Thermal fatigue has been one of the most serious problems for solder joint reliability. Thermo-mechanical fatigue failure is considered to be closely related to micro-structural coarsening (grain/phase growth). Factors that influence the phase growth are studied and measurement methods are discussed, including the preparation of the eutectic solder sample for phase size measurement. Three categories of models used to predict grain growth in polycrystalline materials are presented. Finally, phase growth in solder during high temperature aging and temperature cycling and its use as a damage correlation factor are discussed.