Trimming whiskers

The European Union (EU) will ban the use of lead (and five other hazardous substances) in all electrical and electronic equipment sold in EU nations starting in July 2006. US manufacturers must comply with this requirement in order to market their products overseas.

However, pure electroplated tin and lead-free tin alloys tend to spontaneously grow metallic whiskers (thin filament-like structures often several millimetres long) during service. These defects can lead to electrical shorts and failures across component leads and connectors.

Whiskers--and more benign raised formations called hillocks--are believed to be a metal's means of relieving stress generated by the electroplating process.

US National Institute of Standards and Technology (NIST) researchers - working with the International Electronics Manufacturing Initiative (iNEMI) - have been trying to identify the origins of such stresses and better understand the resulting mechanisms for whisker and hillock growth.

In a recent paper in Acta Materialia, they reported that the surfaces of tin-copper deposits developed extremely long whiskers while pure tin deposits (the simplest lead-free plating finish) only produced hillocks. By comparison, the soon-to-be-banned tin-lead deposits did not form either type of deformity, a characteristic known since the 1960s.