Investigation of interfacial reaction between Au–Sn solder and Kovar for hermetic sealing application

doi:10.1016/j.mee.2007.05.058

Microelectronic Engineering

Volume 84, Issue 11, November 2007, Pages 2634-2639

https://doi.org/10.1016/j.mee.2007.05.058 Get rights and content

Abstract

The microstructural evolution and interfacial reactions of Au/Sn/Au/Au/Ni/Kovar joint were investigated during aging at 180 and 250 °C for up to 1000 h. The Au/Sn combination formed a rapid diffusion system. Even in non-annealed joint, three phases such as AuSn, AuSn₂ and AuSn₄ were formed. After initial aging at 180 °C, the AuSn, AuSn₂, AuSn₄, Au and Sn phases, which were formed after plating, were fully transformed into ζ-phase and δ-phase, and (Ni, Au)₃Sn₂ intermetallic compound (IMC) layer was observed between the ζ-phase and Kovar. As a whole, the microstructure of the joint was stable during aging at 180 °C. On the other hand, the solid-state interfacial reaction was much faster at 250 °C than at 180 °C. During aging at 250 °C, the Ni layer on the Kovar reacted primarily with the δ-phase in the solder, resulting in the formation and growth of the (Au, Ni)Sn IMC layer at the interface. After aging for 48 h, the Fe–Co–Ni–Au–Sn phase was formed underneath the (Au, Ni)Sn IMC layer. Furthermore, cracks were observed inside the interfacial layers after complete consumption of the Ni layer. The study results clearly demonstrate the need for either a thicker Ni layer or an alternative surface finish on Ni, in order to ensure the high temperature reliability of the Au/Sn/Au/Au/Ni/Kovar joint above 250 °C.

Introduction

In many applications, ceramic and metal parts often need to be joined together for enhancement by combining their characteristics. This technique is broadly applied in many fields such as mechanical engineering, atomic energy industry, aerospace facility, and electronic devices [1].

Ceramic/metal joining parts are often running in thermal shock situation. There are great discrepancies of properties between ceramics and metal, especially in terms of the thermal expansion coefficient. Therefore, stress, either high or low, always exists in the joint and can damage the mechanical properties of the joint to various extents. This damage is usually aggravated by thermal shock or thermal cycling. To obtain good thermal shock resistance of ceramic/metal joints, similar thermal expansion coefficients of ceramic and metal are required [1]. Therefore, Kovar^® is widely used because its coefficient of thermal expansion is close to that of the attached ceramic, especially alumina. Kovar has a low expansion coefficient and belongs to the special thermal expansion alloy group with the following composition: Ni: 29%; Co: 17%; Fe: 53.48%; Mn: 0.3%; Si: 0.2%; and C: 0.02% max. Kovar is employed widely in the seals and carriers of ceramic packages for discrete transistors, diodes, and integrated circuits [2], [3].

An appropriate micro-electro-mechanical systems (MEMS)/micro-opto-electro-mechanical systems (MOEMS) package should offer accurate component placement, attachment, sealing, protection and various interconnects, including electrical, optical or fluidic [4]. MOEMS assemblies usually have long shelf-life and require hermetic sealing. Package sealing can be obtained using a variety of materials including epoxies and solders [4]. Among these, Au–Sn solder is very well suited for soldering optoelectronic devices, hermetic sealing applications and MEMS and MOEMS packaging. Au–Sn (80%Au: 20%Sn by weight percent) soldering has historically been employed in the microelectronics industry for fluxless hermetic lid sealing and die attach applications [5], [6], [7], [8], [9]. For hermetic sealing applications, fluxless soldering is desirable. Fluxless soldering covers a broad range of techniques that either reduce the formation of surface oxides or prevent surface oxidation prior to and during the reflow process [4].

Furthermore, Au, Ni and Pt are the most common surface finishes selected to be soldered with Au–Sn solder [10]. Solders bond strongly to the metallized substrate by forming intermetallic compound (IMC) layers between the solder and metallized substrate. Unfortunately, the IMC layer tends to grow with time by solid state diffusion, even at ambient temperatures. The IMC layer may adversely affect the reliability of solder joints due to excessive growth and thermal fatigue during storage and service. They can become sources of mechanical weakness in solder joints due to the brittle nature of the intermetallics or can cause delamination at the interface [11]. Therefore, knowledge of the interfacial reactions between the Au–Sn solder and Au/Ni metallized Kovar in microelectronic and optoelectronic packaging is essential. In the present study, we conduct an interfacial reaction study on Au/Ni/Kovar substrate with the Au–Sn solder. We compare the interfacial reaction behaviors during aging at two different temperatures (180 °C and 250 °C) and interpret the differences in terms of kinetics and thermodynamics. This detailed study on the interfacial reaction linked with microstructural evolution will support the formation of a complete understanding of the thermal stability of the Au–Sn/Au/Ni/Kovar joint.

Section snippets

Experimental procedures

The Au/Sn/Au/Au/Ni/Kovar material was used in this study. Firstly, the Kovar was metallized with 0.3 μm (3000 Å)-thick Ni and 1 μm-thick Au. The Au layer is known to improve corrosion resistance on the package and also promote wetting of the solders used for the attachment process [4]. Subsequently, Au, Sn and Au were electroplated on the Au/Ni metallized Kovar. Fig. 1 presents a schematic illustration (a) and cross-sectional SEM image (b) of the Au/Sn/Au/Au/Ni/Kovar sample used in this study. In

Results and discussion

Fig. 2a shows the cross-sectional SEM image of the as-plated Au/Sn/Au/Au/Ni/Kovar interface without etching process. A back scattered electron (BSE) image mode of SEM was used to clarify the morphology of the phase formed at the interface. A peculiar phenomenon was observed in the as-plated joint. The SEM micrograph confirmed that the Sn plating layer between the two Au plating layers was not only Sn layer but was actually composed of many phases. A close examination of the cross-sectional

Conclusions

We examined the interfacial reactions between the Au/Sn/Au solder and Au/Ni metallized Kovar alloy under various conditions. The Au/Sn combination formed a rapid diffusion system. Even in non-annealed joint, three phases were found: AuSn, AuSn₂ and AuSn₄. After aging at 180 °C for 24 h, the AuSn, AuSn₂, AuSn₄, Au and Sn phases, which were formed after plating, were fully transformed into the ζ- and δ-phases due to the fast diffusion between Au and Sn. In addition, the (Ni, Au)₃Sn₂ IMC layer was

Acknowledgements

This work was supported by Grant no. RTI04-03-04 from the Regional Technology Innovation Program of the Ministry of Commerce, Industry and Energy (MOCIE).

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Investigation of interfacial reaction between Au–Sn solder and Kovar for hermetic sealing application

Abstract

Introduction

Section snippets

Experimental procedures

Results and discussion

Conclusions

Acknowledgements

Ceramics International

Journal of Materials Processing Technology

Engineering Failure Analysis

Materials Science and Engineering A

Materials Letters

Materials Science and Engineering A