PROGRESS IN THE DESIGN OF NEW LEAD-FREE SOLDER ALLOYS

Various alloy design approaches have been employed to develop new lead-free solder alloys that can not only substitute for the lead-tin solders, but also offer significantly improved mechanical properties. Three new alloys are described in this article. In Sn-3.5Ag-1Zn (melting point approximately 217-degrees-C), the solidification structure and the eutectic precipitate morphology are refined by the addition of zinc. As a result, a high-strength, high-ductility solder with significantly improved creep resistance is obtained. In Bi-43Sn+2.5Fe, a eutectic alloy (melting point approximately 137-degrees-C), dispersion hardening by magnetically distributed iron particles retards both high-temperature deformation and microstructural coarsening, thus widening the useful service range of Bi-Sn eutectic alloys to much higher homologous temperatures than are typical for the Sn-Pb eutectic alloy. Lastly, Sn-Zn-In based alloys (melting point approximately 185-degrees-C) have been developed for consideration as a drop-in replacement for the near-eutectic Sn-Pb alloy (melting point approximately 183-degrees-C).