A failure mode is reemerging that has been responsible for the loss of billions of dollars worth of satellites, missiles and other equipment — the culprit is the electrically conductive entities known as ‘tin whiskers’. Now one research group says that tin whiskers may be responsible for the sudden acceleration in Toyota Camry models from the year 2002 and possibly beyond.
Earlier this year we reported that the US Department of Transportation (DOT) said that Toyota’s problem was not in electronics.
Now, University of Maryland’s Center for Advanced Life Cycle Engineering or CALCE researchers have found the potential for tin whiskers in the electronic control module or ECM. Circuits Assembly broke the story, quoting the CALCE report as follows:
“The ECM contains surface mount electronic devices connected with tin-lead solder to a multilayer PCB. … Interconnect terminals of the perimeter leaded devices were found to be plated with tin. In addition, tin plating was found on terminal pins of the edge connections. As previously discussed, tin-finished leads can grow tin whiskers which can lead to unintended electrical shorts.”
“We know whiskers can form on tin finished terminals,” said Michael Osterman, senior research scientist and director of the CALCE Electronic Products and System Consortium. said. “In this case, Toyota has tin plating in a rather sensitive area, where the system relies on changes in resistance to provide a signal for acceleration.”
The studied pedals furthermore have been shown to cause shorts known to spur sudden unintended acceleration.
The odds of tin whiskers: 140/million. Someone known to this blogger recently drove a 2010 Camry and noticed subtle but perceptible decelerations that were not led by the driver. Was it tin whiskering? Hard to say, even CALCE’s study figures that the whiskers would only form in 140 cars per million, which is statistically very significant but als makes it statistically unlikely that my friend’s only Camry experience would be on the wrong side of those odds.
It’s also worth noting that the whisker syndrome is probably not limited to Toyotas. Nonetheless, the spotlight has fallen where it has fallen, and tin whiskers pose a serious problem in that warrants attention.
Tin whiskers. Tin whiskers develop — or may develop — on any product type that uses lead-free pure tin coatings. Thus, in greener, lead-free products, tin whiskers can pose a major safety, reliability and potential liability threats to all makers and users of high reliability electronics and associated hardware. The CALCE brain trust concluded that existing approaches are not sufficient to control tin whiskering in high-reliability systems such as automobile electrical systems.
US Secretary of Transportation said Toyota is “all clear” in February. The official blog of the US Secretary of Transportation on February 8, 2011 stated:
NASA engineers pored over more than 280,000 lines of software code looking for potential flaws that could initiate an unintended acceleration incident. Alongside NHTSA, they bombarded vehicles with electromagnetic radiation to see whether it could make electronics systems cause the cars they control to gain speed.
And today, their verdict is in. There is no electronic cause behind dangerous unintended acceleration incidents in Toyotas.
To read more about it: http://supply-chain-data-mgmt.blogspot.com/2011/02/us-dot-says-toyota-problem-was-not-in.html
We will continue to follow this story.
Are those 140/million odds of finding evidence of tin whiskers on any of the potentiometer accelerator pedal assembly-equipped Toyota vehicles; or 140/million odds of finding a tin whisker actually ‘shorting’ the particular (critical) signal terminals in those assemblies….? Reports from the inspection of a very limited number of potentiometer-equipped pedals indicated many displayed evidence of tin whisker growth. With one old pedal assembly actually still exhibiting a tin whisker shorting the crucial terminals (and with a resistance value falling within the critical range as well).
Great question. Studies determined that the potential for a tin whisker
shorting failure was 140/1 million. “Tin finishes can produce metal whiskers that
are conductive and capable of creating unintended current
leakage paths,” says the documentation. Using a CALCE-developed whisker data modeler that could “assess the failure
risk posed by observed tin whisker formation on the conductor
pairs, it was determined that the potential for a tin whisker
shorting failure was 140/1 million.”
Here’s a reference:
http://www.autosafety.org/sites/default/files/imce_staff_uploads/Tin_whisker.pdf
Cheers
So there is a possible cause after all. Guess Murphy’s law (anything that can go wrong will go wrong). Probably being d.c. circuits would rule out impedance variations, but how about inductive or inductive coupling?
Thanks, Mr. Kawar for following up with that reference. And Karl, excellent question – will look into it, please make a note here if you find anything or hear more.