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A Novel Lead-Free Solder Alloy with Cerium Addition
Keywords: Pb-free, Solder, Cerium
Although Sn-Ag-Cu lead-free solder alloys were widely used as alternatives to Pb-Sn alloys, there still exist problems in industrial applications such as high melting temperatures, oxidation, deterioration of wettability and drop reliability. In order to address these problems, 300ppm Cerium was added into Sn-2.5Ag-0.5Cu solder and a novel lead-free solder alloy was developed. Liquidus of the Cerium doped solder was effectively reduced, according to DSC analysis. Baking test and TGA results show that oxidation of the solder was prominently inhibited by Cerium addition, due to early formation of a compact protective Cerium oxide film on the solder surface. Cerium tends to segregate in phase boundaries, which decrease the boundary energy and surface tension, so trace amount of Cerium addition improved wetting time and wetting force of Sn-2.5Ag-0.5Cu solder. However, over addition of Cerium causes increase of surface oxidation, which degrades wetting properties. The segregation of Cerium also restraints the growth of -Sn grains, Ag3Sn IMC and Cu6Sn5 IMC. This was proved by a fine solder bulk micro structure and a thinner interfacial IMC observed in 300ppm Cerium doped solder joint, whereas big and long dendrite structures were found to be dominant in un-doped ones. Due to the finer micro structure and IMC, solder bulk tensile properties, solder ball shear strength and hardness, solder joint drop reliability were effectively improved by Cerium addition. Sn-2.5Ag-0.5Cu solder with 300ppm Cerium addition shows excellent overall performance in terms of melting temperatures, wettability, anti-oxidation performance and drop reliability. Therefore, it may become a promising new lead-free solder alloy to address the current issues of Sn-Ag-Cu solders.
Le Liang, Research Engineer
Samsung Semiconductor (China) R&D Co., Ltd.
Suzhou, Jiangsu 215021,
P. R. China


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