Investigation on the thermal degradation mechanism of Cu-Sn intermetallic compound in SAC solder joints with Cohesive Zone modeling

The present study belongs to a series of investigations on the degradation mechanism of the Cu-Sn intermetallic compound (IMC) layers in Sn-Ag-Cu (SAC) solder joints under the thermal aging condition. The root cause of IMC strength weakening is identified as the nucleation of the Cu₃Sn grains that turns the microstructure of the IMC layer from a single-stack structure to a multi-stack structure. To demonstrate the process, and also to further investigate the microstructure effect of the Cu₃Sn layer on the fracture behavior of the Cu-Sn IMCs, a meso-scale, grain level finite element modeling analysis was performed. The stochastic geometry of the Cu₃Sn grains was constructed using a Voronoi tessellation diagram. A Cohesive Zone Model (CZM) approach was adopted in the numerical framework by embedding the cohesive interface element at the grain boundaries to simulate the interfacial fracture behavior. The competition between the Cu₆Sn₅/Cu₃Sn interfacial fracture and the intergranular fracture of the Cu₃Sn layer when the Cu₃Sn layer present different microstructures was investigated. The results were used to verify the Cu₃Sn-contolling thermal degradation mechanism.