Tin Whiskers pose a serious reliability risk to electronic assemblies in RoHS Lead-free design, whose components are having pure Tin or Matte Tin. Several instances have been reported where tin whiskers have caused system failures in both earth and space-based applications. There have also been whisker-induced failures in medical devices, weapon systems, power plants, and consumer products.

Reliability Risks/Failures Associated with Tin Whiskers

The general risks fall into four categories:

1. 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.

2. 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.

3. 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 than 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 until other arc extinguishing processes occur. This kind of arcing is happening in the metal vapor. When an arc quenching agent (e.g., air) is present, more power must be installed into the event to replace power lost to the non-interesting processes happening in the quenching agent. Therefore, as air pressure is reduced, less power is required to initiate and sustain a whisker-induced metal vapor arc.

4. Debris/Contamination. Whiskers or parts of whiskers may break loose and bridge isolated conductors or interfere with optical surfaces.