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 ( λ 2 ), eutectic intermetallic spacing ( λ 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 β -Sn in dendrites and β -Sn in eutectic regions containing Ag 3 Sn and Cu 6 Sn 5 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 λ 2 and λ e ) is only localised next to IMCs without growth in size.