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SnAgCu lead-free solders
Materials Science & Engineering A 667 (2016) 87–96
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Design and performance of Ag nanoparticle-modified graphene/ SnAgCu lead-free solders Lianyong Xu a,b, Xi Chen a,b, Hongyang Jing a,b, Lixia Wang a,b, Jun Wei c, Yongdian Han a,b,n a
School of Materials Science and Engineering, Tianjin University, Tianjin 300354, PR China Tianjin Key Laboratory of Advanced Joining Technology, Tianjin 300354, China c Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore b
art ic l e i nf o
a b s t r a c t
Article history: Received 12 February 2016 Received in revised form 26 April 2016 Accepted 27 April 2016 Available online 28 April 2016
In this study, various weight percentages of Ag-nanoparticle-modified graphene (Ag-GNSs) were incorporated into Pb-free Sn-Ag-Cu (SAC) solder matrices via ball milling and mechanical mixing methods to form composite solders. Up to 0.2 wt% of the Ag-GNSs particles was successfully incorporated into the composite solders, and the microstructures and mechanical properties of the solders were investigated. The experimental results demonstrated that as the Ag-GNSs content increased, the number of intermetallic particles gradually decreased in size. In addition, the uniformity of the interfacial structure increased compared to that of the pure SAC solder. Furthermore, the melting points of the SAC/Ag-GNSs composite solders were unchanged compared to that of the pure SAC solder. The experimental results also revealed that the wettability of the composite solders on Cu substrates was greatly improved. Thermomechanical measurements showed that the thermal stability of the composite solders was also improved. In addition, tensile tests showed that the ultimate tensile strength of the composite solders was enhanced compared to that of the pure SAC solder, but the ductility of the composite solders was reduced. In particular, the tensile strength of the composite solders with low Ag-GNSs contents was increased, but the tensile strength then decreased once a critical Ag-GNSs content was reached. For the ball-milled solders, the reinforcement provided by the addition of Ag-GNSs particles was much better than that provided by unmodified graphene. By comparing the properties of solders fabricated with different preparation methods, it was determined that the ball-milled solders were superior to the mechanically mixed solders. & 2016 Elsevier B.V. All rights reserved.
Keywords: Composite solders Microstructure Mechanical testing Ball milling method
1. Introduction Sn-Pb eutectic solders have been widely used in the electronic packaging industry for a number of years [1,2]. However, because of the toxic nature of Pb, the use of Pb-based solders has been banned in many industrial applications. Therefore, there has been a large amount of research focused on developing environmentally friendly Pb-free alloys to replace Pb-Sn solder, especially in the electronic packaging industry. Currently, there are many promising Pb-free solder alternatives, such as Sn-Ag-Cu (SAC) alloys. Even though a number of different near-eutectic SAC alloys are widely used, they have some disadvantages when compared to Pb-Sn alloys, including higher melting points and poor mechanical-shock resistance [3–8]. There are several secondary phases, such as metallic elements, intermetallics, or inter second phases, that are currently used to n
Corresponding author. E-mail address: [email protected] (Y. Han).
http://dx.doi.org/10.1016/j.msea.2016.04.084 0921-5093/& 2016 Elsevier B.V. All rights reserved.
reinforce conventional SAC solders. The effects of reinforcement on the various properties of SAC solders have been widely investigated. Lin et al. [9] added nano-TiO2 and nano-Cu powders to traditional Sn-Pb solders to obtain nanocomposite Pb-based solders. Liu et al. [10] reinforced a SAC solder with Ti, one of the most active transition metals, which yielded a superior drop-test performance because of the reduced elastic modulus of the composite solder. The addition of Ti also improved the dramatic suppression of the undercooling of SAC solders and reduced the growth of interfacial intermetallic compounds (IMCs). Han et al. [11] used Ni-coated carbon nanotubes (CNTs) to reinforce SAC solder joints. The interfacial microstructures and shear strength of the composite solders were improved compared to that of the pure SAC solder, even after 2000 thermal cycles. Graphene, which was discovered in 2004 [12], is the hardest nanomaterial to date, and it is known for its excellent conductivity and thermal and mechanical properties. Several studies have shown that reinforcing conventional solder alloys with graphene improves the overall material performance [11]. However, there are problems with graphene-reinforced SAC solders that must be
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solved, such as the difficulty in obtaining a uniform graphene distribution in the solder matrix and its poor bonding strength with metal substrates. Thus, modifying graphene with Ag nanoparticles (Ag-GNSs) can lead to improved interactions between the Ag-GNSs particles and Sn matrix. Improving the graphene-Sn interactions is essential for ensuring an effective load transfer across the interface, which would significantly improve the properties of graphene-reinforced SAC solders. The methods used to add reinforcements and fabricate composite solders are primarily mechanical alloying and mechanical mixing. Tao et al. [13] prepared Au-Sn eutectic solders via mechanical alloying by using high-energy ball milling equipment, and studied the effect of the milling time on the microstructure of the solders. Nai et al. [14] mechanically mixed a solder and reinforcing agent to obtain a composite solder. Xu et al. [15] used mechanical mixing to add CNTs to a Sn3.5Ag0.5Cu solder, which improved the mechanical properties of the solder. They reported that some of the solder particles underwent deformation after the mixing process, which decreased both the overall particle size and the uniformity of the particle-size distribution. On the other hand, the prolonged agitation of the particles was reduced because of the aggregation caused by van der Waals forces, which produced a uniform distribution of CNTs within the solder matrix. The typical melting point of SAC solders is approximately 183 °C, and most brazing equipment is designed to use 183 °C as the standard temperature. For electronic packaging materials, the maximum temperature that the most commonly used substrates can be subjected to is 230 °C. If the melting point of the solder alloy is too high, it can damage the substrate and adversely affect the performance of the electronic components. Hence, adjusting the melting point of new composite solders to avoid damaging electronic packaging materials could be an effective cost-saving measure. Furthermore, the microstructure of the Ag-GNSs/solder matrix interface must be examined. This is because the main factor that determines the tensile strength of a solder is the uniformity of the interface morphology, which plays an important role in the reliability of solder joints. Therefore, the coefficient of thermal expansion (CTE), wetting angle, and tensile strength of a solder are crucial factors for predicting the reliability of solder joints. In this study, Ag-GNSs/SAC composite solders were successfully prepared via ball milling and mechanical mixing methods. The microstructures of the composite solders were characterized with scanning electron microscopy (SEM). In addition, the wettability of the solder joints, and tensile and thermomechanical properties of
the Ag-GNSs/SAC solders were investigated. The behavior of the composite solder joints was compared to that of the pure SAC solder in order to demonstrate the outstanding properties of the new Ag-GNSs/SAC composite solders.
2. Experimental procedures 2.1. Material processing A 96.5Sn-3.0Ag-0.5Cu powder (particle sizes ¼25–45 mm, as shown in Fig. 1(a)) was used as the solder matrix and Ag-GNSs particles were used as the reinforcement phase. Fig. 1(b) shows an TEM image of the Ag-GNSs particles. Composite solders with various Ag-GNSs contents (0, 0.03, 0.05, 0.10, and 0.20 wt%) were synthesized, which will hereinafter be referred to as SAC, SAC/0.03Ag-GNSs, SAC/0.05Ag-GNSs, SAC/ 0.1Ag-GNSs, and SAC/0.2Ag-GNSs, respectively. The performance of the reinforced solders was investigated as a function of the Ag-GNSs content. In addition, the performance of the SAC/0.05Ag-GNSs solder was compared to an equivalent solder that was only reinforced with graphene (i.e., no Ag nanoparticles). Finally, two different SAC/0.05Ag-GNSs solders were prepared by ball milling and mechanical mixing to investigate the effects of the fabrication process on the resulting solders. For convenience, Table 1 lists all of the samples fabricated in this study; a “Q” in the sample label indicates the sample was ball milled, whereas an “H” indicates the sample was mechanically mixed. Firstly, mixtures of the solder powder and Ag-GNSs particles were pre-weighted to obtain the desired solder compositions. Each composite solder mixture was then added to a sealed ballmilling jar that contained the milling medium (stainless steel ball) and a certain volume of ethanol. The mixtures were milled with a planetary ball mill at 300 rpm for 5 h in a high-purity Ar gas atmosphere to ensure that the solder matrix and reinforcement phase were thoroughly mixed. The milled mixture was then placed in a stainless steel mold (diameter¼ 20 mm), uniaxially compacted with a hydraulic press at 500 MPa, and sintered at 175 °C for 2 h in an inert atmosphere. Finally, the billet was extruded at room temperature into a 6-mm-diameter rod. The SAC solder was fabricated according to the method outlined above, but the blending process was omitted. The mechanical mixing method used to prepare the composite solder with a Ag-GNSs content of 0.05 wt% was as follows. The
Fig. 1. SEM images of (a) SAC powder and TEM images of (b) Ag-GNSs powders.
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and three samples were tested for each composite solder.
Table 1 Abbreviation of the composite solders in this paper. Solder Matrix
Reinforcement Reinforcement content
Sample label
96.5Sn-3.0Ag0.5Cu
N/A Ag-GNSs
SAC QSAC/0.03AgGNSs QSAC/0.05AgGNSs QSAC/0.1Ag-GNSs QSAC/0.2AgGNSs HSAC/0.05AgGNSs QSAC/0.05GNSs
N/A 0.03 wt% 0.05 wt% 0.1 wt% 0.2 wt% 0.05 wt%
GNSs
0.05 wt%
89
2.6. Tensile tests Tensile tests were performed according to the ASTM E8M-04 standard for material testing. The tensile strength and elongation of circular samples (diameter¼4 mm, length ¼ of 2.5 mm) were obtained with an electric tensile-testing machine, which was operated at room temperature with a crosshead speed of 2 mm/min. The fracture surface of each sample was then observed via SEM.
3. Results and discussion 3.1. Microstructural analysis
composite solder mixture was blended for 12 h in a V-blender at a speed of 60 rpm. After the mechanical mixing process, the remainder of the fabrication process was the same as that of ballmilled samples. 2.2. Microstructural analysis The samples were mounted in epoxy and then ground, polished, and etched for a few seconds with a methanol solution containing 8 vol% of hydrochloric acid. The microstructures of the samples were observed via SEM (HITACHI FE-SEM 4800). In addition, the SEM set-up was capable of energy-dispersive X-ray spectroscopy (EDS). 2.3. Melting point measurements The melting points of the solders were determined via differential scanning calorimetry (DSC; TA Instruments). The DSC samples, which were composed of pieces of solder randomly sliced from the extruded rods, were placed in an Ar-filled Al pan that was heated to 250 °C at a heating rate of 10 °C/min. 2.4. Wettability measurements The wetting angles of the solders were measured on Cu substrates to determine their wettability. The methods used were as follows: (1) a 1.5-mm-thick disc was cut from the extruded composite solder. (2) The polished Cu substrate and solder disc were placed on a heating stage and heated to 250 °C. (3) The samples were cooled in air until the solder had completely spread on the Cu substrate. (4) A cross section of the sample was observed with a stereoscopic microscope and the contact angle was measured; a schematic diagram showing the contact angle (θ) of the solder sample on the Cu substrate is shown in Fig. 2. These steps were repeated three times for each composite solder to obtain the average θ value. 2.5. Thermomechanical analysis The CTEs of the extruded samples were obtained by measuring the displacement of the samples as a function of the temperature over a temperature range of 50–120 °C. An automated TMA 2940 thermomechanical analyzer was used for these measurements,
Fig. 2. Wetting angle.
The microstructures of the QSAC/Ag-GNSs solders are shown in Figs. 3(a-e). Image Pro Plus software was used to calculate the average particle size of the resulting IMCs; the results are listed in Table 2. For the ball-milled samples, as the Ag-GNSs content increases, the average IMC particle size gradually decreases. However, when the Ag-GNSs content reaches 0.2 wt%, the average IMC particle size increases slightly. In addition, Table 2 shows that the IMC particle size of the QSAC/0.05Ag-GNSs solder is 8.9% smaller than that of the QSAC/0.05GNSs solder and 36.6% smaller than the SAC samples, implying that the presence of Ag-GNSs particles inhibits grain growth, which results in the formation of finer IMC grains compared to that of the pure SAC solder. Furthermore, Table 2 shows that the QSAC/0.05Ag-GNSs solder has smaller IMC particles than the HSAC/0.05Ag-GNSs solder. Fig. 4 shows SEM images with the corresponding EDS analysis of the SAC solder and its nanocomposite solder. There are three phases present in the EDS spectrum of the nanocomposite solder, namely β-Sn, Ag3Sn, and Cu6Sn5 [16,17], and the composition is similar to what has been reported by previous studies. The light gray, fine grains correspond to Ag3Sn, while the dark gray, bulk grains correspond to a mixture of Ag3Sn and Cu6Sn5. The SEM image shows that the Ag3Sn and Cu6Sn5 particles are dispersed throughout the β-Sn matrix. As the Ag-GNSs content increases, the uniformity of the IMC phases gradually increases. The addition of Ag-GNSs not only slows the growth of the IMC phases, but it also makes the growth more even. Thus, the IMC phases of the composite solders are more uniform than that of the pure SAC solder. These observations can be explained by the growth thermodynamics and kinetics of the IMC phases [18,19]. There are two IMCs dispersed throughout the solder matrix: Ag3Sn and Cu6Sn5. Because of the inherent characteristics of IMCs, such as higher melting points and hardnesses, even distributions of Ag3Sn and Cu6Sn5 grains can reinforce the β-Sn matrix. The crystallization of Cu6Sn5 and Ag3Sn grains is a nucleation and growth process, which requires energy. This energy is provided by the surface energy and interface energy between the solder matrix and the IMC particles. The solder matrix and Ag-GNSs particles are thoroughly mixed during the ball milling process, with the latter becoming embedded in the surface of the solder particles through high-speed collisions, which creates excellent physical connections. The Ag-GNSs particles are uniformly distributed throughout the solder matrix, and these particles provide sites for the nucleation of Cu6Sn5 and Ag3Sn grains. Thus, the distribution of the Ag-GNSs particles within the solder matrix determines the distribution of IMC particles. Therefore, because the distribution of Ag-GNSs particles within the composite solder is uniform, the nucleation and growth of IMC particles is also uniform [20,21]. However, while the growth of IMC phases is possible, they must overcome the interfacial free energy of the system. The Ag-GNSs
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Fig. 3. Microstructure of composite solders: (a) SAC, (b) QSAC/0.03Ag-GNSs, (c) QSAC/0.05Ag-GNSs, (d) QSAC/0.1Ag-GNSs, (e) QSAC/0.2Ag-GNSs, (f) QSAC/0.05GNSs, (g) HSAC/0.05Ag-GNSs.
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Table 2 average IMC particle size of the composite solders. Sample
Average IMC size (μm)
SAC QSAC/0.03Ag-GNSs QSAC/0.05Ag-GNSs QSAC/0.1Ag-GNSs QSAC/0.2Ag-GNSs QSAC/0.05GNSs HSAC/0.05Ag-GNSs
1.94 1.33 1.23 1.21 1.25 1.35 1.30
particles have a high interfacial free energy, which will increase the interfacial free energy of the entire material and inhibit the growth of the IMC phases [22]. In addition, the Ag-GNSs particles dispersed throughout the solder matrix can enhance the atomic diffusion of the solder, which improves the uniformity of the IMC phases. Furthermore, the Ag nanoparticles within the Ag-GNSs
Fig. 5. SEM image showing the morphology of the fracture surface of SAC/Ag-GNSs composite solder.
Fig. 4. SEM images and the corresponding EDS analysis for the (a) SAC and (b) SAC/Ag-GNSs composite solders.(For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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Fig. 6. DSC curves of the solder samples with various Ag-GNSs contents.
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particles can combine with the solder matrix, which will reduce the surface energy of the solder substrate. This is a primary reason why the Ag-GNSs particles are better than the GNSs particles at improving the uniformity of the solders. However, as the Ag-GNSs content increases, the Ag-GNSs particles begin to agglomerate, which leads to the non-uniform composition of the solder matrix. The nucleation of IMC phases occurs mostly at the edges of the Ag-GNSs particles, and thus, the initial formation and growth of the IMC phases becomes uneven, but it remains more uniform than that in the pure SAC solder. This shows that the Ag-GNSs-containing solders have more refined and uniform grains. However, the degree of reinforcement is not proportional to the Ag-GNSs content, and an excess of Ag-GNSs particles will decrease the strengthening effect. By observing the morphology of the fracture surface of a composite solder (Fig. 5), it is evident that the Ag-GNSs particles aggregate at the fracture surface. The microstructure and morphology of the fracture surfaces of the composite solders will be discussed in Section 3.5. Regarding the different preparation methods, ball milling can create better connections between the matrix phase and reinforcement phase, which results in the uniform distribution of the reinforcement phase. Thus, the IMC particles are also uniformly distributed throughout the matrix, which enhances the strengthening effect of the IMC particles. 3.2. Melting point measurements Fig. 6 shows the DSC curves of the various solder samples. From these DSC curves, the endothermic peaks of the SAC and composite solders are located between 223.5 and 225.5 °C, i.e., there are no significant changes in the melting points of the Ag-GNSs-containing solders. In addition, the melting point of the mechanically milled sample has not changed noticeably. There are a few possible explanations for this behavior. First, the melting point of a given material is an inherent physical property of that pure material [23]. Thus, the addition of a small amount of inert Ag-GNSs particles to the solder has a negligible effect on the melting point of the solder. In addition, the melting point of a nanograin is much lower than that of its bulk counterpart because of the larger volume of the latter [24]. Second, because of the low Ag-GNSs contents used in this study, the Ag-GNSs particles are incapable of noticeably affecting the melting points of the composite solders. Hence, neither the preparation method nor the Ag-GNSs content appear to have an effect on the melting point of the composite solders.
Fig. 7. Wetting angle of the composite solders.
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Fig. 8. Wetting angle of the composite SAC solders on a Cu substrate as a function of the Ag-GNSs content.
3.3. Wettability measurements Solder wettability is an important factor in the performance of soldered joints. Good wettability can promote strong connections between the solder and substrate. The indicators for the wettability of Pb-free solders are primarily the wetting angle, spreading area, wet state, wetting force, and wetting time. In this study, the wetting angle was chosen to characterize the wettability of the composite solders, as shown in Fig. 7. In general, the smaller the wetting angle, the better the wettability of a material [25]. The wetting angles of the ball-milled composite solders are shown in Fig. 8. For the QSAC/0.05GNSs solder, the wetting angle on a bare Cu substrate is 29.5° 72°, whereas the wetting angle of the HSAC/ 0.05Ag-GNSs solder is 28° 71.5°. Furthermore, as the Ag-GNSs content increases, the wetting angle of the ball-milled composite solders on the Cu substrate decreases significantly. The wetting angle of the QSAC/0.2Ag-GNSs solder is approximately half of that of the SAC solder. This demonstrates that the addition of Ag-GNSs particles can significantly improve the wettability of SAC solders on Cu substrates. In addition, the wetting angles of the QSAC/ 0.05GNSs and QSAC/0.05Ag-GNSs solders show that the Ag-GNSs particles are better than the GNSs particles at improving the wetting properties of the ball-milled composite solders. Furthermore, the wetting angle of the QSAC/0.05Ag-GNSs solder is smaller than that of the HSAC/0.05Ag-GNSs solder, which indicates that ball milling is superior to mechanical mixing for preparing composite solders. Many factors affect the wettability of a solder on a certain substrate. With a molten solder, the surface tension between the solder and substrate determines the wetting of the system. Therefore, the solder composition and surface tension are important factors for characterizing the wetting angle of a solder. For the composite solders prepared via ball milling, the microstructures exhibit uniform distributions of fine grains. During the ball milling process, strong collisions occur between the solder particles and the milling medium, which generates a large number of defects (vacancies, dislocations, etc.) at the surface where the Ag-GNSs particles can be partially embedded. In addition, the Ag nanoparticles also react with the solder matrix, which establishes strong connections between the Ag-GNSs particles and the matrix. When this is coupled with a high degree of friction, such collisions can accelerate the compounding of the solder and Ag-GNSs particles, which allows fine grains and uniform distributions of IMCs to form within in the solder matrix that further strengthen the solder. Young’s equation for calculating the wetting angle of a liquid on
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Fig. 9. CTE results of the composite solders at 120 °C as a function of the Ag-GNSs content.
Fig. 11. Elongation of the composite solders as a function of the Ag-GNSs content.
wettability. Therefore, compared to the GNSs particles, the binding of the Ag-GNSs particles with the substrate is enhanced, which enhances the wettability of the Ag-GNSs-containing solders. Regarding the different fabrication processes, the composite solders produced via ball milling have a more favorable connection between the Ag-GNSs particles and the solder matrix throughout the manufacturing process. In addition, because the Ag-GNSs particles play a significant role in reducing the surface energy of the solder matrix, the wettability of the ball-milled solders on Cu substrates is improved. 3.4. Thermomechanical analysis
Fig. 10. Tensile strength of the composite solders as a function of the Ag-GNSs content.
a surface is shown in Eq. (1):
cosθ=
γsg − γsl γlg
(1)
where θ is the wetting angle, and γsg, γsl, and γlg are the specific surface free energies of the matrix/flux, IMCs/liquid solder, and liquid solder/flux, respectively. During the wetting process, the AgGNSs particles can migrate to the wet interface, resulting in the aggregation of Ag-GNSs particles. This causes a reduction in the interfacial free energy of the solder, which in turn reduces the wetting angle. Therefore, we believe that the migration of the Ag-GNSs particles affects the wettability between the flux and solder alloy. The main components of flux are resin acids, and at high temperatures, resin acids can be converted to dehydroabietic acid through a dehydrogenation process during brazing. Compared to the nonpolar Ag-GNSs particles that have a benzene ring structure, the non-polar dehydroabietic acid, which has a similar aromatic structure, has better wettability on Sn-based materials [26]. In addition, dehydroabietic acid promotes the migration of Ag-GNSs particles to the interface. Because of these phenomena, both the surface tension and wetting angle of the composite solder on the flux are reduced. The addition of Ag-GNSs particles reduces the specific surface free energy between the IMCs and liquid solder because the value of cosθ increases, which reduces the wetting angles of the composite solders. By increasing the Ag-GNSs content, the specific surface free energy further decreases, which improves the
The CTE indicates the degree of expansion or contraction in a material. The CTE results of the extruded samples at 120 °C are shown in Fig. 9. The results show that the dimensional stability of the composite solders is improved compared to that of the SAC solder. As the Ag-GNSs content increases, the CTE of the composite solders decreases, indicating that the addition of Ag-GNSs particles can increase the thermal stability of the SAC solder. When 0.03 wt% of Ag-GNSs particles is added to the SAC solder, the CTE decreases by approximately 1.2%. The lowest average CTE value is achieved by the QSAC/0.1Ag-GNSs solder, which is 6.8% lower than the CTE value of the SAC solder. This can be attributed to the thermal expansion of a solder because of the lattice vibrations that are caused by an increase in the internal energy. As mentioned above, the Ag-GNSs particles can inhibit the movement of the lattice [6], which decreases the CTE of the composite solders and increases their stability. 3.5. Tensile tests Solder not only serves as an electrical contact, but it is also subjected to mechanical loading while in service. In this regard, the stability of traditional Pb-free solder joints is not adequate. Thus, it is essential to test the mechanical performance of the new composite solders to determine their mechanical viability. The tensile tests measured the tensile strength and elongation of the specimens. The results of these tests show that for a certain Ag-GNSs content, the tensile strength of the ball-milled composite solders increases, but the elongation decreases. Fig. 10 shows the tensile strengths of the nanocomposite solders as a function of the Ag-GNSs content, and it is apparent that the tensile strength of the SAC solder is improved by the presence of Ag-GNSs particles. When 0.03 wt% of Ag-GNSs particles is added to the SAC solder, the tensile strength of the composite solder is 48.40 MPa, which is 6.5% higher than that of the pure SAC solder. As the Ag-GNSs
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Fig. 12. SEM images of fracture surfaces of the composite solders. (a) Macroscopic fracture-surface morphology of the QSAC/0.20Ag-GNSs solder. (b)(c) Microscopic Ag-GNSs particles in the fracture surface of the QSAC/0.20Ag-GNSs solder. (d) Accumulation of Ag-GNSs particles at the fracture surface of the QSAC/0.20Ag-GNSs solder.
Fig. 13. TEM image on reinforcement of Ag-GNSs material.
content increases to 0.05, 0.1, and 0.2 wt%, the tensile strengths of the composite solders are 50.1, 49.50, and 47.18 MPa, respectively, which are increases of 10.6%, 9.3%, and 4.2%, respectively, relative to the pure SAC solder. From these results, it can be concluded that the maximum tensile strength of the composite solders is achieved with a AgGNSs content of 0.05 wt%. However, the tensile strength of the nanocomposite solders initially increases as the Ag-GNSs content increases, but then it subsequently decreases for Ag-GNSs contents above 0.05 wt%. By comparing the tensile strengths of the composite solders fabricated via ball milling with different Ag-GNSs contents, it can be concluded that the Ag-GNSs particles are better than the GNSs particles for improving the tensile strength of the SAC solder. In addition, the tensile strength of the QSAC/0.05AgGNSs solder is 2.6% greater than that of the HSAC/0.05Ag-GNSs
solder. Moreover, during the ball milling process, the Ag-GNSs particles form more connections with the solder matrix than the GNSs particles, which is advantageous for reinforcing the SAC solder matrix. Fig. 11 shows that as the Ag-GNSs content increase, the ductility of the nanocomposite solders decreases, except when an Ag-GNSs content of 0.03 wt% is used. On the other hand, the mechanically mixed composite solder exhibits good ductility. However, when the Ag-GNSs content is 0.03 wt%, the ductility of the nanocomposite solder is improved. The improved tensile strength of the nanocomposite solders can be attributed to the load-transmitting behavior of the Ag-GNSs particles in the composite solder matrix [27]. This load-transmitting behavior is a combination of mechanical interlocking effects and van der Waals forces between the Ag-GNSs particles and the solder matrix, which can be observed in the fracture-surface morphology (Fig. 12). Here, the Ag-GNSs particles are pulled out of the fracture interface during the fracturing of the composite solder. This is supported by the smooth surfaces of the Ag-GNSs particles (see Fig. 13), but obvious fractures are present in the microstructure of the Ag-GNSs particles after the tensile tests. Furthermore, the fracture cracks mainly occur in the Ag-GNSs particles and the solder matrix, which indicates that the fractures mainly occur in the solder matrix rather than in the combined AgGNSs/matrix. Therefore, it can be concluded that reliable connections are generated between the Ag-GNSs particles and the solder matrix. This is consistent with the results of a previous study that utilized CNTs in a similar manner [11]. The refinement of IMC grains also plays a crucial role in the formation of the microstructure. Thermal mismatches and dislocation interactions can also improve the strength of
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nanocomposite solders. Coarse IMC grains can increase the brittleness and decrease the ductility of nanocomposite solders, which results in brittle fractures at the grain boundaries and a decrease in the overall strength of the solder [28]. The difference between the linear expansion coefficients of the Ag-GNSs particles and the solder matrix increases the dislocation density of the nanocomposite solders; the increase is proportional to the surface area of the material. Therefore, because the surface area of the Ag-GNSs particles is very high [29], the dislocation density will be greatly improved, further strengthening the nanocomposite solders. Furthermore, because of the increase in the dislocation density, the dislocations in the material will inhibit the movement of grains, which implies that the plasticity and ductility of the material will decrease. When the Ag-GNSs content is 0.03 wt%, there is a slight increase in the ductility of the solder. However, because it is only a small increase, both the cause of the increase and the presence of movement-inhibiting dislocations are not obvious. In addition, Fig. 10 shows that when the Ag-GNSs content is 0.1 wt%, the tensile strength of the composite solder decreases compared to that of the QSAC/0.05Ag-GNSs solder. These results can be attributed to two different strengthening mechanisms. For the first mechanism, the Ag-GNSs particles are more evenly distributed than the GNSs particles. This is because the Ag-GNSs particles can react with the solder matrix in addition to forming mechanical connections with the solder, which results in the formation of more stable connections. Unfortunately, the details of these interactions are not well understood. Even though the agglomeration of Ag-GNSs particles is reduced, this enhancement is lost after a critical Ag-GNSs content is reached and the graphene sheets in the solder matrix will agglomerate, weakening the strengthening effect of the Ag-GNSs particles. When the van der Waals forces between the graphene sheets are stronger than the attachment between the Ag nanoparticles and graphene sheets, the Ag-GNSs particles will agglomerate [30]. As the AgGNSs content increases, this agglomeration phenomenon becomes more apparent. As the Ag-GNSs have a multi-sheet structure, it is easy for the sheets to slip over each other [31], which decreases the tensile strength of the solder. The other possible mechanism involves the agglomeration of bound Ag-GNSs particles, which would weaken the bonds between the Ag-GNSs particles and the solder matrix. Thus, the reinforcement effect of the Ag-GNSs particles on the composite solder is weakened, which in turn reduces the tensile strength of the composite solder.
4. Conclusions In this study, SAC solders reinforced with Ag-GNSs particles were successfully synthesized via ball milling. The effect of the AgGNSs content on the performance of the composite solders was investigated. In addition, the effects of Ag-GNSs and GNSs particles on the basic properties of the SAC solder were compared. Finally, the effects of ball milling and mechanical mixing processes on the properties of the SAC solder were investigated for a fixed Ag-GNSs content. The microstructures of the composite solders with increased Ag-GNSs contents were uniform, which indicated that the Ag-GNSs particles could inhibit lattice movement in the composite solders. However, because of the eventual agglomeration of the Ag-GNSs particles, less inhibition was observed when the Ag-GNSs content was very large. The melting points of the composite solders did not change very much with the addition of the AgGNSs particles, which is evidence that the Ag-GNSs particles had little effect on the melting point of the pure SAC solder. The tensile
strength of the composite solders was increased by up to 10.6% compared to that of the SAC solder, while the ductility was reduced by up to 6.8%. However, when an excess of Ag-GNSs particles was used, the degree of improvement in the tensile strength of the composite solder decreased. Furthermore, the wettability of the composite solders was greatly increased, and the wettability increased with the Ag-GNSs content. The minimum wetting angle obtained was approximately half that of the SAC solder. After the brazing reaction, the thickness of the IMC layer at the interface significantly decreased, while the uniformity of the layer increased. In addition, the properties of the ball-milled solder were superior to that of the mechanically mixed solder.
Acknowledgements The authors acknowledge the research funding by National Natural Science Foundation of China (Grant No. 51205282) and the Research Fund for the Doctoral Program of Higher Education of China (20120032120019).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.msea.2016.04.084
References [1] X.D. Liu, Y.D. Han, H.Y. Jing, J. Wei, L.Y. Xu, Mater. Sci. Eng. A 562 (2013) 25–32. [2] H. Zhao, D.R. Nalagatla, D.P. Sekulic, J. Electron. Mater. 38 (2009) 284–291. [3] I. Dutta, B.S. Majumdar, D. Pan, W.S. Horton, W. Wright, Z.X. Wang, J. Electron. Mater. 33 (2004) 258–270. [4] G. L, Electron. Des. (1995). [5] A.D. Schino, J.M. Kenny, J. Mater. Sci. Lett. 21 (2002) 1969–1971. [6] L. Xu, L. Wang, H. Jing, X. Liu, J. Wei, Y. Han, J. Alloy. Compd. 650 (2015) 475–481. [7] C.M.L. Wu, D.Q. Yu, C.M.T. Law, L. Wang, Mater. Sci. Eng. R Rep. 44 (2004) 1–44. [8] P.R.C. Amp, W.A. Jesser, Nature 269 (1977) 481–483. [9] S.T. Kao, Y.C. Lin, J.G. Duh, J. Electron. Mater. 35 (2006) 486–493. [10] W. Liu, P. Bachorik, N.C. Lee, Proceedings of the IEEE/CPMT International Electronics Manufacturing Technology (IEMT) Symposium, 2008, pp. 1–9. [11] Y.D. Han, H.Y. Jing, S.M.L. Nai, L.Y. Xu, C.M. Tan, J. Wei, Intermetallics 31 (2012) 72–78. [12] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183–191. [13] J.S. Benjamin, Sciience 234 (5) (1976) 22–27. [14] S.M.L. Nai, J. Wei, M. Gupta, Thin Solid Films 504 (2006) 401–404. [15] S. Xu, Y.C. Chan, K. Zhang, K.C. Yung, J. Alloy. Compd. 595 (2014) 92–102. [16] L. Lu, M.O. Lai, Mater. Des. 16 (1995) 33–39. [17] J. Rawers, D. Govier, D. Cook, Scr. Metall. Mater. 32 (1995) 1319–1324. [18] K.J. Kong, Y. Choi, B.H. Ryu, J.O. Lee, H. Chang, Mater. Sci. Eng. C 26 (2006) 1207–1210. [19] Y.J. Gan, L.T. Sun, F. Banhart, Small 5 (2008) 587–591. [20] X. Zhao, Q. Zhang, D. Chen, Macromolecules 43 (2010) 2357–2363. [21] S.W. Yoon, W.K. Choi, H.M. Lee, Scr. Mater. 40 (1999) 327–332. [22] H. Ma, J.C. Suhling, J. Mater. Sci. 44 (2009) 1141–1158. [23] B.H. Ryu, Y. Choi, H.S. Park, J.H. Byun, K. Kong, J.O. Lee, H. Chang, Colloids Surf. A Physicochem. Eng. Asp. s270–271 (2005) 345–351. [24] K.N. Tu, K. Zeng, Mater. Sci. Eng. R Rep. 34 (2001) 1–58. [25] L. Ji, Z. Tan, T. Kuykendall, E.J. An, Y. Fu, V. Battaglia, Y. Zhang, Energy Environ. Sci. 4 (2011) 3611–3616. [26] R. Mukherjee, A.V. Thomas, A. Krishnamurthy, N. Koratkar, ACS Nano 6 (2012) 7867–7878. [27] K.S. Kim, S.H. Huh, K. Suganuma, J. Alloy. Compd. 352 (2003) 226–236. [28] H.K. C., D.Y. S.-P., J. K., Y. G., M. E., A.J. M., M. O.K., O.M. Y., Nature 427 (2004) 523–527. [29] S. Vadukumpully, J. Paul, N. Mahanta, S. Valiyaveettil, Carbon 49 (2011) 198–205. [30] M.A. Rafiee, J. Rafiee, Z. Wang, H.H. Song, Z.Z. Yu, N. Koratkar, ACS Nano 3 (2009) 3884–3890. [31] R.M. Shalaby, J. Alloy. Compd. 505 (2010) 113–117.
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Design and performance of Ag nanoparticle-modified graphene/ SnAgCu lead-free solders Lianyong Xu a,b, Xi Chen a,b, Hongyang Jing a,b, Lixia Wang a,b, Jun Wei c, Yongdian Han a,b,n a
School of Materials Science and Engineering, Tianjin University, Tianjin 300354, PR China Tianjin Key Laboratory of Advanced Joining Technology, Tianjin 300354, China c Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore b
art ic l e i nf o
a b s t r a c t
Article history: Received 12 February 2016 Received in revised form 26 April 2016 Accepted 27 April 2016 Available online 28 April 2016
In this study, various weight percentages of Ag-nanoparticle-modified graphene (Ag-GNSs) were incorporated into Pb-free Sn-Ag-Cu (SAC) solder matrices via ball milling and mechanical mixing methods to form composite solders. Up to 0.2 wt% of the Ag-GNSs particles was successfully incorporated into the composite solders, and the microstructures and mechanical properties of the solders were investigated. The experimental results demonstrated that as the Ag-GNSs content increased, the number of intermetallic particles gradually decreased in size. In addition, the uniformity of the interfacial structure increased compared to that of the pure SAC solder. Furthermore, the melting points of the SAC/Ag-GNSs composite solders were unchanged compared to that of the pure SAC solder. The experimental results also revealed that the wettability of the composite solders on Cu substrates was greatly improved. Thermomechanical measurements showed that the thermal stability of the composite solders was also improved. In addition, tensile tests showed that the ultimate tensile strength of the composite solders was enhanced compared to that of the pure SAC solder, but the ductility of the composite solders was reduced. In particular, the tensile strength of the composite solders with low Ag-GNSs contents was increased, but the tensile strength then decreased once a critical Ag-GNSs content was reached. For the ball-milled solders, the reinforcement provided by the addition of Ag-GNSs particles was much better than that provided by unmodified graphene. By comparing the properties of solders fabricated with different preparation methods, it was determined that the ball-milled solders were superior to the mechanically mixed solders. & 2016 Elsevier B.V. All rights reserved.
Keywords: Composite solders Microstructure Mechanical testing Ball milling method
1. Introduction Sn-Pb eutectic solders have been widely used in the electronic packaging industry for a number of years [1,2]. However, because of the toxic nature of Pb, the use of Pb-based solders has been banned in many industrial applications. Therefore, there has been a large amount of research focused on developing environmentally friendly Pb-free alloys to replace Pb-Sn solder, especially in the electronic packaging industry. Currently, there are many promising Pb-free solder alternatives, such as Sn-Ag-Cu (SAC) alloys. Even though a number of different near-eutectic SAC alloys are widely used, they have some disadvantages when compared to Pb-Sn alloys, including higher melting points and poor mechanical-shock resistance [3–8]. There are several secondary phases, such as metallic elements, intermetallics, or inter second phases, that are currently used to n
Corresponding author. E-mail address: [email protected] (Y. Han).
http://dx.doi.org/10.1016/j.msea.2016.04.084 0921-5093/& 2016 Elsevier B.V. All rights reserved.
reinforce conventional SAC solders. The effects of reinforcement on the various properties of SAC solders have been widely investigated. Lin et al. [9] added nano-TiO2 and nano-Cu powders to traditional Sn-Pb solders to obtain nanocomposite Pb-based solders. Liu et al. [10] reinforced a SAC solder with Ti, one of the most active transition metals, which yielded a superior drop-test performance because of the reduced elastic modulus of the composite solder. The addition of Ti also improved the dramatic suppression of the undercooling of SAC solders and reduced the growth of interfacial intermetallic compounds (IMCs). Han et al. [11] used Ni-coated carbon nanotubes (CNTs) to reinforce SAC solder joints. The interfacial microstructures and shear strength of the composite solders were improved compared to that of the pure SAC solder, even after 2000 thermal cycles. Graphene, which was discovered in 2004 [12], is the hardest nanomaterial to date, and it is known for its excellent conductivity and thermal and mechanical properties. Several studies have shown that reinforcing conventional solder alloys with graphene improves the overall material performance [11]. However, there are problems with graphene-reinforced SAC solders that must be
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solved, such as the difficulty in obtaining a uniform graphene distribution in the solder matrix and its poor bonding strength with metal substrates. Thus, modifying graphene with Ag nanoparticles (Ag-GNSs) can lead to improved interactions between the Ag-GNSs particles and Sn matrix. Improving the graphene-Sn interactions is essential for ensuring an effective load transfer across the interface, which would significantly improve the properties of graphene-reinforced SAC solders. The methods used to add reinforcements and fabricate composite solders are primarily mechanical alloying and mechanical mixing. Tao et al. [13] prepared Au-Sn eutectic solders via mechanical alloying by using high-energy ball milling equipment, and studied the effect of the milling time on the microstructure of the solders. Nai et al. [14] mechanically mixed a solder and reinforcing agent to obtain a composite solder. Xu et al. [15] used mechanical mixing to add CNTs to a Sn3.5Ag0.5Cu solder, which improved the mechanical properties of the solder. They reported that some of the solder particles underwent deformation after the mixing process, which decreased both the overall particle size and the uniformity of the particle-size distribution. On the other hand, the prolonged agitation of the particles was reduced because of the aggregation caused by van der Waals forces, which produced a uniform distribution of CNTs within the solder matrix. The typical melting point of SAC solders is approximately 183 °C, and most brazing equipment is designed to use 183 °C as the standard temperature. For electronic packaging materials, the maximum temperature that the most commonly used substrates can be subjected to is 230 °C. If the melting point of the solder alloy is too high, it can damage the substrate and adversely affect the performance of the electronic components. Hence, adjusting the melting point of new composite solders to avoid damaging electronic packaging materials could be an effective cost-saving measure. Furthermore, the microstructure of the Ag-GNSs/solder matrix interface must be examined. This is because the main factor that determines the tensile strength of a solder is the uniformity of the interface morphology, which plays an important role in the reliability of solder joints. Therefore, the coefficient of thermal expansion (CTE), wetting angle, and tensile strength of a solder are crucial factors for predicting the reliability of solder joints. In this study, Ag-GNSs/SAC composite solders were successfully prepared via ball milling and mechanical mixing methods. The microstructures of the composite solders were characterized with scanning electron microscopy (SEM). In addition, the wettability of the solder joints, and tensile and thermomechanical properties of
the Ag-GNSs/SAC solders were investigated. The behavior of the composite solder joints was compared to that of the pure SAC solder in order to demonstrate the outstanding properties of the new Ag-GNSs/SAC composite solders.
2. Experimental procedures 2.1. Material processing A 96.5Sn-3.0Ag-0.5Cu powder (particle sizes ¼25–45 mm, as shown in Fig. 1(a)) was used as the solder matrix and Ag-GNSs particles were used as the reinforcement phase. Fig. 1(b) shows an TEM image of the Ag-GNSs particles. Composite solders with various Ag-GNSs contents (0, 0.03, 0.05, 0.10, and 0.20 wt%) were synthesized, which will hereinafter be referred to as SAC, SAC/0.03Ag-GNSs, SAC/0.05Ag-GNSs, SAC/ 0.1Ag-GNSs, and SAC/0.2Ag-GNSs, respectively. The performance of the reinforced solders was investigated as a function of the Ag-GNSs content. In addition, the performance of the SAC/0.05Ag-GNSs solder was compared to an equivalent solder that was only reinforced with graphene (i.e., no Ag nanoparticles). Finally, two different SAC/0.05Ag-GNSs solders were prepared by ball milling and mechanical mixing to investigate the effects of the fabrication process on the resulting solders. For convenience, Table 1 lists all of the samples fabricated in this study; a “Q” in the sample label indicates the sample was ball milled, whereas an “H” indicates the sample was mechanically mixed. Firstly, mixtures of the solder powder and Ag-GNSs particles were pre-weighted to obtain the desired solder compositions. Each composite solder mixture was then added to a sealed ballmilling jar that contained the milling medium (stainless steel ball) and a certain volume of ethanol. The mixtures were milled with a planetary ball mill at 300 rpm for 5 h in a high-purity Ar gas atmosphere to ensure that the solder matrix and reinforcement phase were thoroughly mixed. The milled mixture was then placed in a stainless steel mold (diameter¼ 20 mm), uniaxially compacted with a hydraulic press at 500 MPa, and sintered at 175 °C for 2 h in an inert atmosphere. Finally, the billet was extruded at room temperature into a 6-mm-diameter rod. The SAC solder was fabricated according to the method outlined above, but the blending process was omitted. The mechanical mixing method used to prepare the composite solder with a Ag-GNSs content of 0.05 wt% was as follows. The
Fig. 1. SEM images of (a) SAC powder and TEM images of (b) Ag-GNSs powders.
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and three samples were tested for each composite solder.
Table 1 Abbreviation of the composite solders in this paper. Solder Matrix
Reinforcement Reinforcement content
Sample label
96.5Sn-3.0Ag0.5Cu
N/A Ag-GNSs
SAC QSAC/0.03AgGNSs QSAC/0.05AgGNSs QSAC/0.1Ag-GNSs QSAC/0.2AgGNSs HSAC/0.05AgGNSs QSAC/0.05GNSs
N/A 0.03 wt% 0.05 wt% 0.1 wt% 0.2 wt% 0.05 wt%
GNSs
0.05 wt%
89
2.6. Tensile tests Tensile tests were performed according to the ASTM E8M-04 standard for material testing. The tensile strength and elongation of circular samples (diameter¼4 mm, length ¼ of 2.5 mm) were obtained with an electric tensile-testing machine, which was operated at room temperature with a crosshead speed of 2 mm/min. The fracture surface of each sample was then observed via SEM.
3. Results and discussion 3.1. Microstructural analysis
composite solder mixture was blended for 12 h in a V-blender at a speed of 60 rpm. After the mechanical mixing process, the remainder of the fabrication process was the same as that of ballmilled samples. 2.2. Microstructural analysis The samples were mounted in epoxy and then ground, polished, and etched for a few seconds with a methanol solution containing 8 vol% of hydrochloric acid. The microstructures of the samples were observed via SEM (HITACHI FE-SEM 4800). In addition, the SEM set-up was capable of energy-dispersive X-ray spectroscopy (EDS). 2.3. Melting point measurements The melting points of the solders were determined via differential scanning calorimetry (DSC; TA Instruments). The DSC samples, which were composed of pieces of solder randomly sliced from the extruded rods, were placed in an Ar-filled Al pan that was heated to 250 °C at a heating rate of 10 °C/min. 2.4. Wettability measurements The wetting angles of the solders were measured on Cu substrates to determine their wettability. The methods used were as follows: (1) a 1.5-mm-thick disc was cut from the extruded composite solder. (2) The polished Cu substrate and solder disc were placed on a heating stage and heated to 250 °C. (3) The samples were cooled in air until the solder had completely spread on the Cu substrate. (4) A cross section of the sample was observed with a stereoscopic microscope and the contact angle was measured; a schematic diagram showing the contact angle (θ) of the solder sample on the Cu substrate is shown in Fig. 2. These steps were repeated three times for each composite solder to obtain the average θ value. 2.5. Thermomechanical analysis The CTEs of the extruded samples were obtained by measuring the displacement of the samples as a function of the temperature over a temperature range of 50–120 °C. An automated TMA 2940 thermomechanical analyzer was used for these measurements,
Fig. 2. Wetting angle.
The microstructures of the QSAC/Ag-GNSs solders are shown in Figs. 3(a-e). Image Pro Plus software was used to calculate the average particle size of the resulting IMCs; the results are listed in Table 2. For the ball-milled samples, as the Ag-GNSs content increases, the average IMC particle size gradually decreases. However, when the Ag-GNSs content reaches 0.2 wt%, the average IMC particle size increases slightly. In addition, Table 2 shows that the IMC particle size of the QSAC/0.05Ag-GNSs solder is 8.9% smaller than that of the QSAC/0.05GNSs solder and 36.6% smaller than the SAC samples, implying that the presence of Ag-GNSs particles inhibits grain growth, which results in the formation of finer IMC grains compared to that of the pure SAC solder. Furthermore, Table 2 shows that the QSAC/0.05Ag-GNSs solder has smaller IMC particles than the HSAC/0.05Ag-GNSs solder. Fig. 4 shows SEM images with the corresponding EDS analysis of the SAC solder and its nanocomposite solder. There are three phases present in the EDS spectrum of the nanocomposite solder, namely β-Sn, Ag3Sn, and Cu6Sn5 [16,17], and the composition is similar to what has been reported by previous studies. The light gray, fine grains correspond to Ag3Sn, while the dark gray, bulk grains correspond to a mixture of Ag3Sn and Cu6Sn5. The SEM image shows that the Ag3Sn and Cu6Sn5 particles are dispersed throughout the β-Sn matrix. As the Ag-GNSs content increases, the uniformity of the IMC phases gradually increases. The addition of Ag-GNSs not only slows the growth of the IMC phases, but it also makes the growth more even. Thus, the IMC phases of the composite solders are more uniform than that of the pure SAC solder. These observations can be explained by the growth thermodynamics and kinetics of the IMC phases [18,19]. There are two IMCs dispersed throughout the solder matrix: Ag3Sn and Cu6Sn5. Because of the inherent characteristics of IMCs, such as higher melting points and hardnesses, even distributions of Ag3Sn and Cu6Sn5 grains can reinforce the β-Sn matrix. The crystallization of Cu6Sn5 and Ag3Sn grains is a nucleation and growth process, which requires energy. This energy is provided by the surface energy and interface energy between the solder matrix and the IMC particles. The solder matrix and Ag-GNSs particles are thoroughly mixed during the ball milling process, with the latter becoming embedded in the surface of the solder particles through high-speed collisions, which creates excellent physical connections. The Ag-GNSs particles are uniformly distributed throughout the solder matrix, and these particles provide sites for the nucleation of Cu6Sn5 and Ag3Sn grains. Thus, the distribution of the Ag-GNSs particles within the solder matrix determines the distribution of IMC particles. Therefore, because the distribution of Ag-GNSs particles within the composite solder is uniform, the nucleation and growth of IMC particles is also uniform [20,21]. However, while the growth of IMC phases is possible, they must overcome the interfacial free energy of the system. The Ag-GNSs
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Fig. 3. Microstructure of composite solders: (a) SAC, (b) QSAC/0.03Ag-GNSs, (c) QSAC/0.05Ag-GNSs, (d) QSAC/0.1Ag-GNSs, (e) QSAC/0.2Ag-GNSs, (f) QSAC/0.05GNSs, (g) HSAC/0.05Ag-GNSs.
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Table 2 average IMC particle size of the composite solders. Sample
Average IMC size (μm)
SAC QSAC/0.03Ag-GNSs QSAC/0.05Ag-GNSs QSAC/0.1Ag-GNSs QSAC/0.2Ag-GNSs QSAC/0.05GNSs HSAC/0.05Ag-GNSs
1.94 1.33 1.23 1.21 1.25 1.35 1.30
particles have a high interfacial free energy, which will increase the interfacial free energy of the entire material and inhibit the growth of the IMC phases [22]. In addition, the Ag-GNSs particles dispersed throughout the solder matrix can enhance the atomic diffusion of the solder, which improves the uniformity of the IMC phases. Furthermore, the Ag nanoparticles within the Ag-GNSs
Fig. 5. SEM image showing the morphology of the fracture surface of SAC/Ag-GNSs composite solder.
Fig. 4. SEM images and the corresponding EDS analysis for the (a) SAC and (b) SAC/Ag-GNSs composite solders.(For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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Fig. 6. DSC curves of the solder samples with various Ag-GNSs contents.
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particles can combine with the solder matrix, which will reduce the surface energy of the solder substrate. This is a primary reason why the Ag-GNSs particles are better than the GNSs particles at improving the uniformity of the solders. However, as the Ag-GNSs content increases, the Ag-GNSs particles begin to agglomerate, which leads to the non-uniform composition of the solder matrix. The nucleation of IMC phases occurs mostly at the edges of the Ag-GNSs particles, and thus, the initial formation and growth of the IMC phases becomes uneven, but it remains more uniform than that in the pure SAC solder. This shows that the Ag-GNSs-containing solders have more refined and uniform grains. However, the degree of reinforcement is not proportional to the Ag-GNSs content, and an excess of Ag-GNSs particles will decrease the strengthening effect. By observing the morphology of the fracture surface of a composite solder (Fig. 5), it is evident that the Ag-GNSs particles aggregate at the fracture surface. The microstructure and morphology of the fracture surfaces of the composite solders will be discussed in Section 3.5. Regarding the different preparation methods, ball milling can create better connections between the matrix phase and reinforcement phase, which results in the uniform distribution of the reinforcement phase. Thus, the IMC particles are also uniformly distributed throughout the matrix, which enhances the strengthening effect of the IMC particles. 3.2. Melting point measurements Fig. 6 shows the DSC curves of the various solder samples. From these DSC curves, the endothermic peaks of the SAC and composite solders are located between 223.5 and 225.5 °C, i.e., there are no significant changes in the melting points of the Ag-GNSs-containing solders. In addition, the melting point of the mechanically milled sample has not changed noticeably. There are a few possible explanations for this behavior. First, the melting point of a given material is an inherent physical property of that pure material [23]. Thus, the addition of a small amount of inert Ag-GNSs particles to the solder has a negligible effect on the melting point of the solder. In addition, the melting point of a nanograin is much lower than that of its bulk counterpart because of the larger volume of the latter [24]. Second, because of the low Ag-GNSs contents used in this study, the Ag-GNSs particles are incapable of noticeably affecting the melting points of the composite solders. Hence, neither the preparation method nor the Ag-GNSs content appear to have an effect on the melting point of the composite solders.
Fig. 7. Wetting angle of the composite solders.
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Fig. 8. Wetting angle of the composite SAC solders on a Cu substrate as a function of the Ag-GNSs content.
3.3. Wettability measurements Solder wettability is an important factor in the performance of soldered joints. Good wettability can promote strong connections between the solder and substrate. The indicators for the wettability of Pb-free solders are primarily the wetting angle, spreading area, wet state, wetting force, and wetting time. In this study, the wetting angle was chosen to characterize the wettability of the composite solders, as shown in Fig. 7. In general, the smaller the wetting angle, the better the wettability of a material [25]. The wetting angles of the ball-milled composite solders are shown in Fig. 8. For the QSAC/0.05GNSs solder, the wetting angle on a bare Cu substrate is 29.5° 72°, whereas the wetting angle of the HSAC/ 0.05Ag-GNSs solder is 28° 71.5°. Furthermore, as the Ag-GNSs content increases, the wetting angle of the ball-milled composite solders on the Cu substrate decreases significantly. The wetting angle of the QSAC/0.2Ag-GNSs solder is approximately half of that of the SAC solder. This demonstrates that the addition of Ag-GNSs particles can significantly improve the wettability of SAC solders on Cu substrates. In addition, the wetting angles of the QSAC/ 0.05GNSs and QSAC/0.05Ag-GNSs solders show that the Ag-GNSs particles are better than the GNSs particles at improving the wetting properties of the ball-milled composite solders. Furthermore, the wetting angle of the QSAC/0.05Ag-GNSs solder is smaller than that of the HSAC/0.05Ag-GNSs solder, which indicates that ball milling is superior to mechanical mixing for preparing composite solders. Many factors affect the wettability of a solder on a certain substrate. With a molten solder, the surface tension between the solder and substrate determines the wetting of the system. Therefore, the solder composition and surface tension are important factors for characterizing the wetting angle of a solder. For the composite solders prepared via ball milling, the microstructures exhibit uniform distributions of fine grains. During the ball milling process, strong collisions occur between the solder particles and the milling medium, which generates a large number of defects (vacancies, dislocations, etc.) at the surface where the Ag-GNSs particles can be partially embedded. In addition, the Ag nanoparticles also react with the solder matrix, which establishes strong connections between the Ag-GNSs particles and the matrix. When this is coupled with a high degree of friction, such collisions can accelerate the compounding of the solder and Ag-GNSs particles, which allows fine grains and uniform distributions of IMCs to form within in the solder matrix that further strengthen the solder. Young’s equation for calculating the wetting angle of a liquid on
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Fig. 9. CTE results of the composite solders at 120 °C as a function of the Ag-GNSs content.
Fig. 11. Elongation of the composite solders as a function of the Ag-GNSs content.
wettability. Therefore, compared to the GNSs particles, the binding of the Ag-GNSs particles with the substrate is enhanced, which enhances the wettability of the Ag-GNSs-containing solders. Regarding the different fabrication processes, the composite solders produced via ball milling have a more favorable connection between the Ag-GNSs particles and the solder matrix throughout the manufacturing process. In addition, because the Ag-GNSs particles play a significant role in reducing the surface energy of the solder matrix, the wettability of the ball-milled solders on Cu substrates is improved. 3.4. Thermomechanical analysis
Fig. 10. Tensile strength of the composite solders as a function of the Ag-GNSs content.
a surface is shown in Eq. (1):
cosθ=
γsg − γsl γlg
(1)
where θ is the wetting angle, and γsg, γsl, and γlg are the specific surface free energies of the matrix/flux, IMCs/liquid solder, and liquid solder/flux, respectively. During the wetting process, the AgGNSs particles can migrate to the wet interface, resulting in the aggregation of Ag-GNSs particles. This causes a reduction in the interfacial free energy of the solder, which in turn reduces the wetting angle. Therefore, we believe that the migration of the Ag-GNSs particles affects the wettability between the flux and solder alloy. The main components of flux are resin acids, and at high temperatures, resin acids can be converted to dehydroabietic acid through a dehydrogenation process during brazing. Compared to the nonpolar Ag-GNSs particles that have a benzene ring structure, the non-polar dehydroabietic acid, which has a similar aromatic structure, has better wettability on Sn-based materials [26]. In addition, dehydroabietic acid promotes the migration of Ag-GNSs particles to the interface. Because of these phenomena, both the surface tension and wetting angle of the composite solder on the flux are reduced. The addition of Ag-GNSs particles reduces the specific surface free energy between the IMCs and liquid solder because the value of cosθ increases, which reduces the wetting angles of the composite solders. By increasing the Ag-GNSs content, the specific surface free energy further decreases, which improves the
The CTE indicates the degree of expansion or contraction in a material. The CTE results of the extruded samples at 120 °C are shown in Fig. 9. The results show that the dimensional stability of the composite solders is improved compared to that of the SAC solder. As the Ag-GNSs content increases, the CTE of the composite solders decreases, indicating that the addition of Ag-GNSs particles can increase the thermal stability of the SAC solder. When 0.03 wt% of Ag-GNSs particles is added to the SAC solder, the CTE decreases by approximately 1.2%. The lowest average CTE value is achieved by the QSAC/0.1Ag-GNSs solder, which is 6.8% lower than the CTE value of the SAC solder. This can be attributed to the thermal expansion of a solder because of the lattice vibrations that are caused by an increase in the internal energy. As mentioned above, the Ag-GNSs particles can inhibit the movement of the lattice [6], which decreases the CTE of the composite solders and increases their stability. 3.5. Tensile tests Solder not only serves as an electrical contact, but it is also subjected to mechanical loading while in service. In this regard, the stability of traditional Pb-free solder joints is not adequate. Thus, it is essential to test the mechanical performance of the new composite solders to determine their mechanical viability. The tensile tests measured the tensile strength and elongation of the specimens. The results of these tests show that for a certain Ag-GNSs content, the tensile strength of the ball-milled composite solders increases, but the elongation decreases. Fig. 10 shows the tensile strengths of the nanocomposite solders as a function of the Ag-GNSs content, and it is apparent that the tensile strength of the SAC solder is improved by the presence of Ag-GNSs particles. When 0.03 wt% of Ag-GNSs particles is added to the SAC solder, the tensile strength of the composite solder is 48.40 MPa, which is 6.5% higher than that of the pure SAC solder. As the Ag-GNSs
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Fig. 12. SEM images of fracture surfaces of the composite solders. (a) Macroscopic fracture-surface morphology of the QSAC/0.20Ag-GNSs solder. (b)(c) Microscopic Ag-GNSs particles in the fracture surface of the QSAC/0.20Ag-GNSs solder. (d) Accumulation of Ag-GNSs particles at the fracture surface of the QSAC/0.20Ag-GNSs solder.
Fig. 13. TEM image on reinforcement of Ag-GNSs material.
content increases to 0.05, 0.1, and 0.2 wt%, the tensile strengths of the composite solders are 50.1, 49.50, and 47.18 MPa, respectively, which are increases of 10.6%, 9.3%, and 4.2%, respectively, relative to the pure SAC solder. From these results, it can be concluded that the maximum tensile strength of the composite solders is achieved with a AgGNSs content of 0.05 wt%. However, the tensile strength of the nanocomposite solders initially increases as the Ag-GNSs content increases, but then it subsequently decreases for Ag-GNSs contents above 0.05 wt%. By comparing the tensile strengths of the composite solders fabricated via ball milling with different Ag-GNSs contents, it can be concluded that the Ag-GNSs particles are better than the GNSs particles for improving the tensile strength of the SAC solder. In addition, the tensile strength of the QSAC/0.05AgGNSs solder is 2.6% greater than that of the HSAC/0.05Ag-GNSs
solder. Moreover, during the ball milling process, the Ag-GNSs particles form more connections with the solder matrix than the GNSs particles, which is advantageous for reinforcing the SAC solder matrix. Fig. 11 shows that as the Ag-GNSs content increase, the ductility of the nanocomposite solders decreases, except when an Ag-GNSs content of 0.03 wt% is used. On the other hand, the mechanically mixed composite solder exhibits good ductility. However, when the Ag-GNSs content is 0.03 wt%, the ductility of the nanocomposite solder is improved. The improved tensile strength of the nanocomposite solders can be attributed to the load-transmitting behavior of the Ag-GNSs particles in the composite solder matrix [27]. This load-transmitting behavior is a combination of mechanical interlocking effects and van der Waals forces between the Ag-GNSs particles and the solder matrix, which can be observed in the fracture-surface morphology (Fig. 12). Here, the Ag-GNSs particles are pulled out of the fracture interface during the fracturing of the composite solder. This is supported by the smooth surfaces of the Ag-GNSs particles (see Fig. 13), but obvious fractures are present in the microstructure of the Ag-GNSs particles after the tensile tests. Furthermore, the fracture cracks mainly occur in the Ag-GNSs particles and the solder matrix, which indicates that the fractures mainly occur in the solder matrix rather than in the combined AgGNSs/matrix. Therefore, it can be concluded that reliable connections are generated between the Ag-GNSs particles and the solder matrix. This is consistent with the results of a previous study that utilized CNTs in a similar manner [11]. The refinement of IMC grains also plays a crucial role in the formation of the microstructure. Thermal mismatches and dislocation interactions can also improve the strength of
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nanocomposite solders. Coarse IMC grains can increase the brittleness and decrease the ductility of nanocomposite solders, which results in brittle fractures at the grain boundaries and a decrease in the overall strength of the solder [28]. The difference between the linear expansion coefficients of the Ag-GNSs particles and the solder matrix increases the dislocation density of the nanocomposite solders; the increase is proportional to the surface area of the material. Therefore, because the surface area of the Ag-GNSs particles is very high [29], the dislocation density will be greatly improved, further strengthening the nanocomposite solders. Furthermore, because of the increase in the dislocation density, the dislocations in the material will inhibit the movement of grains, which implies that the plasticity and ductility of the material will decrease. When the Ag-GNSs content is 0.03 wt%, there is a slight increase in the ductility of the solder. However, because it is only a small increase, both the cause of the increase and the presence of movement-inhibiting dislocations are not obvious. In addition, Fig. 10 shows that when the Ag-GNSs content is 0.1 wt%, the tensile strength of the composite solder decreases compared to that of the QSAC/0.05Ag-GNSs solder. These results can be attributed to two different strengthening mechanisms. For the first mechanism, the Ag-GNSs particles are more evenly distributed than the GNSs particles. This is because the Ag-GNSs particles can react with the solder matrix in addition to forming mechanical connections with the solder, which results in the formation of more stable connections. Unfortunately, the details of these interactions are not well understood. Even though the agglomeration of Ag-GNSs particles is reduced, this enhancement is lost after a critical Ag-GNSs content is reached and the graphene sheets in the solder matrix will agglomerate, weakening the strengthening effect of the Ag-GNSs particles. When the van der Waals forces between the graphene sheets are stronger than the attachment between the Ag nanoparticles and graphene sheets, the Ag-GNSs particles will agglomerate [30]. As the AgGNSs content increases, this agglomeration phenomenon becomes more apparent. As the Ag-GNSs have a multi-sheet structure, it is easy for the sheets to slip over each other [31], which decreases the tensile strength of the solder. The other possible mechanism involves the agglomeration of bound Ag-GNSs particles, which would weaken the bonds between the Ag-GNSs particles and the solder matrix. Thus, the reinforcement effect of the Ag-GNSs particles on the composite solder is weakened, which in turn reduces the tensile strength of the composite solder.
4. Conclusions In this study, SAC solders reinforced with Ag-GNSs particles were successfully synthesized via ball milling. The effect of the AgGNSs content on the performance of the composite solders was investigated. In addition, the effects of Ag-GNSs and GNSs particles on the basic properties of the SAC solder were compared. Finally, the effects of ball milling and mechanical mixing processes on the properties of the SAC solder were investigated for a fixed Ag-GNSs content. The microstructures of the composite solders with increased Ag-GNSs contents were uniform, which indicated that the Ag-GNSs particles could inhibit lattice movement in the composite solders. However, because of the eventual agglomeration of the Ag-GNSs particles, less inhibition was observed when the Ag-GNSs content was very large. The melting points of the composite solders did not change very much with the addition of the AgGNSs particles, which is evidence that the Ag-GNSs particles had little effect on the melting point of the pure SAC solder. The tensile
strength of the composite solders was increased by up to 10.6% compared to that of the SAC solder, while the ductility was reduced by up to 6.8%. However, when an excess of Ag-GNSs particles was used, the degree of improvement in the tensile strength of the composite solder decreased. Furthermore, the wettability of the composite solders was greatly increased, and the wettability increased with the Ag-GNSs content. The minimum wetting angle obtained was approximately half that of the SAC solder. After the brazing reaction, the thickness of the IMC layer at the interface significantly decreased, while the uniformity of the layer increased. In addition, the properties of the ball-milled solder were superior to that of the mechanically mixed solder.
Acknowledgements The authors acknowledge the research funding by National Natural Science Foundation of China (Grant No. 51205282) and the Research Fund for the Doctoral Program of Higher Education of China (20120032120019).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.msea.2016.04.084
References [1] X.D. Liu, Y.D. Han, H.Y. Jing, J. Wei, L.Y. Xu, Mater. Sci. Eng. A 562 (2013) 25–32. [2] H. Zhao, D.R. Nalagatla, D.P. Sekulic, J. Electron. Mater. 38 (2009) 284–291. [3] I. Dutta, B.S. Majumdar, D. Pan, W.S. Horton, W. Wright, Z.X. Wang, J. Electron. Mater. 33 (2004) 258–270. [4] G. L, Electron. Des. (1995). [5] A.D. Schino, J.M. Kenny, J. Mater. Sci. Lett. 21 (2002) 1969–1971. [6] L. Xu, L. Wang, H. Jing, X. Liu, J. Wei, Y. Han, J. Alloy. Compd. 650 (2015) 475–481. [7] C.M.L. Wu, D.Q. Yu, C.M.T. Law, L. Wang, Mater. Sci. Eng. R Rep. 44 (2004) 1–44. [8] P.R.C. Amp, W.A. Jesser, Nature 269 (1977) 481–483. [9] S.T. Kao, Y.C. Lin, J.G. Duh, J. Electron. Mater. 35 (2006) 486–493. [10] W. Liu, P. Bachorik, N.C. Lee, Proceedings of the IEEE/CPMT International Electronics Manufacturing Technology (IEMT) Symposium, 2008, pp. 1–9. [11] Y.D. Han, H.Y. Jing, S.M.L. Nai, L.Y. Xu, C.M. Tan, J. Wei, Intermetallics 31 (2012) 72–78. [12] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183–191. [13] J.S. Benjamin, Sciience 234 (5) (1976) 22–27. [14] S.M.L. Nai, J. Wei, M. Gupta, Thin Solid Films 504 (2006) 401–404. [15] S. Xu, Y.C. Chan, K. Zhang, K.C. Yung, J. Alloy. Compd. 595 (2014) 92–102. [16] L. Lu, M.O. Lai, Mater. Des. 16 (1995) 33–39. [17] J. Rawers, D. Govier, D. Cook, Scr. Metall. Mater. 32 (1995) 1319–1324. [18] K.J. Kong, Y. Choi, B.H. Ryu, J.O. Lee, H. Chang, Mater. Sci. Eng. C 26 (2006) 1207–1210. [19] Y.J. Gan, L.T. Sun, F. Banhart, Small 5 (2008) 587–591. [20] X. Zhao, Q. Zhang, D. Chen, Macromolecules 43 (2010) 2357–2363. [21] S.W. Yoon, W.K. Choi, H.M. Lee, Scr. Mater. 40 (1999) 327–332. [22] H. Ma, J.C. Suhling, J. Mater. Sci. 44 (2009) 1141–1158. [23] B.H. Ryu, Y. Choi, H.S. Park, J.H. Byun, K. Kong, J.O. Lee, H. Chang, Colloids Surf. A Physicochem. Eng. Asp. s270–271 (2005) 345–351. [24] K.N. Tu, K. Zeng, Mater. Sci. Eng. R Rep. 34 (2001) 1–58. [25] L. Ji, Z. Tan, T. Kuykendall, E.J. An, Y. Fu, V. Battaglia, Y. Zhang, Energy Environ. Sci. 4 (2011) 3611–3616. [26] R. Mukherjee, A.V. Thomas, A. Krishnamurthy, N. Koratkar, ACS Nano 6 (2012) 7867–7878. [27] K.S. Kim, S.H. Huh, K. Suganuma, J. Alloy. Compd. 352 (2003) 226–236. [28] H.K. C., D.Y. S.-P., J. K., Y. G., M. E., A.J. M., M. O.K., O.M. Y., Nature 427 (2004) 523–527. [29] S. Vadukumpully, J. Paul, N. Mahanta, S. Valiyaveettil, Carbon 49 (2011) 198–205. [30] M.A. Rafiee, J. Rafiee, Z. Wang, H.H. Song, Z.Z. Yu, N. Koratkar, ACS Nano 3 (2009) 3884–3890. [31] R.M. Shalaby, J. Alloy. Compd. 505 (2010) 113–117.