SnAgCu (SAC) solders undergo continuous micro structural coarsening during both storage and service. In this study, we use cross-sectioning and image processing techniques to periodically quantify the effect of isothermal aging quantitatively on phase coarsening and evolution, in SAC305 (Sn3.0Ag0.5Cu) solder. SAC305 alloy is aged for (24–1000) hours at 100°C (∼ 0.7–0.8Tmelt). The characteristic parameters monitored during isothermal aging include size, volume fraction, and inter-particle spacing of both nanoscale Ag3Sn intermetallic compounds (IMCs) and micronscale Cu6Sn5 IMCs, as well as the volume fraction of pure tin dendrites in SAC305 solder.
Effects of above microstructural evolution on secondary creep constitutive response of SAC305 interconnects were modeled using a mechanistic multiscale creep model. The mechanistic phenomena modeled include: (1) dispersion strengthening by coarsened nanoscale Ag3Sn IMCs and reinforcement strengthening by micronscale Cu6Sn5 IMCs, respectively; and (2) load sharing between pure Sn dendrites and the surrounding eutectic Sn-Ag phase.
The coarse-grained polycrystalline Sn micro structure in SAC305 solder was not captured in the above model because isothermal aging did not appear to cause any significant change in the initial grain morphology of SAC305 solder joints. The above model is shown to predict the drop in creep resistance due to the influence of isothermal aging on SAC305 solder joints.