Top: Colorized scanning electron micrograph (SEM) shows a "whisker" on the surface of a tin-copper alloy. The image was taken 176 days after the alloy was electroplated onto a tiny cantilever beam.
Bottom: Micrograph of a "hillock" on an electrodeposited surface of pure tin (10 times the magnification of top image).
Image credit:NIST/Boettinger et al., Acta Materialia,5033-5050
Environmental groups around the world have been campaigning for years to replace lead-containing solders and protective layers on electronic components with non-hazardous metals and alloys. In response, 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. U.S. 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 millimeters 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, so 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).
Michael E. Newman | EurekAlert!
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