Large scale three-dimensional finite element analyses of 68 and 44 pin types Quad Flat I-Leaded packages subjected to −55°C to 150°C temperature cycling are performed to predict the way in which inelastic strains and microstructure evolve in the solder joints. A new set of unified constitutive equations, which correctly accounts for the inelastic strain rate dependency on stress, temperature, and microstructure (internal stresses) and the evolution of the latter with deformation, was used to model the solder joints’ thermo-mechanical behavior. Estimates of the critical inelastic strain ranges in the solder joints were obtained from the calculated inelastic deformation histories. The range of inelastic strain associated with each thermal cycle is small and practically unaffected by the observed ratcheting of the cycles toward positive strains due to the non-zero mean cycle stresses. By combining the numerical results with fatigue life data obtained from leads subjected to the same thermal loading, a fatigue life relation for mean failure probability of the IC packages’ solder joints has been proposed. The analysis correctly predicts the critical locations for crack initiation.

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