Low cycle fatigue of Sn-based lead-free solder joints and the analysis of fatigue life prediction uncertainty

Solder joints of Sn-37Pb, Sn-3.5Ag, Sn-4.0Ag-0.5Cu and Sn-8Zn-3Bi (all compositions in wt%) were tested by means of isothermal low cycle mechanical fatigue at room temperature over a wide range of strain ranges (1%-10%). The eutectic Sn-37wt%Pb was used as a reference. Single lap shear samples with solder joints 0.45mm in height and 1.6mm in diameter were used. The testing was executed in a displacement-controlled mode, at three different amplitudes namely, 30, 40 and 50mum. The frequency used was 0.2Hz. The displacement waveform was triangular, due to its constant strain rate. The Coffin-Manson law was compared to the Morrow's energy model and both the energy density value and the plastic strain range values were calculated in different ways to analyse the uncertainty in the calculation methodology of fatigue life. According to the results, the solder alloy with the best isothermal fatigue properties is the ternary Sn-4.0Ag-0.5Ag. The Sn-3.5Ag-3Bi exhibited quite poor fatigue properties, worse than all the other alloys tested. Regarding the model used for fatigue life prediction, both methods gave consistent results