Tin Pest - A Potential Threat To Technology?

Most of us know a little bit about solder alloys. These are the low melting point materials that bond electrical wires and tabs to one another, allowing current to flow through them. Historically, these alloys have been made of lead and tin, with an occasional additional element added to control melting points, flow characteristics of the liquid solder or other properties.

Recently, things have changed in the world of solder. Since lead has been shown to be extremely damaging to humans and other life, forms, it's been effectively banned from most applications, including solder. The European Union has been the latest to ban lead in solder alloys. So what does this have to do with Tin Pest?

It turns out that pure tin undergoes an allotropic transformation, that is, changes it's crystal structure, when it's cooled below about 56 degrees Fahrenheit. This transformation is not at all rapid, and generally is retarded by the presence of lead or any of a number of other elements. However, if pure or nearly pure tin is used in solder, the transformation can occur at temperatures and time frames that might matter to us in the future. The damage occurs when the tin, in transforming, dramatically expands it's volume (by about 27%). This results in what was once a smooth solid changing into a gray material that erupts on its surfaces, and thereby weakens the solder joints to the point where they could be unusable.

An interesting article by Ronald C Lasky (available online) discusses some of the legends about, and factual episodes of tin pest, and the damage that it can do.

The danger occurs when pure tin is exposed at quite cold temperatures, below about -20 degrees F. These conditions exist in various polar and sub-arctic parts of the globe, and also in outer space. While its unlikely that tin pest will be a problem that most of us will encounter in commercial devices such as computers and cell phones, there exists the potential for this "disease" to occur in equipment exposed to the low temperature extremes noted above, and could prove damaging to that equipment, including military and space electronics. A growing awareness of the problem by industry will likely lead to the development of new solders with alloying elements other than lead. However, its always good to be aware of the unintended consequences of our efforts to be good environmental stewards by banning lead from products.