Microstructure-property relations in as-atomized and as-extruded Sn-Cu (-Ag) solder alloys

Abstract

Rapidly solidified Sn-based solder alloys can provide metallurgical features such as low segregation and fine intermetallic compounds (IMCs). These features can be obtained in a controlled way by Impulse Atomization that provides powders of various sizes corresponding to a variety of cooling rates and undercoolings to which diverse microstructures are associated. In the present investigation, rapid solidification of Sn-0.7 wt%Cu and Sn-0.7 wt%Cu-3.0 wt%Ag alloys have been examined through the production of a wide size range of impulse atomized powders. The microstructures and hardness resulting from the generated powders have been compared with those of directionally solidified (DS) specimens. Regular cells > dendrites and reverse dendrites > cells transitions were identified, and high-speed eutectic cells were found to prevail for the examined Sn-0.7 wt%Cu powders of size smaller than 300 μm. It is shown that the Vickers microhardness of the ternary Sn-3.0 wt%Ag-0.7 wt%Cu alloy is directly influenced by both the presence of tertiary dendrite arms (λ3) and the cooling rate/powder size dependent eutectic fraction. Also, compaction and extrusion of the atomized powders were carried out in order to consolidate the samples so that tensile tests could be carried out. Tensile strength and ductility of samples corresponding to different powder sizes and compositions were thus measured and the results are found to be consistent with their microstructures.