The isothermal mechanical durability properties of two lead-free solder alloys, Sn3.5Ag and Sn3.9Ag0.6Cu, are presented and compared to that of the baseline eutectic Sn37Pb solder. Cyclic mechanical tests are performed at room temperature at various strain-rates and load levels, using a thermo-mechanical-microstructural (TMM) test system developed by the authors. The data is analyzed using standard power-law durability models based on work and inelastic strain range. The Sn3.9Ag0.6Cu lead-free alloy is found to be most durable, followed by the Sn3.5Ag solder and finally the baseline Sn37Pb eutectic alloy, under the test conditions investigated. However, tests at high load levels show a greater difference in durability than tests at low load levels. This trend is the opposite of that reported in the literature for thermal cycling durability. A hypothesis is put forward to explain the observed differences between mechanical cycling and thermal cycling, based on the energy-partitioning damage model.

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