This study aims to use experimental, numerical and statistical methods to investigate Sn–Bi/SAC hybrid solder joints. Three solder paste compositions (Sn42Bi58, Sn42Bi57.6Ag0.4 and Sn64Bi35Ag1), stencil thicknesses, (0.08, 0.10 and 0.12 mm) and reflow temperatures (170 °C, 180 °C and 190 °C) were evaluated. Hybrid joints were fabricated through stencil printing, BGA placement and reflow soldering. Microstructural and compositional analyses were conducted using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and XRD, while mechanical performance was assessed through Vickers microhardness testing. ANSYS Fluent CFD simulations using the volume of fluid (VOF) model were performed to analyze solder diffusion behavior. Taguchi optimization identified the most influential process parameters affecting joint quality and reliability.
The methodology of this study involved a combined experimental, numerical and statistical approach to investigate the diffusion bonding behavior and microstructural evolution of Sn–Bi/SAC hybrid solder joints. Three solder paste compositions (Sn42Bi58, Sn42Bi57.6Ag0.4 and Sn64Bi35Ag1), three stencil thicknesses (0.08 , 0.10 and 0.12 mm) and three reflow peak temperatures (170 °C, 180 °C and 190 °C) were evaluated. The fabricated samples underwent SEM, EDS, XRD and Vickers microhardness characterization to analyze interfacial morphology and mechanical performance. CFD simulations using ANSYS Fluent with the VOF model were conducted to study molten solder diffusion behavior. Taguchi optimization was applied to identify significant process parameters affecting joint reliability.
The findings revealed that reflow temperature and stencil thickness significantly influenced the diffusion bonding and mechanical performance of Sn–Bi/SAC hybrid solder joints. Sn42Bi58 solder paste achieved the highest mixing percentage of 100% and the highest Vickers hardness of 28.2 HV at 190 °C with 0.12 mm stencil thickness, indicating superior interfacial bonding. SEM and EDS analyses confirmed the formation of Cu6Sn5 and Cu3Sn intermetallic compounds and crescent-shaped diffusion interfaces. CFD simulations successfully reproduced molten solder spreading and interfacial evolution, closely matching experimental observations. Taguchi optimization identified reflow peak temperature as the most dominant parameter affecting mixing behavior and joint reliability.
The originality of this study lies in the integration of experimental characterization, computational fluid dynamic simulation and Taguchi statistical optimization to comprehensively investigate Sn–Bi/SAC hybrid solder joints for low-temperature electronic packaging. Unlike previous studies focusing only on microstructural analysis, this work correlates molten solder flow behavior, diffusion bonding and mechanical performance through combined SEM, EDS, XRD, Vickers hardness testing and ANSYS Fluent VOF simulations. The study also introduces a comparative analysis of three different Sn–Bi solder paste compositions under varying stencil thicknesses and reflow temperatures. Furthermore, the application of Taguchi optimization provides a systematic framework for identifying dominant process parameters to improve hybrid solder joint reliability and performance.
