Influence of Indium addition on microstructural and mechanical behavior of Sn solder alloys: Experiments and first principles calculations

The composition of alloying elements in Sn-rich solder plays a pivotal role in determining the performance of solder joints in an electronic package. Recently, Indium (In) has attracted special attention due to its ability to reduce the melting point and its inherent ductile nature. However, the effect of In addition on pure Sn solder microstructure and mechanical properties is not very well understood. This research investigates the influence of small additions of In (2–10 wt%) to pure Sn solder properties. The combination of characterization techniques such as scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and wavelength dispersive spectroscopy (WDS) provided unique insights into the microstructure formation of bulk solder as well as interfacial intermetallic (IMC) layer. The mechanisms behind the coarsening of bulk IMCs and grain refinement of microstructure due to In addition were discussed. Unique surface relief features were observed in Sn-10In solder which exhibited a two-phase microstructure (βSn + γ-(Sn, In)). Moreover, these microstructural changes were correlated with mechanical properties determined non-destructively by using nanoindentation. The IMC scallops were indented in as-reflowed condition. Precise locations of IMC scallops were determined using scanning probe microscopy (SPM). To understand the mechanisms behind changes in IMC properties at the atomistic level, first-principles calculations were carried out. The experimental and calculated modulus values of IMCs followed similar trends.