Extended cohesive zone model for simulation of solder/IMC interface cyclic damage process in Pb-free solder interconnects

The current formulation of stress- and energy-based cohesive zone model (CZM) is extended to account for load reversals. Cyclic degradation of solder/IMC interface properties, namely penalty stiffness, strengths and critical energy release rates follows power-law functions of fatigue cycles. Performance of the extended CZM is examined using finite element (FE) simulation of a single Sn-4Ag-0.5Cu (SAC405) solder interconnect specimen. Strain rate-dependent response of the solder is represented by unified inelastic strain equations (Anand´s model) with optimized model parameters for SAC405 solders. The 3D FE model of the specimen is subjected to cyclic relative displacement (Δδ = 0.003 mm, R = 0) so as to induce shear-dominant fatigue loading. Results show that interface crack initiated at the leading edge of the solder/IMC interface on the tool side of the assembly after 22 cycles have elapsed. Bending stress component induced by the solder stand-off height dominates the interface damage process. A straight interface crack front is predicted indicating the relatively brittle nature of the SAC405/Cu₆Sn₅ interface. The extended formulation of the CZM to account for load reversals has demonstrated the ability to describe the progressive solder/IMC interface damage process consistent with the mechanics of relatively brittle interface fracture.