Microstructures and tensile properties at 233 K, 300 K and 398 K of Sn-3.0 mass%Ag-0.5 mass%Cu (SAC305) and Sn-Ag-Cu-In-Sb solder were investigated by using miniature size specimens with 0.5 mm diameter, which can reproduce the microstructure of the real solder joint. In this study, three kinds of Sn-Ag-Cu-In-Sb solder (SAC305-6.0 mass%In-1.0, 2.0 and 3.0 mass%Sb) were used. The microstructure of SAC305 consisted of a single crystal grain. On the other hand, the microstructures of Sn-Ag-Cu-In-Sb solder consisted of polycrystalline. The number of crystal grains per the cross section of SAC305-6.0In-1.0Sb was stably several tens or more. The tensile strength of Sn-Ag-Cu-In-Sb was improved approximately 2 times that of SAC305. Also, the variation in tensile strength of SAC305 at 233 K was large due to anisotropy of the crystal grain. In contrast, the variation in tensile strength of Sn-Ag-Cu-In-Sb at 233 K was lower than that of SAC305. In particular, that of SAC305-6.0In-1.0Sb was reduced to approximately a sixth of that of SAC305. It seems that the effect of anisotropy of the crystal grain is decreased by polycrystallization in SAC305-6.0In-1.0Sb.

In order to examine the effect of the Bi addition on tensile properties of SnAgCu solder at low temperatures, stress-strain diagrams were acquired by tensile tests at 233 K using miniature size specimens. Stress drops were observed in the stress-strain diagram of SnAgCuBi solder before it lead to a break. Similar phenomenon did not observed in the SnAgCu solder. The stress drops was exceptionally sharp in the SnAgCu solder with added 3 mass% Bi, compared to the solder with added 1 or 2 mass% Bi. The mode of the stress drop is depended on twin deformation. On the contrary, similar stress drop phenomenon was not observed in any stress-strain diagrams at 298 K. From the results of grain map analysis, it was found that many twin deformations occur in the specimen in which exceptional sharp stress drops appear in the stress-strain diagram.

In this study, the microstructure and the thermal fatigue life of Sn3.0 mass%Ag0.5 mass%Cu6.0 mass%In1.0 mass%Sb (SAC3056In 1Sb) were investigated and they were compared with those of Sn3.0 mass%Ag0.5 mass%Cu (SAC305). As the test pieces, chip resistors were joined on a printed wiring board by reflow soldering with each solder. Although precipitates of Ag and Cu were present in both solder joints, the precipitation state and size of Ag precipitates were different for each solder joint. In the solder joint with SAC3056In1Sb, the precipitate of In and Sb was also confirmed. In addition, In and Sb were solid-soluted in Sn. Solder joints with SAC3056In1Sb were polycrystal with approximately 20 crystal grains per cross section of the joint. On the other hand, the joint of SAC305 was consisted of a single crystal. Average of thermal fracture life of SAC3056In1Sb was approximately 4.3 times longer than that of SAC305. In addition, the variation in the life of SAC3056In1Sb was smaller.