Low cycle fatigue testing and simulation of Sn-8Zn-3Bi and Sn-37Pb solder joints

Purpose - Sn-Zn based lead free solders with a melting temperature around 199 degrees C are an attractive alternative to the conventional Sn-Pb solder and the addition of bismuth improves its wetability. Whilst lead-free soldering with Sn-8Zn-3Bi has already been used in the electronics assembly industry, it is necessary to study its low cycle fatigue properties since such data have not been reported up to now. Design/methodology/approach - In this study, displacement-control led low cycle fatigue testing of Sn-8Zn-3Bi and Sn-37Pb solder joints was done on lap shear samples. The test amplitude was varied whilst the frequency was kept constant at 0.2 Hz and failure was defined as a 50 per cent load reduction. Finite element (FE) modelling was used for analysis and the results were compared to the experimental data. Findings - The microstructure of the Sn-8Zn-3Bi solder showed a mixed phase of small cellular-shaped and coarser needle-shaped areas. Au-Zn intermetallic compounds were observed near the interface from the SEM-EDS observation. The average lifetime for the Sn-8Zn-3Bi solder joints was 17 per cent longer compared to the Sn-37Pb solder joints. The cross section observation indicated that the fatigue cracks propagated along the interface between the solder bulk and the Au/Ni layer. The locations of maximum equivalent stress from the FE simulation were found to be at the two opposite corners of the solder joints, coinciding with the experimental observations of crack initiation. Originality/value - This is believed to be the first time, the low cycle fatigue properties of Sn-8Zn-3Bi solder have been reported.