Microstructure and orientation evolution of β-Sn and interfacial Cu6Sn5 IMC grains in SAC105 solder joints modified by Si3N4 nanowires

In this study, Si3N4 nanowires (NWs) with ceramic properties were incorporated into Sn1.0Ag0.5Cu (SAC105) solder to enhance its overall performance. The thermal properties, spreading behavior, microstructure, interface, and mechanical properties of SAC105-xSi3N4 (x=0, 0.2, 0.4, 0.6, 0.8, 1.0 wt%) solders were systematically investigated. The research revealed that doping Si3N4 NWs into SAC105 solder could expand its melting range and decrease undercooling. Notably, the solder alloy containing 0.6 wt% Si3N4 NWs had the lowest thermal expansion coefficient (CTE), which improved the CTE matching of SAC105 solder with Cu substrate. Although the solder with 0.4 wt% Si3N4 NWs exhibited superior wetting properties, it was less effective in refining the microstructure and interfacial intermetallic compounds (IMC) than those containing 0.6 wt% Si3N4 NWs. Meanwhile, when the β-Sn phase in the matrix and the Cu6Sn5 IMC phase at the interface were analyzed by electron back scatter diffraction (EBSD), their grain orientation was found to be optimized by the doping of 0.6 wt% Si3N4 NWs. The <001> direction of the β-Sn grains of SAC105-0.6Si3N4 was perpendicular to the Cu substrate, showing a texture structure. The grain orientation of the interfacial Cu6Sn5 IMC made it well-adapted for three-dimensional (3D) packaging. Finally, the mechanical properties enhancement of the solder joints were analyzed in detail by combining the fracture morphology and the EBSD results of the Sn matrix.