A thermal fatigue life prediction method is proposed for SnAgCu solder joints that incorporates the effect of creep strength reduction due to microstructural coarsening of the solder during thermal cycling. The proposed method was used to predict the thermal fatigue life of solder joints in a BGA (ball grid array) semiconductor package. In the study, the thermal fatigue life was calculated through as follows. A creep constitutive equation for the solder that incorporates the strain-enhanced growth of the intermetallic compounds in the solder was used to update the constitutive equation at each given cycle and the relationship between inelastic strain energy density range and the number of cycles of the solder joint was obtained to calculate the fatigue life. As the number of thermal cycles increased, the intermetallic compounds grew, and in conjunction with this, the inelastic strain energy density range increased, causing the thermal fatigue life of the solder joint to decrease compared with that without the microstructural coarsening effect. Since the growth of intermetallic compounds under field conditions with a long dwell time is large compared with that during accelerated tests, the decrease in thermal fatigue life due to microstructural coarsening was found to be significant under field conditions with a long dwell time.
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