If tin pest were a living thing it might complain, “I can’t get no respect.” Reason: Tin whiskers get so much attention, while tin pest is forgotten.
Although my feeling is that tin whiskers are a greater concern, the number of recorded fails related to tin whiskers is less than 100. Compare this to the number of hard drive fails — about 100 million! With that in mind, let’s learn a little about tin pest.
Tin is a metal that is allotropic, meaning that it has different crystal structures under varying conditions of temperature and pressure. Tin has two allotropic forms. “Normal” or white beta tin has a stable tetragonal crystal structure with a density of 7.31g/cm3. Upon cooling below about 13.2ºC, beta tin turns extremely slowly into alpha tin. “Gray” or alpha tin has a cubic structure and a density of only 5.77g/cm3. Alpha tin is also a semiconductor, not a metal. The expansion of tin from white to gray causes most tin objects, afflicted with tin pest, to crumble.
The macro conversion of white to gray tin takes on the order of 18 months. The photo — likely the most famous modern photograph of tin pest — shows the phenomenon quite clearly.
This photo is titled “The Formation of Beta-Tin into Alpha-Tin in Sn-0.5Cu at T <10ºC" and is referenced from a paper by Y. Karlya, C. Gagg and W.J. Plumbridge, "Tin Pest in Lead-Free Solders," in Soldering and Surface Mount Technology, vol. 13 no. 1. 2000, 39-40.
The tin pest phenomenon has been known for centuries and there are many interesting, probably apocryphal, stories about tin pest. Perhaps the most famous is of the tin buttons on Napoleon’s soldiers’ coats disintegrating on their retreat from Moscow. Since tin pest looks like the tin has become diseased, many in the middle-ages attributed it to Satan as many tin organ pipes in Northern European churches fell victim to the effect.
Initially, tin pest was called “tin disease” or “tin plague.” I believe that the name “tin pest” came from the German translation for the word “plague” (i.e., in German plague is “pest”).
To most people with a little knowledge of materials, the conversion of beta to alpha tin at colder temperatures seems counterintuitive. Usually materials shrink at colder temperatures, not expand. Although it appears that the mechanism is not completely understood, it is likely due to gray alpha tin having lower entropy than white beta tin. With the removal of heat at the lower temperatures a lower entropy state would likely be more stable.
Because the conversion to gray tin requires expansion, the tin pest will usually nucleate at an edge, corner or surface. The nucleation can take scores of months, but once it starts, the conversion can be rapid, causing structural failure within months. The effect is also cumulative, so warming the sample will stop the growth, but it will continue once the sample is cold again.
Although tin pest can form at <13.2ºC, most researchers believe that the kinetics are very sluggish at this temperature. There seems to be general agreement in the literature that the maximum rate of tin pest formation occurs at -30º to -40ºC.
What is the real risk of tin pest in Pb-free electronics? Not great. Modern researchers have had trouble reproducing it, even in the lab. The reason for this is likely that test samples contain small amounts of metal "contaminates" (<0.1%), such as bismuth, antimony, lead and a few other metals. These trace metals solid solution strengthen the solder and inhibit the expansion needed to form tin pest. Unfortunately, copper and silver (the typical Pb-free metals added to tin), do not appear tin inhibit tin pest growth.