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Oxidation behavior of Sn–Zn solders under high-temperature and high-humidity conditions
Journal of Alloys and Compounds 462 (2008) 244–251
Oxidation behavior of Sn–Zn solders under high-temperature and high-humidity conditions Junxiang Jiang a,b,∗ , Jae-Ean Lee c , Keun-Soo Kim b , Katsuaki Suganuma b a
National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Science, 500 Yu Tian Road, Shanghai 200083, PR China b Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki 567-0047, Japan c Department of Adaptive Machine Systems, Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871, Japan Received 20 June 2007; received in revised form 28 July 2007; accepted 2 August 2007 Available online 8 August 2007
Abstract The oxidation behavior of Sn–9Zn and Sn–8Zn–xBi (x = 1, 3, 6) solders under high-temperature and high-humidity conditions has been investigated as a function of exposure time. The evolution of surface and cross-sectional microstructures during exposure has been examined, and an oxidation model for the Sn–Zn alloys was proposed. The poor oxidation resistance of Sn–Zn alloys is attributed to the oxidation of element Zn, which diffuses to the grain boundaries of Sn matrix, forming ZnO along those boundaries. It was also found that Bi forms a solid solution in Sn with a solubility limit of 4.4 wt.%, which causes the Sn matrix to become liable to form cracks, as well as to have a high distortion energy. The solid solution of Sn(Bi) is considered to be one of the reasons for the rapid oxidation of Bi added alloys. © 2007 Elsevier B.V. All rights reserved. Keywords: Metals and alloys; Oxidation; Microstructure; Scanning electron microscopy; SEM
1. Introduction Many lead-free alloys have been studied in recent years as candidates to replace conventional lead-bearing solders, due to increasing environmental and health concerns about lead toxicity [1]. Even though Sn–Ag–Cu has already been established as the standard lead-free solder, there is still a strong requirement for low melting temperature solders. Among alternative leadfree alloys, Sn–Zn eutectic alloy appears to be attractive as a low-temperature solder since its binary eutectic melting temperature (199 ◦ C) is close to that of Sn–Pb eutectic solder (183 ◦ C), which indicates the possibility of using the conventional production infrastructures and electronic components with minor, or even no, modification. Two other great advantages of the Sn–Zn alloy system are its low cost and excellent mechanical properties [2–5]. Hence, a variety of research work has been carried out on this system [2–21].
∗
Corresponding author. Tel.: +86 21 65362990; fax: +86 21 65362911. E-mail address: [email protected] (J. Jiang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.08.007
In spite of the attractive features of Sn–Zn alloys, adoption of these alloys has been quite limited in a number of industries. Practically, there are still some problems that must be solved before wide-spread application can occur. Poor oxidation resistance and poor compatibility with Cu substrate under high-temperature conditions are two of the major problems. A great deal of effort has been devoted to choosing appropriate alloying elements [8–12] and flux [13] to improve the wettability. Of these, alloying some amount of Bi to Sn–Zn near the eutectic composition, e.g. Sn–8Zn–3Bi, was found to provide better wetting properties and a slight reduction of the melting temperature [4,14]. However, Bi added alloys were also found to be rather susceptible to oxidation and corrosion. In any aspect of packaging technology, the reliability of the solder joints plays an important role in determining the lifetime of electronic devices, and this should be investigated under high-temperature and high-humidity conditions, especially for alloys susceptible to oxidation. Although, many reliability tests have been performed on the mechanical properties of the solder joints by considering the interfacial reaction between Sn–Zn solders and various surface finish layers during reflow or
J. Jiang et al. / Journal of Alloys and Compounds 462 (2008) 244–251
high-temperature aging [15–19], there is little research [20,21] focusing on the oxidation behavior of Sn–Zn solders under high-temperature and high-humidity conditions, and they are not sufficient for evaluating the reliability of the solder joints. A systematic investigation is necessary to understand the oxidation behavior of Sn–Zn alloys. This work is devoted to investigating the oxidation behavior of Sn–Zn low-temperature solders under high-temperature and high-humidity conditions and to clarify their oxidation mechanism, and further, to determine the effect of Bi addition on promoting the oxidation of Sn–Zn alloys. 2. Experimental procedures Sn–Zn alloys provided by Senju Metal Co., Ltd. (Tokyo, Japan), with nominal compositions of Sn–9 wt.% Zn, Sn–8 wt.%Zn–1 wt.% Bi, Sn–8 wt.% Zn–3 wt.% Bi and Sn–8 wt.% Zn–6 wt.% Bi (hereafter, ‘wt.%’ is omitted) were
245
used in the present work. The alloy ingots were remelted at 300 ◦ C for 30 min, and then cast into a steel mold in air. Samples 10 mm × 10 mm × 2.5 mm in size were sliced out of the alloys for high-temperature and high-humidity exposure and were polished with 0.05 m Al2 O3 powder. Polished samples were exposed to a constant temperature of 85 ◦ C with relative humidities (RH) of 43% RH, 64% RH or 85% RH, or to a constant absolute humidity of 149 g/m3 , produced by using 65 ◦ C/93% RH, 75 ◦ C/62% RH or 85 ◦ C/43% RH, for 100, 250, 500, 750 or 1000 h in environmental testing equipment (SH-240, ESPEC Co., Ltd.). The microstructure evolution during high-temperature and high-humidity exposure was observed with a scanning electron microscope (SEM, JEOL, JSM-5510S) and the phase evolution was analyzed by electron probe microanalysis (EPMA, JEOL, JXA-8800R) and energy dispersive X-ray spectrometry (EDS, JEOL, JED-2300CL). X-ray diffraction (XRD, RIGAKU, RU-200) analysis and electron backscattering diffraction (EBSD, JEOL, JOIM) analysis were also carried out to identify the phases. The lattice parameters of the Sn matrix of Sn–Zn alloys were measured by XRD using Si powder as an internal standard. The measurement was performed at 25 ◦ C using graphite-monochromated Cu K␣1 radiation at 50 kV and 150 mA with a measurement range of 2θ = 75–140◦ , a scan speed of 0.5◦ /min and a sampling step of 0.002◦ .
Fig. 1. SEM images of Sn–Zn alloys before (as-cast) and after 85 ◦ C/85% RH 1000 h exposure (surface and cross-section).
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3. Results and discussion 3.1. Microstructure evolution during high-temperature and high-humidity exposure The microstructures of the as-cast Sn–Zn alloys are shown in Fig. 1. Two types of Zn morphologies, primary Zn and eutectic Zn, exist in the Sn matrix of all alloys, although their compositions are very close to the eutectic point according to the Sn–Zn equilibrium diagram [22]. The primary and eutectic Zn becomes coarser as the Bi content increases, especially in Sn–8Zn–3Bi and Sn–8Zn–6Bi. The Bi phase begins to segregate as white particles in Sn–8Zn–6Bi, implying that the Bi content exceeds its solubility limit. These results are in accordance with those of XRD analysis as shown in Fig. 2a, which confirms that only Sn and Zn phases are present in the as-cast alloys, except in Sn–8Zn–6Bi, where a Bi phase is also present. An example of the surface morphologies of Sn–Zn alloys after 85 ◦ C/85% RH 1000 h exposure is also shown in Fig. 1. The Zn phase has almost disappeared from the surface and many dark grey speckles have formed. Many flat extrusions formed on the Sn–9Zn surface, and few whiskers were observed. However, ball-like extrusions formed on the surface of Bi added
alloys, and their amount increased with increasing Bi content. An EPMA analysis performed on the surface of Sn–9Zn alloy after 85 ◦ C/85% RH 100 h exposure is shown in Fig. 3. The element mappings suggest that the dark grey speckles are composed of the elements Zn and O, implying that the Zn on the surface oxidized after 100 h of exposure. Fig. 2b presents the results of XRD analysis carried out on the surface of Sn–Zn alloys after 85 ◦ C/85% RH 100 h exposure. It proves that there is no Zn phase remaining and ZnO peaks are detected. Fig. 1 also shows an example of the cross-sectional morphologies of Sn–Zn alloys after 85 ◦ C/85% RH 1000 h exposure. A different layer from that in the as-cast morphology formed near the free surface of all samples, where the Zn phase disappeared and island-like dark grey phases and cracks formed. The thickness of this layer increased sharply with an increasing Bi content. An EBSD analysis was carried out on the crosssection of Sn–8Zn–1Bi alloy, which is marked as broken lines in Fig. 4a. EBSD analysis provides a grain mapping of the Sn matrix in Fig. 4b, as well as a phase mapping of ZnO in Fig. 4c. It is clearly seen that the island-like dark grey phases are ZnO, and they are not distributed in Sn grains as the eutectic Zn phase, but on the grain boundaries of the Sn matrix. This is always accompanied with a Zn-depleted layer between Zn particles and ZnO islands, as seen in cross-sectional images. These results elucidate that Zn massively diffuses to Sn grain boundaries and forms oxides along the grain boundaries during the progress of oxidation. 3.2. Characteristics of oxidation of Sn–Zn alloys
Fig. 2. XRD patterns of Sn–Zn alloys. (a) as-cast, (b) after 85 ◦ C/85% RH 100 h exposure. (1) Sn–9Zn, (2) Sn–8Zn–1Bi, (3) Sn–8Zn–3Bi, (4) Sn–8Zn–6Bi.
3.2.1. Diffusion of Zn in Sn From the above results and analyses, it can be concluded that the oxidation of Sn–Zn alloys is due to the oxidation of Zn, which exhibits strong segregation to the Sn grain boundaries to form oxides. Fast diffusion of Zn or O along the Sn grain boundaries is expected to dominate the oxidation process. However, there was no reference data for diffusion coefficients obtained at the corresponding exposure temperatures. A sandwich type sheet of Sn/Zn/Sn layers (Fig. 5a) was prepared by cold rolling to investigate the diffusion behavior of Zn. The surface and cross-sectional images of the rolled sheet after 85 ◦ C/85% RH 50 h exposure are shown in Fig. 5b and c, respectively. The dark grey morphology along the triangular grain boundary on the surface of the rolled sheet was identified as being composed of Zn and O by EDS element analysis, which implies that Zn diffuses onto the free surface along the grain boundaries of Sn. The cross-sectional image also shows that oxidation proceeds along the Sn grain boundaries, and the oxide ZnO forms on the free surface and extends deep inside, reaching the Zn layer after 50 h of exposure. Fast diffusion of Zn in Sn has also been observed in tin finish plated on brass substrate, in which the element Zn was found to diffuse to the tin-plating surface under ambient preservation [23]. It can be considered that, at the initial stage of oxidation of Sn–Zn alloys, the oxidation progresses with the fast diffusion of Zn to the free surface along the Sn grain boundaries, not the diffusion of oxygen into the alloy.
J. Jiang et al. / Journal of Alloys and Compounds 462 (2008) 244–251
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Fig. 3. EPMA of the surface of Sn–9Zn alloy after 85 ◦ C/85% RH 100 h exposure.
3.2.2. Model of oxidation of Sn–Zn alloys Fig. 6 gives a schematic diagram of the oxidation process of Sn–Zn alloys. As the samples are exposed to a high-temperature and high-humidity atmosphere, the Zn on the free surface will soon react with water vapor or oxygen and form ZnO on the free surface with speckle morphologies as shown in Figs. 1 and 3. At the same time, Zn will also diffuse along the Sn grain boundaries to the free surface and form oxides near the free surface because of its fast diffusion along the Sn grain boundaries. It is known
that the phase transformation from Zn to ZnO is accompanied by a volume expansion. As the oxidation near the free surface progresses, the stress induced by the volume expansion of ZnO, especially along the Sn grain boundaries, increases, which causes cracks to occur along those boundaries (Fig. 6a). The water vapor and oxygen will enter the alloy along these cracks, which reduces the diffusion distance of Zn to the grain boundaries instead of to the free surface (Fig. 6b). Thus, the oxidation will be accelerated and new ZnO will form along the cracks. The
Fig. 4. EBSD analysis of Sn–8Zn–1Bi alloy after 85 ◦ C/85% RH 1000 h exposure. (a) SEM image, (b) grain map of Sn, (c) phase map of ZnO.
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Fig. 6. Schematic diagrams of the oxidation of Sn–Zn alloys. (a) ZnO formed on the Sn grain boundaries near free surface causes crack to occur, (b) water vapor and oxygen come into alloys along cracks and Zn diffuses to cracks forming ZnO, (c) ZnO along the cracks accumulates and causes new cracks to occur.
3.3. Oxidation rate of Sn–Zn alloys
Fig. 5. SEM images of Sn/Zn/Sn sheet. (a) cross-section of the as-rolled, (b) surface and (c) cross-section after 85 ◦ C/85% RH 50 h exposure.
crack tip acts as a new stress-raiser as the oxides along cracks accumulate. As the stress at the crack tip increases, the crack will proceed along the Sn grain boundaries, which allows the water vapor and oxygen to enter even further (Fig. 6c). Here, the oxidation behavior of Sn–Zn alloys can be described as a phenomenon in which oxide formation and crack occurrence proceed alternately and periodically along the Sn grain boundaries.
3.3.1. Oxidized layer thickness and its time dependence The oxidized layer thickness is a mean value obtained for each condition from five maximum thicknesses, which were measured from the free surface to the deepest point. Fig. 7 shows two examples of the time dependence of the oxidized layer thickness for Sn–Zn alloys exposed to 85 ◦ C/43% RH and 85 ◦ C/85% RH atmospheres, respectively. The oxidized layer thickness of all alloys is proportional to the 1/2 order of exposure time over the whole range for 85 ◦ C/43% RH exposure (Fig. 7a), as well as for 75 ◦ C/62% RH and 65 ◦ C/93% RH conditions. This implies that the oxidation of Sn–Zn alloys is controlled by a diffusion process [24]. However, in the case of
J. Jiang et al. / Journal of Alloys and Compounds 462 (2008) 244–251
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Fig. 8. Arrhenius plot of ln(kp ) vs. 1/T for Sn–Zn alloys in the exposures of an absolute humidity of 149 g/m3 . The straight line represents a linear fitting for each alloy.
Fig. 7. Typical relationship between exposure time and oxidized layer thickness under exposure conditions of (a) 85 ◦ C/43% RH and (b) 85 ◦ C/85% RH. The straight line in (a) presents a linear fitting for each alloy and that in (b) represents a best fitting.
higher humidity conditions of 85 ◦ C/64% RH and 85 ◦ C/85% RH, the Sn–9Zn alloy presents a linear line as shown in Fig. 7b, while a rapid increase in the oxidized layer thickness is observed for the Bi added alloys after 500 h of exposure. This rapid increase is attributed to the number of cracks occurring from the free surface, as the cross-sectional images show in Fig. 1, and the reason for this will be discussed in Section 3.3.4. 3.3.2. Effect of temperature on oxidized layer thickness For the three exposure conditions at the same absolute humidity of 149 g/m3 , 85 ◦ C/43% RH, 75 ◦ C/62% RH and 65 ◦ C/93% RH, the relationship between the oxidized layer thickness (d) and the exposure time (t) can be expressed as [24] d = (kp t)1/2
presents the Arrhenius plot of the oxidation rate constant ln(kp ) vs. 1/T, fitted linearly on the basis of least-squares regression. The activation energy obtained from the slope of the straight line is 72 kJ/mol for the oxidation of Sn–9Zn alloy, while a value of 53 kJ/mol was obtained for Bi added alloys. Since the oxidation of Sn–Zn alloys is controlled by the diffusion of Zn and crack formation, the activation energy obtained is considered to be related to the two processes. The reason why Bi added alloys have a decreased activation energy will also be discussed in Section 3.3.4. 3.3.3. Effect of relative humidity on oxidized layer thickness The oxidized layer thickness is presented in Fig. 9 as a function of relative humidity for the Sn–Zn alloys exposed at 85 ◦ C for 1000 h. The oxidized layer thickness of Sn–9Zn alloy increases with increasing relative humidity, while that of Bi added alloys has a peak at 64% RH. The Zn in Sn–Zn alloys can be oxidized by two routes when they are exposed to high-temperature and high-humidity conditions [25]. One is the reaction: Zn + H2 O → Zn(OH)2 + 21 H2
(3-1)
(1)
where kp is a constant representing the oxidation rate and is assumed to have an Arrhenius temperature dependence, kp = A exp(−Q/RT )
(2)
where A is the pre-exponential term representing the frequency factor, Q is the activation energy, R is the gas constant and T is the temperature in Kelvin. The oxidation rate constant kp was obtained from the slope of the straight line as presented in Fig. 7a, which was calculated for each alloy using a linear fit according to Eq. (1). Fig. 8
Fig. 9. Relative humidity dependence of oxidized layer thickness for Sn–Zn alloys exposed at 85 ◦ C for 1000 h.
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accompanied by the subsequent decomposition Zn(OH)2 → ZnO + H2
(3-2)
and the other is a direct reaction between Zn and O2 , Zn + 21 O2 → ZnO
(4)
The reaction rates of the two routes are controlled by the partial pressure of water vapor (pH2 O) and that of oxygen (pO2 ), respectively. However, the oxidation rate of Sn–9Zn alloy only increases with an increasing relative humidity, and appears to be independent of the partial pressure ratio of pH2 O/pO2 . On the other hand, the Bi added alloys seem to be dependent on the partial pressure ratio, and their total oxidation rate shows a peak at 64% RH, where the ratio pH2 O/pO2 has a value of 2.76. Bi addition is expected to change the susceptibilities of Zn to water vapor and oxygen, however, this has not been investigated yet and will be studied in further work. 3.3.4. Effect of Bi addition on the oxidized layer thickness Fig. 10 shows the oxidized layer thickness of Sn–Zn alloys with different Bi contents after 1000 h exposure to identified conditions. It was found that the oxidized layer thickness increases with increasing Bi content even with only 1 wt.% addition, particularly under higher humidity conditions. As mentioned in Section 3.1, the added Bi causes the primary and eutectic Zn to become coarser. This can make the Bi added alloys be apt to form cracks because the Bi phases are brittle. To clarify the effect of Bi, the lattice parameters of the Sn matrix were measured by XRD using silicon for internal calibration. As seen in Fig. 11, both lattice parameters a and c increase with an increasing Bi content and are saturated at a Bi content of 4.4 wt.%. It is known that Bi can dissolve into Sn, forming a solid solution, which can then make the Sn matrix harden and become likely to form cracks. Furthermore, a high distortion energy is expected in the Sn lattice, because the Sn lattice has expanded due to the solid solution formation. The high distortion energy of the Sn matrix can promote Zn diffusion [26]. These facts may result in the decrease of the measured activation energy for the oxidation of Bi added alloys.
Fig. 11. Lattice parameters of Sn matrix of Sn–Zn alloys with different Bi contents.
4. Conclusions The oxidation behavior of Sn–Zn alloys has been investigated under high-temperature and high-humidity conditions. The oxidation of Sn–Zn alloys is attributed to the oxidation of Zn, which diffuses to the Sn grain boundaries and forms ZnO. These oxides are liable to give rise to cracks along the Sn grain boundaries, especially in Bi added alloys. The oxidation of Sn–Zn alloys is controlled by the diffusion and crack formation processes. The oxidized layer thickness of all alloys was found to be in proportion to the 1/2 order of exposure time when the absolute humidity was 149 g/m3 , while an accelerated oxidation rate was observed for Bi added alloys when they were exposed to higher humidity conditions. The effects of temperature and relative humidity on the oxidation rate of Sn–Zn alloys were investigated, as well as the effect of the Bi content. It was found that the activation energy for the oxidation of Bi added alloys is lower than that of Sn–9Zn alloy. The oxidation rate of Sn–9Zn alloy increases with increasing relative humidity (RH) for the exposure at 85 ◦ C, while that of Bi added alloys exhibits a peak at a relative humidity of 64% RH. The oxidized layer thickness was found to increase with increasing Bi content, even with 1 wt.% addition. The solid solution of Sn(Bi), which causes the Sn matrix to become liable to form cracks and promotes the diffusion of Zn, is considered to be one of the reasons for the rapid oxidation of Bi added alloys. Acknowledgements This work was carried out under the both supports of the 21COE program of the Japan Ministry of Education, Culture, Sports, Science and Technology, and of METI (Ministry of Economy, Trade and Industry) Low Temperature Soldering Project organized by JEITA. References
Fig. 10. Oxidized layer thickness of Sn–Zn alloys with different Bi contents after 1000 h of exposure under identified atmospheres.
[1] K. Suganuma, Curr. Opin. Solid State Mater. Sci. 5 (2001) 55–64. [2] M. McCormack, S. Jin, JOM 45 (1993) 36–40. [3] K. Suganuma, K. Niihara, T. Shoutoku, Y. Nakamura, J. Mater. Res. 13 (1998) 2859–2865.
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[17] L.L. Duan, D.Q. Yu, S.Q. Han, H.T. Ma, L. Wang, J. Alloys Compd. 381 (2004) 202–207. [18] A. Sharif, Y.C. Chan, Microelectron Eng. 84 (2007) 328–335. [19] J.W. Yoon, S.B. Jung, J. Mater. Res. 21 (2006) 1590–1599. [20] K.S. Kim, T. Matsuura, K. Suganuma, J. Electron. Mater. 35 (2006) 41–47. [21] K. Suganuma, K.S. Kim, J. Mater. Sci. Mater. Electron. 18 (2007) 121–127. [22] Z. Moser, J. Dutkiewicz, W. Gasior, J. Salawa, Binary Alloy Phase Diagrams, ASM International, 1992. [23] T.A. Woodrow, Proceeding of the SMTA International Conference PC Rosemont, IL, 2006, pp. 24–28. [24] H.H. Uhlig, R.W. Revie, Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, Third ed., John Wiley & Sons. Inc., New York, 1985. [25] C.J. Slunder, W.K. Boyd, Zinc: Its Corrosion Resistance, International Lead Zinc Research Organization Inc., New York, 1983. [26] S. Mrowec, Defects and Diffusion in Solids: An Introduction, Elsevier Scientific, New York, 1980.
Oxidation behavior of Sn–Zn solders under high-temperature and high-humidity conditions Junxiang Jiang a,b,∗ , Jae-Ean Lee c , Keun-Soo Kim b , Katsuaki Suganuma b a
National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Science, 500 Yu Tian Road, Shanghai 200083, PR China b Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki 567-0047, Japan c Department of Adaptive Machine Systems, Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871, Japan Received 20 June 2007; received in revised form 28 July 2007; accepted 2 August 2007 Available online 8 August 2007
Abstract The oxidation behavior of Sn–9Zn and Sn–8Zn–xBi (x = 1, 3, 6) solders under high-temperature and high-humidity conditions has been investigated as a function of exposure time. The evolution of surface and cross-sectional microstructures during exposure has been examined, and an oxidation model for the Sn–Zn alloys was proposed. The poor oxidation resistance of Sn–Zn alloys is attributed to the oxidation of element Zn, which diffuses to the grain boundaries of Sn matrix, forming ZnO along those boundaries. It was also found that Bi forms a solid solution in Sn with a solubility limit of 4.4 wt.%, which causes the Sn matrix to become liable to form cracks, as well as to have a high distortion energy. The solid solution of Sn(Bi) is considered to be one of the reasons for the rapid oxidation of Bi added alloys. © 2007 Elsevier B.V. All rights reserved. Keywords: Metals and alloys; Oxidation; Microstructure; Scanning electron microscopy; SEM
1. Introduction Many lead-free alloys have been studied in recent years as candidates to replace conventional lead-bearing solders, due to increasing environmental and health concerns about lead toxicity [1]. Even though Sn–Ag–Cu has already been established as the standard lead-free solder, there is still a strong requirement for low melting temperature solders. Among alternative leadfree alloys, Sn–Zn eutectic alloy appears to be attractive as a low-temperature solder since its binary eutectic melting temperature (199 ◦ C) is close to that of Sn–Pb eutectic solder (183 ◦ C), which indicates the possibility of using the conventional production infrastructures and electronic components with minor, or even no, modification. Two other great advantages of the Sn–Zn alloy system are its low cost and excellent mechanical properties [2–5]. Hence, a variety of research work has been carried out on this system [2–21].
∗
Corresponding author. Tel.: +86 21 65362990; fax: +86 21 65362911. E-mail address: [email protected] (J. Jiang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.08.007
In spite of the attractive features of Sn–Zn alloys, adoption of these alloys has been quite limited in a number of industries. Practically, there are still some problems that must be solved before wide-spread application can occur. Poor oxidation resistance and poor compatibility with Cu substrate under high-temperature conditions are two of the major problems. A great deal of effort has been devoted to choosing appropriate alloying elements [8–12] and flux [13] to improve the wettability. Of these, alloying some amount of Bi to Sn–Zn near the eutectic composition, e.g. Sn–8Zn–3Bi, was found to provide better wetting properties and a slight reduction of the melting temperature [4,14]. However, Bi added alloys were also found to be rather susceptible to oxidation and corrosion. In any aspect of packaging technology, the reliability of the solder joints plays an important role in determining the lifetime of electronic devices, and this should be investigated under high-temperature and high-humidity conditions, especially for alloys susceptible to oxidation. Although, many reliability tests have been performed on the mechanical properties of the solder joints by considering the interfacial reaction between Sn–Zn solders and various surface finish layers during reflow or
J. Jiang et al. / Journal of Alloys and Compounds 462 (2008) 244–251
high-temperature aging [15–19], there is little research [20,21] focusing on the oxidation behavior of Sn–Zn solders under high-temperature and high-humidity conditions, and they are not sufficient for evaluating the reliability of the solder joints. A systematic investigation is necessary to understand the oxidation behavior of Sn–Zn alloys. This work is devoted to investigating the oxidation behavior of Sn–Zn low-temperature solders under high-temperature and high-humidity conditions and to clarify their oxidation mechanism, and further, to determine the effect of Bi addition on promoting the oxidation of Sn–Zn alloys. 2. Experimental procedures Sn–Zn alloys provided by Senju Metal Co., Ltd. (Tokyo, Japan), with nominal compositions of Sn–9 wt.% Zn, Sn–8 wt.%Zn–1 wt.% Bi, Sn–8 wt.% Zn–3 wt.% Bi and Sn–8 wt.% Zn–6 wt.% Bi (hereafter, ‘wt.%’ is omitted) were
245
used in the present work. The alloy ingots were remelted at 300 ◦ C for 30 min, and then cast into a steel mold in air. Samples 10 mm × 10 mm × 2.5 mm in size were sliced out of the alloys for high-temperature and high-humidity exposure and were polished with 0.05 m Al2 O3 powder. Polished samples were exposed to a constant temperature of 85 ◦ C with relative humidities (RH) of 43% RH, 64% RH or 85% RH, or to a constant absolute humidity of 149 g/m3 , produced by using 65 ◦ C/93% RH, 75 ◦ C/62% RH or 85 ◦ C/43% RH, for 100, 250, 500, 750 or 1000 h in environmental testing equipment (SH-240, ESPEC Co., Ltd.). The microstructure evolution during high-temperature and high-humidity exposure was observed with a scanning electron microscope (SEM, JEOL, JSM-5510S) and the phase evolution was analyzed by electron probe microanalysis (EPMA, JEOL, JXA-8800R) and energy dispersive X-ray spectrometry (EDS, JEOL, JED-2300CL). X-ray diffraction (XRD, RIGAKU, RU-200) analysis and electron backscattering diffraction (EBSD, JEOL, JOIM) analysis were also carried out to identify the phases. The lattice parameters of the Sn matrix of Sn–Zn alloys were measured by XRD using Si powder as an internal standard. The measurement was performed at 25 ◦ C using graphite-monochromated Cu K␣1 radiation at 50 kV and 150 mA with a measurement range of 2θ = 75–140◦ , a scan speed of 0.5◦ /min and a sampling step of 0.002◦ .
Fig. 1. SEM images of Sn–Zn alloys before (as-cast) and after 85 ◦ C/85% RH 1000 h exposure (surface and cross-section).
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3. Results and discussion 3.1. Microstructure evolution during high-temperature and high-humidity exposure The microstructures of the as-cast Sn–Zn alloys are shown in Fig. 1. Two types of Zn morphologies, primary Zn and eutectic Zn, exist in the Sn matrix of all alloys, although their compositions are very close to the eutectic point according to the Sn–Zn equilibrium diagram [22]. The primary and eutectic Zn becomes coarser as the Bi content increases, especially in Sn–8Zn–3Bi and Sn–8Zn–6Bi. The Bi phase begins to segregate as white particles in Sn–8Zn–6Bi, implying that the Bi content exceeds its solubility limit. These results are in accordance with those of XRD analysis as shown in Fig. 2a, which confirms that only Sn and Zn phases are present in the as-cast alloys, except in Sn–8Zn–6Bi, where a Bi phase is also present. An example of the surface morphologies of Sn–Zn alloys after 85 ◦ C/85% RH 1000 h exposure is also shown in Fig. 1. The Zn phase has almost disappeared from the surface and many dark grey speckles have formed. Many flat extrusions formed on the Sn–9Zn surface, and few whiskers were observed. However, ball-like extrusions formed on the surface of Bi added
alloys, and their amount increased with increasing Bi content. An EPMA analysis performed on the surface of Sn–9Zn alloy after 85 ◦ C/85% RH 100 h exposure is shown in Fig. 3. The element mappings suggest that the dark grey speckles are composed of the elements Zn and O, implying that the Zn on the surface oxidized after 100 h of exposure. Fig. 2b presents the results of XRD analysis carried out on the surface of Sn–Zn alloys after 85 ◦ C/85% RH 100 h exposure. It proves that there is no Zn phase remaining and ZnO peaks are detected. Fig. 1 also shows an example of the cross-sectional morphologies of Sn–Zn alloys after 85 ◦ C/85% RH 1000 h exposure. A different layer from that in the as-cast morphology formed near the free surface of all samples, where the Zn phase disappeared and island-like dark grey phases and cracks formed. The thickness of this layer increased sharply with an increasing Bi content. An EBSD analysis was carried out on the crosssection of Sn–8Zn–1Bi alloy, which is marked as broken lines in Fig. 4a. EBSD analysis provides a grain mapping of the Sn matrix in Fig. 4b, as well as a phase mapping of ZnO in Fig. 4c. It is clearly seen that the island-like dark grey phases are ZnO, and they are not distributed in Sn grains as the eutectic Zn phase, but on the grain boundaries of the Sn matrix. This is always accompanied with a Zn-depleted layer between Zn particles and ZnO islands, as seen in cross-sectional images. These results elucidate that Zn massively diffuses to Sn grain boundaries and forms oxides along the grain boundaries during the progress of oxidation. 3.2. Characteristics of oxidation of Sn–Zn alloys
Fig. 2. XRD patterns of Sn–Zn alloys. (a) as-cast, (b) after 85 ◦ C/85% RH 100 h exposure. (1) Sn–9Zn, (2) Sn–8Zn–1Bi, (3) Sn–8Zn–3Bi, (4) Sn–8Zn–6Bi.
3.2.1. Diffusion of Zn in Sn From the above results and analyses, it can be concluded that the oxidation of Sn–Zn alloys is due to the oxidation of Zn, which exhibits strong segregation to the Sn grain boundaries to form oxides. Fast diffusion of Zn or O along the Sn grain boundaries is expected to dominate the oxidation process. However, there was no reference data for diffusion coefficients obtained at the corresponding exposure temperatures. A sandwich type sheet of Sn/Zn/Sn layers (Fig. 5a) was prepared by cold rolling to investigate the diffusion behavior of Zn. The surface and cross-sectional images of the rolled sheet after 85 ◦ C/85% RH 50 h exposure are shown in Fig. 5b and c, respectively. The dark grey morphology along the triangular grain boundary on the surface of the rolled sheet was identified as being composed of Zn and O by EDS element analysis, which implies that Zn diffuses onto the free surface along the grain boundaries of Sn. The cross-sectional image also shows that oxidation proceeds along the Sn grain boundaries, and the oxide ZnO forms on the free surface and extends deep inside, reaching the Zn layer after 50 h of exposure. Fast diffusion of Zn in Sn has also been observed in tin finish plated on brass substrate, in which the element Zn was found to diffuse to the tin-plating surface under ambient preservation [23]. It can be considered that, at the initial stage of oxidation of Sn–Zn alloys, the oxidation progresses with the fast diffusion of Zn to the free surface along the Sn grain boundaries, not the diffusion of oxygen into the alloy.
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Fig. 3. EPMA of the surface of Sn–9Zn alloy after 85 ◦ C/85% RH 100 h exposure.
3.2.2. Model of oxidation of Sn–Zn alloys Fig. 6 gives a schematic diagram of the oxidation process of Sn–Zn alloys. As the samples are exposed to a high-temperature and high-humidity atmosphere, the Zn on the free surface will soon react with water vapor or oxygen and form ZnO on the free surface with speckle morphologies as shown in Figs. 1 and 3. At the same time, Zn will also diffuse along the Sn grain boundaries to the free surface and form oxides near the free surface because of its fast diffusion along the Sn grain boundaries. It is known
that the phase transformation from Zn to ZnO is accompanied by a volume expansion. As the oxidation near the free surface progresses, the stress induced by the volume expansion of ZnO, especially along the Sn grain boundaries, increases, which causes cracks to occur along those boundaries (Fig. 6a). The water vapor and oxygen will enter the alloy along these cracks, which reduces the diffusion distance of Zn to the grain boundaries instead of to the free surface (Fig. 6b). Thus, the oxidation will be accelerated and new ZnO will form along the cracks. The
Fig. 4. EBSD analysis of Sn–8Zn–1Bi alloy after 85 ◦ C/85% RH 1000 h exposure. (a) SEM image, (b) grain map of Sn, (c) phase map of ZnO.
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Fig. 6. Schematic diagrams of the oxidation of Sn–Zn alloys. (a) ZnO formed on the Sn grain boundaries near free surface causes crack to occur, (b) water vapor and oxygen come into alloys along cracks and Zn diffuses to cracks forming ZnO, (c) ZnO along the cracks accumulates and causes new cracks to occur.
3.3. Oxidation rate of Sn–Zn alloys
Fig. 5. SEM images of Sn/Zn/Sn sheet. (a) cross-section of the as-rolled, (b) surface and (c) cross-section after 85 ◦ C/85% RH 50 h exposure.
crack tip acts as a new stress-raiser as the oxides along cracks accumulate. As the stress at the crack tip increases, the crack will proceed along the Sn grain boundaries, which allows the water vapor and oxygen to enter even further (Fig. 6c). Here, the oxidation behavior of Sn–Zn alloys can be described as a phenomenon in which oxide formation and crack occurrence proceed alternately and periodically along the Sn grain boundaries.
3.3.1. Oxidized layer thickness and its time dependence The oxidized layer thickness is a mean value obtained for each condition from five maximum thicknesses, which were measured from the free surface to the deepest point. Fig. 7 shows two examples of the time dependence of the oxidized layer thickness for Sn–Zn alloys exposed to 85 ◦ C/43% RH and 85 ◦ C/85% RH atmospheres, respectively. The oxidized layer thickness of all alloys is proportional to the 1/2 order of exposure time over the whole range for 85 ◦ C/43% RH exposure (Fig. 7a), as well as for 75 ◦ C/62% RH and 65 ◦ C/93% RH conditions. This implies that the oxidation of Sn–Zn alloys is controlled by a diffusion process [24]. However, in the case of
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Fig. 8. Arrhenius plot of ln(kp ) vs. 1/T for Sn–Zn alloys in the exposures of an absolute humidity of 149 g/m3 . The straight line represents a linear fitting for each alloy.
Fig. 7. Typical relationship between exposure time and oxidized layer thickness under exposure conditions of (a) 85 ◦ C/43% RH and (b) 85 ◦ C/85% RH. The straight line in (a) presents a linear fitting for each alloy and that in (b) represents a best fitting.
higher humidity conditions of 85 ◦ C/64% RH and 85 ◦ C/85% RH, the Sn–9Zn alloy presents a linear line as shown in Fig. 7b, while a rapid increase in the oxidized layer thickness is observed for the Bi added alloys after 500 h of exposure. This rapid increase is attributed to the number of cracks occurring from the free surface, as the cross-sectional images show in Fig. 1, and the reason for this will be discussed in Section 3.3.4. 3.3.2. Effect of temperature on oxidized layer thickness For the three exposure conditions at the same absolute humidity of 149 g/m3 , 85 ◦ C/43% RH, 75 ◦ C/62% RH and 65 ◦ C/93% RH, the relationship between the oxidized layer thickness (d) and the exposure time (t) can be expressed as [24] d = (kp t)1/2
presents the Arrhenius plot of the oxidation rate constant ln(kp ) vs. 1/T, fitted linearly on the basis of least-squares regression. The activation energy obtained from the slope of the straight line is 72 kJ/mol for the oxidation of Sn–9Zn alloy, while a value of 53 kJ/mol was obtained for Bi added alloys. Since the oxidation of Sn–Zn alloys is controlled by the diffusion of Zn and crack formation, the activation energy obtained is considered to be related to the two processes. The reason why Bi added alloys have a decreased activation energy will also be discussed in Section 3.3.4. 3.3.3. Effect of relative humidity on oxidized layer thickness The oxidized layer thickness is presented in Fig. 9 as a function of relative humidity for the Sn–Zn alloys exposed at 85 ◦ C for 1000 h. The oxidized layer thickness of Sn–9Zn alloy increases with increasing relative humidity, while that of Bi added alloys has a peak at 64% RH. The Zn in Sn–Zn alloys can be oxidized by two routes when they are exposed to high-temperature and high-humidity conditions [25]. One is the reaction: Zn + H2 O → Zn(OH)2 + 21 H2
(3-1)
(1)
where kp is a constant representing the oxidation rate and is assumed to have an Arrhenius temperature dependence, kp = A exp(−Q/RT )
(2)
where A is the pre-exponential term representing the frequency factor, Q is the activation energy, R is the gas constant and T is the temperature in Kelvin. The oxidation rate constant kp was obtained from the slope of the straight line as presented in Fig. 7a, which was calculated for each alloy using a linear fit according to Eq. (1). Fig. 8
Fig. 9. Relative humidity dependence of oxidized layer thickness for Sn–Zn alloys exposed at 85 ◦ C for 1000 h.
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accompanied by the subsequent decomposition Zn(OH)2 → ZnO + H2
(3-2)
and the other is a direct reaction between Zn and O2 , Zn + 21 O2 → ZnO
(4)
The reaction rates of the two routes are controlled by the partial pressure of water vapor (pH2 O) and that of oxygen (pO2 ), respectively. However, the oxidation rate of Sn–9Zn alloy only increases with an increasing relative humidity, and appears to be independent of the partial pressure ratio of pH2 O/pO2 . On the other hand, the Bi added alloys seem to be dependent on the partial pressure ratio, and their total oxidation rate shows a peak at 64% RH, where the ratio pH2 O/pO2 has a value of 2.76. Bi addition is expected to change the susceptibilities of Zn to water vapor and oxygen, however, this has not been investigated yet and will be studied in further work. 3.3.4. Effect of Bi addition on the oxidized layer thickness Fig. 10 shows the oxidized layer thickness of Sn–Zn alloys with different Bi contents after 1000 h exposure to identified conditions. It was found that the oxidized layer thickness increases with increasing Bi content even with only 1 wt.% addition, particularly under higher humidity conditions. As mentioned in Section 3.1, the added Bi causes the primary and eutectic Zn to become coarser. This can make the Bi added alloys be apt to form cracks because the Bi phases are brittle. To clarify the effect of Bi, the lattice parameters of the Sn matrix were measured by XRD using silicon for internal calibration. As seen in Fig. 11, both lattice parameters a and c increase with an increasing Bi content and are saturated at a Bi content of 4.4 wt.%. It is known that Bi can dissolve into Sn, forming a solid solution, which can then make the Sn matrix harden and become likely to form cracks. Furthermore, a high distortion energy is expected in the Sn lattice, because the Sn lattice has expanded due to the solid solution formation. The high distortion energy of the Sn matrix can promote Zn diffusion [26]. These facts may result in the decrease of the measured activation energy for the oxidation of Bi added alloys.
Fig. 11. Lattice parameters of Sn matrix of Sn–Zn alloys with different Bi contents.
4. Conclusions The oxidation behavior of Sn–Zn alloys has been investigated under high-temperature and high-humidity conditions. The oxidation of Sn–Zn alloys is attributed to the oxidation of Zn, which diffuses to the Sn grain boundaries and forms ZnO. These oxides are liable to give rise to cracks along the Sn grain boundaries, especially in Bi added alloys. The oxidation of Sn–Zn alloys is controlled by the diffusion and crack formation processes. The oxidized layer thickness of all alloys was found to be in proportion to the 1/2 order of exposure time when the absolute humidity was 149 g/m3 , while an accelerated oxidation rate was observed for Bi added alloys when they were exposed to higher humidity conditions. The effects of temperature and relative humidity on the oxidation rate of Sn–Zn alloys were investigated, as well as the effect of the Bi content. It was found that the activation energy for the oxidation of Bi added alloys is lower than that of Sn–9Zn alloy. The oxidation rate of Sn–9Zn alloy increases with increasing relative humidity (RH) for the exposure at 85 ◦ C, while that of Bi added alloys exhibits a peak at a relative humidity of 64% RH. The oxidized layer thickness was found to increase with increasing Bi content, even with 1 wt.% addition. The solid solution of Sn(Bi), which causes the Sn matrix to become liable to form cracks and promotes the diffusion of Zn, is considered to be one of the reasons for the rapid oxidation of Bi added alloys. Acknowledgements This work was carried out under the both supports of the 21COE program of the Japan Ministry of Education, Culture, Sports, Science and Technology, and of METI (Ministry of Economy, Trade and Industry) Low Temperature Soldering Project organized by JEITA. References
Fig. 10. Oxidized layer thickness of Sn–Zn alloys with different Bi contents after 1000 h of exposure under identified atmospheres.
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