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Journal of Alloys and Compounds 647 (2015) 844e856
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Formation and growth of intermetallic phases at the interface in the Cu/ SneZneAgeCu /Cu joints Tomasz Gancarz*, Janusz Pstrus Institute of Metallurgy and Materials Science, Polish Academy of Sciences, Krakow, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 April 2015 Received in revised form 8 June 2015 Accepted 15 June 2015 Available online 20 June 2015
This work presents the effect of soldering time and temperature on the microstructure and electrical properties of Cu/solder/Cu joints, where solder are Sn14.7Zn0.5Ag1.0Cu and Sn14.7Zn1.0Ag1.0Cu0.1Al (at.%) alloys. Soldering of Cu/solder/Cu joints was performed for 3, 8, 15, 30 and 60 min at 250 C, and at 230 and 280 C for 8 min, with the use of flux. Aging of Cu/solder/Cu joints was carried out at 170 C for 1, 10 and 30 days. After soldering, it was found that Cu5Zn8 and AgZn3 are formed at the solder/Cu interface. The thickness of Cu5Zn8 increases with time, while layer of AgZn3 grows up to 15 min and later diminishes. Precipitates of AgZn3 are found in the central part of joints, with their number increasing with soldering temperature. Higher content of Ag and addition of Al hinder growth of Cu5Zn8. During aging Cu6Sn5 starts to appear at the interface on the expense of Cu5Zn8. The electrical resistivity of joints decreases with time and temperature of soldering but increases after aging. © 2015 Elsevier B.V. All rights reserved.
Keywords: SneZneAgeCu Kinetics of growth of IMPs Activation energy Electrical resistivity Lead-free solders
1. Introduction The SneZn eutectic based alloy with the addition of Ag and Cu could be used as a replacement for Pb-bearing solder alloys [1]. The SneZn eutectic alloy shows better mechanical properties, tensile stress and strain compared to Sn37Pb (wt %) [2]. The wettability of SneZnexAl alloys was studied by Zhang et al. [3], and showed that the addition of Al improved the atmosphere and temperature during soldering. Lin and Shih [4] found that the wetting time decreases with increasing temperature, while it increases with Ag content. Saad et al. [5] studied the effect of Ag addition to SneZn on deformation temperature, and the effect of applied stress on the creep characteristics. They found that the addition of 1.5 wt.% Ag to the binary alloy increased its creep resistance. This behaviour was attributed to the formation of the IMPs AgeZn and AgeSn. These intermetallic phases (IMPs) stop the dislocations, and consequently lead to increased creep resistance for the alloy. Hu et al. [6] found that, if the concentration of Ag is lower than 0.3 wt.%, the microstructure of SneZn alloy is not altered and Zn is present in the form of needle-like precipitates. Only Ag
* Corresponding author. E-mail address: [email protected] (T. Gancarz). http://dx.doi.org/10.1016/j.jallcom.2015.06.122 0925-8388/© 2015 Elsevier B.V. All rights reserved.
concentration of between 0.5 and 1.0 wt.% is sufficient for Ag and Zn to react and form AgeZn IMPs visible in the changed microstructure. Chen et al. [7] studied the microstructure of the interface between Sn9Zn1Ag (wt.%) solder and Cu substrate, and found that, after soldering, there is only 0.3 wt.% of Ag left in the solder, and IMPs from CueZn and AgeZn systems are located at the interface, which causes its mechanical properties to deteriorate. The addition of Ag is expected, as shown in Gancarz et al. [8], to limit growth of IMPs. Another approach suggested by Lin and Hsu [9] is a solder that itself acts as a barrier after reflow, thus eliminating the need for a barrier layer. The addition of Al to SneZn solder in contact with the Cu substrate exhibits an ‘‘inherent barrier” [9]. The Al will form a thin layer of Al4.2Cu3.2Zn0.7 compound (Al52Cu39Zn9 at.%) at the liquid/solid interface. This compound, which results from the gathering of Al at the interface, provides a barrier to Sn diffusion towards the Cu substrate, which causes no formation of CueSn compounds [9]. The phase diagram of the SneZneCu system was investigated by Chou et al. [10], and no ternary compound was found in this system. In the wetting studies of SneZn alloy on Cu substrate at the interface, the three IMPs (CuZn [11], Cu5Zn8 and CuZn4 [12]) were found. Chou et al. [10] noted that the addition of Cu to eutectic SnZn alloy has an effect on wetting reactions with Cu substrate, and inhibits the growth of an IMPs layer at the interface. This is due to the
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formation of precipitates from the CueZn system formed in the solder and lack of free Zn to create IMPs at the interface. The same chemical composition was studied by Yu et al. [13], who observed that the addition of Cu to eutectic SnZn, causes lower a wetting angle on the Cu substrate. Both authors [10,13] observed that, with increasing Cu content in the alloy, the formation of Cu6Sn5
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precipitates in the solder. They also observed the formation of a Cu6Sn5 layer on the Cu substrate with increasing wetting time. Huang and Lin [14] investigated the effect of the addition of Al and Ag to eutectic SnZn on the formation of IMPs in soldering of Cu substrate. First, they observed two IMPs: CuZn4 and Cu5Zn8 after soldering eutectic SnZn on the Cu substrate. The addition of Al
Fig. 1. Soldering by SnZn0.5Ag1.0Cu at 250 C for different times: a) 3 min; b) 10 min; c) 15 min; d) 30 min; e) 60 min.
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caused the formation of IMPs from AleCueZn system at the interface. Further addition of Ag resulted in precipitates of AgZn3 being created at the interface. The authors also remarked on the spherical precipitates of AgZn3 occurring in bulk regardless of the cooling rate, but found that, for air-cooling, the number of precipitates was higher. The aim of this work is analysis of the effect of Ag, Al and Cu
content in SneZn solder on IMPs growth and kinetics on Cu substrates versus time and temperature. 2. Experimental For preparation of the alloys, SnZn0.5Ag1.0Cu and SnZn1.0Ag1.0Cu0.1Al (at. %), high purity metals Sn, Zn, Cu, Al
Fig. 2. Soldering by SnZn1.0Ag1.0Cu0.1Al at 250 C for different times: a) 3 min; b) 10 min; c) 15 min; d) 30 min; e) 60 min.
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(99.999%) and Ag (99.99%), were used. The SneZneAgeCu alloys were prepared in a glove box filled with high purity Ar (99.9999%), with levels of O2 and H2O kept below 1 ppm, and lowered N2 content, as presented in Refs. [8,15]. Carefully weighed amounts of metals were melted in graphite crucibles in a resistance furnace and then cast into graphite moulds. For soldering tests, alloys were cold-rolled into 1 mm thick sheets, which were later cut into 10 10 1 mm plates. Soldering tests were performed on Cu (99.9%) substrates, 25 mm long, 10 mm wide, and 4 mm thick [15]. The solder plates were put between the two substrates. Soldering connections of the type Cu/solder/Cu were prepared at 250 C for 3, 8, 15, 30 and 60 min, and at 230 and 280 C for 8 min, respectively. Alu33® flux was used for preparation [15] of all of the types of connections. Characterisation of the joints was performed using scanning electron microscopy (SEM), coupled with energy dispersive spectrometry (EDS), and X-ray diffraction (XRD). For all samples, three measurements were made at different areas to improve counting statistics and to check the homogeneity of the joints. First, an XRD measurement was performed in the centre of the joint and on opposite edges. The stress investigation was carried out with the basic assumptions known from the traditional sin2j method, with the use of the D8 Discover Bruker diffractometer equipped with a parallel-beam primary optics PolyCap system completed a pinhole collimator (1.0 mm aperture) for CoKa radiation (l ¼ 1.790300 Å) and a linear position-sensitive detector LynxEye (range 2.6 on 2q), capable of working in a parallel secondary beam configuration with the use of Soller collimators. The accelerating voltage and the applied current were 40 kV and 35 mA. The SEM observation, EDS and XRDs analysis were used to determine the kinetics of growth and the characteristic of IMPs layers occurring at the interface. Aging of soldered Cu/solder/Cu joints was carried out at the temperature of 170 C for 1, 10 and 30 days. Metallographic crosssections of the aged samples were prepared for the microstructure characterisation and thickness measurement of intermetallic compounds (IMPs), using a scanning electron microscope. All of the joints were subjected to electrical resistivity measurements [15]. Their aim was to assess the quality of joints, and to correlate this with preparation and aging conditions. 3. Results and discussion
the bulk solder. Because the coefficient of diffusion of Cu, which is the driving force of the formation of Cu5Zn8, is four times greater than that of Zn [12], the Cu5Zn8 grows at the expense of AgZn3, as the AgZn3 has less negative Gibbs free energy of formation than does the Cu5Zn8 [8]. Similar results were obtained by Hang et al. [14], who reported a thick layer of IMPs from the AleZneCu system and, directly after the g layer, the growth of precipitates of AgZn3. On the initial stages of soldering (at 3e15 min), the observed microstructures of joints consist of large AgZn3 and Cu5Zn8 precipitates surrounded by Sn. After 30 min of soldering, the microstructures change to eutectic SnZn with small precipitates of AgZn3. The XRD analysis in Fig. 4 a, b, c, d, shows that, after 3 min (Fig. 4 a, b), AgZn3 and Cu5Zn8 occur in both cases, but for SnZn1.0Ag1.0Cu0.1Al solder (Fig. 4b), it is observed that the Al dissolved in the Cu5Zn8 phase. After 60 min (Fig. 4 c, d), Cu6Sn5 precipitates are formed at the interface, and in the corresponding SEM microstructure of the soldered joints small precipitates of AgZn3 and a layer of Cu5Zn8 at the interface of the solder/substrate are observed. In the case of Cu/SnZn1.0Ag1.0Cu0.1Al/Cu joints (Fig. 4d), increased concentration of Ag to 1.0%, and a small addition of Al 0.1 (at.%), caused the creation of larger AgZn3 precipitates without Cu5Zn8 precipitates inside of solder matrix at the beginning of the soldering process. It also caused a smaller increase of Cu5Zn8 andAgZn3 IMPs layers, as presented in Figs. 5 and 6. Therefore, a small addition of Al to the SnZn solder is a valid technique to slow down the growth rates of IMPs between the solder and Cu substrate. The same conclusions were reached by Ref. [17], who have studied the effect of Al addition to SAC alloys. 3.2. Effect of temperature on the microstructure of soldered joints The microstructures of Cu/solder/Cu joints, after joining for 10 min at different temperatures, are presented in Figs. 5 and 6: 230 C (a); 280 C (b); 320 C (c) for solders SnZn0.5Ag1.0Cu (Fig. 5) and SnZn1.0Ag1.0Cu0.1Al (Fig. 6). The microstructures at 230 C show large precipitates of AgZn3 and Cu5Zn8 in the case of the SnZn1.0Ag1.0Cu0.1Al alloy, and AgZn3 precipitates in the eutectic SnZn matrix in the case of the SnZn0.5Ag1.0Cu alloy. At 250 C, the microstructure of joints is similar for both alloys, but for SnZn0.5Ag1.0Cu there are a greater number of small AgZn3
3.1. Effect of time on the microstructure of soldered joints The microstructure of cross-sectioned joints Cu/ SnZn0.5Ag1.0Cu/Cu (Fig. 1 a, b, c, d, e) and Cu/SnZn1.0Ag1.0Cu0.1Al/Cu (Fig. 2 a, b, c, d, e) soldered at 250 C for 3 min (a), 10 min (b), 15 min (c), 30 min (d) and 60 min (e), are shown in Figs. 1 and 2, respectively. At the solder/Cu substrate interface, IMPs AgZn3 and Cu5Zn8 are formed, which was confirmed with EDS analysis (linescane in the figures) and XRD analyses. Fig. 3 illustrates the thickness of AgZn3 and Cu5Zn8IMPs vs. square rooted time. In this case, the process of IMPs growth is a diffusioncontrolled reaction, as reported in Refs. [8,16], whereas in the case of Cu joints soldered by ZnAl with Ag and Cu, n was much higher ~0.85, and diffusion had a mixed character for the Cu5Zn8 phase [15]. This is due to a higher concentration of Zn in ZnAl compared to SnZn, where force was applied in the solder of joints [15], but it also was observed for higher addition Ag (1.0; 1.5%) to ZnAl [8], where the sessile drop method was used. The layers of Cu5Zn8 grow with time, and the thickness of AgZn3 increases with up to 15 min of soldering. However, after 30 min its thickness is significantly reduced, and after 60 min of soldering the AgZn3 phase is not present in the soldered joint. The cause of the observed phenomena is the diffusion of Cu from the substrate into
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Fig. 3. Thickness of IMPs on Cu substrate for different times at 250 C.
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Fig. 4. XRD analysis of soldering joints at 250 C for different times: a, b) 3 min; c, d) 60 min for SnZn0.5Ag1.0Cu and SnZn1.0Ag1.0Cu0.1Al, respectively.
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Fig. 4. (continued).
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precipitates. Moving on to higher temperatures, 280 C and 320 C, the microstructure of joints differs in that, for the SnZn0.5Ag1.0Cu alloy, eutectic SneZn with big AgZn3 and Cu5Zn8 precipitates is observed, while for the SnZn1.0Ag1.0Cu0.1Al alloy, eutectic SneZn with small AgZn3 precipitates is observed. The kinetics of the growth of the IMPs layer at the interface between the solder and Cu substrate is shown in Fig. 7. The calculated constants (the diffusion coefficient for different temperatures (k) and phases (k0), together with the activation energy for the growth of both the AgZn3 and the Cu5Zn8IMPs) are presented in Table 1. The general form of the relation between the diffusion coefficient and thickness of the IMC layer can be expressed by Eq. (1) [18e21].
x¼
pffiffiffiffiffi kt
(1)
where x is the thickness of the IMC layer (mm), k denotes the diffusion coefficient and t is the heating time (s). The obtained values of the coefficient (k0) are of the same magnitude as in the literature data for SneZneAl/Cu joints [19]. The obtained constant coefficient (k0) for Cu5Zn8 was 1.06E-04 (m2/s) for the SnZn0.5Ag1.0Cu alloy and 6.72E-05 (m2/s) for the SnZn1.0Ag1.0Cu0.1Al, respectively. A similar value of (k0) 7.4E-05
(m2/s) was obtained for Cu5Zn8 by Liu [18], in the soldered SneAgeZn/Cu joints. The increase of (k) values with increasing temperature is also observed as in the literature [19,22], and the values are of the same magnitude for Cu5Zn8. The obtained value of activation energy for growth is lower than that determined by Wang et al. [19], which may be caused by the use of flux in the present study. During soldering for 10 min at temperatures from 230 to 320 C, the growth of the Cu5Zn8 IMC layer with temperature was observed. The AgZn3 layer shows a different character, and with increasing temperature is dissolved, which is the reason why the obtained activation energy for growth is negative. The activation energy of growth of Cu5Zn8 is almost identical, i.e. 12.7 and 12.6 (kJ/ mol), respectively, for the Cu/SnZn0.5Ag1.0Cu/Cu and Cu/ SnZn1.0Ag1.0Cu0.1Al/Cu joints. But for the Cu/SnZn1.0Ag1.0Cu0.1Al/Cu joints, faster dissolution of AgZn3 layer was observed. The activation energy of growth for Cu5Zn8 (53.77 kJ/mol) obtained by Wang et al. [19] for the 91Sne8.55Sne0.45Al alloy is much higher than our results. There are two possible reasons of this discrepancy. One is addition of Cu to alloys, which facilitated the formation of the Cu5Zn8 phase [23] in our case, and the other is the different process of soldering used in Ref. [19], where pads were hot-dipped in solder and after that subjected to aging.
Fig. 5. Soldering by SnZn0.5Ag1.0Cu for 10 min at different temperatures: a) 230 C; b) 280 C; c) 320 C.
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Fig. 6. Soldering by SnZn1.0Ag1.0Cu0.1Al for 10 min at different temperatures: a) 230 C; b) 280 C; c) 320 C.
3.3. Effect of aging on the microstructure of soldered joints
Fig. 7. Kinetics of growth of IMPs vs. temperature for SnZn0.5Ag1.0Cu and SnZn1.0Ag1.0Cu0.1Al, respectively.
After 3 min of soldering at 250 C, selected joints were subjected to aging at 170 C at different times (24, 144 and 744 h). The microstructure of cross sections of aged soldered joints, and XRD analysis, are shown in Figs. 8e10, for different aging times. The Cu/ SnZn0.5Ag1.0Cu/Cu and Cu/SnZn1.0Ag1.0Cu0.1Al/Cu joints, after 24 h of aging at 170 C, are presented in Fig. 8 a and b, respectively, and there are differences in the process of the growth of IMPs. For the connection Cu/SnZn0.5Ag1.0Cu/Cu, dissolution of Cu5Zn8 and AgZn3 layers is followed by growth of Cu6Sn5, by dividing the IMC layer into small pieces. For Cu/SnZn1.0Ag1.0Cu0.1Al/Cu, growth of Cu6Sn5 is not observed, only minimal thinning of the existing IMPs layers. The microstructures of solders in the joints are different. In the case of Cu/SnZn0.5Ag1.0Cu/Cu it is an Sn-matrix with small, uniformly distributed precipitates of AgZn3, but in the case of Cu/ SnZn1.0Ag1.0Cu0.1Al/Cu it is an eutectic SnZn matrix with single precipitates of AgZn3. In both cases there are the Cu5Zn8 layers at the interface, but for the Cu/SnZn0.5Ag1.0Cu/Cu joint, layers look discontinuous and dissolving. Descriptions of the phases based on EDS analysis are in agreement with XRD analysis presented on Fig. 8 c and d, respectively. Fig. 9, a and b show the microstructure of cross sections after aging for 144 h, for Cu/SnZn0.5Ag1.0Cu/Cu and Cu/ SnZn1.0Ag1.0Cu0.1Al/Cu, respectively. For Cu/SnZn0.5Ag1.0Cu/Cu,
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the higher propagation of Sn into the layers is observed, as is the formation of Cu6Sn5 precipitates from the side of the Cu substrate. The thickness of IMPs “layers” Cu5Zn8, AgZn3 is greater than after 24 h of aging, and a greater number of precipitates are visible in the solder matrix in the joint. For the Cu/SnZn1.0Ag1.0Cu0.1Al/Cu joint, Cu5Zn8, AgZn3 IMPs layers are nearly invisible, while numerous Cu5Zn8, AgZn3 precipitates are present in the solder in the joint. The XRD analysis of joints after 144 h of aging, shown in Fig. 9 c and d, confirmed the occurrence of the Cu5Zn8, AgZn3 IMPs. Further aging of Cu/SnZn0.5Ag1.0Cu/Cu and Cu/ SnZn1.0Ag1.0Cu0.1Al/Cu joints, for 744 h at the same temperature (170 C), is presented in Fig. 10, with the microstructure of the cross section of Cu/SnZn0.5Ag1.0Cu/Cu in Fig. 10a, and XRD analysis in Fig. 10c. The microstructure and XRD analysis for Cu/ SnZn1.0Ag1.0Cu0.1Al/Cu are presented in Fig. 10b and d, respectively. For Cu/SnZn0.5Ag1.0Cu/Cu, it can be observed that the continued diffusion of Sn to the interface, and also to the Cu substrate, results in the formation of new area of IMPs Cu6Sn5 and Cu3Sn, similarly as reported by Wang et al. [12]. As can be seen, in the microstructure of the solder in the joint, precipitates of Cu6Sn5 start to appear. Samples after soldering were removed from the furnace and put onto metal plates for cooling to room temperature, which is similar to the process in the work of Wang et al. [12], who studied how cooling rates affect the microstructure after casting. Similar conditions give similar effects, i.e. dissolution of the Cu5Zn8 layer and formation of Cu6Sn5 and Cu3Sn precipitates at the interface and in the Cu substrate. For the sample that was cooled in the furnace (slowly), after aging for 696 h [12], the Cu5Zn8 layer exists at the interface. For the Cu/SnZn1.0Ag1.0Cu0.1Al/Cu joint, no Cu3Sn phase is observed. The formation of two individual layers in the soldered interface (Fig. 2) could be interpreted as follows: since the Gibbs free energy (DG) for Cu5Zn8 (31.44 kJ/mol) [8,24] is much lower than that (27.94 kJ/mol) of the AgZn3 [8] at 200 C, the Cu5Zn8 IMPs layer, which is in a metastable state at high temperature [25,26], forms first during soldering [18]. Also, the diffusivity of Sn in CueSn alloys is given by kSn ¼ 1.90 1010 cm2/s at 300 C, and that of Zn in CueZn alloys is kZn ¼ 2.70 1010 cm2/s at 300 C [27]. This explains the formation of g-Cu5Zn8 instead of CueSn compounds. But, during aging, diffusion of Zn is hindered because of the distance between Zn precipitates in the joint, and total amount of Zn in the joint is limited. During aging at 170 C, diffusion of Cu from the substrate to the joint continues, and since the system tends to reach the minimum energy, formation of Cu6Sn5 is possible. Cu6Sn5 precipitates form between the Cu5Zn8 layer and the Cu substrate, and with time they join and form a new IMC layer. 3.4. Electrical resistivity of soldered joints Fig. 11 shows the electrical resistivity of the soldered joints Cu/ SnZn0.5Ag1.0Cu/Cu. From the obtained results for Cu/ SnZn0.5Ag1.0Cu/Cu joints, it is clear that the effect on electrical resistivity of time of joining at 250 C, as well as temperature of
joining for 10 min, is negligible. However, the results after aging, except for an aging time of 24 h, show an increase in electrical resistivity, up to 80% after 744 h of aging. As can be observed in Fig. 11, the electrical resistivity of joints decreases with the increase of time and temperature, which is caused by a greater amount of Cu in the IMPs layers in the joint. The observed increase of the electrical resistivity of joints after aging is not desirable. The formation of IMPs precipitates inside the solder, caused by increases in the number of borders, (observed on Fig. 10 a and b), has a direct effect on the increase of electrical resistivity.
4. Conclusion During soldering at 250 C, the CuZn4 and AgZn3 phases grow at first, and later, after 15 and 30 min, the size of the phase AgZn3 dramatically decreases and the size of CuZn4 decreases minimally. However, after 60 min we can see the complete disappearance of the AgZn3 phase, and more than three times the growth of the Cu5Zn8 phase. Similarly, the growth of phases can be observed for soldering at 10 min at a variable temperature. When describing the kinetics of the IMPs layers appearing at the interface of the solder/ Cu pad, one must also take into account the precipitates inside solder CuZn4, Cu5Zn8 and AgZn3 which may have an influence on the formation of layers. The observed effect for SnZn1.0Ag1.0Cu0.1Al solder was that the higher addition of Ag, Cu and Al causes the reduction in growth of Cu5Zn8, is shown in Fig. 2 d and e. In the soldering process in the investigation solders SnZn0.5Ag1.0Cu and SnZn1.0Ag1.0Cu0.1Al, similar microstructures were obtained, resulting from the same alloying additions that determine the occurrence of IMPs at the interface. However, the Al addition caused a difference to occur in the g - Cu5Zn8 phase, with Al dissolving and g’ e Cu9Al4 being formed, which is consistent with the study by Yu et al. [27]. The occurrence of g0 was determined by the TEM images and selected-area electron diffraction (SAED) patterns. The different content of alloying elements caused, in the SnZn0.5Ag1.0Cu, the occurrence of a eutectic SnZn region in the solder after soldering above 30 min. All these factors resulted in the thickness of the IMPs layer in the SnZn1.0Ag1.0Cu0.1Al alloy to be lower. In the aging study, a difference for the SnZn0.5Ag1.0Cu alloy was observed after one day. The IMPs layer at the interface are discontinued, compared to the SnZn1.0Ag1.0Cu0.1Al alloy. After 10 days, Cu6Sn5 was detected at the interface in the joints using the SnZn0.5Ag1.0Cu alloy, which was not observed for the SnZn1.0Ag1.0Cu0.1Al alloy. The higher amount of precipitates of the CueZn system was caused by the higher addition of Ag and Cu, the same as in the soldering process for the SnZn1.0Ag1.0Cu0.1Al alloy. The results of the electrical resistivity measurement of the soldered joints Cu/solder/Cu show that the effect of soldering time and temperature on the resistance of soldered joints is negligible. In the case of aging at a temperature of 170 C, resistance of soldered joints Cu/solder/Cu increases with increasing aging time. Resistance nearly doubled after 744 h.
Table 1 Calculated constants diffusion coefficient (k) for different temperature and activation energy Q for both IMCs AgZn3 and Cu5Zn8. Solders
IMC
k (m2 s1)
SnZn0.5Ag1.0Cu SnZn-1.0Ag1.0Cu0.1Al
Cu5Zn8 Cu5Zn8
k0 (m2 s1)
Activation energy Q (kJ/mol)
1.06E-04 6.72E-05
12.7 12.6
230 C
250 C
280 C
320 C
4.08E-14 1.47E-14
5.88E-14 2.87E-14
8.00E-14 3.30E-14
1.04E-13 4.08E-14
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Fig. 8. Soldered joints Cu/Cu after 3 min at 250 C, soldering and aging at 170 C for 24 h, for SnZn0.5Ag1.0Cu, a) microstructure of cross section, c) XRD analysis, and for SnZn1.0Ag1.0Cu0.1Al, b) microstructure of cross section, d) XRD analysis.
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Fig. 9. Soldered joints Cu/Cu after 3 min at 250 C, soldering and aging at 170 C for 144 h, for SnZn0.5Ag1.0Cu, a) microstructure of cross section, c) XRD analysis, and for SnZn1.0Ag1.0Cu0.1Al, b) microstructure of cross section, d) XRD analysis.
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Fig. 10. Soldered joints Cu/Cu after 3 min at 250 C, soldering and aging at 170 C for 744 h, for SnZn0.5Ag1.0Cu, a) microstructure of cross section, c) XRD analysis, and for SnZn1.0Ag1.0Cu0.1Al, b) microstructure of cross section, d) XRD analysis.
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Fig. 11. Electrical resistivity of Cu/SnZn0.5Ag1.0Cu/Cu, for different times at 250 C.
Acknowledgement This work was financed within the framework of the project No. POIG.01.01.02-00-015/09, co-funded by the European Regional Development Fund (ERDF) and the Government of Poland under the Innovative Economy Programme. References [1] T. Gancarz, J. Pstrus, Arch. Metall. Mater. 60 (2015) (in press). [2] K.-I. Chen, S-Ch Cheng, S. Wu, K.-L. Lin, J. Alloy Compd. 416 (2006) 98e105. [3] L. Zhang, S.-B. Xue, Li-li Gao, Z. Sheng, H. Ye, Z.-X. Xiao, G. Zeng, Y. Chen, S.L. Yu, J. Mater. Sci. Mater. Electron 21 (2010) 1. [4] K.L. Lin, C.L. Shih, J. Electron Mater. 32 (2003) 95. [5] G. Saad, A. Fawzy, E. Shawky, J. Alloy Compd. 479 (2009) 844. [6] Y.H. Hu, S.B. Xue, W.X. Chen, H. Wang, J. Mater. Eng. 16 (2009) 60. [7] W.X. Chen, S.B. Xue, H. Wang, J.X. Wang, Trans. China Weld. Inst. 30 (2009) 75. ska, J. Alloy Compd. 582 (2014) 313. [8] T. Gancarz, J. Pstrus, P. Fima, S. Mosin [9] K.L. Lin, H.M. Hsu, J. Electron Mater. 30 (2001) 1068.
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C.-Y. Chou, S.-W. Chen, Acta Mater. 54 (2006) 2393. C.-W. Hwang, K.-S. Kim, K. Suganuma, J. Electron Mater. 32 (2003) 1249. J.-Y. Wang, Chih-Fan Lin, Chih-Ming Chen, Scr. Mater. 64 (2011) 633. D.Q. Yu, H.P. Xie, L. Wang, J. Alloy Compd. 385 (2004) 119. C.-W. Huang, K.-L. Lin, J. Electron Mater. 35 (2006) 2135. J. Pstrus, T. Gancarz, J. Mater. Eng. Perform. 23 (2014) 1614e1624. G. Subbarayan, J. Electron Pack. 118 (1996) 127. J.F. Li, P.A. Agykawa, C.M. Johnson, J. Alloy Compd. 545 (2012) 70. Y.C. Liu, J.B. Wan, Z.M. Gao, J. Alloy Compd. 465 (2008) 205. M.-C. Wang, S.-P. Yu, T.-C. Chang, M.-H. Hon, J. Alloy Compd. 381 (2004) 162. K. Suganuma, K. Niihara, T. Shoutoku, Y. Nakamura, J. Mater. Res. 13 (1998) 2859. R.J.K. Wassink, Soldering in Electronics, second ed., Electrochemical Publications, Scotland, 1989, p. 357. M.S. Park, S.L. Gibbons, R. Arroyave, Acta Mater. 60 (2012) 6278. P. Fima, J. Pstrus, T. Gancarz, J. Mater. Eng. Perform. 23 (2014) 1530e1535. M.-C. Wang, S.-P. Yu, T.-C. Chang, M.-H. Hon, J. Alloy Compd. 389 (2005) 133. R.K. Shine, L.W. Tsay, C.L. Lin, J.L. Ou, Microelectron. Reliab. 43 (2003) 453. Y. Nakamura, Y. Sakakibara, Y. Watanabe, Y. Amamoto, Solder Surf. Mt. Tech. 10 (1998) 10. S.-P. Yu, M.-C. Wang, M.-H. .Hon, J. Mater. Res. 16 (2001) 76.
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Formation and growth of intermetallic phases at the interface in the Cu/ SneZneAgeCu /Cu joints Tomasz Gancarz*, Janusz Pstrus Institute of Metallurgy and Materials Science, Polish Academy of Sciences, Krakow, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 April 2015 Received in revised form 8 June 2015 Accepted 15 June 2015 Available online 20 June 2015
This work presents the effect of soldering time and temperature on the microstructure and electrical properties of Cu/solder/Cu joints, where solder are Sn14.7Zn0.5Ag1.0Cu and Sn14.7Zn1.0Ag1.0Cu0.1Al (at.%) alloys. Soldering of Cu/solder/Cu joints was performed for 3, 8, 15, 30 and 60 min at 250 C, and at 230 and 280 C for 8 min, with the use of flux. Aging of Cu/solder/Cu joints was carried out at 170 C for 1, 10 and 30 days. After soldering, it was found that Cu5Zn8 and AgZn3 are formed at the solder/Cu interface. The thickness of Cu5Zn8 increases with time, while layer of AgZn3 grows up to 15 min and later diminishes. Precipitates of AgZn3 are found in the central part of joints, with their number increasing with soldering temperature. Higher content of Ag and addition of Al hinder growth of Cu5Zn8. During aging Cu6Sn5 starts to appear at the interface on the expense of Cu5Zn8. The electrical resistivity of joints decreases with time and temperature of soldering but increases after aging. © 2015 Elsevier B.V. All rights reserved.
Keywords: SneZneAgeCu Kinetics of growth of IMPs Activation energy Electrical resistivity Lead-free solders
1. Introduction The SneZn eutectic based alloy with the addition of Ag and Cu could be used as a replacement for Pb-bearing solder alloys [1]. The SneZn eutectic alloy shows better mechanical properties, tensile stress and strain compared to Sn37Pb (wt %) [2]. The wettability of SneZnexAl alloys was studied by Zhang et al. [3], and showed that the addition of Al improved the atmosphere and temperature during soldering. Lin and Shih [4] found that the wetting time decreases with increasing temperature, while it increases with Ag content. Saad et al. [5] studied the effect of Ag addition to SneZn on deformation temperature, and the effect of applied stress on the creep characteristics. They found that the addition of 1.5 wt.% Ag to the binary alloy increased its creep resistance. This behaviour was attributed to the formation of the IMPs AgeZn and AgeSn. These intermetallic phases (IMPs) stop the dislocations, and consequently lead to increased creep resistance for the alloy. Hu et al. [6] found that, if the concentration of Ag is lower than 0.3 wt.%, the microstructure of SneZn alloy is not altered and Zn is present in the form of needle-like precipitates. Only Ag
* Corresponding author. E-mail address: [email protected] (T. Gancarz). http://dx.doi.org/10.1016/j.jallcom.2015.06.122 0925-8388/© 2015 Elsevier B.V. All rights reserved.
concentration of between 0.5 and 1.0 wt.% is sufficient for Ag and Zn to react and form AgeZn IMPs visible in the changed microstructure. Chen et al. [7] studied the microstructure of the interface between Sn9Zn1Ag (wt.%) solder and Cu substrate, and found that, after soldering, there is only 0.3 wt.% of Ag left in the solder, and IMPs from CueZn and AgeZn systems are located at the interface, which causes its mechanical properties to deteriorate. The addition of Ag is expected, as shown in Gancarz et al. [8], to limit growth of IMPs. Another approach suggested by Lin and Hsu [9] is a solder that itself acts as a barrier after reflow, thus eliminating the need for a barrier layer. The addition of Al to SneZn solder in contact with the Cu substrate exhibits an ‘‘inherent barrier” [9]. The Al will form a thin layer of Al4.2Cu3.2Zn0.7 compound (Al52Cu39Zn9 at.%) at the liquid/solid interface. This compound, which results from the gathering of Al at the interface, provides a barrier to Sn diffusion towards the Cu substrate, which causes no formation of CueSn compounds [9]. The phase diagram of the SneZneCu system was investigated by Chou et al. [10], and no ternary compound was found in this system. In the wetting studies of SneZn alloy on Cu substrate at the interface, the three IMPs (CuZn [11], Cu5Zn8 and CuZn4 [12]) were found. Chou et al. [10] noted that the addition of Cu to eutectic SnZn alloy has an effect on wetting reactions with Cu substrate, and inhibits the growth of an IMPs layer at the interface. This is due to the
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formation of precipitates from the CueZn system formed in the solder and lack of free Zn to create IMPs at the interface. The same chemical composition was studied by Yu et al. [13], who observed that the addition of Cu to eutectic SnZn, causes lower a wetting angle on the Cu substrate. Both authors [10,13] observed that, with increasing Cu content in the alloy, the formation of Cu6Sn5
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precipitates in the solder. They also observed the formation of a Cu6Sn5 layer on the Cu substrate with increasing wetting time. Huang and Lin [14] investigated the effect of the addition of Al and Ag to eutectic SnZn on the formation of IMPs in soldering of Cu substrate. First, they observed two IMPs: CuZn4 and Cu5Zn8 after soldering eutectic SnZn on the Cu substrate. The addition of Al
Fig. 1. Soldering by SnZn0.5Ag1.0Cu at 250 C for different times: a) 3 min; b) 10 min; c) 15 min; d) 30 min; e) 60 min.
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caused the formation of IMPs from AleCueZn system at the interface. Further addition of Ag resulted in precipitates of AgZn3 being created at the interface. The authors also remarked on the spherical precipitates of AgZn3 occurring in bulk regardless of the cooling rate, but found that, for air-cooling, the number of precipitates was higher. The aim of this work is analysis of the effect of Ag, Al and Cu
content in SneZn solder on IMPs growth and kinetics on Cu substrates versus time and temperature. 2. Experimental For preparation of the alloys, SnZn0.5Ag1.0Cu and SnZn1.0Ag1.0Cu0.1Al (at. %), high purity metals Sn, Zn, Cu, Al
Fig. 2. Soldering by SnZn1.0Ag1.0Cu0.1Al at 250 C for different times: a) 3 min; b) 10 min; c) 15 min; d) 30 min; e) 60 min.
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(99.999%) and Ag (99.99%), were used. The SneZneAgeCu alloys were prepared in a glove box filled with high purity Ar (99.9999%), with levels of O2 and H2O kept below 1 ppm, and lowered N2 content, as presented in Refs. [8,15]. Carefully weighed amounts of metals were melted in graphite crucibles in a resistance furnace and then cast into graphite moulds. For soldering tests, alloys were cold-rolled into 1 mm thick sheets, which were later cut into 10 10 1 mm plates. Soldering tests were performed on Cu (99.9%) substrates, 25 mm long, 10 mm wide, and 4 mm thick [15]. The solder plates were put between the two substrates. Soldering connections of the type Cu/solder/Cu were prepared at 250 C for 3, 8, 15, 30 and 60 min, and at 230 and 280 C for 8 min, respectively. Alu33® flux was used for preparation [15] of all of the types of connections. Characterisation of the joints was performed using scanning electron microscopy (SEM), coupled with energy dispersive spectrometry (EDS), and X-ray diffraction (XRD). For all samples, three measurements were made at different areas to improve counting statistics and to check the homogeneity of the joints. First, an XRD measurement was performed in the centre of the joint and on opposite edges. The stress investigation was carried out with the basic assumptions known from the traditional sin2j method, with the use of the D8 Discover Bruker diffractometer equipped with a parallel-beam primary optics PolyCap system completed a pinhole collimator (1.0 mm aperture) for CoKa radiation (l ¼ 1.790300 Å) and a linear position-sensitive detector LynxEye (range 2.6 on 2q), capable of working in a parallel secondary beam configuration with the use of Soller collimators. The accelerating voltage and the applied current were 40 kV and 35 mA. The SEM observation, EDS and XRDs analysis were used to determine the kinetics of growth and the characteristic of IMPs layers occurring at the interface. Aging of soldered Cu/solder/Cu joints was carried out at the temperature of 170 C for 1, 10 and 30 days. Metallographic crosssections of the aged samples were prepared for the microstructure characterisation and thickness measurement of intermetallic compounds (IMPs), using a scanning electron microscope. All of the joints were subjected to electrical resistivity measurements [15]. Their aim was to assess the quality of joints, and to correlate this with preparation and aging conditions. 3. Results and discussion
the bulk solder. Because the coefficient of diffusion of Cu, which is the driving force of the formation of Cu5Zn8, is four times greater than that of Zn [12], the Cu5Zn8 grows at the expense of AgZn3, as the AgZn3 has less negative Gibbs free energy of formation than does the Cu5Zn8 [8]. Similar results were obtained by Hang et al. [14], who reported a thick layer of IMPs from the AleZneCu system and, directly after the g layer, the growth of precipitates of AgZn3. On the initial stages of soldering (at 3e15 min), the observed microstructures of joints consist of large AgZn3 and Cu5Zn8 precipitates surrounded by Sn. After 30 min of soldering, the microstructures change to eutectic SnZn with small precipitates of AgZn3. The XRD analysis in Fig. 4 a, b, c, d, shows that, after 3 min (Fig. 4 a, b), AgZn3 and Cu5Zn8 occur in both cases, but for SnZn1.0Ag1.0Cu0.1Al solder (Fig. 4b), it is observed that the Al dissolved in the Cu5Zn8 phase. After 60 min (Fig. 4 c, d), Cu6Sn5 precipitates are formed at the interface, and in the corresponding SEM microstructure of the soldered joints small precipitates of AgZn3 and a layer of Cu5Zn8 at the interface of the solder/substrate are observed. In the case of Cu/SnZn1.0Ag1.0Cu0.1Al/Cu joints (Fig. 4d), increased concentration of Ag to 1.0%, and a small addition of Al 0.1 (at.%), caused the creation of larger AgZn3 precipitates without Cu5Zn8 precipitates inside of solder matrix at the beginning of the soldering process. It also caused a smaller increase of Cu5Zn8 andAgZn3 IMPs layers, as presented in Figs. 5 and 6. Therefore, a small addition of Al to the SnZn solder is a valid technique to slow down the growth rates of IMPs between the solder and Cu substrate. The same conclusions were reached by Ref. [17], who have studied the effect of Al addition to SAC alloys. 3.2. Effect of temperature on the microstructure of soldered joints The microstructures of Cu/solder/Cu joints, after joining for 10 min at different temperatures, are presented in Figs. 5 and 6: 230 C (a); 280 C (b); 320 C (c) for solders SnZn0.5Ag1.0Cu (Fig. 5) and SnZn1.0Ag1.0Cu0.1Al (Fig. 6). The microstructures at 230 C show large precipitates of AgZn3 and Cu5Zn8 in the case of the SnZn1.0Ag1.0Cu0.1Al alloy, and AgZn3 precipitates in the eutectic SnZn matrix in the case of the SnZn0.5Ag1.0Cu alloy. At 250 C, the microstructure of joints is similar for both alloys, but for SnZn0.5Ag1.0Cu there are a greater number of small AgZn3
3.1. Effect of time on the microstructure of soldered joints The microstructure of cross-sectioned joints Cu/ SnZn0.5Ag1.0Cu/Cu (Fig. 1 a, b, c, d, e) and Cu/SnZn1.0Ag1.0Cu0.1Al/Cu (Fig. 2 a, b, c, d, e) soldered at 250 C for 3 min (a), 10 min (b), 15 min (c), 30 min (d) and 60 min (e), are shown in Figs. 1 and 2, respectively. At the solder/Cu substrate interface, IMPs AgZn3 and Cu5Zn8 are formed, which was confirmed with EDS analysis (linescane in the figures) and XRD analyses. Fig. 3 illustrates the thickness of AgZn3 and Cu5Zn8IMPs vs. square rooted time. In this case, the process of IMPs growth is a diffusioncontrolled reaction, as reported in Refs. [8,16], whereas in the case of Cu joints soldered by ZnAl with Ag and Cu, n was much higher ~0.85, and diffusion had a mixed character for the Cu5Zn8 phase [15]. This is due to a higher concentration of Zn in ZnAl compared to SnZn, where force was applied in the solder of joints [15], but it also was observed for higher addition Ag (1.0; 1.5%) to ZnAl [8], where the sessile drop method was used. The layers of Cu5Zn8 grow with time, and the thickness of AgZn3 increases with up to 15 min of soldering. However, after 30 min its thickness is significantly reduced, and after 60 min of soldering the AgZn3 phase is not present in the soldered joint. The cause of the observed phenomena is the diffusion of Cu from the substrate into
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Fig. 3. Thickness of IMPs on Cu substrate for different times at 250 C.
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Fig. 4. XRD analysis of soldering joints at 250 C for different times: a, b) 3 min; c, d) 60 min for SnZn0.5Ag1.0Cu and SnZn1.0Ag1.0Cu0.1Al, respectively.
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Fig. 4. (continued).
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precipitates. Moving on to higher temperatures, 280 C and 320 C, the microstructure of joints differs in that, for the SnZn0.5Ag1.0Cu alloy, eutectic SneZn with big AgZn3 and Cu5Zn8 precipitates is observed, while for the SnZn1.0Ag1.0Cu0.1Al alloy, eutectic SneZn with small AgZn3 precipitates is observed. The kinetics of the growth of the IMPs layer at the interface between the solder and Cu substrate is shown in Fig. 7. The calculated constants (the diffusion coefficient for different temperatures (k) and phases (k0), together with the activation energy for the growth of both the AgZn3 and the Cu5Zn8IMPs) are presented in Table 1. The general form of the relation between the diffusion coefficient and thickness of the IMC layer can be expressed by Eq. (1) [18e21].
x¼
pffiffiffiffiffi kt
(1)
where x is the thickness of the IMC layer (mm), k denotes the diffusion coefficient and t is the heating time (s). The obtained values of the coefficient (k0) are of the same magnitude as in the literature data for SneZneAl/Cu joints [19]. The obtained constant coefficient (k0) for Cu5Zn8 was 1.06E-04 (m2/s) for the SnZn0.5Ag1.0Cu alloy and 6.72E-05 (m2/s) for the SnZn1.0Ag1.0Cu0.1Al, respectively. A similar value of (k0) 7.4E-05
(m2/s) was obtained for Cu5Zn8 by Liu [18], in the soldered SneAgeZn/Cu joints. The increase of (k) values with increasing temperature is also observed as in the literature [19,22], and the values are of the same magnitude for Cu5Zn8. The obtained value of activation energy for growth is lower than that determined by Wang et al. [19], which may be caused by the use of flux in the present study. During soldering for 10 min at temperatures from 230 to 320 C, the growth of the Cu5Zn8 IMC layer with temperature was observed. The AgZn3 layer shows a different character, and with increasing temperature is dissolved, which is the reason why the obtained activation energy for growth is negative. The activation energy of growth of Cu5Zn8 is almost identical, i.e. 12.7 and 12.6 (kJ/ mol), respectively, for the Cu/SnZn0.5Ag1.0Cu/Cu and Cu/ SnZn1.0Ag1.0Cu0.1Al/Cu joints. But for the Cu/SnZn1.0Ag1.0Cu0.1Al/Cu joints, faster dissolution of AgZn3 layer was observed. The activation energy of growth for Cu5Zn8 (53.77 kJ/mol) obtained by Wang et al. [19] for the 91Sne8.55Sne0.45Al alloy is much higher than our results. There are two possible reasons of this discrepancy. One is addition of Cu to alloys, which facilitated the formation of the Cu5Zn8 phase [23] in our case, and the other is the different process of soldering used in Ref. [19], where pads were hot-dipped in solder and after that subjected to aging.
Fig. 5. Soldering by SnZn0.5Ag1.0Cu for 10 min at different temperatures: a) 230 C; b) 280 C; c) 320 C.
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Fig. 6. Soldering by SnZn1.0Ag1.0Cu0.1Al for 10 min at different temperatures: a) 230 C; b) 280 C; c) 320 C.
3.3. Effect of aging on the microstructure of soldered joints
Fig. 7. Kinetics of growth of IMPs vs. temperature for SnZn0.5Ag1.0Cu and SnZn1.0Ag1.0Cu0.1Al, respectively.
After 3 min of soldering at 250 C, selected joints were subjected to aging at 170 C at different times (24, 144 and 744 h). The microstructure of cross sections of aged soldered joints, and XRD analysis, are shown in Figs. 8e10, for different aging times. The Cu/ SnZn0.5Ag1.0Cu/Cu and Cu/SnZn1.0Ag1.0Cu0.1Al/Cu joints, after 24 h of aging at 170 C, are presented in Fig. 8 a and b, respectively, and there are differences in the process of the growth of IMPs. For the connection Cu/SnZn0.5Ag1.0Cu/Cu, dissolution of Cu5Zn8 and AgZn3 layers is followed by growth of Cu6Sn5, by dividing the IMC layer into small pieces. For Cu/SnZn1.0Ag1.0Cu0.1Al/Cu, growth of Cu6Sn5 is not observed, only minimal thinning of the existing IMPs layers. The microstructures of solders in the joints are different. In the case of Cu/SnZn0.5Ag1.0Cu/Cu it is an Sn-matrix with small, uniformly distributed precipitates of AgZn3, but in the case of Cu/ SnZn1.0Ag1.0Cu0.1Al/Cu it is an eutectic SnZn matrix with single precipitates of AgZn3. In both cases there are the Cu5Zn8 layers at the interface, but for the Cu/SnZn0.5Ag1.0Cu/Cu joint, layers look discontinuous and dissolving. Descriptions of the phases based on EDS analysis are in agreement with XRD analysis presented on Fig. 8 c and d, respectively. Fig. 9, a and b show the microstructure of cross sections after aging for 144 h, for Cu/SnZn0.5Ag1.0Cu/Cu and Cu/ SnZn1.0Ag1.0Cu0.1Al/Cu, respectively. For Cu/SnZn0.5Ag1.0Cu/Cu,
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the higher propagation of Sn into the layers is observed, as is the formation of Cu6Sn5 precipitates from the side of the Cu substrate. The thickness of IMPs “layers” Cu5Zn8, AgZn3 is greater than after 24 h of aging, and a greater number of precipitates are visible in the solder matrix in the joint. For the Cu/SnZn1.0Ag1.0Cu0.1Al/Cu joint, Cu5Zn8, AgZn3 IMPs layers are nearly invisible, while numerous Cu5Zn8, AgZn3 precipitates are present in the solder in the joint. The XRD analysis of joints after 144 h of aging, shown in Fig. 9 c and d, confirmed the occurrence of the Cu5Zn8, AgZn3 IMPs. Further aging of Cu/SnZn0.5Ag1.0Cu/Cu and Cu/ SnZn1.0Ag1.0Cu0.1Al/Cu joints, for 744 h at the same temperature (170 C), is presented in Fig. 10, with the microstructure of the cross section of Cu/SnZn0.5Ag1.0Cu/Cu in Fig. 10a, and XRD analysis in Fig. 10c. The microstructure and XRD analysis for Cu/ SnZn1.0Ag1.0Cu0.1Al/Cu are presented in Fig. 10b and d, respectively. For Cu/SnZn0.5Ag1.0Cu/Cu, it can be observed that the continued diffusion of Sn to the interface, and also to the Cu substrate, results in the formation of new area of IMPs Cu6Sn5 and Cu3Sn, similarly as reported by Wang et al. [12]. As can be seen, in the microstructure of the solder in the joint, precipitates of Cu6Sn5 start to appear. Samples after soldering were removed from the furnace and put onto metal plates for cooling to room temperature, which is similar to the process in the work of Wang et al. [12], who studied how cooling rates affect the microstructure after casting. Similar conditions give similar effects, i.e. dissolution of the Cu5Zn8 layer and formation of Cu6Sn5 and Cu3Sn precipitates at the interface and in the Cu substrate. For the sample that was cooled in the furnace (slowly), after aging for 696 h [12], the Cu5Zn8 layer exists at the interface. For the Cu/SnZn1.0Ag1.0Cu0.1Al/Cu joint, no Cu3Sn phase is observed. The formation of two individual layers in the soldered interface (Fig. 2) could be interpreted as follows: since the Gibbs free energy (DG) for Cu5Zn8 (31.44 kJ/mol) [8,24] is much lower than that (27.94 kJ/mol) of the AgZn3 [8] at 200 C, the Cu5Zn8 IMPs layer, which is in a metastable state at high temperature [25,26], forms first during soldering [18]. Also, the diffusivity of Sn in CueSn alloys is given by kSn ¼ 1.90 1010 cm2/s at 300 C, and that of Zn in CueZn alloys is kZn ¼ 2.70 1010 cm2/s at 300 C [27]. This explains the formation of g-Cu5Zn8 instead of CueSn compounds. But, during aging, diffusion of Zn is hindered because of the distance between Zn precipitates in the joint, and total amount of Zn in the joint is limited. During aging at 170 C, diffusion of Cu from the substrate to the joint continues, and since the system tends to reach the minimum energy, formation of Cu6Sn5 is possible. Cu6Sn5 precipitates form between the Cu5Zn8 layer and the Cu substrate, and with time they join and form a new IMC layer. 3.4. Electrical resistivity of soldered joints Fig. 11 shows the electrical resistivity of the soldered joints Cu/ SnZn0.5Ag1.0Cu/Cu. From the obtained results for Cu/ SnZn0.5Ag1.0Cu/Cu joints, it is clear that the effect on electrical resistivity of time of joining at 250 C, as well as temperature of
joining for 10 min, is negligible. However, the results after aging, except for an aging time of 24 h, show an increase in electrical resistivity, up to 80% after 744 h of aging. As can be observed in Fig. 11, the electrical resistivity of joints decreases with the increase of time and temperature, which is caused by a greater amount of Cu in the IMPs layers in the joint. The observed increase of the electrical resistivity of joints after aging is not desirable. The formation of IMPs precipitates inside the solder, caused by increases in the number of borders, (observed on Fig. 10 a and b), has a direct effect on the increase of electrical resistivity.
4. Conclusion During soldering at 250 C, the CuZn4 and AgZn3 phases grow at first, and later, after 15 and 30 min, the size of the phase AgZn3 dramatically decreases and the size of CuZn4 decreases minimally. However, after 60 min we can see the complete disappearance of the AgZn3 phase, and more than three times the growth of the Cu5Zn8 phase. Similarly, the growth of phases can be observed for soldering at 10 min at a variable temperature. When describing the kinetics of the IMPs layers appearing at the interface of the solder/ Cu pad, one must also take into account the precipitates inside solder CuZn4, Cu5Zn8 and AgZn3 which may have an influence on the formation of layers. The observed effect for SnZn1.0Ag1.0Cu0.1Al solder was that the higher addition of Ag, Cu and Al causes the reduction in growth of Cu5Zn8, is shown in Fig. 2 d and e. In the soldering process in the investigation solders SnZn0.5Ag1.0Cu and SnZn1.0Ag1.0Cu0.1Al, similar microstructures were obtained, resulting from the same alloying additions that determine the occurrence of IMPs at the interface. However, the Al addition caused a difference to occur in the g - Cu5Zn8 phase, with Al dissolving and g’ e Cu9Al4 being formed, which is consistent with the study by Yu et al. [27]. The occurrence of g0 was determined by the TEM images and selected-area electron diffraction (SAED) patterns. The different content of alloying elements caused, in the SnZn0.5Ag1.0Cu, the occurrence of a eutectic SnZn region in the solder after soldering above 30 min. All these factors resulted in the thickness of the IMPs layer in the SnZn1.0Ag1.0Cu0.1Al alloy to be lower. In the aging study, a difference for the SnZn0.5Ag1.0Cu alloy was observed after one day. The IMPs layer at the interface are discontinued, compared to the SnZn1.0Ag1.0Cu0.1Al alloy. After 10 days, Cu6Sn5 was detected at the interface in the joints using the SnZn0.5Ag1.0Cu alloy, which was not observed for the SnZn1.0Ag1.0Cu0.1Al alloy. The higher amount of precipitates of the CueZn system was caused by the higher addition of Ag and Cu, the same as in the soldering process for the SnZn1.0Ag1.0Cu0.1Al alloy. The results of the electrical resistivity measurement of the soldered joints Cu/solder/Cu show that the effect of soldering time and temperature on the resistance of soldered joints is negligible. In the case of aging at a temperature of 170 C, resistance of soldered joints Cu/solder/Cu increases with increasing aging time. Resistance nearly doubled after 744 h.
Table 1 Calculated constants diffusion coefficient (k) for different temperature and activation energy Q for both IMCs AgZn3 and Cu5Zn8. Solders
IMC
k (m2 s1)
SnZn0.5Ag1.0Cu SnZn-1.0Ag1.0Cu0.1Al
Cu5Zn8 Cu5Zn8
k0 (m2 s1)
Activation energy Q (kJ/mol)
1.06E-04 6.72E-05
12.7 12.6
230 C
250 C
280 C
320 C
4.08E-14 1.47E-14
5.88E-14 2.87E-14
8.00E-14 3.30E-14
1.04E-13 4.08E-14
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Fig. 8. Soldered joints Cu/Cu after 3 min at 250 C, soldering and aging at 170 C for 24 h, for SnZn0.5Ag1.0Cu, a) microstructure of cross section, c) XRD analysis, and for SnZn1.0Ag1.0Cu0.1Al, b) microstructure of cross section, d) XRD analysis.
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Fig. 9. Soldered joints Cu/Cu after 3 min at 250 C, soldering and aging at 170 C for 144 h, for SnZn0.5Ag1.0Cu, a) microstructure of cross section, c) XRD analysis, and for SnZn1.0Ag1.0Cu0.1Al, b) microstructure of cross section, d) XRD analysis.
T. Gancarz, J. Pstrus / Journal of Alloys and Compounds 647 (2015) 844e856
855
Fig. 10. Soldered joints Cu/Cu after 3 min at 250 C, soldering and aging at 170 C for 744 h, for SnZn0.5Ag1.0Cu, a) microstructure of cross section, c) XRD analysis, and for SnZn1.0Ag1.0Cu0.1Al, b) microstructure of cross section, d) XRD analysis.
856
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Fig. 11. Electrical resistivity of Cu/SnZn0.5Ag1.0Cu/Cu, for different times at 250 C.
Acknowledgement This work was financed within the framework of the project No. POIG.01.01.02-00-015/09, co-funded by the European Regional Development Fund (ERDF) and the Government of Poland under the Innovative Economy Programme. References [1] T. Gancarz, J. Pstrus, Arch. Metall. Mater. 60 (2015) (in press). [2] K.-I. Chen, S-Ch Cheng, S. Wu, K.-L. Lin, J. Alloy Compd. 416 (2006) 98e105. [3] L. Zhang, S.-B. Xue, Li-li Gao, Z. Sheng, H. Ye, Z.-X. Xiao, G. Zeng, Y. Chen, S.L. Yu, J. Mater. Sci. Mater. Electron 21 (2010) 1. [4] K.L. Lin, C.L. Shih, J. Electron Mater. 32 (2003) 95. [5] G. Saad, A. Fawzy, E. Shawky, J. Alloy Compd. 479 (2009) 844. [6] Y.H. Hu, S.B. Xue, W.X. Chen, H. Wang, J. Mater. Eng. 16 (2009) 60. [7] W.X. Chen, S.B. Xue, H. Wang, J.X. Wang, Trans. China Weld. Inst. 30 (2009) 75. ska, J. Alloy Compd. 582 (2014) 313. [8] T. Gancarz, J. Pstrus, P. Fima, S. Mosin [9] K.L. Lin, H.M. Hsu, J. Electron Mater. 30 (2001) 1068.
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