Thermal aging effects on microstructure, elastic property and damping characteristic of a eutectic Sn–3.5Ag solder

The present work describes the microstructural changes and their impacts on the electrical resistivity, elastic modulus and damping property of a eutectic Sn–3.5wt% Ag solder material when exposed to high temperature. A detail microstructural study was conducted through the scanning electron microscopy with energy-dispersive spectroscopy analysis and electron backscattered diffraction technique. In as-cast eutectic Sn–Ag solder alloy, sub-micrometer size Ag₃Sn intermetallic compound (IMC) particles and bamboo-like dendritic structure with a dimension of length 20–30 µm and width 3–5 µm formed during solidification. However, after thermal aging treatment at 150 °C for 60 days, the fine Ag₃Sn IMC particles and β-Sn grain appeared with coarse microstructure with the formation of twinning having the <100> twin axis and 60° rotation. As a result, microstructure and Sn-crystal orientation of Sn–Ag solder greatly impact on its overall properties and turned inferior. From material properties evaluation, it was confirmed that the electrical resistivity, elastic and shear moduli values were significantly reduced with aging time. Consequently, the values of damping capacity improved due to the reduction of moduli.