Fracture behaviour of Sn-58Bi alloy reinforced by activated bamboo charcoal

\nSn-Bi solders are considered by PV manufacturers as acceptable Pb-free alternative due to its low operating temperature and cost, despite being prone to brittleness caused by coarse Bi phases. Bamboo charcoal (BC) is a sustainable and environment-friendly resource with high surface area and its addition to Sn-Bi solder meets the requirement of a green solder, in line with environmental regulations. This study aims to improve the shear strength and reduce brittleness of the Sn-Bi solder alloy by using a sustainable reinforcement, the activated bamboo charcoal. Sn-58Bi solder paste was reinforced with 0.25, 0.50, 0.75 and 1.00 wt.% of activated BC, respectively and reflowed to create a single lap shear joint. Aging response was determined through accelerated aging at 120 °C for 7 days and 14 days. Shear strength of the joints increased as the activated BC content increased in the as-reflowed condition and there was lower degradation in shear strength for heat-aging specimens compared to pure Sn-Bi solder. No failures occurred via Mode 3 when the as-reflowed samples contained 0.50 wt.% or more of activated BC, and the 7- and 14-day heat-aged samples were free from Mode 3 failures when the activated BC composition was at 0.75 wt.% and above. Fracture surfaces showed transitions from flat and smooth surface to elongated-dimple structures even with just 0.25 wt.% of activated BC addition. While prolonged heat-aging increased cleavage presence, increasing amounts of activated BC reduced facet numbers and dimple sizes, indicating successful suppression of Bi phase segregation.\n