Influence of substrate metallization on diffusion and reaction at the under-bump metallization/solder interface in flip-
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Influence of substrate metallization on diffusion and reaction at the under-bump metallization/solder interface in flip-chip packages F. Zhang,b) M. Li,a) and C.C. Chum Institute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602
K.N. Tu Department of Materials Science and Engineering, University of California—Los Angeles, Los Angeles, California 90095-1595 (Received 8 April 2002; accepted 30 July 2002)
In flip-chip packages, the effect of Ni metallization on the substrate side on interfacial reactions between solders and an Al/Ni(V)/Cu under-bump metallization (UBM) on the chip side was investigated during the reflow process. The Ni substrate metallization greatly accelerated interfacial reactions on the chip side and quickly degraded the thermal stability of the UBM due to a fast consumption of the Ni(V) layer. This phenomenon can be explained in terms of rapid Ni or Sn diffusion in the ternary (Cu,Ni)6Sn5 phase, which was formed in the solder adjacent to the Ni(V) layer and the enhanced dissolution of (Cu,Ni)6Sn5 into the molten solder. Without the Ni metallization on the substrate side, the Al/Ni(V)/Cu UBM remained very stable with both eutectic SnPb and Pb-free solders. With the constant drive for miniaturization of electronic components and high number of input/output (I/O), flip-chip technology has attracted great attention in both the industry and academia.1,2 A solder bumped flipchip interconnection consists of under-bump metallization (UBM) on the Si chip, solder bump, substrate surface metallization, and underfill material. Due to increasing environmental concerns, the electronics industry has been searching for Pb-free solders to replace Pbcontaining solders, which are commonly used in today’s electronic goods.1,2 Sn-based solders with Ag and/or Cu as minor constituents are promising Pb-free substitutes for the conventional eutectic SnPb solders.3,4 However, the Sn concentration is higher in Pb-free solders than in eutectic SnPb solders, and the required reflow temperature is also higher for Pb-free solders. Interfacial reactions between the solder and the UBM as well as the substrate metallization are therefore critical material reliability issues for Pb-free solders. A commonly used substrate metallization is Ni or Nibased solid solution, such as Ni/Au, Ni/Pd, and Ni/Ag, due to the relatively slow reaction between a Pb-free solder and Ni during both device production and device operation.5–8 However, since noble and near-noble metals, such as Au, Pd, or Ag, dissolve rapidly in molten Sn-based solders, the Ni substrate metallization, which is a)
Address all correspondence to this author. e-mail: [email protected] b) Current address: Chartered Semiconductor Manufacturing Ltd., Singapore. J. Mater. Res., Vol. 17, No. 11, Nov 2002
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exposed to the molten solder during reflow, is expected to dissolve, diffuse across the entire solder joint, and then affect interfacial reactions between the solder and UBM on the other s
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