Characterization of tin pest by electrical resistance measurement

In this paper the allotropic transition of β to α-Sn (so called tin pest) was characterized by electrical resistance measurements. The aim was to compare the ability and sensitivity of the usually applied optical observations by the electrical measurements. Samples were prepared from 99Sn1Cu alloy. The samples were inoculated by InSb and were stored at -18°C for 40 days. The transition was checked by optical microscopy on the surface of the samples and electrical resistance of the samples was measured by a microhmmeter. Cross-sections were also prepared to compare the resistance change with the amount of transformed tin in the samples. It was found that the electrical resistance measurement is a much more sensitive characterization method of tin pest growth than any kind of optical observation. It was also observed that in the case of InSb inoculator the nature of the process differs from the results of α-Sn inoculator.

I. Introduction

Tin (Sn) is one of the most important materials in the electronics technology, this is the base metal in most of the solder alloys and used as surface finishing of the solder pads and components leads. Tin has four allotropes: and tin. From these -Sn (white tin) is the metallic form and it is applied in the electronics technology. -Sn is stable between 13.2 and 231.9°C, it has bct (body-centered tetragonal) structure crystallizes in the space-group symmetry I41/amd (No. 141) with lattice parameters Å and Å. The -Sn (gray tin) is a nonmetallic form, which is stable below 13.2°C, it has diamond structure with cubic symmetry Fd3m (No. 227), lattice parameter (Fig. 1). is brittle and has semiconductor properties. The other two allotropes, and , exist only at very harsh circumstances, over 161°C and high pressure [1].