Trans. Nonferrous Met. Soc. China 22(2012) 837-841

Growth kinetics for intermetallic compound layer between molten In-Sn alloy and CuZr-based bulk metallic glass

MA Guo-feng¹, ZHANG Bo², ZHANG Hai-feng², HU Zhuang-qi²

1. Key Laboratory of Advanced Materials Fabrication Technology of Liaoning Province,

Shenyang University, Shenyang 110044, China

2. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

Received 8 April 2011; accepted 17 November 2011

Abstract: The growth kinetics of intermetallic compound layer between molten In-Sn alloy and Cu₄₀Zr₄₄Al₈Ag₈ bulk metallic glass  substrate was examined by solid state isothermal aging at the temperature range between 333 and 393 K. The aged samples were characterized by scanning electron microscopy and energy dispersive spectrometry. It is found that the intermetallic compound layer is composed of Zr, Cu and Sn. The layer growth of the intermetallic compound is mainly controlled by a diffusion mechanism over the temperature range and the value of the time exponent is approximately 0.5. The apparent activation energy for the growth of total intermetallic compound layers is 98.35 kJ /mol calculated by the Arrhenius equation.

Key words: bulk metallic glass; compound layer; kinetics; diffusion mechanism

1 Introduction

Among many bonding techniques, soldering has become an important joining technology in the electronic industry and has been used widely in packaging processes [1,2]. The liquid alloy melt can react with substrates resulting in the formation of intermetallic compounds at the interface during soldering [3,4]. It is desirable to achieve a good metallurgical bond while forming a thin intermetallic compound layer. However, excessive intermetallic compounds growth may have a deleterious effect since the intermetallic compounds are brittle and hard in nature [5,6].

Bulk metallic glasses (BMGs) usually possess high strength, soft magnetic properties and good corrosion resistance, which are considered to have many potential applications as advanced engineering materials [7,8]. In order to adopt BMGs in a broader range of engineering applications, it is very important to establish appropriate joint processes for BMG/BMG and BMG/crystalline metal [9]. Until now, several joint processes have been attempted to join a BMG to a BMG and a BMG to a crystalline metal, and successful joints are obtained by some processes, such as electron-beam welding, friction welding and laser welding [10-13]. However, in the BMG joint, the most important issue is the reformation of glassy phase at high temperature area. Hence, the soldering process is studied to join BMGs in order to avoid the crystallization of the glassy phase. The soldering process between molten In-Sn alloy and Cu₄₀Zr₄₄Al₈Ag₈ BMG was investigated in previous study [14]. It is found that the intermetallic compound exists at the interface between the molten In-Sn alloy and Cu₄₀Zr₄₄Al₈Ag₈ BMG. It is well known that intermetallic compounds at the interface have significant effect on the performance of joints. Therefore, the information of the interfacial growth kinetics between molten alloys and BMGs is fundamentally important for the reliability evaluation of BMG joints.

In this work, the growth of intermetallic compound layer between molten In-Sn alloy and Cu₄₀Zr₄₄Al₈Ag₈ BMG was studied during solid state isothermal aging. The constants of the growth rate for intermetallic compounds were measured as a function of time and temperature, and the activation energy for intermetallic growth was calculated by the Arrhenius equation.

2 Experimental

The eutectic alloy with composition of 49.1Sn-50.9In (mass fraction, %) was prepared by induction melting in high purity argon atmosphere (purity>99.99%). Then the alloy was cut into pieces of about 50 mg. Cu₄₀Zr₄₄Al₈Ag₈ alloy was produced by arc melting under a Ti-gettered Ar atmosphere. The purity of all the elements was above 99.5%. Cu₄₀Zr₄₄Al₈Ag₈ alloys were remelted in a quartz tube by induction melting, followed by casting into copper moulds with plate cavity of 2 mm in thickness. Cu₄₀Zr₄₄Al₈Ag₈ BMG plates were cut into small substrates with dimensions of 20 mm×  20 mm and then polished. Before being subjected to the measurements, both substrates and pieces of In-Sn alloy were cleaned in acetone.

Soldering experiments were performed by sessile drop method in a high vacuum furnace at 473 K for    5 min. For the molten In-Sn alloy/Cu₄₀Zr₄₄Al₈Ag₈ BMG system, the isothermal aging was performed in a high vacuum furnace. The aging temperatures were 333, 353, 373 and 393 K and the time periods were 1-10 h in order to avoid crystallization. The solidified drops were sectioned and polished to examine the possible interfacial reactions using high resolution scanning electron microscopy (HRSEM). The intermetallic phase was analyzed by energy dispersive X-ray spectra (EDS). Thickness measurement of the intermetallic layer was performed by using image analysis software.

3 Results and discussion

Figure 1 shows the HRSEM micrographs of the molten In-Sn alloy/Cu₄₀Zr₄₄Al₈Ag₈ BMG reaction couples after aging for 6 h at various temperatures (333- 393 K). It is found that the intermetallic compound exists at the interface between In-Sn alloy and Cu₄₀Zr₄₄Al₈Ag₈ BMG, as marked by the arrows. The thickness of the intermetallic compound increases with the increase of aging temperature. The cross-section HRSEM image and EDS analysis after aging at 373 K for 10 h are shown in Fig. 2. It is seen that the reaction layer consists of Sn, Zr and Cu by the EDS analysis. Previous XRD results showed that the reactive products were Zr₃Sn, CuZr and InSn compounds, respectively [14].

Figure 3 shows the HRSEM micrographs of the molten In-Sn alloy/Cu₄₀Zr₄₄Al₈Ag₈ BMG reaction couples aged at 393 K for different aging time. The thickness of the intermetallic compound layer increases with the increase of aging time. The intermetallic thickness is approximately 3.5 μm when aged at 393 K for 10 h.

Fig. 1 HRSEM micrographs of In-Sn alloy/Cu₄₀Zr₄₄Al₈Ag₈ BMG interface after aging for 6 h at different temperatures: (a) 333 K;  (b) 353 K; (c) 373 K; (d) 393 K

Fig. 2 Cross-section HRSEM image (a) and EDS analysis (b) of In-Sn alloy/Cu₄₀Zr₄₄Al₈Ag₈ BMG interface after aging at 373 K for 10 h

Fig. 3 HRSEM micrographs of In-Sn alloy/Cu₄₀Zr₄₄Al₈Ag₈ BMG interface aged at 373 K for different aging time: (a) 1 h; (b) 4 h; (c) 8 h; (d) 10 h

Generally, the thickness of a reaction layer in the diffusion couples is expressed by the simple parabolic equation [15]:

d=ktⁿ                                     (1)

where d is the thickness of the reaction layer; k is the growth rate constant; n is the time exponent; t is the reaction time.

The thickness of the intermetallic compound layer as a function of the square root of time for each aging temperature is shown in Fig. 4. These intermetallic thicknesses were measured at every interval of time. The mean thickness of the intermetallic compound is found to increase linearly with the square root of aging time and the growth is faster at higher aging temperature.

The atomic diffusion of Cu, Zr and Sn through the intermetallic compound layer is the controlling process for the intermetallic compound growth during aging. If the growth process is controlled by diffusion mechanism, the thickness increase of the intermetallic compound layer after aging should follow the square root time law, d=kt^0.5 . It is empirically found that n takes the value of 0.5 when the reaction is mainly controlled by diffusion mechanism. From Fig. 4, it can be seen that the growth of intermetallic compounds follows a parabolic law, implying that the growth of the intermetallic layer is diffusion-controlled.

Fig. 4 Thickness of IMC layer vs square root of reaction time at different temperature

The growth rate constant is calculated from a linear 0.5 regression analysis of d versus t, where the slope is k. The calculated growth rate constant k ranges from 2.3×10^-19 to 1.47×10^{-18 m/s1/2}. Most of the linear correlation coefficient values R for these plots are greater than 0.98. This confirms that the growth of the intermetallic compound layers is diffusion-controlled over the temperature range studied.

To evaluate the time exponent, the growth kinetics at each temperature can be represented as [15]:

Y=Atⁿ+B                                    (2)

where Y is the layer thickness; t is reaction time; n is the time exponent; B is the layer thickness at t=0; A is a constant. This equation is converted into a logarithmic expression as:

ln (Y–B)=nln t+ln A                           (3)

The time exponent n can be obtained from the slope of the ln(Y-B) versus ln t. The time exponent is determined by the linear regression analysis of each aging temperature using Eq. (3). The values of the exponents are around 0.53, 0.437, 0.51 and 0.56 at 333, 353, 373 and 393 K, respectively. The diffusion processes appear to be largely responsible for the growth of the intermetallic compound layer.

It could be deduced that the following interfacial reactions between the In-Sn alloy and Cu₄₀Zr₄₄Al₈Ag₈ BMG probably occur as [14]:

3Zr+Sn?Zr₃Sn                             (4)

10Cu+7Zr?Cu₁₀Zr₇                          (5)

Here, the liquid/solid interfacial reaction is only analyzed by using the standpoint of atomic diffusivity. Usually, the atomic diffusivity is made up of two parts in the growth process of the intermetallic compound layer. For the molten In-Sn alloy/Cu₄₀Zr₄₄Al₈Ag₈ BMG system, firstly Sn diffusion in Cu₄₀Zr₄₄Al₈Ag₈ BMG is restrained because of no grain-boundaries nor interfaces in the amorphous alloy. Secondly, Zr and Cu diffusion in molten In-Sn alloy is also affected because of the interfacial crystallization reaction of the molten In-Sn alloy/Cu₄₀Zr₄₄Al₈Ag₈ BMG, as shown in Eq. (5). Hence, the layer growth of the intermetallic compound is mainly controlled by atomic (Sn, Zr and Cu) diffusion.

The following simple Arrhenius relationship is used to determine the activation energy for the total intermetallic compound as:

k=k₀exp(-Q/RT)                          (6)

where k is the square of growth rate constant; k₀ is the frequency factor; Q is the activation energy; R is the gas constant; T is the aging temperature. The activation energy is calculated from the slope of the Arrhenius plot using a linear regression model. Figure 5 shows the Arrhenius plots for the growth of the total intermetallic compound Cu-Zr-Sn. The apparent activation energy calculated for the growth of the intermetallic compound is 98.35 kJ/mol. The thickness of a reaction layer in the diffusion couples can be expressed as:

        (7)

Fig. 5 Arrhenius plot of Cu-Zr-Sn intermetallic compound growth

4 Conclusions

1) In the solder joints between the molten In-Sn alloy and the Cu₄₀Zr₄₄Al₈Ag₈ BMG, the intermetallic compound layer is composed of Zr, Cu and Sn.

2) The growth of the intermetallic layer is controlled by a diffusion mechanism over the temperature range because the value of the time exponent n is approximately 0.5.

3) The thickness of intermetallic compound layer reaches 3.5 μm after solid state isothermal aging at   393 K for 10 h.

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In-Sn熔体合金与CuZr基块状非晶合金界面生长动力学

马国峰¹，张  波²，张海峰²，胡壮麒²

1. 沈阳大学 辽宁省先进材料制备技术重点实验室，沈阳 110044；

2. 中国科学院 金属研究所，沈阳 110016

摘  要：研究In-Sn熔体合金与Cu₄₀Zr₄₄Al₈Ag₈块状非晶合金的界面生长动力学。通过扫描电镜和能谱对时效的样品进行分析，发现界面层由Zr、Cu和Sn组成。在时效温度区间，扩散机制是反应速度的控制步骤，且时间指数值近似为0.5。计算得到的反应激活能为98.35 kJ/mol。

关键词：块状非晶；界面层；动力学；扩散机制

 (Edited by FANG Jing-hua)

Foundation item: Project (2011CB606301) supported by the National Basic Research Program of China; Project (20212339) supported by the Doctor Startup Foundation Program of Shenyang University, China

Corresponding author: ZHANG Hai-feng; Tel: +86-24-23971785; Fax: +86-24-23891320; E-mail: hfzhang@imr.ac.cn

DOI: 10.1016/S1003-6326(11)61253-3