Flux technology for lead-free alloys and its impact on cleaning

Cleaning flux residue of lead-free solder pastes is more challenging than that of Sn/Pb systems. This is primarily due to (1) higher reflow temperature, (2) higher flux capacity, therefore higher flux-induced side reactions, (3) more tin-salts formation. The cleanability decreases from no-clean soft-residue fluxes to water washable fluxes to no-clean hard residue fluxes. Improvement in cleanability may link to improvement in thermal stability of fluxes. Semi-aqueous and/or aqueous solvent sprayable cleaners yield the best cleaning efficiency, if the solvent is selected property. Spray is the most critical mechanical agitation, regardless whether it is spray-in-air or spray-under-immersion. Ultrasonic aid also imparted a significant positive effect. Saponified aqueous spray is one of the top choices for cleaning water washable residues. Better solvency and spray are the directions for further improving cleaning. Improvement in flux thermal stability will help both soldering and cleaning.