“Tin whiskers” is not an imaginative, fanciful term for some aspect of electronics manufacturing.Tin whiskers are real, and they pose a serious problem for electronics of all types. Tin whiskers can could cause short circuits. The switch to alternative solders (away from traditional tin+lead solder) in order to achieve RoHS compliance has created some challenges for the semiconductor industry, especially tin whiskers.
Understanding and mitigating tin whiskers article says that tin whiskers are almost invisible to the human eye and are 10 to 100 times thinner than a human hair. They can bridge fairly large distances between electrical device leads, and in so doing, can short out the conductors. They can grow fairly rapidly; incubation can range from days to years.
Toyota has had problems with uncontrolled acceleration. Earlier report have said that the acceleration problems that led to the recall of nearly eight million Toyota cars and trucks in 2009 and 2010 were mechanical. Yet some believe that the issue goes beyond the floor mats and some new material on that has just surfaced. Toyota accelerations revisited—hanging by a (tin) whisker article tells that a NASA paper delivered at the International Tin Whisker Symposium last year reported on tin whisker growth in Toyota accelerator pedal position sensors that 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 NASA paper gives further insight by detailing tin whisker physical behavior as well as outlining guidelines and methods of detecting whiskers.
Understanding and mitigating tin whiskers paper tells that tin whiskers are real, and they pose a serious problem for electronics of all types. Connectors, passive and active components, switches, and relays now must all be lead-free, and the switch to alternative solders in order to achieve RoHS compliance has created some challenges for the semiconductor industry, especially tin whiskers. Pure, tin-plated electronics have become ubiquitous over just the past five years. When used as a finish material for electronic components, pure tin can spontaneously grow conductive whiskers.
When a whisker grows between two conductors, the whisker usually fuses (disappears), creating a momentary short circuit. In some cases the whisker forms a conductive path, creating false signals at an incorrect location. In very rare cases, rather than disappearing like a fuse link, the whisker can instead form a conductive plasma.
Because the electronics industry does not really know what causes tin whiskers, cannot predict their appearance other than to say whiskers are likely to form on pure tin. Also higher the altitude, the more rapid the whisker formation.
Here are some suggestions for reducing the risk of tin whiskers:
1. Do not use pure tin. Studies have shown that alloying tin with a second metal reduces the propensity for whisker growth. (minimum of 3% lead by weight is acceptable)
2. Do not rely on the order paperwork. Use x-ray fluorescence (XRF) to verify finish on all critical parts.
3. Refinish a pure tin-finished part with a hot-solder dip. Stripping and replating is a recommended approach and is a service offered by some of the original part manufacturers.
4. Use some type of encapsulation or conformal coating. Uralane 5750 conformal can provide some benefit by reducing the growth rate, but tin whiskers can grow through conformal coating.
The shift away from lead solder presents risks for high-reliability applications, particularly in the form of tin whiskers on tin-containing finishes. When a tin-bearing finish is used, conformal coatings have been somewhat effective in fighting this problem. Because of the potentially dangerous and unpredictable risks of pure tin, it is not presently used in medical devices.
NASA has lost satellites due tin whisker. NASA Tin Whisker (and Other Metal Whisker) Homepage has more details on that. Tin Whiskers in Electronics Components white paper from Tyco Electronics is also good reading with lots of pictures.