Thermal fatigue life of Pb-Sn alloys.

概要

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The relationship between composition of Pb-Sn alloys and thermal fatigue lifetime was clarified by two different methods; estimation method from tensile properties of bulk solder and thermal fatigue test of joint model. This system alloys consist of 3 regions in phase diagram; α-Pb solid solution (Sn: 0-19.5wt%), eutectic (Sn: 19.5-97.5wt%) and β-Sn solid solution (Sn: 97.5-100wt%) and they show the maxima of the thermal fatigue lifetime at purePb, Pb-50wt%Sn, Pb - 75wt%Sn and pure Sn. The lifetime of α-solid solution is longer at the higher content of Pb. The longer fatigue lifetime depends on the softness and ductility of the α-solid solution. As the content of Sn in α-solid solution increases, the fatigue lifetime is reduced by the precipitation of β-crystals from α-solid solution. The Pb-50wt% Sn shows the longest lifetime because of fine grains of eutectic solder and ductility of a Pb initial crystals. The Pb- 75 wt%Sn and pure Sn also have relatively long lifetime. The main reason for the long lifetime of Pb-75wt%Sn is considered to be sliding of fine β-Sn crystals and its large shearing stress together with the deformation of fine eutectic solders. It is also concluded that the thermal fatigue lifetime can be estimated well from the tensile properties of bulk solder through the consideration of the cooling rate during casting and the strain rate in tensile tests.
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