M A T E R I A L S C H A R A C T E R I Z A T I O N 6 4 ( 2 0 1 2 ) 2 7 – 3 5

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Stability of molybdenum nanoparticles in Sn–3.8Ag–0.7Cusolder during multiple reflow and their influence on interfacialintermetallic compounds

A.S.M.A. Haseeb⁎, M.M. Arafat1, Mohd Rafie Johan2

Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +603 7967 4492.E-mail addresses: haseeb@um.edu.my (A

(M.R. Johan).1 Tel.: +60 176977652.2 Tel.: +60 3 7967 6873; fax: +60 3 7967 5317

1044-5803/$ – see front matter © 2011 Elseviedoi:10.1016/j.matchar.2011.11.006

A B S T R A C T

Article history:Received 27 August 2011Received in revised form 9November 2011Accepted 11 November 2011

This work investigates the effects of molybdenum nanoparticles on the growth ofinterfacial intermetallic compound between Sn–3.8Ag–0.7Cu solder and copper substrateduring multiple reflow. Molybdenum nanoparticles were mixed with Sn–3.8Ag–0.7Cusolder paste by manual mixing. Solder samples were reflowed on a copper substrate in a250 °C reflow oven up to six times. The molybdenum content of the bulk solder wasdetermined by inductive coupled plasma-optical emission spectrometry. It is found thatupon the addition of molybdenum nanoparticles to Sn–3.8Ag–0.7Cu solder, the interfacialintermetallic compound thickness and scallop diameter decreases under all reflowconditions. Molybdenum nanoparticles do not appear to dissolve or react with the solder.They tend to adsorb preferentially at the interface between solder and the intermetalliccompound scallops. It is suggested that molybdenum nanoparticles impart their influenceon the interfacial intermetallic compound as discrete particles. The intact, discretenanoparticles, by absorbing preferentially at the interface, hinder the diffusion flux of thesubstrate and thereby suppress the intermetallic compound growth.

© 2011 Elsevier Inc. All rights reserved.

Keywords:Nanocomposite solderMo nanoparticlesReflow behaviorIntermetallic compounds (IMCs)

1. Introduction

Regulations restricting the use of lead in electronics haveresulted in a recent upsurge in activity on the developmentof lead free solders. Research done so far had lead to theemergence of tin based alloys as alternatives to lead basedsolder alloys. Among the tin based alloys, tin–silver–copper(Sn–Ag–Cu, SAC) alloys have become popular because oftheir advantages, such as good wetting characteristicswith substrate, good fatigue resistance, and good jointstrength.

However, these alloys suffer from some drawbacks thatraise concerns over their reliability. The microstructure of

.S.M.A. Haseeb), arafat_m

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r Inc. All rights reserved.

SAC alloys has been found to coarsen during use and duringhigh temperature exposure to a great extent than in lead con-taining counterparts [1]. Moreover, tin based solders formthicker intermetallic compound (IMC) layers at the solder/substrate interface than the lead based solders [2]. The inter-facial IMCs in lead free solder also grow at a rate faster thanthat in lead based solders. Coarsening of microstructure andrapid growth of brittle interfacial IMC are known to degradethe properties of lead free solder joints resulting in lowerlong term reliability. Research efforts are therefore underwayto improve the quality of tin based solder alloys.

One of the approaches to improving the properties of tinbased solder is through appropriate additions. Both alloy addi-

ahmood@yahoo.com (M.M. Arafat), mrafiej@um.edu.my

28 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 4 ( 2 0 1 2 ) 2 7 – 3 5

tions [3–6] and particle additions [7,8] are being studied cur-rently. Particles additions to tin based solder leads to the de-velopment 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 beingconsidered.

The rationale behind particle addition is that when appro-priate types of particles are added to the solder, they shouldlead to dispersion strengthening. They are also expected tostabilize themicrostructure by restricting the growth of differ-ent phases in the solder during use. The addition of nanosizedparticles to tin based solders has attracted a great deal ofattention in recent years [8,10]. With the decrease of solderpitch size in electronic packages, the addition of nanoparticlesis becoming even more relevant.

Improvement in bulk mechanical properties such asstrength [11], hardness [15], and creep resistance [14] hasbeen observed in lead free solders reinforced with nanoparti-cles. In particular, the addition of Mo nanoparticles hasresulted in considerable improvement in the bulk mechanicalproperties of solder [16–18]. However, the performance of asolder joint not only depend on its bulk properties, they alsodepend on the properties of the solder/substrate interface. Itis therefore important to understand the effect of the nano-particles on the interfacial characteristics. There are only afew studies available on the influence of nanoparticles onthe interfacial IMC. It was found [10,19] that the addition ofCo, Ni, Pt nanoparticles affect the morphology and thicknessof interfacial IMC. Haseeb and Tay [19] investigated themechanism by which Co nanoparticles exert their influenceon interfacial IMC. By comparing the effects of Co nanoparti-cles on the morphology and growth behavior of interfacialIMC with that of Co alloy addition, they suggested that Conanoparticles actually dissolve during reflow and imparttheir influence as alloy additions. In a preliminary work [20],it was observed that the presence of Mo nanoparticles in liq-uid SAC solder reduces the dissolution of the copper sub-strate. That study also revealed evidence suggesting that Monanoparticles did not dissolve in liquid SAC. The presentpaper investigates the effect of Mo nanoparticles addition onthe morphology and growth of interfacial IMC between SACand copper substrate under multiple reflow conditions. Thestability of Mo nanoparticles during reflow for over six cyclesis evaluated. Based on the result obtained, a mechanismthrough which Mo nanoparticles impart their influence onIMC growth is suggested.

2. Experimental Procedures

The composite solders were prepared by manual mixing ofmolybdenum (Mo) nanoparticles (Aldrich, 99.8% trace metalbasis) with Sn–3.8Ag–0.7Cu (SAC) solder paste (IndiumCorpora-tion of America). Themorphology and size of theMonanoparti-cles were investigated using a Philips CM200 transmissionelectron microscope. For this purpose, a small amount of Monanoparticles were dispersed into distilled water onto a carbonfilm supported by copper girds.

Blending of SAC solder paste with Mo nanoparticles wascarried out for approximately 30 min to ensure uniform distri-bution of nanoparticles. Mo nanoparticles were added atdifferent nominal percentages, e.g., 1 wt.%, 2 wt.% and3 wt.%. Polycrystalline copper (Cu) sheets with a dimensionof 30 mm×30 mm×0.3 mm were used as substrates. Prior tosoldering, the substrates were cleaned and dipped in10 vol.% H2SO4 to remove any oxides present. After that, thesubstrates were washed in deionized water and dried in ace-tone. The composite solder paste was placed on the cleanedsubstrate through a mask having an opening diameter of6.5 mm and a thickness of 1.24 mm (JIS Z3198-3, 2003). Thesolder paste was reflowed in a reflow oven (Forced convection,FT02) at a peak temperature of 250 °C for 45 s. Prepared soldersamples were cleaned with acetone to remove the flux resi-due. After the first reflow, the bulk solder was chemically an-alyzed using an inductively coupled plasma-optical emissionspectrometer (Perkin Elmer Optima 2000 DV) to find measurethe actual amount of molybdenum retained in the solder.

Some of the solder samples were reflowed two, four, andsix times. After reflow, the samples were cross sectioned,mounted in epoxy and polished using standardmetallograph-ic technique. The final finishing step involved polishing with0.02 μm silica particles. The cross-sectional image of the inter-facial intermetallic compound (IMC) was collected with thebackscattered electron detector in a scanning electron micro-scope. Elemental analysis of different phases was carried outby 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 solutioncontaining (93% CH3OH+5% HNO3+2% HCl) solution [21]. Themicrostructure and elemental analysis was carried out in aconventional scanning electronmicroscope, and in a high res-olution field emission scanning electron microscope (ZeissUltra-60) with energy dispersive X-ray spectroscopy (EDAX-Genesis Utilities). The high resolution field emission scanningelectron microscope is necessary to observe and analyze indi-vidual Mo nanoparticles. The secondary electron detector(secondary electrons are generated by backscattered electronsthat returned to the surface after several inelastic collisionevents) was used in the field emission scanning electron mi-croscope at an accelerating voltage of 10 kV. Using these pa-rameters, the samples were magnified up to 50 K formicrostructural investigations. The thickness of the interfa-cial IMC and the scallop diameter were calculated from themicrograph by using image analysis software (OlympusSZX10). IMC thickness was calculated by dividing the area cov-ered by the IMC layer in the cross-sectional micrograph by theIMC length [22]. From each image, one average value for theIMC thickness was calculated by using area analysis. Foreach test condition at least five images at randomly selectedpositions were utilized to calculate the average value of IMCthickness and scallop diameter.

3. Results

Fig. 1 shows a transmission electron microscope (TEM) imageof themolybdenum (Mo) nanoparticles used in this study. Thenanoparticles are almost perfectly spherical. The average

Fig. 1 – TEM micrograph of the Mo nanoparticles.

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particle size was approximately 70 nm. However, the size dis-tribution was quite wide, ranging from 20 nm to as large as200 nm [20].

Fig. 2 shows the field emission scanning electron micro-scope (FESEM) micrographs of the solder paste after beingmixed with Mo nanoparticles. It may be noted that the solderpaste mainly consists of a flux containing Sn–3.8Ag–0.7Cu(SAC) solder balls which have an average diameter of about40 μm. Upon mixing, tiny Mo nanoparticles tend to stick tothe surface of the solder balls (Fig. 2a). The nanoparticles alsoare dispersed in the flux (Fig. 2b) between the solder balls.The nanoparticles are more or less uniformly distributed inthe solder paste after manual mixing.

Upon reflow, the solder balls melted, coalesced and formedthe solder joint. The flux residue remained on the surface of

Fig. 2 – FESEM images of solder paste after mixing, nominally conanoparticles into the solder paste, (b) high resolution image of

the solder joint. In order to find out how much of the Monanoparticles are retained in the solidified solder, the latterwas chemically analyzed by inductive coupled plasma-optical emission spectrometry (ICP-OES). The actual Mo con-tent of the solder is shown in Fig. 3 as a function of nominalamount of Mo nanoparticles added to the solder paste. Forthe addition of 1, 2 and 3 wt.% of Mo nanoparticles into thesolder 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 re-mains in the flux residue [20]. Hereafter, solders containing0.04 and 0.10wt.% Mo will be designated as (SAC+0.04 n-Mo)and (SAC+0.10 n-Mo) respectively with n referring tonanoparticles.

Fig. 4 shows cross sectional scanning electron microscope(SEM) micrographs of SAC and (SAC+0.10 n-Mo) solders afterfirst and sixth reflow. On all samples, Cu6Sn5 with a typicalscallop type morphology formed. The composition of Cu6Sn5

was confirmed by energy dispersive X-ray spectroscopy(EDX). A similar result was obtained by Lee et al. [23] for Snbased solder on Cu substrate during reflow. Below theCu6Sn5 layer, there is a thin flat layer of Cu3Sn having a darkercontrast. Evidence of the formation during reflow of a verythin Cu3Sn IMC layer between the Sn-based solder and theCu substrate can be found in the literature [24,25]. A compari-son of Fig. 4a to b shows that the addition of Mo nanoparticlesresults in a decrease in overall interfacial intermetallic com-pound (IMC) thickness after the first reflow. The effect of Monanoparticles is evident after the sixth reflow as well (Fig. 4cand b). Generally, the thickness of the Cu6Sn5 layer increaseswith the reflow number for both the SAC and Mo nanoparticleaugmented SAC solders. No molybdenum could be detectedwithin the Cu6Sn5 IMC by EDX analysis for either the first orsixth reflow. In Fig. 5, the variation of IMC thickness isshown with respect to the number of reflow cycles for bothSAC and Mo nanoparticles added solders. Lower IMCthickness is observed for all Mo nanoparticle added samples.

In order to reveal the surface morphology of the interfacialIMC, deep etchingwas used to dissolve the solder. Fig. 6 showsa typical low magnification micrograph of the (SAC+0.10 n-Mo) solder sample after deep etching. It can be seen fromthe figure that the visibility of the interfacial IMC in the topview depends on the extent of etching. Where etching was

ntaining 2 wt.% of Mo nanoparticles (a) distribution of Mothe flux.

Fig. 3 – Molybdenum content of the solder analyzed by ICP-OES after reflow.

Fig. 5 – IMC thickness as a function of the number of reflowcycles and percentage of Mo nanoparticles.

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complete, the IMC can be seen clearly as indicated by a whiteoutline in Fig. 6. To compare the surface morphology of inter-facial IMC of different samples, places where etching wascompleted were used. To determine the distribution of Monanoparticles, places where the etching was incompletewere particularly investigated.

Fig. 7 shows the top view of the interfacial IMC for the SACand (SAC+0.10 n-Mo) solder samples after first and sixthreflow. Places undergoing complete etching were chosen tocompare the morphology of the interfacial IMC, as mentioned

Fig. 4 – Backscattered electronmicrographs of the cross sectional(b) (SAC+0.10 n-Mo) after first reflow, (c) SAC after six reflows an

earlier. In each case, it was found that the morphology of theinterfacial IMC was scallop type. EDX analysis on thesescallops confirmed that these are Cu6Sn5. From Fig. 7 it isseen that as the number of reflow cycle is increased, thediameter of the scallop is increased for SAC and (SAC+0.10n-Mo) solder. It is also seen that the diameter of the scallopsis smaller for (SAC+0.10 n-Mo) solder than for SAC for all re-flow conditions. In Fig. 8, the average scallop diameter is

view of the solder/substrate interface (a) SAC after first reflow,d (d) (SAC+0.10 n-Mo) after six reflows.

Fig. 6 – SEMmicrograph of (SAC+0.10 n-Mo) sample showingthe extent of etching [2× reflow]. Fig. 8 – Scallop diameter of interfacial IMC as a function of

number of reflows and amount of Mo nanoparticles.

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presented as a function of reflow number and percentage ofMo nanoparticles in the samples. It is clear from Figs. 7 and8 that the diameter of the Cu6Sn5 scallop decreases as theamount of Mo nanoparticles increases under all reflowconditions.

Fig. 9 shows a typical micrograph of the top view of Cu6Sn5

in (SAC+0.04 n-Mo) solder after four reflow cycles. The areapresented in Fig. 9 underwent incomplete etching. Two typesof white particles can be seen in the micrograph. One type of

Fig. 7 – Top view of the interfacial IMC (a) SAC after first reflow, (band (d) (SAC+0.10 n-Mo) after sixth reflow [after deep etching].

particles has a bright appearance with an irregular shape(Marked ‘X’). These types of irregularly shaped particles werefound in both SAC and Mo nanoparticle-added SAC solders.EDX spot analysis confirmed that these irregularly shapedparticles are Ag3Sn. The formation of Ag3Sn on Cu6Sn5 afterreflow has been observed by others for Sn-based solder pre-pared on Cu or Ni substrate [26]. The second type of bright par-ticles (marked ‘Y’) have an almost perfectly spherical shape

) (SAC+0.10 n-Mo) after first reflow, (c) SAC after sixth reflow,

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) EDXspectrum on ‘Y’.

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and were found only in Mo nanoparticle-added solders. EDXspot analysis revealed the occurrence of predominately Moin these particles. This suggests that these particles are Monanoparticles. It should be noted that the microscopy andelemental spot analysis on the Mo nanoparticles were donein an ultra high resolution field emission scanning electronmicroscope (FESEM, Zeiss Ultra-60) equipped with EDX(EDAX-Genesis Utilities) which provides good imaging andanalytical resolution.

Fig. 10 shows an FESEM image and elemental maps depict-ing the presence of Mo nanoparticles on the interfacial IMC of(SAC+0.10 n-Mo) solder after sixth reflow. The presence ofspherical Mo nanoparticles is clearly visible in the micro-graph. The FESEM image together with the elemental mapsfor Mo, Sn and Cu given in Fig. 10 suggest that spherical Monanoparticles are concentrated at the boundaries betweenIMC scallops. Random EDX area analysis at multiple spots onthe IMC surface yielded a Mo concentration of about3–3.5 wt.% on the partially etched IMC surface. It may benoted that the average concentration of Mo in the bulk solderfor this sample is only 0.1wt.%. Therefore it is believed thatMo nanoparticles tend to concentrate on the IMC surfaceduring reflow.

4. Discussion

In the present work, paste mixing was used to incorporate Monanoparticles into the solder. In this method only a fraction ofMo nanoparticles entered the solidified solder. Similar resultshave been obtained for Co nanoparticles [19]. However, in thecase of Co nanoparticles, the fraction of nanoparticlesretained in the solder was higher. The incorporation of nano-particles into solder will mainly depend on the interactionsbetween nanoparticles and the solder. It has been suggestedthat a reinforcing particle can be pushed (rejected), engulfedor entrapped at the particle–liquid metal interface dependingupon the interaction mechanisms [27,28]. The incorporationof a lower amount of Mo in SAC suggests that Mo nanoparti-cles experience rejection by the liquid SAC interface to agreater extent. Poor wetting of Mo and SAC could be a reasonfor higher rejection. Notwithstanding the rejection, theamount of Mo nanoparticles retained in the solder has a defi-nite influence on interfacial IMC growth characteristics, aswill be discussed later.

It has been observed that the morphology of the interfacialCu6Sn5 layer does not change with the addition of Mo

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) overlapof elemental maps for Sn, Ag, Cu and Mo.

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nanoparticles into the SAC solder. The usual scallop type mor-phology is preserved even when Mo nanoparticles are added(Fig. 7). However, the addition of Mo nanoparticles suppressesthe growth of Cu6Sn5 (Figs. 4 and 5). Both the thickness and scal-lop diameter of the interfacial IMC become smaller when Monanoparticles are added to the SAC solder (Figs. 4 and 7). For ex-ample, after first reflow, addition of 0.10 wt.% Mo nanoparticlescauses a decrease in Cu6Sn5 layer thickness from 1.5 μm to0.95 μm (Fig. 4a, b) and a decrease in scallop diameter from2.2 μm to 1.3 μm (Fig. 7a, b). The presence of Mo nanoparticlesclearly influences the interfacial IMC growth characteristics.

The exact mechanism(s) through which Mo nanoparticlessuppresses the growth of interfacial IMC thickness andscallop diameter is not clear. However, several scenarios canbe hypothesized. At one extreme, nanoparticles may remainas discrete, unaltered particles during reflow. At the other ex-treme, they may be completely consumed in some reaction(s)or through dissolution within the molten solder. Actual alter-ation of the nanoparticles will depend on a number of factors,including their melting point and chemical interaction(s) withthe 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 duringreflow. Comparing Figs. 1 and 10, it is found that the originalnearly perfectly spherical shape of Mo nanoparticles is pre-served even after six reflow cycles. This suggests that thenanoparticles did not undergo any significant physical orchemical change during multiple reflows. Referring to the

Mo–Sn phase diagram, Mo has negligible solubility in Sn. Thephase diagram shows that as many as three IMCs e.g.,Mo3Sn, Mo2Sn3/Mo3Sn2, and MoSn2 can exist in the Mo–Snsystem [29]. But no evidence of Mo–Sn compound formationwas found onMo nanoparticles by EDX. For instance, in the el-emental map (Fig. 10), Sn signals does not appear at the placeswhere Mo particles are present. It should be noted that Modoes not form any compound with Cu and Ag at 250 °C, andhas no solubility in these elements [30,31].

The above results suggest that when Mo nanoparticles aremixed with SAC solder paste and reflowed at 250 °C, they re-main as stable solid particles. So the retardation of the growthof IMC thickness and scallop diameter is mainly due to the ef-fect of Mo nanoparticles as discrete particles. There are threepossible mechanisms through which Mo nanoparticles sup-press the thickness and reduce the diameter of IMC scallops:

i) Mo nanoparticles can act as heterogeneous nucleationsites for the formation of Cu6Sn5 nucleus. This can in-crease the density of nucleation of Cu6Sn5 grains,

ii) Mo nanoparticles may have a pinning effect on thegrowing front of Cu6Sn5 scallops, and

iii) Both (i) and (ii).

For a particle to act as a heterogeneous nucleation site, theinterfacial energy between the liquid and solid particlesshould be low. In other words, the wetting angle of the liquidsolder at the solid Mo surface should be low. No data for theinterfacial energy and wetting angle between liquid Cu6Sn5

Fig. 11 – Schematic diagram of scallop growth in (a) SAC sol-der, (b) Mo nanoparticles added SAC solder preferentiallyabsorbed at the IMC scallops.

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and solid Mo is available in the literature. Available data showthat 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 factorfor the heterogeneous nucleation of Cu–Sn compounds onMo nanoparticles. Further, if Mo nanoparticles acted as het-erogeneous nucleation sites, these particles would likely befound as inclusions inside the Cu6Sn5 scallops. Extensive ex-amination of multiple samples in cross-section under highresolution SEM could not identify any such inclusion. Thisevidence leads us to suggest that Mo nanoparticles areunlikely to act as heterogeneous nucleation sites. It is, there-fore, believed that the influence of Mo nanoparticles on theCu6Sn5 layer is due to their effect on the growth process.

Experimental results obtained in the high resolutionFESEM confirmed the presence of Mo nanoparticles on thesurface of IMC (Fig. 9) and at channels between the IMC scal-lops (Fig. 10). In fact, a higher percentage of Mo was foundon the IMC surface (3–3.5wt.%) than in the solder(<0.14wt.%). This indicates that the nanoparticles tend tostay preferentially on the IMC scallops. Preferential segrega-tion of Mo nanoparticles at the liquid/IMC interface may belinked with higher interfacial energy between liquid tin andmolybdenum [32]. It should be noted that the substrate wasplaced in a horizontal position during reflow. Thus the densitydifference 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 nanoparticlesare believed to have a retarding effect on the IMC growth.

Themechanism through which Mo nanoparticles suppressthe interfacial IMC thickness and scallop diameter may beexplained by referring to Fig. 11. Kim and Tu [34] suggestedthat liquid channels exist between the Cu6Sn5 scallops.These channels extend all the way to Cu3Sn/Cu interface.These channels serve as fast diffusion and dissolution pathsfor Cu and thus feed the interfacial reaction. Ignoring thepresence of Cu3Sn IMC and all other chemical reactions forconvenience, Kim and Tu [34] suggested that two kinds offluxes are responsible for the growth of the scallops. One isflux of ripening (JR) and the other is flux of interfacial reaction(JI). At the start of reflow, Cu6Sn5 scallops nucleate at the sol-der/substrate interface. The scallops grow with the increaseof reflow as Cu is supplied by the fluxes, JR and JI. Cu6Sn5 scal-lops grow both in thickness and diameter with time. JI feedsthe thickness growth, while JR causes coalescence and in-creases the diameter of the scallops [Fig. 11a].

When Mo nanoparticles are added to the solder, they blockthe channels between the IMC scallops and are preferentiallyabsorbed at the growth front of IMC scallops [Fig. 11b]. Theblocking of the channels obstructs the movement of copperatoms from the substrate to the liquid solder and hence re-duces JI. This helps to reduce the thickness of IMC. Theadsorbed Mo nanoparticles on the IMC surface will reduce JR

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 becomparedwith that of Co nanoparticles as described in the lit-erature [19]. The addition of Co nanoparticles was found to in-crease the thickness of Cu6Sn5 but decrease Cu3Sn thickness[19]. This was very similar to the case when Co is added as

alloy addition [5]. It was, therefore, suggested that Co nano-particles dissolve during reflow and exert their influence onIMC through alloying effect [19]. This is in contrast with Monanoparticles, which remain intact as seen in this study. Mo-lybdenum, a refractory metal with a high melting point andlow reactivity, remains stable during reflow. These particlesdo not undergo any detectable alteration during reflow. It istherefore suggested that Mo nanoparticles exert their influ-ence on the interfacial IMC growth as discrete particles.

5. Conclusion

The addition of Mo nanoparticles to SAC solder has a pro-nounced effect on the growth of interfacial IMC between SACsolder and Cu substrate. The following conclusions can bedrawn based on this study:

01. Mo nanoparticles do not dissolve or react with the SACsolder and remain stable during multiple reflow cyclesat 250 °C.

02. The addition of Mo nanoparticles to SAC solder causes adecrease in the thickness and diameter of interfacialCu6Sn5 scallops.

03. Mo nanoparticles influence the interfacial IMC throughdiscrete particle effect by preferentially absorbing atthe grain boundaries of interfacial IMC scallops.

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04. Mo nanoparticles influence IMC growth rather thannucleation.

Acknowledgments

The authors acknowledge the financial support of FundamentalResearch Grant Scheme (FRGS, Project No. FP013/2010B) andUniversity of Malaya (IPPP, with the project no. PS072-2009B).

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