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Interfacial stability of eutectic SnPb solder and composite 60Pb40Sn solder on Cu/Ni(V)/Ti under-bump metallization

Published online by Cambridge University Press:  03 March 2011

Albert T. Wu  and
F. Hua
Albert T. Wu*
Affiliation:
Institute of Materials Science and Engineering, National Taipei University of Technology, Taipei City 106, Taiwan, Republic of China; and Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei City 106, Taiwan, Republic of China
F. Hua
Affiliation:
Intel Corp., Santa Clara, California 95052
*
a) Address all correspondence to this author. e-mail: atwu@ntut.edu.tw This experiment was conducted at Intel Corp. and a United States patent has been filed.
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Abstract

Eutectic SnPb solder has been widely used in packaging for several decades. The stability of the interface between solder and under-bump metallization (UBM) is an important issue that has led to many studies. Even though Ni atoms dissolve much slower into SnPb solder than Cu, the intermetallic compound, Ni3Sn4, which forms when eutectic SnPb solder reacts with Ni(V)/Ti UBM, is not stable on Ti layer, creates V-rich zone, and causes spalling. To prevent the phenomenon, and the resulting reduction of mechanical reliability in solder joints, we propose the addition of a layer of Cu thin film to serve as a sacrificial layer. Both eutectic SnPb solder and composite solder (high-Pb solder with eutectic SnPb solder) were studied in severe reflow conditions to simulate the worst case of die attach and later reflow process. Cu film first was consumed completely to form a compound. Due to lower interfacial energy between Cu6Sn5 and Ni(V), the interface was stable and no spalling occurred. However, the same thickness of Cu was insufficient to prevent Ni from diffusing into solder or compound. Not only diffusion of Ni atoms was observed; Sn atoms also diffused into the Ni(V) layer. The Sn–Ni reaction caused the interface between the compound and Ni(V) to retreat into the Ni(V) layer. The compound was not stable at the interface, and spalling could be seen. Due to the interdiffusion of Ni and Sn, many Kirkendall voids were also observed at both side of the interface.

Keywords

KineticsPackaging
Type
Articles
Information
Journal of Materials Research , Volume 22 , Issue 3 , March 2007 , pp. 735 - 741
Copyright
Copyright © Materials Research Society 2007

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