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Location: Pantego, Texas, United States

Wednesday, March 12, 2008

The greenies forced a change in solder used in electronic devices that has not worked well. This problem was just surfacing as I was about to retire from almost 50 years in Aerospace. The whole thing seemed silly to me, with the solution probably worse than the alleged problem. Here is a comment about this issue from the blog "greenie watch:"

Tin whiskers: Sometimes going green hurts more than it helps

If you own almost any electricity-powered device, this concerns you

[Tin whiskers affect] all of your soldered devices that are two years old or less. Most of these are now assembled using solder joints that have no lead in an effort to save our groundwater and our health. The fact that the lead has been generally replaced with silver or bismuth, both of which are actually greater health risks than lead, well we'll leave that one for Ralph Nader if he decides not to run for President. The longer-term trend is toward all-tin connections, anyway, but they don't work very well, either.

I wrote a column about this back in 2004 (it's in this week's links) that was heavy on information and therefore low on readership. Everything in that column has come to pass and more. Where's my Pulitzer Prize? Costs have gone up, mean time between failures (MTBF) has gone down (accelerated MTBF tests, which are the only MTBF tests we do anymore, don't reliably pick this up, by the way), and reliability has suffered. Since we don't fix things anymore, it's hard to say whether your gizmo failed because of bad solder or not, but the problem is becoming worse as a greater percentage of total circuits in use have lead-free solder. The military was especially concerned, even before the whisker crisis.

We're talking about tin whiskers, single crystals that mysteriously grow from pure tin joints but not generally from tin-lead solder joints. Nobody knows how or why these whiskers grow and nobody knows how to stop them, except through the use of lead solder. Whiskers can start growing in a decade or a year or a day after manufacture. They can grow at up to nine millimeters per year. They grow in any atmosphere including a pure vacuum. They grow in any humidity condition. They just grow. And when they get long enough they either touch another joint, shorting out one or more connections, or they vaporize in a flash, creating a little plasma cloud that can carry for an instant hundreds of amps and literally blow your device to pieces.

Since 2006 we have been exclusively manufacturing soldered connections thousands of times more likely to create tin whiskers than previous generation joints made with tin-lead solder. Because of the universal phase-in of the new solder technology and the fact that the solder technologies can't reliably be mixed (old solders mess with new solder joints in the same device through simple outgassing) this means that it is practically impossible to use older, more reliable technology just for mission-critical (even life-critical) connections. So we're all in this tin boat together.

Some experts confidently say that the disparity of joint reliability we are seeing today will go away and that the new joints will become as reliable or even more reliable than the old tin-lead joints as we gain experience with the new processes. What's disturbing, though, is that these experts don't actually know how this increased reliability is likely to be achieved. Just like extrapolating a Moore's Law curve to figure out how fast or how cheap technology is likely to be a decade from now, they have no idea how these gains will be made, just confidence that they will be.

What if the experts are wrong? Tin whiskers can take out your iPod or your network. They can stop your car cold. They can take down an entire airport or Citibank. They are much more common than most people -- even most experts -- think. The reason for this is that most tin whiskers can't even be seen.

"Maybe it is worth adding," said one expert who prefers to remain anonymous, "that whisker diameters range from 0.1 um to 10 um, while the diameter of a human hair is 70 um to 100 um --- so the largest whisker is only some 15 percent of the diameter of a thin hair, and most are less than 5 percent. A good fraction (of these are) so thin that light waves just pass them by, scattering a bit but not reflecting. So the optical microscope images that (typically used to illustrate whiskers) show only a small fraction of what is really there. Scanning electron microscope (SEM) images are a bit better, but only show a small zone of the sample; also, not many folks are able to acquire SEM images of their equipment. So all too many folks have the idea that whiskers are something that happens to someone else, but never to them. This is an expensive misconception."

What I wonder is whether a cost-benefit analysis of this solder technology changeover was ever done? I haven't seen one. And if you think this problem is minor, I have been told that just the cost of changing to lead-free solder stands right now at $280 BILLION and climbing. That cost is borne by all of us. Maybe dumping lead solder was absolutely the right thing to do. Maybe it was absolutely the wrong thing to do. The truth is we haven't the slightest idea the answer to that question and anyone who claims to know is wrong. We didn't know what would happen when we started this and we don't know what we'll get out of it, either, or whether it will be worth the cost. All we know for sure is that a bumpy ride lies ahead.

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