A thermo-mechanical cohesive zone model for solder joint lifetime prediction
Résumé
A Cohesive zone model (CZM) is developed for simulating the thermo-mechanical fatigue of solders in electronic chip packages subjected to active power cycling. Repeated on-off switching of the device is the source of thermal strains, resulting in the initiation and the propagation of a fatigue crack in the solder joint. The fatigue failure in the solder material may cause local intensification of the temperature gradient due to partial or total degradation of the thermal conductance in the cracked region. In this paper, the coupling between fatigue cracking and crack heat transfer is accounted for by using a thermo-mechanically coupled cohesive zone element. Thus, the quantity of heat flux transferred across the interface will be dependent on the degradation level of the interface thermal conductance. Typically, the cohesive zone conductance is taken as a function of the separation between the cohesive surfaces, meaning that the thermal conductance can be restored across the interface during crack closure. It may alternately be chosen as being dependent on the damage's level of the cohesive zone element, describing in this way an irreversible thermal degradation in the solder joint. Simulations are carried out in order to analyze and predict the solder fatigue under active power cyclic loading. The numerical results reproduce qualitatively the desired behavior for the cracked interface and demonstrate how the temperature field and the heat transfer in the system are radically changed. Then, the evolution of the average damage level in the cohesive layer, reflecting the crack propagation level during power cycling, is used to estimate the number of cycles to failure at which the device becomes faulty.