A study of fatigue and creep behaviour of four high temperature solders

Creep and cyclic deformation behavior of two lead-free high temperature solder alloys, 95Sn-5Ag and 99Sn-1.0Cu, a high lead alloy 97.5Pb-1.5Ag-1.0Sn, and an Ag-modified eutectic alloy 62.5Sn-36.1Pb-1.4Ag, were studied. Room temperature and high (100 degrees C and 150 degrees C) temperature fatigue tests (with cyclic strain amplitude up to 6.0%) for the four solders were conducted, with the fatigue lives ranging from a few cycles to more than 100,000 cycles. It is shown that among the alloys studied, 62.5Sn-36.1Pb-1.4Ag (the modified Sn-Pb eutectic alloy) has the lowest fatigue resistance in term of low cycle fatigue life (strain controlled). The high lead alloy, 97.5Pb-1.5Ag-1.0Sn, has the highest strain fatigue resistance in the large strain region (Delta epsilon > 2.0%). Temperature has a significant effect on alloys 95Sn-5Ag and 99Sn-1.0Cu, but has a negligible effect on the Ag modified Sn-Pb eutectic alloy 62.5Sn-36.1Pb-1.4Ag and 97.5Pb-1.5Ag-1.0Sn. Creep studies show that these alloys generally have a very significant primary creep regime (up to 20%); thus, any realistic constitutive relation has to take such a primary creep phase into consideration. Cyclic deformation of alloy 95Sn-5Ag was simulated by using a constitutive relation built upon a 2-cell model, which covers both primary and secondary creep. This model provides a good estimate of the peak stresses (the minimum stress and the maximum stress in each cycle); it agrees with experimental results when the applied cyclic strain is small and/or the applied strain rate is very low.