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Effect of Ag addition on the creep characteristics of Sn–8.8 wt%Zn solder alloy
Journal of Alloys and Compounds 479 (2009) 844–850
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of Ag addition on the creep characteristics of Sn–8.8 wt%Zn solder alloy G. Saad ∗ , A. Fawzy, E. Shawky Department of Physics, Faculty of Education, Ain Shams University, Elmakrezy/ Roxy, Cairo 11575, Egypt
a r t i c l e
i n f o
Article history: Received 27 October 2008 Received in revised form 13 January 2009 Accepted 21 January 2009 Available online 31 January 2009 PACS: 62.20.Fe 61.82.Bg 61.66.Dk Keywords: Metals and alloys Precipitation Microstructure X-ray diffraction
a b s t r a c t Full implementation of the new generation of lead-free solders requires detailed knowledge and understanding of their mechanical behavior. The materials used in the present study are Sn–8.8 wt%Zn (binary) and Sn–8.8 wt%Zn–1.5 wt%Ag (tertiary) alloys. Effect of Ag addition, deformation temperature T and the applied stress, , on the creep characteristics have been studied. Creep tests were performed under the effect of different stresses ranged from 17.8 to 26 MPa at the deformation temperatures 291, 303, 323 and 343 K. The transient creep parameters ˇ and n were found to be markedly affected by the creep test conditions, T and . The parameter ˇ was found to be decreased by increasing T and/or while n was found to increase by increasing T irrespective of the applied stress . The steady-state creep rate ε´ st was found to increase with increasing both T and in both solder alloys. The steady-state creep rate ε´ st was related to the stress with the relationship ε´ st = C m where m = (∂ln ε´ st /∂ln ) is the stress exponent. This exponent is decreased with increasing T in both alloys. Addition of 1.5 wt%Ag to the binary alloy increased its creep resistance. This behavior was attributed to the formation of the intermetallic compounds (IMCs) AgZn and Ag3 Sn during solidification. These IMCs played the role of pinning action for the moving dislocations and consequently leading to the increase of its creep resistance. Micro-structural changes were investigated by scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analysis. © 2009 Published by Elsevier B.V.
1. Introduction It is well known that joining materials (solders) provide electrical, thermal and mechanical continuity in electronic assemblies. The near eutectic Sn–Pb solders are extensively used through whole assemblies because they have many advantages. They are inexpensive; they have good processing characteristics such as low melting temperature (456 K), high strength, high ductility, good thermal and electrical properties. However, in the early nineteenth-century, lead poisoning was in paint pigment workers in USA and France since the use of lead has been gradually banned worldwide in plumbing, paints, and gasoline through the enactment of various legislations. In the electronic industry, the lead generated by the disposal of electronic assemblies is considered as hazardous to the environment. So, the European Union has officially designated 1 July 2006 as the date when directive on the restriction of hazardous substances in electrical and electrical equipment will require be phased out. Hence, electronic manufacturing now have only one choice which is the use of Pb-free solder alloys and subsequently close attention is paid lately to substitution of the Pb-rich solders by lead-free ones. Pb-free solder must form joints with accept-
∗ Corresponding author. E-mail address: [email protected] (G. Saad). 0925-8388/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jallcom.2009.01.069
able strength, and withstand over the projected operational life of assembly. Mechanical properties and microstructure changes are among the main parameters to be controlled in the candidate solders in an industrial process. Although creep is the most common and important deformation mechanism of solders, the creep resistance of various Pb-free solder alloys is still in question. The softness of tin metal due to its low mechanical strength leads to the idea of strengthening it by adding hardening alloying elements to meet its wide applications [1,2]. Pb-free solders are typically tin-based alloys. Among types of Pb-free solder alloys are Sn–Bi [3], Sn–Cu [4,5], Sn–Ag [6] and Sn–Zn [7]. Sn–Zn [8,9] based alloys are still under development to overcome the shortfall of Sn–9 wt%Zn alloys. Other results showed that Sn–9 wt%Zn alloys with Ag content higher than 0.5 wt%Ag exhibited low strength at low solidification rates while an acceptable strength was obtained at higher solidification rate due to the presence of the hard AgZn intermetallic compounds (IMC) besides the refined and modified distribution of the Zn-rich needles [10]. Moreover increasing Ag content in Sn–9 wt%Zn tends to diminish the eutectic structure due to the formation of larger volume fractions of the AgZn IMC [11]. The morphology and distribution of such phases was found to strongly affect the creep behavior of solder alloy. This was attributed to the action of the alloying addition that may leads to a fine and uniform dispersion of small seeded particles of the second phase. Hence,
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Table 1 Chemical compositions of the alloys: Sn–8.8 wt%Zn and Sn–8.8 wt%Zn–1.5 wt%Ag. Alloy
Sn
Zn
Ag
Pb
Sn–8.8 wt%Zn Sn–8.8 wt%Zn–1.5 wt%Ag
91.11 89.61
8.82 8.82
0.00 1.52
0.05 0.04
the improvement of mechanical properties is closely related to the amount and type of the alloying elements [1]. On the other hand Ag addition to Sn–Bi solder alloy resulted in the decrease of its mechanical strength [3]. This was explained as due to the surface activity of the alloying elements. Between softening and hardening effects of Ag addition to the Sn-based alloys, the present work is devoted to get a more insight into the effect of Ag addition, temperature and applied stress on the creep characteristics of Sn–8.8 wt%Zn alloy.
Fig. 1. Phase diagram for Sn–Zn binary system.
2. Experimental procedures Sn–8.8 wt%Zn and Sn–8.8 wt%Zn–1.5 wt%Ag alloys were prepared from high purity Sn, Zn and Ag of purity 99.99%. The appropriate weights of the elements for the binary alloy were well mixed with CaCl2 flux to prevent oxidation in a graphite mold and kept at 100 K above the melting temperature of Ag. Casting into stainless steel molds was done in air with a cooling rate of 10 K/s. An appropriate weight of silver was added to portion of the binary alloy, while the temperature is kept at 100 K above the melting temperature of Ag to form the second alloy (Sn–8.8 wt%Zn–1.5 wt%Ag). Chemical analysis of the two solder alloys was found to be close to the desired constitution (see Table 1). The ingots of the two alloys were in the form of rods 12 mm in diameter. After casting, each of them was cold drawn into 0.6-mm diameter wire. Specimens with a gauge length of 50 mm were prepared for tensile testing. The experiments were carried out using a conventional creep testing machine equipped with a strain resolution equal to10−4 . Isothermal strain-time experiments were performed under
constant applied stresses ranging between 17.9 and 26 MPa at different deformation temperatures 291, 308, 323, and 343 K. During tensile tests, the deformation temperatures were controlled to within ±2 K. The microstructure of the as-cast samples was investigated using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analysis. Prior to the SEM observations of the as-cast alloys, samples of 3-mm diameter were mechanically and then electrically polished using solutions containing 80% acetic acid and 20% perchloric acid.
3. Experimental results According to the Sn–Zn binary phase diagram (see Fig. 1), the equilibrium microstructure of the eutectic Sn–8.8 wt%Zn consists
Fig. 2. (a) XRD pattern for the as-cast Sn–8.8 wt%Zn and (b) SEM micrographs for the as-cast Sn–8.8 wt%Zn.
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G. Saad et al. / Journal of Alloys and Compounds 479 (2009) 844–850
Fig. 3. (a) X-ray diffraction patterns for the as-cast Sn–8.8 wt%Zn–1.5 wt%Ag solder alloy, (b) SEM micrographs for the as-cast Sn–8.8 wt%Zn–1.5 wt%Ag solder alloy, and (c) EDX pattern for the as-cast Sn–8.8 wt%Zn–1.5 wt%Ag solder alloy.
of the eutectic mixture of Sn-rich (close to pure Sn) phase and the Zn-rich phase. As mentioned in the preceding section, addition of Ag allows predicting microstructure containing the IMC AgZn [8]. Examination of the present Ag-free solder alloy by XRD analysis and SEM examination revealed the same microstructure as predicted before [8] in which it consists of Sn-rich and Znrich phases as shown in Fig. 2a and b respectively. Concerning the present Ag-containing solder alloy, XRD analysis showed similar microstructure obtained by [10] beside the additional IMC Ag3 Sn as shown in the diffractogram of Fig. 3a. It should be noted that the addition of Ag in this work resulted in the refinement of the Zn-rich phase (compared with that obtained in Fig. 2b) as clearly observed in the micrograph of Fig. 3b. Creep tests for the binary and tertiary alloys were performed at the deformation temperatures 291, 308, 323, 343 K under the effect of 17.8, 19.5, 21.2, 24.4, and 26 MPa. Fig. 4a is a representa-
tive example for isothermal creep curves of samples tested at the deformation temperatures 291, 308, 323, 343 K under the effect of 17.8 MPa for both binary and tertiary alloys showing the effect of deformation temperature. Fig. 4b is a representative example for creep curves of samples tested at the deformation temperature of 291 K under the effect of different stresses17.8, 19.5, 21.2, 24.4, and 26 MPa for both alloys showing the effect of applied stresses. Fig. 4c is a representative comparative example for creep curves of samples tested under the stress of 17.8 MPa at the deformation temperature 291 K for both alloys showing the effect of Ag addition. It is to be noted that the results obtained in Fig. 4a and b, indicated that the creep curves of both are sensitive to the deformation temperature, T and the applied stress showing monotonic shift towards higher strains with increasing the deformation temperature and/or the applied stress. On the other hand a lower level of
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Fig. 5. Representative example for the relation between ln εtr and ln t at = 19.48 MPa at different deformation temperatures indicated on the figure.
Fig. 5. Slopes of these straight lines gave the values of the transient creep time exponent n while their intercepts at ln t = 0 gave the transient creep parameter ˇ. It was found that n increases with increasing deformation temperature for both alloys (Fig. 6a). The parameter ˇ was found to decrease with increasing temperature and/or decreasing the applied stress as shown in Fig. 6b. 5. Steady-state creep stage The steady-state strain rate ε´ st of the tested samples is calculated from the slopes of the linear parts of the obtained creep curves. It increases with increasing both the T and/or in both solder alloys (Fig. 4a and b). It is also seen that under the same test conditions the tertiary alloy showed higher creep resistance compared with that of the binary one (Fig. 3c).
Fig. 4. Representative creep curves for Sn–8.8Zn and Sn–8.8Zn–1.5 wt%Ag wires: (a) tested at different temperatures under constant applied stress of 17.86 MPa, (b) tested at 291 K under the effect of different stresses as indicated, and (c) tested at 308 K under17.86 MPa Showing the effect of Ag addition.
creep strain was observed in the Ag-containing strain-time curves (Fig. 4c). 4. Transient stage Concerning the transient stage of the creep curve, the transient creep strain is generally described by the well-known equation [12]: εtr = ˇt n where εtr and t are the transient creep strain, and time, ˇ and n are constants depending on the experimental test conditions. In the present work the relation between ln εtr and ln t is constructed showing straight lines as shown from the representative example in
Fig. 6. (a) Temperature dependence of the transient creep exponent, n and (b) the temperature dependence of the transient creep parameter, ˇ for both binary and tertiary solder alloys.
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Fig. 7. (a) The relation between ln εst and ln for both binary and tertiary solder alloys at different deformation temperatures indicated on the figure and (b) the variation of the stress exponent m with the deformation temperature T.
It is well known that the steady-state creep rate ε´ st is related to the applied stress according to the relation [13]
where C is a constant and m is the stress exponent and both of them depend on the testing conditions. A relation between ln ε´ st and ln was constructed and straight lines for both alloys were obtained as shown in Fig. 7a. Slopes of the obtained straight lines gave the values of the stress exponent m, at different deformation temperatures. The temperature dependence of m given in Fig. 7b shows that it increases with increasing deformation temperature. Besides, values of m for the tertiary alloy was found higher than those of the binary one indicating that the tertiary alloy is harder. In the present creep tests a linear dependence of ln ε´ st on at different deformation temperatures is obtained (Fig. 8a) and values of the activation volume, V = ∂log ε´ st /∂ [14], are calculated from the slopes of the obtained straight lines. The activation volume, V was found to increase with increasing deformation temperature in both solder alloys, as shown in Fig. 8b. It is to be noticed also that, V for the binary alloy is higher than those of the tertiary one at all testing temperatures. 6. Discussion It is generally well known that the internal structure of an alloy controls and modifies the nature and mode of interaction of the existing defects and the alloy components. So, the most important result of the creep tests is the strong dependence of the creep parameters on the micro-structural changes as well as the
Fig. 8. (a) ln εst versus at different deformation temperatures for Sn–8.8 wt%Zn and Sn–8.8 wt%Zn–1.5 wt%Ag solder alloy and (b) temperature dependence of the activation volume, V for both alloys.
alloying element. In the present investigation the constitutional diagram of Sn–Zn system [15] at the eutectic composition predicts a microstructure consisting of Zn-rich phase in the form of Zn platelike dispersed in the ˇ-Sn matrix [11]. Addition of 1.5 wt%Ag seems to change the microstructure of the binary alloy and the plate-like Zn phase appears in refined form as shown in Fig. 2b beside Ag3 Sn fine particles which were detected by (i) XRD analysis as shown in Fig. 3a, this led to (ii) detect the elemental composition for different zones from the dark phase appeared in the SEM by means of EDX which revealed the existence of Ag with its maximum concentration as illustrated in Fig. 3c. This means that the bright phase (Sn-rich phase) is nearly depleted from Ag. Moreover, the profile of XRD patterns reveals that Ag-containing solder alloy showed a remarkable higher intensity of Sn (1 1 0) and Sn (2 0 0) crystallographic texture while a lower intensity for Zn (0 0 2), Zn (1 0 0) and Zn (1 0 1) was observed (Fig. 3a). In other words, the intensity of Sn in Ag-containing alloy (Fig. 3a) is greater than that of Sn in Ag-free one (Fig. 2a) while the intensity of Zn in the Ag-free alloy (Fig. 2a) is greater than that in the Ag-containing alloy (Fig. 3a). This result is due to the addition of 1.5 wt%Ag which increased the solubility of Zn in the Sn matrix. This led to reducing sizes and increased the number of these plate-like Zn-rich particles. Hence a uniform and fine microstructure is attained as has been observed in Fig. 3b compared with that obtained in Fig. 2b. This was in agreement with the results obtained in other work [15] which reported that the Zn-rich phase decreases on the expense of the increase of the solubility of Zn in the Sn matrix by Ag addition, which enhance the corrosion resistance of Sn–Zn eutectic alloy [16]. In view of these considerations, Ag addition is expected to modify the creep behavior of the Sn–Zn binary alloy due to the presence of AgZn and Ag3 Sn IMCs. Consequently, the creep strain in the Agcontaining would be always lower than the Ag-free one.
G. Saad et al. / Journal of Alloys and Compounds 479 (2009) 844–850
Taking the above remarks into consideration, the creep behavior of the present materials can be interpreted as follows. 7. The transient creep stage It is well known that the transient creep parameter n determines the dependence of the density of mobile dislocations on the mean internal stress i and the relaxed shear modulus G according to the relation = ( i /nGb)2 as reported before [17] where b is the Berger’s vector of the involved dislocations. At high normalized stress (/G) > 10−4 one can assume that the internal stress i does not change. In view of this consideration increasing the deformation temperature T, the dislocation density would decreases due to their annihilation. Consequently the transient creep parameter n will increase which is clearly seen in Fig. 6a. In addition increasing the deformation temperature T resulted in: (i) increasing thermal agitation for the moving dislocations leading to faster annihilation and/or (ii) decreasing the pinning effect of each alloy components. Hence the increase of n with T can be accounted for. The higher value of the transient creep parameter n for the Agcontaining alloy might be due to the finer microstructure of the IMCs (Ag–Zn and Ag3 Sn) which are expected to be easily agitated more than the Zn-plate like particles in the binary alloy. In order to determine the energy activating the transient creep process Etr , one can assume that values of ˇ satisfy the Arrhenius type form [18]: ˇ = ˇ0 exp −Etr /KT where ˇ0 is a constant, K is the Boltzman constant. The activation energy Etr is calculated from the slopes of the straight lines of Fig. 9. Etr was found to have a mean value of ∼0.25 eV in agreement with that needed for the dislocation motion [3]. 8. The steady-state stage In this stage, raising deformation temperature affected the creep behavior as previously observed in Fig. 4a from which the creep strain in both alloy specimens increased with increasing T while the applied stress is kept constant. As mentioned before, the Ag-containing solder alloy was found to have a superior strength compared to Sn–8.8 wt%Zn binary alloy (see Figs. 4c) due to the difference in the microstructure in both solder alloys. Ag-containing solder alloy is characterized by more fine and uniform dispersion of AgZn besides the presence of Ag3 Sn particles in the Sn matrix. Therefore, the observed lower creep strain values in the Ag-contained solder alloy may be attributed to the pin-
Fig. 9. The best fitting straight lines relating ln ˇ and (1000/T) at different applied stresses for both solder alloys.
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ning action of the finely dispersed AgZn and Ag3 Sn fine particles [19–21]. The increase in the creep strain by increasing T (Fig. 4a) for the two solder alloys under investigation may be due to the decrease in density of the effective pinning centers with increasing T allowing higher slip distances traveled by the moving dislocations and/or the decrease in the density of mobile dislocations due to their annihilation at the ˇ-Sn dendrites in the two solder alloy specimens. 9. Stress exponent In creep deformation, upon the application of stress at different deformation temperatures the obtained steady-state creep rate seems to depend mainly on structure of the tested sample and the deformation temperature. With increasing the applied stress, the interaction between dislocations and the other phases may lead to climb, glide and/or viscous motion of the mobile dislocations on their slip planes. It is generally known that the value of the stress exponent m helps in judging the controlling mechanism. In the present study, the stress exponent m was increased with increasing the deformation temperature, from ∼8.7 to ∼11.3 in the tertiary alloy and from ∼8.5 to ∼10.3 in the binary one (Fig. 7b) indicating that the rate controlling mechanism may be glide and/or climb [3]. From Fig. 7b, it is clear also that m for the tertiary alloy is higher than that in the binary one due to the presence of the IMCs, AgZn and Ag3Sn. It has been previously reported by [14] that activation volume is directly proportional to the average spacing of dislocations. The variation of the activation volume V with T (Fig. 8b) was found to be consistent with the variation of n with T (Fig. 6a). In the present study, addition of Ag decreased the volume fraction of the Zn platelike precipitates in the Sn matrix (see Figs. 2b and 3b) leading to the increase of the average distances swept by moving dislocations. On the other hand, addition of 1.5 wt%Ag appears to refine the microstructure of Zn-rich phase from Zn plate-like (dark phase) as shown in Fig. 2b into a finer and more uniform distribution of these plates-like particles as shown Fig. 3b leading to increase its pinning action. Between the two contradictory effects of the Ag addition, increasing the deformation temperature leads to increase the thermal agitation for the moving dislocations. Hence, the increase of n as well as V could be accounted for. From Figs. 6a, 7b and 8b, it should be noted that the different values for n between the Ag-free and Ag-containing solder alloys decrease with increasing the deformation temperature T while the difference values for both m and V decrease with increasing T. This contradiction in behaviour between n, m and V with respect to the deformation temperature T for the two solder alloys may be due to the difference in their microstructure. The activation energy of the steady-state creep, Est , for both alloys can be calculated using an Arrhenius type equation of the form [18],
where K and T have the usual meanings as previously mentioned. The relation between ln ε´ st and 1000/T as illustrated in Fig. 10 gave straight lines for the different applied stresses. It shows that the calculated energy needed for the operating mechanism in the steady-state creep is ∼0.5 eV. This value was found to be close to that of viscous glide motion of dislocations on their slip planes [3]. This is may be due the fact that upon heating, if a gliding dislocation was held by an obstacle, a small amount of climb might permit it to surmount the obstacle, allowing it to glide to the next set of obstacles where the process was repeated. The glide step produces almost all of the strain. The climb process of dislocations requires atomic diffusion. As diffusion processes are more difficult in the
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10. Conclusions The main conclusions to be drawn from this work may be summarized as follows:
Fig. 10. The best fitting straight lines relating ln ε´ st and (1000/T) at different applied stresses for both solder alloys.
(i) The transient creep parameter n increases with increasing the deformation temperature while ˇ decreased. (ii) Both of the stress exponent m and activation volume V increase with increasing the deformation temperature. (iii) The difference values for n decreases while the difference values for both m and V increase with increasing the deformation temperature. (iv) Addition of Ag to the Sn matrix led to: (i) The formation of the IMCs (AgZn and Ag3 Sn). (ii) Reduced the volume fraction of the plate-like Zn phase. (v) A significant improvement in the creep resistance is achieved by 1.5 wt%Ag addition to the binary Sn–8.8 wt%Zn solder alloy. (vi) The action of the alloying addition still needs more and more attention. References
Fig. 11. The relation between ln ˇ and ln ε´ st for for both solder alloy.
ordered lattice [22] the activation energies for the formation and migration of vacancy increase with order, resulting in higher activation energies for diffusion and for creep, and therefore lower creep strain [23]. Finally, the correlation between the transient and steady-state creep in the present study is obtained by constructing the relation between ln ε´ st and ln ˇ at different stresses as shown in Fig. 11. The obtained straight lines proved the validity of the relation [24] with the exponent independent on the stress applied. The value of (=ı ln ˇ/ı ln ε´ st ) was found to be ∼0.9 indicating that the dependence of ε´ st on ˇ seemed to be induced by the dislocation process which enhanced both the transient and steady-state processes.
[1] B.T.K. Barry, C.J. Thwattes, Tin and Its Alloys and Compounds, Wiley, New York, 1983, p. 280 (Chapters 3 and 4). [2] G.S. Al-Ganainy, M.T. Mostafa, Egyptian Journal of Solids 23 (2) (2000) 333–340. [3] C.H. Reader, D. Mitlin, R.W. Messler, Journal of Materials Science 33 (1998) 4503–4508. [4] H.G. Song, J.W. Morris Jr., F. Hua, Materials Transactions 43 (2002) 1847–1853. [5] K. Suganuma, MRS Bulletin 26 (2001) 880–884. [6] A. Fawzy, Materials Characterization 58 (2007) 323–331. [7] J. Jiang, J.-E. Lee, K.-S. Kim, K. Suganuma, Journal of Alloys and Compounds 462 (2008) 244–251. [8] M.-H. Hon, T.-C. Chang, M.-C. Wang, Journal of Alloys and Compounds 458 (2008) 189–199. [9] W.-L. Li, Y.-R. Chen, K.-M. Chang, C.-Y. Liu, M.-H. Hon, M.-C. Wang, Journal of Alloys and Compounds 461 (2008) 160–165. [10] T.-C.M.-C. Chang, M.-M. Hon, Journal of Crystal Growth 250 (2003) 236–243. [11] T.-C. Chang, M.-H. Hon, Moo-Chin Wang 352 (2003) 168–174. [12] J. Friedel, Dislocations, Pergamon Press, London, 1964, p. 304. [13] A. Fawzy, N. Habib, M. Sobhy, E. Nassr, G. Saad, Materials Science and Technology 24 (2008) 488–494. [14] F. Abd El-Salam, A.M. Abd El-Khalek, R.H. Nada, A. Fawzy, Materials Characterization 59 (2008) 9–17. [15] F. Abd El-Salam, R.H. Nada, A.M. Abd El-Khalek, Materials Science and Engineering A 448 (2007) 171–176. [16] T.C. Chang, M.H. Hon, M.C. Wang, Journal of the Electrochemical Society 15 (2004) 484–491. [17] G. Saad, G. Graiss, A. Fawzy, Indian Journal of Pure and Applied Physics 29 (1991) 344–347. [18] A.M. Abd El-Khalek, Physica B 315 (2002) 7–12. [19] C.M.L. Wu, M.L. Huang, Journal of Electronic Materials 31 (2002) 442–447. [20] M. Mc Cormack, S. Jin, Journal of Electronic Materials 23 (1994) 635–640. [21] Y. Kariya, M. Otsuka, Journal of Electronic Materials 27 (1998) 866–870. [22] G. Sharma, R.V. Ramanujan, T.R.G. Kutty, G.P. Tiwari, Materials Science and Engineering A 278 (2000) 106–112. [23] C.G. McKamey, P.J. Maziasz, J.W. Jones, Journal of Materials Research 7 (1992) 2089–2106. [24] G.S. AL-Ganainy, M.R. Nagy, B.A. Khalifa, R. Afify, Phys Status Solidi (a) 158 (1996) 463–470.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of Ag addition on the creep characteristics of Sn–8.8 wt%Zn solder alloy G. Saad ∗ , A. Fawzy, E. Shawky Department of Physics, Faculty of Education, Ain Shams University, Elmakrezy/ Roxy, Cairo 11575, Egypt
a r t i c l e
i n f o
Article history: Received 27 October 2008 Received in revised form 13 January 2009 Accepted 21 January 2009 Available online 31 January 2009 PACS: 62.20.Fe 61.82.Bg 61.66.Dk Keywords: Metals and alloys Precipitation Microstructure X-ray diffraction
a b s t r a c t Full implementation of the new generation of lead-free solders requires detailed knowledge and understanding of their mechanical behavior. The materials used in the present study are Sn–8.8 wt%Zn (binary) and Sn–8.8 wt%Zn–1.5 wt%Ag (tertiary) alloys. Effect of Ag addition, deformation temperature T and the applied stress, , on the creep characteristics have been studied. Creep tests were performed under the effect of different stresses ranged from 17.8 to 26 MPa at the deformation temperatures 291, 303, 323 and 343 K. The transient creep parameters ˇ and n were found to be markedly affected by the creep test conditions, T and . The parameter ˇ was found to be decreased by increasing T and/or while n was found to increase by increasing T irrespective of the applied stress . The steady-state creep rate ε´ st was found to increase with increasing both T and in both solder alloys. The steady-state creep rate ε´ st was related to the stress with the relationship ε´ st = C m where m = (∂ln ε´ st /∂ln ) is the stress exponent. This exponent is decreased with increasing T in both alloys. Addition of 1.5 wt%Ag to the binary alloy increased its creep resistance. This behavior was attributed to the formation of the intermetallic compounds (IMCs) AgZn and Ag3 Sn during solidification. These IMCs played the role of pinning action for the moving dislocations and consequently leading to the increase of its creep resistance. Micro-structural changes were investigated by scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analysis. © 2009 Published by Elsevier B.V.
1. Introduction It is well known that joining materials (solders) provide electrical, thermal and mechanical continuity in electronic assemblies. The near eutectic Sn–Pb solders are extensively used through whole assemblies because they have many advantages. They are inexpensive; they have good processing characteristics such as low melting temperature (456 K), high strength, high ductility, good thermal and electrical properties. However, in the early nineteenth-century, lead poisoning was in paint pigment workers in USA and France since the use of lead has been gradually banned worldwide in plumbing, paints, and gasoline through the enactment of various legislations. In the electronic industry, the lead generated by the disposal of electronic assemblies is considered as hazardous to the environment. So, the European Union has officially designated 1 July 2006 as the date when directive on the restriction of hazardous substances in electrical and electrical equipment will require be phased out. Hence, electronic manufacturing now have only one choice which is the use of Pb-free solder alloys and subsequently close attention is paid lately to substitution of the Pb-rich solders by lead-free ones. Pb-free solder must form joints with accept-
∗ Corresponding author. E-mail address: [email protected] (G. Saad). 0925-8388/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jallcom.2009.01.069
able strength, and withstand over the projected operational life of assembly. Mechanical properties and microstructure changes are among the main parameters to be controlled in the candidate solders in an industrial process. Although creep is the most common and important deformation mechanism of solders, the creep resistance of various Pb-free solder alloys is still in question. The softness of tin metal due to its low mechanical strength leads to the idea of strengthening it by adding hardening alloying elements to meet its wide applications [1,2]. Pb-free solders are typically tin-based alloys. Among types of Pb-free solder alloys are Sn–Bi [3], Sn–Cu [4,5], Sn–Ag [6] and Sn–Zn [7]. Sn–Zn [8,9] based alloys are still under development to overcome the shortfall of Sn–9 wt%Zn alloys. Other results showed that Sn–9 wt%Zn alloys with Ag content higher than 0.5 wt%Ag exhibited low strength at low solidification rates while an acceptable strength was obtained at higher solidification rate due to the presence of the hard AgZn intermetallic compounds (IMC) besides the refined and modified distribution of the Zn-rich needles [10]. Moreover increasing Ag content in Sn–9 wt%Zn tends to diminish the eutectic structure due to the formation of larger volume fractions of the AgZn IMC [11]. The morphology and distribution of such phases was found to strongly affect the creep behavior of solder alloy. This was attributed to the action of the alloying addition that may leads to a fine and uniform dispersion of small seeded particles of the second phase. Hence,
G. Saad et al. / Journal of Alloys and Compounds 479 (2009) 844–850
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Table 1 Chemical compositions of the alloys: Sn–8.8 wt%Zn and Sn–8.8 wt%Zn–1.5 wt%Ag. Alloy
Sn
Zn
Ag
Pb
Sn–8.8 wt%Zn Sn–8.8 wt%Zn–1.5 wt%Ag
91.11 89.61
8.82 8.82
0.00 1.52
0.05 0.04
the improvement of mechanical properties is closely related to the amount and type of the alloying elements [1]. On the other hand Ag addition to Sn–Bi solder alloy resulted in the decrease of its mechanical strength [3]. This was explained as due to the surface activity of the alloying elements. Between softening and hardening effects of Ag addition to the Sn-based alloys, the present work is devoted to get a more insight into the effect of Ag addition, temperature and applied stress on the creep characteristics of Sn–8.8 wt%Zn alloy.
Fig. 1. Phase diagram for Sn–Zn binary system.
2. Experimental procedures Sn–8.8 wt%Zn and Sn–8.8 wt%Zn–1.5 wt%Ag alloys were prepared from high purity Sn, Zn and Ag of purity 99.99%. The appropriate weights of the elements for the binary alloy were well mixed with CaCl2 flux to prevent oxidation in a graphite mold and kept at 100 K above the melting temperature of Ag. Casting into stainless steel molds was done in air with a cooling rate of 10 K/s. An appropriate weight of silver was added to portion of the binary alloy, while the temperature is kept at 100 K above the melting temperature of Ag to form the second alloy (Sn–8.8 wt%Zn–1.5 wt%Ag). Chemical analysis of the two solder alloys was found to be close to the desired constitution (see Table 1). The ingots of the two alloys were in the form of rods 12 mm in diameter. After casting, each of them was cold drawn into 0.6-mm diameter wire. Specimens with a gauge length of 50 mm were prepared for tensile testing. The experiments were carried out using a conventional creep testing machine equipped with a strain resolution equal to10−4 . Isothermal strain-time experiments were performed under
constant applied stresses ranging between 17.9 and 26 MPa at different deformation temperatures 291, 308, 323, and 343 K. During tensile tests, the deformation temperatures were controlled to within ±2 K. The microstructure of the as-cast samples was investigated using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analysis. Prior to the SEM observations of the as-cast alloys, samples of 3-mm diameter were mechanically and then electrically polished using solutions containing 80% acetic acid and 20% perchloric acid.
3. Experimental results According to the Sn–Zn binary phase diagram (see Fig. 1), the equilibrium microstructure of the eutectic Sn–8.8 wt%Zn consists
Fig. 2. (a) XRD pattern for the as-cast Sn–8.8 wt%Zn and (b) SEM micrographs for the as-cast Sn–8.8 wt%Zn.
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Fig. 3. (a) X-ray diffraction patterns for the as-cast Sn–8.8 wt%Zn–1.5 wt%Ag solder alloy, (b) SEM micrographs for the as-cast Sn–8.8 wt%Zn–1.5 wt%Ag solder alloy, and (c) EDX pattern for the as-cast Sn–8.8 wt%Zn–1.5 wt%Ag solder alloy.
of the eutectic mixture of Sn-rich (close to pure Sn) phase and the Zn-rich phase. As mentioned in the preceding section, addition of Ag allows predicting microstructure containing the IMC AgZn [8]. Examination of the present Ag-free solder alloy by XRD analysis and SEM examination revealed the same microstructure as predicted before [8] in which it consists of Sn-rich and Znrich phases as shown in Fig. 2a and b respectively. Concerning the present Ag-containing solder alloy, XRD analysis showed similar microstructure obtained by [10] beside the additional IMC Ag3 Sn as shown in the diffractogram of Fig. 3a. It should be noted that the addition of Ag in this work resulted in the refinement of the Zn-rich phase (compared with that obtained in Fig. 2b) as clearly observed in the micrograph of Fig. 3b. Creep tests for the binary and tertiary alloys were performed at the deformation temperatures 291, 308, 323, 343 K under the effect of 17.8, 19.5, 21.2, 24.4, and 26 MPa. Fig. 4a is a representa-
tive example for isothermal creep curves of samples tested at the deformation temperatures 291, 308, 323, 343 K under the effect of 17.8 MPa for both binary and tertiary alloys showing the effect of deformation temperature. Fig. 4b is a representative example for creep curves of samples tested at the deformation temperature of 291 K under the effect of different stresses17.8, 19.5, 21.2, 24.4, and 26 MPa for both alloys showing the effect of applied stresses. Fig. 4c is a representative comparative example for creep curves of samples tested under the stress of 17.8 MPa at the deformation temperature 291 K for both alloys showing the effect of Ag addition. It is to be noted that the results obtained in Fig. 4a and b, indicated that the creep curves of both are sensitive to the deformation temperature, T and the applied stress showing monotonic shift towards higher strains with increasing the deformation temperature and/or the applied stress. On the other hand a lower level of
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Fig. 5. Representative example for the relation between ln εtr and ln t at = 19.48 MPa at different deformation temperatures indicated on the figure.
Fig. 5. Slopes of these straight lines gave the values of the transient creep time exponent n while their intercepts at ln t = 0 gave the transient creep parameter ˇ. It was found that n increases with increasing deformation temperature for both alloys (Fig. 6a). The parameter ˇ was found to decrease with increasing temperature and/or decreasing the applied stress as shown in Fig. 6b. 5. Steady-state creep stage The steady-state strain rate ε´ st of the tested samples is calculated from the slopes of the linear parts of the obtained creep curves. It increases with increasing both the T and/or in both solder alloys (Fig. 4a and b). It is also seen that under the same test conditions the tertiary alloy showed higher creep resistance compared with that of the binary one (Fig. 3c).
Fig. 4. Representative creep curves for Sn–8.8Zn and Sn–8.8Zn–1.5 wt%Ag wires: (a) tested at different temperatures under constant applied stress of 17.86 MPa, (b) tested at 291 K under the effect of different stresses as indicated, and (c) tested at 308 K under17.86 MPa Showing the effect of Ag addition.
creep strain was observed in the Ag-containing strain-time curves (Fig. 4c). 4. Transient stage Concerning the transient stage of the creep curve, the transient creep strain is generally described by the well-known equation [12]: εtr = ˇt n where εtr and t are the transient creep strain, and time, ˇ and n are constants depending on the experimental test conditions. In the present work the relation between ln εtr and ln t is constructed showing straight lines as shown from the representative example in
Fig. 6. (a) Temperature dependence of the transient creep exponent, n and (b) the temperature dependence of the transient creep parameter, ˇ for both binary and tertiary solder alloys.
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Fig. 7. (a) The relation between ln εst and ln for both binary and tertiary solder alloys at different deformation temperatures indicated on the figure and (b) the variation of the stress exponent m with the deformation temperature T.
It is well known that the steady-state creep rate ε´ st is related to the applied stress according to the relation [13]
where C is a constant and m is the stress exponent and both of them depend on the testing conditions. A relation between ln ε´ st and ln was constructed and straight lines for both alloys were obtained as shown in Fig. 7a. Slopes of the obtained straight lines gave the values of the stress exponent m, at different deformation temperatures. The temperature dependence of m given in Fig. 7b shows that it increases with increasing deformation temperature. Besides, values of m for the tertiary alloy was found higher than those of the binary one indicating that the tertiary alloy is harder. In the present creep tests a linear dependence of ln ε´ st on at different deformation temperatures is obtained (Fig. 8a) and values of the activation volume, V = ∂log ε´ st /∂ [14], are calculated from the slopes of the obtained straight lines. The activation volume, V was found to increase with increasing deformation temperature in both solder alloys, as shown in Fig. 8b. It is to be noticed also that, V for the binary alloy is higher than those of the tertiary one at all testing temperatures. 6. Discussion It is generally well known that the internal structure of an alloy controls and modifies the nature and mode of interaction of the existing defects and the alloy components. So, the most important result of the creep tests is the strong dependence of the creep parameters on the micro-structural changes as well as the
Fig. 8. (a) ln εst versus at different deformation temperatures for Sn–8.8 wt%Zn and Sn–8.8 wt%Zn–1.5 wt%Ag solder alloy and (b) temperature dependence of the activation volume, V for both alloys.
alloying element. In the present investigation the constitutional diagram of Sn–Zn system [15] at the eutectic composition predicts a microstructure consisting of Zn-rich phase in the form of Zn platelike dispersed in the ˇ-Sn matrix [11]. Addition of 1.5 wt%Ag seems to change the microstructure of the binary alloy and the plate-like Zn phase appears in refined form as shown in Fig. 2b beside Ag3 Sn fine particles which were detected by (i) XRD analysis as shown in Fig. 3a, this led to (ii) detect the elemental composition for different zones from the dark phase appeared in the SEM by means of EDX which revealed the existence of Ag with its maximum concentration as illustrated in Fig. 3c. This means that the bright phase (Sn-rich phase) is nearly depleted from Ag. Moreover, the profile of XRD patterns reveals that Ag-containing solder alloy showed a remarkable higher intensity of Sn (1 1 0) and Sn (2 0 0) crystallographic texture while a lower intensity for Zn (0 0 2), Zn (1 0 0) and Zn (1 0 1) was observed (Fig. 3a). In other words, the intensity of Sn in Ag-containing alloy (Fig. 3a) is greater than that of Sn in Ag-free one (Fig. 2a) while the intensity of Zn in the Ag-free alloy (Fig. 2a) is greater than that in the Ag-containing alloy (Fig. 3a). This result is due to the addition of 1.5 wt%Ag which increased the solubility of Zn in the Sn matrix. This led to reducing sizes and increased the number of these plate-like Zn-rich particles. Hence a uniform and fine microstructure is attained as has been observed in Fig. 3b compared with that obtained in Fig. 2b. This was in agreement with the results obtained in other work [15] which reported that the Zn-rich phase decreases on the expense of the increase of the solubility of Zn in the Sn matrix by Ag addition, which enhance the corrosion resistance of Sn–Zn eutectic alloy [16]. In view of these considerations, Ag addition is expected to modify the creep behavior of the Sn–Zn binary alloy due to the presence of AgZn and Ag3 Sn IMCs. Consequently, the creep strain in the Agcontaining would be always lower than the Ag-free one.
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Taking the above remarks into consideration, the creep behavior of the present materials can be interpreted as follows. 7. The transient creep stage It is well known that the transient creep parameter n determines the dependence of the density of mobile dislocations on the mean internal stress i and the relaxed shear modulus G according to the relation = ( i /nGb)2 as reported before [17] where b is the Berger’s vector of the involved dislocations. At high normalized stress (/G) > 10−4 one can assume that the internal stress i does not change. In view of this consideration increasing the deformation temperature T, the dislocation density would decreases due to their annihilation. Consequently the transient creep parameter n will increase which is clearly seen in Fig. 6a. In addition increasing the deformation temperature T resulted in: (i) increasing thermal agitation for the moving dislocations leading to faster annihilation and/or (ii) decreasing the pinning effect of each alloy components. Hence the increase of n with T can be accounted for. The higher value of the transient creep parameter n for the Agcontaining alloy might be due to the finer microstructure of the IMCs (Ag–Zn and Ag3 Sn) which are expected to be easily agitated more than the Zn-plate like particles in the binary alloy. In order to determine the energy activating the transient creep process Etr , one can assume that values of ˇ satisfy the Arrhenius type form [18]: ˇ = ˇ0 exp −Etr /KT where ˇ0 is a constant, K is the Boltzman constant. The activation energy Etr is calculated from the slopes of the straight lines of Fig. 9. Etr was found to have a mean value of ∼0.25 eV in agreement with that needed for the dislocation motion [3]. 8. The steady-state stage In this stage, raising deformation temperature affected the creep behavior as previously observed in Fig. 4a from which the creep strain in both alloy specimens increased with increasing T while the applied stress is kept constant. As mentioned before, the Ag-containing solder alloy was found to have a superior strength compared to Sn–8.8 wt%Zn binary alloy (see Figs. 4c) due to the difference in the microstructure in both solder alloys. Ag-containing solder alloy is characterized by more fine and uniform dispersion of AgZn besides the presence of Ag3 Sn particles in the Sn matrix. Therefore, the observed lower creep strain values in the Ag-contained solder alloy may be attributed to the pin-
Fig. 9. The best fitting straight lines relating ln ˇ and (1000/T) at different applied stresses for both solder alloys.
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ning action of the finely dispersed AgZn and Ag3 Sn fine particles [19–21]. The increase in the creep strain by increasing T (Fig. 4a) for the two solder alloys under investigation may be due to the decrease in density of the effective pinning centers with increasing T allowing higher slip distances traveled by the moving dislocations and/or the decrease in the density of mobile dislocations due to their annihilation at the ˇ-Sn dendrites in the two solder alloy specimens. 9. Stress exponent In creep deformation, upon the application of stress at different deformation temperatures the obtained steady-state creep rate seems to depend mainly on structure of the tested sample and the deformation temperature. With increasing the applied stress, the interaction between dislocations and the other phases may lead to climb, glide and/or viscous motion of the mobile dislocations on their slip planes. It is generally known that the value of the stress exponent m helps in judging the controlling mechanism. In the present study, the stress exponent m was increased with increasing the deformation temperature, from ∼8.7 to ∼11.3 in the tertiary alloy and from ∼8.5 to ∼10.3 in the binary one (Fig. 7b) indicating that the rate controlling mechanism may be glide and/or climb [3]. From Fig. 7b, it is clear also that m for the tertiary alloy is higher than that in the binary one due to the presence of the IMCs, AgZn and Ag3Sn. It has been previously reported by [14] that activation volume is directly proportional to the average spacing of dislocations. The variation of the activation volume V with T (Fig. 8b) was found to be consistent with the variation of n with T (Fig. 6a). In the present study, addition of Ag decreased the volume fraction of the Zn platelike precipitates in the Sn matrix (see Figs. 2b and 3b) leading to the increase of the average distances swept by moving dislocations. On the other hand, addition of 1.5 wt%Ag appears to refine the microstructure of Zn-rich phase from Zn plate-like (dark phase) as shown in Fig. 2b into a finer and more uniform distribution of these plates-like particles as shown Fig. 3b leading to increase its pinning action. Between the two contradictory effects of the Ag addition, increasing the deformation temperature leads to increase the thermal agitation for the moving dislocations. Hence, the increase of n as well as V could be accounted for. From Figs. 6a, 7b and 8b, it should be noted that the different values for n between the Ag-free and Ag-containing solder alloys decrease with increasing the deformation temperature T while the difference values for both m and V decrease with increasing T. This contradiction in behaviour between n, m and V with respect to the deformation temperature T for the two solder alloys may be due to the difference in their microstructure. The activation energy of the steady-state creep, Est , for both alloys can be calculated using an Arrhenius type equation of the form [18],
where K and T have the usual meanings as previously mentioned. The relation between ln ε´ st and 1000/T as illustrated in Fig. 10 gave straight lines for the different applied stresses. It shows that the calculated energy needed for the operating mechanism in the steady-state creep is ∼0.5 eV. This value was found to be close to that of viscous glide motion of dislocations on their slip planes [3]. This is may be due the fact that upon heating, if a gliding dislocation was held by an obstacle, a small amount of climb might permit it to surmount the obstacle, allowing it to glide to the next set of obstacles where the process was repeated. The glide step produces almost all of the strain. The climb process of dislocations requires atomic diffusion. As diffusion processes are more difficult in the
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10. Conclusions The main conclusions to be drawn from this work may be summarized as follows:
Fig. 10. The best fitting straight lines relating ln ε´ st and (1000/T) at different applied stresses for both solder alloys.
(i) The transient creep parameter n increases with increasing the deformation temperature while ˇ decreased. (ii) Both of the stress exponent m and activation volume V increase with increasing the deformation temperature. (iii) The difference values for n decreases while the difference values for both m and V increase with increasing the deformation temperature. (iv) Addition of Ag to the Sn matrix led to: (i) The formation of the IMCs (AgZn and Ag3 Sn). (ii) Reduced the volume fraction of the plate-like Zn phase. (v) A significant improvement in the creep resistance is achieved by 1.5 wt%Ag addition to the binary Sn–8.8 wt%Zn solder alloy. (vi) The action of the alloying addition still needs more and more attention. References
Fig. 11. The relation between ln ˇ and ln ε´ st for for both solder alloy.
ordered lattice [22] the activation energies for the formation and migration of vacancy increase with order, resulting in higher activation energies for diffusion and for creep, and therefore lower creep strain [23]. Finally, the correlation between the transient and steady-state creep in the present study is obtained by constructing the relation between ln ε´ st and ln ˇ at different stresses as shown in Fig. 11. The obtained straight lines proved the validity of the relation [24] with the exponent independent on the stress applied. The value of (=ı ln ˇ/ı ln ε´ st ) was found to be ∼0.9 indicating that the dependence of ε´ st on ˇ seemed to be induced by the dislocation process which enhanced both the transient and steady-state processes.
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