Microstructure and mechanical properties of lead-free solders and solder joints used in microelectronic applications

The replacement of lead (Pb)-bearing solders used in the electronic industry with Pb-free solders will become a reality in the near future. Several promising Pb-free solders have recently been identified, including Sn-0.7Cu, Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and Sn-3.5Ag-4.8Bi (in wt.% with slight variations in composition). These are all Sn-rich solders with melting temperatures between 210°C and 227°C, and are recommended for various soldering applications, including surface mount technology (SMT), plated-through-hole (PTH), ball grid array (BGA), flip-chip bumping, and others. Although a considerable amount of information on Pb-free solders has been published in the last few years, the database on these new materials is still at an infant stage compared with that for Pb-containing solders. This paper addresses several aspects of the current fundamental materials understanding associated with Pb-free solders and various issues regarding their imminent use in electronic interconnect applications, including microstructure-processing-property relations, mechanical properties, interfacial reactions, and the thermal-fatigue life and failure mechanisms of Pb-free solder joints.