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

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 an 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. 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.