Tin Pest and Tin Oxidation on Tin-Rich Lead-Free Alloys Investigated by Electron Microscopy Methods

Abstract

Soldering of joints in electronic circuits is performed in all branches of electronic industry. At temperatures below 13.2°C, an allotropic transformation of white β-tin into gray α-tin called tin pest may occur, leading to the degradation of mechanical properties or even a total disintegration of the alloy. Presence of some chemical elements in the alloy can inhibit the transformation, while other can promote it, e.g. a significant Pb addition prevents the transformation. However, the Restriction of Hazardous Substances Directive adopted by the European Union since 2006, limits Pb amount to 0.1 wt.%, leading to tin pest phenomenon in tin rich materials. The energy dispersive X-ray spectroscopy (EDXS) in scanning electron microscope was performed on samples of tin-rich lead-free alloys subjected to accelerated low temperature stress and after months of storage at -18 °C showed the tin pest occurrence. Much higher oxygen content in EDXS spectrum was revealed at old regions of transformed α-tin than at new α-tin and non-transformed β-tin regions. The tin pest oxidation is much accelerated in comparison to β-tin and can finally lead to the consuming of the whole tin in deteriorated places by the tin oxide.