  The heterogeneous evolution of microstructure in a single Cu/SAC305/Cu solder
joint is investigated using in-situ thermal cycling combined with electron
backscatter diffraction (EBSD). Local deformation due to thermal expansion
mismatch results in heterogeneous lattice rotation, localised towards the
corners of the joint and decreases towards the centre of the joint. This
deformation is induced by the constraint from the CTE between the \b{eta}-Sn,
Cu6Sn5 and Cu at interfaces. The formation of subgrains with continuous
increase in misorientation is revealed during deformation, implying the
accumulation of plastic slip at the strain-localised regions and the activation
of slip systems (1-10)[111]/2 and (11-0)[111]/2.
