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Thermal fatigue is a common failure mode in electronic solder joints, yet therole of microstructure is incompletely understood. Here, we quantify theevolution of microstructure and damage in Sn-3Ag-0.5Cu joints throughout a ballgrid array (BGA) package using electron backscatter diffraction (EBSD) mappingof localised subgrains, recrystallisation and heavily coarsened Ag3Sn. We theninterpret the results with a multi-scale modelling approach that links from acontinuum model at the package/board scale through to a crystal plasticityfinite element model at the microstructure scale accounting for the anisotropicthermal expansion, elastic and plastic properties of beta-Sn with embeddedAg3Sn and Cu6Sn5 particles. We measure and explain the dependence of damageevolution on (i) the beta-Sn crystal orientation(s) in single and multigrainjoints, and (ii) the coefficient of thermal expansion (CTE) mismatch betweentin grains in cyclic twinned multigrain joints. We further explore the relativeimportance of the solder microstructure versus the joint location in the array.The results provide a basis for designing optimum solder joint microstructuresfor thermal fatigue resistance.

Microstructure effects on the thermal fatigue of solder joints: coupling damage measurements with multi-scale modelling