  Thermal fatigue is a common failure mode in electronic solder joints, yet the
role of microstructure is incompletely understood. Here, we quantify the
evolution of microstructure and damage in Sn-3Ag-0.5Cu joints throughout a ball
grid array (BGA) package using EBSD mapping of localised subgrains,
recrystallisation and heavily coarsened Ag3Sn. We then interpret the results
with a multi-scale modelling approach that links from a continuum model at the
package/board scale through to a crystal plasticity finite element model at the
microstructure scale. We measure and explain the dependence of damage evolution
on (i) the beta-Sn crystal orientation(s) in single and multigrain joints, and
(ii) the coefficient of thermal expansion (CTE) mismatch between tin grains in
cyclic twinned multigrain joints. We further explore the relative importance of
the solder microstructure versus the joint location in the array. The results
provide a basis for designing optimum solder joint microstructures for thermal
fatigue resistance.
