The paper compares theoretical calculations with experimental data, to identify the metallurgical mechanisms with respect to the rework or repair that may be encountered in the transition period from Sn-Pb to Pb-free soldering. Thermodynamic calculations have been carried out to study material behaviour and possible formation of intermetallic precipitates during the reaction between Sn-Pb and Sn-Ag-Cu Pb-free alloys. Two Sn-Ag-Cu alloys that are relevant to current industrial interests, namely Sn-3.0Ag-0.6Cu* (known as '405 alloy' in Europe and North America), and Sn-3.0Ag-0.5Cu (known as '305' alloy in Asia), were reacted with different contamination levels of eutectic Sn-37Pb solder. The variables examined included those related to both the materials and processes, such as composition, temperature and cooling rate. Together these are the primary drivers with respect to the resultant solder microstructures, which were studied using scanning electron microscopy (SEM). Nanoindentation, which is suitable for the ultra-fine and complex microstructures, was also used to investigate the micromechanical properties, including hardness and elastic modulus, at both ambient and elevated temperatures. The results from this work provide guidance as to the consequence for microstructural evolution and hence mechanical integrity when small amounts of Pb exist in Pb-free alloys.