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20Sn solder alloy
Journal of Alloys and Compounds 476 (2009) 138–141
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Creep behavior of eutectic 80Au/20Sn solder alloy G.S. Zhang a , H.Y. Jing a , L.Y. Xu a,∗ , J. Wei b , Y.D. Han a a b
School of Material Science and Engineering, Tianjin University, Tianjin 300072, China Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore
a r t i c l e
i n f o
Article history: Received 28 July 2008 Received in revised form 20 August 2008 Accepted 3 September 2008 Available online 31 October 2008 Keywords: Eutectic 80Au/20Sn solder alloy Creep Constitutive model Creep mechanism
a b s t r a c t Eutectic 80Au/20Sn solder alloy is widely used in high power electronics and optoelectronics packaging in which the creep property of the solder joint is essential to meet the global demand for longer operating lifetime in their applications. In this study, the tensile creep behavior of bulk eutectic 80Au/20Sn solder alloy is reported and compared with 63Sn37Pb solder joint. The creep strain rate increases and creep lifetime decreases as the applied stress level and temperature increase. The 80Au/20Sn solder alloy shows a superior anti-creep performance over the 63Sn37Pb solder joint. The experimental data were successfully fit with Dorn model and Garofalo model. However, the application of Garofalo model resulted in a lower estimated variance of error terms as compared to the Dorn model. Grain boundary sliding is the possible creep mechanism within the given stress level and temperature. The nucleation, accumulation and further growth of microvoids lead to the creep rupture. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The traditional tin–lead solders have been widely employed as electrical interconnects in electronic industries. However, the toxic Pb can cause harmful influence on environment and health. On the other hand, the tin–lead solders would no longer satisfy the reliability requirement in high power electronic and optoelectronic components with the smaller size of solder joint and higher mechanical, thermal and electrical load in which the excellent creep resistance is essential. Hence, it is urgent to develop the lead-free solders with excellent creep and fatigue resistance. In the recent decade, a great amount of lead-free solders have been reported and the promising candidates mainly include the Sn-based alloys such as Sn–Ag, Sn–Cu, Sn–Zn, Sn–Bi, Sn–Sb, Au–Sn, and Sn–Ag–Cu systems. Among the lead-free candidate solders, the eutectic 80Au/20Sn solder alloy is particularly attractive in high power electronics and optoelectronics as hermetic sealing and die attachment material because it has excellent high-temperature performance, high mechanical strength, high electrical and thermal conductivity, and can also offer fluxless soldering [1–6]. The manufacturing process for such applications has been investigated extensively, with emphasis on the bonding quality, the metallurgical interaction and formation of Au–Sn solder bump [7–11]. Besides the manufacturing and solderability, the thermomechanical properties of 80Au/20Sn solder alloy have been reported [12–14].
∗ Corresponding author. Tel.: +86 22 2740 2439; fax: +86 22 2740 5020. E-mail address: [email protected] (L.Y. Xu). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.009
Nevertheless, the creep behavior of 80Au/20Sn solder has been seldom reported in the literature. The 80Au/20Sn solder has obvious creep response because of its lower melting point. The creep deformation can lower the reliability and performance of the electrical components significantly and creep damage is vital for the reliability of solder joint. Therefore, it is necessary to gain an insight into the creep behavior of eutectic 80Au/20Sn solder alloy. Creep characteristics of solder alloys can be studied by different experimental techniques such as conventional tensile creep [15], uniaxial tensile [16], stress relaxation [17], impression [18], and indentation testing methods [14]. The conventional tensile creep test is time-consuming and difficult for the material with the tendency of geometric instabilities, but it is very important because the tensile stress state promotes the solder joint failure. In this study, the tensile creep behavior of bulk eutectic 80Au/20Sn solder alloy was investigated at different temperatures and stress levels and the experimental results were compared with 63Sn37Pb solder joint. The creep constitutive models for description of high temperature creep behavior and low cycle fatigue property of 80Au/20Sn solder alloy and the possible creep deform mechanisms were proposed. 2. Experimental procedure The material employed was as-cast 80Au/20Sn solder alloy with a melting point of 279.4 ± 0.1 ◦ C. In order to get the similar microstructure with the solder joints in the actual packages, the air cooling method was used when the solder bars were cast. Subsequently, small rectangular specimens were prepared by machining from the high purity as-cast solder bars. The specimens had a total length of 30 mm, a gauge length of 20 mm, and a central cross-section of 0.4 mm × 1.55 mm. Next, the gauge section of each specimen was ground carefully with SiC papers (#220, 500, 1200, and 2400) and polished using 1 m diamond past. Afterwards, the spec-
G.S. Zhang et al. / Journal of Alloys and Compounds 476 (2009) 138–141 imens were annealed at 60 ◦ C for 24 h in a N2 atmosphere to eliminate the residual stresses. The tensile creep tests were performed on a micro-force test system (Tytron 895.20A-02). The system has the capability to maintain a constant loads (load variation <5%) for a long time. The system was equipped with a specially designed environmental chamber rated from −65 to 250 ◦ C for running creep tests at low and high temperatures. TestStarII software was used to control the system and collect the data. The tests were conducted at three different temperatures of 25, 75 and 125 ◦ C with a constant stress control. An extensometer with a gauge of 10 mm and resolution of 0.1 m was employed to measure the strain of the specimen. The specimens were held at the test temperature for 30 min before loading. Three samples were tested for each condition and the results were averaged. Following creep testing, metallurgical examinations were conducted for the specimens. The standard procedures of metallographic preparation were sample mounting, grinding and polishing, chemical etching, microscope examination and measurements. The bonded samples were cold-mounted with epoxy for 8 h and polished. The mold was ground using SiC paper of grit 600 to expose the interface then SiC paper of grit 1200 was used to further grind the materials until the cross-section of the sample was visible. Next, the sectioned area was polished with Type K alumina suspension (liquid diamond) of 3, 1, and 0.5 m sequentially until the interface and internal microstructure of the solder joint can be seen clearly under an optical microscope. A final touchup using polishing suspensions (COL-K) allowed better surface finishing for microstructural analysis. The samples were then examined with a combination of optical microscope and SEM/EDX.
3. Results and discussion 3.1. Creep curves Fig. 1 shows the creep curves of 80Au/20Sn solder alloy. It can be seen that the creep curves display typical primary and secondary steady-state creep stages. The tertiary creep deformation characteristic can also be seen clearly at 125 ◦ C when the applied stress is 25 MPa. The creep characteristics distinguish obviously at different stress levels and temperatures. The creep strain increases with an increase in applied stress level and temperature. Then creep
139
Table 1 The steady-state creep rate and creep lifetime of 80Au/20Sn solder alloy. Temperature (K)
Creep stress (MPa)
Creep rate (1/s)
Creep lifetime (h)
298 298 298 298 298 298 298 348 348 348 348 348 348 348 398 398 398 398 398 398 398
100 125 150 175 200 250 300 25 37.5 50 60 75 87.5 100 5 7.5 10 12.5 15 16.7 25
1.58e−8 2.84e−8 4.55e−8 8.80e−8 9.26e−8 1.32e−7 2.28e−7 1.98e−7 4.88e−7 8.49e−7 1.20e−6 2.14e−6 2.79e−6 3.61e−6 7.04e−7 2.30e−6 6.34e−6 8.74e−6 1.57e−5 2.01e−5 6.57e−5
132.215 123.52 114.45 101.20 49.90 47.26 42.67 145.67 97.66 64.61 53.11 13.31 12.03 9.02 119.70 113.54 47.25 22.96 17.34 16.23 3.05
strain rate at any given time can be determined by differentiating creep strain versus time and the minimum rate was taken as the creep strain rate of steady-state stage. The total time from the beginning of primary stage to the end of tertiary stage is defined as the “creep lifetime”. The effect of stress level and temperature on the steady-state creep rate and creep lifetime are shown in Table 1. It indicates that the creep strain rate increases and creep lifetime decreases sharply with increasing applied stress level and temperature. 3.2. Constitutive model The homologous temperature of 80Au/20Sn solder alloy at room temperature exceeds 0.5 so the creep is considered as the dominant deformation-controlling mechanism. Considering that the steady-state stage takes up the most of the creep time of final failure, the secondary creep strain region is chosen to represent the entire process in most of the constitutive models. In the steadystate creep stage, the creep strain rate can be generally described by Weertman–Dorn equation [19] as follows: ε˙ =
AGb kT
b p n G
d
Q
D0 exp −
kT
(1)
where ε˙ is the steady-state creep strain rate, G is the shear modulus, b is the Burgers vector, k is Boltzmann’s constant, T is the absolute temperature, A is a constant, d is the grain size, p is the grain size exponent, is the applied stress, D0 is the frequency factor, n is the creep stress exponent and Q is the activation energy of the deformation process. From the above equation, it can be seen clearly that the steady-state creep strain rate is related to the properties of the solder alloy itself (shear modulus, grain size, etc.) and service conditions (service temperature and applied stress). The smaller the grain size, or the higher the service temperature, or the higher the applied stress, the higher the steady-state creep strain rate and the shorter the creep lifetime. As the Boltzmann constant is equal to the ratio of the gas constant (R) to the Avogadro constant, and some constants can be converted into a new constant, the above equation can be simplified as the Dorn power law constitutive model [20] given by
Q
ε˙ = A1 n exp − Fig. 1. Tensile creep curves of 80Au/20Sn solder alloy. (a) Under different stress levels at 125 ◦ C; (b) at different temperatures with the applied stress of 25 MPa.
RT
where A1 is a complex constant.
(2)
140
G.S. Zhang et al. / Journal of Alloys and Compounds 476 (2009) 138–141
It is assumed that the deformation is dominated by one creep mechanism in the whole stress range applied, and therefore the stress exponent can be taken as a constant at any given temperature. Then we have the calculation as follows. Taking the natural logarithmic transformation on both sides of Eq. (2), then we have: ln ε˙ = ln A1 + nln −
Q RT
(3)
It is clear that at a given temperature the creep stress exponent can be calculated by linear regression of the experimental data. For the bulk 80Au/20Sn solder alloy, the calculated stress exponents at 25, 75 and 125 ◦ C are 2.37, 2.10 and 2.77, respectively. The creep stress exponent and creep activation energy are stress level applied and temperature dependent in Dorn model, the average value can be calculated by fitting all the experimental results together. Eq. (4) shows the fitted Dorn model:
ε˙ = 8.37 × 105 2.07 exp
−
1.02 × 105 RT
(4)
The hyperbolic sine type Garofalo model [18] describing creep flow for low and high stresses where the simple power law breaks down is also used to predict the steady-state creep behavior of solders. The model is given by
Q
ε˙ = A2 [sinh(˛)]n exp −
(5)
RT
In this equation, ˛ is the stress coefficient. Data are fit by iterative multivariable nonlinear regression method, the following fit was obtained: ε˙ = 4.62 × 1015 [sinh(2 × 10−5 )]
2.07
exp
−
1.02 × 105 RT
(6)
Fig. 2 compares the Dorn model and Garofalo model for the steady creep rate versus applied stress curves. Clearly, both models are able to provide an acceptable description of eutectic 80Au/20Sn solder alloy experimental results over the present experimental stress–temperature range. However, the Garofalo model exhibits a lower estimated variance of error terms as compared to the Dorn model with the present data. The better agreement between Garofalo model and experimental results is evident. For the eutectic 80Au/20Sn solder alloy, the calculated creep stress exponent and creep activation energy is 2.07 and 102 kJ/mol, respectively by tensile creep test, which are different from n = 2.55, and Q = 18.95 kcal/mol (79.29 kJ/mol) obtained by Morgan based on uniaxial compression creep testing [21]. The variations can be explained by differences in testing methods, microstructure, specimen preparation, measuring errors, and data processing method.
Fig. 2. Comparison on steady-state creep rate obtained from experimental results and fit models. (a) Dorn model; (b) Garofalo model.
morphology of the specimens after testing. The grain boundary sliding along the loading direction and the cuplike depressions across the specimen can be observed. The size of the depressions depends on the number and distribution of microvoids that are nucleated. It is known that the creep deformation by microvoids growth and coalescences mechanisms generally relates to intergranular creep fracture process [23]. The fractography does not vary significantly but for the reduction of fracture surface. This verifies the creep mechanism which does not change and indicates high mechanical strength of 80Au/20Sn solder even at high temperature and applies stress level. From the discussion above, the grain boundary sliding can be the dominant creep mechanism. At high homologous temperature
3.3. Creep failure mechanism When the Dorn power law creep relation is valid, the value of creep stress exponent is often used to identify the creep deformation-controlling mechanism. The creep stress exponent does not change significantly over the wide applied stress level and the testing temperature range. This suggests that the change in applied stress and temperature does not change the creep rate controlling mechanism. Grivas et al. investigated the deformation behavior of Pb–Sn solder and pointed out that the deformation was controlled by grain boundary (GB) sliding near the creep stress exponent close to 2 [22]. Further studies should be performed to clarify the creep mechanism of the eutectic 80Au/20Sn solder alloy. The microstructure often changes during the creep process; the creep deformation mechanisms can be largely clarified by analyzing the creep fractography. Fig. 3 shows a typical fractography
Fig. 3. Typical morphology of the fractography of eutectic 80Au/20Sn solder alloy.
G.S. Zhang et al. / Journal of Alloys and Compounds 476 (2009) 138–141
141
steady-state creep rate and higher creep resistance compared with the 63Sn37Pb solder. (2) The Dorn power law creep stress exponent does not change significantly with increasing the stress, and therefore both Dorn model and Garofalo model are suitable for the eutectic 80Au/20Sn solder over the tested stress and temperature ranges. However, the latter fits the experimental data more successfully. (3) The grain boundary sliding is probably the most dominant creep mechanism in this experiment and the final intergranular fracture via nucleation, accumulation and further growth of microvoids. Acknowledgements Fig. 4. Comparison of the steady-state creep model curves between 80Au20Sn and 63Sn37Pb solder.
and under the action of applied stress, grain boundary sliding takes place and leads to the formation of microvoids. The nucleation, accumulation and further growth of microvoids finally results in the intergranular fracture of typical creep. 3.4. Comparison with 63Sn37Pb solder The creep behavior of the traditional eutectic 63Sn37Pb solder has been widely reported by many researchers. For instance, the constant-load creep tests of eutectic 63Sn37Pb solder were performed at temperatures of 25, 75 and 125 ◦ C by Zhang et al. [24] The comparison between the fit steady-state creep constitutive model curves of 63Sn37Pb and eutectic 80Au/20Sn solder is shown in Fig. 4. It can be seen that the eutectic 80Au/20Sn solder has lower steady-state creep rate and the steady-state creep rate does not increase as fast as 63Sn37Pb solder with an increase in the applied stress. This attributes to the good mechanical properties of ␦(AuSn) and (Au5 Sn) phase in eutectic 80Au/20Sn solder. The tiny ␦ and phases strengthen pure Sn grain boundaries and block grain boundary sliding, thus the eutectic 80Au/20Sn solder has superior creep resistance and higher creep activation energy than the 63Sn37Pb eutectic solder. 4. Conclusions Relationships between tensile creep behavior and microstructure of eutectic 80Au/20Sn solder alloy was studied and compared with the 63Sn37Pb solder. Two steady-state creep constitutive models were employed to describe the observed creep behaviors. The study supports the following conclusions: (1) The eutectic 80Au/20Sn solder alloy exhibits the typical creep deformation characteristics. The creep strain increases and creep lifetime decreases with the improved applied stress level and temperature. The 80Au/20Sn solder alloy exhibits a lower
This work is sponsored by the Doctor Subject Foundation of China under grant no. 20050056035 and the Natural Science Foundation of China under grant no. 50575160. References [1] A. Larsson, S. Forouhar, J. Cody, R.J. Lang, IEEE Photon. Technol. Lett. 2 (1990) 307–309. [2] G.W. Yang, R.J. Hwu, Z.T. Xu, X.Y. Ma, IEEE J. Quant. Electron. 35 (1999) 1535–1541. [3] W. Pittroff, G. Erbert, G. Beister, et al., IEEE Trans. Adv. Packag. 24 (2001) 434–441. [4] J. Zhao, L. Li, W.M. Wang, Y.C. Lu, IEEE Photon. Technol. Lett. 15 (2003) 1507–1509. [5] Y. Qu, S. Yuan, C.Y. Liu, B.X. Bo, G.J. Liu, H.L. Jiang, IEEE Photon. Technol. Lett. 16 (2004) 389–391. [6] X.S. Liu, M.H. Hu, H.K. Nguyen, C.G. Caneau, M.H. Rasmussen, IEEE Trans. Adv. Packag. 27 (2004) 640–646. [7] D.S. Ellis, J.M. Xu, IEEE J. Select. Top. Quant. Electron. 3 (1997) 640–648. [8] D.A. Ackerman, P.A. Morton, R.F. Kazarinov, T. Tanbun-Ek, R.A. Logan, Appl. Phys. Lett. 66 (1995) 466–468. [9] S.J. Sweeney, L.J. Lyons, A.R. Adams, D.A. Lock, IEEE J. Select. Top. Quant. Electron. 9 (2003) 1325–1331. [10] J.W.R. Teo, F.L. Ng, L.S.K. Goi, et al., Microelectron. Eng. 85 (2008) 512–517. [11] W. Jiang, Cryst. Res. Technol. 41 (2006) 150–153. [12] A. Yamauchi, Y. Arai, Proceedings of the Electronic Components and Technology Conference, 2001, pp. 242–246. [13] G. Chen, Phys. Rev. B 57 (1998) 14958–14973. [14] Y.C. Liu, J.W.R. Teo, S.K. Tung, K.H. Lam, J. Alloys Compd. 448 (2008) 340–343. [15] M.D. Mathew, H. Yang, S. Movva, K.L. Murty, Metall. Mater. Trans. 36A (2005) 99–105. [16] X. Chen, G. Chen, M. Sakane, IEEE Trans. Compon. Packag. Technol. 28 (2005) 111–116. [17] S.G. Jadhav, T.R. Bieler, K.N. Subramanian, J.P. Lucas, J. Electron. Mater. 30 (2001) 1197–1205. [18] J.J. Stephens, D.R. Frear, Metall. Mater. Trans. A 30 (1999) 1301–1313. [19] A. Schubert, H. Walter, R. Dudek, et al., Proceedings of the International Symposium on Advanced Packaging Materials Processes, 2001, pp. 129–134. [20] R.S. Whitelaw, R.W. Neu, D.T. Scott, Trans. ASME, J. Electron. Packag. 121 (1999) 99–107. [21] H.S. Morgan, Trans. ASME, J. Electron. Packag. 113 (1995) 350–354. [22] D. Grivas, K.L. Murty, J.W. Morris Jr., Acta Metall. 27 (1979) 731–737. [23] A.J. McEvily, Metal failures: mechanisms, in: Analysis and Prevention, John Wiley & Sons, New York, 2002, pp. 163-180. [24] Q. Zhang, A. Dasgupta, P. Haswell, ASME Electron. Photon. Packag. 3 (2003) 215–224.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Creep behavior of eutectic 80Au/20Sn solder alloy G.S. Zhang a , H.Y. Jing a , L.Y. Xu a,∗ , J. Wei b , Y.D. Han a a b
School of Material Science and Engineering, Tianjin University, Tianjin 300072, China Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore
a r t i c l e
i n f o
Article history: Received 28 July 2008 Received in revised form 20 August 2008 Accepted 3 September 2008 Available online 31 October 2008 Keywords: Eutectic 80Au/20Sn solder alloy Creep Constitutive model Creep mechanism
a b s t r a c t Eutectic 80Au/20Sn solder alloy is widely used in high power electronics and optoelectronics packaging in which the creep property of the solder joint is essential to meet the global demand for longer operating lifetime in their applications. In this study, the tensile creep behavior of bulk eutectic 80Au/20Sn solder alloy is reported and compared with 63Sn37Pb solder joint. The creep strain rate increases and creep lifetime decreases as the applied stress level and temperature increase. The 80Au/20Sn solder alloy shows a superior anti-creep performance over the 63Sn37Pb solder joint. The experimental data were successfully fit with Dorn model and Garofalo model. However, the application of Garofalo model resulted in a lower estimated variance of error terms as compared to the Dorn model. Grain boundary sliding is the possible creep mechanism within the given stress level and temperature. The nucleation, accumulation and further growth of microvoids lead to the creep rupture. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The traditional tin–lead solders have been widely employed as electrical interconnects in electronic industries. However, the toxic Pb can cause harmful influence on environment and health. On the other hand, the tin–lead solders would no longer satisfy the reliability requirement in high power electronic and optoelectronic components with the smaller size of solder joint and higher mechanical, thermal and electrical load in which the excellent creep resistance is essential. Hence, it is urgent to develop the lead-free solders with excellent creep and fatigue resistance. In the recent decade, a great amount of lead-free solders have been reported and the promising candidates mainly include the Sn-based alloys such as Sn–Ag, Sn–Cu, Sn–Zn, Sn–Bi, Sn–Sb, Au–Sn, and Sn–Ag–Cu systems. Among the lead-free candidate solders, the eutectic 80Au/20Sn solder alloy is particularly attractive in high power electronics and optoelectronics as hermetic sealing and die attachment material because it has excellent high-temperature performance, high mechanical strength, high electrical and thermal conductivity, and can also offer fluxless soldering [1–6]. The manufacturing process for such applications has been investigated extensively, with emphasis on the bonding quality, the metallurgical interaction and formation of Au–Sn solder bump [7–11]. Besides the manufacturing and solderability, the thermomechanical properties of 80Au/20Sn solder alloy have been reported [12–14].
∗ Corresponding author. Tel.: +86 22 2740 2439; fax: +86 22 2740 5020. E-mail address: [email protected] (L.Y. Xu). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.009
Nevertheless, the creep behavior of 80Au/20Sn solder has been seldom reported in the literature. The 80Au/20Sn solder has obvious creep response because of its lower melting point. The creep deformation can lower the reliability and performance of the electrical components significantly and creep damage is vital for the reliability of solder joint. Therefore, it is necessary to gain an insight into the creep behavior of eutectic 80Au/20Sn solder alloy. Creep characteristics of solder alloys can be studied by different experimental techniques such as conventional tensile creep [15], uniaxial tensile [16], stress relaxation [17], impression [18], and indentation testing methods [14]. The conventional tensile creep test is time-consuming and difficult for the material with the tendency of geometric instabilities, but it is very important because the tensile stress state promotes the solder joint failure. In this study, the tensile creep behavior of bulk eutectic 80Au/20Sn solder alloy was investigated at different temperatures and stress levels and the experimental results were compared with 63Sn37Pb solder joint. The creep constitutive models for description of high temperature creep behavior and low cycle fatigue property of 80Au/20Sn solder alloy and the possible creep deform mechanisms were proposed. 2. Experimental procedure The material employed was as-cast 80Au/20Sn solder alloy with a melting point of 279.4 ± 0.1 ◦ C. In order to get the similar microstructure with the solder joints in the actual packages, the air cooling method was used when the solder bars were cast. Subsequently, small rectangular specimens were prepared by machining from the high purity as-cast solder bars. The specimens had a total length of 30 mm, a gauge length of 20 mm, and a central cross-section of 0.4 mm × 1.55 mm. Next, the gauge section of each specimen was ground carefully with SiC papers (#220, 500, 1200, and 2400) and polished using 1 m diamond past. Afterwards, the spec-
G.S. Zhang et al. / Journal of Alloys and Compounds 476 (2009) 138–141 imens were annealed at 60 ◦ C for 24 h in a N2 atmosphere to eliminate the residual stresses. The tensile creep tests were performed on a micro-force test system (Tytron 895.20A-02). The system has the capability to maintain a constant loads (load variation <5%) for a long time. The system was equipped with a specially designed environmental chamber rated from −65 to 250 ◦ C for running creep tests at low and high temperatures. TestStarII software was used to control the system and collect the data. The tests were conducted at three different temperatures of 25, 75 and 125 ◦ C with a constant stress control. An extensometer with a gauge of 10 mm and resolution of 0.1 m was employed to measure the strain of the specimen. The specimens were held at the test temperature for 30 min before loading. Three samples were tested for each condition and the results were averaged. Following creep testing, metallurgical examinations were conducted for the specimens. The standard procedures of metallographic preparation were sample mounting, grinding and polishing, chemical etching, microscope examination and measurements. The bonded samples were cold-mounted with epoxy for 8 h and polished. The mold was ground using SiC paper of grit 600 to expose the interface then SiC paper of grit 1200 was used to further grind the materials until the cross-section of the sample was visible. Next, the sectioned area was polished with Type K alumina suspension (liquid diamond) of 3, 1, and 0.5 m sequentially until the interface and internal microstructure of the solder joint can be seen clearly under an optical microscope. A final touchup using polishing suspensions (COL-K) allowed better surface finishing for microstructural analysis. The samples were then examined with a combination of optical microscope and SEM/EDX.
3. Results and discussion 3.1. Creep curves Fig. 1 shows the creep curves of 80Au/20Sn solder alloy. It can be seen that the creep curves display typical primary and secondary steady-state creep stages. The tertiary creep deformation characteristic can also be seen clearly at 125 ◦ C when the applied stress is 25 MPa. The creep characteristics distinguish obviously at different stress levels and temperatures. The creep strain increases with an increase in applied stress level and temperature. Then creep
139
Table 1 The steady-state creep rate and creep lifetime of 80Au/20Sn solder alloy. Temperature (K)
Creep stress (MPa)
Creep rate (1/s)
Creep lifetime (h)
298 298 298 298 298 298 298 348 348 348 348 348 348 348 398 398 398 398 398 398 398
100 125 150 175 200 250 300 25 37.5 50 60 75 87.5 100 5 7.5 10 12.5 15 16.7 25
1.58e−8 2.84e−8 4.55e−8 8.80e−8 9.26e−8 1.32e−7 2.28e−7 1.98e−7 4.88e−7 8.49e−7 1.20e−6 2.14e−6 2.79e−6 3.61e−6 7.04e−7 2.30e−6 6.34e−6 8.74e−6 1.57e−5 2.01e−5 6.57e−5
132.215 123.52 114.45 101.20 49.90 47.26 42.67 145.67 97.66 64.61 53.11 13.31 12.03 9.02 119.70 113.54 47.25 22.96 17.34 16.23 3.05
strain rate at any given time can be determined by differentiating creep strain versus time and the minimum rate was taken as the creep strain rate of steady-state stage. The total time from the beginning of primary stage to the end of tertiary stage is defined as the “creep lifetime”. The effect of stress level and temperature on the steady-state creep rate and creep lifetime are shown in Table 1. It indicates that the creep strain rate increases and creep lifetime decreases sharply with increasing applied stress level and temperature. 3.2. Constitutive model The homologous temperature of 80Au/20Sn solder alloy at room temperature exceeds 0.5 so the creep is considered as the dominant deformation-controlling mechanism. Considering that the steady-state stage takes up the most of the creep time of final failure, the secondary creep strain region is chosen to represent the entire process in most of the constitutive models. In the steadystate creep stage, the creep strain rate can be generally described by Weertman–Dorn equation [19] as follows: ε˙ =
AGb kT
b p n G
d
Q
D0 exp −
kT
(1)
where ε˙ is the steady-state creep strain rate, G is the shear modulus, b is the Burgers vector, k is Boltzmann’s constant, T is the absolute temperature, A is a constant, d is the grain size, p is the grain size exponent, is the applied stress, D0 is the frequency factor, n is the creep stress exponent and Q is the activation energy of the deformation process. From the above equation, it can be seen clearly that the steady-state creep strain rate is related to the properties of the solder alloy itself (shear modulus, grain size, etc.) and service conditions (service temperature and applied stress). The smaller the grain size, or the higher the service temperature, or the higher the applied stress, the higher the steady-state creep strain rate and the shorter the creep lifetime. As the Boltzmann constant is equal to the ratio of the gas constant (R) to the Avogadro constant, and some constants can be converted into a new constant, the above equation can be simplified as the Dorn power law constitutive model [20] given by
Q
ε˙ = A1 n exp − Fig. 1. Tensile creep curves of 80Au/20Sn solder alloy. (a) Under different stress levels at 125 ◦ C; (b) at different temperatures with the applied stress of 25 MPa.
RT
where A1 is a complex constant.
(2)
140
G.S. Zhang et al. / Journal of Alloys and Compounds 476 (2009) 138–141
It is assumed that the deformation is dominated by one creep mechanism in the whole stress range applied, and therefore the stress exponent can be taken as a constant at any given temperature. Then we have the calculation as follows. Taking the natural logarithmic transformation on both sides of Eq. (2), then we have: ln ε˙ = ln A1 + nln −
Q RT
(3)
It is clear that at a given temperature the creep stress exponent can be calculated by linear regression of the experimental data. For the bulk 80Au/20Sn solder alloy, the calculated stress exponents at 25, 75 and 125 ◦ C are 2.37, 2.10 and 2.77, respectively. The creep stress exponent and creep activation energy are stress level applied and temperature dependent in Dorn model, the average value can be calculated by fitting all the experimental results together. Eq. (4) shows the fitted Dorn model:
ε˙ = 8.37 × 105 2.07 exp
−
1.02 × 105 RT
(4)
The hyperbolic sine type Garofalo model [18] describing creep flow for low and high stresses where the simple power law breaks down is also used to predict the steady-state creep behavior of solders. The model is given by
Q
ε˙ = A2 [sinh(˛)]n exp −
(5)
RT
In this equation, ˛ is the stress coefficient. Data are fit by iterative multivariable nonlinear regression method, the following fit was obtained: ε˙ = 4.62 × 1015 [sinh(2 × 10−5 )]
2.07
exp
−
1.02 × 105 RT
(6)
Fig. 2 compares the Dorn model and Garofalo model for the steady creep rate versus applied stress curves. Clearly, both models are able to provide an acceptable description of eutectic 80Au/20Sn solder alloy experimental results over the present experimental stress–temperature range. However, the Garofalo model exhibits a lower estimated variance of error terms as compared to the Dorn model with the present data. The better agreement between Garofalo model and experimental results is evident. For the eutectic 80Au/20Sn solder alloy, the calculated creep stress exponent and creep activation energy is 2.07 and 102 kJ/mol, respectively by tensile creep test, which are different from n = 2.55, and Q = 18.95 kcal/mol (79.29 kJ/mol) obtained by Morgan based on uniaxial compression creep testing [21]. The variations can be explained by differences in testing methods, microstructure, specimen preparation, measuring errors, and data processing method.
Fig. 2. Comparison on steady-state creep rate obtained from experimental results and fit models. (a) Dorn model; (b) Garofalo model.
morphology of the specimens after testing. The grain boundary sliding along the loading direction and the cuplike depressions across the specimen can be observed. The size of the depressions depends on the number and distribution of microvoids that are nucleated. It is known that the creep deformation by microvoids growth and coalescences mechanisms generally relates to intergranular creep fracture process [23]. The fractography does not vary significantly but for the reduction of fracture surface. This verifies the creep mechanism which does not change and indicates high mechanical strength of 80Au/20Sn solder even at high temperature and applies stress level. From the discussion above, the grain boundary sliding can be the dominant creep mechanism. At high homologous temperature
3.3. Creep failure mechanism When the Dorn power law creep relation is valid, the value of creep stress exponent is often used to identify the creep deformation-controlling mechanism. The creep stress exponent does not change significantly over the wide applied stress level and the testing temperature range. This suggests that the change in applied stress and temperature does not change the creep rate controlling mechanism. Grivas et al. investigated the deformation behavior of Pb–Sn solder and pointed out that the deformation was controlled by grain boundary (GB) sliding near the creep stress exponent close to 2 [22]. Further studies should be performed to clarify the creep mechanism of the eutectic 80Au/20Sn solder alloy. The microstructure often changes during the creep process; the creep deformation mechanisms can be largely clarified by analyzing the creep fractography. Fig. 3 shows a typical fractography
Fig. 3. Typical morphology of the fractography of eutectic 80Au/20Sn solder alloy.
G.S. Zhang et al. / Journal of Alloys and Compounds 476 (2009) 138–141
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steady-state creep rate and higher creep resistance compared with the 63Sn37Pb solder. (2) The Dorn power law creep stress exponent does not change significantly with increasing the stress, and therefore both Dorn model and Garofalo model are suitable for the eutectic 80Au/20Sn solder over the tested stress and temperature ranges. However, the latter fits the experimental data more successfully. (3) The grain boundary sliding is probably the most dominant creep mechanism in this experiment and the final intergranular fracture via nucleation, accumulation and further growth of microvoids. Acknowledgements Fig. 4. Comparison of the steady-state creep model curves between 80Au20Sn and 63Sn37Pb solder.
and under the action of applied stress, grain boundary sliding takes place and leads to the formation of microvoids. The nucleation, accumulation and further growth of microvoids finally results in the intergranular fracture of typical creep. 3.4. Comparison with 63Sn37Pb solder The creep behavior of the traditional eutectic 63Sn37Pb solder has been widely reported by many researchers. For instance, the constant-load creep tests of eutectic 63Sn37Pb solder were performed at temperatures of 25, 75 and 125 ◦ C by Zhang et al. [24] The comparison between the fit steady-state creep constitutive model curves of 63Sn37Pb and eutectic 80Au/20Sn solder is shown in Fig. 4. It can be seen that the eutectic 80Au/20Sn solder has lower steady-state creep rate and the steady-state creep rate does not increase as fast as 63Sn37Pb solder with an increase in the applied stress. This attributes to the good mechanical properties of ␦(AuSn) and (Au5 Sn) phase in eutectic 80Au/20Sn solder. The tiny ␦ and phases strengthen pure Sn grain boundaries and block grain boundary sliding, thus the eutectic 80Au/20Sn solder has superior creep resistance and higher creep activation energy than the 63Sn37Pb eutectic solder. 4. Conclusions Relationships between tensile creep behavior and microstructure of eutectic 80Au/20Sn solder alloy was studied and compared with the 63Sn37Pb solder. Two steady-state creep constitutive models were employed to describe the observed creep behaviors. The study supports the following conclusions: (1) The eutectic 80Au/20Sn solder alloy exhibits the typical creep deformation characteristics. The creep strain increases and creep lifetime decreases with the improved applied stress level and temperature. The 80Au/20Sn solder alloy exhibits a lower
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