Flux technology for lead-free alloys differs considerably from that for eutectic Sn/Pb solder systems, mainly for soldering and cleaning purpose. For most of the lead-free solders, paste handling is not an issue. Although dust has not settled yet, it becomes clear that the flux needed should have higher flux capacity, higher oxygen barrier capability, and higher thermal stability. Halide-containing fluxes may prevail, due to the high flux efficiency per unit flux volume. The oxygen barrier capability can be reduced if inert reflow atmosphere is available. RMA flux, no-clean flux with high solid content, and water washable systems exhibit a greater prospect to be upgraded for lead-free applications. For 58Bi/42Sn solder, flux with low activation temperature is needed. For Sn/Zn/Bi solders, the demand on flux capacity and oxygen barrier capability is even greater than other lead-free solder systems. In addition, the fluxes should be stable enough to retard the reaction with Zn. 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 properly. 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.
Key words: Lead-free, solder, soldering, flux, cleaning, solder paste, residue.