Interface reactions and phase equilibrium between Ni/Cu under-bump metallization and eutectic SnPb flip-chip solder bump
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Ni-based under-bump metallization (UBM) for flip-chip application is widely used in today’s electronics packaging. In this study, electroplated Ni UBM with different thickness was used to evaluate the interfacial reaction during multiple reflow between Ni/Cu UBM and eutectic Sn–Pb solders in the 63Sn–37Pb/Ni/Cu/Ti/Si3N4/Si multilayer structure. During the first cycle of reflow, Cu atoms diffused through electroplated Ni and formed the intermetallic compound (IMC) (Ni1−x,Cux)3Sn4. After more than three times of reflow, Cu atoms further diffused through the boundaries of (Ni1−x,Cux)3Sn4 IMC and reacted with Ni and Sn to form another IMC of (Cu1−y,Niy)6Sn5. After detailed quantitative analysis by electron probe microanalysis, the values of y were evaluated to remain around 0.4; however, the values of x varied from 0.02 to 0.35. The elemental distribution of IMC in the interface of the joint assembly could be correlated to the Ni–Cu–Sn ternary equilibrium. In addition, the mechanism of (Cu1−y,Niy)6Sn5 formation was also probed.
I. INTRODUCTION
Flip-chip technology provides many beneficial advantages, including high I/O connections, low cost assembly, and high reliability. It has thus attracted a great deal of attention for advanced electronic packaging in microprocessor or consumer products.1–3 In this technology, Si chips with solder bumps are placed face to connect with a metallized substrate. An under-bump metallization (UBM) is used to provide adhesion and acts as a diffusion barrier between solders and Si chips. In fact, the material selection for UBM is a critical issue in flip-chip technology. The Ni-based UBMs have been widely used as a diffusion barrier due to the rapid reaction of Sn with the Cu conductor and the spalling problem associated with Cu–Sn intermetallic compounds (IMCs).4–10 It is thus essential to investigate the interfacial reactions between solders and Ni/Cu UBM during reflow. Recently, the phenomenon of IMCs transformation in the Ni/Cu UBM interface and the effect of Ni thickness and reflow times on IMCs transformation have been reported.11 However, the mechanism of IMCs transformation in Ni/Cu UBM is not yet clear. To reduce (Cu,Ni)6Sn5 IMC formation for reliability purpose, the details of IMC transformation should be understood.
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 4, Apr 2003
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The aim of this study is to further investigate the detailed interfacial reactions and related IMC equilibrium behavior between solders and Ni/Cu UBM during reflow. A 63Sn–37Pb/Ni/Cu/Ti/Si3N4/Si multilayer structure was used, in which Ti played a role of adhesion layer, and Cu was the conductor, while Ni acted as wetting layer and diffusion barrier. Solder joints with different thicknesses of Ni (1, 3, 5, 6, and 9 m) followed by multiple reflows of one, three, five, and ten times, respectively, were used for microstructural evaluation. According to the phase equilibrium in the related system o
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