Effects of Co Addition on Bulk Properties of Sn-3.5Ag Solder and Interfacial Reactions with Ni-P UBM

• Published in: Journal of electronic materials 2009, Vol.38 (1), p.39-45
• Main Authors: Kim, Dong Hoon, Cho, Moon Gi, Seo, Sun-Kyoung, Lee, Hyuck Mo
• Format: Article
• Language: English
• Subjects: Applied sciences; Brazing. Soldering; Characterization and Evaluation of Materials; Chemistry and Materials Science; Cobalt; Electronics and Microelectronics; Exact sciences and technology; Instrumentation; Intermetallic compounds; Joining, thermal cutting: metallurgical aspects; Lead free solders; Materials Science; Metallurgy; Metals. Metallurgy; Microhardness; Nickel; Optical and Electronic Materials; Solid State Physics
• ISSN: 0361-5235; 1543-186X
• DOI: 10.1007/s11664-008-0529-x

The effects of Co addition on the undercooling, microstructure, and microhardness of Sn-3.5Ag solder (all in wt.% unless specified otherwise) and interfacial reactions with Ni-P under bump metallurgy (UBM) are investigated when the Co content varies from 0.01 wt.% to 0.7 wt.%. When more than 0.02 wt.% Co was added to Sn-3.5Ag solder, the undercooling of the Sn-3.5Ag solder was significantly reduced and the microstructures coarsened with the increased eutectic region. In addition, the hardness value increased as the Co content in Sn-3.5Ag increased. In the interfacial reactions with Ni-P UBM, a spalling phenomenon of intermetallic compounds (IMCs) during reflow was prevented in the Sn-3.5Ag- x Co ( x  ≥ 0.02 wt.%). However, when more than 0.05 wt.% Co was added to Sn-3.5Ag, the IMC morphology changed from a bulky shape to a plate-like shape. The bulky IMCs were Ni 3 Sn 4 and the plate-like IMCs were Sn-Ni-Co ternary compounds. The main issues discussed include the relations between the morphological changes and the IMC phases, the effects of Co addition on the prevention of IMC spalling, and the optimum level of Co addition.