Publication: The influence of indium on microstructure evolution during isothermal aging of Sn-Cu solder
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Date
2024-02
Authors
Saidah Najihah binti Mohd Ghazali
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Abstract
The increasing environmental concerns regarding the toxicity of lead have prompted the widespread search for a suitable lead-free alternative to replace traditional lead solder. The replacement requires a lead-free solder that possesses comparable melting characteristics, excellent wettability, and superior physical and mechanical properties. Despite promising potential, Sn-Cu alloy exhibits higher melting point and wetting angle compared to lead solder, leading to potential risks to the sensitive polymeric based substrate. This research aims to investigate the influence of Indium (In) and Bismuth (Bi) on microstructure evolution, wettability and mechanical properties of Sn0.7Cu exposed to isothermal aging. The solder joints are
characterized by microstructure observation, joint strength, and thermal behaviour using differential scanning calorimetry (DSC). Through the addition of indium in varying weight percentages (1.0 wt.%to 7.0 wt%), the melting temperature of the solder alloy was successfully reduced from 231.04°C to 217.65°C. The wettability of solder alloys exhibited improvement with increasing In content as the wetting angle decreased 20.4% from that of Sn0.7Cu solder. Analysis of the bulk microstructure revealed a refinement of β-Sn grains with higher indium concentrations, and In addition also led to reduced IMC layer thickness at solder interface. The mechanical properties observed showed improved microhardness of as cast solder alloy where addition of Bi gave 27.89% increment and this trend continued with addition of indium (1.0 wt.%to 7.0 wt%) with the highest value at 20.28Hv. The hardness decreased when subjected to isothermal aging at 180℃ for 50, 200, and 500 hours due to grains growth. The reduction of hardness however was lowest in Sn0.7Cu-7In alloy after isothermal aging, indicating that higher indium addition enhanced thermal stability of solder. In addition, the shear strength of single lap joint solder improved 96.5% at 44.089 MPa with 7 wt.% In compared to that of Sn0.7Cu solder at 22.34 MPa. The results showed beneficial effect of addition In and Bi into Sn 0.7 Cu solder alloy.