[W1SMH] Tin Whiskers

[Clayton Coleman]

Some of you may have heard of this phenomenon but for those who have
not, here's an article worth reading.  It keeps resurfacing in a lot
of Internet groups of which I'm a member.  With a lot of "going green"
in manufacturing, the use of lead-free solder is already showing signs
of trouble.

73,
Clayton
W5PFG
Ex-KB5TBB

http://www.pbs.org/cringely/pulpit/2008/pulpit_20080221_004346.html

Leadfoot: Sometimes going green hurts more than it helps.
By Robert X. Cringely
bob at cringely.com

If you have ever seen my show Triumph of the Nerds well then you've
also seen my car, a 1966 Ford Thunderbird convertible very similar to
the car used in the movie Thelma & Louise (I play the role of Thelma).
It is in almost every way a fabulous car. It's going up in value, for
one thing. It looks cool. It goes like hell with its 428 cubic-inch
V-8 engine. It is heavier than anything else on the road, so in a
collision with anything less than a dump truck I win. And thanks to
analog technology that handily predates the Clean Air Act, it somehow
manages to do all this while getting 22 miles per gallon on the
highway, 16 in town. Everything is good about my T-Bird, in fact,
except for its wires. My car has bad wires. And shortly YOUR car will
have bad wires, too, as will everything else you own that has soldered
electrical connections. Everything. Prepare to share my pain.

My T-Bird was built at the absolute apex of 20th century
electromechanical automotive technology. A convertible, it has a fully
automatic electric top that relies on eight electrical relays firing
in sequence to put the top up or down. Here's the typical (and
inevitable) failure mode. I'm at the beach with the top down. It
starts to rain. Quickly I run to the car, start it, and hit the button
to raise the top. First the suicide trunk lid opens until it is fully
vertical. Next the electric tonneau cover opens until it is vertical,
too. Then the canvas top begins to retract, raising until it, too, is
vertical, at which point everything grinds to a halt and I drive my
car home in the pouring rain at five miles per hour, traffic honking
behind me, and all three broken parts sticking eight feet into the
sky.

Victim to a succession of good and bad mechanics, some
well-intentioned but all in it for the money, I have spent thousands
of dollars over the years replacing electrical parts -- window motors,
switches, relays for both the top and the sequential tail lights --
all to little avail. New parts failed as quickly as old parts.
Eventually I abandoned all hope of viewing my car as a restoration and
replaced the relays with brilliant little computers from British
Columbia. Now the hydraulics worked beautifully but all it really
meant was that the top no longer failed in mid-sequence: it would
either work fine or not at all.

When you've replaced everything else the problem has to be with what's
left, which in the case of my car was the wires, themselves. Over the
years the wires had somehow corroded inside their insulation and the
terminals had lost their mojo. I had been replacing perfectly good
switches and motors (and knowledgeable folks had been selling me
switches and motors) that would have been helped more by simply
replacing the terminals or, better still, the wires. Some experts
think Ford just got a bad batch of wire back in 1966 -- that this
problem is isolated -- but I don't care. So what if my car is two
years older than my wife? All her parts seem to be working just fine,
why shouldn't my T-Bird?

Which brings me to you, or rather to 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.

Fortunately I have new shock absorbers (and a new wiring harness) on my T-Bird.