Study On The Non-eutectic Sn-Bi Solders With Low Melting Point

Current lead-free solders, the most promising and popular ones, such as Sn/Ag, Sn/Cu, Sn/Zn and Sn/Ag/Cu, are unwillingly accepted by many electronics manufacturing services (EMS). Except for their high costs, the key problem is that their melting points are 34～38℃higher than that of Sn-37Pb, The EMS have to replace both their soldering equipment and technology process to fit these solders, furthermore, take a risk of the wetting and reliability. The melting point of Sn-Bi systems are close to Sn63/Pb37 solder, but the dendritic segregation and grain coarsening of Bi in welding process is an insolvable problem for a long time.In this paper, as the substitutes of Sn/Pb alloy, Sn-20Bi-0.7Ag-0.1In-0.5Ge-0.5Ce- 0.5Sb-0.3Ga and Sn-14Bi-2.4Ag-0.5In-0.5Sb lead-free solder alloys have been studied by alloying method. The influence of rapid cooling, aging and adding alloy element contents in correspondence to Sn-Bi alloy on their microstructure, physical properties, mechanical properties and interfacial behavior have been observed. The results list as follows:1. Bi can reduce melting point of solder .Based on Sn-20Bi, Ag In Ge Ce and Sb were added in order to improve its soldering property. When the content of Ag was 0.7%, and In was 0.1% the melting points of the solder reduced respectively and the segregation of Bi decreased. In and Ga reduced the melting point of the solder. Ga, Ce and Sb facilitated the segregation of Bi. Sb enhanced its mechanical behaviour. Ce can be grain refiner of solder. The solder have spheroidized Bi when Ge was being added.2. Sn-20Bi-0.7Ag-0.1In-0.5Ge-0.5Ce-0.5Sb-0.3Ga and Sn-14Bi-2.4Ag-0.5In-0.5 Sb lead-free solder alloys improved the brittleness of Sn-20Bi alloy, reduced the segregation of Bi, their melting point and mechanical properties were close to Sn63/Pb37 solder, but the wettability was worse than Sn63/Pb37.3. The rapid cooling can restrain the segregation of Bi, fine the microstructure, and enhance the soldering and mechanical properties; Sn-20Bi-X alloy can be used for manual soldering.4. The morphology of Intermetallic Compound (IMC) becomes from irregular shape to planar during aging process, and with the increase of aging time, the thickness of IMC increased. Bi accumulates at the interface with the aging temperature and aging time increasing. The thickness of IMC cannot exceed 3μm with aging at 125℃for 100 hours. The microstructures of Sn-20Bi-X solder were even and refined, and the mechanical properties of solder joints were better with aging at 125℃for 20 hours.The structures is stable after aging. It's necessary and effective to aging for Sn-20Bi solder at 120℃for 20 hours.