The present invention discloses an in-situ targeted alloying method to reactively dissolve metallic nanoparticles at the solder-substrate interface to control the morphology and growth of intermetallic compounds (IMC) thickness by dispersing metallic nanoparticles into a carrier in lead-free microelectronic assemblies. An in-situ targeted alloying method comprises placing the carrier dispersed with metallic nanoparticles on the substrate, followed by placing and heating lead-free solder alloys on top of the nanoparticles doped carrier to form desired solder joint at the solder-substrate interface.