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铁道科学与工程学报

JOURNAL OF RAILWAY SCIENCE AND ENGINEERING

Vol. 16    No. 1    February 2006

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Effects of cooling rates on microstructure and
microhardness of lead-free Sn-3.5%Ag solders
SHEN Jun(沈 骏), LIU Yong-chang(刘永长), HAN Ya-jing(韩雅静), GAO Hou-xiu(高后秀),
WEI Chen(韦 晨), YANG Yu-qin(杨渝钦)

College of Materials Science and Engineering, Tianjin University, Tianjin 300072, China

Abstract:The microstructure and microhardness of Sn-3.5%Ag solders were explored in the cooling rate ranging from 0.08 to 104 K/s. Under rapid cooling condition, the strong kinetic undercooling effect leads to the actual solidification process starting at the temperature lower than the equilibrium eutectic point, and the actual metastable eutectic point shifts to the higher Ag concentration. Hence, the higher the applied cooling rate is, the more the volume fraction of primary β-Sn crystal forms. At the same time, the separation of primary β-Sn crystal favors restraining the formation of bulk Ag3Sn intermetallic compounds (IMCs) in solder due to the mismatch crystalline orientation relationship, those Ag3Sn phase separating through the eutectic reaction could hardly cling to the primary β-Sn crystal and grow up. Additionally, the Vickers hardness test shows that fine β-Sn and spherical Ag3Sn phase in the rapidly solidified alloy strongly improves the microhardness of the Sn-3.5%Ag solder.

 

Key words: lead-free solder; Sn-3.5%Ag solder; eutectic reaction; intermetallic compounds; microhardness

ISSN 1672-7029
CN 43-1423/U

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