Abstract
The effect of In addition on the binary eutectic Sn–3.5Ag (compositions are all in mass% unless specified otherwise) solder alloy was investigated. The phase equilibrium and the thermodynamic behaviors were examined up to an indium level of 11.5. The addition of about 9In lowered the liquidus temperature below 210°C, and the microstructures of the solder and the morphology of secondary phases in the solder matrix changed accordingly. The secondary phases coarsened in aging at 150°C up to 500 h. However, the coarsening had little effect on the hardness of the solder alloys. Though the surface tension between the solder and the vapor increased with an increase in In content, the wetting angle over the Cu substrate decreased. This was explained by the increased driving force for formation of the intermetallic compound (IMC) at the interface between the Sn–3.5Ag–In solder and the Cu substrate and thereby the decreased surface tension between the solder and the substrate.
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