| In recent years, the electronic devices became more integration, miniaturization andintellectuallization, and this trend put forward higher requirements especially inworking temperature for the solder alloys which act as the connection material. Inorder to develop a high temperature lead-free solder paste, we chose the Ag-In system(Ag-24.5In, wt%) as our candidate in this paper. The solidification process andmicrostructure under different cooling rates were systematically investigated by thedifferential thermal analysis, microstructure observation, scanning electronmicroscope, X-ray diffraction instrument and transmission electron microscope. Inaddition, the effects of aging treatments on the relevant microstructure stabilities andproperties were also explored. Finally, the effects of different cooling rates and agingtreatments on the mechanical properties of Ag-24.5In alloy were studied by thevariation of Vickers hardness. More details were given as follows:The influence of cooling rates on the room-temperature microstructure ofAg-24.5In alloy was investigated by changing the cooling medium. The resultsindicated that the microstructure were consisted of the primary phase and the ζphase whether on equilibrium-cooling or not, and the primary phase was presentedin dendritic morphology, and the typical characteristics of peritectic morphologycould not be seen in the microstructure owing to the incomplete peritectic reaction andthe peritectoid reaction at670℃. In addition, the solution of In made the latticeconstant of Ag bigger, so the whole diffraction spectrum shifted to the left comparedwith pure silver.We also studied the influence of different aging treatments on the stability andmicrostructure of the Ag-24.5In alloy. The results showed that the Ag-24.5In alloywas very stable at room temperature, after aging at room temperature for120d onlythe Vickers hardness value decreased a little. After aging at185℃for40h themicrostructure coarsened and the hardness decreased. The alpha phase grew upattaching to the grain boundary of primary phase and surrounded it. After agingtreatments the ductility and melting point were both increased, so we considered thatthe high temperature aging treatments is good for the service process of this alloy.Based on the results of Vickers hardness we found that the hardness was increased with the solidification rate speeding up. The reason is that the microstructure becamesmall and the relative content of phase (soft phase) reduced with the increase ofsolidification rate. In general, the hardness of Ag-24.5In alloy was decreased afteraging treatments and the faster the cooling rate the greater the falling of hardness.After high temperature aging a new grain boundary was generated by the grew upphase and the precipitated ξ phase, which will increased the hardness to a certainextent, but in the process of aging the hardness decreasing factors were still dominant,so the hardness value was still falling but slowly. |
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