Creep of directionally solidified Sn-3Ag-0.5Cu wt.% dog-bone samples with a controlled or fibre texture is investigated under constant load tensile testing (stress level: 17 - 44 MPa) and at a range of temperatures (298 – 473 K). The tensile creep strain rate and the localised strain gradient are studied by two-dimensional optical digital image correlation. A transition of creep mechanisms from climb-controlled dislocation creep to lattice-limited diffusion creep is found beyond ~ T/T_M = 0.7 to 0.74 which changes depending on microstructure length-scale. The dominating creep mechanisms explored using electron backscatter diffraction, which enables the understanding of the heterogeneity in microstructural evolution at different strain levels, temperatures and strain rates. This heterogeneity is associated with grain boundaries and two distinct microstructural regions, i.e. the -Sn in dendrites and eutectic containing Ag3Sn and Cu6Sn5. Formation of subgrains and recrystallisation are observed with increasing strain levels, and these correlate with local lattice rotation. Tensile deformation of grains with [100] along the loading direction deform less than grains oriented with [110] along the loading direction. For [110] grains, the (11 ̅0)[001] slip system activates. For [100] grains, the (110)[11 ̅1]/2 and (11 ̅0)[1 ̅1 ̅1]/2 slip systems activate.
The heterogeneous evolution of microstructure for a single Cu/SAC305/Cu solder joint is investigated using in-situ thermal cycling. Local deformation due to thermal mismatch results in heterogeneous lattice rotation, localised towards the corners of the Cu metal substrate and decreases towards the centre of the solder joint. This deformation is induced due to the constraint from the coefficient of thermal expansion mismatch between the β-Sn, Cu6Sn5 and Cu at the interface. Formation of subgrains with continuous increase in misorientation is revealed during deformation, implying that the accumulation of plastic slip at the strain-localised regions play a significant role in grain recrystallisation and crack nucleation which likely leads to joint failure.Open Acces
