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Creep and cyclic deformation behavior of two lead-free high temperature solder alloys, 95Sn-5Ag and 99Sn-l.OCu, a high lead alloy 97.5Pb-1.5Ag-l.OSn. and an Ag-modified eutectic alloy 62.5Sn-36.1Pb-I.4Ag, were studied. Room temperature and high (100°C and 150°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, OOO 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-l.5Ael.OSn, has the highest strain fatigue resistance in the large strain region (A& > 2.0%). Temperature has a significant effect on alloys 95Sn-5Ag and 99Sn-l.OCu, hut has a negligible effect on the Ag modified Sn-Pb eutectic alloy 62.5Sn-36.1Pte1.4Ag and 97.5Pb1.5Ag-l.OSn. 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.

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Creep of two lead-free high temperature solder alloys, 95Sn-5Ag and 99Sn-1.0Cu was studied in this investigation. Room and high temperature creep tests were performed to examine deformation mechanisms and to establish mathematical models of creep deformation for the alloys. A state variable creep model was introduced to model both primary and secondary creep deformation of these two alloys which show a very significant primary creep. Fatigue life models of the alloys were established based on an energy-based failure criteria, which was deduced from variable strain amplitude tests at a constant strain rate of 0.003/sec, and from variable strain rates tests with two constant strain amplitudes of 0.005 and 0.01. Applications of the creep model includes determination of peak loads and hysteresis strain energy density in strain-controlled fatigue tests. It is demonstrated that the creep model can be very helpful to establish fatigue failure criteria and to assess fatigue lives of these two alloys.

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