Evaluation of Thermal Fatigue Crack Initiation in Sn-3.0Ag-0.5Cu Solder Joints by Phase Growth Approach

Abstract

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. First of all, SEM observations make it clear that both the β-Sn and the Ag₃Sn phases grow as a number of thermal cycles N increases. This phase growth is characterized by phase growth parameter S, which is defined as the average phase size to the 4th power. Furthermore, a simple power law relation exists between the average number of cycles leading to the initiation of thermal fatigue cracks N_i and the average increase in the parameter per cycle ΔS. Especially, using the phase growth parameter in the β-Sn phase, one common evaluation curve is obtained regardless of the thermal cycle condition or the chemical composition. This relation will lead us to a practical evaluation method of thermal fatigue crack initiation in Sn-Ag-Cu solder joints.