Design Article
Toyota accelerations revisited—hanging by a (tin) whisker
Rick DeMeis
1/10/2012 12:08 AM EST
A NASA paper delivered at the International Tin Whisker Symposium last year reported on tin whisker growth in Toyota accelerator pedal position sensors that, depending on pedal rate of movement, could lead to unintended acceleration.
The whiskers were found in a "failed" sensor from a 2003 Camry and at least two other similar units that did not malfunction. The sensor in question was used on the Camry from 2002 to 2006 model years, and the specific Camry with the defective sensor (which was furnished to NASA) had 82,000 miles on it. The vehicle owner reported the car as "undrivable," with throttle response as: "I couldn't get any 'gas', and then the car would jerk forward at a rapid rate." The sensor had at least 17 whiskers in it, only one of which shorted contacts.
While the inspected sensors noted above could be typical of the entire lot, these incidents of whisker growth and shorting give cause for safety concern across the industry.
The NASA paper gives further insight by detailing tin whisker physical behavior as well as outlining guidelines and methods of detecting whiskers. But rather than summarize the report here, readers can access the easily read, but complete, 30 page NASA presentation by clicking here.
The whiskers were found in a "failed" sensor from a 2003 Camry and at least two other similar units that did not malfunction. The sensor in question was used on the Camry from 2002 to 2006 model years, and the specific Camry with the defective sensor (which was furnished to NASA) had 82,000 miles on it. The vehicle owner reported the car as "undrivable," with throttle response as: "I couldn't get any 'gas', and then the car would jerk forward at a rapid rate." The sensor had at least 17 whiskers in it, only one of which shorted contacts.
While the inspected sensors noted above could be typical of the entire lot, these incidents of whisker growth and shorting give cause for safety concern across the industry.
The NASA paper gives further insight by detailing tin whisker physical behavior as well as outlining guidelines and methods of detecting whiskers. But rather than summarize the report here, readers can access the easily read, but complete, 30 page NASA presentation by clicking here.
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eembedded_janitor
1/10/2012 4:56 PM EST
Faults like tin whiskers are exactly the reason why the pedal sensor has two pots. The values of the two pots are compared against an expected profile and if there is any discrepancy, the sensor is deemed to be faulty and power is limited to "limp home" mode.
By the way, more modern sensors use hall effect sensing which is not prone to mechanical damage like pot wipers.
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MN
1/11/2012 5:15 AM EST
It's becoming increasingly obvious that the RoHS push for lead-free solder and platings was extremely poorly thought out.
As well as the tin whisker problem, lead-free solder requires higher temperature soldering (putting more stress on components), and solder joints are much more likely to suffer mechanical fatigue.
Just another example of environmentalists creating huge problems in the "real world".
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feepingcreatures
1/12/2012 2:25 AM EST
Reduction of lead waste in the environment is the right thing to do. The problem was the way it was done. In hindsight, perhaps it would have been better to enforce rigorous re-collection of discarded PCBs for solder recycling (as is done with lead-acid batteries). It's not the environmentalists' fault if polluting industries fail to develop (or propose) better solutions for cleaning up their pollution.
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kalpak
1/12/2012 4:36 AM EST
In case of Lead Acid batteries, then Lead is always in nice big chunks, so recycling is easy.
On a PCB it is in form of an alloy melted onto other metals, with a conformal coating on the top.
This makes recycling far more challenging.
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feepingcreatures
1/14/2012 4:08 AM EST
yes, sure, but that doesn't mean we shouldn't do it, just because it's too difficult. Manufacturers can't have it both ways: lead is a persistent, dangerous pollutant and MUST be kept out of the environment. We can either recycle it, or stop using it. One or the other.
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Etmax
1/11/2012 6:39 AM EST
Please don't forget that apart from needing almost twice the energy in production (I thought carbon emissions were to be avoided) lead free PCB's generally don't survive rework/repair due to copper migration from the PCB surface to the tin. This results in more scrap/waste. Also I have read that lead doesn't leach out of solder in land fill (one of the excuses for RoHS) and there has been no noticeable increase in lead levels of people performing materials recovery (the other excuse for RoHS) That said, many of the other substances banned under RoHS legislation are a problem and the world is better without them, eg. Chromium VI and Cadmium.
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FRANKOK
1/11/2012 7:32 AM EST
No doubt Prof. Gilbert was correct and tin whiskers could be one of the causes for rare cases of sudden acceleration.
http://www.thetruthaboutcars.com/2010/02/dr-gilbert-explains-his-research-into-toyota-electronics/
excerpt
"What I have done is, I have shown that in the fault detection strategy of the Toyota systems, there is a window of opportunity where [an error] could occur and not be detected."
also see discussions in:
http://townhall-talk.edmunds.com/WebX/.f20003b/2011
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tomkawal
1/13/2012 8:38 AM EST
Lead-free solder enforcement is a very poor thought decision. It should be limited to low reliability short-life toys.
The reasons are: the reliability and the fact that lead going properly to landfill is not a big problem. The harm is in compounds created, when incinerated.
So, with the toys likely to be thrown to furnace, the lead elimination is OK.
But not at all with properly recycled automotive electronics.
cheers
tomkawal
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WKetel
2/15/2012 2:06 PM EST
So just exactly how much lead are these regulations actually keeping out of landfills? The only numbers that I have heard implied that there were about six pounds of lead in each PC that was scrapped. That sort of damaged the credibility of the claimers, at least as far as my belief in their veracity. I would ask the same questions about lead leaching out of landfills from electronics. My guess is that we see a lot more lead from demolished building materials that contained lead paint, which was in a more soluble form to start with. Besides all of that, putting a deposit on electronics the same as on deposit bottles, would have been a far less damaging way to keep them out of landfills. A $50 deposit on every cell phone and Ipod and similar devices would be much better for keeping that half gram of lead from reaching anyplace except a recycler. Am I the only one who has thought up this idea? Yes, of course the logistics might be a bother, but we could keep a higher level of reliability in all our electronic toys.
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jsk2pa
3/14/2012 11:20 AM EDT
Go back to using lead!
For those of you suggesting how dangerous lead is to the environment: Really? How many people should die as a result of electronic failures due to the lack of lead? How many have already died? The original reason given to pull lead from electronics was the suggestion that lead leaches out of electronics in landfills and gets into the ground water. More recent testing indicates that was wrong (even in areas with the worst acid rain on the planet). Therefore, as long as you don't eat it, lead is not nearly the problem in electronic assemblies that most folks were led to believe.
Go back to using lead and require all electronics to be recycled - oh yeah, that's right, even with lead free electronics; we are still required to recycle.
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J3rrYcid
7/12/2012 11:18 PM EDT
It's amazing how something so small, like a tin whisker would cause the car to be so undrivable. If toyota does not confront this issue head on, they could soon see many angry car owners sending their cars back for a refund. They already had some trouble a few years back with this safety issue, and I cannot imagine that they haven't learnt their lesson.
Thomas - http://www.carid.com/
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