Screaming Circuits: October 2010

Pad is as Pad Does

I've recently written a bit about soldermask and pads relative to BGAs. In most cases, we recommend NSMD (Non Solder Mask Defined), or copper pad defined, pad for BGAs. With the BGAs, the NSMD pads will allow the BGA to sag down just a bit more and adhere to both the top and the sides of the pad, resulting in a better mechanical connection. The exception seems to be 0.4mm pitch BGAs with a straight matrix alignment as in the illustration the link above. Ti, with their Beagleboard project found that NSMD pads tended to lead to bridging and had much better results with SMD pads. Staggered BGA lands should still use NSMD pads though.

Along with the 0.4mm BGAs, not all parts need or want NSMD pads. International Rectifier has a package called "DirectFET" which is designed to use solder-mask-defined layouts. In this package, the FET source and gate connections are directly on the FET die. The drain connection is a plated copper can directly bonded to the drain side of the silicon die. This system gives a very low-loss capable part with great thermal conduction properties.

Internal Rectifier recommends solder-mask-defined pad layouts. Take a look at their application note 1035 for complete details on designing with this package. I might try the form-factor out myself some time. It always bugs me that a 100 Amp MOSFET might only, in practice, be able to pass a small chunk of that amount of current because the leads or internal interconnects would otherwise melt. The DirectFET package should aleviatemuch of the melting problem.

Duane Benson
Melting is good if you're talking about toasted cheese

Spam, Spam, Eggs and Spam

I normally expend most of my writing words on challenges our customers and other engineers might face in their day to day design and layout activities. But not today. Today, it's about a specific challenge faced by your typical blogger. Off and on for the last couple of weeks, I've come into work in the morning, opened up the blog and found three spammy comments. Here's today's three"

"Compare to the majority of the other blogs, your site tend to be so fantastic. Therefore nice to examining the post. If I've a probability, I would like to research along with you because I think that my potential haven't yet achieved the excellent amount."

"You may remember the three proverbs: Laugh, and the world laughs with you; Weep, and you weep lone. Life is measured by thought and action not by time. Long absent, soon forgotten."

"You may remenber [sic] the three proverbs: Laugh, and the world laughs with you; Weep, and you weep lone. Life is measured by thought and action not by time. Long absent, soon forgotten."

Now, it is a bit flattering to hear that my site tend to be so fantastic. Not just "fantastic", but "so fantastic"! But perhaps the subject matter could be a bit more on topic. The author noted that if he have probability, he would like to research along with me. I could always use some extra help, but I don't have any probability to pass on. Perhaps a call to Zaphod would be in order.

I'm not sure I agree completely with the second supposition. I'd bet that a lot of unemployed or underemployed folks are weeping right now. Probably enough that they could be considered to have a world weeping with them. I know I would. I'm also not sure what the deal is with the third one. That author just copied from the one above it. How rude.

All is not always as it seems though. After reading these this morning, I did as I always do and fed the three comments into my netlist confabulator. It turns out that the text in these three comments is actually a turbo-encoded form of the design of the Constellation spacecraft. If I had checked the IP address prior to marking the comments as spam, I wonder if I would have found that this is a desperate rocket scientist tying to smuggle his decade of work home before the lights go out and the servers get recycled.

Duane Benson
Have you got anything without spam?

Random Via-in-Pad Myth #7

Myth #7: In regards to via-in-pad, all PCB finishes are the same  Ant_wideweb__430x317

Well, it might seem so, but let's look a little closer. No. Not that close. Back the camera up a bit. 

Here's a good example: In some cases, it's okay to seal off the via with soldermask on the opposite side of the board. It's not the optimal way to do it, but when the geometries aren't that small, it can work. It needs to be a part where voiding isn't an issue, because the solder may still go down the via and cause some of those voids. "Void" may be accepted in C code, but it's usually bad form in a PCB.

Getting back to the subject... Immersion silver gives a nice smooth surface. It's fairly easy to solder and provided the boards are used promptly or stored properly, it's a good RoHS choice.

BGA via in pad Silver But, it's not a good choice for a situation where you cap a via with soldermask on the underside of the PCB. The immersion silver finish will likely out-gas a bit and when contained, as in the sealed off space between the solder on the top and the soldermask on the bottom, that outgassing can be corrosive and lead to reliability issues sometime during the life of the product. So if you do need to have vias that are capped on the bottom side, you should consider a surface finish other than immersion silver.

Duane Benson
No more silver on Walden Pond

The Sky Is Falling

Or - The top ten things to do if you're depressed about the economy.

10: Tell every young person you know not to get into engineering because it's a dead-end job. Wait. No. Don't do that. Scold yourself if you do.

9: Put yourself into a drunken stupor until the Mayans destroy the Earth in 2012.

8: Meditate. Go to the top of a mountain. Sit cross legged for three days straight. Get hypothermia because it's cold up there.

7: Invent a time machine and go back in time to those halcyon days of the mid-70's to early 80's when engineering was at it's prime. Wait. Didn't we go from double-digit inflation to double digit mortgage interest rates back then? Weren't we having our economic clock cleaned by Japan back then? Didn't gas double in price overnight twice in that span?

6: Invent a time machine and go 20 years into the future when engineering will be at its prime again. Just make sure you time your arrival well or you'll run into another one of these points when the world is coming to an end. Plus your skills will be obsolete, if they aren't already.

5: Obsolete? Who's obsolete? If you're feeling obsolete, go take some college classes or find a way study up on something new.

4: Just about every blasted job coming up these days wants both analog and digital experience, as well as software. Ugh. If you only know one, go learn something about the others.

3: Exercise. Eat well. Sleep well. You'll feel better and if you have a job, you'll be more productive and less likely to be cut. If you don't have a job, you'll look happier and more employable in your interviews.

2: Call your self "Open Source." It's the buzz word of the decade and everyone will think you're cool. Plus anyone can take all of your ideas without guilt and without compensating you in any way.

1: And the number one thing to do if you're depressed about the economy, out of a job, out of luck and out of answers - go find a few other people in the same boat with you and start something. Build robots or aerial drones or solar power stuff. You're an engineer and engineers solve problems. So take this problem and solve it.

Duane Benson
Tired of being depressed...
Or is that tired of being recessed? I can't remember.

Window Pane in not a Pain

QFN parts (also known as MLF or Micro Lead Frame) parts used to cause a lot of problems a few years ago, as evidenced by the number of blog posts covering the subject.

Can I use my own blog as cited evidence to justify my own conclusion? Doing so is probably bad form, but I'm doing it anyway. Interestingly, if you look up "citations" in Wikipedia, the entry (as of this writing) has a note indicating that the article on citations has insufficient in-line citations. Hmmm.

Screaming_QFN_Fig1 Anyway, it seems that the industry in catching up with the proper manufacturing methodology for use of the technology. It's important enough though that it bears repeating now and then. The key to successful QFN and DFN manufacturing really is in the solder paste stencil pattern. Consult the data sheet for the part, but if you can't find the datasheet or if it doesn't cover the stencil layer, use the window pane technique, or "segmenting" for the stencil layer when you're making the library part for your CAD software.

If you leGood QFN stencil bave the full thermal pad area fully open, you'll most likely end up with too much solder in that area. The part will ride higher than it should and may very well float too high for all of the pads on the  side to connect. See the top  part on the above right illustration.

Shoot for 50 - 75% paste coverage by segmenting the stencil as in this illustration on the left here. That'll ensure that the center pad and the side signal lands will be at the same level. You'll get much better yields and reliability.

Duane Benson
The strangest sight I've ever seen
2 buffalos, 2 buffalos, buffalos on my lawn.

Mismasked PCB

Stencil w mask Personally, I think this PCB and stencil is prettier with the green showing through. It breaks up all of the boring silver color. It adds some life in.

...unless you are a chip wanting to be soldered down. If that's the case, then it doesn't look so attractive.

Whomever really, truly and universally solves the library problem should get a Nobel prize or a Pulitzer. Maybe a free latte. Something tells me the problem won't be solved in my lifetime though.

It really shouldn't be that difficult. How many different package form-factors are there? Yes, a lot, but a manageble lot. The problem comes in when you have to match those footprints to the millions of schematic symbols. Maybe there could be a way to decouple the specific footprint from the schematic symbol.

The schematic could have its pins defined to an abstraction layer and then that abstraction layer could be automatically connected by the layout CAD software pin to pin on the specific footprint selected. Maybe. We can dream, can't we?

Duane Benson
I'm happy I live in a split-level head

Reference Designators

Not a subject I give much thought to.

For one, we here at Screaming Circuits don't really care too much what convention you use for your components. We want them to match and be properly formatted when in your BOM, of course. But because we program our SMT machines electronically, we don't really care if you mix things up. i.e. O for resistor instead of R or F for capacitor. It's not a good idea to do that, but we can still build it.

But, if we can build it, shouldn't anybody be able to build it? And, if anyone can build it, why should it matter? Well, in theory, it shouldn't matter at all. In practice though, people tend to be human and humans tend to be error prone. That's why we have standards, conventions and test procedures - to reduce the chances for errors. We also have conventions for the purpose of distributing bad, overpriced food and educational sessions, but that's probably a different convention.

It would be kind of like if you drove into a small town and there was a sign at the city limits indicating that in this town, red means go and green means stop. You would have all of the information needed safely traverse the town, but you would still be very prone to go with the green light.

I just recently saw a design where the connectors were labeled U1, U2... Again, we can build this and we did. But, if it comes time to do any rework, or if you want to make some design mods in-house, of if someone else needs to work with the board, they'll see "U something" and think you're talking about an IC instead of some sort of connector.

There are some specific industry standard documents covering the reference designator conventions, but I bet it's one of those things that most people just sort of know, but don't have the official document to go with it. Wikipedia has a list and a lot of companies probably have their own conventions.

It is easy enough to find these lists of conventions, but it does leave me wondering how some of them came to be. I get "R" for resistor and "C" for capacitor. "Q" for transistor even makes sense, although it's derived from a property of the device instead of the name as are R and C. But, why "U" for integrated circuit? It used to be "IC", but that's fallen out of favor now. Really weird is the inductor. It starts with "I", it's inductance value is measured in "henries" and henries are indicated by "L." Go figure.

Duane Benson
U take the high road and L take the low road


Dripping Wet Is Too Much

Well, that's 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 Orlville, but rather, like his pop corn. 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 MSD logo 75 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.

So, the message here is that if your parts are labeled as moisture sensitive, don't open the moisture barrier packaging before sending them to Screaming Circuits. Or, if you have to open the package, please let us know. We'll bake them at the proper temperature

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

To Lead or not to Lead. That is the question

Back 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 intermetalic 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 degrees 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 re-balled 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

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