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For pt. 1 see ibid., p.681-94, (2007). In Part 1, a novel two-dimensional model was presented for multi-axial thermal stresses, elastic strains, creep strains, and creep energy density at the interfaces of solder joints in leadless chip resistor (LCR) assemblies. In this paper, the model is used to characterize the creep performance of SnPb and SAC lead-free solder joints in LCRs. For both the SAC lead-free and eutectic tin-lead solder joints the predicted cyclic stresses and strains exhibit ratcheting behavior, in good agreement with finite-element predictions. The model is also employed to assess the role of ramp rate and temperature variations in accelerated thermal cycling (ATC) tests. The predictions of the present model correlate well with the experimentally measured number of cycles-to-failure using the Coffin-Manson strain-based model for the SnPb solder and energy-based life prediction model for the SAC solder joints.
Date of Publication: Nov. 2007