The effect of plant-based reinforcement on shear strength and microstructure of Sn58Bi solder

Plant-based activated carbon (AC), a low-cost and eco-friendly resource with high surface area could be a sustainable alternative to conventional metallic reinforcements in improving properties of solders. This study investigates the failure modes, shear strength, and interfacial microstructures of Sn58Bi solder alloy containing plant-based AC, aimed to reduce its brittleness. Sn58Bi-xAC solder pastes were prepared and applied onto lap joints for shear testing in the as-reflowed and accelerated-aging conditions at 120°C. The exact formulation is not disclosed due to UTAR Trade Secret DCC/TS/004 (2025). Failure mode transitioned from IMC failure mode to solder failure mode in reinforced samples while Sn58Bi samples failed predominantly by IMC failure mode. Shear strength increased by 19.86% and 21.64%, respectively in as-reflowed and after 336 hrs of accelerated aging for the highest addition of plant-based AC. Features such as: dimpled structures of the Sn-phases, intergranular coarsened Bi-phases/rock-candy structured IMC and transgranular Bi-phases with cleavages defined the nature of fracture. By superimposing the EDX mapping of C onto the fractured surface and the interfacial microstructure, boundary pinning by plant-based AC was confirmed. Other interfacial morphology improvements include: absence of voids in the intermetallic compound (IMC) layer, reduced IMC layer thickness by 24% and notable refinement of solder microstructures. Diffusivity for Sn58Bi was 1.18×10 -6 μm 2 s -1 and reduced to 2.00×10 -7 μm 2 s -1 with the maximum plant-based AC addition, likely to be attributed to absence of Kirkendall voids within the IMC layer due to plant-based AC particles providing obstruction in the diffusion channels.