I wrote not long ago and not far, far away about the type of pad recommended for 0.4mm pitch BGAs, as in non-soldermask defined vs. soldermask defined (NSMD vs SMD). The wisdom being that with the pads being so close on a 0.4 mm pitch land, the BGA balls may be too close together and may bridge if the pads are NSMD.

I got a comment from an engineer stating that not all 0.4mm pitch BGAs are  created equal, that some have staggered lands which would still require NSMD pads. I searched and couldn’t find a picture of such a part or corresponding land — too many millions to go through — so I simulated what that might look like in this image.

IPC calls this an “Interspersed array” (IPC-7095B, March 2008, Figure 6-13) or a “Staggered Matrix” (IPC-7351A, February 2007, Figure 14-8). The thought being that, when staggered like this, the center to center distance between the pads is 0.57mm even though the column center to center distance is 0.4mm and that would put it back in the realm of getting better results with NSMD pads because 0.5’s and above generally want NSMD pads.

Hmmm. Makes sense. Thanks Mr. Pythagoras for the extra 0.1656854mm of space.

Duane Benson
Use the reflow, Luke. Use the reflow

http://blog.screamingcircuits.com/

Quick: If you haven’t had your chance to answer the question, go back and read it first.

Done? Okay.

#1. This is an OSP (organic surface preservative) finished PCB. That in itself isn’t a problem. However, take notice of the two different colors of pads. The darker pads are oxidized or otherwise contaminated — not a well-preserved surface. And that means that this board isn’t going to work.

#2. Oops. I already answered #2 in with my answer to #1. It’s got some oxidized/contaminated pads. Perhaps the PCBs weren’t stored properly, were kept too long or the OSP finish wasn’t well applied at the fab house.

#3. Planet 10.

#4. Real Soon.

And, no it can’t be used as is. The solder most likely just wont take on those bad pads.

A couple of other notes: The mask registration isn’t bad. Not perfect, but not bad. Big pitch BGAs like this do tend to work best with NSMD (non solder mask defined) pads like this. That permits the BGA ball to sag a bit and grasp the side edges of the copper pad for improved adhesion. (Note that for extra fine pitch BGAs, like 0.4 mm pitch, you need soldermask defined pads) And, note that there is a good web of soldermask between the pad and the via. This will keep the solder from being sucked down the via.

Duane Benson
I’ve been ionized, but I’m okay now

http://blog.screamingcircuits.com/

Yesterday, I wrote about how much faster we can get things done in the prototype world than we could back when many of us were starting our careers. I mentioned that the decrease in time from weeks to days needed to respin a board and get a new prototype run completed is a double-edged sword, but I kind of only mentioned the benefits — time savings, cost savings, quicker time to revenue generation. I didn’t mention the downside.

Here’s a common scenario: You have an array of small components. Maybe some SOT23 transistors or a set of LEDs. On one side, you have wires and chips and stuff hooked up all over the place. On the other side, you have a ground plane.

The easy route would just plop the grounded pad of the part right on the ground plane. You would get better heatsinking if needed. You get a much more direct path to ground. It would be quicker to lay out.

But — and there’s almost always a “but” to such questions — you could get tombstoning. Especially if the parts are 0402s or smaller. You would also likely have soldering problems because the plane will act like a heatsink and may keep the solder paste from melting.

If you really need to, you could do the pad directly on plane thing, but you’d probably have to hand retouch each connection on the big pad and maybe rework tombstoned or crooked parts.

Much better would be to do something like the image on the right. You could also use thermal pads in the plane. With really small parts though, you might still be opening yourself up to soldering problems because of the heatsinking of the plane. The thermal pads would typically have three connections to the plane in a setup like this and that could still be an unequal amount of copper connecting on one side vs the other. You generally want the same amount of copper on both sides of the small parts.

You could also just run the eight traces straight to the plane. How would you approach this seemingly simple but surprisingly error-prone layout?

Duane Benson
You’ll take the left road and I’ll take the right road
And I’ll be in reflow before you

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 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 } }.

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 instigated? If not, is it human exacerbated? If it’s primarily human instigated, 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 off the combustion engine while stopped and using an electric motor to re-start and accelerate through the least efficient first few miles per hour, you 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.

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 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 adrenaline 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 10 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 that we haven’t run across. Figure out those ones 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

http://blog.screamingcircuits.com