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To improve the interfacial and mechanical properties of Sn-0.7Cu solder, we tried to add minor Ge into it and investigated the interfacial behavior and mechanical properties using ball solder joints during high-temperature isothermal aging and thermal cycling. After soldering, Ge addition promoted the formation of SnGe solid solution, improved the shear strength of solder joints, and also decreased the thickness of interfacial intermetallic compounds (IMCs) in solder joints. After the following isothermal aging, the evolution on IMC thickness showed that the IMC growth followed linear with square root of aging time, and Ge addition slowed down the diffusion rates of Sn and Cu atoms and the growth of interfacial Cu6Sn5 and Cu3Sn IMCs. During thermal cycling, the addition of Ge also improved the shear strength and reliability of solder joints by inhibiting the growth of interfacial IMCs. Furthermore, the growth rate of IMCs during thermal cycling was much slower than that in isothermal aging. We defined the difference between them with factor K because it can be attributed to the fact that the activation of Cu and Sn atoms was difficult enough to make the diffusion impossible in the low-temperature stage of thermal cycling. [ABSTRACT FROM AUTHOR]

In this work, the atomic migration behavior in Sn-58Bi solder joints was investigated under the condition of a temperature gradient. The effect of different substrates including Cu, Ni and Co on microstructural evolution in the solder was also compared. It was found that the main migrating element in Sn-58Bi solder was Bi in the direction of temperature gradient to produce certain Bi-rich phases at the cold end. With the change of hot end from Cu to Ni or Co, the thermal migration rate of Bi phase and the growth rate of interfacial Intermetallic compounds (IMC) were significantly depressed, which can be attributed to the decrease of the temperature gradient in the solder joint. In particular, there is no obvious asymmetric growth during the growth of IMC at both ends of the interface, which is different from other Sn-based Pb-free solders. Finally, finite element simulation analysis also verified the excellent thermal migration resistance of Co and Ni substrates. [ABSTRACT FROM AUTHOR]

Sn-15Bi solid solution solder provides a potential replacement for Sn-40Pb solder due to lower cost and similar melting point. Sn-15Bi solder was compared with Sn-40Pb solder to assess the microstructure, wettability, bulk tensile properties, interfacial microstructures in solder joints with a Cu substrate, the interfacial evolution in joints during isothermal aging, and the shear strength on ball solder joints with the effect of aging conditions. The microstructure of Sn-15Bi bulk solder was composed of the primary β-Sn phase with Bi in solid solution and with precipitated Bi phases along the grain boundaries. The wettability of solder alloys was evaluated with wetting balance testing, and the results showed that Sn-15Bi solder had a lower wetting force and longer wetting time than Sn-40Pb solder. The poorer wettability of Sn-15Bi solder was not influenced by different types of soldering flux, but improved with increasing temperature. Tensile tests on bulk solder alloys indicated that Sn-15Bi solder had a higher tensile strength but lower elongation than Sn-40Pb solder. Also, the tensile strength of Sn-Bi solder decreased with decreasing strain rate and with increasing temperature, while the elongation of Sn-Bi solder was independent of the temperature and strain rate. In as-soldered joints, Sn-15Bi solder produced a thicker Cu6Sn5 layer than Sn-40Pb solder. Following isothermal aging, the thickness of the interfacial Cu-Sn intermetallic compound (IMC) increased with the aging time, and the growth rate of the IMC layer in Sn-15Bi solder joints was slightly higher than that in Sn-40Pb solder joints during aging. Ball shear test on solder joints illustrated that Sn-15Bi solder joint had a higher shear strength than Sn-40Pb solder joint, and that with increased aging time, the solder became stronger due to precipitation of Bi, leading to a change in the fracture mode from ductile failure in the solder to brittle fracture at the IMC interface. [ABSTRACT FROM AUTHOR]

A comparative analysis between the microstructure and tensile properties of eutectic Sn-58Bi, Sn-3.0Ag-0.5Cu and SnBi-SnAgCu composition mixed solders including 20, 25 or 50 wt% Sn-Ag-Cu in Sn-Bi solder was conducted, and the three point flexural bending properties of the solders as Cu/solder/Cu joints were evaluated with the effect of loading rate. With Sn-Ag-Cu added into Sn-Bi solder, the content of eutectic Sn-Bi phase decreased while Cu 6 Sn 5 IMC and Sn-Ag-Bi eutectic structure were produced in the microstructure. Tensile results on the solder alloys indicated that SB-20SAC and SB-25SAC mixed solders showed an increased ductility with higher elongation and lower tensile strength compared with Sn-Bi solder, while SB-50SAC showed a brittle characteristic with the greatest strength but lowest elongation among all the mixed solders. The bending tests on the solder joints illustrated that composition mixing was helpful to increase the toughness of Sn-Bi solder, and the bending fracture energy of joints was dependent on the loading rate. A relationship between strain rate, crack propagation and fracture toughness was developed. Sn-Bi solder always had a brittle fracture mode. With Sn-Ag-Cu mixed into Sn-Bi solder, the fracture toughness of joints increased. The crack propagation was then decided by the strain rate, in which ductile failure in the solder matrix was occurred in the bending with a lower strain rate while brittle failure along IMC/solder interface was produced in a higher strain rate. [ABSTRACT FROM AUTHOR]

Sn-58Bi eutectic solder is the most recommended low temperature Pb-free solder but is also limited from the interfacial embrittlement of Bi segregation. Since the quaternary Sn-38Bi-1.5Sb-0.7Ag solder provides a similar melting point as Sn-58Bi eutectic, this paper systematically investigated the properties of this solder from wettability, bulk tensile properties, interfacial microstructure in solder joints with a Cu substrate, interfacial evolution in joints during isothermal aging and the shear strength on ball solder joints with effect of aging conditions. The results were also compared with Sn-58Bi solder. The wettability of solder alloys was evaluated with wetting balance testing, and the quaternary Sn-38Bi-1.5Sb-0.7Ag solder had a better wettability than Sn-58Bi solder on the wetting time. Tensile tests on bulk solder alloys indicated that the quaternary Sn-38Bi-1.5Sb-0.7Ag solder had a higher tensile strength and similar elongation compared with Sn-58Bi solder due to the finely distributed SnSb and Ag3Sn intermetallics in the solder matrix. The tensile strength of solder decreased with a decrease in the strain rate and with an increase in temperature, while the elongation of solder was independent of the temperature and strain rate. When soldering with a Cu substrate, a thin Cu6Sn5 intermetallic compound (IMC) is produced at the interface in the solder joint. Measurement on IMC thickness showed that the quaternary Sn-38Bi-1.5Sb-0.7Ag had a lower IMC growth rate during the following isothermal aging. Ball shear test on solder joints illustrated that the quaternary Sn-38Bi-1.5Sb-0.7Ag solder joints had higher shear strength than Sn-58Bi solder joints. Compared with the serious deterioration on shear strength of Sn-58Bi joints from isothermal aging, the quaternary Sn-38Bi-1.5Sb-0.7Ag solder joints presented a superior high temperature stability. Therefore, the quaternary Sn-38Bi-1.5Sb-0.7Ag solder provides better performances and the possibility to replace Sn-58Bi solder to realize low temperature soldering. [ABSTRACT FROM AUTHOR]