Evaluation of Microstructural Evolution and Thermal Fatigue Crack Initiation in Sn-Ag-Cu Solder Joints

Thermal fatigue damage in solder joints is believed to be closely related to microstructural evolution. In this study, a microstructural approach will be taken to evaluate the lifetime of Sn-3.0Ag-0.5Cu lead-free solder joints before the initiation of thermal fatigue cracks. In order to clarify the relation between the microstructural change and the initiation of thermal fatigue cracks, a series of thermal cycle tests were performed using fabricated PCBs on which various chip resistors were mounted. The following results were obtained via SEM observations. First, both the β-Sn and the Ag₃Sn phases grow as the number of cycles N increases. This phase growth is characterized by phase growth parameters S_s and S_A in the β-Sn phase and the Ag₃Sn phase, respectively, which are defined as the average phase size to the 4th power. The phase growth proceeds such that the parameters increase proportionally to N. Furthermore, simple relations exist between the average number of cycles leading to the initiation of thermal fatigue cracks N_i and the average increase in the parameters per cycle ΔS_s or ΔS_A. That is, power law relations ΔS_s = C₂N_i^−β and ΔS_A = C₃N_i^−γ are determined. These relationships will enable us to evaluate the lifetime of the solder joints before the initiation of thermal cracks based on observations of microstructural evolution.