Solidification Microstructure and Properties of Low-temperatureSn-xBi-1Ag Solder Alloys

Author of the article:HU Xiaojuan1, ZENG Guang1, LIAO Jieli1, FAN Jiangkun2

Author's Workplace：1. School of Materials Science and Engineering, Central South University, Changsha 410083, China; 2. State Key Laboratoryof Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China

Key Words：Sn-xBi-1Ag alloy; solidification microstructure; thermogravimetric analysis; coefficient of thermal expansion;nanoindentation hardness

Abstract：The solidification microstructure and properties of Sn-Bi-Ag alloys significantly affect the reliability of solderjoints. This study investigates the effects of Bi content and cooling rate on the solidification microstructure andnanohardness of Sn-10Bi-1Ag and Sn-57Bi-1Ag (wt.% ) alloys and compares their thermal stability and coefficient ofthermal expansion, thereby establishing the microstructure-property relationships. The results show that both alloys consistof β-Sn, Bi, and Ag3Sn phases, with the Bi content exerting a greater influence on the microstructure and nanoindentationhardness than the cooling rate. Under two different cooling rates, the nanohardness hardness of the Sn-10Bi-1Ag alloyincreases by ~32% (10 ℃/min) and ~22% (40 ℃/min) compared with that of the Sn-57Bi-1Ag alloy. This enhancement inthe Sn-10Bi-1Ag alloy is attributed to the fine dispersion of Bi particles impeding local plastic deformation of the β-Snmatrix, providing significant reinforcement. Additionally, the mechanical properties of the Sn-57Bi-1Ag alloy are influencedby the β-Sn/Bi lamellar spacing. Furthermore, compared with the Sn-10Bi-1Ag alloy, the Sn-57Bi-1Ag alloy exhibitsgreater thermal stability but a lower average thermal expansion coefficient