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The microstructural change and hardness, which strongly influence the life of soldered joints, were investigated by microstructure observations for three typical promising alloys as Pb free solder, Sn–3.5 mass%Ag, Sn–8.8 mass%Zn and Sn–57 mass%Bi alloys, and Sn–38.1 mass%Pb for comparison. The solders of Sn–Ag and Sn–Zn eutectic alloys and Sn–Pb eutectic alloy were found to age-soften during storage at 25–100°C (298–373 K). It seems that the coarsening of the microstructure and re-crystallization caused the age-softening of the Sn–Ag and Sn–Zn alloys. The hardness of the Sn–Bi eutectic alloy hardly changed during storage at 25–100°C (298–373 K) in spite of remarkable microstructural coarsening. It seems that the hardening effect caused by solid-solution hardening of Bi into Sn-rich solution and/or the precipitation hardening of Bi in the Sn-rich solution compensated the softening caused by the remarkable microstructural coarsening of the Sn–Bi alloy.
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Miyazawa et al. (2001) studied this question.
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