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Microstructure, mechanical, and thermal properties of the Sn–1Ag–0.5Cu solder alloy bearing Fe for electronics applications
Materials Science and Engineering A 551 (2012) 160–168
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Microstructure, mechanical, and thermal properties of the Sn–1Ag–0.5Cu solder alloy bearing Fe for electronics applications Dhafer Abdul-Ameer Shnawah a,∗ , Suhana Binti Mohd Said b , Mohd Faizul Mohd Sabri a , Irfan Anjum Badruddin a , Fa Xing Che c a
Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Electrical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia c Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research), 11 Science Park Road, Singapore Science Park II, Singapore 117685, Singapore b
a r t i c l e
i n f o
Article history: Received 7 January 2012 Received in revised form 26 April 2012 Accepted 27 April 2012 Available online 11 May 2012 Keywords: Sn–1Ag–0.5Cu alloy Fe addition Mechanical properties Microstructure properties Thermal properties
a b s t r a c t This work investigates the effect of Fe addition on the microstructural, mechanical, and thermal properties of the Sn–1Ag–0.5Cu (SAC105) solder alloy. The addition of Fe leads to the formation of large circular FeSn2 intermetallic compound (IMC) particles, which produce a weak interface with the -Sn matrix. The addition of Fe also leads to the inclusion of Fe in the Ag3 Sn and Cu6 Sn5 IMC particles. Moreover, Fe-bearing solders have been shown to form large primary -Sn grains. The weak interface between the large FeSn2 IMC particles and the -Sn matrix together with the presence of the large primary Sn grains results in a significant reduction on the elastic modulus and yield strength of the Fe-bearing solders. Moreover, the improved plasticity of the large primary -Sn grains causes the Fe-bearing solders to exhibit large total elongation. The addition of Fe also significantly reduces the effect of aging. After aging at 100 ◦ C and 180 ◦ C, it has been observed that the Fe-bearing solders significantly suppress the coarsening of the Ag3 Sn IMC particles; consequently, they exhibit stable mechanical properties. This effect can be attributed to the inclusion of Fe in the Ag3 Sn IMC particles. In addition, fracture surface analysis indicates that the addition of Fe to the SAC105 solder alloy does not affect the mode of fracture, and all tested solders exhibited large ductile-dimples on the fracture surface. Moreover, the addition of Fe did not produce any significant effect on the melting behavior. As a result, the use conditions of the Fe-bearing solders are consistent with the conditions for conventional Sn–Ag–Cu solder alloys. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Modern electronic assemblies must operate with high reliability in elevated temperature ranges, under high stresses, and under occasional shock loading [1–4]. In the automotive industry, where modern automobiles offer extended warranties of 100,000 miles and beyond, the electronic components in the engine compartment become increasingly complex [5]. Automated engine controls and sensors are incorporated to ensure engine operation at high temperatures, vibration levels, and stresses. These demanding operation conditions require electrical connections which possess high strength-enhanced creep resistance and increased thermal–mechanical fatigue resistance of up to 175 ◦ C [5,6]. High temperatures dramatically reduce the firmness of the solder joint during aging and/or thermal cycling because of the greater plastic deformation and grain coarsening of the solder [7–9]. Thus, the
∗ Corresponding author. Fax: +603 7967 5317/603 7967 7643. E-mail address: dhafer [email protected] (D.A.-A. Shnawah). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.04.115
mechanical and microstructural stability of the solder joint at high temperatures is of great concern. On the other hand, the portable electronics industry, which includes products such as mobile phones and digital cameras, exhibit an increasing trend toward miniaturization and high functionality [10–12]. This trend entails internal electronic components which are of higher density and smaller dimensions. Correspondingly, solder technology must keep up with market developments, as exemplified by the smaller solder joints provided by the ball grid array (BGA). However, the smaller solder joints of BGA packages are more vulnerable to thermal cycling and drop impact loading conditions [10–14]. In addition, the insertion of additional functionality in a smaller package leads to induce higher heat density. This increase in heat leads to grain coarsening, and thus mechanical degradation of the solder joints [10,13]. It is also of notable mention that the market boom in portable electronics products has grown hand in hand with the current demand for the solder industry to migrate into adopting lead-free solders in electronics. Sn–Pb solder alloy has remained the principal soldering material for a long time because of its superior thermal and mechanical
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Table 1 Chemical composition of the alloys (wt.%). Solder alloy
Sn
Ag
Cu
Fe
Pb
Sb
Al
As
Bi
Cd
Zn
In
Ni
SAC105 SAC105–0.1Fe SAC105–0.3Fe
98.3815 98.2544 98.0889
1.0446 1.0957 1.0676
0.5070 0.5230 0.5200
0.0021 0.1099 0.3064
0.0067 0.0059 0.0061
0.0042 0.0045 0.0046
0.0005 0.0005 0.0005
<0.0001 0.0015 0.0013
0.0019 0.0021 0.0022
0.0001 0.0001 0.0001
0.0002 0.0001 0.0001
0.0019 0.0022 0.0022
0.0493 – –
performance and its low cost in modern electronic packaging [15]. However, the inherent toxicity of Pb has raised serious environmental and public health concerns [16]. The Sn–Ag–Cu alloys with high-Ag-content, such as Sn–4 wt.%Ag–0.5 wt.%Cu (SAC405) and Sn–3 wt.%Ag–0.5 wt.%Cu (SAC305), are considered to be promising replacements for Sn–Pb solder alloy in microelectronics applications because of their low available melting temperatures, near-eutectic compositions, and good thermal mechanical fatigue properties [17,18]. However, because of the rigidity of the high-Agcontent Sn–Ag–Cu alloys compared with that of the Sn–Pb solder alloy, more failures occur in drop and high impact applications in portable electronic products [19–21]. Moreover, the high-Agcontent Sn–Ag–Cu alloys renders the cost of these solder alloys to be relatively high, and the world market for Ag is hard pressed to sustain the supply of Ag for the solder industry [22]. Low-Ag-content Sn–Ag–Cu alloys, such as Sn–1 wt.%Ag–0.5 wt.%Cu (SAC105), have been considered a solution to both of these issues. Reducing the Agcontent of the Sn–Ag–Cu alloy has been shown to increase its elastic compliance and plastic energy dissipation ability, which are identified as key factors to enhance the drop resistance [23,24]. However, this improvement in drop impact performance has been recognized to accompany degradation of thermal mechanical fatigue properties and aging resistance [25]. In view of the survey of existing solder materials, it would be desirable to improve the thermal mechanical fatigue properties and aging resistance without sacrificing the drop impact performance. A small number of studies have revealed that the addition of fourth alloying elements, such as Mn, Ce, Ni, Ti, or Bi, to low Ag-content Sn–Ag–Cu alloys provide a marked improvement in their microstructural modifications and mechanical properties, and these alloys have attracted considerable attention [26–30]. The net result of these minor alloying additions is to (1) alter the bulk alloy microstructure and mechanical properties and (2) control the interfacial intermetallic layer(s). Fe is a low-cost element, and the use of Fe is considered to be environmentally friendly because Fe is a nonhazardous material. Furthermore, recent studies have revealed that Fe-bearing solders significantly reduce the interface IMC layer growth and increase the shear strength of the solder joint [31–36]. However, to the best of our knowledge, the effect of Fe on the microstructure and mechanical properties of low-Agcontent SAC105 solder has not yet been investigated. The effect of Fe is expected to be significant because the Fe has little solubility in the -Sn matrix (and vice versa) below 200 ◦ C [37–40]. Moreover, the FeSn2 phase may precipitate in the solder [31,32,39]. It is well known that FeSn2 IMC particles undergo minimal growth and coarsening [31–34,39,41]. Therefore, this work investigates the effects of small additions (0.1 and 0.3 wt.%) of Fe on the microstructural, mechanical, and thermal properties of the Sn–1Ag–0.5Cu (SAC105) solder alloy. 2. Experimental procedures Bulk solder specimens of Sn–1Ag–0.5Cu (SAC105), Sn–1Ag–0.5Cu–0.1Fe (SAC105–0.1Fe), and Sn–1Ag–0.5Cu–0.3Fe (SAC105–0.3Fe) with flat dog-bone shapes were used in this study. The dimensions of the gauge sections of the tensile test specimens were 5.0 mm thick × 5.0 mm wide × 21 mm long. The alloys were prepared by melting pure ingots of Sn, Ag, Cu, and Fe in an induction furnace at more than 1000 ◦ C for 40 min. Then, the
molten alloys were mixed with liquid pure Sn in a melting furnace at 290–300 ◦ C for 60 min. Subsequently, the molten alloys were cast to disk shaped ingots and sent to a third party lab (SGS) to verify the Fe element concentration. Chemical composition analyses were carried out to determine the exact composition of the casting ingots (see Table 1). Then, the molten alloys were poured into stainless steel molds that were pre-heated at 120–130 ◦ C, and the molds were air cooled naturally to room temperature (25 ◦ C). The molds were disassembled, and the dog-bone samples were removed and visually inspected to ensure that the surface of the parallel area was without damage or voids. The melting and solidification behavior of the solder alloys was investigated using differential scanning calorimetry (DSC). The sample size was of approximately 2 mg, and the scanning rate was 5 ◦ C/min. For each alloy, the sample was first scanned from ambient temperature up to 300 ◦ C, followed by cooling down to ambient temperature at the same rate. Before the tensile testing, the specimens were annealed at 100 ◦ C for 2 h to decrease the residual stress that was induced during the sample preparation. Then, the solder bar was set onto a testing grip at two ends of the specimen using an Instron 5569A universal testing machine. An extensometer was secured onto the specimen surface to measure the strain of the solder. In this study, a length of 10 mm was used as a gauge length. The tensile force applied to the specimen was measured by a load cell for stress calculation. Fifteen samples were tested under the same testing conditions for each solder specimen in order to obtain reliable and repeatable results. Subsequently, the tensile properties were obtained by averaging the test data. In order to study the effect of aging on the bulk alloy microstructure and tensile properties, the specimens were isothermally aged at 100 ◦ C for 30 days and 180 ◦ C for 1 day. The tensile tests were conducted at room temperature (25 ◦ C) for the SAC SAC105 and Fe-bearing SAC105 solders under a strain rate of 10−3 s−1 to investigate the effects of the added Fe on the mechanical properties of the solder, such as the elastic modulus, yield stress, ultimate tensile strength (UTS), and elongation. In this paper, the elastic modulus, which is also called the Young’s modulus of the solder, was obtained from the elastic part of the tensile stress–strain curve. The yield stress of the solder was considered to be the stress value at which 0.2% plastic strain occurred. The UTS of the solder was considered to be the maximum stress in the stress–strain curve. A scanning electron microscope (SEM) with a backscattered electron detector was used to examine the microstructures. Additionally, energy dispersive X-ray spectroscopy was adopted to determine the phase compositions. Electron backscatter diffraction analysis was also carried out to determine the IMC phases. To obtain the microstructure, the solder samples were prepared by dicing, resin molding, grinding and polishing. The samples were ground with four grades of SiC paper (#800, #1200, #2400 and #4000) and then mechanically polished with a diamond suspension (3 m). Finally, the specimens were polished with a colloidal silica suspension (0.04 m). 3. Results and discussion 3.1. Tensile test The stress–strain curves of the as-cast SAC105, SAC105–0.1Fe, and SAC105–0.3Fe bulk solders are shown in Fig. 1. The figure shows
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Fig. 1. Stress–strain curves of as-cast SAC105, SAC105–0.1Fe, and SAC105–0.3Fe solders.
that the small additions (0.1 and 0.3 wt.%) of Fe to the SAC105 solder have a significant effect on the mechanical properties. Fig. 2 shows the elastic modulus, the 0.2% proof stresses, the ultimate tensile strengths (UTS), and the total elongation of the tested solders. The addition of Fe decreases the elastic modulus, yield strength, and UTS, whilst the total elongation is still maintained at SAC105 level. These changes increase the bulk compliance and the plastic energy dissipation ability of the solder joints. The elastic compliance and plastic energy dissipation ability are identified as key material properties to be optimized for extrinsic toughening mechanisms, which are believed to play an important role in the drop impact performance enhancement. Extrinsic toughening refers to a toughening mechanism that is attained by reducing the effective crack driving force experienced by the crack tip through various energy dissipation processes, without increasing the inherent fracture resistance of the material or the interface [42]. Specifically, high elastic compliance and high plastic energy dissipation ability effectively toughen the crack tip during crack propagation and
prolong the time required to reach the critical stress for fracture under high strain rate loading conditions. The strain rate experienced by solder joints during drop impact testing is estimated to be 102 s−1 , which falls within the dynamic-to-impact loading condition. Under this condition, metallic materials are generally subjected to a strain rate sensitivity phenomenon [23,24]. In other words, metallic materials (including solders) become stronger and plastic deformation becomes more difficult as the strain rate increases. Because solders typically operate at high homologous temperatures (approximately 0.6) (even at room temperature), they exhibit considerable strain-rate sensitivity. Therefore, the plastic deformation seems to be at least a secondary process in solder joints under drop impact loading conditions, and the elastic compliance becomes a significant material property that governs solder joint deformation. Fig. 3 shows the elastic part of the tensile stress–strain curve of the SAC105, SAC105–0.1Fe, and SAC105–0.3Fe bulk solders. The curve shows that the Fe-bearing solders possess higher elastic compliance than that of the SAC105 solder; as a result, the stress on the Fe-bearing bulk solders is lower than that on the SAC105 solder at the same strain. Thus, the Fe-bearing bulk solders with higher elastic compliance are expected to exhibit longer strain to failure than the SAC105 under high strain loading conditions. The mechanical properties of the SAC105, SAC105–0.1Fe, and SAC105–0.3Fe bulk solders after aging at 100 ◦ C for 30 days and 180 ◦ C for 1 day are shown in Fig. 2. It is clear that the elastic modulus and yield strength of the SAC105 solder decrease significantly and that the elongation increases whereas there were only slight changes in the elastic modulus, yield strength, and elongation for the Fe-bearing solders. The SAC105 solder experiences dramatic changes in its mechanical properties whereas the Fe-bearing solders exhibit stable mechanical properties with aging. The elastic modulus and yield strength of the Fe-bearing solders exceed those of the SAC105 solder after 180 ◦ C aging. Thus, the addition of small amounts (0.1 and 0.3 wt.%) of Fe can effectively enhance
Fig. 2. Tensile properties of SAC105, SAC105–0.1Fe, and SAC105–0.3Fe solders before and after aging: (a) elastic modulus, (b) yield stress, (c) UTS, and (d) total elongation.
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Fig. 3. The elastic part of stress–strain curves of as-cast SAC105, SAC105–0.1Fe, and SAC105–0.3Fe solders.
and stabilize the mechanical properties of SAC105 solder, which are important for thermal cycling survivability. 3.2. Bulk alloy microstructure properties The as-cast microstructure of the SAC105 solder alloy is composed of primary -Sn grains and eutectic regions that consist of two IMC particles dispersed within the Sn-rich matrix, as shown in Fig. 4. Using EBSD analysis, the bright IMC particles (approximately 0.11–0.72 m) are identified as Ag3 Sn, and the gray IMC particles (approximately 0.46–2.90 m) are identified as Cu6 Sn5 . These results are consistent with those of other studies [15,17]. The Ag3 Sn and Cu6 Sn5 phases are well known to possess a much higher strength than the bulk material in the SAC alloys whereas the primary -Sn phase has the lowest elastic modulus and lowest yield strength among the bulk constituent phases of the SAC alloys [23,24]. Hence, the high fraction of the primary -Sn phase reduces the elastic modulus and yield strength, which produce a soft and highly compliant bulk solder. On the other hand, the existence of large amounts of the Ag3 Sn and Cu6 Sn5 phases increases the elastic modulus and yield strength, which produce a stiff bulk solder. However, the volume fraction of Cu6 Sn5 in the SAC105 solder is much smaller than that of Ag3 Sn. Hence, the effect of Cu6 Sn5 on the mechanical properties is smaller than that of the Ag3 Sn. Therefore, it is sufficient to consider the effect of Ag3 Sn only. The microstructure of the SAC105 alloy, which is shown in Fig. 4, consists of large primary -Sn grains and fine Ag3 Sn IMC particles sparsely distributed within the eutectic regions. The presence of the large primary -Sn grains together with the sparse distribution
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of the Ag3 Sn IMC particles causes the SAC105 solder to exhibit a low elastic modulus and yield strength and a large elongation, which produce a soft and highly compliant bulk solder. In turn, these properties cause high dynamic energy to be dissipated through bulk solder deformation during drop impact loading conditions, reducing the dynamic stress transferred to the interface IMC layers. Therefore, electronic assemblies with low-Ag-content SAC105 solder joints have high drop lifetimes when subjected to drop impacts, which is consistent with the testing results reported by Syed et al. [14]. The as-cast microstructures of the SAC105–0.1Fe and SAC105–0.3Fe solders, which are shown in Fig. 5a and b, consist of large primary -Sn grains and eutectic regions of three distinct types of IMC particles dispersed in the Sn-rich matrix. The magnified micrographs of the eutectic regions of the SAC105–0.1Fe and SAC105–0.3Fe solders are shown in Fig. 5c and d, respectively. The EDS analysis results indicate that the gray circular particles (approximately 7.50–20.08 m) are Fe24.74–Sn75.26 (see Fig. 6a), the bright circular and rod-like IMC particles (approximately 0.095–0.68 m) are Ag47.49–Sn50.90–Fe01.61 (see Fig. 6b), and the gray circular and rod-like IMC particles (approximately 0.61–3.81 m) are Cu39.42–Sn56.70–Fe03.88 (see Fig. 6c). Using EBSD analysis, the large gray particles are identified as FeSn2 IMC, the fine bright particles are identified as Ag3 Sn IMC with a small amount of Fe, and the coarsened gray particles are identified as Cu6 Sn5 IMC with a small amount of Fe. Fe has little solubility in the -Sn matrix (and vice versa) below 200 ◦ C. Consequently, most of the Fe precipitates as the FeSn2 phase or other forms, such as pure Fe in the eutectic regions. However, in the present work, only the FeSn2 phase was identified in addition to the inclusion of Fe in the Ag3 Sn and Cu6 Sn5 phases. The large circular FeSn2 IMC particles are sparsely distributed within the bulk alloy microstructure located in the eutectic regions; hence, these FeSn2 IMC particles are not always observed in the eutectic regions. The fine Ag3 Sn IMC particles are sparsely distributed within the eutectic regions. Moreover, Fe-bearing solders have been shown to possess large primary -Sn grains (see Fig. 5). Fine particles in alloys are well known to effectively impede dislocation movement and produce an alloy with greater yield strength. When these particles grow in size, the yield strength decreases. In addition, when the coherence of the particles within the matrix is gradually lost with particle growth, the yield strength is further decreased [43]. In light of this mechanism, the formation of large FeSn2 IMC particles is believed to develop a weak interface with the -Sn matrix, facilitating crack initiation at this weak interface during tensile testing. Moreover, the Fe-bearing solders exhibit larger degree of undercooling than the SAC105 solder, as
Fig. 4. SEM micrographs of SAC105 solder.
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Fig. 5. SEM micrographs of Fe-bearing SAC105 solders.
Fig. 6. EDS analysis results of the IMC particles in the Fe-bearing SAC105 bulk solders.
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Fig. 7. SEM micrographs of SAC105 and Fe-bearing SAC105 solders after aging: (a and b) at 100 ◦ C for 30 days and (c and d) at 180 ◦ C for 1 day.
discussed in Section 3.3; consequently, these Fe-bearing solders are expected to possess larger primary -Sn grains that have the lowest elastic modulus and lowest yield strength among the bulk constituent phases. Thus, the weak interface between the large FeSn2 IMC particles and the -Sn matrix together with the presence of large primary -Sn grains causes the Fe-bearing solders to significantly reduce the elastics modulus and yield strength. It is interesting to note that the formation of large circular FeSn2 IMC particles in the eutectic regions does not affect the high plasticity of the large primary -Sn grains, which in turn causes the Fe-bearing solders to exhibit large total elongation. The microstructure of the SAC105, SAC105–0.1Fe, and SAC105–0.3Fe solders after 100 ◦ C and 180 ◦ C aging is shown in Fig. 7. Microstructural coarsening is visible for the SAC105 solder alloy. The Ag3 Sn and Cu6 Sn5 IMC particles in the SAC105 solder alloy clearly undergo Ostwald ripening during aging, and the primary -Sn grains coarsen as a result. Ostwald ripening is defined as the growth of larger crystals by the dissolution of those of smaller size that have a higher interfacial enthalpy than the larger ones. This phenomenon describes the growth of large particles through the dissolution of small particles [44]. Thus, the numbers of Ag3 Sn and Cu6 Sn5 IMC particles in the SAC105 solder decrease drastically after aging compared with the as-cast microstructures because of this Ostwald ripening process. It is well known that the coarsened second phases are less able to block dislocation movements, causing a loss of strength. Thus, the significant decrease of the elastic modulus and yield strength of the SAC105 solder after aging can be attributed to the coarsening of the Ag3 Sn and Cu6 Sn5 IMC particles. However, the contribution of Cu6 Sn5 coarsening to the evolution of the mechanical properties of the SAC105 solder is minimal compared with that of the Ag3 Sn. In comparison, from the micrographs shown in Fig. 7, it is evident
that the SAC105–0.1Fe and SAC105–0.3Fe solders significantly suppress the coarsening of the IMC particles during both 100 ◦ C and 180 ◦ C aging. Therefore, it can be deduced that the addition of Fe suppresses the coarsening of the IMC particles. The growth and coarsening of the FeSn2 IMC particles is minimal, and this result is consistent with previous reports [31–34,39,41]. This observation can be explained by the low solubility and diffusivity of Fe in Sn (and even non-reactivity with Sn), making the particles impervious to coarsening. In addition, the Ag3 Sn IMC particles in the Fe-bearing solders significantly resist the IMC particle coarsening compared with those in the SAC105 solder. This coarsening resistance can be attributed to the inclusion of a small amount of Fe in the Ag3 Sn IMC particles, as indicated in the EDS analysis results (see Fig. 6b). The inclusion of Fe is believed to reduce the vacancy diffusion rate that is required for IMC coarsening. It can be concluded that a stable IMC structure, which lead to stable microstructure, is the primary contributing factor to the stable mechanical properties exhibited during aging of the Fe-bearing solders. To further study the effect of Fe addition to the SAC105 solder alloy, the fracture surfaces of the SAC105, SAC105–0.1Fe, and SAC105–0.3Fe specimens were examined after the tensile tests. The necking phenomenon can be seen clearly in all tested specimens, indicating that the fracture is ductile (see Fig. 8a, c, and e). The SEM micrographs of the fracture surfaces of the tested specimens are shown in Fig. 8b, d, and f. The dimpled pattern is represented in all fracture surfaces. The fracture surface of the SAC105 specimen consists of large ductile-dimples, as shown in Figs. 8b. The fracture surface of the Fe-bearing solders specimens also consists of large ductile-dimples, in addition to a few large voids, as shown in Fig. 8d and f. Thus, the addition of Fe to the SAC105 alloy does not affect the mode of fracture. This observation can be explained as follows: the formation of large circular FeSn2 IMC particles produces
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Fig. 8. SEM fractographs of the alloys after tensile tests: (a and b) SAC105; (c and d) SAC105–0.1Fe; (e and f) SAC105–0.3Fe solders.
a weak interface with the -Sn matrix. Thus, in the early tensile stage, cracks may easily initiate at these weak interfaces, thus acting as nucleation sites for failure. However, the large FeSn2 IMC particles are sparsely distributed within the solder alloy microstructure. Moreover, in a limited time, the cracks surrounding these large FeSn2 IMC particles may not able to depart it from the propagation path of the boundary crack. Thus, these cracks are expected to be isolated, and as a result, they do not dominate the failure mechanism. On further straining, under the influence of the special structure of primary -Sn grains, these cracks are expected to be controlled to some extent in sizes and depths. Specifically, the presence of large primary -Sn grains can effectively toughen the crack tip and prolong the time required to reach the critical stress for failure through various energy dissipation processes activated by the high plasticity of the -Sn grains. With increasing loading time, the primary -Sn grains will be prolonged along the loading direction. The formation of large FeSn2 IMC particles in the form
of circular shape located in the eutectic regions does not affect the plastic deformation of the primary -Sn grains. The plastic deformations of the -Sn matrix and the IMC particles are not associated with each other. As a result, internal stress occurs at the interface between the matrix and the particles. The internal stress increases with the applied stress, and a microvoid nucleates at the interface when the stress becomes large enough. The IMC particles are the source of the microvoid nucleation. After microvoid nucleation, the microvoid grows under the applied stress in both the longitudinal and cross directions. Then, the microvoids coalesce, and the particles separate from the matrix completely. Finally, the matrix is drawn further until the sample necks down. 3.3. Thermal behavior DSC analysis was carried out in order to determine the effect of Fe addition on the thermal behavior of the SAC105 alloy. Fig. 9a
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Table 2 Differential scanning calorimetry (DSC) test results of the alloys. Alloys
Solidus temp. Ts (◦ C)
Liquidus temp. Tl (◦ C)
Pasty range
Onset soldifi. temp. Tc (◦ C)
Under cooling, Ts − Tl (◦ C)
SAC105 SAC105–0.1Fe SAC105–0.3Fe
217.52 217.40 217.63
226.97 226.42 226.48
9.45 9.02 8.85
207.82 194.35 197.34
9.70 23.05 20.29
and b shows the DSC endothermic and exothermic peaks of the samples upon heating and cooling, respectively. Fig. 9a reveals that the Fe-bearing solder alloys exhibit two endothermic peaks similar to those of the SAC105 alloy upon heating. The solidus and liquidus temperatures and pasty range of the solder alloys are measured and listed in Table 2. As can be seen, the addition of Fe in the SAC105 solder alloy has a negligible effect on the melting behavior. Consequently, the use conditions of the SAC105–0.1Fe and SAC105–0.3Fe solder alloys are consistent with the conditions for use of the conventional SAC alloy. However, Fig. 9b reveals that the Fe-bearing solders were observed to solidify at lower temperatures than the SAC105 alloy during cooling. The onset solidification temperatures and the degree of undercooling of the solder alloys were also measured; they are listed in Table 2. The degree of undercooling decreased from SAC105 containing 0.1 wt.% Fe to SAC105 containing 0.3 wt.% Fe, and then to SAC105. It is well known that the difficult nucleation of -Sn during solidification of Sn and Sn-based lead-free solder alloys can result in high degrees of undercooling of the liquid prior to solidification [45,46]. In other words, -Sn phase requires a large degree of undercooling to nucleate and solidify finally. Consequently, the Fe-bearing solders are expected to possess large primary -Sn grains because of its significant degree of undercooling in comparison with the SAC105 solder. However, more fundamental studies of the relationship between the undercooling and the microstructural behavior are under investigation.
4. Conclusion 1. The addition of Fe led to the formation of a small number of large circular FeSn2 IMC particles located in the eutectic regions in addition to the inclusion of Fe in the Ag3 Sn and Cu6 Sn5 IMC particles. The formation of these large FeSn2 IMC particles produces a weak interface with the -Sn matrix. 2. The Fe-bearing solders have been shown to form large primary -Sn grains. 3. The weak interface between the large FeSn2 IMC particles and the -Sn matrix together with the presence of the large primary -Sn grains has a great effect on the reduction of the elastic modulus and yield strength. 4. The inclusion of Fe in the Ag3 Sn IMC particles suppresses their IMC coarsening, which causes the Fe-bearing solders to exhibit stable mechanical properties with aging. 5. The addition of Fe to the Sn–1Ag–0.5Cu solder does not affect the mode of fracture, and all tested solders exhibited large ductiledimples on the fracture surface. 6. The addition of Fe did not cause any significant effect on the melting behavior, allowing the use of the Fe-containing Sn–1Ag–0.5Cu alloy to be consistent with the conditions of usage for the conventional Sn–Ag–Cu solder alloys. Acknowledgements The authors would like to acknowledge the financial support provided by University of Malaya (UM) under the IPPP Fund Project No.: PS117/2010B and the UMRG Fund under project No.: RG101/10AET. References
Fig. 9. DSC thermographs of SAC105 and Fe-bearing SAC105 solders.
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Microstructure, mechanical, and thermal properties of the Sn–1Ag–0.5Cu solder alloy bearing Fe for electronics applications Dhafer Abdul-Ameer Shnawah a,∗ , Suhana Binti Mohd Said b , Mohd Faizul Mohd Sabri a , Irfan Anjum Badruddin a , Fa Xing Che c a
Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Electrical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia c Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research), 11 Science Park Road, Singapore Science Park II, Singapore 117685, Singapore b
a r t i c l e
i n f o
Article history: Received 7 January 2012 Received in revised form 26 April 2012 Accepted 27 April 2012 Available online 11 May 2012 Keywords: Sn–1Ag–0.5Cu alloy Fe addition Mechanical properties Microstructure properties Thermal properties
a b s t r a c t This work investigates the effect of Fe addition on the microstructural, mechanical, and thermal properties of the Sn–1Ag–0.5Cu (SAC105) solder alloy. The addition of Fe leads to the formation of large circular FeSn2 intermetallic compound (IMC) particles, which produce a weak interface with the -Sn matrix. The addition of Fe also leads to the inclusion of Fe in the Ag3 Sn and Cu6 Sn5 IMC particles. Moreover, Fe-bearing solders have been shown to form large primary -Sn grains. The weak interface between the large FeSn2 IMC particles and the -Sn matrix together with the presence of the large primary Sn grains results in a significant reduction on the elastic modulus and yield strength of the Fe-bearing solders. Moreover, the improved plasticity of the large primary -Sn grains causes the Fe-bearing solders to exhibit large total elongation. The addition of Fe also significantly reduces the effect of aging. After aging at 100 ◦ C and 180 ◦ C, it has been observed that the Fe-bearing solders significantly suppress the coarsening of the Ag3 Sn IMC particles; consequently, they exhibit stable mechanical properties. This effect can be attributed to the inclusion of Fe in the Ag3 Sn IMC particles. In addition, fracture surface analysis indicates that the addition of Fe to the SAC105 solder alloy does not affect the mode of fracture, and all tested solders exhibited large ductile-dimples on the fracture surface. Moreover, the addition of Fe did not produce any significant effect on the melting behavior. As a result, the use conditions of the Fe-bearing solders are consistent with the conditions for conventional Sn–Ag–Cu solder alloys. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Modern electronic assemblies must operate with high reliability in elevated temperature ranges, under high stresses, and under occasional shock loading [1–4]. In the automotive industry, where modern automobiles offer extended warranties of 100,000 miles and beyond, the electronic components in the engine compartment become increasingly complex [5]. Automated engine controls and sensors are incorporated to ensure engine operation at high temperatures, vibration levels, and stresses. These demanding operation conditions require electrical connections which possess high strength-enhanced creep resistance and increased thermal–mechanical fatigue resistance of up to 175 ◦ C [5,6]. High temperatures dramatically reduce the firmness of the solder joint during aging and/or thermal cycling because of the greater plastic deformation and grain coarsening of the solder [7–9]. Thus, the
∗ Corresponding author. Fax: +603 7967 5317/603 7967 7643. E-mail address: dhafer [email protected] (D.A.-A. Shnawah). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.04.115
mechanical and microstructural stability of the solder joint at high temperatures is of great concern. On the other hand, the portable electronics industry, which includes products such as mobile phones and digital cameras, exhibit an increasing trend toward miniaturization and high functionality [10–12]. This trend entails internal electronic components which are of higher density and smaller dimensions. Correspondingly, solder technology must keep up with market developments, as exemplified by the smaller solder joints provided by the ball grid array (BGA). However, the smaller solder joints of BGA packages are more vulnerable to thermal cycling and drop impact loading conditions [10–14]. In addition, the insertion of additional functionality in a smaller package leads to induce higher heat density. This increase in heat leads to grain coarsening, and thus mechanical degradation of the solder joints [10,13]. It is also of notable mention that the market boom in portable electronics products has grown hand in hand with the current demand for the solder industry to migrate into adopting lead-free solders in electronics. Sn–Pb solder alloy has remained the principal soldering material for a long time because of its superior thermal and mechanical
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Table 1 Chemical composition of the alloys (wt.%). Solder alloy
Sn
Ag
Cu
Fe
Pb
Sb
Al
As
Bi
Cd
Zn
In
Ni
SAC105 SAC105–0.1Fe SAC105–0.3Fe
98.3815 98.2544 98.0889
1.0446 1.0957 1.0676
0.5070 0.5230 0.5200
0.0021 0.1099 0.3064
0.0067 0.0059 0.0061
0.0042 0.0045 0.0046
0.0005 0.0005 0.0005
<0.0001 0.0015 0.0013
0.0019 0.0021 0.0022
0.0001 0.0001 0.0001
0.0002 0.0001 0.0001
0.0019 0.0022 0.0022
0.0493 – –
performance and its low cost in modern electronic packaging [15]. However, the inherent toxicity of Pb has raised serious environmental and public health concerns [16]. The Sn–Ag–Cu alloys with high-Ag-content, such as Sn–4 wt.%Ag–0.5 wt.%Cu (SAC405) and Sn–3 wt.%Ag–0.5 wt.%Cu (SAC305), are considered to be promising replacements for Sn–Pb solder alloy in microelectronics applications because of their low available melting temperatures, near-eutectic compositions, and good thermal mechanical fatigue properties [17,18]. However, because of the rigidity of the high-Agcontent Sn–Ag–Cu alloys compared with that of the Sn–Pb solder alloy, more failures occur in drop and high impact applications in portable electronic products [19–21]. Moreover, the high-Agcontent Sn–Ag–Cu alloys renders the cost of these solder alloys to be relatively high, and the world market for Ag is hard pressed to sustain the supply of Ag for the solder industry [22]. Low-Ag-content Sn–Ag–Cu alloys, such as Sn–1 wt.%Ag–0.5 wt.%Cu (SAC105), have been considered a solution to both of these issues. Reducing the Agcontent of the Sn–Ag–Cu alloy has been shown to increase its elastic compliance and plastic energy dissipation ability, which are identified as key factors to enhance the drop resistance [23,24]. However, this improvement in drop impact performance has been recognized to accompany degradation of thermal mechanical fatigue properties and aging resistance [25]. In view of the survey of existing solder materials, it would be desirable to improve the thermal mechanical fatigue properties and aging resistance without sacrificing the drop impact performance. A small number of studies have revealed that the addition of fourth alloying elements, such as Mn, Ce, Ni, Ti, or Bi, to low Ag-content Sn–Ag–Cu alloys provide a marked improvement in their microstructural modifications and mechanical properties, and these alloys have attracted considerable attention [26–30]. The net result of these minor alloying additions is to (1) alter the bulk alloy microstructure and mechanical properties and (2) control the interfacial intermetallic layer(s). Fe is a low-cost element, and the use of Fe is considered to be environmentally friendly because Fe is a nonhazardous material. Furthermore, recent studies have revealed that Fe-bearing solders significantly reduce the interface IMC layer growth and increase the shear strength of the solder joint [31–36]. However, to the best of our knowledge, the effect of Fe on the microstructure and mechanical properties of low-Agcontent SAC105 solder has not yet been investigated. The effect of Fe is expected to be significant because the Fe has little solubility in the -Sn matrix (and vice versa) below 200 ◦ C [37–40]. Moreover, the FeSn2 phase may precipitate in the solder [31,32,39]. It is well known that FeSn2 IMC particles undergo minimal growth and coarsening [31–34,39,41]. Therefore, this work investigates the effects of small additions (0.1 and 0.3 wt.%) of Fe on the microstructural, mechanical, and thermal properties of the Sn–1Ag–0.5Cu (SAC105) solder alloy. 2. Experimental procedures Bulk solder specimens of Sn–1Ag–0.5Cu (SAC105), Sn–1Ag–0.5Cu–0.1Fe (SAC105–0.1Fe), and Sn–1Ag–0.5Cu–0.3Fe (SAC105–0.3Fe) with flat dog-bone shapes were used in this study. The dimensions of the gauge sections of the tensile test specimens were 5.0 mm thick × 5.0 mm wide × 21 mm long. The alloys were prepared by melting pure ingots of Sn, Ag, Cu, and Fe in an induction furnace at more than 1000 ◦ C for 40 min. Then, the
molten alloys were mixed with liquid pure Sn in a melting furnace at 290–300 ◦ C for 60 min. Subsequently, the molten alloys were cast to disk shaped ingots and sent to a third party lab (SGS) to verify the Fe element concentration. Chemical composition analyses were carried out to determine the exact composition of the casting ingots (see Table 1). Then, the molten alloys were poured into stainless steel molds that were pre-heated at 120–130 ◦ C, and the molds were air cooled naturally to room temperature (25 ◦ C). The molds were disassembled, and the dog-bone samples were removed and visually inspected to ensure that the surface of the parallel area was without damage or voids. The melting and solidification behavior of the solder alloys was investigated using differential scanning calorimetry (DSC). The sample size was of approximately 2 mg, and the scanning rate was 5 ◦ C/min. For each alloy, the sample was first scanned from ambient temperature up to 300 ◦ C, followed by cooling down to ambient temperature at the same rate. Before the tensile testing, the specimens were annealed at 100 ◦ C for 2 h to decrease the residual stress that was induced during the sample preparation. Then, the solder bar was set onto a testing grip at two ends of the specimen using an Instron 5569A universal testing machine. An extensometer was secured onto the specimen surface to measure the strain of the solder. In this study, a length of 10 mm was used as a gauge length. The tensile force applied to the specimen was measured by a load cell for stress calculation. Fifteen samples were tested under the same testing conditions for each solder specimen in order to obtain reliable and repeatable results. Subsequently, the tensile properties were obtained by averaging the test data. In order to study the effect of aging on the bulk alloy microstructure and tensile properties, the specimens were isothermally aged at 100 ◦ C for 30 days and 180 ◦ C for 1 day. The tensile tests were conducted at room temperature (25 ◦ C) for the SAC SAC105 and Fe-bearing SAC105 solders under a strain rate of 10−3 s−1 to investigate the effects of the added Fe on the mechanical properties of the solder, such as the elastic modulus, yield stress, ultimate tensile strength (UTS), and elongation. In this paper, the elastic modulus, which is also called the Young’s modulus of the solder, was obtained from the elastic part of the tensile stress–strain curve. The yield stress of the solder was considered to be the stress value at which 0.2% plastic strain occurred. The UTS of the solder was considered to be the maximum stress in the stress–strain curve. A scanning electron microscope (SEM) with a backscattered electron detector was used to examine the microstructures. Additionally, energy dispersive X-ray spectroscopy was adopted to determine the phase compositions. Electron backscatter diffraction analysis was also carried out to determine the IMC phases. To obtain the microstructure, the solder samples were prepared by dicing, resin molding, grinding and polishing. The samples were ground with four grades of SiC paper (#800, #1200, #2400 and #4000) and then mechanically polished with a diamond suspension (3 m). Finally, the specimens were polished with a colloidal silica suspension (0.04 m). 3. Results and discussion 3.1. Tensile test The stress–strain curves of the as-cast SAC105, SAC105–0.1Fe, and SAC105–0.3Fe bulk solders are shown in Fig. 1. The figure shows
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Fig. 1. Stress–strain curves of as-cast SAC105, SAC105–0.1Fe, and SAC105–0.3Fe solders.
that the small additions (0.1 and 0.3 wt.%) of Fe to the SAC105 solder have a significant effect on the mechanical properties. Fig. 2 shows the elastic modulus, the 0.2% proof stresses, the ultimate tensile strengths (UTS), and the total elongation of the tested solders. The addition of Fe decreases the elastic modulus, yield strength, and UTS, whilst the total elongation is still maintained at SAC105 level. These changes increase the bulk compliance and the plastic energy dissipation ability of the solder joints. The elastic compliance and plastic energy dissipation ability are identified as key material properties to be optimized for extrinsic toughening mechanisms, which are believed to play an important role in the drop impact performance enhancement. Extrinsic toughening refers to a toughening mechanism that is attained by reducing the effective crack driving force experienced by the crack tip through various energy dissipation processes, without increasing the inherent fracture resistance of the material or the interface [42]. Specifically, high elastic compliance and high plastic energy dissipation ability effectively toughen the crack tip during crack propagation and
prolong the time required to reach the critical stress for fracture under high strain rate loading conditions. The strain rate experienced by solder joints during drop impact testing is estimated to be 102 s−1 , which falls within the dynamic-to-impact loading condition. Under this condition, metallic materials are generally subjected to a strain rate sensitivity phenomenon [23,24]. In other words, metallic materials (including solders) become stronger and plastic deformation becomes more difficult as the strain rate increases. Because solders typically operate at high homologous temperatures (approximately 0.6) (even at room temperature), they exhibit considerable strain-rate sensitivity. Therefore, the plastic deformation seems to be at least a secondary process in solder joints under drop impact loading conditions, and the elastic compliance becomes a significant material property that governs solder joint deformation. Fig. 3 shows the elastic part of the tensile stress–strain curve of the SAC105, SAC105–0.1Fe, and SAC105–0.3Fe bulk solders. The curve shows that the Fe-bearing solders possess higher elastic compliance than that of the SAC105 solder; as a result, the stress on the Fe-bearing bulk solders is lower than that on the SAC105 solder at the same strain. Thus, the Fe-bearing bulk solders with higher elastic compliance are expected to exhibit longer strain to failure than the SAC105 under high strain loading conditions. The mechanical properties of the SAC105, SAC105–0.1Fe, and SAC105–0.3Fe bulk solders after aging at 100 ◦ C for 30 days and 180 ◦ C for 1 day are shown in Fig. 2. It is clear that the elastic modulus and yield strength of the SAC105 solder decrease significantly and that the elongation increases whereas there were only slight changes in the elastic modulus, yield strength, and elongation for the Fe-bearing solders. The SAC105 solder experiences dramatic changes in its mechanical properties whereas the Fe-bearing solders exhibit stable mechanical properties with aging. The elastic modulus and yield strength of the Fe-bearing solders exceed those of the SAC105 solder after 180 ◦ C aging. Thus, the addition of small amounts (0.1 and 0.3 wt.%) of Fe can effectively enhance
Fig. 2. Tensile properties of SAC105, SAC105–0.1Fe, and SAC105–0.3Fe solders before and after aging: (a) elastic modulus, (b) yield stress, (c) UTS, and (d) total elongation.
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Fig. 3. The elastic part of stress–strain curves of as-cast SAC105, SAC105–0.1Fe, and SAC105–0.3Fe solders.
and stabilize the mechanical properties of SAC105 solder, which are important for thermal cycling survivability. 3.2. Bulk alloy microstructure properties The as-cast microstructure of the SAC105 solder alloy is composed of primary -Sn grains and eutectic regions that consist of two IMC particles dispersed within the Sn-rich matrix, as shown in Fig. 4. Using EBSD analysis, the bright IMC particles (approximately 0.11–0.72 m) are identified as Ag3 Sn, and the gray IMC particles (approximately 0.46–2.90 m) are identified as Cu6 Sn5 . These results are consistent with those of other studies [15,17]. The Ag3 Sn and Cu6 Sn5 phases are well known to possess a much higher strength than the bulk material in the SAC alloys whereas the primary -Sn phase has the lowest elastic modulus and lowest yield strength among the bulk constituent phases of the SAC alloys [23,24]. Hence, the high fraction of the primary -Sn phase reduces the elastic modulus and yield strength, which produce a soft and highly compliant bulk solder. On the other hand, the existence of large amounts of the Ag3 Sn and Cu6 Sn5 phases increases the elastic modulus and yield strength, which produce a stiff bulk solder. However, the volume fraction of Cu6 Sn5 in the SAC105 solder is much smaller than that of Ag3 Sn. Hence, the effect of Cu6 Sn5 on the mechanical properties is smaller than that of the Ag3 Sn. Therefore, it is sufficient to consider the effect of Ag3 Sn only. The microstructure of the SAC105 alloy, which is shown in Fig. 4, consists of large primary -Sn grains and fine Ag3 Sn IMC particles sparsely distributed within the eutectic regions. The presence of the large primary -Sn grains together with the sparse distribution
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of the Ag3 Sn IMC particles causes the SAC105 solder to exhibit a low elastic modulus and yield strength and a large elongation, which produce a soft and highly compliant bulk solder. In turn, these properties cause high dynamic energy to be dissipated through bulk solder deformation during drop impact loading conditions, reducing the dynamic stress transferred to the interface IMC layers. Therefore, electronic assemblies with low-Ag-content SAC105 solder joints have high drop lifetimes when subjected to drop impacts, which is consistent with the testing results reported by Syed et al. [14]. The as-cast microstructures of the SAC105–0.1Fe and SAC105–0.3Fe solders, which are shown in Fig. 5a and b, consist of large primary -Sn grains and eutectic regions of three distinct types of IMC particles dispersed in the Sn-rich matrix. The magnified micrographs of the eutectic regions of the SAC105–0.1Fe and SAC105–0.3Fe solders are shown in Fig. 5c and d, respectively. The EDS analysis results indicate that the gray circular particles (approximately 7.50–20.08 m) are Fe24.74–Sn75.26 (see Fig. 6a), the bright circular and rod-like IMC particles (approximately 0.095–0.68 m) are Ag47.49–Sn50.90–Fe01.61 (see Fig. 6b), and the gray circular and rod-like IMC particles (approximately 0.61–3.81 m) are Cu39.42–Sn56.70–Fe03.88 (see Fig. 6c). Using EBSD analysis, the large gray particles are identified as FeSn2 IMC, the fine bright particles are identified as Ag3 Sn IMC with a small amount of Fe, and the coarsened gray particles are identified as Cu6 Sn5 IMC with a small amount of Fe. Fe has little solubility in the -Sn matrix (and vice versa) below 200 ◦ C. Consequently, most of the Fe precipitates as the FeSn2 phase or other forms, such as pure Fe in the eutectic regions. However, in the present work, only the FeSn2 phase was identified in addition to the inclusion of Fe in the Ag3 Sn and Cu6 Sn5 phases. The large circular FeSn2 IMC particles are sparsely distributed within the bulk alloy microstructure located in the eutectic regions; hence, these FeSn2 IMC particles are not always observed in the eutectic regions. The fine Ag3 Sn IMC particles are sparsely distributed within the eutectic regions. Moreover, Fe-bearing solders have been shown to possess large primary -Sn grains (see Fig. 5). Fine particles in alloys are well known to effectively impede dislocation movement and produce an alloy with greater yield strength. When these particles grow in size, the yield strength decreases. In addition, when the coherence of the particles within the matrix is gradually lost with particle growth, the yield strength is further decreased [43]. In light of this mechanism, the formation of large FeSn2 IMC particles is believed to develop a weak interface with the -Sn matrix, facilitating crack initiation at this weak interface during tensile testing. Moreover, the Fe-bearing solders exhibit larger degree of undercooling than the SAC105 solder, as
Fig. 4. SEM micrographs of SAC105 solder.
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Fig. 5. SEM micrographs of Fe-bearing SAC105 solders.
Fig. 6. EDS analysis results of the IMC particles in the Fe-bearing SAC105 bulk solders.
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Fig. 7. SEM micrographs of SAC105 and Fe-bearing SAC105 solders after aging: (a and b) at 100 ◦ C for 30 days and (c and d) at 180 ◦ C for 1 day.
discussed in Section 3.3; consequently, these Fe-bearing solders are expected to possess larger primary -Sn grains that have the lowest elastic modulus and lowest yield strength among the bulk constituent phases. Thus, the weak interface between the large FeSn2 IMC particles and the -Sn matrix together with the presence of large primary -Sn grains causes the Fe-bearing solders to significantly reduce the elastics modulus and yield strength. It is interesting to note that the formation of large circular FeSn2 IMC particles in the eutectic regions does not affect the high plasticity of the large primary -Sn grains, which in turn causes the Fe-bearing solders to exhibit large total elongation. The microstructure of the SAC105, SAC105–0.1Fe, and SAC105–0.3Fe solders after 100 ◦ C and 180 ◦ C aging is shown in Fig. 7. Microstructural coarsening is visible for the SAC105 solder alloy. The Ag3 Sn and Cu6 Sn5 IMC particles in the SAC105 solder alloy clearly undergo Ostwald ripening during aging, and the primary -Sn grains coarsen as a result. Ostwald ripening is defined as the growth of larger crystals by the dissolution of those of smaller size that have a higher interfacial enthalpy than the larger ones. This phenomenon describes the growth of large particles through the dissolution of small particles [44]. Thus, the numbers of Ag3 Sn and Cu6 Sn5 IMC particles in the SAC105 solder decrease drastically after aging compared with the as-cast microstructures because of this Ostwald ripening process. It is well known that the coarsened second phases are less able to block dislocation movements, causing a loss of strength. Thus, the significant decrease of the elastic modulus and yield strength of the SAC105 solder after aging can be attributed to the coarsening of the Ag3 Sn and Cu6 Sn5 IMC particles. However, the contribution of Cu6 Sn5 coarsening to the evolution of the mechanical properties of the SAC105 solder is minimal compared with that of the Ag3 Sn. In comparison, from the micrographs shown in Fig. 7, it is evident
that the SAC105–0.1Fe and SAC105–0.3Fe solders significantly suppress the coarsening of the IMC particles during both 100 ◦ C and 180 ◦ C aging. Therefore, it can be deduced that the addition of Fe suppresses the coarsening of the IMC particles. The growth and coarsening of the FeSn2 IMC particles is minimal, and this result is consistent with previous reports [31–34,39,41]. This observation can be explained by the low solubility and diffusivity of Fe in Sn (and even non-reactivity with Sn), making the particles impervious to coarsening. In addition, the Ag3 Sn IMC particles in the Fe-bearing solders significantly resist the IMC particle coarsening compared with those in the SAC105 solder. This coarsening resistance can be attributed to the inclusion of a small amount of Fe in the Ag3 Sn IMC particles, as indicated in the EDS analysis results (see Fig. 6b). The inclusion of Fe is believed to reduce the vacancy diffusion rate that is required for IMC coarsening. It can be concluded that a stable IMC structure, which lead to stable microstructure, is the primary contributing factor to the stable mechanical properties exhibited during aging of the Fe-bearing solders. To further study the effect of Fe addition to the SAC105 solder alloy, the fracture surfaces of the SAC105, SAC105–0.1Fe, and SAC105–0.3Fe specimens were examined after the tensile tests. The necking phenomenon can be seen clearly in all tested specimens, indicating that the fracture is ductile (see Fig. 8a, c, and e). The SEM micrographs of the fracture surfaces of the tested specimens are shown in Fig. 8b, d, and f. The dimpled pattern is represented in all fracture surfaces. The fracture surface of the SAC105 specimen consists of large ductile-dimples, as shown in Figs. 8b. The fracture surface of the Fe-bearing solders specimens also consists of large ductile-dimples, in addition to a few large voids, as shown in Fig. 8d and f. Thus, the addition of Fe to the SAC105 alloy does not affect the mode of fracture. This observation can be explained as follows: the formation of large circular FeSn2 IMC particles produces
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Fig. 8. SEM fractographs of the alloys after tensile tests: (a and b) SAC105; (c and d) SAC105–0.1Fe; (e and f) SAC105–0.3Fe solders.
a weak interface with the -Sn matrix. Thus, in the early tensile stage, cracks may easily initiate at these weak interfaces, thus acting as nucleation sites for failure. However, the large FeSn2 IMC particles are sparsely distributed within the solder alloy microstructure. Moreover, in a limited time, the cracks surrounding these large FeSn2 IMC particles may not able to depart it from the propagation path of the boundary crack. Thus, these cracks are expected to be isolated, and as a result, they do not dominate the failure mechanism. On further straining, under the influence of the special structure of primary -Sn grains, these cracks are expected to be controlled to some extent in sizes and depths. Specifically, the presence of large primary -Sn grains can effectively toughen the crack tip and prolong the time required to reach the critical stress for failure through various energy dissipation processes activated by the high plasticity of the -Sn grains. With increasing loading time, the primary -Sn grains will be prolonged along the loading direction. The formation of large FeSn2 IMC particles in the form
of circular shape located in the eutectic regions does not affect the plastic deformation of the primary -Sn grains. The plastic deformations of the -Sn matrix and the IMC particles are not associated with each other. As a result, internal stress occurs at the interface between the matrix and the particles. The internal stress increases with the applied stress, and a microvoid nucleates at the interface when the stress becomes large enough. The IMC particles are the source of the microvoid nucleation. After microvoid nucleation, the microvoid grows under the applied stress in both the longitudinal and cross directions. Then, the microvoids coalesce, and the particles separate from the matrix completely. Finally, the matrix is drawn further until the sample necks down. 3.3. Thermal behavior DSC analysis was carried out in order to determine the effect of Fe addition on the thermal behavior of the SAC105 alloy. Fig. 9a
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Table 2 Differential scanning calorimetry (DSC) test results of the alloys. Alloys
Solidus temp. Ts (◦ C)
Liquidus temp. Tl (◦ C)
Pasty range
Onset soldifi. temp. Tc (◦ C)
Under cooling, Ts − Tl (◦ C)
SAC105 SAC105–0.1Fe SAC105–0.3Fe
217.52 217.40 217.63
226.97 226.42 226.48
9.45 9.02 8.85
207.82 194.35 197.34
9.70 23.05 20.29
and b shows the DSC endothermic and exothermic peaks of the samples upon heating and cooling, respectively. Fig. 9a reveals that the Fe-bearing solder alloys exhibit two endothermic peaks similar to those of the SAC105 alloy upon heating. The solidus and liquidus temperatures and pasty range of the solder alloys are measured and listed in Table 2. As can be seen, the addition of Fe in the SAC105 solder alloy has a negligible effect on the melting behavior. Consequently, the use conditions of the SAC105–0.1Fe and SAC105–0.3Fe solder alloys are consistent with the conditions for use of the conventional SAC alloy. However, Fig. 9b reveals that the Fe-bearing solders were observed to solidify at lower temperatures than the SAC105 alloy during cooling. The onset solidification temperatures and the degree of undercooling of the solder alloys were also measured; they are listed in Table 2. The degree of undercooling decreased from SAC105 containing 0.1 wt.% Fe to SAC105 containing 0.3 wt.% Fe, and then to SAC105. It is well known that the difficult nucleation of -Sn during solidification of Sn and Sn-based lead-free solder alloys can result in high degrees of undercooling of the liquid prior to solidification [45,46]. In other words, -Sn phase requires a large degree of undercooling to nucleate and solidify finally. Consequently, the Fe-bearing solders are expected to possess large primary -Sn grains because of its significant degree of undercooling in comparison with the SAC105 solder. However, more fundamental studies of the relationship between the undercooling and the microstructural behavior are under investigation.
4. Conclusion 1. The addition of Fe led to the formation of a small number of large circular FeSn2 IMC particles located in the eutectic regions in addition to the inclusion of Fe in the Ag3 Sn and Cu6 Sn5 IMC particles. The formation of these large FeSn2 IMC particles produces a weak interface with the -Sn matrix. 2. The Fe-bearing solders have been shown to form large primary -Sn grains. 3. The weak interface between the large FeSn2 IMC particles and the -Sn matrix together with the presence of the large primary -Sn grains has a great effect on the reduction of the elastic modulus and yield strength. 4. The inclusion of Fe in the Ag3 Sn IMC particles suppresses their IMC coarsening, which causes the Fe-bearing solders to exhibit stable mechanical properties with aging. 5. The addition of Fe to the Sn–1Ag–0.5Cu solder does not affect the mode of fracture, and all tested solders exhibited large ductiledimples on the fracture surface. 6. The addition of Fe did not cause any significant effect on the melting behavior, allowing the use of the Fe-containing Sn–1Ag–0.5Cu alloy to be consistent with the conditions of usage for the conventional Sn–Ag–Cu solder alloys. Acknowledgements The authors would like to acknowledge the financial support provided by University of Malaya (UM) under the IPPP Fund Project No.: PS117/2010B and the UMRG Fund under project No.: RG101/10AET. References
Fig. 9. DSC thermographs of SAC105 and Fe-bearing SAC105 solders.
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