Investigation of microstructure and mechanical properties of Sn-xCu solder alloys

Accepted Manuscript Investigation of microstructure and mechanical properties of Sn-xCu solder alloys A.F. Abd El-Rehim, H.Y. Zahran PII:

S0925-8388(16)33854-3

DOI:

10.1016/j.jallcom.2016.11.371

Reference:

JALCOM 39868

To appear in:

Journal of Alloys and Compounds

Received Date: 30 June 2016 Revised Date:

22 November 2016

Accepted Date: 26 November 2016

Please cite this article as: A.F. Abd El-Rehim, H.Y. Zahran, Investigation of microstructure and mechanical properties of Sn-xCu solder alloys, Journal of Alloys and Compounds (2016), doi: 10.1016/ j.jallcom.2016.11.371. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Investigation of microstructure and mechanical properties of Sn-xCu solder alloys A. F. Abd El-Rehim a,b,* , H. Y. Zahran a,b a

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Physics Department, Faculty of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia. b Physics Department, Faculty of Education, Ain Shams University, P.O. Box 5101, Heliopolis 11771, Roxy, Cairo, Egypt ∗ [email protected]

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Abstract The microstructure and mechanical properties of five binary pre-aged Sn-Cu alloys with varying Cu contents were investigated. The Cu compositions examined varied from 1 wt% to 5 wt%. Serial aging heat treatment processes were performed at 373, 393, 413 and 433 K with a holding time of 2 h at each temperature. The microstructural characteristics of the tested alloys have been investigated using scanning electron microscope (SEM) and X-rays diffraction (XRD). Typical indentation creep curves were derived from hardness values. The minimum creep rate is increased as the Cu content increased up to 4 wt%, but above this level the trend is reversed. Moreover, increasing aging temperature resulted in an increase in the minimum creep rate for the all tested alloys. The mean values of energy activating the creep process are in agreement with that quoted for dislocation core diffusion mechanism.

1. INTRODUCTION

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Key words Sn-Cu solder alloys; indentation creep properties; microstructure; activation energy

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Tin-Lead (Sn-Pb) solder alloys are widely used in microelectronics packaging applications due to their acceptable mechanical properties, good wettability and low melting point. Recently, health and environmental issues about Pb toxicity have led to ban the use of Pbcontaining solders in the electronics industry [1-4]. Now, many numbers of Pb-free solders such as Sn-Zn, Sn-Cu, Sn-Ag and Sn-Au have been developed. Out all the available alternatives, the Sn–Cu solder alloys are found to be a potential alternative due to their good wettability, high electrical conductivity, good mechanical properties and low-cost [5]. To date, there are limited data available on the microstructure and mechanical characteristics of Sn-Cu solder alloys [6-9]. The effect of solidification process on the rod eutectic microstructure of Sn-1.2 and 3.0 wt% Cu solder alloys has been investigated [10]. It was found that the inter-rod spacing has been influenced by solidification parameters. The microstructural changes induced by the cooling rate and Cu concentration of the solidified Sn-Cu solder alloys have been studied by differential thermal analysis (DTA) and theoretical calculations [11]. It has been reported that the existence of η-precipitates (Cu6Sn5 1

ACCEPTED MANUSCRIPT intermetallic compound) in the Sn–1wt% Cu alloy is associated with lower cooling rate condition. The eutectic point of binary Sn–Cu solder alloy shifts to the direction of higher Cu content at higher cooling rate condition. Corrosion [12] and tensile experiments [13] have been carried out with a Sn-2.8wt% Cu alloy. It was found that higher cooling rates during liquid/solid transformation allow the development of uniformly distributed Cu6Sn5 and Cu3Sn intermetallic compounds (IMCs).

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The indentation creep tests have been done on many soft metals and solder alloys such as pure tin [14], the eutectic Sn-37.8wt% Pb [15-17], Sn-3.5wt% Ag [18], Pb-9wt% Sn [19], Sn-5%Sb [20], Pb-Sb [21] and Sn–0.7wt% Cu-In [22]. The creep properties of the Sn-Cu solder alloys were investigated by conventional creep tests [7,23,24].

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Although Sn-Cu solder alloys were studied by many researchers there is a limited amount of data about the effect of heat treatment and Cu concentration on the microstructure and mechanical properties of Sn–Cu solder alloys. Thus, the aim of the current work is to shed more light on the effect of aging temperature and Cu-content on the microstructure development and mechanical characteristics of Sn–Cu solder alloys.

2. EXPERIMENTAL PROCEDURES

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Sn–xCu (x = 1, 2, 3, 4 and 5 wt%) solder alloys used in the present work have been prepared by melting Sn and Cu of 99.99% purity. The pre-weighted materials were melted in a clean mild steel crucible in an induction furnace under argon gas atmosphere at 573 K for 60 min. The molten alloys with different contents of Cu were poured into mild steel moulds. The ingots were heated at 453 K for 24 h for homogenization and then rolled to 1.2 mm-thick sheets followed by slow furnace cooling to room temperature. Chemical analysis of the cast Sn-xCu alloys revealed that the composition of alloys is very close to the alloys composition required. The sheet samples aged at different temperatures (T = 373, 393, 413 and 433 K) for 2 h and quenched into iced water at 273 K. The accuracy of the temperature measurements is of the order of ±1 K. The microhardness of samples was measured using the MH3 Vickers Microhardness indenter developed by Metkon with a load of 100 g for different dwell times ranging from 5 to 90 s. The pyramidal indenter made of diamond with an included angle of 136° has been used. The test machine forced the indenter into the surface of the material and the resulting indentation is viewed using a low magnification optical microscope. The two diagonal lengths of the indentation area left in the surface of the material are measured using the optical microscope and their average calculated. The Vickers hardness number is calculated from: (1) 2

ACCEPTED MANUSCRIPT where F is the applied load in N and d is the average diagonal length in mm. The average of ten separate readings taken randomly on the surface of the specimens has been calculated. The distance between any two successive indentations should be at least 3mm away from each other.

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For microstructure investigations, the samples were ground using SiC papers wet with water, down to grit size 1200, and polished with 3 µm and 0.25 µm diamond paste. Finally the samples were etched in a solution consisting of 2ml HCl, 3ml HNO3 and 95ml C2H5OH. The microstructure characterization was performed using a JEOL JSM-6360LV scanning electron microscope (SEM) coupled to an Energy Dispersive Spectroscope (EDS). A Shimadzu D6000 X-ray diffractometer using Cu-Kα was employed for the identification of the phases that were formed in the test alloys. 3. RESULTS AND DISCUSSION

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Figure 1 (a-d) shows typical indentation creep curves, for the five Sn-Cu solder alloys, evaluated from the obtained hardness values using the Atkin’s model [25] at different aging temperatures (T = 373, 393, 413 and 433 K). As the (1/H)1.5 is equated with the strain, the values of strain are presented in arbitrary units. It is important to observe that the shape of all the indentation creep curves of the present alloys is similar to conventional creep curves. The curves of each alloy exhibit two stages; a normal primary creep stage, followed by a steady state creep stage, with a constant creep rate (minimum creep rate). It is not possible to record a third stage of the creep curve because the hardness test is actually a compression test therefore necking and fracture of the specimen do not occur [26]. It can be seen that the creep strength of the tested alloys increases with increasing Cu content. Among the alloys investigated, the Sn-4 wt% Cu alloy exhibits the best creep resistance. As can be inferred from Fig. 1, the lowest minimum creep rate belongs to the Sn-4 wt% Cu alloy and the highest to the Sn-1wt% Cu alloy, with the remaining alloys lying between. Figure 2 demonstrates the variation of minimum creep rate at all aging temperatures investigated as a function of Cu content. According to Fig. 2, as the Cu content increased up to 4wt%, the minimum creep rate decreased, but above this percentage it increased. Moreover, it can also be inferred from Fig. 2, at different Cu contents, the greater the aging temperature, the higher minimum creep rate. The correlation between the mechanical properties and microstructural features is very important as the mechanical properties such as hardness and creep strength are dependent on the microstructure evolution, which is in turn affected by alloy compositions and heat treatment conditions. For soldering application purposes, phase equilibria at the Sn-rich corner are more important. The Sn-rich corner of the Sn-Cu phase diagram is illustrated in Fig. 3 [13]. This diagram shows that there are 3 intermetallic compounds (IMCs), η-Cu6Sn5, 3

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ε-Cu3Sn, and η՝ -Cu6Sn5. The eutectic Sn-0.7 wt% Cu alloy having a melting temperature of 227 °C. The melting point of the five Sn-Cu alloys ranging from 227 to 260 °C [27,28]. In the temperature range 385 to 500 K, the tracer diffusivity of Cu in Sn has been investigated [29]. It was found that the diffusivity of Cu in Sn was much faster than vice versa. Furthermore, Cu diffusion by interstitial diffusion was more likely to be the dominant mechanism at the lower temperatures (below 423 K). In Sn-Cu solder alloys, after initial formation of the ηCu6Sn5 phase from the reaction between liquid Sn and Cu, thermal aging induces Sn and Cu interdiffusion to form additional η-phase at the solder/η interface, leading to η-phase growth (see Fig. 4). At higher temperatures, Sn could diffuse through the η-phase to react with Cu and form the ε-phase. Therefore, Sn diffusion by vacancy-diffusion mechanisms was predominant. According to the Sn-Cu binary phase diagram, both intermetallic phases exist in the temperature range from room temperature to 500 K. However, experiments have found that Cu6Sn5 (η-phase) precipitates first followed by the precipitation of Cu3Sn (ε-phase) after a certain period of incubation. During the initial stages of isothermal heat treatment, both the Cu6Sn5 and Cu3Sn IMCs grew, even though the former is the dominant one. After the consumption of all available Sn, the Cu3Sn IMC grew reactively at the expense of Cu and Cu6Sn5 at the Cu/η-phase interface [30]. Consequently, the higher Cu content, the higher is the Cu3Sn IMC formed [31]. Shen et al. [11] reported that the hypereutectic Sn-1.0 wt% Cu alloy developed both Cu6Sn5 and Cu3Sn IMCs. Tu [32] studied the diffusion of Cu in Sn-Cu soldering system and observed the formation of ε-phase precipitates upon annealing above 333 K. It has been reported [33] that the onset temperature for ε-phase formation in Sn-Cu system at 448 K from the reaction between η-phase and Cu. Tu and Thompson [34] investigated the formation and growth of ε-phase in Sn-Cu system at temperatures ranging from 388 K to 453 K. They concluded that the ε-phase grows at the expense of η-phase at a parabolic rate until the η-phase is fully consumed.

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Fig. 4a and b reveals representative SEM images of the Sn–1 w% Cu alloy aged for 2 h at 393 and 433 K respectively. The microstructure is sufficiently homogeneous and consisting of Sn-rich phase and two kinds of intermetallic compounds (IMCs) - Cu6Sn5 (η-phase, as a dark gray phase) and Cu3Sn (ε-phase, as a white phase). The chemical composition of each phase was determined by EDS analysis and given in Fig. 4. The η-phase develops following typical H-shaped or M-shaped morphologies while ε-phase is characterized by spheroids morphologies [35,36]. Similar microstructure has been reported by Spinelli and Garcia on Sn2.8wt% Cu alloy [13]. The experimental results depicted in Fig.2 show that the minimum creep rate values decreased as the Cu content increased up to 4 wt% in Sn-matrix, i.e. the Sn–4wt% Cu samples exhibited the lowest minimum creep rate values. This behavior could be rendered as being due to the presence of denser, harder and stiffer η-phase and ε-phase IMCs (see Fig. 4 and 5) which act as potential sites for obstructing localized plastic deformation of 4

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the Sn-matrix during localized indentation (see Fig. 4 and 5). These results agree well with the data previously reported for tin based solders in the literature [22,37,38]. When the Cu content exceeds 4 wt%, the hardness of Sn-Cu alloys declines more obviously and the minimum creep rate values are drastically increased. This could be attributed to the dissolution of Cu6Sn5 precipitates and growth and/or coarsening of intermetallic compound Cu3Sn (see Fig.6) [30]. Our results are in a fair agreement with the view of Yang et al. [39] who concluded that the hardness of Cu6Sn5 precipitates is greater than that of Cu3Sn precipitates. The enhancement in minimum creep rate values with increasing aging temperature from 373 to 433 K (see Fig. 2) for all alloys investigated could be referred to the coalescence and dissolution of Cu6Sn5 and Cu3Sn precipitates (see Figs 5 and 6). Such coalescence results in an increase in precipitates size accompanying with a decrease in their numbers. Hence, the number of barriers for dislocation motion will widely be reduced and the recovery of dislocations occurs by a higher rate, resulting in higher creep rates [37].

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The SEM results discussed above have been confirmed by XRD results. XRD analysis of the investigated alloys showing the existence of three phases, namely: Sn-rich phase, η-phase (Cu6Sn5) and ε-phase (Cu3Sn). These results were confirmed by coinciding the obtained XRD data with the literature values of (JCPDS 65-0296) for Sn-rich phase, (JCPDS 65-2303) for Cu6Sn5 (η-phase) and (JCPDS 01-1240) for Cu3Sn (ε-phase). The XRD results are in agreement with those reported in literature [11,12]. The relative intensities for the two IMCs are tabulated in Table 1. From Table 1, it is clear that the relative intensities of the diffraction peaks of η-phase decreased with increasing aging temperature for Sn-3wt% Cu alloy, while ε-phase showed the opposite trend. This can be explained by the fact that at higher aging temperatures, Sn can diffusion by vacancy-diffusion mechanism through the η-precipitates to react with Cu and form the ε- precipitates. In the case of Sn-5wt% Cu alloy, the diffraction peaks of η-phase disappeared completely at higher aging temperature (413 and 433 K). That could explain the present experimental data, i.e. the minimum creep rate gradually decreased with increasing Cu concentration, reaching its minimum value at 4 wt% Cu and then increased upon further addition of Cu. Typical X-ray diffraction pattern of the five Sn-Cu samples aged at 373 and 433 K is shown in Fig. 7a and b respectively. It should be pointed out that the diffraction lines of the η-Cu6Sn5 that appeared at low aging temperature approximately disappeared at higher aging temperatures (413 and 433 K) for Sn-5wt% Cu alloy. Moreover, the relative intensity of the diffraction peaks of η-phase increased with increasing Cu content up to 4wt%, but the trend reversed above this level (for samples aged at 373 K). These intriguing results can be explained by the fact that the ε-phase had a significantly higher growth rate, presumably at the expense of the η-phase, at the higher temperature (433 K), while at the lower temperature (373 K), the rate of growth of ε-phase was significantly lower because of the lower driving force for ε- phase formation [40].

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Figure 8 shows the variation of full width at half maximum intensity, ∆2θ for (211) plane, and lattice parameter, a, of Sn-rich phase with aging temperature. It is obvious from Fig. 8 that both ∆2θ and a increased with increasing aging temperature. During thermal aging, the Sn-rich phase lattice parameter, a, increases from values around 5.652 Å to about 5.850 Å. This increase is caused by the precipitation of Cu atoms from the Sn-rich phase to form Cu6Sn5 and Cu3Sn IMCs. This is largely due to thermal stresses in the matrix caused by misfitting inclusions (IMCs). When the Cu content increases to 5 wt%, the Sn-rich phase lattice parameter, a, decreased dramatically between 1w% and 2 wt% Cu (see Fig. 8). It seems that, in more concentrated Cu-containing alloys (Sn-5wt% Cu), the high Cu content enhances the formation of ε-phase. The increasing in aging temperature leading to the coarsening and dissolution of ε-phase (see Fig. 6). This causes the relief of internal strains and stresses during the dissolution of ε-phase in the Sn-rich phase. The defects associating the dissolution of the ε-phase and the inhomogeneity of structure raise the value of the lattice parameter, a, as shown in Fig. 8.

σ0 (nCtε&0 )1 / n

(2)

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H (t ) =

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Figure 9 (a-d) shows the hardness values versus dwell time, both on logarithmic scales, for the five Sn-Cu solder alloys at different aging temperatures ranging from 373 to 433 K. It was found that the hardness values decreased as the dwell time increased as a linear function. The stress exponent, n, can be determined using Sargent and Ashby approach [41] in which the variation of hardness with dwell time can be expressed as:

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where n is the stress exponent, t is a dwell time, εo is the strain rate at a reference stress σo, and C is a constant. From Eq. (2), the slopes of the straight lines relating ln H and ln t at different temperatures equal to − 1/n. The values of n calculated in this manner are plotted against Cu content (see Fig. 10). They ranged from 7 to 12. These results are consistent with those reported by Wu and Huang [23]. According to the power-law relation for the creep, a decrease in stress exponent values with increasing aging temperature would result in an increase in creep rate due to a decrease in yield strength [42,43]. Therefore, Sn-4wt% Cu solder alloy with higher n values is more resistant to indentation creep compared with the other four Sn-Cu solder alloys. In order to determine the energy activating creep process, variations of minimum creep rate in terms of 1000/T are drawn under different Cu contents as shown in Fig. 11. The slopes of straight lines representing the creep activation energy. The calculated values of energy activating creep process ranged from 59.5 to 64.5 kJ/mol for the five Sn-Cu solder alloys. These values of activation energy are consistent with that reported in the literature on Snbased alloys where the energy activating creep process ranged from 55 to 77 kJ/mol [44,45]. 6

ACCEPTED MANUSCRIPT Moreover, they compare well with that quoted for dislocation core diffusion in pure Sn [46,47]. Therefore, for the entire range of aging temperature, dislocation core diffusion seems to be the dominant diffusion mechanism during indentation creep tests in the Sn-Cu alloys.

4. CONCLUSIONS

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The effect of Cu content and aging temperature on the microstructure and indentation creep properties of five Sn-Cu solder alloys has been studied. The following conclusions could be drawn from the present study: i. The Sn-4wt% Cu solder alloy exhibited the best creep resistance.

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ii. With increasing aging temperature, the stress exponent varies between 7 and 12.

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iii. The calculated values of activation energy for the creep process of the tested alloys ranged from 59.5 to 64.5 kJ/mol. These values indicated that the rate-controlling mechanism is dislocation core diffusion mechanism.

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1: Typical indentation creep curves for the five Sn-Cu solder alloys aged at different temperatures.

Fig. 3: The Sn-rich corner of the Sn-Cu phase diagram.

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Fig. 2: Variation of minimum creep rate with Cu content at different aging temperatures.

Fig. 4: SEM micrographs of Sn-1 wt% Cu alloy aged at a) 393 K, and b) 433 K showing the coarsening of η-phase at higher aging temperature.

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Fig. 5: SEM micrographs of Sn-3 wt% Cu alloy aged at a) 393 K, and b) 433 K.

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Fig. 6: SEM micrographs of Sn-5 wt% Cu alloy aged at a) 393 K, and b) 433 K showing the dissolution of η-phase and coarsening of ε-phase at higher aging temperature (433 K). Fig. 7: Representative XRD patterns of the tested alloys aged at a) 373, and b) 433 K. Fig. 8: The aging temperature dependence of a) X-ray full width at half maximum intensity, ∆2θ, and b) lattice parameter, a, for Sn-rich phase.

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Fig. 9: Hardness-dwell time ln-ln plots for the five Sn-Cu solder alloys aged at different temperatures. Fig. 10: Variation of stress exponent, n, with Cu content at different aging temperatures.

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Fig. 11: Plots of minimum creep rate versus 1000/T for the five Sn-Cu solder alloys.

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Sn-5 wt% Cu

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ε-phase

η-phase

ε-phase

74 59 37 34

13 15 17 20

28 13 0 0

15 13 8 6

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373 393 413 433

Sn-3 wt% Cu

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Aging temperature (K)

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Relative intensity

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ACCEPTED MANUSCRIPT • The effect of Cu content on the microstructure and mechanical properties of Sn-Cu alloys has been studied. • The results showed that the Sn-4wt% Cu solder alloy exhibited the best creep resistance.

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• The values of activation energy were found to be consistent with dislocation core diffusion mechanism.