August 23, 2011
A new technical paper from the research scientists at the University of Maryland’s Center for Advanced Life Cycle Engineering (CALCE) buttresses the findings of the National Highway Traffic Safety Administration and NASA’s Engineering Safety Center investigation into Toyota unintended acceleration: Toyota vehicles with potentiometer type accelerator pedal position sensors have a propensity to grow tin whiskers that can and do cause shorts in a highly sensitive engine management area.
Researchers Bhanu Sood, Michael Osterman and Michael Pecht studied a pedal assemblies performed a physical analysis of an engine control system from a 2005 Camry XLE, V-6 and an accelerator pedal assembly from a defunct 2002 Camry. The 2005 engine control system included the ECM, an accelerator pedal unit, throttle body, electrical connectors and electrical connecting cables.
This tear-down of the accelerator pedal position sensors (APPS) in both Camrys revealed tin whisker formations. Tin whiskers are crystalline structures emanating from tin solder that can produce electrical shorts and current leakage, and have been associated with numerous electronic failures. The trio of researchers did not have access to the vehicles’ history, so it was not known if the presence of tin whiskers was associated with any malfunctions during the life of either car.
“We were looking at the overall manufacturing of assembly circuit and looking for what level of construction had the potential for defects throughout the entire engine control system,” says Osterman, Senior Research Scientist and the director of the CALCE Electronic Products and System Consortium.
CALCE’s analysis, published in the current issue of the international journal Circuit World, lends support to the work of NASA scientists who found tin whiskers growing in the accelerator pedal unit of every potentiometer they examined. The February report, Technical Support to the National Highway Traffic Safety Administration on the Reported Toyota Motor Corporation Unintended Acceleration Investigation, was unclear on the subject of the total sample; NASA found tin whiskers growing in the APPSs of either three or four Camrys. One was associated with a vehicle in which the consumer reported that her pedal was ‘jumpy” and that the vehicle was “completely undriveable.” However, based on an analysis of warranty data which was performed by Toyota’s defense expert, Exponent, NHTSA concluded that the presence of tin whiskers did not represent a safety hazard. (see How NHTSA and NASA Gamed the Toyota Data)
On this point, the CALCE scientists sharply diverged. CALCE researchers have been examining the tin whisker phenomenon since 2002, looking at mitigation strategies, growth patterns and tin whisker failures. In addition, they have published widely on the subject of intermittent failures in automotive electrical environments and the difficulties manufacturers face in isolating their root causes. On this study, researchers found as many as six tin whiskers growing on one APPS. Unlike NESC, which used warranty data (secret, time-limited, and otherwise unreliable) as the basis for determining the prevalence of tin whiskers in the fleet and its effect on safety, CALCE used its algorithm and came up with the opposite conclusion:
“In our analysis, a significant number of tin whiskers were found. Using the CALCE Whisker Risk Calculator (CALCE) Tin Whisker Risk Calculator, 2005) to 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. Considering the number of vehicles on the road, it is expected that this would present a significant safety hazard.”
In addition to tin whisker formation in the APPS, the CALCE researchers found the potential for tin whisker formation in the ECM:
“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,” Osterman 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.”
In their discussion about the printed circuit board manufacturing processes of Toyota Camrys, CALCE scientists questioned the lack of a safety standard regarding automotive electronics, given broad range of whisker-induced failures. They were openly critical of NHTSA’s lack of action.
“It is highly likely that tin whiskers could induce a failure that is later undetected. For this reason, best practices for electronics design stipulate that tin not be used as a plating material. It is very questionable why the National Highway Traffic Safety Administration, with a stated mission to ‘save lives, prevent injuries and reduce economic costs due to road traffic crashes, through education, research, safety standards, and enforcement activity,’ has not come out with a requirement that no electronics use pure tin as a material component, since the potential for tin whiskers presents an unreasonable and unnecessary risk.”