Using nanoindentation to investigate the temperature cycling of Sn–37Pb solders

Using nanoindentation and energy dispersive X-ray spectrometry (EDS), we have conducted an investigation into corner failures to elucidate not only the nanomechanical properties of Sn–37Pb solder balls but also the effects of temperature cycling tests (TCTs). We found that the hardness of Sn–37Pb solder balls was greater in central locations [1.18 ± 0.05 GPa for room-temperature (RT) sample; 1.3 ± 0.05 GPa for TCT sample], but had standard values in corner locations (> 0.2 ± 0.02 GPa). The modulus increased after the TCTs. Nevertheless, the mechanical properties were closely related to the average area of the α-Pb phase. The average area of the Pb-rich region was more stable after the TCTs than that of the RT sample, due to the enhanced mechanical properties of the Sn–37Pb solder, suggesting good reliability. From an analysis of average areas in the RT sample, it appears that the Pb-rich solid solution that formed led to weak Sn–Pb bonds near the corner locations. Electron back-scattered diffraction measurements revealed that grains with grain boundaries formed as a result of accelerated TCT cycling. We conclude that Sn–Pb recrystallization was initiated and propagated after the TCTs, followed by propagation to the interfacial region.