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Nanoparticles of SnAgCu lead-free solder alloy with an equivalent melting temperature of SnPb solder alloy
Journal of Alloys and Compounds 484 (2009) 777–781
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Nanoparticles of SnAgCu lead-free solder alloy with an equivalent melting temperature of SnPb solder alloy Yulai Gao a,∗ , Changdong Zou a , Bin Yang a , Qijie Zhai a , Johan Liu b,c , Evgeny Zhuravlev d , Christoph Schick d a
Shanghai Key Laboratory of Modern Metallurgy & Materials Processing, Shanghai University, Yanchang Road 149, Shanghai 200072, PR China Bionano Systems Laboratory, Department of Microtechnology and Nanoscience, MC2, Chalmers University of Technology, SE - 412 96, Göteborg, Sweden SMIT Center, School of Mechatronics and Automation, Shanghai University, Yanchang Road 149, Shanghai 200072, PR China d Institute of Physics, University of Rostock, Universitätsplatz 3, 18051 Rostock, Germany b c
a r t i c l e
i n f o
Article history: Received 6 April 2009 Received in revised form 4 May 2009 Accepted 9 May 2009 Available online 18 May 2009 Keywords: Lead-free solder Nanoparticles Melting temperature depression Arc technique SnAgCu
a b s t r a c t Nanoparticles were prepared with a consumable-electrode direct current arc (CDCA) technique. The results showed that the calorimetric melting onset temperature of the nanoparticles of SnAgCu solder alloy could be as low as 179 ◦ C, equivalent to that of the traditionally used SnPb eutectic alloy (183 ◦ C). Moreover, the homogenous melting model (HMM) and Gibbs–Thomson equation were employed to theoretically estimate the size-dependent melting temperature of the as-prepared nanoparticles. The structure and morphology of the nanoparticles were analyzed with a high-resolution transmission electron microscopy (HRTEM). The CDCA technique showed promising prospect in manufacturing large amounts of nanoparticles with controlled shape, small size, narrow particle size distribution and nearly oxide-free composition. This undoubtedly puts forward a novel feasible approach to manufacture high quality lead-free solders for electronic products. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Electronic packaging is a manufacturing technology used for electronic products. Packaging provides a medium for electronic interconnections and mechanical support, and solder alloys provide the electrical and mechanical connections between the die (chip) and the bonding pads. The selection of materials for solder alloys is, therefore, critical and plays an important role in solder joint reliability. The Sn–Pb alloy has been the most widely used solder alloy system as an interconnection material in the electronic packaging industry. The Pb-containing alloys are reliable, well tested and quite inexpensive, and Sn–Pb alloys over the whole composition range can be used as solders. There are, however, disadvantages with Pbcontaining solders. Apart from the undeniable toxicity of damaging human’s nervous system, Pb is also harmful to the environment by causing ground water contamination. As a result, Pb-containing solder alloys are either being banned or phased out from the electronic industry. Legislations, such as the Waste from Electrical and Electronic Equipment (WEEE) and the Restriction of the use of Hazardous Substances (RoHS) have been implemented in the European
∗ Corresponding author. Tel.: +86 21 56332144. E-mail address: [email protected] (Y. Gao). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.042
Union since 2006. Therefore, an urgent necessity exists for more appropriate substitutes for the Sn–Pb solders. It is also important to ensure that the physical and mechanical properties of the substitutes are comparable or even superior to those of Sn–Pb solder. From a manufacturing point of view, the melting temperature of the solder alloy is a crucial factor that has to be taken into account in order to achieve high package quality. A large variety of lead-free solders have already been developed, mainly involving the Sn–Cu, Sn–Ni and Sn–Ag systems [1]. These new lead-free solders have been identified as the most promising alternatives to the eutectic Sn–Pb solder. However, the higher melting temperatures of these alloys, comparing to that of the eutectic Sn–Pb solder, limit their applications in the electronic industry. For that matter, new methods have been explored to decrease the melting temperature of solder alloys. In 1954, Takagi carried out the first experimental investigation dealing with the effect of size on the melting of very small particles [2]. Utilizing electron microscope, Sambles observed the lowering of the melting point of the gold particles [3]. In 1976, Buffat also described that the melting temperature could be depressed by decreasing the particle size to nanometer scale [4]. Later, nanocalorimetry was employed to characterize the melting temperature of a small number of nanoparticles [5,6], and hot stage transmission electron microscopy was used to observe the melting behavior of isolated nanoparticles
778
Y. Gao et al. / Journal of Alloys and Compounds 484 (2009) 777–781
[7]. Wong et al. reported the size-dependent melting temperature depression of Sn nanoparticles [8] by traditional differential scanning calorimetry (DSC). They also reported size-dependent melting temperature depression of Sn–3.5Ag nanoparticles synthesized by a chemical reduction method [9]. However, the production rate of nanoparticles was limited by this method. Hsaio and Duh manufactured and studied nanoparticles of Sn–3.5Ag–xCu (x = 0.2, 0.5, 1.0), synthesized for lead-free solder applications [10]. They did not, however, observe any obvious melting temperature depression in their investigation. When the particle size was further decreased until to infinitesimal size, e.g. in which only tens or hundreds of atoms were contained, an anomalous size dependence of the melting temperature was observed and it was deemed that not only the cluster size but also structural features governed their correlations [11]. Nevertheless, no such correlation has been observed for the particle size scale of interest for solder applications. Banhart et al. [12] observed that the melting temperature of Sn and Pb nanocrystals might be increased to a value even higher than the corresponding bulk alloy if exterior shells exist. Embedded in some matrix, some nanoparticles revealed even opposite changing tendency of the melting temperatures [13–16], indicating that the exterior structure of nanoparticles played a significant role in their melting behavior. Premelting, which occurs before the whole melting of the clusters or nanoparticles, is easily produced near crystal defects [17]. Except the large specific surface area, the presence of large interfaces in nanoparticles was another crucial factor to influence the melting temperature and should be taken into account [18]. Pusey and co-workers [19,20] observed that premelting at a lower temperature was produced along grain boundaries before complete melting of the crystalline colloids. Riegler and Kohler [21] also thought that the interfacial properties would strongly affect the phase transition behavior of the small particles due to their large interfaces, and premelting would occur on surrounding interfaces. However, little information was available about grain boundary premelting of metals due to their high temperature and the resultant difficulty for simultaneous observations. Fortunately, the recently developed coherent X-ray diffraction imaging technique might be a possible approach to study the premelting of nanocrystal interfaces owing to its diffraction patterns and latest third-generation synchrotron radiation sources to obtain full three-dimensional images [22]. In contrast to the lower melting temperature (183 ◦ C) of SnPb eutectic solder alloy [23], therefore, the lead-free solder alloys usually cause a raised temperature of about 30–40 ◦ C during electronic assembling. And the increased temperature reduces the integrity, reliability and functionality of printed wiring boards, components and other attachment [24]. As a result, much effort has been put lately on finding possible ways to solve some of the issues imposed by lead-free solders. Lin et al. employed the supernatant process to obtain the Sn–3.5Ag solder alloy nanoparticles, yet no obvious melting temperature depression was observed possibly due to the aggregation of those nanoparticles [25]. Therefore, it remains an urgent necessity to conduct an indepth study of feasible processes to lower the melting temperature of the currently used lead-free solder alloy systems. 2. Experimental procedures The lead-free master alloy with a nominal composition of Sn–3.0Ag–0.5Cu (wt.%) was prepared by an induction melting method. Fig. 1 shows the DSC melting curve of the master alloy. The melting temperature was 217.8 ◦ C, which equals the equilibrium melting temperature. Integration of the peak yields 67 J/g for the heat of fusion. Several models described the size dependence of the melting temperature of nano-sized particles. Generally it is assumed that the melting is initiated by a continuous vibrational lattice instability on the solid surface or in a solid-solid interface [18]. For nano-scale particles with large surfaces, the instability becomes obvious
Fig. 1. DSC curve of the SnAgCu bulk alloy at the heating rate of 30 ◦ C/min.
and usually the nanoparticles will lose stability at lower temperature than their bulk precursor. The size-dependent melting temperature of nanoparticles could be estimated using the homogenous melting model (HMM) [4]. This model was established based on the assumption that the solid and liquid particles of the same mass are in equilibrium with their common vapor. The free energy of a solid particle with a radius r is modified by an extra term: 2V/r, where , V and r are the surface tension, mole volume and radius of the particle, respectively. In other words, increasing the free energy of the particles causes depression of the melting temperature. The size-dependent melting temperature of nanoparticles is then given by the following equation: Tm (r) =
bulk Tm
−
bulk + 273.15) 2(Tm
Hfbulk s r
2/3
sv − lv
s l
(1)
where r represents the radius of a spherical particle and Tm (r) is its melting temperbulk is the melting temperature of the bulk alloy, Hfbulk is ature (in degree Celsius), Tm the heat of fusion for the bulk alloy, s and l are the solid and liquid phase densities of the bulk alloy, and sv and lv are the solid–vapor and liquid–vapor interfacial energies, respectively. The melting temperature depression for small crystals can also be evaluated by the Gibbs–Thomson equation [26]: bulk − Tm (r) = Tm
bulk + 273.15)sl 2(Tm
(2)
Hfbulk s r
where sl is the solid–liquid interfacial energy. For many metals, sl ≈ sv − lv [27]. As a matter of fact, if the value of s was deemed the same as that of l , then sl = sv − lv [28]. That is to say, under these conditions the HMM and Gibbs–Thomson equations could be expressed by the same formula. The melting temperature and the heat of fusion of the bulk master alloy were obtained by the DSC measurement shown in Fig. 1. However, some necessary thermodynamic parameters of Sn–3.0Ag–0.5Cu alloy were unavailable, thus those of the similar Sn–3.5Ag (wt.%) system were used [23]. The data for calculation are listed in Table 1, and the dependence of melting temperature on nano-sized particles was calculated by the HMM and Gibbs–Thomson equation, as shown in Fig. 2. The solid curve denotes the melting temperatures calculated by the HMM and Gibbs–Thomson equations, the upper and lower horizontal dashed lines correspond to the bulk melting temperatures of SnAgCu alloy and SnPb eutectic solder, respectively. According to Fig. 2, particles of about 10 nm diameter should show a melting temperature comparable to that of SnPb eutectic solder (183 ◦ C). Nanoparticles were prepared from the master alloy with a recently developed consumable-electrode direct current arc (CDCA) technique as shown schematically in Fig. 3. Liquid paraffin was chosen as a dielectric protection media. The anode and cathode, with a diameter of 6 mm, were made from the master alloy by means of a suction casting technique. The anode and the cathode electrodes were fixed inside the CDCA container and connected to a power supply. When the two electrodes get close enough, an arc discharge could be produced between them, causing a local melting and breakdown of the solder material into nanoparticles. The obtained particles were rinsed with chloroform for several times to remove the liquid paraf-
Table 1 Data used in the calculation of size-dependent melting temperatures [23]. bulk ◦ Tm ( C)
Hfbulk (J/g)
s (g/cm3 )
sv (J/m2 )
lv (J/m2 )
sl = sv – lv (J/m2 )
217.8
67
7.39
0.51
0.43
0.082
Y. Gao et al. / Journal of Alloys and Compounds 484 (2009) 777–781
Fig. 2. Melting temperature dependence of SnAgCu nanoparticles on particle diameter. fin. Furthermore, the rinsed particles were centrifugally separated at 4000 rpm for 45 min to remove the possibly existed large particles. The phases of the prepared nanoparticles were examined on a Rigaku D/Max2200 X-ray diffractometer (XRD). The morphology and size distribution of the nanoparticles were revealed by means of JSM-6700F type field-emission scanning electron microscopy (FE-SEM). The melting temperatures of the bulk master alloy and the nanoparticles were measured with PerkinElmer Pyris 1 differential scanning calorimeter (DSC) under the protection of high-purity nitrogen. The detailed structure and morphology of the nanoparticles was further studied with JEM-2010F type high-resolution transmission electron microscopy (HRTEM).
3. Results and discussion Unlike the chemical reduction method applied by Wong and co-workers [9,10], the CDCA technique is suitable for most conducting materials, and the manufacturing process could be continuous allowing preparation of lead-free solder nanoparticles at high production rates. During the preparation of the nanoparticles, discharge and solder alloy breakdown took place inside the dielectric media, protecting the nanoparticles from serious oxidation. The XRD pattern of the as-prepared nanoparticles is shown in Fig. 4. The Sn, Ag3 Sn and Cu6 Sn5 phases were detected, implying that a good alloying was produced during the particle preparation process. In addition, no oxide traces were found by the XRD measurement, indicating that the nanoparticles were not significantly oxidized. Arc current is one of the crucial parameters to control nanoparticle formation, especially the particle size and size distribution. The morphology and sizes of the prepared nanoparticles at 20 A arc current were obtained from SEM images, as shown in Fig. 5. The nanoparticles were in spherical shape on the whole, but particle agglomeration was also detected. The size distribution of the
Fig. 3. Schematic of the CDCA setup for nanoparticles preparation (1) cathode, (2) anode (connected to a high current and low voltage power source), (3) and (4) bulk alloy electrodes, (5) arc discharge taking place between the electrodes, and (6) dielectric coolant.
779
Fig. 4. XRD pattern of the nanoparticles prepared by CDCA technique under the protection of liquid paraffin.
nanoparticles was determined by image analysis with Image-Pro plus software, and the size distribution histogram is shown in the inset of Fig. 5. It was found that the size of most particles was in the range 15–60 nm with an average of about 30 nm. The melting temperature of the aforementioned nanoparticles was determined with DSC at a heating rate of 30 ◦ C/min. To reduce the possible experimental error, the DSC measurement was repeated for three cycles, as shown in Fig. 6. It was found that the melting peak was shifted to lower temperatures and broadened. The peak onset temperature was decreased to about 197 ◦ C, 20 ◦ C lower than that of the bulk master alloy. Increasing the arc current to 50 A led to a significantly narrower size distribution of the nanoparticles, as shown in Fig. 7. The particle size was concentrated in the range 25–42 nm, again with an average of about 30 nm. The nanoparticles prepared with an arc current of 50 A were measured in the DSC at heating rate 30 ◦ C/min for five cycles, presented in Fig. 8. The results show that the melting onset temperature is lowered below 190 ◦ C and remained nearly unchanged even after repeated heating–cooling cycles. Due to the large specific surface area of the nanoparticles, it was hard to avoid oxidation dur-
Fig. 5. SEM image of the nanoparticles of the SnAgCu alloy prepared by CDCA technique at 20 A arc current. The inset shows the size distribution histogram of the nanoparticles.
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Y. Gao et al. / Journal of Alloys and Compounds 484 (2009) 777–781
Fig. 6. DSC curves for nanoparticles of SnAgCu alloy prepared with current value of 20 A heated at 30 ◦ C/min for three cycles. The vertical dashed lines correspond to the expected melting temperatures according Eq. (1) for the particle diameters given on top. Curves are displaced vertically for clarity.
ing preparation, storage and the measurement. That is to say, some oxidation layer was generally present on the particle surface. As a result, two opposite effects are expected during these repeated DSC measurements. The first effect was the further oxidation of the nanoparticles, which would decrease the size of the metallic core in the nanoparticles. Oxidation causes a continuous size reduction of the metal core and therefore a further decreasing melting temperature. In parallel to the decreasing core size the amount of metal undergoing the phase transition would decrease. This could explain the slight decrease of the peak area in Fig. 8 with thermal cycling. The second effect was the possible agglomeration of the nanoparticles during the melting in the former measurement cycles, which would increase the real particle size. An increasing particle size would lead to an increasing melting temperature at constant peak area. According to the present repeated DSC measurements, it seems that slight oxidation dominated the experiments. But the results also revealed that the beginning of melting of the nanoparti-
Fig. 7. SEM image of the nanoparticles of SnAgCu alloy prepared by CDCA technique with arc current of 50 A. The inset shows the size distribution histogram of the nanoparticles.
Fig. 8. DSC curves for nanoparticles of SnAgCu alloy prepared with current value of 50 A heated at 30 ◦ C/min for five cycles. The vertical dashed lines correspond to the expected melting temperatures according Eq. (1) for the particle diameters given on top. Curves are displaced vertically for clarity.
cles of the SnAgCu solder alloy could be decreased to the equivalent value of the traditionally used SnPb eutectic alloy, which was the aim of the present study. For the nanoparticles produced by the two different arc currents a broad melting range is observed. It spans from temperatures comparable with the melting temperature of SnPb eutectic solder alloy up to the bulk melting temperature of the SnAgCu alloy. In Fig. 8 even a small peak at the equilibrium melting temperature is present. According the particle size distribution one would expect a maximum around 212 ◦ C in the DSC curves, which is not present in Fig. 8. The maximum at about 205 ◦ C corresponds to a particle size of about 20 nm. Also the observed peak onset temperature of about 195 ◦ C and 180 ◦ C in Figs. 6 and 8, respectively, cannot be explained with the particle size distribution. Either the metallic core of the particles is significantly smaller than the outer size of the particles or there are inner structures present yielding a lower melting temperature. HRTEM was employed to detect the detailed structure and morphology of the nanoparticles. Fig. 9 shows the HRTEM image of one as-prepared nanoparticle prepared under the arc current of 50 A. It was interesting to notice that several particles with different phase structures formed agglomerates. The XRD result shown in Fig. 4 indicates that the nanoparticles had a structure of Ag3 Sn. The HRTEM structure analysis also proved the presence of the Ag3 Sn phase. In particular, the HRTEM image demonstrated that the single nanoparticle observed by SEM was actually a polyparticle or a polycrystal. That is to say, the seeming “single nanoparticle” observed by SEM were possibly agglomerated particles or crystals. Whether the inner interfaces cause some premelting or the effective particle size is smaller than that seen in the SEM pictures is not known yet. Both effects may explain the low melting temperatures seen in Figs. 6 and 8, which are not completely in accordance with the expected values from the size distributions shown in Figs. 5 and 7. Another explanation of the low melting temperature could be a reduction of the metallic core size of the nanoparticle due to some oxide layer. The curves in Fig. 8 show a change in shape as well as a reduction of the peak area. Changes in curve shape could be explained by changes in the internal structure of the particles, e.g. reduction of internal surface area. But such processes
Y. Gao et al. / Journal of Alloys and Compounds 484 (2009) 777–781
781
work does offer the direct evidences of the feasibility to decrease the melting temperature of Sn–3.0Ag–0.5Cu (wt.%) sloder alloy to the equivalent temperature of the traditional SnPb eutectic solder alloy without significant problems due to oxidation of the nanoparticles at ambient conditions. The CDCA technique, suitable for most electrical conducting materials, showed high potential in manufacturing large amounts of nanoparticles with controlled shape, small size, narrow particle size distribution and nearly oxide-free composition. This undoubtedly puts forward a novel feasible approach to manufacture high-grade electronic products using lead-free solders. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.50571057), AM Foundation of STCSM (Grant No. 08520740500) and Robert Bosch Foundation (Grant No. 32.5.8003.0025.0/MA01). Liu acknowledges the Swedish National Science Foundation support (Grant No. 621-2007-4660). References
Fig. 9. HRTEM image of one as-prepared SnAgCu nanoparticle, arc current 50 A.
cannot explain the decrease in peak area. Oxidation seems to be a more plausible explanation for the observed changes. The smallest particles resulting in the lowest melting temperatures may be completely oxidized and disappear in the melting curves, as seen in Fig. 8. At the same time less metal is present and the entire peak decreases. Assuming an oxide layer of about 5 nm would explain the shift in the peak maximum in Fig. 8 and seems to be reasonable. 4. Conclusions Broad endothermic peaks were observed from the DSC measurements (Figs. 6 and 8) of nanoparticles produced from a Sn–3.0Ag–0.5Cu (wt.%) master alloy with a consumable-electrode direct current arc (CDCA) technique. A fraction of the produced nanoparticles showed a melting temperature similar to that of eutectic SnPb solder. Most interestingly, they partly survived several melting crystallization cycles, indicating that the oxidation of the nanoparticles was continued slowly but the liquid particles did not totally agglomerate even under a series of heating and cooling measurements. The data indicate a broad size distribution of the nanoparticles, which melt step by step. Nanoparticles not melted at the anticipated low processing temperature may cause difficulties in the soldering process. Notwithstanding these limitations, this
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Nanoparticles of SnAgCu lead-free solder alloy with an equivalent melting temperature of SnPb solder alloy Yulai Gao a,∗ , Changdong Zou a , Bin Yang a , Qijie Zhai a , Johan Liu b,c , Evgeny Zhuravlev d , Christoph Schick d a
Shanghai Key Laboratory of Modern Metallurgy & Materials Processing, Shanghai University, Yanchang Road 149, Shanghai 200072, PR China Bionano Systems Laboratory, Department of Microtechnology and Nanoscience, MC2, Chalmers University of Technology, SE - 412 96, Göteborg, Sweden SMIT Center, School of Mechatronics and Automation, Shanghai University, Yanchang Road 149, Shanghai 200072, PR China d Institute of Physics, University of Rostock, Universitätsplatz 3, 18051 Rostock, Germany b c
a r t i c l e
i n f o
Article history: Received 6 April 2009 Received in revised form 4 May 2009 Accepted 9 May 2009 Available online 18 May 2009 Keywords: Lead-free solder Nanoparticles Melting temperature depression Arc technique SnAgCu
a b s t r a c t Nanoparticles were prepared with a consumable-electrode direct current arc (CDCA) technique. The results showed that the calorimetric melting onset temperature of the nanoparticles of SnAgCu solder alloy could be as low as 179 ◦ C, equivalent to that of the traditionally used SnPb eutectic alloy (183 ◦ C). Moreover, the homogenous melting model (HMM) and Gibbs–Thomson equation were employed to theoretically estimate the size-dependent melting temperature of the as-prepared nanoparticles. The structure and morphology of the nanoparticles were analyzed with a high-resolution transmission electron microscopy (HRTEM). The CDCA technique showed promising prospect in manufacturing large amounts of nanoparticles with controlled shape, small size, narrow particle size distribution and nearly oxide-free composition. This undoubtedly puts forward a novel feasible approach to manufacture high quality lead-free solders for electronic products. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Electronic packaging is a manufacturing technology used for electronic products. Packaging provides a medium for electronic interconnections and mechanical support, and solder alloys provide the electrical and mechanical connections between the die (chip) and the bonding pads. The selection of materials for solder alloys is, therefore, critical and plays an important role in solder joint reliability. The Sn–Pb alloy has been the most widely used solder alloy system as an interconnection material in the electronic packaging industry. The Pb-containing alloys are reliable, well tested and quite inexpensive, and Sn–Pb alloys over the whole composition range can be used as solders. There are, however, disadvantages with Pbcontaining solders. Apart from the undeniable toxicity of damaging human’s nervous system, Pb is also harmful to the environment by causing ground water contamination. As a result, Pb-containing solder alloys are either being banned or phased out from the electronic industry. Legislations, such as the Waste from Electrical and Electronic Equipment (WEEE) and the Restriction of the use of Hazardous Substances (RoHS) have been implemented in the European
∗ Corresponding author. Tel.: +86 21 56332144. E-mail address: [email protected] (Y. Gao). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.042
Union since 2006. Therefore, an urgent necessity exists for more appropriate substitutes for the Sn–Pb solders. It is also important to ensure that the physical and mechanical properties of the substitutes are comparable or even superior to those of Sn–Pb solder. From a manufacturing point of view, the melting temperature of the solder alloy is a crucial factor that has to be taken into account in order to achieve high package quality. A large variety of lead-free solders have already been developed, mainly involving the Sn–Cu, Sn–Ni and Sn–Ag systems [1]. These new lead-free solders have been identified as the most promising alternatives to the eutectic Sn–Pb solder. However, the higher melting temperatures of these alloys, comparing to that of the eutectic Sn–Pb solder, limit their applications in the electronic industry. For that matter, new methods have been explored to decrease the melting temperature of solder alloys. In 1954, Takagi carried out the first experimental investigation dealing with the effect of size on the melting of very small particles [2]. Utilizing electron microscope, Sambles observed the lowering of the melting point of the gold particles [3]. In 1976, Buffat also described that the melting temperature could be depressed by decreasing the particle size to nanometer scale [4]. Later, nanocalorimetry was employed to characterize the melting temperature of a small number of nanoparticles [5,6], and hot stage transmission electron microscopy was used to observe the melting behavior of isolated nanoparticles
778
Y. Gao et al. / Journal of Alloys and Compounds 484 (2009) 777–781
[7]. Wong et al. reported the size-dependent melting temperature depression of Sn nanoparticles [8] by traditional differential scanning calorimetry (DSC). They also reported size-dependent melting temperature depression of Sn–3.5Ag nanoparticles synthesized by a chemical reduction method [9]. However, the production rate of nanoparticles was limited by this method. Hsaio and Duh manufactured and studied nanoparticles of Sn–3.5Ag–xCu (x = 0.2, 0.5, 1.0), synthesized for lead-free solder applications [10]. They did not, however, observe any obvious melting temperature depression in their investigation. When the particle size was further decreased until to infinitesimal size, e.g. in which only tens or hundreds of atoms were contained, an anomalous size dependence of the melting temperature was observed and it was deemed that not only the cluster size but also structural features governed their correlations [11]. Nevertheless, no such correlation has been observed for the particle size scale of interest for solder applications. Banhart et al. [12] observed that the melting temperature of Sn and Pb nanocrystals might be increased to a value even higher than the corresponding bulk alloy if exterior shells exist. Embedded in some matrix, some nanoparticles revealed even opposite changing tendency of the melting temperatures [13–16], indicating that the exterior structure of nanoparticles played a significant role in their melting behavior. Premelting, which occurs before the whole melting of the clusters or nanoparticles, is easily produced near crystal defects [17]. Except the large specific surface area, the presence of large interfaces in nanoparticles was another crucial factor to influence the melting temperature and should be taken into account [18]. Pusey and co-workers [19,20] observed that premelting at a lower temperature was produced along grain boundaries before complete melting of the crystalline colloids. Riegler and Kohler [21] also thought that the interfacial properties would strongly affect the phase transition behavior of the small particles due to their large interfaces, and premelting would occur on surrounding interfaces. However, little information was available about grain boundary premelting of metals due to their high temperature and the resultant difficulty for simultaneous observations. Fortunately, the recently developed coherent X-ray diffraction imaging technique might be a possible approach to study the premelting of nanocrystal interfaces owing to its diffraction patterns and latest third-generation synchrotron radiation sources to obtain full three-dimensional images [22]. In contrast to the lower melting temperature (183 ◦ C) of SnPb eutectic solder alloy [23], therefore, the lead-free solder alloys usually cause a raised temperature of about 30–40 ◦ C during electronic assembling. And the increased temperature reduces the integrity, reliability and functionality of printed wiring boards, components and other attachment [24]. As a result, much effort has been put lately on finding possible ways to solve some of the issues imposed by lead-free solders. Lin et al. employed the supernatant process to obtain the Sn–3.5Ag solder alloy nanoparticles, yet no obvious melting temperature depression was observed possibly due to the aggregation of those nanoparticles [25]. Therefore, it remains an urgent necessity to conduct an indepth study of feasible processes to lower the melting temperature of the currently used lead-free solder alloy systems. 2. Experimental procedures The lead-free master alloy with a nominal composition of Sn–3.0Ag–0.5Cu (wt.%) was prepared by an induction melting method. Fig. 1 shows the DSC melting curve of the master alloy. The melting temperature was 217.8 ◦ C, which equals the equilibrium melting temperature. Integration of the peak yields 67 J/g for the heat of fusion. Several models described the size dependence of the melting temperature of nano-sized particles. Generally it is assumed that the melting is initiated by a continuous vibrational lattice instability on the solid surface or in a solid-solid interface [18]. For nano-scale particles with large surfaces, the instability becomes obvious
Fig. 1. DSC curve of the SnAgCu bulk alloy at the heating rate of 30 ◦ C/min.
and usually the nanoparticles will lose stability at lower temperature than their bulk precursor. The size-dependent melting temperature of nanoparticles could be estimated using the homogenous melting model (HMM) [4]. This model was established based on the assumption that the solid and liquid particles of the same mass are in equilibrium with their common vapor. The free energy of a solid particle with a radius r is modified by an extra term: 2V/r, where , V and r are the surface tension, mole volume and radius of the particle, respectively. In other words, increasing the free energy of the particles causes depression of the melting temperature. The size-dependent melting temperature of nanoparticles is then given by the following equation: Tm (r) =
bulk Tm
−
bulk + 273.15) 2(Tm
Hfbulk s r
2/3
sv − lv
s l
(1)
where r represents the radius of a spherical particle and Tm (r) is its melting temperbulk is the melting temperature of the bulk alloy, Hfbulk is ature (in degree Celsius), Tm the heat of fusion for the bulk alloy, s and l are the solid and liquid phase densities of the bulk alloy, and sv and lv are the solid–vapor and liquid–vapor interfacial energies, respectively. The melting temperature depression for small crystals can also be evaluated by the Gibbs–Thomson equation [26]: bulk − Tm (r) = Tm
bulk + 273.15)sl 2(Tm
(2)
Hfbulk s r
where sl is the solid–liquid interfacial energy. For many metals, sl ≈ sv − lv [27]. As a matter of fact, if the value of s was deemed the same as that of l , then sl = sv − lv [28]. That is to say, under these conditions the HMM and Gibbs–Thomson equations could be expressed by the same formula. The melting temperature and the heat of fusion of the bulk master alloy were obtained by the DSC measurement shown in Fig. 1. However, some necessary thermodynamic parameters of Sn–3.0Ag–0.5Cu alloy were unavailable, thus those of the similar Sn–3.5Ag (wt.%) system were used [23]. The data for calculation are listed in Table 1, and the dependence of melting temperature on nano-sized particles was calculated by the HMM and Gibbs–Thomson equation, as shown in Fig. 2. The solid curve denotes the melting temperatures calculated by the HMM and Gibbs–Thomson equations, the upper and lower horizontal dashed lines correspond to the bulk melting temperatures of SnAgCu alloy and SnPb eutectic solder, respectively. According to Fig. 2, particles of about 10 nm diameter should show a melting temperature comparable to that of SnPb eutectic solder (183 ◦ C). Nanoparticles were prepared from the master alloy with a recently developed consumable-electrode direct current arc (CDCA) technique as shown schematically in Fig. 3. Liquid paraffin was chosen as a dielectric protection media. The anode and cathode, with a diameter of 6 mm, were made from the master alloy by means of a suction casting technique. The anode and the cathode electrodes were fixed inside the CDCA container and connected to a power supply. When the two electrodes get close enough, an arc discharge could be produced between them, causing a local melting and breakdown of the solder material into nanoparticles. The obtained particles were rinsed with chloroform for several times to remove the liquid paraf-
Table 1 Data used in the calculation of size-dependent melting temperatures [23]. bulk ◦ Tm ( C)
Hfbulk (J/g)
s (g/cm3 )
sv (J/m2 )
lv (J/m2 )
sl = sv – lv (J/m2 )
217.8
67
7.39
0.51
0.43
0.082
Y. Gao et al. / Journal of Alloys and Compounds 484 (2009) 777–781
Fig. 2. Melting temperature dependence of SnAgCu nanoparticles on particle diameter. fin. Furthermore, the rinsed particles were centrifugally separated at 4000 rpm for 45 min to remove the possibly existed large particles. The phases of the prepared nanoparticles were examined on a Rigaku D/Max2200 X-ray diffractometer (XRD). The morphology and size distribution of the nanoparticles were revealed by means of JSM-6700F type field-emission scanning electron microscopy (FE-SEM). The melting temperatures of the bulk master alloy and the nanoparticles were measured with PerkinElmer Pyris 1 differential scanning calorimeter (DSC) under the protection of high-purity nitrogen. The detailed structure and morphology of the nanoparticles was further studied with JEM-2010F type high-resolution transmission electron microscopy (HRTEM).
3. Results and discussion Unlike the chemical reduction method applied by Wong and co-workers [9,10], the CDCA technique is suitable for most conducting materials, and the manufacturing process could be continuous allowing preparation of lead-free solder nanoparticles at high production rates. During the preparation of the nanoparticles, discharge and solder alloy breakdown took place inside the dielectric media, protecting the nanoparticles from serious oxidation. The XRD pattern of the as-prepared nanoparticles is shown in Fig. 4. The Sn, Ag3 Sn and Cu6 Sn5 phases were detected, implying that a good alloying was produced during the particle preparation process. In addition, no oxide traces were found by the XRD measurement, indicating that the nanoparticles were not significantly oxidized. Arc current is one of the crucial parameters to control nanoparticle formation, especially the particle size and size distribution. The morphology and sizes of the prepared nanoparticles at 20 A arc current were obtained from SEM images, as shown in Fig. 5. The nanoparticles were in spherical shape on the whole, but particle agglomeration was also detected. The size distribution of the
Fig. 3. Schematic of the CDCA setup for nanoparticles preparation (1) cathode, (2) anode (connected to a high current and low voltage power source), (3) and (4) bulk alloy electrodes, (5) arc discharge taking place between the electrodes, and (6) dielectric coolant.
779
Fig. 4. XRD pattern of the nanoparticles prepared by CDCA technique under the protection of liquid paraffin.
nanoparticles was determined by image analysis with Image-Pro plus software, and the size distribution histogram is shown in the inset of Fig. 5. It was found that the size of most particles was in the range 15–60 nm with an average of about 30 nm. The melting temperature of the aforementioned nanoparticles was determined with DSC at a heating rate of 30 ◦ C/min. To reduce the possible experimental error, the DSC measurement was repeated for three cycles, as shown in Fig. 6. It was found that the melting peak was shifted to lower temperatures and broadened. The peak onset temperature was decreased to about 197 ◦ C, 20 ◦ C lower than that of the bulk master alloy. Increasing the arc current to 50 A led to a significantly narrower size distribution of the nanoparticles, as shown in Fig. 7. The particle size was concentrated in the range 25–42 nm, again with an average of about 30 nm. The nanoparticles prepared with an arc current of 50 A were measured in the DSC at heating rate 30 ◦ C/min for five cycles, presented in Fig. 8. The results show that the melting onset temperature is lowered below 190 ◦ C and remained nearly unchanged even after repeated heating–cooling cycles. Due to the large specific surface area of the nanoparticles, it was hard to avoid oxidation dur-
Fig. 5. SEM image of the nanoparticles of the SnAgCu alloy prepared by CDCA technique at 20 A arc current. The inset shows the size distribution histogram of the nanoparticles.
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Y. Gao et al. / Journal of Alloys and Compounds 484 (2009) 777–781
Fig. 6. DSC curves for nanoparticles of SnAgCu alloy prepared with current value of 20 A heated at 30 ◦ C/min for three cycles. The vertical dashed lines correspond to the expected melting temperatures according Eq. (1) for the particle diameters given on top. Curves are displaced vertically for clarity.
ing preparation, storage and the measurement. That is to say, some oxidation layer was generally present on the particle surface. As a result, two opposite effects are expected during these repeated DSC measurements. The first effect was the further oxidation of the nanoparticles, which would decrease the size of the metallic core in the nanoparticles. Oxidation causes a continuous size reduction of the metal core and therefore a further decreasing melting temperature. In parallel to the decreasing core size the amount of metal undergoing the phase transition would decrease. This could explain the slight decrease of the peak area in Fig. 8 with thermal cycling. The second effect was the possible agglomeration of the nanoparticles during the melting in the former measurement cycles, which would increase the real particle size. An increasing particle size would lead to an increasing melting temperature at constant peak area. According to the present repeated DSC measurements, it seems that slight oxidation dominated the experiments. But the results also revealed that the beginning of melting of the nanoparti-
Fig. 7. SEM image of the nanoparticles of SnAgCu alloy prepared by CDCA technique with arc current of 50 A. The inset shows the size distribution histogram of the nanoparticles.
Fig. 8. DSC curves for nanoparticles of SnAgCu alloy prepared with current value of 50 A heated at 30 ◦ C/min for five cycles. The vertical dashed lines correspond to the expected melting temperatures according Eq. (1) for the particle diameters given on top. Curves are displaced vertically for clarity.
cles of the SnAgCu solder alloy could be decreased to the equivalent value of the traditionally used SnPb eutectic alloy, which was the aim of the present study. For the nanoparticles produced by the two different arc currents a broad melting range is observed. It spans from temperatures comparable with the melting temperature of SnPb eutectic solder alloy up to the bulk melting temperature of the SnAgCu alloy. In Fig. 8 even a small peak at the equilibrium melting temperature is present. According the particle size distribution one would expect a maximum around 212 ◦ C in the DSC curves, which is not present in Fig. 8. The maximum at about 205 ◦ C corresponds to a particle size of about 20 nm. Also the observed peak onset temperature of about 195 ◦ C and 180 ◦ C in Figs. 6 and 8, respectively, cannot be explained with the particle size distribution. Either the metallic core of the particles is significantly smaller than the outer size of the particles or there are inner structures present yielding a lower melting temperature. HRTEM was employed to detect the detailed structure and morphology of the nanoparticles. Fig. 9 shows the HRTEM image of one as-prepared nanoparticle prepared under the arc current of 50 A. It was interesting to notice that several particles with different phase structures formed agglomerates. The XRD result shown in Fig. 4 indicates that the nanoparticles had a structure of Ag3 Sn. The HRTEM structure analysis also proved the presence of the Ag3 Sn phase. In particular, the HRTEM image demonstrated that the single nanoparticle observed by SEM was actually a polyparticle or a polycrystal. That is to say, the seeming “single nanoparticle” observed by SEM were possibly agglomerated particles or crystals. Whether the inner interfaces cause some premelting or the effective particle size is smaller than that seen in the SEM pictures is not known yet. Both effects may explain the low melting temperatures seen in Figs. 6 and 8, which are not completely in accordance with the expected values from the size distributions shown in Figs. 5 and 7. Another explanation of the low melting temperature could be a reduction of the metallic core size of the nanoparticle due to some oxide layer. The curves in Fig. 8 show a change in shape as well as a reduction of the peak area. Changes in curve shape could be explained by changes in the internal structure of the particles, e.g. reduction of internal surface area. But such processes
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work does offer the direct evidences of the feasibility to decrease the melting temperature of Sn–3.0Ag–0.5Cu (wt.%) sloder alloy to the equivalent temperature of the traditional SnPb eutectic solder alloy without significant problems due to oxidation of the nanoparticles at ambient conditions. The CDCA technique, suitable for most electrical conducting materials, showed high potential in manufacturing large amounts of nanoparticles with controlled shape, small size, narrow particle size distribution and nearly oxide-free composition. This undoubtedly puts forward a novel feasible approach to manufacture high-grade electronic products using lead-free solders. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.50571057), AM Foundation of STCSM (Grant No. 08520740500) and Robert Bosch Foundation (Grant No. 32.5.8003.0025.0/MA01). Liu acknowledges the Swedish National Science Foundation support (Grant No. 621-2007-4660). References
Fig. 9. HRTEM image of one as-prepared SnAgCu nanoparticle, arc current 50 A.
cannot explain the decrease in peak area. Oxidation seems to be a more plausible explanation for the observed changes. The smallest particles resulting in the lowest melting temperatures may be completely oxidized and disappear in the melting curves, as seen in Fig. 8. At the same time less metal is present and the entire peak decreases. Assuming an oxide layer of about 5 nm would explain the shift in the peak maximum in Fig. 8 and seems to be reasonable. 4. Conclusions Broad endothermic peaks were observed from the DSC measurements (Figs. 6 and 8) of nanoparticles produced from a Sn–3.0Ag–0.5Cu (wt.%) master alloy with a consumable-electrode direct current arc (CDCA) technique. A fraction of the produced nanoparticles showed a melting temperature similar to that of eutectic SnPb solder. Most interestingly, they partly survived several melting crystallization cycles, indicating that the oxidation of the nanoparticles was continued slowly but the liquid particles did not totally agglomerate even under a series of heating and cooling measurements. The data indicate a broad size distribution of the nanoparticles, which melt step by step. Nanoparticles not melted at the anticipated low processing temperature may cause difficulties in the soldering process. Notwithstanding these limitations, this
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