Metallurgy and kinetics of liquid-solid interfacial reaction during lead-free soldering

The wetting of a molten solder on metallic surfaces is a rather complex phenomenon. In addition to physical spreading due to surface tension reduction. there are interfacial metallurgical and flux chemical reactions with the metallic substrate surface. Substrate dissolution and intermetallic formation take place rapidly during soldering. Since lead-free soldering requires substantially higher soldering temperatures (around 250 degrees C), the rates of intermetallic growth and substrate dissolution for lead-free solders are expected to be significantly greater than those for the current Sn-Pb eutectic solder. This study systematically investigates the metallurgy of the solid-liquid interface reactions and intermetallic growth kinetics for three lead-free solders: Sn-Ag eutectic (96.5%Sn-3.5%Ag), Sn-Cu eutectic (99.3%Sn-0.7%Cu) and Sn-Ag-Cu eutectic (Sn-3.8Ag4.7Cu, SAC 387) with three metallic substrates: Cu, Ni, and Alloy 42 (42%Ni-52%Fe) over temperatures ranging from 225 to 280 degrees C for reaction time from 10s to 16h. Wetting behavior of these three alloys on PCBs with OSP, immersion Sn, and Ni/Au finishes, was also examined from 220 degrees C up to 260 degrees C. A thorough understanding of lead-free solder/substrate interfacial reactions should give guidance to the optimum lead-free soldering processes and to the optimum lead-free coating thicknesses for component and PCB terminal finishes, as well as for under-chip metallurgical coatings for flip-chip and BGA applications.