Abstract: Due to the enhancement of environmental protection in our world, the research on lead-free solders has become the important role in electronic industry, the addition of rare earth can improve the properties of lead-free solders, based on the soldering testing of lead-free solders bearing rare earth Ce, the scanning electron microscope and energy dispersive X-ray spectroscopy technology are used to investigate the effect mechanism of rare earth Ce on the microstructure of SnAgCu solder joints systematically. The results indicated that CeSn3 with different morphologies appear in the solder matrix. The chemical affinity is utilized to describe the correlation between Ce and Sn, Ag, Cu elements, it can be demonstrated that the rare earth Ce had higher affinity for Sn in the SnAgCu system in theoretically. The adsorption effect of rare earth is investigated based on ULF principle, it can be used to explain the intermetallic compounds(IMCs) refinement of SnAgCu solder joints bearing rare earth Ce. With the microstructure analysis of SnAgCuCe solder joints, it is found that the order of IMCs particles sizes is CeSn3>Cu6Sn5>Ag3Sn, in the theory, it is confirmed that the Ag3Sn particles plays an important role in the strengthen of SnAgCu solder joints, which can provide the theory support for the research of lead-free solders.

Key words: Adsorption effect, Lead-free solders, Rare earth, Solder joints