Tin Pest

What is it?

Tin pest describes the phenomenon where tin changes from a white shiny metal to a grey powdery non-metallic form as temperatures decrease below 13.2°C. The change progresses, with the rate reaching a maximum at about minus 30°C (-22°F). It is also known as tin disease, tin blight or tin plague.

Why does it happen?

Tin has two physical forms, (allotropes) and it undergoes what is known as an allotropic phase transformation at 13.2°C. Below this temperature, the familiar white shiny metallic form converts to a grey powdery non-metallic form. The reaction is autocatalytic i.e. the presence of the grey form accelerates the process.

Investigations into tin pest occurring in Lead-Free solders have found sufficient evidence to confirm that it does indeed occur in some of these alloys.

What problems can it cause?

Because of the relatively high concentrations of tin in Lead-Free solders, there is a concern that tin pest may become a problem. The use of tin rich soldering alloys such a SAC and tin-copper could lead to long term reliability issues for electronics used in cold environments. If tin pest does form in Lead-Free solders exposed to lower temperatures it will lead to both mechanical and electrical failures in the affected joints.

How can it be prevented?

The presence of lead in an alloy at greater than around 5%, is sufficient to suppress the occurrence of tin pest. This is obviously not possible with Lead-Free solders. However, levels of antimony and bismuth as low as 0.5% have been shown to be effective in suppressing tin pest. Therefore a suitable Lead-Free solder alloy containing these elements might be preferable to the standard SAC and tin-copper alloys in applications where sustained low temperature operation is required.

NB: If the melting point of this bismuth alloy is considered too low an alternative would be a tin-antimony alloy such as the 95%Sn/5%Sb alloy which melts at around 235°C.