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What are the Risks/Failure Mechanisms Associated with Tin Whiskers?

What are the Risks/Failure Mechanisms Associated with Tin Whiskers? Tin whiskers pose a serious reliability risk to electronic assemblies. Several instances have been reported where tin whiskers have caused system failures in both earth and space-based applications. To date, there are reports of at least three tin whisker induced short circuits that resulted in complete failure of on-orbit commercial satellites . The general risks fall into four categories: Stable short circuits in low voltage, high impedance circuits. In such circuits there may be insufficient current available to fuse the whisker open and a stable short circuit results. Depending on a variety of factors including the diameter and length of the whisker, it can take more than 50 milliamps (mA) to fuse open a tin whisker. Transient short circuits . At atmospheric pressure, if the available current exceeds the fusing current of the whisker, the circuit may only experience a transient glitch as the whisker fuses open. Metal Vapor Arc If a tin whisker initiates a short in an application environment possessing high levels of current and voltage, then a VERY DESTRUCTIVE phenomenon known as a Metal Vapor Arc can occur. The ambient pressure, temperature and the presence of arc suppressing materials also affect metal vapor arc formation. In a metal vapor arc, the solid metal whisker is vaporized into a plasma of HIGHLY CONDUCTIVE metal ions (more conductive that the solid whisker itself). This plasma can form an ARC capable of carrying HUNDREDS OF AMPERES . Such arcs can be sustained for long duration (several seconds) until interrupted by circuit protection devices (e.g., fuses, circuit breakers) or other arc extinguishing processes occur. In addition to occurring under atmospheric conditions commonly found on earth, past research has also confirmed that tin whisker induced metal vapor arcs can form in reduced atmospheric pressure (e.g., vacuum conditions in space applications). These experiments demonstrated that at atmospheric pressures of about 150 torr, a tin whisker could initiate a sustained metal vapor arc where the supply voltage was approximately 13 Volts (or greater) and supply current was 15 Amps (or greater). Tin (or other materials) from the adjacent surfaces can help to sustain the arc until the available material is consumed or the supply current is interrupted. Metal vapor arcs in vacuum are reported to have occurred on at least three commercial satellites resulting in blown fuses that rendered the spacecraft non-operational. Debris/Contamination. Whiskers or parts of whiskers may break loose and bridge isolated conductors or interfere with optical surfaces NASA Goddard Space Flight Center Tin Whisker (and Other Metal Whisker) Homepage PFmeeting @ 2004-02-27 00:50 引用(Trackback0) 评论 发表评论