Thermodynamic reassessment of Au–Ni–Sn ternary system H.Q. Dong n , V. Vuorinen, T. Laurila, M. Paulasto-Kröckel Department of Electronics, Aalto University School of Electrical Engineering, FIN-02601 Espoo, Finland article info Article history: Received 6 June 2013 Received in revised form 30 September 2013 Accepted 6 October 2013 Available online 22 October 2013 Keywords: Thermodynamic modeling Au–Ni–Sn Diffusion couples Diffusion path abstract For having a better understanding on the formation and evolution of metal bonding interconnection microstructures the Au–Ni–Sn ternary system was reassessed on the basis of experimental results and the recently reported thermodynamical description for Au–Sn and Ni–Sn systems. In this paper, the thermodynamic parameters of Ni 3 Sn 4 phase were modified in order to achieve a better agreement with experimentally determined phase boundaries. Further, a self-consistent set of thermodynamic para- meters for the Au–Ni–Sn system were obtained, which were able to reproduce most of the available experimental data. The Ni|80Au20Sn (wt%) diffusion couples were annealed at 320 1C for 10,000 s and at 150 1C for 2500 h. The microstructures of these samples were studied with SEMþEDS technique. The reaction interface between Ni and near eutectic AuSn alloy consisted of Ni 3 Sn/(Ni, Au) 3 Sn 2 /AuSn/Au 5 Sn layers. This experimentally observed diffusion path of Ni against Au–20 wt% Sn solder at 150 1C was rationalized on the basis of the thermodynamically calculated isothermal section. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction Metal bonding method is becoming more popular in micro- electro-mechanical systems (MEMS) wafer level packaging. It is now gradually taking over the traditional glass based MEMS bonding pro- cesses due to several advantages over other bonding methods: not only significantly reducing products costs, but also capable of provid- ing electrical interconnection and higher levels of hermeticity [1]. With respect to various metal bonding techniques, the transient liquid phase bonding (TLP) is considered very promising due to several advantages: (1) the final bond has a potential of having closely related mechanical, electrical and thermal properties to those of the base materials, owing to the formation of the interlayer (i.e., alloys containing the base materials as a solvent and a solute); (2) the bonding process can be performed at a low temperature, which enables the integration of temperature sensi- tive devices, and has the capability of reducing the cost and minimizing the stresses induced by a thermal mismatch between the materials; and (3) it permits higher processing temperatures for further assembly steps as well as higher operational temperatures in the end-application due to the higher melting point of the formed interlayer tin comparison to the bonding temperature [2–4]. The Au–20 wt% Sn alloy is continually used in TLP bonding technique owing to its relatively low bonding temperature, low elastic mod- ulus, high thermal conductivity and high strength. Here the Sn element acts as the solder material, and Au is the base metal [5]. The element, Ni, just as in other bonding technologies [6,7], still plays the role of diffusion barrier in Sn-related bonding systems using TLP bonding [5]. Thus, it is essential to understand the microstructure of the Au–Sn/Ni bonding joint which governs its mechanical properties. However, the microstructure after the bonding joint is determ- ined not only by its composition and thermal history, but also by the reactions that occur at the solder/substrate interface [8]. A comp- rehensive understanding on the formation and evolution of the interconnection microstructure requires obtaining a consistent ther- modynamic description of the various metal bonding systems, in order to gain better understanding on the changes occurring during fabrication, reliability testing and field usage [9, 10]. Therefore, in this work, the phase equilibria of Au–Ni–Sn ternary were recalculated on the basis of the newly updated thermodynamic description of two binary systems. Besides, microstructure of the diffusion couple, Au–20 wt% Sn/bulk Ni, at 150 1C was also investi- gated via different annealing period. The paper is organized as follows: the experimental method will be introduced in Section 2. A brief review of previous work is presented in Section 3. The applied methods and models that were used will be described in Section 4. The results obtained and the related discussions can be found in Section 5. Finally, conclusions drawn from this simulation are summarized in Section 6. 2. Materials and methods The reaction couples were manufactured by placing a 100 mm thick near eutectic Au80/Sn20 (wt%) [Au70.7/Sn29.3 (at%)] alloy Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/calphad CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 0364-5916/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.calphad.2013.10.001 n Corresponding author. Tel.: þ358 409311239. E-mail address: hongqun.dong@aalto.fi (H.Q. Dong). CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 43 (2013) 61–70