Influence of La2O3 nanoparticle additions on microstructure, wetting, and tensile characteristics of Sn–Ag–Cu alloy

Materials and Design 87 (2015) 370–379

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Influence of La2O3 nanoparticle additions on microstructure, wetting, and tensile characteristics of Sn–Ag–Cu alloy Ashutosh Sharma a, B.G. Baek b, Jae Pil Jung a,⁎ a b

Department of Materials Science and Engineering, University of Seoul, Seoul 130-743, Republic of Korea KD One Co. Ltd., 22 Gukhoedaero 76-gil, Seoul 150-729, Republic of Korea

a r t i c l e

i n f o

Article history: Received 12 November 2014 Received in revised form 12 June 2015 Accepted 25 July 2015 Available online 1 August 2015 Keywords: Lead free Wettability Composite Nanoparticle Microstructure Tensile

a b s t r a c t In this study, the effect of La2O3 nanoparticles (0, 0.01, 0.03, 0.05 and 0.1 wt.%) has been investigated in Sn–3.0Ag–0.5Cu (SAC-305) alloy. The various soldering properties have been tested, such as wettability, microstructural evolution, intermetallic compound formation, micro-hardness, tensile strength, and fracture analysis of tensile tested samples. La 2O 3 nanoparticles are added in the Sn–3.0Ag–0.5Cu alloy by mechanical mixing of powders and melting. The structural and morphological features of the samples are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and electron probe micro-analyzer (EPMA). The experimental results indicate that the best combination of microstructural, wetting and tensile properties is obtained at 0.05 wt.% La2O3 in the solder matrix. The sample reinforced with 0.05 wt.% La2O3 i.e., SAC-0.05 La2O3 exhibits ~ 18% increase in microhardness, ~ 26% increase in the ultimate tensile strength (UTS), and ~ 14% elongation due to the adsorption of high surface energy of La2O3 nanoparticles in the matrix. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Lead free soldering is not an exception nowadays in microelectronic packaging industries. There has been an enormous amount of research activities on the development of lead free solders all over the world. A major issue of environmental hazard associated with micro-joining is the use of solders having lead as the prime alloying material. Therefore, highly toxic lead is now regulated in microelectronic industries due to the WEEE and RoHS directives [1]. Rapid growth in consumer electronic appliances in the last two decades has accelerated the lead free trends in device manufacturing. Solders provide connections among various interconnecting networks in the device and maintain their integrity. Various lead free systems for soldering applications are developed such as Sn–Cu, Sn–Ag, Sn–Bi, Sn–Zn, Sn–Ag–Cu, and Sn–Ag–Bi–In to provide a cost effective replacement of the Pb–Sn solder [1–6]. The most popular alloy in terms of similar properties and cost is the Sn–Ag–Cu system having a low melting point and better wettability; however the mechanical properties are still a concern due to the formation of large Ag3Sn and Cu6Sn5 intermetallic compounds [5]. These IMCs may have a serious concern over long term reliability of the device, if formed in excess they may cause failure of the joint leading to the failure of the entire device [7,8]. Hence, in order to fulfill these demands, a new solder has to be designed using the modern nanotechnology where harder nanoparticles are added in a soft metallic matrix. The ⁎ Corresponding author. E-mail address: [email protected] (J.P. Jung).

http://dx.doi.org/10.1016/j.matdes.2015.07.137 0264-1275/© 2015 Elsevier Ltd. All rights reserved.

solderability of the desired solder alloy is also an important factor for Cu or Ni based metallization. Haseeb et al. [9] studied the effect of Mn nanoparticles on wettability between Sn–3.8Cu–0.7Cu and Cu substrate, and reported a decrease in wetting of the solder. On the other hand, addition of Cu nanoparticles has been reported to improve the wettability of solders. However, addition of copper may result in extra Cu6Sn5 IMCs [10]. The addition of Ag nanoparticles has been also shown to increase the solderability [11]. However, due to the increasing price of silver in international market, use of Ag is not justified. The uses of rare earth elements have also been shown to improve the solderability of the matrix [12]. Among ceramic additives, Si3N4 particulates have been added in Sn–0.7Cu solder and an improved wetting has been obtained [13]. Ceramic additives have an advantage over metallic nanoparticles, and are generally used due to their inert nature, i.e., they do not react with the solder matrix. Recently, carbon nanotubes (CNT), and graphene reinforced solders have shown promising solderability [14, 15]. However, the dispersion of these nano-reinforcements needs extra care while fabrication, as they may float on the substrate easily [16]. The novel properties obtained by the use of nanoparticles have driven research community to develop nanocomposite solders. A few researchers have already adopted composite approach to produce tin based nanocomposite type solders such as, Sn–Cu/Al2O3, Sn–Cu/Si3N4, Sn–Ag/SnO2, Sn–Ag–Cu/TiO2 Sn–Ag–Cu/SiC, Sn–Ag–Cu/ZrO2, Sn–Ag– Cu/ZnO, and Sn–Ag–Cu/CNT [17–23]. Rare earth oxides are also used as reinforcement to improve microstructural and mechanical properties of the solder matrix composites, already available in recent reports on Sn/CeO2, Sn–Ag/CeO2, and Sn–Ag–Cu/CeO2 [24–26]. Among rare earth

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oxides, La2O3 oxide has been used rarely to reinforce the Sn–Ag–Cu solder matrix, which can be a promising candidate for developing a nanocomposite solder. The main advantages of these rare earth oxides are higher density as compared to their other counterparts, i.e., density of Al2O3: 3.97 g/cm3, TiO2: 3.89 g/cm3, and for Sn–3Ag– 0.5Cu: 7.11 g/cm3 which is close to that of La2O3: 6.51 g/cm3; and a greater hardness compared with that of a Sn–3Ag–0.5Cu matrix. In the present study, an attempt has been made to reinforce La2O3 nanoparticles in the Sn–3.0Ag–0.5Cu (SAC 305) alloy matrix using the conventional mixing and casting of the solder paste. The amount of the La2O3 nanoparticles has been varied from 0, 0.01, 0.03, 0.05, to 0.1 wt.% in the matrix. The wetting and spreading properties, microstructural properties, and microhardness of the solders reinforced with La2O3 nanoparticles are also investigated. The ultimate tensile strength, elongation percentage as well as the fractured microstructure of the samples have also been analyzed.

samples with different La2O3 nanoparticles and Sn–3.0 wt.%Ag–0.5 wt.%Cu samples are reported. 2.5. Spreading test The spreading test is performed following a JIS-Z-3197 standard [27]. A copper (Cu) coupon (99.99% pure) of 30 mm × 30 mm × 0.3 mm size is used as a test specimen. The Cu coupon is finely polished and cleaned with alcohol followed by drying. After drying, the coupon is heated to a temperature of 150 °C in a low temperature oven for 1 h to produce a uniform oxidized surface. After that, 0.3 g solder powder mixed with approximately 0.03 g of flux is applied to the center of the Cu coupon. The coupon is then mounted on a solder bath maintained at 300 °C. After some time the solder placed at the center of the coupon begins to melt. The copper plate is taken off the solder bath after 30 s to cool at room temperature. The measurement of spread area ratio is performed according to the relation:

2. Experimental procedure 2.1. Materials Lanthanum oxide powder (La2O3, Dae-Jung Chemicals Co. Ltd., Gyeonggi-do, South Korea, N 99.8% purity) with an average particle size of 30 nm is chosen as reinforcement material. The solder matrix material consists of Type IV, Sn–3.0 wt.%Ag–0.5 wt.%Cu (SAC 305) alloy powder (N99.8% purity) having an average particle size of 20–38 μm. 2.2. Mixing of powders The composite powder is prepared by manually mixing the Sn–3.0 wt.%Ag–0.5 wt.%Cu powder with La2O3 nanopowder in compositions varying from 0.01 to 0.1 wt.% in a crucible for 1 h. After that the powder mixture is transferred to a planetary ball mill (Fritsch Pulverisette P-6, Germany) for uniform mixing of the powders. The mill is operated at 200 rpm and the mixing is carried out for 1 h at room temperature. 2.3. Composite fabrication After mixing, the powder mixture is taken out from the ball mill, and mixed with soldering flux (Sparkle Flux, SENJU WF-6063 M5) in an alumina crucible. The solder and the flux are mixed in a ratio of 9:1, with a spatula manually for 5 min to form a uniformly dispersed solder paste. The solder paste thus obtained is melted in a vacuum melting furnace 30 min at 500 °C with 10 °C per minute heating rate, followed by furnace cooling to room temperature. After the solder is solidified, around 5 g of the solder is mounted in an epoxy resin for further characterization. 2.4. Characterization details The mounted sample is subjected to metallographic grinding, polishing and etching treatments. Grinding is carried out using different 400#, 800#, 1200#, 2400# and 2400# grades of SiC papers. After grinding, the sample is polished using a fine alumina suspension (~ 1 μm) followed by diamond polishing. After polishing, the samples are etched with 92 vol.% CH3OH–5vol.% HNO3–3 vol.% HCl solution for 30 s so that the discernible features can be revealed under the microscope. The structural characterization is done with an X-ray diffractometer (XRD, D8 Discover operating at 40 kV and 40 mA with Cu Target). The morphology of the samples is observed using a field emission scanning electron microscope (Hitachi 4800 FE-SEM) to obtain a clear observation of the intermetallic compounds (IMCs) and grains. The various IMCs present in the matrix are characterized by electron probe microanalyzer (EPMA, JEOL, JXA-8500 F). The microstructural observation of

371

S¼

  D−H  100 D

ð1Þ

where S = solder spread ratio, H = height of the solder spread, and D = diameter of the solder when it is assumed to be a sphere (D = 1.24 V1/3, where V is the mass/specific gravity of the solder). 2.6. Wetting balance test A wetting balance tester (RESCA SAT 5000) is used to study the wetting behavior of the samples investigated in this study. A copper plate (Cu, 99.99% pure) in dimensions of 30 mm × 10 mm × 1 mm is selected as a parent material for the measurement of wettability. The Cu specimen is prepared by mechanical polishing with SiC papers (Grit #4000) and diamond polishing, followed by ultrasonic cleaning to remove the adherent impurities. The prepared Cu specimen is slightly coated with flux, and is further activated by warming it over a solder bath for 30 s. The Cu specimen is immersed in molten solder maintained at 250 °C at a dipping speed of 2.5 mm/s up to a depth of 2 mm, for 5 s [28]. The mean zero-cross time (ZCT) of the different samples observed in the wetting tests is analyzed. The wetting force in the wetting balance test is given by Fw ¼ p σ lv cosθ–ρgV

ð2Þ

where, g is the acceleration due to gravity, p is the perimeter of Cu specimen, V is the volume of Cu specimen immersed in the solder bath, ρ is the density of the solder, and σlv is the surface tension of the molten solder [28]. The molten solder with a flat surface is assumed as the most stable with highest wettability, which corresponds to a zero wetting angle. The surface tension of the molten solder is given by σ lv ¼ ð Fwd þ ρgVÞ=p

ð3Þ

Fw = Fwd at zero wetting angle. Here, Fwd represents the maximum withdrawal force. 2.7. Microhardness The Vickers' microhardness of different nanocomposite solders produced is measured with a Mitutoyo MVK H1 microhardness testing machine. The measurements are performed at a load of 100 gf for a period of 10 s. The recorded indentation diameter values are converted to microhardness value according to the relation: Hardness ¼ 1:854 

F D2

;

ð4Þ

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3.1.2. Composite fabrication

Fig. 1. A schematic diagram for the size of prepared tensile samples.

where F is the load in kgf and D is the diameter of indentation in mm. A total of 7 measurements are selected and average microhardness values are reported. 2.8. Tensile testing Different samples for tensile testing are prepared using ASTM E8 M-01 standards. The sample size and dimensions are shown in Fig. 1. The vertical cross section (T) indicates the thickness of the specimen. L is the gauge length, R is the radius of the fillet, P is the length of the reduced section, T is the thickness of the cross section, W is the width of the gauge section, and B is the width of the grip section. The overall length of the sample is 80 mm. The tensile testing is done by using a universal testing machine (UTM MTS 810, USA) at a crosshead speed set at 0.5 mm/min, and a strain rate of 0.001/s. The ultimate tensile strength (UTS) and elongation% values are calculated from the plot of stress strain curves in Microcal Origin 8.0 software module. 2.9. Fracture analysis The fractured samples after tensile testing are etched with the etchant prepared for microstructural analysis (92 vol.% CH3OH–5 vol.% HNO3–3 vol.% HCl) and the fractured morphology is observed in the FESEM. The various modes of the failure of the different solder samples are discussed. 3. Results and discussion 3.1. Characterization studies 3.1.1. Reinforcement Fig. 2(a) shows the X-ray diffraction pattern of the La2O3 nanopowder. The peaks are very broad depicting a nanocrystalline nature of the powder. The particle size of the La2O3 powder is around 30 nm which is also shown in the FESEM image (Fig. 2(b)).

3.1.2.1. Phase analysis. Any change in material property is reflected in the XRD analysis of monolithic SAC and the samples containing La2O3. A comparison of the XRD patterns of the Sn–3.0 wt.%Ag–0.5 wt.%Cu and SAC-0.05 La2O3 is presented in Fig. 3(a–b). The diffraction patterns show the formation of Ag3Sn and Cu6Sn5 IMCs in the solder matrix. The presence of the IMCs in Sn–Ag based solders has also been established by other researchers [20–23]. There is a shift in the diffraction peaks of SAC-La2O3 in comparison to monolithic SAC indicating a change in the physical structure of the matrix. It should be noted that the intensity of the Cu6Sn5 IMCs seems to decrease in composite samples as compared to monolithic SAC sample (Fig. 3b). A reduced intensity of Cu6Sn5 may indicate an inhibited copper diffusion and/or restricted growth of Cu6Sn5 inside the matrix [13]. To confirm this behavior, EPMA depth profile analysis is performed across the whole sample as shown in Fig. 3c. It is noticed that the total number of counts for copper is around 1650, which decreases to 1250 in composite SAC0.05 La2O3 sample. A decreasing content of copper in the matrix indicates reduced growth of large Cu6Sn5 rods in the solder matrix [29]. It has been already reported that when hard ceramic particulates are added into a ductile matrix, an annular plastic zone around these particulates is created due to a high residual stresses around the particles, and the resultant stress gets modified [30]. Indyka et al. has also observed that the stress level changes considerably with the introduction of Al2O3 nanoparticles in Ni-based coatings [31]. The minimization of copper diffusion in the composite samples may therefore be related to the development of localized stress fields surrounding the La2O3 nanoparticles, when incorporated in the matrix. As a consequence, a distortion of the lattice after addition of La2O3 inside the SAC alloy may occur hindering the movement of copper Cu atoms in the matrix. Therefore, it can be inferred that Cu diffusion may be restricted significantly in the presence of the La2O3 nanoparticles. The result is consistent with the recent report by Sharma et al. and his co-workers where the reinforcement particles significantly minimized the Cu–Sn diffusion in the solder matrix [25]. 3.1.2.2. Surface morphology. The surface morphology of La2O3 nanoparticles embedded in Sn–3.0 wt.%Ag–0.5 wt.%Cu (SAC) solders is shown in Fig. 4 (a–f). It is observed that the microstructure of monolithic SAC as well as composite solder matrix consist of Sn-rich phases surrounded by needle shaped Ag3Sn, and rounded Cu6Sn5 is also present in the matrix. Most of the Ag3Sn IMC phases are found at the boundaries of Sn-rich phases. It is also observed that the size and growth of grains and IMCs are affected after the addition of La2O3 nanoparticles in the solder matrix. It is observed that an increase in concentration of La2O3 nanoparticles in the matrix up to 0.5 wt.% leads to fine grained and compact morphology. The particle incorporation increases the number of

Fig. 2. (a) X-ray diffraction pattern of La2O3 nanopowder, and (b) morphology of La2O3 nanopowder particles.

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Fig. 3. X-ray diffraction patterns of (a) monolithic SAC, and SAC-0.05 La2O3, and (b) A magnified view of the Cu6Sn5 IMC (111) peak of (a); and (c) EPMA depth profile analysis for copper across the SAC and SAC-0.05 La2O3 samples.

nucleation sites and also limits the grain growth of the matrix resulting in a fine grained microstructure [24]. In the present case, the grain size of Sn is reduced with an addition of La2O3 but still it lies in the micrometer range. The best morphology of the SAC-La2O3 composite is obtained when it is reinforced with the 0.05 wt.% La2O3 (Fig. 4d). It is also noted that the IMCs are thicker in case of SAC-0.1 La2O3 (Fig. 4e). A high resolution image is also observed after etching the sample to confirm this behavior (Fig. 4f). It is observed that the nanoparticles are present in agglomerated form causing the formation of cracks and pores in the composite as shown in the encircled regions. This can be correlated to the fact that due to a high concentration of La2O3 (0.1 wt.%) in molten matrix, their interparticle distance decreases and the particles come closer to form loose agglomerates and easily get segregated out. As seen from the Fig. 4f, the big agglomerates are nearly of size 150–200 nm, indicated

by arrows. The different entities in the microstructure are confirmed through the EPMA analysis, as summarized in Fig. 5(a) and (b). These results confirm that the thin flake like structure is Ag3Sn while the rounded structures are Cu6Sn5; similar observations are also reported in Ref. [22]. Further, effects of La2O3 nanoparticles on the IMC thickness are also performed as follows. 3.1.2.3. Matrix grain size and Ag3Sn IMC thickness. The effect of La2O3 nanoparticles on the solder matrix grain size and Ag3Sn IMC thickness are measured at different locations randomly and averaged out (Fig. 6). It should be noted that the grain size and the IMC thickness decrease up to a 0.05 wt.% La2O3 in the matrix. The average grain size observed for monolithic SAC and SAC-0.05 La2O3 solder is approximately ~30 and ~20 μm respectively. The IMC thickness also decreases up to ~2.17 μm in SAC-0.05 La2O3 as compared to the ~2.95 μm of monolithic

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Fig. 4. FESEM image of (a) Sn–3.0Ag–0.5Cu solder, (b) SAC-0.01 La2O3, (c) SAC-0.03 La2O3, (d) SAC-0.05 La2O3, (e) SAC-0.1 La2O3, and (f) high resolution image of (e).

SAC solder. An estimate of the IMC thickness provides an insight into the strengthening of the nanocomposite solders. According to the adsorption theory, an increase in adsorption elements in the matrix decreases the surface energy and growth velocity of the IMCs. Generally, the plane of the IMCs with the maximum surface tension grows fastest, while the amount of adsorption of the surface-active material in this plane is maximized [20,21,25]. For composite solders, the size of the IMCs (~of the order of 2–3 μm) is larger than the size of the La2O3 nanoparticles (~ of the order of 30 nm). The presence of the La2O3 nanoparticles in the solder matrix refines the IMCs which can be understood by the adsorption of surface active material (i.e., La2O3 nanoparticles) at the IMC grain boundaries. Thus, La2O3 particles can easily cling to the growing IMCs and restrict their growth by lowering their surface energy. This can be explained as follows. The surface of any phase can be considered as a planar defect, because of the incomplete bonding of the surface atoms. Surface atoms of a crystal are in a different environment compared to those embedded inside the crystal. Therefore, the

surface atoms have a higher surface energy than those of the deep inside the crystal. The work required to make a new surface over an area is the excess surface energy that it costs to create a new surface. Generally, a crystal is bounded by closed packed planes that grow most slowly, since fast growing planes grow out of existence. The impurity atoms may modify the morphology and growth rate of planes by modifying the relative surface energies. These impurity atoms affect the growth rate in two ways: (1) they can either get adsorbed and remain fixed on the active sites, i.e., steps on the plane, and (2) the impurities can also get adsorbed and diffuse modifying the surface diffusion of incorporating atoms on the surface. The relative mobility of impurity atoms thus determines the crystal growth rate, surface structure and morphology [32]. Therefore, it can be concluded that the velocity of IMCs is dependent on the adsorption elements. A thick Ag3Sn is observed in SAC-0.01 La2O3 because of the insignificant concentration of La2O3 in the matrix. Due to the minute concentration, the behavior of the SAC-0.01 La2O3 is more or less similar to the monolithic SAC alloy. Moreover, a lesser amount of La2O3 (0.01 wt.%),

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375

Fig. 5. EPMA images of (a) Sn–3.0Ag–0.5Cu solder, and (b) SAC-0.05 La2O3.

the dispersion is non uniform and inter-particle distance is larger. Therefore, the adsorption of nanoparticles on the IMC and the grain is not significant to bring any change in the IMC thickness.

However, the thickness of IMCs increases beyond 0.05 wt.% La2O3 in the matrix. A thick Ag3Sn is obtained in SAC-0.1 La2O3 because the concentration of La2O3 increases much (0.1 wt.%) in the matrix. An enormous amount of addition of the nanoparticles reduces the interparticle distance and they come closer due to their high surface energy in molten metal. This results in the formation of big lumps or agglomerates/segregation [25]. This type of segregation results in non-uniform dispersion and generally believed to generate porosity/cracks and weaken the microstructural refinement and strengthening mechanism. As a consequence, the IMC thickness keeps on increasing. 3.2. Evaluation of properties

Fig. 6. Grain size and Ag3Sn IMC thickness in monolithic SAC and composite solders.

3.2.1. Wetting balance tests-wettability The wetting force curves obtained from the wetting balance tests are shown in Fig. 7 (a–b). The wetting of the solders on Cu specimen begins at the value of zero cross time (ZCT) of the curve. There is a steep rise in the wetting force beyond ZCT which is caused by an increase in the upthrust force by the withdrawal procedure. The wetting angle falls to zero followed by solder necking and detachment from the copper specimen. The wetting force increases up to SAC-0.05 La2O3 sample showing an improvement in wetting after addition of La2O3 nanoparticles. The wetting force is lowest for SAC-0.1 La2O3 which shows a dewetting of solder on Cu specimen. A higher wetting force indicates a decreased wetting angle and enhanced wettability [28]. Therefore,

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Fig. 7. (a) Wetting force time curve diagram for monolithic SAC and composite solders, and (b) high magnification image of (a).

SAC-0.05 La2O3 has the best wetting behavior among all the samples studied. The wettability of a material is also dependent on the surface tension of the material, and the surface tension depends on the surface energy of the material. The addition of La2O3 in the matrix lowers the interfacial energy and hence better wetting is achieved. To quantify the influence of the nanoparticles on the wetting, the ZCT of all the solders is plotted in Fig. 8. The ZCTs are significantly different between the monolithic SAC and composite specimens. It is observed that ZCT decreases continuously with the addition of the La2O3 in the matrix up to 0.05 wt.% La2O3, and then dewetting occurs. The ZCT for the SAC is 1.08 s, and decreases slightly up to 0.988 s for SAC-0.01 La2O3. The ZCT decreases significantly up to 0.285 s for SAC-0.05 La2O3. This confirms a best wetting behavior of the SAC-0.05 La2O3 composite solder. However, the ZCT increased to a maximum value of 1.41 s for the SAC-0.1 La2O3 solder indicating the dewetting phenomenon. The optimal solderability is obtained at a 0.05 wt.% La2O3 in the solder matrix. The dewetting phenomenon is generally attributed to the presence of the enormous number of nanoparticles available in the molten solder bath; the inter-nanoparticle distance decreases and they get segregated and/or agglomerated in due course of melting and solidification. The interaction of these nanoparticles with the substrate and their segregation at the surface can hinder the penetration of the molten solder to the surface pores. This may be manifested by the decreased wetting force caused by an increase in surface tension. This type of observations is also reported in recent reports [23,28].

Fig. 8. Zero cross time of monolithic SAC and different composite solders.

3.2.2. Spreadability The spreading images of the solidified solder on Cu specimen are shown in Fig. 9. It is clear that a higher spreading is noted in the case of solder matrix reinforced up to 0.05 wt.% La2O3. The spreading is degraded in 0.1 wt.% La2O3 solder as expected, which is evidenced by the presence of voids created in due course of cooling (Fig. 9). The addition of too much nanoparticles causes them to flow out of the melt and gets segregated. To assess the spreadability quantitatively, the spread ratio is calculated using Eq. (1) for each sample and compared (Fig. 10). It is observed that the solder reinforced with 0.05 La2O3 shows the highest spreading ratio ~ 87% among all the solders. The spread ratio increases up to the SAC-0.05 La2O3 solder and decreases thereafter. The sample with 0.1 wt.% La2O3 has a least spread ratio. This type of behavior in the presence of a high amount of reinforcements obstructs the melt flow by increasing the viscosity of the melt [21]. From preceding sections, the solderability tests such as wetting and spreading behavior show the beneficial effect of adding nanoparticles up to an optimal amount in to the solder matrix. It is inferred from these observations that an addition of 0.05 wt.% La2O3 in the SAC alloy improves the solderability on a copper substrate. However, it is also concluded that a higher amount of nano reinforcements are deteriorating to the solderability of the SAC alloy. 3.2.3. Microhardness The microhardness of monolithic SAC and composite samples are shown in Fig. 11. It is observed that the microhardness values of composite solders are improved as compared to that of the monolithic sample. The microhardness varies from ~ 11 to ~ 13 Hv, being highest for SAC-0.05 La2O3 solder. This indicates around ~18% in the microhardness of composite solder with 0.05 wt.% La2O3. One of the reasons for this could be related to the fine microstructure brought about by the presence of fine Ag3Sn IMCs. The hard and finely dispersed phases such as Ag3Sn resist indentation much more effectively than the broad and widely spread Ag3Sn present in the monolithic SAC. The increase in microhardness in case of composite solders can be related to: (1) the presence of harder La2O3 nanoparticles in the matrix, and (2) hindrance to the localized matrix deformation due to the presence of La2O3 nanoparticles. This result is consistent with studies on other metallic matrix nanocomposite solders [17,18,20,24–26]. 3.2.4. Tensile testing The stress–strain curves, ultimate tensile strength (UTS), and the percent elongation of the monolithic SAC and composite samples are shown in Fig. 12(a–c). It is observed that the UTS is increased significantly from 62.73 to 79.02 MPa showing an increase of ~ 26% when reinforced with 0.05 wt.% of La2O3 nanoparticles (Fig. 12(b)). Similar results on the influence of nano-reinforcement on the tensile strength

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Fig. 9. Spreading images of monolithic SAC and different composite solders.

of composite solder have been reported by Nai et al. [33], and Mohan et al. [34] for composite solders fabricated by powder metallurgy. Improvements in tensile strength can be attributed to the following factors: (1) generation of dislocations in order to counter balance thermal and elastic mismatch between solder matrix and nanoparticles, (2) the presence of the hard nanoceramic phase increases the load bearing capacity of the matrix, and (3) Orowan dislocation bypassing mechanism where dislocation loops form as dislocations by-pass the La2O3 nanoparticles [18,23]. The tensile strength of a reinforced matrix composite can be defined by [35]: σ c ¼ σ m þ Δσ

ð5Þ

where σm and σc are the yield strength of the monolithic SAC and composite solder matrix, respectively. Δσ represents the total increment in yield stress of the composite matrix given by [36]: 2

2

ðΔσ Þ2 ¼ Δσ ðthermal mismatchÞ þ Δσ ðelastic modulusÞ þ Δσ ðorowanÞ2 :

ð6Þ

It is also noted that the tensile strength of SAC-0.1 La2O3 is degraded (Fig. 12 a–b). This can be attributed to the weakening in the strengthening mechanism caused by the segregation and agglomerated nanoparticles in the matrix. Moreover, it has been suggested by other researchers that amount of reinforcement should not exceed the certain amount otherwise the properties will be affected adversely [18,20,24,25]. The addition of La2O3 also affects the elongation percentage after tensile testing as shown in Fig. 12(c). There is a minute difference in

Fig. 10. Spread ratio of monolithic SAC and different composite solders.

elongation percentage of SAC and SAC-0.01 La2O3 sample (~ 11.88%), while the elongation percentage jumps to highest value of 14.13% in the SAC-0.05 La2O3 sample. It is surprisingly contradictory to those reported by researchers where they have found a decrease in elongation percentage with increasing nanoceramic reinforcements [18,34,35]. This simultaneous increase of composite ductility with tensile strength may be correlated to the slip mode transition produced by the presence of La2O3 nanoparticles, which depends on the interaction of La2O3 nanoparticles with the solder matrix interface [37,38]. Since La2O3 are very fine (~ 30 nm), the plastic deformation gets changed from dislocation reinforcement shearing to dislocation reinforcement bypassing. La2O3 nanoparticles impede the dislocation motion and collapse them around the matrix-reinforcement interface [39]. Such cross-slip modes are responsible the breaking up of agglomerates and distribute particles, leading to an increased ductility. This result is consistent to those of other research works on metallic alloys [40–42]. Samples reinforced with 0.1 wt.% La2O3 have lower values of elongation percentage due to the unrecovered and agglomerated La2O3 particles as explained in earlier sections.

3.2.5. Fracture analysis The fractured surface morphology of monolithic SAC and nanocomposite specimens after tensile tests is shown in Fig. 13 (a–e). A number of dimpling structures are observed for monolithic SAC representing ductile fracture (Fig. 13a). Similarly, SAC-0.01 La2O3 shows a number of dimples without much change in the microstructure (Fig. 13b). The size of the dimples is decreased in samples SAC-0.03 La2O3 and SAC-0.05 La2O3, where a large number of dimples are observed with a few of the cleaved planes. A cleaved plane usually represents a fractured Ag3Sn IMC in the matrix. Few of the voids have also undergone considerable growth and their coalescence leads to shallow dimples (Fig. 13 c–d). This shows a typical mixed ductile and brittle fracture,

Fig. 11. Vickers microhardness of monolithic SAC and different composite solders.

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Fig. 12. (a) Stress strain curves of the monolithic SAC and different composite solders, (b) UTS of the monolithic SAC and different composite solders, and (c) percent elongation of the monolithic SAC and different composite solders.

which is predominantly ductile in nature. However, the fracture surface of the sample containing 0.1 wt.% La2O3 displayed worst ductility. Several cleavage planes associated with the fractured IMCs are found which indicate a brittle fracture (Fig. 13e). Therefore, in the SAC-0.1 La2O3 sample, the Orowan strengthening mechanism does not hold

good where large Ag3Sn needles may cause cracking and a drastic reduction in ductility of alloys [43]. Therefore, from the preceding sections it may be inferred that a combination of tensile strength and elongation percentage can be achieved in 0.05 wt.% La2O3 reinforced solder specimen.

Fig. 13. Tensile fractured microstructures of (a) Sn–3.0Ag–0.5Cu, (b) SAC-0.01 La2O3, (c) SAC-0.03 La2O3, (d) SAC-0.05 La2O3, and (e) SAC-0.1 La2O3.

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4. Conclusions The effect of nanosized La2O3 particles on wetting, microstructure and tensile properties of Sn–3.0 wt.% Ag–0.5 wt.% Cu solder alloy is studied. The important conclusions can be summarized as: 1. The La2O3 nanoparticles have been successfully reinforced in the Sn–3.0 wt.%Ag–0.5 wt.%Cu alloy by mechanically blending and melting. 2. Microstructural investigations reveal that the addition of nanosized 0.05 wt.% La2O3 particles to Sn–3.0 wt.%Ag–0.5 wt.%Cu solder inhibited the growth of the grain size as well as the IMCs due to the adsorption of La2O3 nanoparticles in the matrix during solidification. 3. The zero cross time of the Sn–3.0 wt.%Ag–0.5 wt.%Cu solder reinforced with up to 0.05 wt.% La 2 O3 is continuously decreased (~ 0.285 s) as compared to monolithic Sn–3Ag–0.5Cu solder (~ 1.08). A dewetting phenomenon is also observed for the solder with 0.1 wt.% La2 O3 , which shows a higher zero cross time (~ 1.41 s) due to the increase in the surface tension and or/wetting angle caused by increase in viscosity. The best combination of wetting and spreading is achieved at 0.05 wt.% La2O3 in the Sn–3.0 wt.%Ag–0.5 wt.% Cu matrix. 4. The microhardness increases up to the 0.05 wt.% La2O3 in the solder matrix which is about 18% increase compared to the unreinforced SAC alloy due to the presence of finer IMCs acting as a secondary reinforcement phase in the matrix. 5. Tensile tests revealed that the addition of 0.05 wt.% nanosized La2O3 particles increased the ultimate tensile strength up to 26% as compared to the Sn–3.0 wt.%Ag–0.5 wt.%Cu alloy. This significantly strengthening is due to the thermal and elastic modulus mismatch of the solder and reinforcement as well as by the Orowan strengthening of the matrix. The percentage elongation is also increased around 14% with the addition of 0.05 wt.% nanosized La2O3 particles, due to the activation of cross slip mode transition in the matrix brought about by La2O3 nanoparticles. Acknowledgments This work was supported by the Technology Innovation Program (10051436, Development and mass prodution of 25% reduced prices nano-micro compound Pb-free solder paste for automotive devices to respond to ELV Directive) funded by the Ministry of Trade, industry & Energy (MI, Korea). References [1] M. Abtew, G. Selvaduray, Lead-free solders in microelectronics, Mater. Sci. Eng. R 27 (2000) 95–141. [2] Y.S. Ki, H.-I.I. Kim, J.M. Kim, Y.E. Shin, Prediction of thermal fatigue life on mBGA solder joint using Sn–3.5Ag, Sn–3.5Ag–0.7Cu, Sn–3.5Ag–3.0In–0.5Bi solder alloys, JWJ 21 (2003) 92–98. [3] J.H. Park, H.Y. Lee, J.H. Jhun, C.S. Cheon, J.P. Jung, Characteristics of Sn–1.7Bi–0.7Cu– 0.6In lead-free solder, JWJ 26 (2008) 43–48. [4] J.W. Moon, M.-I.I. Kim, J.P. Jung, A study on the soldering characteristics of Sn–Ag– Bi–In ball in BGA, JWJ 20 (2002) 99–103. [5] K. Zeng, K.N. Tu, Six cases of reliability study of Pb-free solder joints in electronic packaging technology, Mater. Sci. Eng. R 38 (2002) 55–105. [6] A. Sharma, S. Malik, N.N. Ekere, J.P. Jung, Printing morphology and rheological characteristics of lead-free Sn–3Ag–0.5Cu (SAC) solder pastes, J. Microelectron. Packag. Soc. 21 (4) (2014) 1–7. [7] T.Y. Lee, W.J. Choi, K.N. Tu, J.W. Jang, S.M. Kuo, J.K. Lin, D.R. Frear, K. Zeng, J.K. Kivilahti, Morphology, kinetics, and thermodynamics of solid-state aging of eutectic SnPb and Pb-free solders (Sn–3.5Ag, Sn–3.8Ag–0.7Cu and Sn–0.7Cu) on Cu, J. Mater. Res. 17 (2002) 291–301. [8] K.S. Kim, S.H. Huh, K. Suganuma, Effects of intermetallic compounds on properties of Sn–Ag–Cu lead-free soldered joints, J. Alloys Compd. 352 (2003) 226–236. [9] K.K. Xiang, A.S.M.A. Haseeb, M.M. Arafat, G. Yingxin, Effects of Mn nanoparticles on wettability and intermetallic compounds in between Sn–3.8Ag–0.7Cu and Cu substrate during multiple reflow, Proceedings of 4th Asia Symposium on Quality Electronic Design, Penang, Malaysia: IEEE 2012, pp. 297–301. [10] A. Nadia, A.S.M.A. Haseeb, Understanding the effects of addition of copper nanoparticles to Sn–3.5Ag solder, Solder Surf. Mt. Technol. 23 (2011) 68–74.

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