Multi-scale viscoplastic modeling of Pb-free Sn3.0Ag0.5Cu solder interconnects

A mechanistic multiscale modeling framework is proposed, to capture the dominant creep mechanisms and the influence of key microstructural features on the measured secondary creep response of microscale as fabricated Sn3.0Ag0.5Cu (SAC305) solder interconnects. At the smallest length scale, mechanistic dislocation creep models are used to capture the creep strengthening mechanisms in the Sn-Ag eutectic phase. These models account for the strengthening from the microscale Cu ₆ Sn ₅ intermetallics as well as the nanoscale dimension Ag ₃ Sn intermetallic particles. At the next length scale, these models are combined to capture the load-sharing between Sn dendrites and intermetallic phases. The next higher length scale (Sn grains) is not addressed here since secondary creep response is empirically found to be insensitive to grain microstructure. The model constants of the proposed framework are obtained from secondary creep measurements of SAC305 solder, using a modified lap-shear microscale solder specimen and a custom-built Thermo-Mechanical Microscale (TMM) test setup (Figure 1). The calibrated model is used to study the effect of alloy composition and aging loads on SAC solders, by accounting for the changes in the eutectic Sn-Ag region, IMCs and the Sn dendrites. The results show that the multi-scale model predictions provide the right qualitative trends, and, quantitatively match the SAC105 (Figure 2) and prior measurements from SAC387 (not shown here) very well. The model also captures the degradation in the creep properties of as-fabricated SAC305 solder subject to isothermal aging (not shown here). The model effectively captures the effect of alloy composition and aging loads on SAC solders, thereby aiding in the effective design and optimization of the viscoplastic behavior of SAC alloys.