Tin Whiskers growing between pure tin-plated hook terminals of an electromagnetic relay. Image courtesy of nepp.nasa.gov
Shortly after I began my current position, the Pb-free solder initiative titled RoHS was enacted. This new legislation restricted the use of leaded solders and a wave of new alloys, all tin-based and Pb-free, were their replacement. These new solder alloys were scrutinized and discussed in every electronics assembly meeting and forum, and one major point of interest were Tin Whiskers, believed by some to be the result of Pb-Free solders.
Fortunately, years of testing have disproved this theory. Tin Whiskers are formed out of pure tin under high stress, such as applications which experience sub-zero temperatures. My understanding is that the tin whiskers problem should be nearly abated since the tin plating companies now dope their tin plating with enough lead to prevent whisker formation.
Pb-free solders are almost always alloys of tin, silver, copper and/or antimony; never pure tin. During the initial Pb-free transisiton, the number of cited tin whisker cases grew because the Pb-free solder alloys stress a tin plating more than leaded solder, so whiskers were more likely to occur. Now that the tin platers are using doped tin, the issue should be mitigated. Other alternatives would be to use another solderable surface finish such as copper, nickel, or gold.
Based on these details, I feel the need to defend Pb-free solder alloys. I do not believe that they are the mass murderers they are accused of. It is possible that tin whiskers are the cause of the crash on the Red Line between the Takoma and Fort Totten stations, or even the airbus crash, however, if that is true, I doubt that Pb-free solder alone caused these whiskers.