Screaming Circuits: February 2010


More Thermal Mass Issues

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

Over heated caps

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

Thermal Mass

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 up. 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 then the other - it can even be a problem Two part tombstone even if it's an inner layer 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

Toyota is as Toyota does

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 LED via-in-padissues 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. In summary, I really don'tSilk on pad 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 lot's 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.

IPC-A-610 Class III Assembly

The Screaming Circuits website will now offers instant online quoting and ordering of prototype and short-run production PCB assembly built and inspected to IPC-A-610 Class III standards. By default, Screaming Circuits assembles and inspects to IPC-A-610 Class II, but with the new capability, the higher reliability standard can be quoted and ordered just as easily.

Just what is IPC Class III and why should you care? IPC-A-610 covers workmanship for electronic assemblies - the boards you design and get built up. The higher the number, the more stringent the build and inspection requirements are. Class II is the general commercial standard: items produced and inspected to IPC-A-610 Class II are deemed quality for typical commercial applications. Class III are deemed to be appropriate for hi-reliability and mission critical applications.

For most prototype and small volume applications, IPC Class II workmanship and inspection will be just fine. However, for some with tighter reliability requirements, e.g. aerospace, military, medical, harsh environments, class III workmanship and inspection may be required. Class III service is available as an option on the Screaming Circuits "Full Proto" and "Short-Run" assembly services.

Duane Benson

Foot Prints

I write a fair amount about component foot prints / land patterns / CAD library components - whatever you call them. What are you supposed to call them anyway? I've heard them called all three of those things and a few other names as well. Seems like a good case for some terminology standardization. Or maybe I just need to do some more research on that specific subject.

PCB123 cut out capture I'm curious as to how common it is that a designer needs to create a custom footprint for a part. I seem to have to create at least one for every project I do and I rarely use new parts or complex packages. Of course, I don't use CAD packages that cost gobs of money either. Just Sunstone's PCB123 and Cadsoft Eagle. PCB123 has all of the NXP parts so that's good, but it's not just the big components that get you.Eagle library design

In a board I 'm just finishing up - 22 BOM line items, I had to make a custom library part for a crystal and modify a library part for my microcontroller. I had to create some custom footprints for a couple of connectors too, but half way through the layout, I upgraded Eagle and discovered that someone else had just done the same thing I needed for the same connector, and uploaded it to the Eagle site, so I used their work. Hopefully I haven't made any of the mistakes I always write about.

I really am curious as to how many designs typically require at least one custom footprint and how many custom footprints per design would be a typical rule of thumb.

Duane Benson
Aquatone

Is Geek Cool?

When I was young, "Geek" was not cool. Neither was "Nerd". Working on cars was cool as was logging and shooting Bambi's uncles with high powered rifles, at least where I came from things were that way. On the other hand, every little town had a Radio Shack where you could buy tubes, transistors, ICs and other assorted electronic components. You don't see that so much anymore. Grocery stores sold publications like Byte Magazine, 101 Electronics Projects and Radio Electronics. Those magazines were about building things. People who read and wrote those and others like them created an industry in their garages, basements and bedrooms. They started a new Industrial Revolution.

Still, back then, tech folks were more likely thought of as mad monks and strange people like Eddie Deezen as "Mr Potato head" (Malvin) in the 1983 movie War Games. You didn't want to be one. I like to think that attitudes have changed over the years, and I think the signs are there.

The FIRST Lego league with its robotics tournaments has created a legitimate "sports like" atmosphere for geek-types in school. 50,000 plus Arduinos being sold shows that the electronics hobbyist world is moving again like it did in the 80's. The maker and bender communities illustrated by Hackaday, Makezine and supported by companies like Adafruit and SparkFun show that creating with chips is as alive as it was in the late 70's and 80's. TV shows like Mythbusters, Jimmy Neutron and Prototype This have glorified the geek.

And why do we care? Because the more engineers we build out of the masses, the better we can design and build our economy. The more mainstream and acceptably technology is, the more educators will work to encourage and foster the environment and attitudes that allowed Apple, Dell, Google and SparkFun to thrive. We need that. We need robotics competitions to be as socially acceptable as football games.

Duane Benson
The rooms were so much colder then

Parts Too Close For Comfort

Another tale from the sometimes kind of kooky world of prototypes. I think what happened here is that the board was originally laid out for caps with a smaller voltage rating (or even a smaller capacitance value). We're seeing things like this more often these days due to availability issues. The part gets swapped at the last minute because the exact one is out of stock. Operationally, it won't hurt and if the board had been laid out with more space, this wouldn't have caused any issues at all. Of course, then, it may not have fit in the box.

Caps interfere with eachother

The moral of this story is that with last-minute swaps, don't forget to double check things like the part package and board spacing. This board works fine, but it won't be meeting up with any IPC standards as long as these parts are like this.

Duane Benson
Parallel parking is hard. I'd much rather diagonal park.

Another Land Pattern Mixup

PCB fab error - L2 pads are not correct Here's an interesting error that doesn't show up very often. Notice that the opening in the solder mask matches about what the copper land pads should be, but the pads themselves are really tiny. To start with, it's not an exact match. It would fit the part a little better if the pads were closer together, but more than that, the pads are just the wrong size. The stencil looks okay and the solder mask opening looks okay (except that it's a little too too wide).

I'm not really sure what went wrong here. Maybe one  person started making a custom library part and PCB Fab error - L2 Pads - Stencil and PCB do not match (stencil correct)someone else finished it, or maybe the component was changed between starting and finishing the library part.

If the stencil photo looks like it's made of cloth - it's not. We just photographed a CRT view of it.

Duane Benson
Need more chocolate.

Close Only Counts...

DSCN1975DSCN1971 I think this one has both some land pattern issues and some schematic issues. It's unfortunately pretty common to see footprints that are close, but not close enough to work. Looking at the data sheet here, there may be a schematic issue as well. The only connections on to the part on this board are ground and P3. I don't know the part, so I suppose it's possible that all it needs is P3 and ground, but it looks more like a case of the footprint being wrong and the pin connections being wrong. Pins 1, 2, 3, 5 and 7 are all ground, but on the board, 1 and 3 go some place else. Bummer.

Making custom CAD parts can be pretty annoying and it seems to be a function disproportionately prone to error. Why is that?

Duane Benson
in horse shoes, hand grenades and sometimes atom bombs.

The Heartbreak Of Units Mis-match

A little extra checking may take a little extra time, but it can save a lot in the end. This sort of thing just bums me out. I've done it and similar things a few times myself. I've used a library for a connector with the same footprint, but smaller diameter pins than my chosen connector. That was a bummer too.

Connector Metric vs SAE

My best guess is that it's a .1 inch (2.54mm) spaced footprint on the PCB and a 2.5mm pitch part. It almost works. You might not even notice if it were a three or four pin part. At six, you'd certainly notice and at something from there on, the part simply will not fit.

In a prototype world, you may be able to get away with bending the pins a bit and forcing them a little way in, but maybe not. In a production electronics manufacturing environment, most certainly not.

Duane Benson
In the world according to Google, it's only 72,000 beard-seconds off.

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