This paper presents the mechanical properties and microstructure of Sn-Ag-Bi Pb-free solder. We evaluated the effects of Bi content on the mechanical properties of Sn-Ag-Bi solder such as tensile strength, elongation and deformation behavior at cross-head speeds of 0.1 mm/min and 500 mm/min. The experimental results show that at low cross-head speeds, the addition of Bi to Sn-Ag solder initially increases the tensile strength and decreases elongation due to solid-solution hardening of Sn-phase. As the Bi content is increased to 10 mass% and more, however, elongation increases to a maximum at Sn-Ag-Bi solder containing 57 mass%Bi. Deformation of Sn-Ag solder is governed by slip within the Sn phase, and for high-Bi solders (about 57 mass%Bi) deformation occurs due to slip at Sn-Bi grain boundaries. Intermediate-Bi solders, on the other hand, do not slip in either the Sn phase or at Sn-Bi grain boundaries. At high cross-head speeds, the elongation of both intermediate-Bi solders and high-Bi solders was low and almost constant, indicating slip at Sn-Bi grain boundaries becomes difficult. The impact resistance of these solders was investigated through charpy impact tests, and it is found that Bi has a marked effect on impact resistance. The impact absorption energy of Sn-Ag solder decreases rapidly with the addition of Bi.
This paper presents low-temperature Pb-free soldering technology using Sn-57Bi-1Ag (mass%). Here, the effects of hightemperature annealing on the mechanical properties of the solder such as tensile strength and elongation are investigated. The experimental results show that during annealing, the sizes of both of Sn and Bi phases coarsen, however the mechanical properties do not deteriorate. The deformation behavior of Sn-57Bi-1Ag is found to be dependent on sliding at grain boundaries between Sn and Bi phases, and this behavior remains consistent even after coarsening. The creep strength of solder joint at high temperature is also studied, and it is found that Sn-57Bi-1Ag exhibits superior creep strength at temperature below approximately 100°C compared to the Sn-37Pb (mass%) solder.The thermal cycling test of Sn-57Bi-1Ag solder joint is also conducted under the condition between 0°C and 90°C. The result shows that the length of crack is shorter than Sn-37Pb in the same conditions, which means Sn-57Bi-1Ag is an effective material for low temperature soldering.
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