- Email: [email protected]
Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates
Microelectronics Reliability xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates Jiaxing Liang, Tingbi Luo, Anmin Hu ⇑, Ming Li State Key Laboratory of Metal Matrix Composites, Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, School of Material Science and Engineering, Shanghai Jiao Tong University, No. 800 Dongchuan Rd., Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 1 May 2013 Received in revised form 19 August 2013 Accepted 20 August 2013 Available online xxxx
a b s t r a c t In this paper, the formation and growth of intermetallic compounds (IMCs) of Sn–8Zn–3Bi–0.3Cr solder on Cu, Ni and Ni–W substrates have been investigated. For the Cu substrate, only Cu5Zn8 intermetallic compound was observed. For the Ni substrate, a Ni5Zn21 film formed at the interface due to the fast reaction between Ni and Zn. For the Ni–W substrate, a thin Ni5Zn21 film appeared between the solder and Ni– W layer, whose thickness decreases with the increase of W content. A bright layer was also found to form below the Ni5Zn21 layer as aging time extended, which is caused by the diffusion of Zn into Ni–W layer. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Sn–Pb solders have been widely used in electronics manufacturing for many decades. However, there are environmental and health issues concerning the toxicity of Pb present in the Sn–Pb solder alloys. These concerns have inspired a great deal of research to find suitable lead free replacement alloys [1,2]. In all kinds of lead free solder, the Sn–Zn alloy is highly recommended as the promising lead free solder of the next generation, owing to its low cost, favorable melting point and excellent mechanical properties [3–5]. However, the Sn–9Zn alloy is poor in wettability and Zn is easy to be oxidized, which is a major obstacle to its applications [6,7]. Previous studies show that: adding Bi can reduce the surface tension of the solder alloy; adding a small amount of Cr can significantly improve the oxidation resistance of Sn–Zn–Bi alloy and restrain the growth rate of intermetallic compounds (IMCs) because Cr reacts with Zn and forms the Sn–Zn–Cr phase which block the diffusion of Zn atom to the interface and slow down the IMCs growth rate [8–10]. Interfacial reactions in solder/substrate systems are of particular importance to the manufacturability and reliability of electronic products. In this regard, the morphology, growth behavior and properties of IMC formed at the interface play a key role. The brittle nature of IMC formed in solder/substrate systems can affect solder joint reliability, such as by a loss of mechanical strength, particularly under high loading rate environments [11]. The soldering technology in electronic packaging has traditionally relied upon copper as the substrate material. However, when the Sn–Zn alloy is soldered on a bare copper substrate, copper reacts quickly with zinc, forming thick, irregular IMC layer(s) which impacts the solder ⇑ Corresponding author. Tel./fax: +86 21 34202748. E-mail address: [email protected] (A. Hu).
joint reliability [12]. A thin nickel film layer is usually used as a barrier film between lead free solder and copper substrate to limit the outward diffusion of copper atoms, because of its lower reaction rates with solder and copper [13,14]. However, the nickel film is consumed faster with lead free solders as compared with lead based solders. Tungsten, as an alloy addition to Ni, has attracted a lot of attention in recent years, because it makes the alloys structurally stable even when exposed to elevated temperatures. It was suggested that low diffusion coefficient of W atoms in Ni and their segregation to nanograin boundaries make these alloys more stable [15,16]. However, knowledge of the reactions between Ni–W film and the Sn–Zn lead free solder is still insufficient. In this study, the growth behavior of interfacial IMC between Sn–8Zn–3Bi–0.3Cr and Cu, Ni or Ni–W barrier film (Ni–3 at.% W and Ni–10 at.% W) has been investigated through the examination of the morphologies at different aging times (15 min, 30 min, 1 h, 4 h) at 250 °C. The effect of W content in the barrier film on the interfacial IMC growth behavior and morphologies has also been investigated. 2. Experimental procedures Sn–8 wt.%Zn–3 wt.%Bi-0.3 wt.%Cr lead free solder was prepared from ingots of pure Sn, Zn, Bi and Cr (with a purity of 99.99%). The solder constituents were then weighed and melted in a crucible followed by natural cooling in air. The solder was cut into pieces weighing 0.1 g, and melted to make solder balls in liquid colophony at 250 °C. Copper sheet of dimension 0.5 mm 20 mm 50 mm was used as the substrate. Ni and Ni–W alloy films were prepared on copper substrate by electrodeposition. Before soldering or electrodepositing, the Cu substrate was cleaned with detergent and dipped in 20 vol.% H2SO4 to clean the surface and get rid of the
0026-2714/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2013.08.008
Please cite this article in press as: Liang J et al. Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates. Microelectron Reliab (2013), http://dx.doi.org/10.1016/j.microrel.2013.08.008
2
J. Liang et al. / Microelectronics Reliability xxx (2013) xxx–xxx
Table 1 Composition of electrolytic solution of Ni electrodeposition. Components
Concentration (g/L)
Orthoboric acid, H3BO3 Nickel(II)sulfate hexahydrate, NiSO46H2O Nickel(II) chloride hexahydrate, NiCl26H2O Sodium dodecyl sulfate, NaC12H25SO4
50.0 300.0 40.0 0.1
Table 2 Composition of electrolytic solution of Ni–W alloy electrodeposition. Components
Concentration (g/L)
Nickel (II) sulphate hexahydrate, NiSO46H2O Sodium citrate dihydrate, Na3C6H5O72H2O Sodium tungstate dihydrate, Na2WO42H2O Ammonium chloride, NH4Cl Sodium bromide, NaBr
57.8–86.7 129.0 3.3–23.0 26.7 15.4
Table 3 Tungsten content of Ni–W alloy electrodeposits obtained from various baths. Bath contents
and parameters for Ni–W alloy electrodeposition. The bath pH was adjusted to 8.5 and temperature maintained at 80 °C. Ni and Ni–W alloy films were deposited to a thickness about 2.0 lm. The solder ball was soldered on the substrate at 250 °C using 30 wt.% colophony and 5 wt.% ethylene glycol ethanol solutions as flux. The soldering process lasted for 10 s then cooled in the air environment. The samples were placed in the furnace and aged at 250 °C for 15 min, 30 min, 1 h and 4 h. Samples were prepared for cross sectional examination by standard metallographic technique. Scanning electron microscopy with the backscattering electron image (BEI) was used for microstructural examination. The average thickness of the interfacial IMC layer was measured by dividing the area covered by the layer by its length with Adobe Photoshop CS4. For each experimental condition, the average thickness was calculated from at least four micrographs taken at randomly selected position on the cross sectional samples. The elemental analysis of different phases and element line distribution were carried out by energy dispersive X-ray spectroscopy (EDX) (see Table 3). 3. Results and discussion
Deposit W content (at.%)
NiSO46H2O (g/L)
Na2WO42H2O(g/L)
86.7 57.8
3.3 23.0
3.0 10.0
oxide layer. It was rinsed with distilled water followed by drying with acetone. Table 1 shows the bath composition and parameters for Ni electrodeposition. The bath pH was adjusted to 4.5 and temperature maintained at 45 °C. Table 2 shows the bath composition
Fig. 1 shows typical cross-sectional SEM images of solder/Cu, solder/Ni and solder/Ni–W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 10 s. The microstructure of the Sn–8Zn– 3Bi–0.1Cr solder consists of a typical Sn–Zn eutectic region, some needle-like Zn-rich phase and Bi precipitated phase. Zn–Cr phases were found in the b-Sn matrix [17]. For the solder/Cu system, a thin continuous reaction layer of thickness of 1.6 lm is formed at the interface (Fig. 1a). A line analysis was carried out on the interface and the result is shown in Fig. 2. Elemental ratio of the reaction layer determined by EDX mainly contains Zn (64.94 at.%) and Cu
Fig. 1. SEM images of solder/Cu, solder/Ni and solder/Ni–W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 10 s: (a) Cu; (b) Ni; (c) Ni–3 at.%W and (d) Ni– 10 at.%W.
Please cite this article in press as: Liang J et al. Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates. Microelectron Reliab (2013), http://dx.doi.org/10.1016/j.microrel.2013.08.008
J. Liang et al. / Microelectronics Reliability xxx (2013) xxx–xxx
3
Fig. 2. EDX analysis of solder/Cu interface reflowed at 250 °C for 10 s.
Fig. 3. SEM images of solder/Cu, solder/Ni and solder/Ni–W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 15 min: (a) Cu; (b) Ni; (c) Ni–3 at.%W and (d) Ni– 10 at.%W.
(35.06 at.%). According to the atomic ratio and the relevant studies [11,18], it is presumed that the phase is Cu5Zn8. Cu–Sn IMCs are not found in the solder joint interface or within the solder, because Zn has a higher affinity for reaction with Cu than Sn. For the solder/ Ni system, the electroplated Ni layer is flat with thickness of about 2 lm and no obvious IMC is observed at the interface in Fig. 1b, which indicates that Ni layer can effectively limit the reaction of the solder and copper. For the solder/Ni–W system, the shape of Ni–W layers is smooth with thickness of approximately 2 lm. No obvious IMCs are observed at the interface for both Ni–3 at.%W and Ni–10 at.%W layers (Fig. 1c and d).
Fig. 3 shows SEM images of solder/Cu, solder/Ni and solder/Ni– W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 15 min. A 10–11 lm thick layer of IMCs is noticed at the interface of solder/Cu system, which is identified as Cu5Zn8 (Fig. 3a). For the solder/Ni system, the interface microstructure is different from that in the solder/Cu system. A thin layer of IMC with the thickness of 0.5–0.8 lm can be observed at the interface (Fig. 3b). As analyzed by EDX, the composition of the IMC is Ni:Zn = 18.01:81.99 at.%. Based on the compositions and the results from the similar previous studies [19], the IMC is identified as the Ni5Zn21 phase. For the solder/Ni–W system, there is no massive
Please cite this article in press as: Liang J et al. Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates. Microelectron Reliab (2013), http://dx.doi.org/10.1016/j.microrel.2013.08.008
4
J. Liang et al. / Microelectronics Reliability xxx (2013) xxx–xxx
irregular IMC at the interface, but thin and compact IMC layers covering on the barrier layers which thickness is only 0.2– 0.3 lm. The composition of the IMC layer is analyzed by EDX and it can be identified as Ni5Zn21. Fig. 4 shows SEM images of solder/Cu, solder/Ni and solder/Ni– W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 30 min. After 30 min reaction in the molten condition, the Cu5Zn8 IMC layer at the interface grew thicker with the increase of aging time (Fig. 4a). The thickness of Cu5Zn8 IMC has reached 15– 16 lm. In the solder/Ni system, the morphology of electroplated Ni layer remain stable and the Ni5Zn21 IMC layer grew to 2 lm thick (Fig. 4b). For the solder/Ni–W system, the thickness of Ni5Zn21 IMC layer did not increase significantly, which indicates that Ni–W barrier layer is more stable than Ni barrier layer under high temperature aging. On the other hand, there appears a bright layer between the Ni5Zn21 and Ni–10% W film, which has been reported in earlier study [20] as an amorphous film formed at the interface between Sn and Ni–W (Fig. 4d). Referring to the element line-distribution, it reveals that Zn atoms and Ni atoms diffuse into each other, while W atoms stay in the barrier layer; the content ratio of W on nickel rises up, becoming much higher than the original ratio of the barrier film. Other research shows that if W content is more than 25 at.% in Ni, the resulting alloy is amorphous [21]. Fig. 5 shows SEM images of solder/Cu, solder/Ni and solder/Ni– W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 1 h. With increasing the aging time, some Cu5Zn8 IMC at the solder/Cu interface was observed to spall off to float into the bulk solder near the interface. And some voids can be observed between the solder and Cu5Zn8 IMC layer (Fig. 5a). It might be ascribed to spalling of Cu5Zn8. These voids would reduce the mechanical and electrical reliability of the solder joints. For the solder/Ni system, the thick-
ness of Ni5Zn21 IMC layer was about 4–5 lm, which was much thinner than Cu5Zn8 IMC in solder/Cu system. Moreover, Ni5Zn21 did not scatter into the bulk solder (Fig. 5b). Compared with these two different W content Ni–W alloy layers in Fig. 5c and d, the morphology of Ni–10 at.%W layer was planer than Ni–3 at.%W layer after aging for 1 h. The interfacial reactions between the solder and Ni–W substrates after long-term aging were quite different from that after short-term aging. We prolonged the aging time to 4 h and an interesting phenomenon was found in Fig. 6. For the solder/Cu system, Zn atoms continuously diffused to the solder/IMC interface to form Cu5Zn8 IMC and the thickness has reached about 25–26 lm (Fig. 6a). For the solder/Ni system, the thickness of electroplated Ni layer reduced to 1 lm. However, the Ni layer was still plane and complete and the atoms from both solder and copper sides have not crossed the barrier film to form Cu–Sn or Cu–Zn IMC (Fig. 6b). Fig. 6c and d shows that the Ni–W layers cracked and the thickness of IMCs increased rapidly after 4 h aging. Zn atoms crossed the Ni–W barrier layer and diffused towards to the copper substrate to form Cu5Zn8. That is why the thickness of IMCs increased rapidly. To learn more about the cracking of Ni–W layer, we observed the surface micro-topography of these two different W content Ni–W layers (Fig. 7). It can be seen that lots of voids formed on the Ni–W layer surface before aging, which may lead to high stress and promotes the cracking for Ni–W layer after long-term aging. Related literatures [22] showed that hydrogen evolution has a significant influence on crack density of Ni–W alloys. As hydrogen gas formation during plating process increases, the probability of hydrogen diffusion into coated layer increases spontaneously. The release of hydrogen results in voids and high tensile stress, which promotes cracking of Ni–W layer. We have
Fig. 4. SEM images of solder/Cu, solder/Ni and solder/Ni–W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 30 min: (a) Cu; (b) Ni; (c) Ni–3 at.%W and (d) Ni– 10 at.%W.
Please cite this article in press as: Liang J et al. Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates. Microelectron Reliab (2013), http://dx.doi.org/10.1016/j.microrel.2013.08.008
J. Liang et al. / Microelectronics Reliability xxx (2013) xxx–xxx
5
Fig. 5. SEM images of solder/Cu, solder/Ni and solder/Ni–W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 1 h: (a) Cu; (b) Ni; (c) Ni–3 at.%W; (d) Ni–10 at.%W.
Fig. 6. SEM images of solder/Cu, solder/Ni and solder/Ni–W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 4 h: (a) Cu; (b) Ni; (c) Ni–3 at.%W and (d) Ni– 10 at.%W.
Please cite this article in press as: Liang J et al. Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates. Microelectron Reliab (2013), http://dx.doi.org/10.1016/j.microrel.2013.08.008
6
J. Liang et al. / Microelectronics Reliability xxx (2013) xxx–xxx
Fig. 7. SEM images of Ni–W layers surface: (a) Ni–3 at.%W and (b) Ni–10 at.%W.
Fig. 8. SEM/EDX and XRD analyses of bright layer of solder/Ni–10 at.%W interface: (a) SEM/EDX and (b) XRD patterns.
observed the microstructure and composition of the bright layer using SEM, EDX and XRD, as shown in Fig. 8. According to the EDX analysis, Ni and W remain in the bright layer and the atomic ratio is Ni:W:Cu:Sn:Zn = 11.05:17.24:12.01:54.74:4.96 at.% (Fig. 8a). The content ratio of W is higher than the original ratio of Ni–W film. X-ray diffraction was conducted to confirm the structure of the bright layer (Fig. 8b). According to XRD patterns, the sample contained Ni, Ni5Zn21, Cu5Zn8, Cu6Sn5 and Cu. The apparent Cu peaks were a result of Cu substrate. There was no evidence that the bright layer was amorphous. Combined the results of EDX and XRD, we considered that the bright layer between Ni5Zn21 and Ni– 10 at.%W film was a W atoms enrichment layer. The relationship between the IMC thickness and aging time can be represented by the following equation [23]:
X ðtÞ ¼ X 0 þ At n expðQ=RtÞ
ð1Þ
where t is the aging time, X(t) is the IMCs thickness at t time, X0 is the initial IMCs thickness, Q is the activity energy, A is a constant, n is the time constant. Since the growth is diffusion-controlled mechanism, n is taken as 0.5 as reported by many researchers [24]. Our results are quite consistent with the results reported by them. So the Eq. (1) can be simplified as follows:
pffiffiffiffiffi X X 0 ¼ Kt
ð2Þ
where X is the IMCs thickness at t time, X0 is the initial as-soldered IMCs thickness, K is the growth rate constant of IMC growth rate which can be determined by the slope. Fig. 9 shows the linear relationship between the thickness of IMCs and the square root of aging time (hour1/2). K values of Sn– 8Zn–3Bi–0.3Cr solder joints with Cu, Ni and Ni–W substrates at 250 °C were shown at Table 4. The long-term aging (4 h) data
Fig. 9. IMCs growth with increased aging time.
was not plotted because the Ni–W layers were failure and Cu5Zn8 IMC formed. The type and growth behavior of IMCs were quite different from short-term specimens’. The short-term aging result indicated that IMC growth at the solder/Ni–W interface was the slowest and the solder/Ni interface’s was slower than that at the solder/Cu interface. In addition, the thickness of IMC at the solder/Ni–W interfaces decreased with the increase of W content in the Ni–W barrier film. It was suggested that Ni–W barrier film’s stability at high temperature is due to low diffusion coefficient of W atoms in Ni and their segregation to nanograin boundaries. Re-
Please cite this article in press as: Liang J et al. Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates. Microelectron Reliab (2013), http://dx.doi.org/10.1016/j.microrel.2013.08.008
J. Liang et al. / Microelectronics Reliability xxx (2013) xxx–xxx Table 4 The growth rate of IMCs on different barrier layer. Cu 2
K (m /s)
2.23 10
Ni 14
Ni–W (3 at%) 15
5.27 10
16
5.03 10
Ni–W (10 at%) 3.15 1016
lated literatures [16] show that Ni grains are nanocrystalline and W-enriched zones are grain boundaries in Ni–W alloy. After annealing, the nanocrystalline structure is mainly preserved and the W content in the Ni grains increased to form intermetallic compound Ni4W. This might slow down the IMC growth rate. On the other hand, long-term aging at high temperature, Ni–W barrier films cracked because of the voids and high stress formed on the surface during plating process. 4. Conclusion In this study, interfacial reactions between Sn–8Zn–3Bi–0.3Cr solder and different substrates (Cu, Ni, Ni–3 at.% W, Ni–10 at.% W) were studied. On the Cu substrate, Sn–8Zn–3Bi–0.3Cr reacted with Cu which formed Cu5Zn8 IMC. On the Ni substrate, Ni5Zn21 IMC formed at the solder and substrate interface. On the Ni–W substrates, Ni5Zn21 was found as the main IMC between solder and substrates, and its thickness decreases with the increase of W content in the Ni–W layer. As the aging time extended, Ni–W barrier film cracked and Zn atoms diffused to Cu substrate to form Cu5Zn8 due to high stress of Ni–W film which formed during electroplating process. So long-term reliability of Sn–8Zn–3Bi–0.3Cr solder on Ni–W barrier layer requires further investigation. Acknowledgments This work is sponsored by National Natural Science foundation of China (61176097, 61376107). We thank the Instrumental Analysis Center of Shanghai Jiao Tong University, for the use of the SEM and XRD equipment. References [1] Abtew M, Selvaduray G. Lead-free solders in microelectronics. Mater Sci Eng R 2000;27:95–141. [2] Zeng K, Tu KN. Six cases of reliability study of Pb-free solder joints in electronic packaging technology. Mater Sci Eng R 2002;38:55–105. [3] Garcia LeonardoR, Peixoto LeandroC. Globular-to-needle Zn-rich phase transition during transient solidification of a eutectic Sn–9%Zn solder alloy. Mater Lett 2009;63:1314–6.
7
[4] Osório WisleiR. Microstructure and mechanical properties of Sn–Bi, Sn–Ag and Sn–Zn lead-free solder alloys. J Alloys Compd 2013;572:97–106. [5] Chen Kang-I, Cheng Shou-Chang. Effects of small additions of Ag, Al, and Ga on the structure and properties of the Sn–9Zn eutectic alloy. J Alloys Compd 2006;416:98–105. [6] McCormack M. New lead-free, Sn–Zn–In solder alloys. J Electron Mater 1994;23:687–90. [7] Song Jenn-Ming, Lui Truan-Sheng. Compositional effects on the microstructure and vibration fracture properties of Sn–Zn–Bi alloys. J Alloys Compd 2005;403:191–6. [8] Kim Young-Sun, Kim Keun-Soo. Effect of composition and cooling rate on microstructure and tensile properties of Sn–Zn–Bi alloys. J Alloys Compd 2003;352:237–45. [9] Chen X, Li M, Ren X, Hu A, Mao DL. Effect of small additions of alloying elements on the properties of Sn–Zn eutectic alloy. J Electron Mater 2006;35:1734–9. [10] Bi J, Hu A, Hu J, Luo T. Effect of Cr additions on interfacial reaction between the Sn–Zn–Bi solder and Cu/electroplated Ni substrates. Microelectron Reliab 2011;51:636–41. [11] Lee Chang-Yong, Yoon Jeong-Won. Interfacial reactions and joint reliability of Sn–9Zn solder on Cu or electrolytic Au/Ni/Cu BGA substrate. Microelectron Eng 2005;82:561–8. [12] Chi-Yang Yu, Chen Wei-Yu, Duh Jenq-Gong. Suppressing the growth of Cu–Sn intermetallic compounds in Ni/Sn–Ag–Cu/Cu–Zn solder joints during thermal aging. Intermetallics 2012;26:11–7. [13] Chiu MY, Wang SS, Chuang TH. Intermetallic compounds formed during interfacial reactions between liquid Sn–8Zn–3Bi solders and Ni substrates. J Electron Mater 2002;31:494–9. [14] Alam MO, Chan YC, Tu KN. Effect of reaction time and P content on mechanical strength of the interface formed between eutectic Sn–Ag solder and Au/ electroless Ni(P)/Cu bond pad. J Appl Phys 2003;94:4108–15. [15] Choi P, Al-Kassab T. Thermal stability of nanocrystalline nickel–18 at.% tungsten alloy investigated with the tomographic atom probe. Mater Sci Eng A 2003;353:74–9. [16] Choi IS, Detor AJ. Mechanics of indentation of plastically graded materials – II: Experiments on nanocrystalline alloys with grain size gradients. J Mech Phys Solids 2008;56:172–83. [17] Luo T, Hu A, Hu J, Li M, Mao D. Microstructure and mechanical properties of Sn–Zn–Bi–Cr lead-free solder. Microelectron Reliab 2012;52:585–8. [18] Mohan Kumar K, Kripesh V. Nanoindentation study of Zn-based Pb free solders used in fine pitch interconnect applications. Mater Sci Eng A 2006;423:57–63. [19] Wang CH, Chen HH. Kinetic analysis of Ni5Zn21 growth at the interface between Sn–Zn solders and Ni. Intermetallics 2012;22:166–75. [20] Haseeb ASMA, Chew CS, Johan Mohd Rafie. Interfacial reactions between Sn– 3.5 Ag solder and Ni–W alloy films. J Mater Sci Mater Electron 2011;22:1372–7. [21] Królikowski Andrzej, Płon´ska Ewelina. Effects of compositional and structural features on corrosion behavior of nickel–tungsten alloys. J Solid State Electrochem 2009;13:263–75. [22] Alimadadi H, Ahmadi M. Corrosion properties of electrodeposited nanocrystalline and amorphous patterned Ni–W alloy. Mater Des 2009;30:1356–61. [23] Kim DG, Jung SB. Interfacial reactions and growth kinetics for intermetallic compound layer between In–48Sn solder and bare Cu substrate. J Alloys Compd 2005;386:151–6. [24] Liu NS, Lin KL. Evolution of interfacial morphology of Sn–8.5Zn–0.5Ag–0.1Al– xGa/Cu system during isothermal aging. J Alloys Compd 2008;456:466–73.
Please cite this article in press as: Liang J et al. Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates. Microelectron Reliab (2013), http://dx.doi.org/10.1016/j.microrel.2013.08.008
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates Jiaxing Liang, Tingbi Luo, Anmin Hu ⇑, Ming Li State Key Laboratory of Metal Matrix Composites, Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, School of Material Science and Engineering, Shanghai Jiao Tong University, No. 800 Dongchuan Rd., Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 1 May 2013 Received in revised form 19 August 2013 Accepted 20 August 2013 Available online xxxx
a b s t r a c t In this paper, the formation and growth of intermetallic compounds (IMCs) of Sn–8Zn–3Bi–0.3Cr solder on Cu, Ni and Ni–W substrates have been investigated. For the Cu substrate, only Cu5Zn8 intermetallic compound was observed. For the Ni substrate, a Ni5Zn21 film formed at the interface due to the fast reaction between Ni and Zn. For the Ni–W substrate, a thin Ni5Zn21 film appeared between the solder and Ni– W layer, whose thickness decreases with the increase of W content. A bright layer was also found to form below the Ni5Zn21 layer as aging time extended, which is caused by the diffusion of Zn into Ni–W layer. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Sn–Pb solders have been widely used in electronics manufacturing for many decades. However, there are environmental and health issues concerning the toxicity of Pb present in the Sn–Pb solder alloys. These concerns have inspired a great deal of research to find suitable lead free replacement alloys [1,2]. In all kinds of lead free solder, the Sn–Zn alloy is highly recommended as the promising lead free solder of the next generation, owing to its low cost, favorable melting point and excellent mechanical properties [3–5]. However, the Sn–9Zn alloy is poor in wettability and Zn is easy to be oxidized, which is a major obstacle to its applications [6,7]. Previous studies show that: adding Bi can reduce the surface tension of the solder alloy; adding a small amount of Cr can significantly improve the oxidation resistance of Sn–Zn–Bi alloy and restrain the growth rate of intermetallic compounds (IMCs) because Cr reacts with Zn and forms the Sn–Zn–Cr phase which block the diffusion of Zn atom to the interface and slow down the IMCs growth rate [8–10]. Interfacial reactions in solder/substrate systems are of particular importance to the manufacturability and reliability of electronic products. In this regard, the morphology, growth behavior and properties of IMC formed at the interface play a key role. The brittle nature of IMC formed in solder/substrate systems can affect solder joint reliability, such as by a loss of mechanical strength, particularly under high loading rate environments [11]. The soldering technology in electronic packaging has traditionally relied upon copper as the substrate material. However, when the Sn–Zn alloy is soldered on a bare copper substrate, copper reacts quickly with zinc, forming thick, irregular IMC layer(s) which impacts the solder ⇑ Corresponding author. Tel./fax: +86 21 34202748. E-mail address: [email protected] (A. Hu).
joint reliability [12]. A thin nickel film layer is usually used as a barrier film between lead free solder and copper substrate to limit the outward diffusion of copper atoms, because of its lower reaction rates with solder and copper [13,14]. However, the nickel film is consumed faster with lead free solders as compared with lead based solders. Tungsten, as an alloy addition to Ni, has attracted a lot of attention in recent years, because it makes the alloys structurally stable even when exposed to elevated temperatures. It was suggested that low diffusion coefficient of W atoms in Ni and their segregation to nanograin boundaries make these alloys more stable [15,16]. However, knowledge of the reactions between Ni–W film and the Sn–Zn lead free solder is still insufficient. In this study, the growth behavior of interfacial IMC between Sn–8Zn–3Bi–0.3Cr and Cu, Ni or Ni–W barrier film (Ni–3 at.% W and Ni–10 at.% W) has been investigated through the examination of the morphologies at different aging times (15 min, 30 min, 1 h, 4 h) at 250 °C. The effect of W content in the barrier film on the interfacial IMC growth behavior and morphologies has also been investigated. 2. Experimental procedures Sn–8 wt.%Zn–3 wt.%Bi-0.3 wt.%Cr lead free solder was prepared from ingots of pure Sn, Zn, Bi and Cr (with a purity of 99.99%). The solder constituents were then weighed and melted in a crucible followed by natural cooling in air. The solder was cut into pieces weighing 0.1 g, and melted to make solder balls in liquid colophony at 250 °C. Copper sheet of dimension 0.5 mm 20 mm 50 mm was used as the substrate. Ni and Ni–W alloy films were prepared on copper substrate by electrodeposition. Before soldering or electrodepositing, the Cu substrate was cleaned with detergent and dipped in 20 vol.% H2SO4 to clean the surface and get rid of the
0026-2714/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2013.08.008
Please cite this article in press as: Liang J et al. Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates. Microelectron Reliab (2013), http://dx.doi.org/10.1016/j.microrel.2013.08.008
2
J. Liang et al. / Microelectronics Reliability xxx (2013) xxx–xxx
Table 1 Composition of electrolytic solution of Ni electrodeposition. Components
Concentration (g/L)
Orthoboric acid, H3BO3 Nickel(II)sulfate hexahydrate, NiSO46H2O Nickel(II) chloride hexahydrate, NiCl26H2O Sodium dodecyl sulfate, NaC12H25SO4
50.0 300.0 40.0 0.1
Table 2 Composition of electrolytic solution of Ni–W alloy electrodeposition. Components
Concentration (g/L)
Nickel (II) sulphate hexahydrate, NiSO46H2O Sodium citrate dihydrate, Na3C6H5O72H2O Sodium tungstate dihydrate, Na2WO42H2O Ammonium chloride, NH4Cl Sodium bromide, NaBr
57.8–86.7 129.0 3.3–23.0 26.7 15.4
Table 3 Tungsten content of Ni–W alloy electrodeposits obtained from various baths. Bath contents
and parameters for Ni–W alloy electrodeposition. The bath pH was adjusted to 8.5 and temperature maintained at 80 °C. Ni and Ni–W alloy films were deposited to a thickness about 2.0 lm. The solder ball was soldered on the substrate at 250 °C using 30 wt.% colophony and 5 wt.% ethylene glycol ethanol solutions as flux. The soldering process lasted for 10 s then cooled in the air environment. The samples were placed in the furnace and aged at 250 °C for 15 min, 30 min, 1 h and 4 h. Samples were prepared for cross sectional examination by standard metallographic technique. Scanning electron microscopy with the backscattering electron image (BEI) was used for microstructural examination. The average thickness of the interfacial IMC layer was measured by dividing the area covered by the layer by its length with Adobe Photoshop CS4. For each experimental condition, the average thickness was calculated from at least four micrographs taken at randomly selected position on the cross sectional samples. The elemental analysis of different phases and element line distribution were carried out by energy dispersive X-ray spectroscopy (EDX) (see Table 3). 3. Results and discussion
Deposit W content (at.%)
NiSO46H2O (g/L)
Na2WO42H2O(g/L)
86.7 57.8
3.3 23.0
3.0 10.0
oxide layer. It was rinsed with distilled water followed by drying with acetone. Table 1 shows the bath composition and parameters for Ni electrodeposition. The bath pH was adjusted to 4.5 and temperature maintained at 45 °C. Table 2 shows the bath composition
Fig. 1 shows typical cross-sectional SEM images of solder/Cu, solder/Ni and solder/Ni–W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 10 s. The microstructure of the Sn–8Zn– 3Bi–0.1Cr solder consists of a typical Sn–Zn eutectic region, some needle-like Zn-rich phase and Bi precipitated phase. Zn–Cr phases were found in the b-Sn matrix [17]. For the solder/Cu system, a thin continuous reaction layer of thickness of 1.6 lm is formed at the interface (Fig. 1a). A line analysis was carried out on the interface and the result is shown in Fig. 2. Elemental ratio of the reaction layer determined by EDX mainly contains Zn (64.94 at.%) and Cu
Fig. 1. SEM images of solder/Cu, solder/Ni and solder/Ni–W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 10 s: (a) Cu; (b) Ni; (c) Ni–3 at.%W and (d) Ni– 10 at.%W.
Please cite this article in press as: Liang J et al. Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates. Microelectron Reliab (2013), http://dx.doi.org/10.1016/j.microrel.2013.08.008
J. Liang et al. / Microelectronics Reliability xxx (2013) xxx–xxx
3
Fig. 2. EDX analysis of solder/Cu interface reflowed at 250 °C for 10 s.
Fig. 3. SEM images of solder/Cu, solder/Ni and solder/Ni–W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 15 min: (a) Cu; (b) Ni; (c) Ni–3 at.%W and (d) Ni– 10 at.%W.
(35.06 at.%). According to the atomic ratio and the relevant studies [11,18], it is presumed that the phase is Cu5Zn8. Cu–Sn IMCs are not found in the solder joint interface or within the solder, because Zn has a higher affinity for reaction with Cu than Sn. For the solder/ Ni system, the electroplated Ni layer is flat with thickness of about 2 lm and no obvious IMC is observed at the interface in Fig. 1b, which indicates that Ni layer can effectively limit the reaction of the solder and copper. For the solder/Ni–W system, the shape of Ni–W layers is smooth with thickness of approximately 2 lm. No obvious IMCs are observed at the interface for both Ni–3 at.%W and Ni–10 at.%W layers (Fig. 1c and d).
Fig. 3 shows SEM images of solder/Cu, solder/Ni and solder/Ni– W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 15 min. A 10–11 lm thick layer of IMCs is noticed at the interface of solder/Cu system, which is identified as Cu5Zn8 (Fig. 3a). For the solder/Ni system, the interface microstructure is different from that in the solder/Cu system. A thin layer of IMC with the thickness of 0.5–0.8 lm can be observed at the interface (Fig. 3b). As analyzed by EDX, the composition of the IMC is Ni:Zn = 18.01:81.99 at.%. Based on the compositions and the results from the similar previous studies [19], the IMC is identified as the Ni5Zn21 phase. For the solder/Ni–W system, there is no massive
Please cite this article in press as: Liang J et al. Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates. Microelectron Reliab (2013), http://dx.doi.org/10.1016/j.microrel.2013.08.008
4
J. Liang et al. / Microelectronics Reliability xxx (2013) xxx–xxx
irregular IMC at the interface, but thin and compact IMC layers covering on the barrier layers which thickness is only 0.2– 0.3 lm. The composition of the IMC layer is analyzed by EDX and it can be identified as Ni5Zn21. Fig. 4 shows SEM images of solder/Cu, solder/Ni and solder/Ni– W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 30 min. After 30 min reaction in the molten condition, the Cu5Zn8 IMC layer at the interface grew thicker with the increase of aging time (Fig. 4a). The thickness of Cu5Zn8 IMC has reached 15– 16 lm. In the solder/Ni system, the morphology of electroplated Ni layer remain stable and the Ni5Zn21 IMC layer grew to 2 lm thick (Fig. 4b). For the solder/Ni–W system, the thickness of Ni5Zn21 IMC layer did not increase significantly, which indicates that Ni–W barrier layer is more stable than Ni barrier layer under high temperature aging. On the other hand, there appears a bright layer between the Ni5Zn21 and Ni–10% W film, which has been reported in earlier study [20] as an amorphous film formed at the interface between Sn and Ni–W (Fig. 4d). Referring to the element line-distribution, it reveals that Zn atoms and Ni atoms diffuse into each other, while W atoms stay in the barrier layer; the content ratio of W on nickel rises up, becoming much higher than the original ratio of the barrier film. Other research shows that if W content is more than 25 at.% in Ni, the resulting alloy is amorphous [21]. Fig. 5 shows SEM images of solder/Cu, solder/Ni and solder/Ni– W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 1 h. With increasing the aging time, some Cu5Zn8 IMC at the solder/Cu interface was observed to spall off to float into the bulk solder near the interface. And some voids can be observed between the solder and Cu5Zn8 IMC layer (Fig. 5a). It might be ascribed to spalling of Cu5Zn8. These voids would reduce the mechanical and electrical reliability of the solder joints. For the solder/Ni system, the thick-
ness of Ni5Zn21 IMC layer was about 4–5 lm, which was much thinner than Cu5Zn8 IMC in solder/Cu system. Moreover, Ni5Zn21 did not scatter into the bulk solder (Fig. 5b). Compared with these two different W content Ni–W alloy layers in Fig. 5c and d, the morphology of Ni–10 at.%W layer was planer than Ni–3 at.%W layer after aging for 1 h. The interfacial reactions between the solder and Ni–W substrates after long-term aging were quite different from that after short-term aging. We prolonged the aging time to 4 h and an interesting phenomenon was found in Fig. 6. For the solder/Cu system, Zn atoms continuously diffused to the solder/IMC interface to form Cu5Zn8 IMC and the thickness has reached about 25–26 lm (Fig. 6a). For the solder/Ni system, the thickness of electroplated Ni layer reduced to 1 lm. However, the Ni layer was still plane and complete and the atoms from both solder and copper sides have not crossed the barrier film to form Cu–Sn or Cu–Zn IMC (Fig. 6b). Fig. 6c and d shows that the Ni–W layers cracked and the thickness of IMCs increased rapidly after 4 h aging. Zn atoms crossed the Ni–W barrier layer and diffused towards to the copper substrate to form Cu5Zn8. That is why the thickness of IMCs increased rapidly. To learn more about the cracking of Ni–W layer, we observed the surface micro-topography of these two different W content Ni–W layers (Fig. 7). It can be seen that lots of voids formed on the Ni–W layer surface before aging, which may lead to high stress and promotes the cracking for Ni–W layer after long-term aging. Related literatures [22] showed that hydrogen evolution has a significant influence on crack density of Ni–W alloys. As hydrogen gas formation during plating process increases, the probability of hydrogen diffusion into coated layer increases spontaneously. The release of hydrogen results in voids and high tensile stress, which promotes cracking of Ni–W layer. We have
Fig. 4. SEM images of solder/Cu, solder/Ni and solder/Ni–W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 30 min: (a) Cu; (b) Ni; (c) Ni–3 at.%W and (d) Ni– 10 at.%W.
Please cite this article in press as: Liang J et al. Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates. Microelectron Reliab (2013), http://dx.doi.org/10.1016/j.microrel.2013.08.008
J. Liang et al. / Microelectronics Reliability xxx (2013) xxx–xxx
5
Fig. 5. SEM images of solder/Cu, solder/Ni and solder/Ni–W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 1 h: (a) Cu; (b) Ni; (c) Ni–3 at.%W; (d) Ni–10 at.%W.
Fig. 6. SEM images of solder/Cu, solder/Ni and solder/Ni–W (Ni–3 at.%W, Ni–10 at.%W) interfaces reflowed at 250 °C for 4 h: (a) Cu; (b) Ni; (c) Ni–3 at.%W and (d) Ni– 10 at.%W.
Please cite this article in press as: Liang J et al. Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates. Microelectron Reliab (2013), http://dx.doi.org/10.1016/j.microrel.2013.08.008
6
J. Liang et al. / Microelectronics Reliability xxx (2013) xxx–xxx
Fig. 7. SEM images of Ni–W layers surface: (a) Ni–3 at.%W and (b) Ni–10 at.%W.
Fig. 8. SEM/EDX and XRD analyses of bright layer of solder/Ni–10 at.%W interface: (a) SEM/EDX and (b) XRD patterns.
observed the microstructure and composition of the bright layer using SEM, EDX and XRD, as shown in Fig. 8. According to the EDX analysis, Ni and W remain in the bright layer and the atomic ratio is Ni:W:Cu:Sn:Zn = 11.05:17.24:12.01:54.74:4.96 at.% (Fig. 8a). The content ratio of W is higher than the original ratio of Ni–W film. X-ray diffraction was conducted to confirm the structure of the bright layer (Fig. 8b). According to XRD patterns, the sample contained Ni, Ni5Zn21, Cu5Zn8, Cu6Sn5 and Cu. The apparent Cu peaks were a result of Cu substrate. There was no evidence that the bright layer was amorphous. Combined the results of EDX and XRD, we considered that the bright layer between Ni5Zn21 and Ni– 10 at.%W film was a W atoms enrichment layer. The relationship between the IMC thickness and aging time can be represented by the following equation [23]:
X ðtÞ ¼ X 0 þ At n expðQ=RtÞ
ð1Þ
where t is the aging time, X(t) is the IMCs thickness at t time, X0 is the initial IMCs thickness, Q is the activity energy, A is a constant, n is the time constant. Since the growth is diffusion-controlled mechanism, n is taken as 0.5 as reported by many researchers [24]. Our results are quite consistent with the results reported by them. So the Eq. (1) can be simplified as follows:
pffiffiffiffiffi X X 0 ¼ Kt
ð2Þ
where X is the IMCs thickness at t time, X0 is the initial as-soldered IMCs thickness, K is the growth rate constant of IMC growth rate which can be determined by the slope. Fig. 9 shows the linear relationship between the thickness of IMCs and the square root of aging time (hour1/2). K values of Sn– 8Zn–3Bi–0.3Cr solder joints with Cu, Ni and Ni–W substrates at 250 °C were shown at Table 4. The long-term aging (4 h) data
Fig. 9. IMCs growth with increased aging time.
was not plotted because the Ni–W layers were failure and Cu5Zn8 IMC formed. The type and growth behavior of IMCs were quite different from short-term specimens’. The short-term aging result indicated that IMC growth at the solder/Ni–W interface was the slowest and the solder/Ni interface’s was slower than that at the solder/Cu interface. In addition, the thickness of IMC at the solder/Ni–W interfaces decreased with the increase of W content in the Ni–W barrier film. It was suggested that Ni–W barrier film’s stability at high temperature is due to low diffusion coefficient of W atoms in Ni and their segregation to nanograin boundaries. Re-
Please cite this article in press as: Liang J et al. Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates. Microelectron Reliab (2013), http://dx.doi.org/10.1016/j.microrel.2013.08.008
J. Liang et al. / Microelectronics Reliability xxx (2013) xxx–xxx Table 4 The growth rate of IMCs on different barrier layer. Cu 2
K (m /s)
2.23 10
Ni 14
Ni–W (3 at%) 15
5.27 10
16
5.03 10
Ni–W (10 at%) 3.15 1016
lated literatures [16] show that Ni grains are nanocrystalline and W-enriched zones are grain boundaries in Ni–W alloy. After annealing, the nanocrystalline structure is mainly preserved and the W content in the Ni grains increased to form intermetallic compound Ni4W. This might slow down the IMC growth rate. On the other hand, long-term aging at high temperature, Ni–W barrier films cracked because of the voids and high stress formed on the surface during plating process. 4. Conclusion In this study, interfacial reactions between Sn–8Zn–3Bi–0.3Cr solder and different substrates (Cu, Ni, Ni–3 at.% W, Ni–10 at.% W) were studied. On the Cu substrate, Sn–8Zn–3Bi–0.3Cr reacted with Cu which formed Cu5Zn8 IMC. On the Ni substrate, Ni5Zn21 IMC formed at the solder and substrate interface. On the Ni–W substrates, Ni5Zn21 was found as the main IMC between solder and substrates, and its thickness decreases with the increase of W content in the Ni–W layer. As the aging time extended, Ni–W barrier film cracked and Zn atoms diffused to Cu substrate to form Cu5Zn8 due to high stress of Ni–W film which formed during electroplating process. So long-term reliability of Sn–8Zn–3Bi–0.3Cr solder on Ni–W barrier layer requires further investigation. Acknowledgments This work is sponsored by National Natural Science foundation of China (61176097, 61376107). We thank the Instrumental Analysis Center of Shanghai Jiao Tong University, for the use of the SEM and XRD equipment. References [1] Abtew M, Selvaduray G. Lead-free solders in microelectronics. Mater Sci Eng R 2000;27:95–141. [2] Zeng K, Tu KN. Six cases of reliability study of Pb-free solder joints in electronic packaging technology. Mater Sci Eng R 2002;38:55–105. [3] Garcia LeonardoR, Peixoto LeandroC. Globular-to-needle Zn-rich phase transition during transient solidification of a eutectic Sn–9%Zn solder alloy. Mater Lett 2009;63:1314–6.
7
[4] Osório WisleiR. Microstructure and mechanical properties of Sn–Bi, Sn–Ag and Sn–Zn lead-free solder alloys. J Alloys Compd 2013;572:97–106. [5] Chen Kang-I, Cheng Shou-Chang. Effects of small additions of Ag, Al, and Ga on the structure and properties of the Sn–9Zn eutectic alloy. J Alloys Compd 2006;416:98–105. [6] McCormack M. New lead-free, Sn–Zn–In solder alloys. J Electron Mater 1994;23:687–90. [7] Song Jenn-Ming, Lui Truan-Sheng. Compositional effects on the microstructure and vibration fracture properties of Sn–Zn–Bi alloys. J Alloys Compd 2005;403:191–6. [8] Kim Young-Sun, Kim Keun-Soo. Effect of composition and cooling rate on microstructure and tensile properties of Sn–Zn–Bi alloys. J Alloys Compd 2003;352:237–45. [9] Chen X, Li M, Ren X, Hu A, Mao DL. Effect of small additions of alloying elements on the properties of Sn–Zn eutectic alloy. J Electron Mater 2006;35:1734–9. [10] Bi J, Hu A, Hu J, Luo T. Effect of Cr additions on interfacial reaction between the Sn–Zn–Bi solder and Cu/electroplated Ni substrates. Microelectron Reliab 2011;51:636–41. [11] Lee Chang-Yong, Yoon Jeong-Won. Interfacial reactions and joint reliability of Sn–9Zn solder on Cu or electrolytic Au/Ni/Cu BGA substrate. Microelectron Eng 2005;82:561–8. [12] Chi-Yang Yu, Chen Wei-Yu, Duh Jenq-Gong. Suppressing the growth of Cu–Sn intermetallic compounds in Ni/Sn–Ag–Cu/Cu–Zn solder joints during thermal aging. Intermetallics 2012;26:11–7. [13] Chiu MY, Wang SS, Chuang TH. Intermetallic compounds formed during interfacial reactions between liquid Sn–8Zn–3Bi solders and Ni substrates. J Electron Mater 2002;31:494–9. [14] Alam MO, Chan YC, Tu KN. Effect of reaction time and P content on mechanical strength of the interface formed between eutectic Sn–Ag solder and Au/ electroless Ni(P)/Cu bond pad. J Appl Phys 2003;94:4108–15. [15] Choi P, Al-Kassab T. Thermal stability of nanocrystalline nickel–18 at.% tungsten alloy investigated with the tomographic atom probe. Mater Sci Eng A 2003;353:74–9. [16] Choi IS, Detor AJ. Mechanics of indentation of plastically graded materials – II: Experiments on nanocrystalline alloys with grain size gradients. J Mech Phys Solids 2008;56:172–83. [17] Luo T, Hu A, Hu J, Li M, Mao D. Microstructure and mechanical properties of Sn–Zn–Bi–Cr lead-free solder. Microelectron Reliab 2012;52:585–8. [18] Mohan Kumar K, Kripesh V. Nanoindentation study of Zn-based Pb free solders used in fine pitch interconnect applications. Mater Sci Eng A 2006;423:57–63. [19] Wang CH, Chen HH. Kinetic analysis of Ni5Zn21 growth at the interface between Sn–Zn solders and Ni. Intermetallics 2012;22:166–75. [20] Haseeb ASMA, Chew CS, Johan Mohd Rafie. Interfacial reactions between Sn– 3.5 Ag solder and Ni–W alloy films. J Mater Sci Mater Electron 2011;22:1372–7. [21] Królikowski Andrzej, Płon´ska Ewelina. Effects of compositional and structural features on corrosion behavior of nickel–tungsten alloys. J Solid State Electrochem 2009;13:263–75. [22] Alimadadi H, Ahmadi M. Corrosion properties of electrodeposited nanocrystalline and amorphous patterned Ni–W alloy. Mater Des 2009;30:1356–61. [23] Kim DG, Jung SB. Interfacial reactions and growth kinetics for intermetallic compound layer between In–48Sn solder and bare Cu substrate. J Alloys Compd 2005;386:151–6. [24] Liu NS, Lin KL. Evolution of interfacial morphology of Sn–8.5Zn–0.5Ag–0.1Al– xGa/Cu system during isothermal aging. J Alloys Compd 2008;456:466–73.
Please cite this article in press as: Liang J et al. Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu, Ni and Ni–W substrates. Microelectron Reliab (2013), http://dx.doi.org/10.1016/j.microrel.2013.08.008