Characterization of Tin Pest Phenomenon in a Low Ag Content SAC Solder Alloy

In the electronics technology, the metallic p-Sn (white tin) is the basic material of solder alloys and surface finishes. The “tin pest” phenomenon is the spontaneous allotropic transition of fi-Sn to the semiconductor a-Sn (gray tin) under the temperature of 13.2°C. In this work, the tin pest susceptibility of the widely used Sn99Ag0.3Cu0.7 solder alloy was investigated and compared to Sn99Cu1 alloy (as a well know reference). Bulk solder bars were prepared by metal casting and the samples were inoculated by InSb, CdTe and a-Sn powders to enhance the allotropic transition process. The inoculator materials were pressed onto the surfaces of the samples by a mechanic lamination. The samples were stored at -10°C temperature for 8 weeks. The allotropic transition was monitored by optical inspection and by electrical resistance measurements. The microstructural changes of the samples - caused by the transition of crystal structure of Sn - were investigated by metallurgical cross-sections. The results showed, that in all cases the Sn99Ag0.3Cu0.7 solder alloy is much less susceptible to tin pest development than the Sn991Cu, which might be explained by the suppression effect of the Ag alloying. Furthermore, it was found that the process of transition highly depends on the applied inoculator material.