From the early days of electronics, it was found that some metallic components, especially the lead-based (Pb) and tin-based (Sn) soldered connections, tend to slowly grow conductive 'metal whiskers' (MWs) which can cause short-circuits and severely affect the reliability of systems.

Even though lead has been largely removed from solder composition, it's been found that the replacement metals also grow whiskers. In addition, the very extensive miniaturisation of electronic components (and the tiny 'solder pads' they're connected to) means that even extremely short whiskers can cause problems.

The whiskers - which vary in diameter from tens of nanometers to microns in diameter, and hundreds of nanometers to millimeters in length, have now been found to also grow on zinc, cadmium, indium, gold, silver, and many other metals.

They grow from accumulations of metal atoms at the root of the whisker rather than the tip.

There is currently no agreed theory on the physics behind MW growth.

Current theories and test methods DO NOT have predictive power of the time-dependence of Whisker Density, Length or Thickness Distributions. A useful theory should identify what we must control to make confident predictions. Such a theory has remained elusive.“

Source: The Art of Metal Whisker Appreciation: A Practical Guide for Electronics Professionals NASA Goddard Space Flight Center.

It's currently thought that whisker growth is encouraged by compressive mechanical stresses - but the exact mechanism is unknown despite intensive research over many decades.

Further technical reading :Physics and applications of conductive filaments in electronic structures: from metal whiskers to solid state memory PhD thesis from Dipesh Niraula, University of Toledo, 2019.

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