Response of 60Sn-40Pb Under Thermal and Mechanical Cycling

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An understanding of the internal mechanisms responsible for the enhanced deformation and degradation under combined thermal and mechanical fatigue in solder alloys is sought. Inelastic deformation can accumulate simply during unconstrained thermal cycling, -55°C to 125°C, in solder alloys due to the heterogeneous microstructure, the variation in properties, primarily the coefficient of thermal expansion, among the different phase and crystallographic grains, the low flow strength, and microstructural instability. To separate the effects driven by thermal cycling versus those driven by mechanical cycling, the local deformation and microstructural changes are tracked using independent interrupted thermal cycling and isothermal fatigue experiments. The deformation is measured by tracking surface roughness using a confocal scanning laser microscope. The accumulated deformation in 60Sn-40Pb under a -55°C to 125°C thermal cycle is significant. The accumulated deformation observed in two Sn-Ag base solder alloys as well as polycrystalline Sn under the same thermal cycle is notably smaller.