around operating conditions. Will the part generate too much heat? Will there be enough airflow or is there enough surrounding material for adequate conduction cooling? All of those are pretty important — especially with the obvious like fast processors and big honking power components. But there are a lot of thermal issues related to manufacturing that have to be considered as well.

Reflow soldering is supposed to gradually and evenly warm the PCB and parts. Then, the temperature will spike up just high enough and long enough to melt all of the solder before dropping again. This is were thermal mass trickery comes in to play.

If you have a tiny passive part and one pad has a lot more copper than the other — it can even be a problem, even if it’s an innerlayer with more copper under one pad then the other. That extra thermal mass can delay the solder melt on that side of the part slightly. That delay in melting can cause the surface tension on the side that did melt to pop the part up like a tombstone. Placing a very large component too close to one side of a very small part can also cause the same problem.

If you have really tiny parts, give your layout (inner and outer layers both) and placement a scan to make sure you haven’t inadvertently created a heat sink on one pad and not the other.

Duane Benson
Quick. Call Wilford Brimley

http://blog.screamingcircuits.com/

Toyota is as Toyota Does

Posted on by

Everyone else seems to be writing about Toyota sudden acceleration problems, so I should probably do that too.

Or should I? Personally, I have absolutely no solid information about what’s going on with Toyota cars. There’s an awful lot written, much of if by people that also don’t have any real information on the subject. Here’s what I do know though:

  • Some people (some with actual knowledge and some without) are speculating that electronics might have something to do with the problems.

That’s about all I know relative to the specific concerns. On the soft side, I do know that people tend to pick on the big guy. Funny how none of this was big news until Toyota became the #1 car maker in the world. Coincidence? Maybe. Maybe not. I also know that in any system there are gobs of places where issues can lead to failures. Of course, to counter that, I know that good, well thought out design – both in the hardware and the software, can produce a quality product that will keep working. Of course, to counter that, I know that good, well thought out design – both in the hardware and the software, can produce a quality product that will keep working. In summary, I really don’t know anything about the Toyota issues

However, any time some sort of actual or potential technical problem makes big news, it’s not a bad idea for those that design and build things to take a step back and evaluate our design practices. I’ve got software in my past, so I’d have to suggest a good solid code review, if you don’t already do one, but today, I’m talking about hardware so I’ll sample just a few things to double check.     

* Those pesky land patterns: Does the land pattern fit the part? Will the copper area and stencil opening allow for a good solid IPC-passing solder joint? It’s so common (as you well know if you read here regularly) to re use or create new CAD part foot prints. Make sure the foot print, stencil, mask and silk layers fit properly.

* Vias in pads: Plug them and plate over them when using small parts. If the solder surface is big enough, like with a power component, you might be able to just cap them, but don’t leave the vias open. In some cases, you may be able to leave very tiny vias open on thermal pads, but it’s best never to.

* Thermal mass: This is important both for operation and for assembly. If you’ve got components that sink and/or generate lots of heat, make sure there is enough air flow to cool them during operation and make sure that the assembly house can build it. Put a couple of high thermal mass parts too close together and an otherwise perfect PCB assembly may end up with some cold solder joints or damaged components that later come back to bite you or your customers.

There are lots of other things to check out too, but those three are just some of the more common traps to keep tabs on.

Duane Benson

I don’t have a Toy Yoda. If I did, I’d sell in on eBay.

http://blog.screamingcircuits.com/