I am still surprised that so few people are concerned about tin pest in lead-free assembly. Below is an excerpt from a paper I wrote on the topic at SMTAI in September of 2004. The entire article is attached as a PDF.
WHAT IS 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 13.2 C, beta tin turns extremely slowly into alpha tin. "Grey" 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 grey causes most tin objects to crumble.
The macro conversion of white to grey tin takes on the order of 18 months . The photo, which is 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 <10 C" 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, 13/1  39-40.
This 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 while 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 counter intuitive. Usually materials shrink at colder temperatures, not expand. Although it appears that the mechanism is not completely understood, it is likely due to grey 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.
Since the conversion to grey tin requires expansion, the tin pest will usually nucleate at an edge, corner, or surface. The nucleation can take 10s of months, but once it starts, the conversion can be rapid, causing structural failure within months.
Although tin pest can form at <13.2C, 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 -30 C to -40 C.