| Based on Sn-3.8Ag-0.7Cu lead-free solder, effects of rare earth Cerium on the properties and reliability of soldered joints were studied by the spreading test and shear strength test. The results show that the wettability and the shear strength of soldered joints can be improved by adding small amount of Cerium and the optimum value of Ce content is about 0.03wt%.The microstructures of Sn-3.8Ag-0.7Cu lead-free solder with different contents of Cerium were analyzed by means of SEM, EDX, EMPA. It is turned out that the microstructures can be refined and the intermetallic compounds can be suppressed because of the addition of small amount of Cerium. When a large amount of Cerium containing to Sn-3.8Ag-0.7Cu alloy, Ce-Sn intermetallic compounds were found by EDX; Based on phase diagrams and thermodynamic calculations, the forming mechanism of intermetallic compounds in the microstructures of Sn-3.8Ag-0.7Cu-Ce is explained and the "affinity with Sn" of Ce is approved.The microstructures of soldered joint were further analyzed by using SEM and EDX. The results show that adding small amount of cerium to Sn-3.8Ag-0.7Cu lead free solder, the shear strength of the soldered joints is enhanced by restraining the growth of coarse Ag3Sn phase on interfacial compound Cu6Sn5 and refined grains of interfacial area. When a large amount of Cerium adding to the alloy, the mechanical properties are decreased with the formation of Ce-Sn bulk intermetallic compounds near the interface; The formation mechanism of the microstructures of soldered joints is explained by phase diagrams and metallographic theory.
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