Aimtec’s App note on addressing product failures caused by Tin whiskers. Link here
Tin whiskers are slim metallic filaments that emanate from the surface of tin plating. As they are conductive, they can cause shorts across adjacent conductors which could result in failure.
Whiskers occurs due to the presence of compressive stress or so called “stress gradient”. This compressive stress drives the preferential diffusion of tin atoms. For whiskers to form and grow, there must be more things present, but in the absence of such stress, they will not occur.
The conditions of the surrounding environment (ambient temperature, thermal cycling, elevated humidity, and bending) simply initiates or accelerates the stresses present within the plating.