The Role of Lengthscale in the Creep of Sn-3Ag-0.5Cu Solder Microstructures

Creep of directionally solidified Sn-3Ag-0.5Cu wt.% (SAC305) samples with near- orientation along the loading direction and different microstructural lengthscale is investigated under constant load tensile testing and at a range of temperatures. The creep performance improves by refining the microstructure, i.e. the decrease in secondary dendrite arm spacing (lambda(2)), eutectic intermetallic spacing (lambda(e)) and intermetallic compound (IMC) size, indicating a longer creep lifetime, lower creep strain rate, change in activation energy (Q) and increase in ductility and homogeneity in macro- and micro-structural deformation of the samples. The dominating creep mechanism is obstacle-controlled dislocation creep at room temperature and transits to lattice-associated vacancy diffusion creep at elevated temperature (T/T-M > 0.7 to 0.75). The deformation mechanisms are investigated using electron backscatter diffraction and strain heterogeneity is identified between beta-Sn in dendrites and beta-Sn in eutectic regions containing Ag3Sn and Cu6Sn5 particles. The size of the recrystallised grains is modulated by the dendritic and eutectic spacings; however, the recrystalised grains in the eutectic regions for coarse-scaled samples (largest lambda(2) and lambda(e)) is only localised next to IMCs without growth in size.

Automated summary

Optimizing the microstructure can improve the creep performance of Sn-3Ag-0.5Cu alloy samples, reducing creep rate, extending creep lifetime, and increasing ductility. The dominant creep mechanism at room temperature is obstacle-controlled dislocation creep, while it transitions to lattice-associated vacancy diffusion creep at elevated temperatures.