Screaming Circuits: June 2010

Ten Things for an Engineer to consider now that summer is here

Now that summer is here...

I should caveat that a bit. Summer just started last week here in the Pacific Northwest. It's been one of the wettest and coldest springs in quite some time. I should caveat that too. "cold" here in the Pacific Northwest means like 40 degrees. I realize that some places don't really consider it to be cold unless it drops below 255.3722, but we're a little more weather intolerant than that around these parts. Now I have to back out of my recursive caveats. </CAVEAT #2> </CAVEAT #1>. That would have been much shorter in C - 22 characters shorter at just } }.

RCA12ax7_sq_arms Now that summer is here, what can an engineer do to keep productive despite all of the distractions outside? I've got a couple of suggestions. Mostly things that roll through my head when the mercury rolls up.

X - Contemplate global warming and question whether we should try to do something about it. In my mind, there is no dispute that global warming is happening. The problem is that the difference between causality and correlation has been politicized. That means that it's very difficult to find any real information that isn't biased based on someone's personal agenda. So, we have a number of questions to muse on: Is it human caused? If not, is it human exacerbated? If it's primarily human caused, is it too late to stop it? If it's primarily a natural phenomenon, should we try to mitigate it? If we try, will we just make it worse? Can we ever get past the politics and agendas and really examine all the facts using the scientific method?

IX - Decide if hybrid vehicles really help or if they are currently designed in such a way as to really help. Taking an economy box that could reasonably get 40 MPG with an efficient gas or diesel engine and simply giving it more power at the same MPG doesn't really help with the fossil fuel problem. On the other hand, if you take a large vehicle that gets 10 MPG and increase that to 15 MPG by turning the combustion engine off while stopped and using an electric motor to re-start and accelerate through the least efficient first few miles per hour could save 15 billion gallons of fuel per year (based on some quick very rough calculations). That's a lot of french fries.

VIII -Think energy storage and retrieval. Petroleum is just about the most compact energy storage medium and the most that is currently practical to use in small quantities. The problem, of course, is that it's easy to get the energy out, but it's a one way trip. We won't really replace petrol until we can find another storage medium that's at least 70% as efficient in terms of energy extraction and can be refilled just as easily.

Linux-penguin-big_origpreview VII - What about locomotion in general? The bicycle is just about the most calorically efficient method of transportation ever devised. It's use can be practical in many situations, such as cities designed to accommodate large numbers of bikes, but is woefully impractical in other situations - hills, long distances, cargo. Can we take anything from the bicycle and apply it to other forms of transportation?

VI - How can we take our economy back from the money grubbers? Profits built this country, but at various times in our history, the unrestrained pursuit of profit above all else has nearly destroyed it. It's a repeating cycle and I think that at the moment, we're in one of the eve-of-destruction points. Even in recovery, the financial institutions, to the best of my knowledge, seem to be more interesting in finding new quick-flip money making loop holes than in creating a strong foundation for the future. Teddy Roosevelt busted the big monopolies. Ten years of great depression and WWII busted the cycle a few decades after that. How can we break this cycle of ruin without a real depression and war?

V - Can we remain free in an increasingly tight surveillance society? We have technology and resources that would have made Orwell's Big Brother drool and that technology isn't going away. It will only get cheaper, smaller and more pervasive. The technology itself isn't inherently bad, but the misuse of it tends to be incredibly tempting. Being a good steward of things that can be used for good or for ill takes a lot of work and a lot of personal and group-think restraint. Are we mature enough a society to maintain our humanity in the face of such tools?

IV - What do we do about the impending loss of fun and adventurous careers like being a pilot? Knights of the air - the fighter pilot has long been the ultimate in high adrenalin jobs, but even today, outside of training, it's more button pushing than envelope pushing. It won't be long before it's all robot drones. In the civilian world, my bet is we have less than ten years before most cargo flights are unpiloted and passenger flights won't be far behind.

III - Speaking of robots, when will someone build something that's actually practical for consumers to use? I know there's the little robot vacuum, but that's just the tiniest of entry points into the consumer world. We're at 1979 in terms of the evolution of the personal computer. Let's get moving and get some real-world personal robots going.

II - What's left to put embedded computing into? Microcontrollers are into just about everything already. But there have to be a few good killer embedded applications left that we haven't run across. Figure those ones out and build another industry. Start your own company to do it and create some good jobs.

I - And, finally, where's my flying car? Okay, this one is really dream-world until we can figure out the energy storage and retrieval problem (see VIII above). If you think it's inefficient to push a car around on the ground, add fuel for lift generation into the equation. Ugh. Fix that problem Batman and then we'll be somewhere.

Duane Benson
Help us Barry McGuire

Vias between BGA pads

I ran across a question posted on the Xilinx forum recently about whether vias in BGA pads needed to be capped, as suggested in my post about that subject. We do recommend that the vias be filled and capped with copper in BGA pads. There really isn't enough land surface to not plug the via holes. Some people are okay with an unfilled microvia that's only one layer deep, but even that can lead to excessive voiding.

If you've got a good board house, you may be just fine with vias between to pads down to a BGA ball pitch of 0.5mm (20 mil). Just make sure that there is solder mask between the land pad and the via as is noted in the illustration below.

BGA on HASL close with vias between pads

Duane Benson
Consider that two wrongs never make a right... but that three do.

Extra Fine Pitch BGA pads

One of the annoyances of the world of trade secrets and proprietariness is that we can't all learn from each others' experiences. That is important and even generally necessary in a competitive world. If you put in some hard work, you should get the first right to profit from it. Otherwise what incentive would you have to put in that hard work?

There are times, however, when it would be helpful for the industry or the economy in general if we can all learn from someone else's challenges. Times when, for example, the entire auto industry and therefor the safety of the general public would benefit if all companies shared what they have learned about the reliability of electronic throttle systems.

Bb Good SMD pads Here's another chance for open source hardware to shine. Take the Beagleboard. The Ti folks who designed it pushed technology in a number of areas and by presenting what they have created as open source, we can all benefit from it. Even stepping outside of the great work in the schematic, they have done great service in the areas of manufacturing complex devices as well.

A while back, I wrote about soldermask defined (SMD pads) vs non-soldermask defined (NSMD) pads on .4mm pitch BGAs. The basic idea is that while with most Bb Bad NSMD pads BGAs, you want NSMD pads for better mechanical strength, with the really small BGAs, like the .4mm pitch OMAP processor, you want SMD pads to prevent shorts.

The messages that the Beagleboard team learned here are, first, it's true that you want SMD pads and second, make sure that your PCB fab house follows your instructions in that regard.

Many fab houses have their own rules and will set the soldermask up based on what they feel is best. They may have never used your part though. Make sure the board house does what you need. By insisting on closing up the soldermask, the Beagleboard team went from 90% failure with the NSMD pads to 96% good and no BGA shorts with the SMD pads. (This info and the photos come from the Beagleboard ESC presentation by Gerald Coley).

Duane Benson
The worms do.

Another Via-in-Pad reason

Just the other day - No not that one. The other one - I was reading through some of the open source BB Empty pcb via in connector pads Beagleboard information again and I came across an interesting tid bit. In one of the early revisions, they had some issues with SMT connectors ripping off the board. The pads detached from the board. I know that's not  a common issue, but it does happen.

Their solution was to put vias in the pads to strengthen the connection between the pads and the PCB. I hadn't thought of that, but it makes perfect sense. Note the four dimples in each of the pads on the audio connector footprint in this image. Also note that they are small and closed off.

If you've got some concern about losing your SMT connectors, you might want to consider the via-in-pad solution. Do, please, cap or plug them though.

Duane Benson
Who plays pinochle on your snout?

How to Fill a Via

We here at Screaming Circuits keep telling people to fill and plate over vias in BGA pads. The same goes for small passives too, like 0402s and 0201s. As I closed in this post, best option is to fill and plate over. Bigger parts can use some of the solder mask techniques, but BGAs and tiny passives really do require filled and plated vias.

The question of the day, however, is: "Just how do I get those vias filled?" Here again is another place where our old Gerber file format falls a little short.

Filled vias 

Copper filled via on the left and conductive epoxy filled on the right.

There are quite a number of different methods for filling vias: Copper plugs, copper plate, surface material (i.e.  HASL), conductive epoxy, non-conductive epoxy. Maybe others. But, the Gerbers don't have a method for specifying this.

So here's what you do. 1) You need to contact your fab house to determine what techniques they have available and what they recommend for your fab project. 2) You have to create a set of fab drawings or special instructions that point out which vias need to be filled and with what material.

Duane Benson
Maybe raspberry jelly filling?

Via in pad - tenting the bottom side

BGA via in pad bottom via capSpeaking of vias in pads, here's a reason we're not terribly fond of the technique of using solder mask to tent the bottom side of a via in a BGA pad.

As I mentioned in my last post, capping the bottom of the via with soldermask is sometimes an acceptable method for dealing with a via in pad. It's not a desirable method, but it does sometimes work. However, with BGAs, the top-open via can still wick solder down to fill the space. At the very least, you will end up with a hefty void in the BGA solder ball. At worst, you will end up with an open, as illustrated here.

Best option: Fill and plate over the via.

Duane Benson
Does cheap mass production = this perfect day?

Large Via In Pad

Large QFN vias I haven't written about via in pad in a while, but the issue hasn't quite gone away yet. This particular QFN, to the left, has the vias tented, which is good. However, it could be better. If you look close, you'll see that they're tented on the bottom of the board with solder mask.

Tenting on the bottom will usually prevent solder spillage out on the back side of the PCB, but with vias this large, the solder will probably flow down into the space, leaving quite a bit of voiding under the part. Sometimes outgassing will pop open the little tents too causing the spillage. And with immersion silver boards, outgassing can cause corrosion in the vias if you have the bottom tented and the top also sealed - like by the part.

If it's a low speed, low temperature QFN that just needs a little ground connection to the center pad, that voiding might not matter. But, in most cases with QFNs, you need minimal voiding for thermal or noise reasons.

The best option for manufactureability is always to have the vias filled and plated over at the board house, but that can be expensive. If you are going to tent with solder mask, this next image illustrates the three Soldermasked vias for blog b ways to do it.

A is the best: a cap on the component side about 100 to 125 microns bigger than the via. B, a larger cap on the component side, or C, a cap on the bottom, will also work but both come with a greater risk of excessive voiding.

Duane Benson
Do solder mask tents need a rain fly?
In Oregon - probably yes.

Underness as a Common Theme Again

I didn't mean to, but here I seem to be with a common theme again. Like yesterday, and again not too long before that - parts under parts seems to be the thing of the hour.

Cap under con
Maybe the caps are shy and are just trying to hide. Or maybe we've been writing too much about package on package lately. Dunno.

I do wonder though, how many of these issues are caused by parts substitution (the cap originally chosen was narrower), the wrong CAD library footprint, or just a rushed layout. Who wants to vote on that one?

Duane Benson
Just push on it harder. We'll make it fit.

Underlapping Parts

Overlapping parts Sometimes, it's the simple things in life that we must stop and appreciate. Like the fact that just because there aren't any pads in a spot, doesn't mean that you can use the space.

Take this PCB, for example. U3 is placed so that one and a half of its legs will be underneath a big RF module. The module isn't shown in the picture, but the pads and outline are shown. U3 is right into the lower left corner of the module as you can see by the rows of pads and the silk outline. The module that U3 steps under is one of those module on PCB using castellated mounting holes.

If U3 were your hot little sports car and the big module were a dump truck, you most certainly wouldn't do this. You'd look first. That's what mirrors are for.

C6 up on top is pretty darn too-close to the module too. Move it a few mils to the right please.

Duane Benson
Mirror mirror on the wall
Who is the overlappingest of them all


C154 Tant. cap direction questionI remember back in the olden days when we didn't have any space and everything needed to be abbreviated or put into obscure symbols. We had eight characters for file names, plus a reserved extension. So, for example, on disk, this photo might have to be called "APTC6010.BMP".

And, years had to be saved in databases as "82" instead of "1982" because space was expensive. I think what happened, is that we all got in the habit of being obscure and obtuse because of that. We've since passed that habit to newer generations too, so that today, even when there is room available, we still do the "PC LOAD LETTER" thing instead of just being nice and clear.

This cap footprint, pictured to the left, does have a polarity indication in the silk screen. Either that, or the silk screen had some printability issues and was cut off on one side. It's probably positive up, but just to be sure, why not put a little + sign on the positive side to remove that bit of ambiguity. There's plenty of room on this board. If space was tight, I could see leaving it off, but what's wrong with a bit of extra safety when you can get it?

Duane Benson
Baseeball been very very good to me

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