Nano-scale mechanistic model for microstructural reliability in reactive hybrid solder joints

Hybrid solder joints with upgraded functional performance and reliability were processed by applying reactive Fe-nanoparticles doped into the flux at the interface in order to control and engineer the formation and growth kinetics of the intermetallic compound (IMC) layer. In this framework, the reflow solidification of a Sn- 3.5Ag solder alloy on a copper substrate was studied at high spatial resolution by scanning transmission electron microscopy (STEM) analyses concerning the atomic scale mechanisms underlying the behavior of the reactive iron nanoparticles that were applied at the interface of the solder joint. Elemental mapping distributions displayed the contributing mechanisms suppressing the IMC layer growth by the applied Fe-nanoparticles via segregation in front of the Cu6Sn5 layer, in situ reactions to form FeSn2 nanophases distributed inside the Cu6Sn5 layer and iron elemental diffusion ahead of the Cu3Sn layer through the lattice of the more extensive IMC layer. These effects of the Fe-nanoparticles that led to a hindrance of the IMC layer growth yielded an energy barrier of up to ∼60–70 kJ/mol, thereby effectively stabilizing the solder joint against detrimental phase formation.