Microstructure evolution and tensile creep behavior of

Journal of Materials Science: Materials in Electronics https://doi.org/10.1007/s10854-018-0492-0

Microstructure evolution and tensile creep behavior of Sn–0.7Cu leadfree solder reinforced with ZnO nanoparticles A. F. Abd El‑Rehim1,2 · H. Y. Zahran1,2 · A. M. Yassin2 Received: 13 October 2018 / Accepted: 30 November 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018

Abstract This paper presents the influence of aging temperature as well as ZnO nanoparticles addition on the properties of Sn–0.7Cu solder. A series of Sn–0.7Cu–ZnO composite solders with ZnO nanoparticles traces (0, 0.1, 0.25, 0.5 and 1.0 wt%) has been fabricated. After being solution heat treated at 443 K for 2 h, specimens were cooled at 273 K by water quenching. All the specimens were isochronally aged for 2 h at temperatures up to 423 K. Subsequently, all samples were quenched into iced water at 273 K. The microstructure evolution, the tensile creep properties and the thermal behavior of the new fabricated solder alloys were studied. Differential scanning calorimetry measurements indicated that the ZnO nanoparticles addition increases slightly the melting point of the investigated composite solders within the range of 227.7–229.2 °C with less than 1.6 °C temperature difference. Microstructural evolutions revealed the efficient refinement of the Cu6Sn5 and Cu10Sn3 intermetallic compounds by the addition of ZnO nanoparticles. Tensile creep tests showed that the creep rate at the steady state stage increases with increasing both the aging temperature and the applied stress. The improvement of the solders creep resistance has been achieved by the increasing of the nanoparticles content up to 0.25 wt%. The deficiency of the creep resistance occurred with the excessive addition of ZnO particles. The mean values of the stress exponents and activation energies indicated that the steady state creep stage is controlled by dislocation-pipe diffusion in the tin matrix as the dominant operating mechanism.

1 Introduction The eutectic Sn–37Pb solder composites have been applied extensively in the various electronic fields because of their good mechanical properties, low melting point, and excellent wettability. However, recently the use of these solders is restricted due to their recognized damage effects for human health. These issues have promoted many researches on lead-free solder alloys for electronics industry [1, 2]. The eutectic Sn–0.7Cu solders are of interest among all the available lead-free alternatives due to their relatively low melting point, inexpensive, high electrical conductivity and relatively good solderability compared to other lead-free solders [3–5]. The presence of a small amount of copper in * H. Y. Zahran [email protected] 1

Physics Department, Faculty of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia

Physics Department, Faculty of Education, Ain Shams University, Heliopolis, Roxy, P.O. Box 5101, Cairo 11771, Egypt

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the tin-based solders causes an obvious enhancement in their creep resistance as a result of the formation of C u6Sn5 and Cu3Sn intermetallic compounds (IMCs) [6]. Some studies have interested in the examination of the mechanical behavior and the microstructure evolution of Sn–Cu based lead-free solders. Shen et al. [7] studied the effect of Cu content and cooling rate on the microstructure development of the solidified Sn–Cu solders. The results indicated to the presence of Cu6Sn5 IMC in the microstructure of the eutectic Sn–0.7Cu solder while the hypereutectic Sn–1Cu alloy developed two kinds of IMCs; Cu6Sn5 (η-phase) and Cu3Sn (ε-phase). On the other hand, Shalaby [8] reported that the microstructure of the eutectic Sn–0.7Cu solder consists of primary β-Sn phase, Cu6Sn5 and Cu10Sn3 (ξ-phase) IMCs. Lai and Ye [9] inspected the variation of the microstructure evolutions and mechanical characteristics of Sn–0.7Cu solder under the influence of Al addition. They found that the ultimate tensile strength and elongation of Sn–0.7Cu–Al solders were improved continuously with increasing the weight percentage of Al due to the refined IMCs. It was mentioned by Wu et al. [10] that the addition of (Ce and La)

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as rare earth elements to the Sn–0.7Cu solder effectively improved the creep resistance, hardness, and tensile strength of the studied solder. Recently, the addition of nanoparticles to the solder in order to create a composite solder is believed to be an economically affordable approach to enhance the performance of conventional lead-free solders. Tsao et al. [11] illustrated that the addition TiO2 nanoparticles to Sn–0.7Cu solders can significantly refine the microstructure of β-Sn and C u6Sn5 IMC. As the T iO2 nanoparticles content increased, the ultimate tensile strength and microhardness of composite solders have been promoted. Zhang and Gupta [12] prepared Sn–0.7Cu alloy with Al2O3 nanoparticles added in different ratios (wt%) via the powder metallurgy route. They found that the increase in the content of Al2O3 did not change the characteristics of β-Sn phase and Cu6Sn5 IMC. Moreover, the number of pores and their irregularity increased with increasing the concentration of Al2O3 nanoparticles. Fathian et al. [13] studied the microstructural development and mechanical characteristics of the Sn–0.7Cu solders reinforced with different concentrations of amorphous silica (SiO2) nanoparticles. The results showed that the optimum mechanical properties of the composite solders have been achieved as the SiO2 concentration is about 1.5 wt%. However, there are few reports on the variation of Sn–0.7Cu solder properties under the influence of nanoparticles addition. To the best of authors’ knowledge, no systematic study has been investigated the influence of ZnO nanoparticles addition on the thermal behavior, microstructure evolution and tensile creep properties of Sn–0.7Cu alloy. The lack of such study motivated the current work.

2 Experimental procedures The ZnO nanoparticles of purity (99.99) with 100 nm (average particle size) were applied in the present study. Figure 1a reveals a typical SEM micrograph for the ZnO nanoparticles. As seen in Fig. 1a, the shape of ZnO particles was a little bit irregular with polygon surface. Figure 1b exhibits the XRD pattern of ZnO nanoparticles. The intensity data were collected over a 2θ range of 10°–90°. The characteristic diffraction peaks for the hexagonal wurtzite structure of ZnO nanoparticles appear in the pattern with lattice constants a = b = 3.25 Å and c = 5.21 Å as compared to JCPDS card No. 36-1451. Metal ingots (99.99% pure) of Sn and Cu were employed to prepare the Sn–0.7 wt% Cu (SC07) solder. The pure metals in proper weight proportions were melted in a high-purity graphite crucible under vacuum in an induction furnace at 673 K for 1 h. The molten was chill cast into a stainless mold at room temperature to form cylindrical ingots of 10 mm in diameter. The Sn–0.7Cu composite solders containing ZnO

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Journal of Materials Science: Materials in Electronics

Fig. 1 a SEM micrograph of the ZnO nanoparticles and b X-ray diffraction spectra of the ZnO nanoparticles Table 1 Chemical compositions of the solders investigated (wt%) Solder

Sn–0.7Cu Sn–0.7Cu–0.1ZnO Sn–0.7Cu–0.25ZnO Sn–0.7Cu–0.5ZnO Sn–0.7Cu–1.0ZnO

Element content (wt%) Cu

ZnO

Sn

0.71 0.70 0.70 0.69 0.67

0 0.09 0.23 0.48 0.97

Bal. Bal. Bal. Bal. Bal.

nanoparticles were prepared primarily by mechanically dispersing different contents (0, 0.1, 0.25, 0.5 and 1.0 wt%) of ZnO nanoparticles at 673 K in a furnace for 1 h and casting in stainless steel molds. The ratio of Cu to Sn for all the composite solders was kept the same as the eutectic Sn–0.7Cu composition. The chemical compositions of the composite solders are listed in Table 1. The ingots were homogenized after annealing at 443 K for 4 h then swaged and cold drawn into (i) wires of 0.8 mm in diameter for tensile creep tests and (ii) sheets of 0.5 mm in thickness (by rolling) for microstructure examinations. After solution heat treatment at 443 K for 2 h to remove the residual stress and defects induced during the machining of ingots, specimens were quenched into iced water at 273 K and immediately


aged at various temperatures (Ta = 348, 373, 398 and 423 K) for 2 h then quenched at 273 K in water. Temperature fluctuation was within ± 1 K. The tensile creep tests have been performed at room temperature (300 K) under constant applied loads corresponding to stresses ranging from 9.8 to 13.7 MPa using a computerized tensile creep testing machine described elsewhere [14]. The applied stress range was chosen to produce experimentally convenient creep rates over the aging temperature range investigated. To investigate the melting characteristics of the five composite solders, differential scanning calorimeter (DSC Q10) was utilized. For microstructure investigations, the samples were etched in a solution containing 95 ml C 2H5OH, 3 ml HNO3 and 2 ml HCl similar to [3]. A JEOL JSM-6360LV scanning electron microscope (SEM) equipped with Energy Dispersive Spectroscope (EDS) operating at 20 kV was used to investigate the microstructural evolutions of the investigated composite solders. A Shimadzu D6000 X-ray diffractometer using Cu-Kα was utilized to identify the phases in the present solders.

3 Results and discussion 3.1 Thermal analysis The DSC scans were performed to investigate the fundamental thermal effect during the heating process of the composite solders. Figure 2 demonstrates the typical DSC curves of the five composite solders through the heating process. The details of DSC results are tabulated in Table 2. Only one single endothermic peak is observed for each composite solder on the DSC curve. The DSC curve of SC07 solder without any addition of ZnO nanoparticles exhibits an endothermic eutectic peak at 227.7 °C (Fig. 2a) corresponding to the eutectic temperature (227 °C) of the SC07 solder. As the ratio of ZnO nanoparticles increased, the melting (peak) temperatures slightly shifted towards the right (Fig. 2b–e). The results showed that the ZnO addition has a little effect on the peak temperature which is less than 1.6 °C. The reason may be that the reinforcement addition into the SC07 solder could change the physical properties of grain boundary/interfacial characteristics and vary the surface instability [15]. Because of the high melting point of ZnO, there is a slight increase in the melting temperature of the composite solders reinforced with nano- ZnO particles. Our findings agree with the results previously reported in the literature [6, 16]. The pasty range (liquidus temperature minus solidus temperature) is slightly widened with ZnO additions when compared with Sn–Pb eutectic. The pasty range of the five composite solders lies in the range of 4.6–5.3 °C, while it reaches 11.5 °C for Sn–Pb eutectic. From the DSC results,

one can conclude that no significant variations occur after the addition of zinc oxide to the SC07 solder and hence the standard reflow profiles can be used in the assembly of this solder.

3.2 Creep properties and microstructure evolutions Creep behavior of the lead-free solders is crucial to their successful applications in the electronic packaging, especially above 0.5 of the homologous temperatures (T/Tm where Tm is the melting point in Kelvin). In order to evaluate the effect of ZnO nanoparticles addition on the creep characteristics of SC07 solder, typical creep curves of SC07–xZnO composite solders (x = 0, 0.1, 0.25, 0.5 and 1.0 wt%) were obtained for samples aged at different temperatures (Ta = 348, 373, 398 and 423 K) for 2 h. Representative creep curves of the five composite solders aged at different temperatures under a constant applied stress of 11.7 MPa are illustrated in Fig. 3. It is obviously seen that all the creep curves display typical transient stage with a decreasing creep rate, steady state stage characterized by an unchanged strain rate, and tertiary stage where the strain rate increases until fracture occurs [17]. As can be inferred from Fig. 3, the levels of the curves are shifted monotonically towards higher values of strain and lower fracture times with increasing aging temperature. The curves seem to be in irregular sequence with respect to ZnO nanoparticles content. The SC07–0.25ZnO composite solder provides the highest creep resistance while the lowest one belongs to the SC07 solder, with the remaining solders lying between. The steady state creep rate, έst, was determined by taking the derivative of the creep strain with creep time for the straight parts of the creep curves (shown in Fig. 3). The dependence of έst values on the content of ZnO at different aging temperatures, Ta, under various applied stresses on a semi-log scale is shown in Fig. 4. As seen from this Figure for all aging temperatures and applied stresses, the steady state creep rate values are characterized by two distinct stages. In the first stage, the values of steady state creep rate are lowered continuously with the elevation of the ZnO weight percentage up to 0.25 wt%. However, when the content of ZnO exceeded 0.25 wt%, a second stage characterized by the increase in έst values is observed. Furthermore, the έst values are enhanced with increasing the applied stress and/ or aging temperature at any given content of ZnO. Two SEM images are illustrated in Fig. 5a, b for the SC07 solder aged at 348 and 423 K for 2 h respectively. As can be inferred from Fig. 5a, the microstructure of SC07 contains a primary β-Sn phase, a dark gray color C u6Sn5 phase (η-phase), and a white color Cu10Sn3 phase (ξ-phase). The EDS analysis confirmed that the dark gray phase is the η-phase while the white phase is the ξ-phase. The observed increase in έst values with the increase of Ta from 348 to 423 K (Fig. 4) for the SC07 solder could be attributed to the

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Fig. 2 DSC results of a SC07, b SC07–0.1ZnO, c SC07–0.25ZnO, d SC07–0.5ZnO, and e SC07–1.0ZnO composite solders during heating process

Table 2 Solidus temperature (TS), liquidus temperature (TL), melting temperature (Tm) and pasty range (ΔT) of lead-free Sn–0.7Cu–xZnO composite solders Solder

Ts (°C)

TL (°C)

Tm (°C)

ΔT (°C)

SC07 SC07–0.1ZnO SC07–0.25ZnO SC07–0.5ZnO SC07–1.0ZnO

225.8 226.1 226.5 227.2 227.6

230.4 230.8 231.4 232.5 232.9

227.7 228.1 228.5 228.9 229.2

4.6 4.7 4.9 5.3 5.3

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dissolution of C u6Sn5 precipitates and the growth of C u10Sn3 precipitates (see Fig. 5b). SEM analysis indicates that, with further heating at 423 K, the microstructure consists of primary β-Sn phase and ξ-phase (Fig. 5b). Eventually, with the growth and coarsening of Cu10Sn3 precipitates the spacing between the precipitates becomes large that the dislocations are able to bow between precipitates and the creep resistance decreases. With the addition of 0.1 and 0.25 ZnO nanoparticles to the SC07 solder, the values of steady state creep rate decreased. Typical microstructures of SC07–0.25ZnO composite solder aged at 348 and 423 K are presented in


Fig. 3 Representative creep curves for the five composite solders aged at different temperatures for 2 h under constant applied stress of 11.7 MPa. ε0 is the instantaneous creep at time = 0

Fig. 4 Variation of steady state creep rate with the content of ZnO at different aging temperatures under various applied stresses

Fig. 6a, b respectively. Comparing Figs. 5 and 6, it is clearly seen that the ZnO nanoparticles addition into the SC07 solder influences the microstructures of composite solders.

Worthy of notice is that the average sizes of more C u6Sn5 and Cu10Sn3 IMCs are decreased with the addition of ZnO nanoparticles. Therefore, the growth rate of IMCs has been

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Fig. 5 SEM micrographs and the corresponding EDS spectra of SC07 solder aged for 2 h at a 348 K and b 423 K

Fig. 6 SEM micrographs of SC07–0.25ZnO composite solder aged for 2 h at a 348 K and b 423 K

successfully suppressed with the ZnO nanoparticles addition. Figure 6a, b shows that the SC07–0.25ZnO composite solder has a refined microstructure compared to the SC07 solder under same test conditions. Two main reasons can explain the enhancement of the creep resistance of SC07 solder reinforced with 0.1 and 0.25 ZnO nanoparticles: (i) Surface adsorption theory can be utilized to explain the refinement of the microstructure of the SC07 solder by the presence of the nano-sized ZnO particles. The ZnO nanoparticles incorporated into the SC07 solder cannot wet the β-Sn matrix during the solidification process of the composite solders because they do not form IMCs in the solder matrix and they are

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non-coarsening and non-reacting. The refinement of microstructure may be due to the high surface free energy on the solidified grain surfaces through the matrix which adsorbs the ZnO nanoparticles during the solidification process. Furthermore, they can u10Sn3 accumulate at the interfaces of C u6Sn5 and C IMCs to refine them in the composite solders. The surface free energy and the growth velocity of these IMCs decrease by the presence of such adsorbed ZnO particles [14, 16]. This explains the reasons for the significant inhibitory effect of ZnO nanoparticles on the growth of IMCs during the solidification process of the composite solders. The homogeneously dispersed IMCs within the Sn-matrix prevents the


slipping of dislocations and thus lead to better creep resistance of the composite solders. (ii) The creep resistance of the solder matrix can be enhanced by the existence of second particles (IMCs and ZnO) which agrees with the theory of dispersion strengthening [18]. The particulates act as barriers for dislocation motion; thus, the stress acting on the particle can be expressed with piling of linear dislocation. Therefore the yield stress, σo, of the solder has been determined from the relation [18]:

𝜎o =

√

Gb𝜎 𝜋 (1 − v) L

(1)

where G is the shear elastic modulus of the solder, b is the Burgers vector, σ is the stress at the particle surface, ν is the Poisson’s ratio, and L is the average distance between the second particles. Due to the addition of ZnO nanoparticles, the average sizes of Cu6Sn5 and Cu10Sn3 IMCs in the β-Sn matrix becomes fine. Therefore, the spacing L between the IMCs is significantly reduced. According to the Eq. (1) it can be drawn that the yield stress of the composite solder, σo, is increased. The dislocations cannot pass through the hard ZnO particles, only bypass them when applied certain stress. For the composite solders with 0.1 and 0.25 ZnO, the refined microstructure of IMCs and the dispersion of ZnO nanoparticles play a significant role of strengthening, which reflects the decrease of the steady state creep rate values as the ZnO addition increased (see Fig. 4). Our experimental results are in good agreement with the previous work of other investigators [11, 12, 16] As the aging temperature is increased to 423 K, the creep resistance of the composite solders decreased

(Fig. 4). This behavior was ascribed to the increase in size and spacing of Cu6Sn5 and Cu10Sn3 IMCs (Fig. 6b). Consequently, the IMCs cannot pin the dislocations gliding and climbing over them, thereby reducing the creep resistance. Figure 4 demonstrates that έst values first decreased significantly and then increased slightly with the increasing of the ZnO nanoparticles content. In other words, there is an appropriate nano-sized ZnO particles content in order to enhance the creep resistance of the composite solders. The optimal ZnO content is 0.25 wt%. The excessive addition of nano-ZnO particles resulted in a decrease in the creep resistance. This behavior could be attributed to the agglomeration of nanoparticles in the solders which enables the possibility for nanoparticles to touch each other and agglomerate to form clusters. At a higher percentage of ZnO, the clusters of nanoparticles may be formed which increases the microcracks levels and creep rates. Similar explanations have also been offered previously in the literature [13, 19]. Figure 7a, b presents the microstructures of the SC07–1.0ZnO solder alloys after aging at 348 and 423 K respectively. The composite solder aged at 423 K has the majority of the crack paths, which cause the elevation of έst values than those obtained for the specimens aged at 348 K. From Fig. 4, it should be noted that the έst values for the five prepared solders rise with increasing the applied stress at any given aging temperature or ZnO content. As the applied stress is increased the dislocations could overcome the second phase precipitates ( Cu6Sn5 and C u10Sn3 IMCs) which act as obstacles; consequently, the steady state creep rate values are enhanced. The SEM observations explored earlier were assured by X-ray diffraction (XRD) results. The XRD patterns of SC07–xZnO composite solders presented in Fig. 8 reflects the existence of the tetragonal β-Sn phase according to (JCPDS card no.4-0673). Both the monoclinic C u 6Sn 5 and the hexagonal Cu10Sn3 phases have diffraction peaks appeared in the XRD patterns at the lower and higher

Fig. 7 SEM micrographs of SC07–1.0 ZnO composite solder aged for 2 h at a 348 K and b 423 K

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Fig. 8 Representative XRD patterns of the investigated composite solders aged at a 348 and b 423 K

aging temperatures (348 K and 423 K respectively). These two IMCs are embedded in the Sn-matrix and the presence of them is confirmed by (JCPDS card no. 45-1488) and (JCPDS card no. 65-3632) respectively. Our results agree with those reported by Shalaby [8] who detected Cu6Sn5 and C u10Sn3 IMCs in the eutectic Sn–0.7Cu solder. The relative intensities of the diffraction peaks for the two IMCs increase with increasing nano-ZnO ratio up to 0.25 wt% and then decreased upon further addition of the nano-oxide. Figure 8b indicates that the diffraction peaks of C u6Sn5 phase disappeared completely at a higher aging temperature of 423 K for SC07 solder. With the addition of ZnO nanoparticles, the diffraction peaks of Cu6Sn5 phase start to appear again. A small diffraction peak was established at 31.6° for the nano-ZnO particles in accordance with (JCPDS card no. 01-1136). Fouda and Eid [20] detected only two small peaks for the ZnO when they successfully prepared Sn–Sb–Cu composite solders reinforced with ZnO nanoparticles. The vanishing of the other nano-ZnO peaks could be rendered to the overlap of the Sn peaks with ZnO peaks. Further SEM observation was also performed to confirm that the ZnO nanoparticles

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were successfully incorporated into the composite solders. A typical SEM micrograph of the SC07–0.25ZnO sample with a higher resolution confirmed the uniform distribution of ZnO nanoparticles within the β-Sn matrix (see Fig. 9).

Fig. 9 Typical SEM micrograph showing the distribution of ZnO nanoparticles within the SC07–0.25ZnO solder aged at 423 K


For Sn-rich phase which has a tetragonal structure, the interplanar distance, d, between (101) and (211) planes can be related to the lattice parameters a and c as follow:

1 h2 + k 2 l2 = + 2 2 2 d a c

(2)

where h, k, and l are the Miller indices of the diffraction plane. Figure 10 shows the dependence of the lattice parameter, a, on the ZnO content at various Ta values. It is clearly seen the lattice parameter, a, decreased as the ZnO concentration increased up to 0.25 wt%, but above this level the trend is reversed. For example, at lower aging temperature of 348 K (Fig. 10a) the values of a decreased from the initial value 5.829–5.821 Å with increasing ZnO concentration from 0 to 0.25 wt% respectively, but they elevated to reach a value of 5.824 Å as the level of ZnO reached 1.0 wt%. On the other hand, the lattice parameter, a, increased with increasing the aging temperature from 348 to 423 K. The nano-ZnO content dependence of the ratio c/a is depicted in

Fig. 11. It can be seen that the dependence of the ratio c/a on the ZnO concentration is in contrary to the behavior of the lattice parameter, a. The decrease in the lattice parameter, a, and the corresponding increase in the ratio c/a of the Sn-rich phase with increasing the nano-ZnO percentage up to 0.25 wt% could be attributed to that the addition of ZnO nanoparticles to the Sn–Cu solder refines the grain sizes of Sn-rich phase during the solidification process. The abnormal increase in the lattice parameter, a, and the decrease in the ratio c/a with increasing ZnO content from 0.25 to 1.0 wt% is believed to be due to the coalescence and partial dissolution of Cu6Sn5 and C u10Sn3 IMCs in the β-Sn matrix. On the other hand, the increase in the lattice parameter, a, and the corresponding decrease in the ratio c/a with increasing the aging temperature for the five composite solders may be due to the dissolution of the second phases (Cu6Sn5 and Cu10Sn3) in the Sn-rich phase leading to the increase in homogeneity of the distribution of Cu atoms in

Fig. 10 The variation of the lattice parameter, a, for Sn-rich phase with nano-ZnO content for the investigated composite solders aged at a 348 K and b 423 K

Fig. 11 The variation of c/a ratio for Sn-rich phase with nano-ZnO content for the investigated composite solders aged at a 348 K and b 423 K

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the Sn-matrix. Abd El-Rehim and Zahran [3] detected the same trend for the lattice parameter, a, values with increasing the aging temperature.

3.3 Stress exponent and activation energy Inspection of the creep data demonstrates that the steady state creep rate, έst, is related to the applied stress, σ, by the following power law creep equation [21]:

𝜖́st = A𝜎 n exp(−Q∕RTa )

(3) where A is a structure-dependent constant, σ is the applied stress, n is the stress exponent, Q is the creep activation energy, R is the universal gas constant and Ta is the aging temperature in Kelvin. A logarithmic plot of the steady state creep rate, έst, versus the applied stress, σ, is presented in Fig. 12 for the five composite solders at different aging temperatures. It is clearly seen that the datum points for any given composite solder or temperature can be approximated by a straight line, whose slope is defined as the stress exponent, n. The mean values of stress exponent were found to


be 7. The stress exponents reported in the literature for Sn and Sn-rich solders scatter widely from 5 to 11 [22–24]. In the present work, the value of n ≈ 7 is consistent with published creep data obtained in previous studies of solders of the same nominal composition [16, 25, 26]. The activation energy of the steady state creep was calculated from the relation between ln έst and 1000/Ta (K−1) (Fig. 13) [27]. The activation energy yielded from the slopes of the straight lines in Fig. 13 has the mean value of 80.2 kJ/mol. The combination of the average stress exponent of 7 and activation energies of 80.2 kJ/mol suggests that the predominant creep mechanism of all the composite solders is the dislocationpipe diffusion in the tin matrix [23, 26].

4 Conclusions The addition of ZnO nanoparticles and the variation of the aging temperature have a significant effect on the properties of Sn–0.7Cu lead-free solder. Five composite solders were prepared by mechanically dispersing different contents

Fig. 12 Double-log plots of steady state creep rate versus applied stress at different aging temperatures

Fig. 13 Plots of ln έst versus 1000/Ta for the five composite solders. Applied stresses are indicated

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(0, 0.1, 0.25, 0.5 and 1.0 wt%) of ZnO into the eutectic Sn–0.7Cu solder. The following conclusions could be drawn from the present study: (i) The addition of ZnO nanoparticles increases slightly the melting point of the composite solders. (ii) The steady state creep rate values of the composite solders decreased with increasing ZnO concentration up to 0.25 wt%, above which the trend is reversed. (iii) The addition of ZnO nanoparticles was found to control the microstructure of Sn–0.7Cu solder. (iv) The values steady state creep rate increased with increasing the aging temperature and/or applied stress at any given content of ZnO. (v) The calculated values of stress exponent and activation energy indicated that the rate-controlling mechanism is dislocation-pipe diffusion in the tin matrix. Acknowledgements The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through General Research Project under Grant Number (G.R.P- 240-39).

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