A highly reliable Sn-Ag-Bi-In-Cu lead-free solder alloy was developed by improving the melting property, the wettability and the mechanical property of the Sn-Ag solder alloy which has a high thermal fatigue resistance due to the addition of bismuth, indium and copper. The optimum alloy composition for the Sn matrix is 2.5-3.0mass%Ag, 3.0mass%Bi, 1.0mass%In and 0.2-0.5mass%Cu.

The additional elemental bismuth contributed to the control of the solidus and liquidus temperatures and the improvement of the wettability and tensile strength. Indium contributed to the control of the solidus and liquidus temperatures and the improvement of wettability. Copper contributed to the improvement of the ductility which is degraded by the addition of bismuth. In comparison with the Sn-3.5Ag eutectic solder, this new lead-free solder alloy has a 19°C lower solidus temperature, 6-7°C lower liquidus temperature, 4% higher spreading factor and 200% higher tensile strength. Its elongation is equal to and above Sn-3.5Ag at 125°C though it is half as long as that of Sn-3.5Ag at 25°C.

It was confirmed that this new lead-free solder joint has a high thermal fatigue resistance and the same electrochemical reliability in comparison with the conventional Sn-37Pb solder joint.

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