http://www.guardian.co.uk/technology...ch.engineering
Within a whisker of failure
Removing lead from solder may seem a smart idea environmentally, but the
resulting microscopic growths called tin whiskers could be just as
problematic
* Kurt Jacobsen
* The Guardian,
* Thursday April 3 2008
* Article history
This article appeared in the Guardian on Thursday April 03 2008 on p1 of the
Technology news & features section. It was last updated at 00:05 on April 03
2008.
Tin whiskers
On April 17 2005, the Millstone nuclear generating plant in Connecticut shut
down when a circuit board monitoring a steam pressure line short-circuited.
In 2006, a huge batch of Swatch watches, made by the eponymous Swiss
company, were recalled at an estimated cost of $1bn (£500m). In both cases,
"tin whiskers" - microscopic growths of the metal from soldering points on a
circuit board - were blamed for causing the problems.
It's not the first time these mysterious growths have been blamed for
electronics failures. In 1998 the Galaxy IV communications satellite
sputtered out after just five years; engineers diagnosed its failure as due
to "whiskers".
The US military blamed them for malfunctioning F-15 radar systems and
misguided Phoenix and Patriot missiles. In 1986, the US Food and Drug
Administration recalled a number of pacemakers because of these same
whiskers. In fact, they've been known about since the 1940s, and happen with
cadmium and zinc, too: during the second world war, similar whiskers would
short the cadmium tuning capacitors in aircraft radios. A decade later,
tin-based relays in AT&T telephone switching centres were found to cause
shorts.
The solution to "whiskering"? Mix lead into the solder, as was done from the
1950s. Colin Hughes, a physicist who worked on the first British nuclear
bomb, told me that the whiskering problem never came up during his career.
But now the lead is gone, by legal mandate, and whiskers are back - causing
potential problems for us all.
Since 2006, lead has been banned from solder in the European Union under the
2003 Reduction of Hazardous Substance (RoHS) directive, which gave
manufacturers three years to phase out lead.
The logic seemed reasonable. Removing lead from petrol (where it was used to
prevent engine mistiming) brought clear environmental and health benefits,
taking a harmful chemical that can affect intelligence out of the
atmosphere. Removing lead from solder, the 37% lead, 63% tin alloy used to
join metal objects in everything from plumbing to circuit boards, was an
obvious next step to prevent it leaching into ground water from dumped items
in landfills.
Meanwhile, the US and Japan have also been moving to lead-free solders. It's
a huge shift; the US Environmental Protection Agency (EPA) estimates that
80m kilograms of lead solder was used worldwide in 2002. Environmental
groups have applauded the move. "In the US we've been surviving without lead
solder for many years," says Rick Hind, legislative director of Greenpeace's
toxics campaign. "With less exposure to lead we will all benefit by being
smarter and making safer and more durable products." (The US has not made
lead-free solder obligatory, but does offer tax benefits for doing so.)
But without lead to tame it, tin behaves oddly on circuit boards. Left
alone, tin plating, like cadmium and zinc, spontaneously generates
microscopic shreds of metal - about one to five microns in diameter, or less
than one-tenth as wide as a human hair - which push up from the base. If
they grow far enough to touch another current-carrying location, they'll
cause a short that can wreck the equipment while leaving barely any trace.
The cause is becoming clearer. "I believe the mechanism of whisker formation
is now understood: it is due to compressive stress - caused by, say,
diffusion of copper into the tin - being built up in the tin layer which
breaks through the tin oxide barrier layer [to the air]," says Steve Jones
of Circatex, in South Shields. Critics cite reports that solder
substitutes - pure tin, tin-zinc, tin-silver-copper - simply cannot match
the lead mixture for reliability, coverage ("wetting" terminals), and cost
(silver is especially pricey). Therefore, the US military, Nasa and medical
and high-level research equipment are exempt from what authorities view as
untrustworthy commercial components.
"I still use lead-tin solder - it works better," says John Ketterson, a
solid state physicist at Northwestern University in Illinois. He notes the
tradeoffs of "cost, materials, strength of the solder and all that" during
this mandated changeover, and that manufacturers "have to get an experience
base" with new processes.
{ snipped as lengthy }
Tin whiskers: coming to a PC near you?
· They can grow at ambient temperature and humidity, or in vacuum
· They can grow in steady or varying temperatures (though the latter may
encourage growth)
· Whiskers' tips are atom-sharp. They will push through any coating, given
time
· They are a prevalent cause, only now being identified, of many past
equipment failures
· One whisker can carry about 30mA - more than enough to cause havoc in
digital circuits
· Silver-tin-copper ("SAC") solder slows but doesn't stop whisker growth
· SAC solder has more environmental impact than the lead-tin version
· Older 37%-63% lead-tin solder mix merely deforms, reducing stress and
hence minimising whiskering
· Whiskers can grow indefinitely
Source: Howard Johnson, Signal Consulting
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