Research On Effects Of Minute Amount Of Rare-earth Element Ce On Reliability Of The Joints Soldered With Sn-Cu-Ni Solder

The research in lead-free solder alloy has been a popular topic in recent years, but a lot of problems in full commercial production and application of lead-free solder must be studied in detail. Based on Sn-0.5Cu-0.05Ni lead-free solder, effects of rare earth Cerium on physical properties, wettability and reliability of soldered joints were studied.The addition of Cerium has little effect on melting temperature and density of Sn-Cu-Ni lead-free solder. Its resistivity is lower than traditional Sn-Pb solder, which indicated that the alloy has an excellent electrical transmit ability and it can reduce the heat produced in the electrocircuit.The wettability of Sn-Cu-Ni-Ce solder on Cu substrate was investigated by spreading area method and wetting balance testing method. The results show that the wettability of solder can be improved by adding minute amount of Cerium under the condition of same temperature and atmosphere. Both N2 atmosphere and heading up of the temperature can improve the wettability, but the temperature can't be too high, or it will reduce the wettability.Mechanical properties of soldered joints and the microstructure of Sn-Cu-Ni-Ce solder were investigated respectively. The results indicate that with the increase of the content of Ce added to the Sn-Cu-Ni solder, the microstructure of Sn-Cu-Ni solder is fine and uniform, so the mechanical properties of soldered joints are improved observably. Experimental results also show that the optimum mechanical properties of joints are obtained when the content of Ce is about 0.05% which added to the Sn-Cu-Ni solder. However, when the content of Ce is over 0.05%, some black and brickle compounds show up, and the mechanical properties of soldered joints deteriorate as well.The influence of thermal cycling on mechanical property of the chip resistor joints which soldered with Sn-Cu-Ni-Ce solder was studied. The results indicate that with the increase of thermal cycling times, the shear forces of the chip resistor joints soldered with Sn-Cu-Ni-Ce solder decreased gradually, and the reliability of soldered joints decreased as well after long time cycling as a result of the existence of crack in soldered joint. The research results have favorable guidance meaning for the development and application of novel Sn-Cu-Ni-Ce lead free solder.