Publication: Investigation of diffusion bonding and microstructure evolution in low temperature solder sn-bi hybrid joints with sac solder balls
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Date
2025-08-08
Authors
Kong, Xin Zu
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Abstract
This research investigated the microstructure evolution, diffusion bonding, and mechanical integrity in low-temperature Sn-Bi hybrid solder joints with SAC solder balls. The aim was to optimize their formation for modern electronics manufacturing. Sn-Bi solder paste was chosen as it showed excellent properties compared to traditional SAC solder, despite issues like brittleness and thick IMC formation. Three important variables were used in this research, such as solder paste types (Sn42Bi58, Sn42Bi57.6Ag0.4, Sn64Bi35Ag1), stencil thicknesses (0.08 mm, 0.10 mm, 0.12 mm), and reflow peak temperatures (170°C, 180°C, 190°C). As a result, SEM, EDS, and XRD analyses revealed a crescent shaped interfacial morphology and confirm the composition of diffused regions and IMC layers. Quantitative results showed that higher temperatures and thicker stencils enhanced Sn-Bi interdiffusion, with Sn42Bi58 reaching 100% mixing at 190°C and 0.12 mm stencil, followed by Sn42Bi57.6Ag0.4 (81.7%) and Sn64Bi35Ag1 (54.8%). Next, mechanical testing was used to measure the Vickers hardness, which related to the strengthening effect of bismuth. Vickers hardness of Sn42Bi58 reached 28.2HV, followed by Sn42Bi57.6Ag0.4 at 23.97HV, and Sn64Bi35Ag1 at 11.23HV at 190°C and 0.12 mm stencil thickness. Additionally, a simulation was carried out using Ansys Fluent for CFD to make a comparison with the experimental results. Process optimization using the Taguchi method identified peak temperature as the most significant factor contributing to both mixing percentage and Vickers hardness, followed by solder paste type, while stencil thickness had the least impact.