Microstructure, wetting property of Sn-Ag-Cu-Bi-xCe solder and IMC growth at solder/Cu interface during thermal cycling

The effects of adding small amounts of cerium (Ce) to Sn-3.3Ag-0.2Cu-4.7Bi solder on microstructure, wettability characteristic, interfacial morphology and the growth of interfacial intermetallic compound (IMC) during thermal cycling were investigated by optical microscopy (OM), scanning electron microscopy (SEM) and solderability tester. It is found that the beta-Sn phase, Ag3Sn phase and Cu6Sn5 phase in the solder are refined and the wetting force increases. Ce is an active element; it more easily accumulates at the solder/flux interface in the molten state, which decreases the interfacial surface energy and reduces the driving force for IMC formation on Cu substrate; therefore, the thickness of IMC at the solder/Cu interface decreases when appropriate Ce was added into the solder. Moreover, the Ce-containing solders have lower growth rate of interfacial IMC than solders without Ce during the thermal cycling between -55 and 125 degrees C. When the Ce content is 0.03 wt% in the Sn-3.3Ag-0.2Cu-4.7Bi solder, the solder has the best wettability and the minimum growth rate of interfacial IMC layer.