Improved electrical and thermal properties of solder alloys are achieved by the use of micro-additives in solder alloys to engineer the electrical and thermal properties of the solder alloys and the properties of the reaction layers between the solder and the metal surfaces. The electrical and thermal conductivity of alloys and that of the reaction layers between the solder and the -metal surfaces can be controlled over a wide range of temperatures. The solder alloys produce stable microstructures wherein such stable microstructures of these alloys do not exhibit significant changes when exposed to changes in temperature, compared to traditional interconnect materials.