Robert X. Cringley writes:

We’re talking about tin whiskers, single crystals that mysteriously grow from pure tin joints but not generally from tin-lead solder joints. Nobody knows how or why these whiskers grow and nobody knows how to stop them, except through the use of lead solder. Whiskers can start growing in a decade or a year or a day after manufacture. They can grow at up to nine millimeters per year. They grow in any atmosphere including a pure vacuum. They grow in any humidity condition. They just grow. And when they get long enough they either touch another joint, shorting out one or more connections, or they vaporize in a flash, creating a little plasma cloud that can carry for an instant hundreds of amps and literally blow your device to pieces.