Nucleation and twinning in tin droplet solidification on single crystal intermetallic compounds

Abstract

βSn nucleation is a key step in the formation of microstructure in electronic solder joints. Here, the heterogeneous nucleation of βSn is studied in undercooled tin droplets spread on the facets of various intermetallic compounds (IMCs). Nucleation undercoolings are measured in solidifying droplets and are linked to orientation relationships (ORs) measured by electron backscatter diffraction (EBSD). Preferred ORs developed on all IMCs studied. For the more potent nucleants (αCoSn3, IrSn4, PtSn4, PdSn4) the ORs represent relatively simple atomic matches. ORs with lower potency nucleants (Cu6Sn5, Ag3Sn, Ni3Sn4) had more complex atomic matches that are explored based on matching of the closest packed atomic rows. βSn solidification twinning is shown to be more complex than has been reported previously: both nucleation on an IMC facet and cyclic twinning of that grain occurred in many droplets on Cu6Sn5, Ag3Sn, Ni3Sn4; in all twinned droplets the <100>Sn twinning axis occurred along a direction on the IMC with the lowest linear atomic disregistry; and interrelated cyclic twins formed consisting of up to five rings of cyclic twins all related by shared <100>Sn axes.