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Cu interface
Journal of Alloys and Compounds 389 (2005) 133–139
Formation and morphology of the intermetallic compounds formed at the 91Sn–8.55Zn–0.45Al lead-free solder alloy/Cu interface Moo-Chin Wanga,b,∗ , Shan-Pu Yuc , Tao-Chih Changc , Min-Hsiung Honc b
a Department of Materials Science and Engineering, National United University, 1 Lien-Da, Kung-Ching Li, Miao-Li 36003, Taiwan Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kung Road, Kaohsiung 80782, Taiwan c Department of Materials Science and Engineering, National Cheng Kung University, 1 Ta-Hsueh Road, Tainan 70101, Taiwan
Received 15 June 2004; received in revised form 4 August 2004; accepted 5 August 2004
Abstract Formation and morphology of the intermetallic compounds (IMCs) formed at the 91Sn–8.55Zn–0.45Al lead-free solder alloy/Cu interface has been studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron diffraction (ED). The XRD analysis indicates that the ␥-Cu5 Zn8 IMC is found at the solder/Cu interface in the as-wetted sample. When heat treated at 423 K for 100 or 400 h, the IMC phases still remain as ␥-Cu5 Zn8 and Cu6 Sn5 . As the aging time increases from 400 to 1000 h, besides ␥-Cu5 Zn8 and Cu6 Sn5 , ␥ -Cu9 Al4 also appears. Planar-shaped ␥-Cu5 Zn8 and Cu6 Sn5 are found near the solder and the Cu substrate, respectively. However, scallop-shaped ␥-Cu5 Zn8 is found at the eutectic 91Sn–8.55Zn–0.45Al solder alloy/Cu interface when aged at 423 K. © 2004 Elsevier B.V. All rights reserved. Keywords: Intermetallic compounds; 91Sn–8.55Zn–0.45Al lead-free solder alloy; Planar-shaped; Scallop-shaped; ␥-Cu5 Zn8 ; Cu6 Sn5 ; ␥ -Cu9 Al4
1. Introduction For electronic parts and devices, solder joints provide electrical conductivity and suitable mechanical strength [1]. Although a lot of solder alloys can be chosen, eutectic Sn–Pb solder alloys with 37–40 wt.% lead are widely used for microsoldering in electronic assembles because of their excellent wettability and other necessary properties, such as appropriate melting temperature and surface tension. However, it has been found that when lead or its compound is inhaled, its toxicity is very harmful to human health. The use of Pb alloys is hence prohibited [2–4], resulting in an emergent research on lead-free solders in electronic industry for substituting the Pb–Sn system. McCormack et al. [5] have pointed out that the nontoxic binary Pb-free solder close to the eutectic composition (91Sn–9Zn) has a melting temperature of 471 K. The Sn–Zn alloy has excellent mechanical properties but is susceptible ∗
Corresponding author. Tel.: +886 37 381710; fax: +886 37 324047. E-mail address: [email protected] (M.-C. Wang).
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to oxidation and corrosion. Al has been incorporated with Zn to enhance the atmospheric corrosion resistance of the conventional galvanizing coating for steels. Al forms a solid solution with Zn and Sn and has an eutectic point at 470 K as reported by Sebaoun et al. [6], who have discussed the diffusion path of various Sn–Zn–Al systems at various isotherms. In addition to Al, certain transition metals such as Cr, Ti and Zr also assist in improving the oxidation and corrosion resistance of the alloys in view of the passivation behavior of these elements. Nevertheless, these elements have high melting points and do not form low melting eutectic alloys with Sn and Zn, and thus are excluded from consideration as Pb substitutes. Various lead-free solder alloys hot-wetted on Cu substrates have been reported by Yu et al. [7–11]. This method offers the benefit that optimal bulk quantities such as adhesion strength [7], process parameters [8], wetting characteristics [9], composition and heat-treatment effects [10], and IMCs formation [11] can be obtained. The present study is a continuation of our work to evaluate the formation and microstructure of the intermetallic compounds (IMCs) layer formed at the eutetic
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91Sn–8.55Zn–0.45Al lead-free solder alloy/Cu substrate interface.
2. Experimental procedures 2.1. Sample preparation The 91Sn–8.55Zn–0.45Al lead-free solder alloy was prepared by melting a Zn–5Al master alloy with tin metal. Accordingly, the solder composition is Sn–9(5Al–Zn), where 9 represents the weight percentage of Zn–5Al. A Cu plate (about 99.9% pure) of approximately 65 mm × 20 mm × 2.5 mm in size, was degreased in an alkaline solution of NaOH (5 wt.%) for 15 s, followed by rinsing in deionized (DI) water for 10 s. Then the Cu substrate was pickled in an HCl solution (5 vol.%) for 10 s, followed by rinsing in DI water again. The substrate was then dipped in a dimethylammonium chloride (DMAHCl) flux (2.5 g DMAHCl/100 C2 H5 OH) for 10 s. After being fluxed, the sample was immersed into the 91Sn–8.55Zn–0.45Al solder bath at 573 K for 5 s as shown in Fig. 1. All samples were aged at 393, 423, and 453 K for various aging times, respectively, in a furnace capable of maintaining the temperature to ±3 K. 2.2. Sample characterization The IMCs were exposed by etching out the un-reacted solder with a solution of 10% 100 g/l FeCl3 /6H2 O, 10% 100 g/l CrO3 /6H2 O, 40% ethanol, 10% HNO3 and 30% HCl, and were analytically determined by X-ray diffraction (XRD).
Fig. 1. Schematic diagram of the apparatus for wetting test.
The XRD patterns of the IMCs were obtained using an Xray diffractometer (Model Rad II A, Rigaku, Tokyo, Japan) with Cu K␣ radiation and an Ni filter, operated at 30 kV, 200 mA and a scanning rate (2θ) of 0.25◦ /min. The thickness of the IMCs layer was determined by scanning electron microscopy (SEM, JEOL 840, Tokyo, Japan). The sample was cross-sectioned and the segment was mounted and prepared to determine the thickness of the IMCs layer. Twenty thickness measurements were conducted for each run and the average value was obtained. The morphology of the IMCs at the lead-free solder alloy/Cu interface was observed by SEM (JEOL840, Tokyo, Japan) and transmission electron microscopy (TEM, H700H, Hitachi, Tokyo, Japan). The specimen for TEM observation was prepared by cutting with a diamond saw, grinding and polishing to 50 m thick, and finally ion milling with Ar+ ions. Electron diffraction (ED) examination was performed on the carefully thinned specimen.
3. Results and discussion 3.1. Phase identification of the IMC layer formed at the 91Sn–8.55Zn–0.45Al lead-free solder alloy/Cu interface Interfacial phase transformation at the solder alloy/substrate joint affects the IMC growth significantly. At the interface between the lead-free solder alloy and copper substrate, the formation of the IMC is accompanied by interfacial reactions and spalling [12–14]. Complex reactions occur at the interface, and the precipitates in the lead-free solder near the interface are accumulated to form the intermetallic layers. The XRD pattern of the sample heated at 573 K for 2.5 min at a rate of 11.8 mm/s and then etched to eliminate the unreacted solder alloy is shown in Fig. 2. It indicates that the ␥-Cu5 Zn8 is found at the 91Sn–8.55Zn–0.45Al lead-free solder/Cu interface. The ␥-Cu5 Zn8 is also found both at the eutectic Sn–Zn/Cu interface [8] and in the Sn–Zn solder alloy on the Cu substrate [15,16]. However, no Cu–Sn compound
Fig. 2. XRD pattern of the sample dipped at 573 K for 2.5 min and a rate of 11.8 mm/s, and then etched to eliminate unreacted solder alloy. () ␥Cu5 Zn8 ; (♦) Sn; () Cu6 Sn5 ; (+) ␥ -Cu9 Al4 ; (䊉) Cu.
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forms at the interface between the Sn–Zn–Al solder alloy and copper substrate, which differs from the previous reports [16–21]. Since the Gibbs free energy of formation for ␥-Cu5 Zn8 is −12.34 kJ/mol, which is much lower than those of -Cu3 Sn (G = −7.78 kJ/mol) and -Cu6 Sn5 (G = −7.42 kJ/mol) [22], it would be more stable. Moreover, the diffusivity of Sn in Cu–Sn alloys is given by DSn = 1.90 × 10−10 cm2 /s at 573 K, and that of Zn in Cu–Zn alloys is DZn = 2.70 × 10−10 cm2 /s at 573 K [11]. Those data justify the formation of ␥-Cu5 Zn8 instead of Cu–Sn compounds. The XRD patterns of the eutectic 91Sn–8.55Zn–0.45Al solder alloy hot-wetted on the Cu substrate at 573 K and heat treated at 423 K for various times are shown in Fig. 3. As heattreated for 100 h, the ␥-Cu5 Zn8 and Cu6 Sn5 phases are found at the solder alloy/Cu interface as shown in Fig. 3(a). As heat treated for 250 and 400 h, the IMCs phases still remain as ␥-Cu5 Zn8 and Cu6 Sn5 , as shown in Fig. 3(b). In Fig. 3(c), when the heat treatment time increases from 400 to 1000 h, besides, ␥-Cu5 Zn8 and Cu6 Sn5 , ␥ -Cu9 Al4 also appears. 3.2. Morphology of the IMCs formed at the 91Sn–8.55Zn–0.45Al lead-free solder alloy/Cu interface The SEM micrographs of the 91Sn–8.55Zn–0.45Al leadfree solder alloys as-wetted on the copper substrate and heated at 423 K for various times are shown in Fig. 4. The IMC is observed at the interface of the lead-free solder alloy and copper substrate. Comparing with Fig. 4(a), the ␥Cu5 Zn8 IMC layer of the sample heated at 423 K for 100 h (Fig. 4(b)) is thicker than the as-wetted one (Fig. 4(a)). Moreover, we observed growth of the ␥-Cu5 Zn8 and Cu6 Sn5 IMC phases. The scallop-shaped IMC grains and discontinuous IMC layer are shown in Fig. 4(b). As aging time increases to 250 h (Fig. 4(c)), the interaction between the 91Sn–8.55Zn–0.45Al lead-free solder alloy and Cu substrate should be enhanced, and the growth of the ␥-Cu5 Zn8 and Cu6 Sn5 IMC phases is observed, which induces the formation of the continuous scallop-shaped IMC layer. When heated for 400 h, the scallop-shaped IMC grains and Kirkendall voids in Fig. 4(d) are bigger than that in Fig. 4(c). Especially, after heating at 423 K for 1000 h (Fig. 4(e)), the cracks formed from the interconnected Kirkendall voids are more obvious between the ␥-Cu5 Zn8 and Cu substrate. On the other hand, the cracks also form between the IMC layer and Cu substrate. According to Fig. 3(c), for more than 400 h, the ␥ -Cu9 Al4 IMC grows and results in microcracks [11]. Simultaneously, Zn diffuses to expedite the growth of ␥-Cu5 Zn8 . Frear et al. [23] have pointed out that the diffusional process which forms an intermetallic layer requires all elements in the solder joint (i.e. Cu and Zn) to diffuse. If one element diffuses more quickly than the other, excessive vacancies will be formed within it, which eventually accumulate and coalesce in a compositional plane defined by the relative diffusional fluxes. This effect is known as Kirkendall voids which reduce the effective cross-sectional area and
Fig. 3. XRD patterns of the eutectic 91Sn–8.55Zn–0.45Al solder alloy hotwetted on the Cu substrate at 573 K and heat treated at 423 K for (a) 100, (b) 400, and (c) 1000 h. () ␥-Cu5 Zn8 ; (♦) Sn; () Cu6 Sn5 ; (+) ␥ -Cu9 Al4 ; (䊉) Cu.
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Fig. 4. SEM micrographs of the cross-sectioned samples for as-dipped and aged at 423 K for various times: (a) as-wetted, (b) 100, (c) 250, (d) 400, and (e) 1000 h.
Table 1 Comparison of the morphology and phases of the intermetallic compound layer formed at the interface between solder alloy and Cu substrate System
Process condition
Heat treatment (temperature/time)
Layer near solder
Layer near Cu
Reference
91Sn–9Zn/Cu 91Sn–9Zn/Cu 91Sn–9Zn/Cu 91Sn–9Zn/Cu 62Sn–36Pb–2Ag/Cu 91Sn–9Zn/Cu 91Sn–9Zn/Cu 91Sn–9Zn/Cu 91Sn–8.55Zn–0.45Al/Cu 91Sn–8.55Zn–0.45Al/Cu 91Sn–8.55Zn–0.45Al/Cu 91Sn–8.55Zn–0.45Al/Cu
Wet test at 523 K/30 s Wet test at 523 K/30 s Wet test at 523 K/30 s As soldered at 543 K ≥462 K, ≥0.5 min Bulk solder Bulk solder Hot-dipped Hot-dipped Hot-dipped Hot-dipped Hot-dipped
373 K/20 h 403 K/20 h 433 K/20 h – – 423 K/300 h 423 K/600 h 423 K/600 h 423 K/100 h 423 K/250 h 423 K/400 h 423 K/1000 h
-Cu3 Sn (scallop) – ␥-Cu5 Zn8 (scallop) ␥-Cu5 Zn8 (scallop) -Cu6 Sn5 (scallop) ␥-Cu5 Zn8 (planar) ␥-Cu5 Zn8 (planar) ␥-Cu5 Zn8 (scallop) ␥-Cu5 Zn8 (scallop) ␥-Cu5 Zn8 (scallop) ␥-Cu5 Zn8 (scallop) ␥-Cu5 Zn8 (planar)
␥-Cu5 Zn8 (planar) ␥-Cu5 Zn8 (planar) -Cu6 Sn5 (inverted trigonal)  -CuZn (planar) -Cu3 Sn (planar) -Cu6 Sn5 (isolate) -Cu6 Sn5 (inverted trigonal) -Cu6 Sn5 (inverted trigonal) – – ␥ -Cu9 Al4 (planar) ␥ -Cu9 Al4 (planar)
[24] [24] [24] [15] [25] [8] [8] [8] This study This study This study This study
M.-C. Wang et al. / Journal of Alloys and Compounds 389 (2005) 133–139
cause mechanical weakening [11]. In addition, after heating at 423 K for more than 200 h, the adhesion strength decreases obviously from above 7 MPa to below 5 MPa for the 91Sn–8.55Zn–0.45Al solder alloy/Cu substrate system [11]. In general, one of the main causes of breakdown in solder joints has been attributed to the excessive growth of IMCs formed at the interface. The comparison of the morphology and phases of the IMC layer formed at the interface between the various solder alloys and the copper substrate is listed in Table 1. Lee et al. [24] have pointed out that in the wet test of the 91Sn–9Zn/Cu system at 423 K for 30 s and heating at 373 K for 20 h, the scallopshaped -Cu3 Zn and planar ␥-Cu5 Zn8 are found near the solder and the copper, respectively, but no -Cu3 Zn was found when heating at 403 K for 20 h. After heating at 433 K for 400 h, the scallop-shaped ␥-Cu5 Zn8 and inverted trigonal Cu6 Sn5 are found near the solder and the copper, respectively. Yu et al. [8] have also pointed out that in the 91Sn–9Zn/Cu system, planar and scallop-shaped ␥-Cu5 Zn8 are found near the solder, and the isolated inverted triangular -Cu6 Sn5 is found in the copper substrate. The morphology of the IMCs is affected by the process condition and heat treatment. In
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the present study, after prolonged heating at 423 K for times longer than 1000 h, the ␥ -Cu9 Al4 IMC (Figs. 3(c) and 6) continuously grows. When heating at 423 K for 1000 h, the planar ␥-Cu5 Zn8 is found in the solder alloy and the ␥ -Cu9 Al4 is found near Cu. However, the scallop-shaped ␥-Cu5 Zn8 is found in the eutectic 91Sn–8.55Zn–0.45Al solder alloy/Cu system when heated at 423 K for 100 and 250 h, respectively, but no ␥ -Cu9 Al4 is formed in the heat treatment process. 3.3. Microstructure at the interface of 91Sn–8.55Zn–0.45Al lead-free solder alloy/Cu substrate Fig. 5 shows the TEM micrographs (bright and dark fields) and ED pattern of the interface region of the as-wetted sample. The ED pattern also provides the criteria for the presence of the ␥-Cu5 Zn8 in this system. Fig. 5 shows the individual grains with subangular and somewhat bell-shaped morphology. This result may be partly due to the impingement of small crystallite, and the enhanced isotropy is aided by Zn which diffuses more quickly than others. The TEM images and ED pattern of the 91Sn– 8.55Zn–0.45Al lead-free solder alloy wetted on Cu substrate
Fig. 5. TEM micrograph and ED pattern of the 91Sn–8.55Zn–0.45Al lead-free solder alloy as-wetted on the Cu substrate for: (a) BF image, (b) DF image, and (c) ED pattern of ␥-Cu5 Zn8 .
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Fig. 6. TEM images and ED patterns of the 91Sn–8.55Zn–0.45Al lead-free solder alloy wetted on Cu substrate and aging at 423 K for 1000 h, for (a) BF image, (b) DF image, and (c) ED pattern of ␥-Cu9 Al4 .
and aged at 423 K for 1000 h are shown in Fig. 6. Fig. 6(a) and (b) are the BF and DF images of ␥ -Cu9 Al4 . This result indicates that the heat treatment enhances the Al enrichment [7] at the ␥-Cu5 Zn8 /Cu interface and Al interacts with Cu to form ␥ -Cu9 Al4 IMC [11]. Fig. 6(c) shows the ED pattern of the ␥ -Cu9 Al4 . In conjunction with the XRD analysis, the BF and DF images of TEM together with the corresponding ED pattern provide the evidence for the formation of the ␥ -Cu9 Al4 IMC at the 91Sn–8.55Zn–0.45Al lead-free solder alloy/Cu interface.
4. Conclusions The formation and morphology of the intermetallic compounds formed at the 91Sn–8.55Zn–0.45Al lead-free solder
alloy/Cu interface has been investigated with XRD, SEM, TEM and ED. The following conclusions are obtained: (1) The ␥-Cu5 Zn8 IMC is found at the 91Sn–8.55Zn–0.45Al lead-free solder/Cu interface by the XRD analysis of the as-wetted sample. (2) As heat treated at 423 K for 400 h, the IMC phases still remain as ␥-Cu5 Zn8 and Cu6 Sn5 . When the aging time increases from 400 to 1000 h, besides ␥-Cu5 Zn8 and Cu6 Sn5 , the ␥ -Cu9 Al4 also appears. (3) When aged at 423 K for 1000 h, the planar ␥-Cu5 Zn8 is found in the solder alloy and the ␥ -Cu9 Al4 is found near the copper substrate. However, the scallop-shaped ␥Cu5 Zn8 is found in the 91Sn–8.55Sn–0.45Al solder/Cu system when heated at 423 K for 100 and 250 h, respectively, but no ␥ -Cu9 Al4 is formed in the heat treatment
M.-C. Wang et al. / Journal of Alloys and Compounds 389 (2005) 133–139
process. (4) The ␥-Cu5 Zn8 near the solder alloy and ␥ -Cu9 Al4 near the Cu substrate are found by TEM observation at the interface between the 91Sn–8.55Zn–0.45Al solder alloy and the copper substrate.
Acknowledgements This work was supported by the National Science Council, the Republic of China under Contract No. NSC85-2216E-151-005 and NSC86-2216-E-151-006, which is gratefully acknowledged.
References [1] G. Ghosh, M. Loomans, M.E. Fine, J. Electron. Mater. 23 (1994) 741. [2] Environment Protection Agency, Strategy for Reducing Lead Exposure, 2 February 1991. [3] P.J. Walitsky, F.G. Yost, The relevance of current environmental issue to solder joints in microelectronic applications, in: Proceedings of the NEPCON West, Anaheim, CA, 23–27 February, 1992, p. 1671. [4] H. Reid, D. Moynihan, J. Leiberman, B. Bradley, Toxic Lead Reduction Act of 1990, p. S-2637. [5] M. McCormack, S. Jin, H.S. Chen, J. Electron. Mater. 23 (1994) 687.
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[6] A. Sebaoun, D. Vincent, D. Treheux, Mater. Sci. Technol. 3 (1987) 241. [7] S.P. Yu, M.H. Hon, M.C. Wang, J. Electron. Mater. 29 (2000) 237. [8] S.P. Yu, H.J. Lin, M.H. Hon, M.C. Wang, J. Mater. Sci.-Mater. Electron. 11 (2000) 36. [9] S.P. Yu, C.L. Liao, M.H. Hon, M.C. Wang, J. Mater. Sci. 35 (2000) 4217. [10] S.P. Yu, H.C. Wang, M.H. Hon, M.C. Wang, JOM 52 (6) (2000) 36. [11] S.P. Yu, M.C. Wang, M.H. Hon, J. Mater. Res. 16 (2001) 76. [12] D. Yao, J.K. Shang, Metall. Mater. Trans. A 26 (1995) 2677. [13] A.J. Sunwoo, J.W. Morries Jr., G.K. Lucey, J.K. Shang, Metall. Mater. Trans. A 23 (1992) 1323. [14] D. Yao, J.K. Shang, IEEE Trans. Component B 19 (1996) 154. [15] K. Suganuma, K. Niihara, T. Shoutoku, Y. Nakamura, J. Mater. Res. 13 (1998) 2859. [16] S.W. Yoon, J.R. Soh, B.L. Lee, H.M. Lee, Acta Mater. 45 (1997) 951. [17] Z. Mei, J.W. Morris Jr., J. Electron. Mater. 21 (1992) 73. [18] Z. Mei, J.W. Morris Jr., J. Electron. Mater. 21 (1992) 599. [19] P.T. Vianco, D.R. Frear, JOM 45 (7) (1993) 14. [20] J.W. Morris Jr., J.L. Frear Goldstein, Z. Mei, JOM 45 (7) (1993) 25. [21] L.F. Felton, C.H. Reader, D.B. Knorr, J. Met. 45 (1993) 28. [22] R. Hultgren, P.D. Desai, D.T. Hawkins, M. Gileiser, K.K. Kelley, Selected Values of the Thermodynamic Properties of Binary Alloy, ASM, Metals Park, OH, 1973. [23] D.R. Frear, W.B. Jones, K.R. Kinsman (Eds.), Solder Mechanics: A State of the Art Assessment, TMS, Warrendale, PA, 1991, p. 57. [24] H.M. Lee, S.W. Yoon, B.J. Lee, J. Electron. Mater. 27 (1998) 1161. [25] M. Schaefer, R.A. Fournelle, J. Liang, J. Electron. Mater. 27 (1998) 1167.
Formation and morphology of the intermetallic compounds formed at the 91Sn–8.55Zn–0.45Al lead-free solder alloy/Cu interface Moo-Chin Wanga,b,∗ , Shan-Pu Yuc , Tao-Chih Changc , Min-Hsiung Honc b
a Department of Materials Science and Engineering, National United University, 1 Lien-Da, Kung-Ching Li, Miao-Li 36003, Taiwan Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kung Road, Kaohsiung 80782, Taiwan c Department of Materials Science and Engineering, National Cheng Kung University, 1 Ta-Hsueh Road, Tainan 70101, Taiwan
Received 15 June 2004; received in revised form 4 August 2004; accepted 5 August 2004
Abstract Formation and morphology of the intermetallic compounds (IMCs) formed at the 91Sn–8.55Zn–0.45Al lead-free solder alloy/Cu interface has been studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron diffraction (ED). The XRD analysis indicates that the ␥-Cu5 Zn8 IMC is found at the solder/Cu interface in the as-wetted sample. When heat treated at 423 K for 100 or 400 h, the IMC phases still remain as ␥-Cu5 Zn8 and Cu6 Sn5 . As the aging time increases from 400 to 1000 h, besides ␥-Cu5 Zn8 and Cu6 Sn5 , ␥ -Cu9 Al4 also appears. Planar-shaped ␥-Cu5 Zn8 and Cu6 Sn5 are found near the solder and the Cu substrate, respectively. However, scallop-shaped ␥-Cu5 Zn8 is found at the eutectic 91Sn–8.55Zn–0.45Al solder alloy/Cu interface when aged at 423 K. © 2004 Elsevier B.V. All rights reserved. Keywords: Intermetallic compounds; 91Sn–8.55Zn–0.45Al lead-free solder alloy; Planar-shaped; Scallop-shaped; ␥-Cu5 Zn8 ; Cu6 Sn5 ; ␥ -Cu9 Al4
1. Introduction For electronic parts and devices, solder joints provide electrical conductivity and suitable mechanical strength [1]. Although a lot of solder alloys can be chosen, eutectic Sn–Pb solder alloys with 37–40 wt.% lead are widely used for microsoldering in electronic assembles because of their excellent wettability and other necessary properties, such as appropriate melting temperature and surface tension. However, it has been found that when lead or its compound is inhaled, its toxicity is very harmful to human health. The use of Pb alloys is hence prohibited [2–4], resulting in an emergent research on lead-free solders in electronic industry for substituting the Pb–Sn system. McCormack et al. [5] have pointed out that the nontoxic binary Pb-free solder close to the eutectic composition (91Sn–9Zn) has a melting temperature of 471 K. The Sn–Zn alloy has excellent mechanical properties but is susceptible ∗
Corresponding author. Tel.: +886 37 381710; fax: +886 37 324047. E-mail address: [email protected] (M.-C. Wang).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.08.009
to oxidation and corrosion. Al has been incorporated with Zn to enhance the atmospheric corrosion resistance of the conventional galvanizing coating for steels. Al forms a solid solution with Zn and Sn and has an eutectic point at 470 K as reported by Sebaoun et al. [6], who have discussed the diffusion path of various Sn–Zn–Al systems at various isotherms. In addition to Al, certain transition metals such as Cr, Ti and Zr also assist in improving the oxidation and corrosion resistance of the alloys in view of the passivation behavior of these elements. Nevertheless, these elements have high melting points and do not form low melting eutectic alloys with Sn and Zn, and thus are excluded from consideration as Pb substitutes. Various lead-free solder alloys hot-wetted on Cu substrates have been reported by Yu et al. [7–11]. This method offers the benefit that optimal bulk quantities such as adhesion strength [7], process parameters [8], wetting characteristics [9], composition and heat-treatment effects [10], and IMCs formation [11] can be obtained. The present study is a continuation of our work to evaluate the formation and microstructure of the intermetallic compounds (IMCs) layer formed at the eutetic
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91Sn–8.55Zn–0.45Al lead-free solder alloy/Cu substrate interface.
2. Experimental procedures 2.1. Sample preparation The 91Sn–8.55Zn–0.45Al lead-free solder alloy was prepared by melting a Zn–5Al master alloy with tin metal. Accordingly, the solder composition is Sn–9(5Al–Zn), where 9 represents the weight percentage of Zn–5Al. A Cu plate (about 99.9% pure) of approximately 65 mm × 20 mm × 2.5 mm in size, was degreased in an alkaline solution of NaOH (5 wt.%) for 15 s, followed by rinsing in deionized (DI) water for 10 s. Then the Cu substrate was pickled in an HCl solution (5 vol.%) for 10 s, followed by rinsing in DI water again. The substrate was then dipped in a dimethylammonium chloride (DMAHCl) flux (2.5 g DMAHCl/100 C2 H5 OH) for 10 s. After being fluxed, the sample was immersed into the 91Sn–8.55Zn–0.45Al solder bath at 573 K for 5 s as shown in Fig. 1. All samples were aged at 393, 423, and 453 K for various aging times, respectively, in a furnace capable of maintaining the temperature to ±3 K. 2.2. Sample characterization The IMCs were exposed by etching out the un-reacted solder with a solution of 10% 100 g/l FeCl3 /6H2 O, 10% 100 g/l CrO3 /6H2 O, 40% ethanol, 10% HNO3 and 30% HCl, and were analytically determined by X-ray diffraction (XRD).
Fig. 1. Schematic diagram of the apparatus for wetting test.
The XRD patterns of the IMCs were obtained using an Xray diffractometer (Model Rad II A, Rigaku, Tokyo, Japan) with Cu K␣ radiation and an Ni filter, operated at 30 kV, 200 mA and a scanning rate (2θ) of 0.25◦ /min. The thickness of the IMCs layer was determined by scanning electron microscopy (SEM, JEOL 840, Tokyo, Japan). The sample was cross-sectioned and the segment was mounted and prepared to determine the thickness of the IMCs layer. Twenty thickness measurements were conducted for each run and the average value was obtained. The morphology of the IMCs at the lead-free solder alloy/Cu interface was observed by SEM (JEOL840, Tokyo, Japan) and transmission electron microscopy (TEM, H700H, Hitachi, Tokyo, Japan). The specimen for TEM observation was prepared by cutting with a diamond saw, grinding and polishing to 50 m thick, and finally ion milling with Ar+ ions. Electron diffraction (ED) examination was performed on the carefully thinned specimen.
3. Results and discussion 3.1. Phase identification of the IMC layer formed at the 91Sn–8.55Zn–0.45Al lead-free solder alloy/Cu interface Interfacial phase transformation at the solder alloy/substrate joint affects the IMC growth significantly. At the interface between the lead-free solder alloy and copper substrate, the formation of the IMC is accompanied by interfacial reactions and spalling [12–14]. Complex reactions occur at the interface, and the precipitates in the lead-free solder near the interface are accumulated to form the intermetallic layers. The XRD pattern of the sample heated at 573 K for 2.5 min at a rate of 11.8 mm/s and then etched to eliminate the unreacted solder alloy is shown in Fig. 2. It indicates that the ␥-Cu5 Zn8 is found at the 91Sn–8.55Zn–0.45Al lead-free solder/Cu interface. The ␥-Cu5 Zn8 is also found both at the eutectic Sn–Zn/Cu interface [8] and in the Sn–Zn solder alloy on the Cu substrate [15,16]. However, no Cu–Sn compound
Fig. 2. XRD pattern of the sample dipped at 573 K for 2.5 min and a rate of 11.8 mm/s, and then etched to eliminate unreacted solder alloy. () ␥Cu5 Zn8 ; (♦) Sn; () Cu6 Sn5 ; (+) ␥ -Cu9 Al4 ; (䊉) Cu.
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forms at the interface between the Sn–Zn–Al solder alloy and copper substrate, which differs from the previous reports [16–21]. Since the Gibbs free energy of formation for ␥-Cu5 Zn8 is −12.34 kJ/mol, which is much lower than those of -Cu3 Sn (G = −7.78 kJ/mol) and -Cu6 Sn5 (G = −7.42 kJ/mol) [22], it would be more stable. Moreover, the diffusivity of Sn in Cu–Sn alloys is given by DSn = 1.90 × 10−10 cm2 /s at 573 K, and that of Zn in Cu–Zn alloys is DZn = 2.70 × 10−10 cm2 /s at 573 K [11]. Those data justify the formation of ␥-Cu5 Zn8 instead of Cu–Sn compounds. The XRD patterns of the eutectic 91Sn–8.55Zn–0.45Al solder alloy hot-wetted on the Cu substrate at 573 K and heat treated at 423 K for various times are shown in Fig. 3. As heattreated for 100 h, the ␥-Cu5 Zn8 and Cu6 Sn5 phases are found at the solder alloy/Cu interface as shown in Fig. 3(a). As heat treated for 250 and 400 h, the IMCs phases still remain as ␥-Cu5 Zn8 and Cu6 Sn5 , as shown in Fig. 3(b). In Fig. 3(c), when the heat treatment time increases from 400 to 1000 h, besides, ␥-Cu5 Zn8 and Cu6 Sn5 , ␥ -Cu9 Al4 also appears. 3.2. Morphology of the IMCs formed at the 91Sn–8.55Zn–0.45Al lead-free solder alloy/Cu interface The SEM micrographs of the 91Sn–8.55Zn–0.45Al leadfree solder alloys as-wetted on the copper substrate and heated at 423 K for various times are shown in Fig. 4. The IMC is observed at the interface of the lead-free solder alloy and copper substrate. Comparing with Fig. 4(a), the ␥Cu5 Zn8 IMC layer of the sample heated at 423 K for 100 h (Fig. 4(b)) is thicker than the as-wetted one (Fig. 4(a)). Moreover, we observed growth of the ␥-Cu5 Zn8 and Cu6 Sn5 IMC phases. The scallop-shaped IMC grains and discontinuous IMC layer are shown in Fig. 4(b). As aging time increases to 250 h (Fig. 4(c)), the interaction between the 91Sn–8.55Zn–0.45Al lead-free solder alloy and Cu substrate should be enhanced, and the growth of the ␥-Cu5 Zn8 and Cu6 Sn5 IMC phases is observed, which induces the formation of the continuous scallop-shaped IMC layer. When heated for 400 h, the scallop-shaped IMC grains and Kirkendall voids in Fig. 4(d) are bigger than that in Fig. 4(c). Especially, after heating at 423 K for 1000 h (Fig. 4(e)), the cracks formed from the interconnected Kirkendall voids are more obvious between the ␥-Cu5 Zn8 and Cu substrate. On the other hand, the cracks also form between the IMC layer and Cu substrate. According to Fig. 3(c), for more than 400 h, the ␥ -Cu9 Al4 IMC grows and results in microcracks [11]. Simultaneously, Zn diffuses to expedite the growth of ␥-Cu5 Zn8 . Frear et al. [23] have pointed out that the diffusional process which forms an intermetallic layer requires all elements in the solder joint (i.e. Cu and Zn) to diffuse. If one element diffuses more quickly than the other, excessive vacancies will be formed within it, which eventually accumulate and coalesce in a compositional plane defined by the relative diffusional fluxes. This effect is known as Kirkendall voids which reduce the effective cross-sectional area and
Fig. 3. XRD patterns of the eutectic 91Sn–8.55Zn–0.45Al solder alloy hotwetted on the Cu substrate at 573 K and heat treated at 423 K for (a) 100, (b) 400, and (c) 1000 h. () ␥-Cu5 Zn8 ; (♦) Sn; () Cu6 Sn5 ; (+) ␥ -Cu9 Al4 ; (䊉) Cu.
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Fig. 4. SEM micrographs of the cross-sectioned samples for as-dipped and aged at 423 K for various times: (a) as-wetted, (b) 100, (c) 250, (d) 400, and (e) 1000 h.
Table 1 Comparison of the morphology and phases of the intermetallic compound layer formed at the interface between solder alloy and Cu substrate System
Process condition
Heat treatment (temperature/time)
Layer near solder
Layer near Cu
Reference
91Sn–9Zn/Cu 91Sn–9Zn/Cu 91Sn–9Zn/Cu 91Sn–9Zn/Cu 62Sn–36Pb–2Ag/Cu 91Sn–9Zn/Cu 91Sn–9Zn/Cu 91Sn–9Zn/Cu 91Sn–8.55Zn–0.45Al/Cu 91Sn–8.55Zn–0.45Al/Cu 91Sn–8.55Zn–0.45Al/Cu 91Sn–8.55Zn–0.45Al/Cu
Wet test at 523 K/30 s Wet test at 523 K/30 s Wet test at 523 K/30 s As soldered at 543 K ≥462 K, ≥0.5 min Bulk solder Bulk solder Hot-dipped Hot-dipped Hot-dipped Hot-dipped Hot-dipped
373 K/20 h 403 K/20 h 433 K/20 h – – 423 K/300 h 423 K/600 h 423 K/600 h 423 K/100 h 423 K/250 h 423 K/400 h 423 K/1000 h
-Cu3 Sn (scallop) – ␥-Cu5 Zn8 (scallop) ␥-Cu5 Zn8 (scallop) -Cu6 Sn5 (scallop) ␥-Cu5 Zn8 (planar) ␥-Cu5 Zn8 (planar) ␥-Cu5 Zn8 (scallop) ␥-Cu5 Zn8 (scallop) ␥-Cu5 Zn8 (scallop) ␥-Cu5 Zn8 (scallop) ␥-Cu5 Zn8 (planar)
␥-Cu5 Zn8 (planar) ␥-Cu5 Zn8 (planar) -Cu6 Sn5 (inverted trigonal)  -CuZn (planar) -Cu3 Sn (planar) -Cu6 Sn5 (isolate) -Cu6 Sn5 (inverted trigonal) -Cu6 Sn5 (inverted trigonal) – – ␥ -Cu9 Al4 (planar) ␥ -Cu9 Al4 (planar)
[24] [24] [24] [15] [25] [8] [8] [8] This study This study This study This study
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cause mechanical weakening [11]. In addition, after heating at 423 K for more than 200 h, the adhesion strength decreases obviously from above 7 MPa to below 5 MPa for the 91Sn–8.55Zn–0.45Al solder alloy/Cu substrate system [11]. In general, one of the main causes of breakdown in solder joints has been attributed to the excessive growth of IMCs formed at the interface. The comparison of the morphology and phases of the IMC layer formed at the interface between the various solder alloys and the copper substrate is listed in Table 1. Lee et al. [24] have pointed out that in the wet test of the 91Sn–9Zn/Cu system at 423 K for 30 s and heating at 373 K for 20 h, the scallopshaped -Cu3 Zn and planar ␥-Cu5 Zn8 are found near the solder and the copper, respectively, but no -Cu3 Zn was found when heating at 403 K for 20 h. After heating at 433 K for 400 h, the scallop-shaped ␥-Cu5 Zn8 and inverted trigonal Cu6 Sn5 are found near the solder and the copper, respectively. Yu et al. [8] have also pointed out that in the 91Sn–9Zn/Cu system, planar and scallop-shaped ␥-Cu5 Zn8 are found near the solder, and the isolated inverted triangular -Cu6 Sn5 is found in the copper substrate. The morphology of the IMCs is affected by the process condition and heat treatment. In
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the present study, after prolonged heating at 423 K for times longer than 1000 h, the ␥ -Cu9 Al4 IMC (Figs. 3(c) and 6) continuously grows. When heating at 423 K for 1000 h, the planar ␥-Cu5 Zn8 is found in the solder alloy and the ␥ -Cu9 Al4 is found near Cu. However, the scallop-shaped ␥-Cu5 Zn8 is found in the eutectic 91Sn–8.55Zn–0.45Al solder alloy/Cu system when heated at 423 K for 100 and 250 h, respectively, but no ␥ -Cu9 Al4 is formed in the heat treatment process. 3.3. Microstructure at the interface of 91Sn–8.55Zn–0.45Al lead-free solder alloy/Cu substrate Fig. 5 shows the TEM micrographs (bright and dark fields) and ED pattern of the interface region of the as-wetted sample. The ED pattern also provides the criteria for the presence of the ␥-Cu5 Zn8 in this system. Fig. 5 shows the individual grains with subangular and somewhat bell-shaped morphology. This result may be partly due to the impingement of small crystallite, and the enhanced isotropy is aided by Zn which diffuses more quickly than others. The TEM images and ED pattern of the 91Sn– 8.55Zn–0.45Al lead-free solder alloy wetted on Cu substrate
Fig. 5. TEM micrograph and ED pattern of the 91Sn–8.55Zn–0.45Al lead-free solder alloy as-wetted on the Cu substrate for: (a) BF image, (b) DF image, and (c) ED pattern of ␥-Cu5 Zn8 .
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Fig. 6. TEM images and ED patterns of the 91Sn–8.55Zn–0.45Al lead-free solder alloy wetted on Cu substrate and aging at 423 K for 1000 h, for (a) BF image, (b) DF image, and (c) ED pattern of ␥-Cu9 Al4 .
and aged at 423 K for 1000 h are shown in Fig. 6. Fig. 6(a) and (b) are the BF and DF images of ␥ -Cu9 Al4 . This result indicates that the heat treatment enhances the Al enrichment [7] at the ␥-Cu5 Zn8 /Cu interface and Al interacts with Cu to form ␥ -Cu9 Al4 IMC [11]. Fig. 6(c) shows the ED pattern of the ␥ -Cu9 Al4 . In conjunction with the XRD analysis, the BF and DF images of TEM together with the corresponding ED pattern provide the evidence for the formation of the ␥ -Cu9 Al4 IMC at the 91Sn–8.55Zn–0.45Al lead-free solder alloy/Cu interface.
4. Conclusions The formation and morphology of the intermetallic compounds formed at the 91Sn–8.55Zn–0.45Al lead-free solder
alloy/Cu interface has been investigated with XRD, SEM, TEM and ED. The following conclusions are obtained: (1) The ␥-Cu5 Zn8 IMC is found at the 91Sn–8.55Zn–0.45Al lead-free solder/Cu interface by the XRD analysis of the as-wetted sample. (2) As heat treated at 423 K for 400 h, the IMC phases still remain as ␥-Cu5 Zn8 and Cu6 Sn5 . When the aging time increases from 400 to 1000 h, besides ␥-Cu5 Zn8 and Cu6 Sn5 , the ␥ -Cu9 Al4 also appears. (3) When aged at 423 K for 1000 h, the planar ␥-Cu5 Zn8 is found in the solder alloy and the ␥ -Cu9 Al4 is found near the copper substrate. However, the scallop-shaped ␥Cu5 Zn8 is found in the 91Sn–8.55Sn–0.45Al solder/Cu system when heated at 423 K for 100 and 250 h, respectively, but no ␥ -Cu9 Al4 is formed in the heat treatment
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process. (4) The ␥-Cu5 Zn8 near the solder alloy and ␥ -Cu9 Al4 near the Cu substrate are found by TEM observation at the interface between the 91Sn–8.55Zn–0.45Al solder alloy and the copper substrate.
Acknowledgements This work was supported by the National Science Council, the Republic of China under Contract No. NSC85-2216E-151-005 and NSC86-2216-E-151-006, which is gratefully acknowledged.
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