Synthesis and thermal properties of low melting temperature tin/indium (Sn/In) lead-free nanosolders and their melting behavior in a vapor flux

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Low melting temperature tin/indium (Sn/In) nanosolders with a wide composition range were synthesized by a surfactant-assisted chemical reduction method in an aqueous solution at ambient conditions. The parameters that affect the synthesis process were studied to control the size, shape and composition of the Sn/In nanosolders. It was found that hydrolysis of the metal ions significantly affected the nanosolder formation and inhibition of hydrolysis resulted in better nanosolder quality. Under optimized conditions, the Sn/In nanosolders around 50/50 (wt%) composition had a size range of 30–90nm. SEM, TEM, EDS and XRD were used to determine the morphology, composition, and crystal structure of the nanosolders. It was found that the Sn/In nanosolders were mainly composed of InSn4 at low In content (30wt% In). When the In content further decreased to 20%, a mixture of InSn4 and β-Sn exist; however, when the In content increased to 40% or higher, pure In appeared in the nanosolders, and the nanosolders have an increasing amount of In nanocrystals with the increase of the In ratio. DSC measurements have been conducted to understand the melting behavior of the nanosolders, and low melting temperatures were achieved at a wide range of In compositions (from 30% to 70%). The lowest melting temperature was found to be 115.5°C, indicating a new eutectic point around 30% In. Finally, the temperature and time effects on the melting behavior and solder reflow property of the nanosolders have been investigated in a vapor flux environment.