A damage integral approach to thermal fatigue of solder joints

Thermal cycling of electronic assemblies produces repeated mechanical loading of connecting solder joints through differential thermal expansion among assembly members. Such cyclic loading can lead to the fatigue failure of these solder joints. The instantaneous fatigue damage rate during this is determined by the solder stress, temperature, and environment. The applicability of a damage integral approach that provides a numerical accounting of accumulated fatigue damage is examined. The stress is calculated using state-variable constitutive relations for inelastic deformation. The fatigue damage rate is described by a phenomenological crack-growth law expressed in terms of a nominal stress-intensity factor, determined from isothermal fatigue data. Damage integral calculations are performed for solder joints fatigued to failure with either isothermal or thermomechanical cycles. The results indicate that the phenomenology of fatigue damage may be equivalent for these two loading modes