Atomic-scale study of Cu$_3$Sn and Cu$_6$Sn$_5$ intermetallic layers growth after soldering

Advanced microscopy and elemental analysis techniques were employed to investigate the atomic-scale characteristics of intermetallic compound (IMC) layers of Cu$_3$Sn and Cu$_6$Sn$_5$, which commonly form on copper-based surfaces during reflow soldering with lead-free Sn-based alloys in electronic packaging applications. The study also focused on the influence of iron nanoparticles—introduced via the soldering flux and localized at the interface during solidification—on the growth and coarsening behavior of these IMCs during both reflow and aging processes. High-resolution analyses, comprising atom probe tomography (APT) and scanning transmission electron microscopy (STEM), were used to characterize nanoscale precipitates at interfaces between Cu$_3$Sn and Cu$_6$Sn$_5$, as well as between Cu$_6$Sn$_5$ and the solder joint. These analyses aimed to elucidate the mechanisms by which iron nanoparticles suppress IMC growth during thermal processing. In addition, nanobeam diffraction mapping (NBDM) was used to quantify nanoscale strain distributions at intermetallic interfaces resulting from solid-state phase transformations during soldering. ... mehrIn the thin, fine-grained Cu$_3$Sn layer, the dominant suppression mechanism was identified as nanoscale segregation of elemental iron. Conversely, in the thicker, coarse-grained Cu$_6$Sn$_5$ layer, multiple mechanisms were observed. These included solid-solutioning of iron within the complex Cu$_6$Sn$_5$ crystal structure and in-situ reactions with tin, leading to nanoscale FeSn$_2$ precipitates both within the Cu$_6$Sn$_5$ layer and extending into the solder matrix. Together, these mechanisms contribute to inhibiting IMC growth during soldering and subsequent thermal aging.