Stability of molybdenum nanoparticles in Sn 3.8Ag 0.7Cu solder during multiple reflow and their influence on interfacial intermetallic compounds

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1 Available online at Stability of molybdenum nanoparticles in Sn 3.8Ag 0.7Cu solder during multiple reflow and their influence on interfacial intermetallic compounds A.S.M.A. Haseeb, M.M. Arafat 1, Mohd Rafie Johan 2 Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia ARTICLE DATA Article history: Received 27 August 2011 Received in revised form 9 November 2011 Accepted 11 November 2011 Keywords: Nanocomposite solder Mo nanoparticles Reflow behavior Intermetallic compounds (IMCs) ABSTRACT This work investigates the effects of molybdenum nanoparticles on the growth of interfacial intermetallic compound between Sn 3.8Ag 0.7Cu solder and copper substrate during multiple reflow. Molybdenum nanoparticles were mixed with Sn 3.8Ag 0.7Cu solder paste by manual mixing. Solder samples were reflowed on a copper substrate in a 250 C reflow oven up to six times. The molybdenum content of the bulk solder was determined by inductive coupled plasma-optical emission spectrometry. It is found that upon the addition of molybdenum nanoparticles to Sn 3.8Ag 0.7Cu solder, the interfacial intermetallic compound thickness and scallop diameter decreases under all reflow conditions. Molybdenum nanoparticles do not appear to dissolve or react with the solder. They tend to adsorb preferentially at the interface between solder and the intermetallic compound scallops. It is suggested that molybdenum nanoparticles impart their influence on the interfacial intermetallic compound as discrete particles. The intact, discrete nanoparticles, by absorbing preferentially at the interface, hinder the diffusion flux of the substrate and thereby suppress the intermetallic compound growth Elsevier Inc. All rights reserved. 1. Introduction Regulations restricting the use of lead in electronics have resulted in a recent upsurge in activity on the development of lead free solders. Research done so far had lead to the emergence of tin based alloys as alternatives to lead based solder alloys. Among the tin based alloys, tin silver copper (Sn Ag Cu, SAC) alloys have become popular because of their advantages, such as good wetting characteristics with substrate, good fatigue resistance, and good joint strength. However, these alloys suffer from some drawbacks that raise concerns over their reliability. The microstructure of SAC alloys has been found to coarsen during use and during high temperature exposure to a great extent than in lead containing counterparts [1]. Moreover, tin based solders form thicker intermetallic compound (IMC) layers at the solder/ substrate interface than the lead based solders [2]. The interfacial IMCs in lead free solder also grow at a rate faster than that in lead based solders. Coarsening of microstructure and rapid growth of brittle interfacial IMC are known to degrade the properties of lead free solder joints resulting in lower long term reliability. Research efforts are therefore underway to improve the quality of tin based solder alloys. One of the approaches to improving the properties of tin based solder is through appropriate additions. Both alloy addi- Corresponding author. Tel.: addresses: haseeb@um.edu.my (A.S.M.A. Haseeb), arafat_mahmood@yahoo.com (M.M. Arafat), mrafiej@um.edu.my (M.R. Johan). 1 Tel.: Tel.: ; fax: /$ see front matter 2011 Elsevier Inc. All rights reserved. doi: /j.matchar

2 28 MATERIALS CHARACTERIZATION 64 (2012) tions [3 6] and particle additions [7,8] are being studied currently. Particles additions to tin based solder leads to the development of the composite solders with superior properties. Different types and sizes of particles are under investigations. Particles types investigated so far include metallic [9,10], ceramics [11,12] and carbon nanotubes [13]. Both micrometer [7] and nanometer [14] sized particles are currently being considered. The rationale behind particle addition is that when appropriate types of particles are added to the solder, they should lead to dispersion strengthening. They are also expected to stabilize the microstructure by restricting the growth of different phases in the solder during use. The addition of nanosized particles to tin based solders has attracted a great deal of attention in recent years [8,10]. With the decrease of solder pitch size in electronic packages, the addition of nanoparticles is becoming even more relevant. Improvement in bulk mechanical properties such as strength [11], hardness [15], and creep resistance [14] has been observed in lead free solders reinforced with nanoparticles. In particular, the addition of Mo nanoparticles has resulted in considerable improvement in the bulk mechanical properties of solder [16 18]. However, the performance of a solder joint not only depend on its bulk properties, they also depend on the properties of the solder/substrate interface. It is therefore important to understand the effect of the nanoparticles on the interfacial characteristics. There are only a few studies available on the influence of nanoparticles on the interfacial IMC. It was found [10,19] that the addition of Co, Ni, Pt nanoparticles affect the morphology and thickness of interfacial IMC. Haseeb and Tay [19] investigated the mechanism by which Co nanoparticles exert their influence on interfacial IMC. By comparing the effects of Co nanoparticles on the morphology and growth behavior of interfacial IMC with that of Co alloy addition, they suggested that Co nanoparticles actually dissolve during reflow and impart their influence as alloy additions. In a preliminary work [20], it was observed that the presence of Mo nanoparticles in liquid SAC solder reduces the dissolution of the copper substrate. That study also revealed evidence suggesting that Mo nanoparticles did not dissolve in liquid SAC. The present paper investigates the effect of Mo nanoparticles addition on the morphology and growth of interfacial IMC between SAC and copper substrate under multiple reflow conditions. The stability of Mo nanoparticles during reflow for over six cycles is evaluated. Based on the result obtained, a mechanism through which Mo nanoparticles impart their influence on IMC growth is suggested. 2. Experimental Procedures The composite solders were prepared by manual mixing of molybdenum (Mo) nanoparticles (Aldrich, 99.8% trace metal basis) with Sn 3.8Ag 0.7Cu (SAC) solder paste (Indium Corporation of America). The morphology and size of the Mo nanoparticles were investigated using a Philips CM200 transmission electron microscope. For this purpose, a small amount of Mo nanoparticles were dispersed into distilled water onto a carbon film supported by copper girds. Blending of SAC solder paste with Mo nanoparticles was carried out for approximately 30 min to ensure uniform distribution of nanoparticles. Mo nanoparticles were added at different nominal percentages, e.g., 1 wt.%, 2 wt.% and 3 wt.%. Polycrystalline copper (Cu) sheets with a dimension of 30 mm 30 mm 0.3 mm were used as substrates. Prior to soldering, the substrates were cleaned and dipped in 10 vol.% H 2 SO 4 to remove any oxides present. After that, the substrates were washed in deionized water and dried in acetone. The composite solder paste was placed on the cleaned substrate through a mask having an opening diameter of 6.5 mm and a thickness of 1.24 mm (JIS Z3198-3, 2003). The solder paste was reflowed in a reflow oven (Forced convection, FT02) at a peak temperature of 250 C for 45 s. Prepared solder samples were cleaned with acetone to remove the flux residue. After the first reflow, the bulk solder was chemically analyzed using an inductively coupled plasma-optical emission spectrometer (Perkin Elmer Optima 2000 DV) to find measure the actual amount of molybdenum retained in the solder. Some of the solder samples were reflowed two, four, and six times. After reflow, the samples were cross sectioned, mounted in epoxy and polished using standard metallographic technique. The final finishing step involved polishing with 0.02 μm silica particles. The cross-sectional image of the interfacial intermetallic compound (IMC) was collected with the backscattered electron detector in a scanning electron microscope. Elemental analysis of different phases was carried out by using energy dispersive X-ray spectroscopy. To expose the top surface of the intermetallic compound, solder samples were chemically etched for 24 h in a solution containing (93% CH 3 OH+5% HNO 3 +2% HCl) solution [21]. The microstructure and elemental analysis was carried out in a conventional scanning electron microscope, and in a high resolution field emission scanning electron microscope (Zeiss Ultra-60) with energy dispersive X-ray spectroscopy (EDAX- Genesis Utilities). The high resolution field emission scanning electron microscope is necessary to observe and analyze individual Mo nanoparticles. The secondary electron detector (secondary electrons are generated by backscattered electrons that returned to the surface after several inelastic collision events) was used in the field emission scanning electron microscope at an accelerating voltage of 10 kv. Using these parameters, the samples were magnified up to 50 K for microstructural investigations. The thickness of the interfacial IMC and the scallop diameter were calculated from the micrograph by using image analysis software (Olympus SZX10). IMC thickness was calculated by dividing the area covered by the IMC layer in the cross-sectional micrograph by the IMC length [22]. From each image, one average value for the IMC thickness was calculated by using area analysis. For each test condition at least five images at randomly selected positions were utilized to calculate the average value of IMC thickness and scallop diameter. 3. Results Fig. 1 shows a transmission electron microscope (TEM) image of the molybdenum (Mo) nanoparticles used in this study. The nanoparticles are almost perfectly spherical. The average

3 29 Fig. 1 TEM micrograph of the Mo nanoparticles. particle size was approximately 70 nm. However, the size distribution was quite wide, ranging from 20 nm to as large as 200 nm [20]. Fig. 2 shows the field emission scanning electron microscope (FESEM) micrographs of the solder paste after being mixed with Mo nanoparticles. It may be noted that the solder paste mainly consists of a flux containing Sn 3.8Ag 0.7Cu (SAC) solder balls which have an average diameter of about 40 μm. Upon mixing, tiny Mo nanoparticles tend to stick to the surface of the solder balls (Fig. 2a). The nanoparticles also are dispersed in the flux (Fig. 2b) between the solder balls. The nanoparticles are more or less uniformly distributed in the solder paste after manual mixing. Upon reflow, the solder balls melted, coalesced and formed the solder joint. The flux residue remained on the surface of the solder joint. In order to find out how much of the Mo nanoparticles are retained in the solidified solder, the latter was chemically analyzed by inductive coupled plasmaoptical emission spectrometry (ICP-OES). The actual Mo content of the solder is shown in Fig. 3 as a function of nominal amount of Mo nanoparticles added to the solder paste. For the addition of 1, 2 and 3 wt.% of Mo nanoparticles into the solder paste, the actual content in the solder is only 0.04, 0.10 and 0.14 wt.% of Mo, respectively. The rest of the Mo remains in the flux residue [20]. Hereafter, solders containing 0.04 and 0.10wt.% Mo will be designated as (SAC+0.04 n-mo) and (SAC+0.10 n-mo) respectively with n referring to nanoparticles. Fig. 4 shows cross sectional scanning electron microscope (SEM) micrographs of SAC and (SAC+0.10 n-mo) solders after first and sixth reflow. On all samples, Cu 6 Sn 5 with a typical scallop type morphology formed. The composition of Cu 6 Sn 5 was confirmed by energy dispersive X-ray spectroscopy (EDX). A similar result was obtained by Lee et al. [23] for Sn based solder on Cu substrate during reflow. Below the Cu 6 Sn 5 layer, there is a thin flat layer of Cu 3 Sn having a darker contrast. Evidence of the formation during reflow of a very thin Cu 3 Sn IMC layer between the Sn-based solder and the Cu substrate can be found in the literature [24,25]. A comparison of Fig. 4a to b shows that the addition of Mo nanoparticles results in a decrease in overall interfacial intermetallic compound (IMC) thickness after the first reflow. The effect of Mo nanoparticles is evident after the sixth reflow as well (Fig. 4c and b). Generally, the thickness of the Cu 6 Sn 5 layer increases with the reflow number for both the SAC and Mo nanoparticle augmented SAC solders. No molybdenum could be detected within the Cu 6 Sn 5 IMC by EDX analysis for either the first or sixth reflow. In Fig. 5, the variation of IMC thickness is shown with respect to the number of reflow cycles for both SAC and Mo nanoparticles added solders. Lower IMC thickness is observed for all Mo nanoparticle added samples. In order to reveal the surface morphology of the interfacial IMC, deep etching was used to dissolve the solder. Fig. 6 shows a typical low magnification micrograph of the (SAC+0.10 n- Mo) solder sample after deep etching. It can be seen from the figure that the visibility of the interfacial IMC in the top view depends on the extent of etching. Where etching was Fig. 2 FESEM images of solder paste after mixing, nominally containing 2 wt.% of Mo nanoparticles (a) distribution of Mo nanoparticles into the solder paste, (b) high resolution image of the flux.

4 30 MATERIALS CHARACTERIZATION 64 (2012) Fig. 3 Molybdenum content of the solder analyzed by ICP- OES after reflow. complete, the IMC can be seen clearly as indicated by a white outline in Fig. 6. To compare the surface morphology of interfacial IMC of different samples, places where etching was completed were used. To determine the distribution of Mo nanoparticles, places where the etching was incomplete were particularly investigated. Fig. 7 shows the top view of the interfacial IMC for the SAC and (SAC+0.10 n-mo) solder samples after first and sixth reflow. Places undergoing complete etching were chosen to compare the morphology of the interfacial IMC, as mentioned Fig. 5 IMC thickness as a function of the number of reflow cycles and percentage of Mo nanoparticles. earlier. In each case, it was found that the morphology of the interfacial IMC was scallop type. EDX analysis on these scallops confirmed that these are Cu 6 Sn 5. From Fig. 7 it is seen that as the number of reflow cycle is increased, the diameter of the scallop is increased for SAC and (SAC+0.10 n-mo) solder. It is also seen that the diameter of the scallops is smaller for (SAC+0.10 n-mo) solder than for SAC for all reflow conditions. In Fig. 8, the average scallop diameter is Fig. 4 Backscattered electron micrographs of the cross sectional view of the solder/substrate interface (a) SAC after first reflow, (b) (SAC+0.10 n-mo) after first reflow, (c) SAC after six reflows and (d) (SAC+0.10 n-mo) after six reflows.

5 31 Fig. 6 SEM micrograph of (SAC+0.10 n-mo) sample showing the extent of etching [2 reflow]. Fig. 8 Scallop diameter of interfacial IMC as a function of number of reflows and amount of Mo nanoparticles. presented as a function of reflow number and percentage of Mo nanoparticles in the samples. It is clear from Figs. 7 and 8 that the diameter of the Cu 6 Sn 5 scallop decreases as the amount of Mo nanoparticles increases under all reflow conditions. Fig. 9 shows a typical micrograph of the top view of Cu 6 Sn 5 in (SAC+0.04 n-mo) solder after four reflow cycles. The area presented in Fig. 9 underwent incomplete etching. Two types of white particles can be seen in the micrograph. One type of particles has a bright appearance with an irregular shape (Marked X ). These types of irregularly shaped particles were found in both SAC and Mo nanoparticle-added SAC solders. EDX spot analysis confirmed that these irregularly shaped particles are Ag 3 Sn. The formation of Ag 3 Sn on Cu 6 Sn 5 after reflow has been observed by others for Sn-based solder prepared on Cu or Ni substrate [26]. The second type of bright particles (marked Y ) have an almost perfectly spherical shape Fig. 7 Top view of the interfacial IMC (a) SAC after first reflow, (b) (SAC+0.10 n-mo) after first reflow, (c) SAC after sixth reflow, and (d) (SAC+0.10 n-mo) after sixth reflow [after deep etching].

6 32 MATERIALS CHARACTERIZATION 64 (2012) Fig. 9 (a) FESEM image of (SAC+0.04 n-mo) solder after four times reflow, (b) EDX spectrum taken on particle X and (c) EDX spectrum on Y. and were found only in Mo nanoparticle-added solders. EDX spot analysis revealed the occurrence of predominately Mo in these particles. This suggests that these particles are Mo nanoparticles. It should be noted that the microscopy and elemental spot analysis on the Mo nanoparticles were done in an ultra high resolution field emission scanning electron microscope (FESEM, Zeiss Ultra-60) equipped with EDX (EDAX-Genesis Utilities) which provides good imaging and analytical resolution. Fig. 10 shows an FESEM image and elemental maps depicting the presence of Mo nanoparticles on the interfacial IMC of (SAC+0.10 n-mo) solder after sixth reflow. The presence of spherical Mo nanoparticles is clearly visible in the micrograph. The FESEM image together with the elemental maps for Mo, Sn and Cu given in Fig. 10 suggest that spherical Mo nanoparticles are concentrated at the boundaries between IMC scallops. Random EDX area analysis at multiple spots on the IMC surface yielded a Mo concentration of about wt.% on the partially etched IMC surface. It may be noted that the average concentration of Mo in the bulk solder for this sample is only 0.1wt.%. Therefore it is believed that Mo nanoparticles tend to concentrate on the IMC surface during reflow. 4. Discussion In the present work, paste mixing was used to incorporate Mo nanoparticles into the solder. In this method only a fraction of Mo nanoparticles entered the solidified solder. Similar results have been obtained for Co nanoparticles [19]. However, in the case of Co nanoparticles, the fraction of nanoparticles retained in the solder was higher. The incorporation of nanoparticles into solder will mainly depend on the interactions between nanoparticles and the solder. It has been suggested that a reinforcing particle can be pushed (rejected), engulfed or entrapped at the particle liquid metal interface depending upon the interaction mechanisms [27,28]. The incorporation of a lower amount of Mo in SAC suggests that Mo nanoparticles experience rejection by the liquid SAC interface to a greater extent. Poor wetting of Mo and SAC could be a reason for higher rejection. Notwithstanding the rejection, the amount of Mo nanoparticles retained in the solder has a definite influence on interfacial IMC growth characteristics, as will be discussed later. It has been observed that the morphology of the interfacial Cu 6 Sn 5 layer does not change with the addition of Mo

7 33 Fig. 10 (a) FESEM image of (SAC+0.10 n-mo) solder after six reflow cycles, and elemental maps for (b) Mo, (c) Sn, and (d) overlap of elemental maps for Sn, Ag, Cu and Mo. nanoparticles into the SAC solder. The usual scallop type morphology is preserved even when Mo nanoparticles are added (Fig. 7). However, the addition of Mo nanoparticles suppresses the growth of Cu 6 Sn 5 (Figs. 4 and 5). Both the thickness and scallop diameter of the interfacial IMC become smaller when Mo nanoparticles are added to the SAC solder (Figs. 4 and 7). For example, after first reflow, addition of 0.10 wt.% Mo nanoparticles causes a decrease in Cu 6 Sn 5 layer thickness from 1.5 μm to 0.95 μm (Fig. 4a, b) and a decrease in scallop diameter from 2.2 μm to 1.3 μm (Fig. 7a, b). The presence of Mo nanoparticles clearly influences the interfacial IMC growth characteristics. The exact mechanism(s) through which Mo nanoparticles suppresses the growth of interfacial IMC thickness and scallop diameter is not clear. However, several scenarios can be hypothesized. At one extreme, nanoparticles may remain as discrete, unaltered particles during reflow. At the other extreme, they may be completely consumed in some reaction(s) or through dissolution within the molten solder. Actual alteration of the nanoparticles will depend on a number of factors, including their melting point and chemical interaction(s) with the solder. Molybdenum has a relatively high melting point (2623 C) compared with the reflow temperature (250 C) used in this study. So, under these experimental conditions, Mo nanoparticles are not expected to physically melt during reflow. Comparing Figs. 1 and 10, it is found that the original nearly perfectly spherical shape of Mo nanoparticles is preserved even after six reflow cycles. This suggests that the nanoparticles did not undergo any significant physical or chemical change during multiple reflows. Referring to the Mo Sn phase diagram, Mo has negligible solubility in Sn. The phase diagram shows that as many as three IMCs e.g., Mo 3 Sn, Mo 2 Sn 3 /Mo 3 Sn 2, and MoSn 2 can exist in the Mo Sn system [29]. But no evidence of Mo Sn compound formation was found on Mo nanoparticles by EDX. For instance, in the elemental map (Fig. 10), Sn signals does not appear at the places where Mo particles are present. It should be noted that Mo does not form any compound with Cu and Ag at 250 C, and has no solubility in these elements [30,31]. The above results suggest that when Mo nanoparticles are mixed with SAC solder paste and reflowed at 250 C, they remain as stable solid particles. So the retardation of the growth of IMC thickness and scallop diameter is mainly due to the effect of Mo nanoparticles as discrete particles. There are three possible mechanisms through which Mo nanoparticles suppress the thickness and reduce the diameter of IMC scallops: i) Mo nanoparticles can act as heterogeneous nucleation sites for the formation of Cu 6 Sn 5 nucleus. This can increase the density of nucleation of Cu 6 Sn 5 grains, ii) Mo nanoparticles may have a pinning effect on the growing front of Cu 6 Sn 5 scallops, and iii) Both (i) and (ii). For a particle to act as a heterogeneous nucleation site, the interfacial energy between the liquid and solid particles should be low. In other words, the wetting angle of the liquid solder at the solid Mo surface should be low. No data for the interfacial energy and wetting angle between liquid Cu 6 Sn 5

8 34 MATERIALS CHARACTERIZATION 64 (2012) and solid Mo is available in the literature. Available data show that wetting angle between liquid tin on solid molybdenum [32] as well as between liquid copper on solid molybdenum [33] is high. This may be considered an unfavorable factor for the heterogeneous nucleation of Cu Sn compounds on Mo nanoparticles. Further, if Mo nanoparticles acted as heterogeneous nucleation sites, these particles would likely be found as inclusions inside the Cu 6 Sn 5 scallops. Extensive examination of multiple samples in cross-section under high resolution SEM could not identify any such inclusion. This evidence leads us to suggest that Mo nanoparticles are unlikely to act as heterogeneous nucleation sites. It is, therefore, believed that the influence of Mo nanoparticles on the Cu 6 Sn 5 layer is due to their effect on the growth process. Experimental results obtained in the high resolution FESEM confirmed the presence of Mo nanoparticles on the surface of IMC (Fig. 9) and at channels between the IMC scallops (Fig. 10). In fact, a higher percentage of Mo was found on the IMC surface (3 3.5wt.%) than in the solder (<0.14wt.%). This indicates that the nanoparticles tend to stay preferentially on the IMC scallops. Preferential segregation of Mo nanoparticles at the liquid/imc interface may be linked with higher interfacial energy between liquid tin and molybdenum [32]. It should be noted that the substrate was placed in a horizontal position during reflow. Thus the density difference between liquid tin (6.99 g.cm 3 ) and molybdenum (10.28 g.cm 3 ) could also contribute to this segregation. Through their presence on the IMC surface, Mo nanoparticles are believed to have a retarding effect on the IMC growth. The mechanism through which Mo nanoparticles suppress the interfacial IMC thickness and scallop diameter may be explained by referring to Fig. 11. Kim and Tu [34] suggested that liquid channels exist between the Cu 6 Sn 5 scallops. These channels extend all the way to Cu 3 Sn/Cu interface. These channels serve as fast diffusion and dissolution paths for Cu and thus feed the interfacial reaction. Ignoring the presence of Cu 3 Sn IMC and all other chemical reactions for convenience, Kim and Tu [34] suggested that two kinds of fluxes are responsible for the growth of the scallops. One is flux of ripening (J R ) and the other is flux of interfacial reaction (J I ). At the start of reflow, Cu 6 Sn 5 scallops nucleate at the solder/substrate interface. The scallops grow with the increase of reflow as Cu is supplied by the fluxes, J R and J I.Cu 6 Sn 5 scallops grow both in thickness and diameter with time. J I feeds the thickness growth, while J R causes coalescence and increases the diameter of the scallops [Fig. 11a]. When Mo nanoparticles are added to the solder, they block the channels between the IMC scallops and are preferentially absorbed at the growth front of IMC scallops [Fig. 11b]. The blocking of the channels obstructs the movement of copper atoms from the substrate to the liquid solder and hence reduces J I. This helps to reduce the thickness of IMC. The adsorbed Mo nanoparticles on the IMC surface will reduce J R and act as a barrier to the coalescence of neighboring scallops. This causes a reduction in scallop diameter. The effect of Mo nanoparticles on the IMC growth could be compared with that of Co nanoparticles as described in the literature [19]. The addition of Co nanoparticles was found to increase the thickness of Cu 6 Sn 5 but decrease Cu 3 Sn thickness [19]. This was very similar to the case when Co is added as Fig. 11 Schematic diagram of scallop growth in (a) SAC solder, (b) Mo nanoparticles added SAC solder preferentially absorbed at the IMC scallops. alloy addition [5]. It was, therefore, suggested that Co nanoparticles dissolve during reflow and exert their influence on IMC through alloying effect [19]. This is in contrast with Mo nanoparticles, which remain intact as seen in this study. Molybdenum, a refractory metal with a high melting point and low reactivity, remains stable during reflow. These particles do not undergo any detectable alteration during reflow. It is therefore suggested that Mo nanoparticles exert their influence on the interfacial IMC growth as discrete particles. 5. Conclusion The addition of Mo nanoparticles to SAC solder has a pronounced effect on the growth of interfacial IMC between SAC solder and Cu substrate. The following conclusions can be drawn based on this study: 01. Mo nanoparticles do not dissolve or react with the SAC solder and remain stable during multiple reflow cycles at 250 C. 02. The addition of Mo nanoparticles to SAC solder causes a decrease in the thickness and diameter of interfacial Cu 6 Sn 5 scallops. 03. Mo nanoparticles influence the interfacial IMC through discrete particle effect by preferentially absorbing at the grain boundaries of interfacial IMC scallops.

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