Fabrication of Sn-Ag/CeO2 Electro-Composite Solder by Pulse Electrodeposition

Fabrication of Sn-Ag/CeO2 Electro-Composite Solderby Pulse Electrodeposition

ASHUTOSH SHARMA, SUMIT BHATTACHARYA, SIDDHARTHA DAS,and KARABI DAS

The Sn-Ag/CeO2 nanocomposite solders have been pulse electrodeposited from an aqueouscitrate bath containing varying concentrations of CeO2 nanopowders (1 to 30 g/L). Micro-structural characterization, hardness, melting point, electrical conductivity, wear resistance, andresidual stress measurement of the composite coatings indicate that the composite depositedfrom an electrolyte containing 15 g/L CeO2 possesses the optimum properties and thus can havepotential applications in solder joints and packaging.

DOI: 10.1007/s11661-013-1894-5� The Minerals, Metals & Materials Society and ASM International 2013

I. INTRODUCTION

IN recent decades, researchers have been looking forlead-free solders because of the various legislations andthe imposed ban on the usage of Pb-bearing materials.Various lead-free solders, either binary or multicompo-nent alloys, e.g., Sn-Cu, Sn-Ag, Sn-Zn, Sn-Bi, Sn-Ag-Cu, Sn-Ag-RE, Sn-Zn-Al, Sn-Ag-Cu-In, etc., are beingdeveloped and studied.[1–3] For the microdevices wheresoldering materials are employed, i.e., flip chip packag-ing and ball grid arrays, it is necessary to ensure that thesolder joints have adequate strength.[4–6] As the size ofthe interconnecting network in microelectronic packag-ing devices is getting finer over time, their reliability iscausing a serious concern. The miniaturization of thepackaging devices will lead to an increase in currentdensity, and it may finally be damaged.[7] One of theattractive routes to ensure the solder joint’s reliability isvia co-electrodeposition approach from aqueous sus-pension of ceramic micro/nanoparticles, nanotubes, orwhiskers in an electrolyte.[8–11] Shin et al.[12] showed thatwith the incorporation of SiC nanoparticles in Sn-Bimatrix, the composite solder exhibited a finer eutecticmicrostructure with enhanced shear strength comparedwith Sn-Bi. On the other hand, Park et al.[13] demon-strated that the addition of carbon particles in the Snmatrix greatly increased their cycle performance. Re-cently, Choi et al.[14] co-electrodeposited multiwalledCNT in Sn matrix and found shear energy beingimproved by 50 pct and thus pointed out the noveltyof the electrodeposition for producing the reliable solderjoints. Various ceramic reinforcements, such as Al2O3,ZrO2, TiO2, SnO2, SiC, Y2O3 etc., have been alreadyadded into a solder alloy.[15–17] These reinforcementsnot only refine the microstructure and intermetallic

compounds, but also increase the mechanical andthermal property, wear and corrosion resistance withoutsacrificing much the electrical performance of thematerials. Currently, rare earth oxides are also beingconsistently used as reinforcing materials. Among therare earth members, CeO2 is being widely used inmechanical, tribological, and microelectronic domain ofresearch.[18–20] The CeO2 not only possesses excellentmechanical properties, but also attractive thermal andelectrical properties. However, it is to be noted thatmicron-sized ceramic particles result in weaker particlematrix bonding that can deteriorate the mechanical,physical, and electrical properties of the nanocompositesolders.[21] Therefore, the prime objective of thisresearch is to design a novel nanocomposite solder,near-eutectic Sn-Ag-containing nanocrystalline CeO2

particles for the first time by pulse co-electrodepositionroute. The developed composites will be evaluated fortheir microhardness, melting point, electrical resisitivity,wear and friction behavior, and residual stress.

II. EXPERIMENTAL PROCEDURE

A. Synthesis of Reinforcement (CeO2)

As-received CeO2 powder (Loba Chemie, 99.9 pct) issubjected to high-energy ball milling in a vario-planetarymilling machine (Fritsch Pulverisette-4, Germany)which consists of WC twin vials rotating simultaneouslyon a revolving WC main disk. The mill is operated at amain disk speed of (X) 300 rpm and a planet speed of(x) 540 rpm with a transmission ratio of �1.8, and theball-to-powder weight ratio is 10:1. After each hour ofmilling, the mill is allowed to cool for between 45 and60 minutes. Toluene is used as a process control agentthat prevents excessive cold welding of the powderparticles. The CeO2 powder is milled for 20 hours, andthe samples are collected regularly during milling toobserve the progress of the size reduction. After drying,the CeO2 powder is washed with alcohol, followed bydistilled water. The powder is kept at room temperaturefor characterization.

ASHUTOSH SHARMA, Research Scholar, SUMITBHATTACHARYA,GraduateStudent, SIDDHARTHADAS,Professorand Head, and KARABI DAS, Professor, are with the Department ofMetallurgical and Materials Engineering, IIT Kharagpur, Kharagpur721302, India. Contact e-mail: [email protected]

Manuscript submitted November 9, 2011.

METALLURGICAL AND MATERIALS TRANSACTIONS A

B. Synthesis of Nanocomposites Through PulseElectrodeposition

The tin plating bath used for the electrodeposition isthe citrate-based aqueous solution. The compositionand bath parameters used in the experiment are shownin Table I. To prepare the plating solution, tri-ammo-nium citrate (100 g/L) is dissolved in deionized water,followed by the addition of SnCl2Æ2H2O (50 g/L) andAgNO3 (0.24 g/L) as a source of tin and silver ions,respectively. In this bath, thiourea (0.1 g/L) is added,which acts as a complexing agent. The addition ofthiourea is necessary for the stabilization of the platingbath containing Ag+ and Sn2+ ions, because Ag+

precipitation occurs if it is not added. The effect ofthiourea is ascribed to its complex formation behaviorwith Ag+ ions.[22] The solution is stirred until theSnCl2Æ2H2O salt is fully dissolved and forms a clearsolution. The pH of the prepared bath is maintained at~4.3. For composite, the co-electrodeposition bath hasbeen prepared by adding the nano-sized CeO2 powder(~30 nm) to the plating bath. Triton X-100 (0.1 g/L) isalso added as a dispersing agent for CeO2 nanoparticles.The suspension thus formed is subjected to ultrasonicvibration (Sartorius Labsonic M) for 2 hours followedby magnetic stirring at 300 rpm before the experiment toavoid the settling of the dispersed nanoparticles duringthe electrodeposition process.

A pure tin plate with dimensions 10 cm 9 3 cm 92.5 cm (Merck Specialties Pvt. Ltd, electrolytic grade,99.8 pct) and a steel specimen with dimensions5 cm 9 3 cm 9 0.2 cm are used as anode and cathode,respectively. The cathode substrate is metallographicallypolished and degreased in ultrasonicator for 30 minutesto remove the dust particles and foreign impurities. TheDC current waveforms are provided from a potentio-stat/galvanostat (Autolab PGSTAT 302 N) which iscontrolled thorough a personal computer, and thicknessof the deposit is maintained at 40 lm. All the current/potential curves are referred to a Ag/AgCl (3M KCl)electrode. The electrochemical measurements are per-formed by Ecochemie software module applicationswith Autolab. The deposited films are stripped off fromthe cathode under a running stream of water andwashed with alcohol and dried in air before the

evaluation of properties. The different amounts ofCeO2 in solution are 0, 1, 2, 5, 10, 15, 20, 25, and30 g/L, and the corresponding deposited samples aredesignated as D0, D1, D2, D5, D10, D15, D20, D25,and D30, respectively.

C. Cyclic Voltammetry Analysis

Three-electrode cell geometry has been used for cyclicvoltammetry measurements. The experiment is per-formed using Autolab PGSTAT 302 N at a scan rateof 1 mV/s. The copper is used as a working electrode.The saturated Ag/AgCl in 3 M KCl solution andplatinum rod are used as reference and counter elec-trodes, respectively. The electrolytic bath with or with-out the CeO2 particles dispersed in the electrolyte is usedfor the measurements. The copper is polished with 1.0,0.3, and 0.05 lm alumina slurry, and then cleanedultrasonically with de-ionized water to give it a mirrorfinish.

D. Microstructural Characterizations

1. X-ray diffraction (XRD)The XRD experiments are carried out in an X-ray

diffractometer (Brucker D8 Advance) using Cu-Ka

radiation, 40 kV voltage, and 35 mA current. Thephases formed are identified by comparing the recordeddiffraction peaks with the standard ICDD databaseusing Philips X’Pert HighScore software.The crystallite size analysis has been done by

Williamson–Hall method.[23] This method assumes thatboth size and strain broadening of profiles follow theLorentzian distribution. According to this assumption,volume-weighted average crystallite size (Dv) and thelattice strain (e) can be given as

b cos hk¼ 1

Dvþ 2e

2 sin hk

� �; ½1�

where b is the integral breadth, h is the Bragg angle, andk is the wavelength of the radiation.

2. Scanning electron microscopy (SEM)The morphology of the coatings and distribution of

CeO2 nanoparticles in the electrodeposited coatings areanalyzed using scanning electron microscope (Carl ZeissSupra EVO 60) coupled with energy dispersive X-rayspectrometer (EDS), which detects the energy of thecharacteristic X-rays. This is used to detect the elementaldistribution present in the sample. The high-resolutionfield emission scanning electron microscope (FESEM,Zeiss Supra 40) is also utilized for the observation ofCeO2 nanoparticles.

3. Transmission electron microscopy (TEM)A transmission electron microscope (Philips Technai

2 Twin G220S) operating at 200 kV is used tocharacterize the electrodeposited film. The samples areprepared by twin-jet electropolishing machine (FishioneModel 120). The twin-jet electropolishing is carried outin an electrolyte containing 75 pct ethanol and 25 pct

Table I. Bath Compositions and Operating Parameters

Experimental Parameters Values

SnCl2Æ2H2O 50 g/LC6H17N3O7 100 g/LAgNO3 0.24 g/LThiourea 0.1 g/LTriton X-100 0.1 g/LNanosized Ceria 0 to 30 g/LpH 4.3Current Density 0.2 A/cm2

Bath Temperature 301 K (28 �C)Duration 10 minAnode (99.8 pct) tin plateAgitation 300 rpmTon, Toff 0.001 s, 0.01 s


phosphoric acid (by volume) at �12 �C and 5 V. Theelectropolished samples are dried with water followed byalcohol, and then stored at room temperature forcharacterization.

E. Evaluation of Properties

1. MicrohardnessThe microhardness measurements are taken by a

Leica Vicker’s microhardness testing machine under aload of 25 g for a dwell time of 20 seconds. The finalmicrohardness values reported are the average of 10measurements performed at different locations of thefilms.

2. Melting pointA Pyris Diamond DSC (Perkin Elmer) is used to

measure the melting point of the developed coatings.The calorimetric investigation is carried out usingalumina (Al2O3) pans containing from 5 to 10 mg ofthe material during testing. The samples are heated from323 K to 523 K (50 �C to 250 �C) at a rate of10 K min�1 (10 �C min�1), under a N2 atmosphere witha flow rate of 50 mL min�1.

3. Electrical resistivityThe resistance (DV/I) of the as-deposited films of Sn

and Sn-based composites is measured using a four-probesetup (Keithley Model 2400), and resistivity is calculatedusing the formula:

For, h � a:

q ¼Q

lnð2Þ hDVI

� �; ½2�

where h is the thickness of the film, a is the distancebetween the two probes, V is the voltage, and I is thecurrent. A probe with a spacing of 1.5 cm and operatingwith an 1 A source meter and 10 mV nanovoltmeter isutilized.

4. Wear and frictionWear and friction tests of the samples are carried out

using a standard ball-on-disk wear tester (DUCOM,TR-208-M1) with a hardened steel ball of 2-mmdiameter, and employing different loads (4 to 10 N)for a total time of 1800 seconds. The roughness, trackpenetration depth, and volume loss are obtained using aprofilometer (Veeco Dektak 150). The wear rate iscalculated using the formula: wear rate = [V/NL]mm3/Nm, where V is the wear volume loss, N is theload in Newton, and L is the sliding distance in meter.

5. Residual stress measurementThe residual film stresses of the selected samples on

copper substrate are measured by sin2w method ofX-ray diffraction analysis using Philips PanalyticalX’Pert Pro (PW 3040/60, Netherland) with a stressgoniometer attached to it. This method is performed bymeasuring the diffraction angle 2h in the w tilt axis(w = 0 deg, 45 deg, and 90 deg) against the direction ofstress measurement and generating sin2w diagrams.[24]

All stress measurements are done along the same in-plane direction in the specimen frame of reference, andthus only one component, r|| (stress parallel to thesurface), of the stress tensor will be reported. The sin2wmethod is suitable for isotropic materials, but whenapplied to anisotropic materials, a reflection peak thatexists at a higher diffraction angle is selected for residualstress measurement.[25] The peak intensity correspond-ing to the (312) plane of the Sn structure is measured.This has been selected previously in the literature by anumber of researchers.[26,27] The final residual stress iscalculated using X’pert Stress Software.

III. RESULTS

A. Characterization of CeO2 Particles

Figure 1 shows the XRD patterns of the CeO2

powder milled for 0, 5, 10, 15, and 20 hours. FromXRD patterns, it is observed that the peaks are gettingbroadened as the milling time increases because of thereduction of the crystallite size and increase in the latticestrain. The inset clearly shows the broadening of the(111) peak confirming the nanocrystallite formation.The crystallite size and lattice strain calculated from

the Williamson Hall method for different ball-milledCeO2 samples are shown in Table II. It is revealed thatthe crystallite size decreases from 191 to 32 nm, and thelattice strain increases from 0.49 9 10�3 to 1.34 9 10�3

because of the continuous fracturing of the CeO2

particles with the increasing milling time.

B. Cyclic Voltammetry

Figure 2 shows cyclic voltammograms recorded forthe plating bath with and without CeO2 in the potential

Fig. 1—XRD patterns of CeO2 powders produced by (a) 0, (b) 5, (c) 10,(d) 15, and (e) 20 h ball milling. The inset shows the (111) CeO2 peakbroadening with milling time.


range of �1.2 to 0.4 V measured at a scan rate of 1 mV/s.At the open circuit potential, no current flows. Duringforward scan, as the voltage increases slowly, thecathodic current also rises that signifies an efficientdeposition process. After the cathodic peak the currentdensity again decreases because of the inherent hydro-gen evolution mechanisms. As the potential is increased

further to more negative in reverse scan, the drasticincrease in the anodic current indicates an efficient tindissolution.[28] It is noticed that the plating bath in thepresence of CeO2 shows a lesser cathodic currentcompared with the absence of CeO2. This may be dueto the adsorption of CeO2 nanoparticles on the activesites of the cathode surface, according to Guglielmimodel.[29] The presence of inert nanoparticles influencesthe metal deposition in many ways. An incompleteembedding of particles blocks the cathode surface areaand, furthermore, new surfaces develop in this processwhen they are completely submerged in the metalmatrix.[30] As a consequence, a decrease in effectivecathodic current density is observed. The current densityis also observed to be lower in anodic region with theincorporation of CeO2 nanoparticles, which indicatesthat the dissolution process of the tin becomes moredifficult in the presence of CeO2 in the coating. Thus, theincorporation of CeO2 nanoparticles in the coatingimproves their stability to the external environment.

C. Pulse Electrodeposition of Monolithic Sn-Ag Alloy

The monolithic Sn-Ag alloy has been electrodepositedfrom the bath containing no CeO2 nanoparticles. Toobtain the near-eutectic composition (Sn-3.5 wt pct Ag),the variation of Ag concentration in the deposit with theAgNO3 concentration in the electrolyte at a fixedcurrent density of 0.2 A/cm2 has been investigated andplotted in Figure 3(a).From Figure 3(a), it is clear that the eutectic compo-

sition is achieved at AgNO3 concentration of 0.24 g/L inthe electrolyte and at a current density 0.2 A/cm2. In aneffort to investigate the effect of current density on theamount of Ag in the deposits, experiments are per-formed by varying the current density from 0.05 to0.4 A/cm2 while keeping the AgNO3 concentration inthe electrolyte constant at 0.24 g/L (Figure 3(b)). Theobserved high amount of Ag in the low-current densityregion (below 0.2 A/cm2) is attributed to the relatively

Table II. Crystallite Size and Lattice Strain of CeO2

Particles as Obtained from the Williamson–Hall Method

Samples (h)CrystalliteSize (nm)

Lattice Strain(910�3)

0 191 0.495 69 0.6510 43 0.7915 39 1.0620 32 1.34

Fig. 3—The Ag content in the deposit as a function of (a) AgNO3 concentration in the electrolyte, and (b) current density.

Fig. 2—Cyclic voltammograms of plating bath in the absence (D0)and the presence (D15) of CeO2 nanoparticles.


positive reduction potential of Ag+ (+0.799 V) com-pared with Sn2+ (�0.134 V). The Ag concentrationdecreases at higher current density because at the higheroverpotential, the reduction of Sn2+ becomes easier andit continues to be deposited, but the supply of the Ag+

ions in the electrolyte becomes limited.[31]

D. Pulse Electrodeposition of Nanocomposites

1. XRDFigure 4 shows the XRD patterns of monolithic Sn-

Ag alloy and Sn-Ag/CeO2 nanocomposite coatingssynthesized by the process of pulse electrodeposition.The XRD pattern shows the presence of (111) CeO2

peak in the nanocomposite. The peaks of Ag3Snintermetallic compound are also observed in all thesamples. Ahat et al.[32] have also reported the formationof Ag3Sn in a Sn-3.5Ag solder alloy, which improves themechanical properties.

2. SEMThe surface morphology of the monolithic Sn-Ag

alloy and Sn-Ag/CeO2 composites is shown in Figure 5.The microstructure of the deposits shows a tendency tobe finer with the increase in CeO2 nanoparticles. Thesample deposited from the electrolyte containing with15 g/L CeO2 (D15) exhibits the best morphology amongall the deposits. On the addition of the CeO2 nanopar-ticles, the grain size is reduced, and it adopts a moreregular shape. This can be correlated to the two factors:(1) the presence of more number of nucleation centersdue to CeO2 incorporation during the co-electrodepos-ition, and (2) the adsorption of CeO2 on the cathodesurface increases cathodic polarization which results infiner grains. Figures 5(f) and (j) depict a uniformdistribution of CeO2 nanoparticles which are white incolor and have a size ranging from about 20 to 30 nm.However, as noticed from the SEM micrographs, theAg3Sn compound is not clearly visible. This type of

observations on the morphology of deposits is consistentwith the earlier study on electrodeposited Sn-Agsolder.[31]

It is also observed from the SEM micrographs that asthe concentration of CeO2 in the electrolyte increasesbeyond 15 g/L, CeO2 nanoparticles exist in the form ofagglomerates in the coating. As the concentration ofCeO2 particles in electrolyte increases, their interparticledistance decreases resulting in an increase in Van derWalls interaction, and eventually leading to the forma-tion of agglomerates and lumps. Hence, an agglomer-ated/non uniform deposit is observed. As a result, theporosities and cracks are formed in the microstructure,encircled in Figure 5(g). The high magnification micro-graph (Figure 5(k)) confirms the formation of CeO2

agglomerates in the matrix.The variation of the CeO2 content in the composites

with different CeO2 concentrations in the electrolyte isgiven in Table III. It is clear that an addition of CeO2 inthe electrolyte up to 15 g/L increases the amount ofCeO2 in the deposit. However, as the CeO2 content inthe electrolyte is more than 15 g/L, the interparticledistance decreases as the amount of CeO2 in bath is toohigh. This results in the formation of agglomerates andlumps, which have difficulties in reaching the cathodeand the amount of CeO2 in the deposit decreases. TheAg concentration has been found to vary from 3 to 3.6wt pct in the deposits confirming a near-eutecticcomposition of the composites produced.The XRD analysis (Figure 4) reveals the presence of

Ag3Sn in all the composites as well as in monolithic Sn-Ag alloy. However, as noticed from the SEM micro-graphs (Figure 5), the Ag3Sn intermetallic compound ishardly visible. Therefore, the sample with the bestmorphology (D15) and monolithic Sn-Ag alloy (D0) arechosen for further study. The samples are mechanicallypolished with 1-lm diamond paste and etched with asolution of 5 vol pct HNO3+95 vol pct C2H5OH. Thistype of electrolyte has already been employed in the pastto reveal the eutectic microstructures of the Sn-Agsolders.[33] After the etching treatment, the soft phase Sngets dissolved, and Ag3Sn is observed clearly in themicrostructure as shown in Figure 6. The point EDSanalysis of these samples also confirms these particlesto be Ag3Sn intermetallic compound, as shown inFigures 6(c) and (d).The needle-like particles in monolithic Sn-Ag alloy

are confirmed to be Ag3Sn. It is also observed that thesize of Ag3Sn in the composite is finer compared withmonolithic alloy and is visible only at very highmagnification. This type of phenomena has beenexplained in terms of surface adsorption theory in thepast by several authors.[33–35] It states that the amountof surface adsorption of active materials is different fordifferent planes. The plane with maximum surfacetension grows the fastest, and the adsorption amountof surface-active material is maximized. However, anincrease in the amount of surface-active materialdecreases its surface energy and therefore decreases thegrowth velocity of this plane. The surface energy of thewhole crystalloid is given by

Fig. 4—XRD patterns of Sn-Ag alloy and Sn-Ag/CeO2 compositesprepared with different concentrations of CeO2 in the electrolyte.


Xi

cicAi ¼Xi

cio � RT

Zc

o

Ci

cdc

0@

1AAi; ½3�

where Ci is the adsorption coefficient of surface-activematerials at ith plane, c is the concentration of surface-active material, R is the ideal gas constant, and T is theabsolute temperature. cic is the surface tension of the ithplane with adsorption, cio is the initial surface tension ofith plane without adsorption, and Ai is the area of theplane i.For a constant volume,

Pi c

ioAi is constant, as it is

independent of concentration. Thus, for a minimumsurface energy, from Eq. [3] we get, RT

Rco

Ci

c dc! max;which implies that the plane, where the nanoparticles areadsorbed with a maximum adsorption amount Ci, is themost effective in minimizing the free energy of the wholeinterface. Since the CeO2 nanoparticles are smaller thanAg3Sn particles, they can easily accumulate on theseAg3Sn particles and on the Sn matrix.[36] Hence,adsorption of CeO2 nanoparticles would refine the Sngrains and restrict the growth of Ag3Sn intermetallic.

Table III. Amount of Co-deposited CeO2 in the Sn-AgNanocomposite Coatings

Sample Amount of CeO2 (Wt Pct)

D0 0D1 2.4D2 5.3D5 7.8D10 10.7D15 11.5D20 4.8D25 4.2D30 3.6

Fig. 5—SEM images of (a) D0, (b) D1, (c) D2, (d) D5, (e) D10, (f) D15, (g) D20, (h) D25, (i) D30, and (j) magnified view of (f), (k) magnifiedview of (i).


3. TEMFigure 7(a) shows the bright-field image of the sample

D15 confirming the presence of the CeO2 nanoparticlesin the Sn-Ag matrix. The Ag3Sn compound is alsopresent in the matrix and has needle-shaped structure asshown in Figure 7(b). The CeO2 nanoparticles are foundto be about ~20 nm in size and randomly oriented withno significant agglomeration.

E. Evaluation of Properties

1. MicrohardnessThe microhardness of samples is measured using

Vicker’s Microhardness Tester as shown in Figure 8. Itis clear from Figure 8 that compared with the mono-lithic alloy, the composites are having a higher micro-hardness. The increase in microhardness with theincorporation of CeO2 nanoparticles can be ascribedto (1) the higher hardness of CeO2, (2) the dispersionstrengthening effect of CeO2 particles in the Sn-Agmatrix, and (3) the hindrance to grain growth of thematrix owing to the presence of CeO2.

It is also noticed that the microhardness values of Sn-Ag/CeO2 composites are much higher than that of

monolithic Sn-Ag alloy. The XRD and SEM studiesreported in the previous section, and also the binaryphase diagram of Sn-Ag system[37] suggest the formationof Ag3Sn compound having a hardness of ~300 Hv inboth the monolithic alloy and composites.[38] However,the size of the intermetallic particles is much smaller inthe composite compared with the monolithic alloy, asseen in Figure 6. Since the CeO2 nanoparticles aresmaller than Ag3Sn intermetallics, according to theadsorption theory described in Section III–D–2, a refinedAg3Sn is formed in the presence of CeO2 nanoparticleswith a significant rise in the microhardness.It is also observed that the composites have lower

hardness when they are deposited from the electrolytecontaining more than 15 g/L CeO2. The microhardnessvalues of D20, D25, and D30 are also lower than that ofD15. It is already mentioned that in these samples, thetotal amount of CeO2 particles incorporated is quite lesscompared with the corresponding composite sampleD15. Moreover, the CeO2 is present in the agglomeratedform, as observed from the SEM micrographs(Figure 5). These lead to a weakening in the describedstrengthening mechanisms and thus composite micro-hardness.

Fig. 6—SEM micrographs of pulse electrodeposited (a) Sn-Ag Alloy, (b) Sn-Ag/CeO2 nanocomposite, after etching with the 5 vol pct HNO3+95 vol pct C2H5OH, (c) point EDS analysis of (a), and (d) point EDS analysis of (b).


2. Melting pointIt is observed that the melting temperature of the

composite samples differs slightly from that of themonolithic sample, which agrees well with the resultsreported for other Sn-based solders reinforced withnanoceramic particles.[39–41] As shown in Figure 9, themelting point for monolithic D0 sample is observed at497.8 K (224.8 �C) which decreases to 495.2 K(222.2 �C) for sample D15. This decrease in the meltingpoint compared with monolithic D0 is around 2.6 K(2.6 �C). The decrease in melting point of a metal oralloy is an inherent physical property and depends onthe material itself. According to the Lindemann crite-rion, a crystal will melt when the root mean-squaredisplacement of the atoms in the crystal exceeds acertain fraction of the interatomic distance.[42] Since thesurface atoms of a crystal usually have low coordinationnumbers, they experience different bonding forces from

those of atoms in the bulk. The combined effect ofincreasing the number of surface atoms and the surfacephonon softening will significantly increase the atomicmean-square displacements, and then there will be avery slight decrease in the melting temperature of thealloy. Therefore, when an alloy is on a nanosize scale orthe refinement in grain occurs by the particle incorpo-ration, the melting temperature of the composite maydecrease. The grain refinement of Sn-Ag matrix in thecomposite brought about by the addition of CeO2

nanoparticles and finer Ag3Sn particles compared withthe monolithic alloy depresses the melting point up tocomposite D15. However, the melting points increaseslightly for the samples D20, D25, and D30 to 498.3 K(225.3 �C), 498.26 K (225.26 �C), and 498.8 K (225.8 �C),respectively. This slight increase may be due to thepresence of agglomerated CeO2 particles causing a reduc-tion in grain refinement mechanism in these samples.Thus, D15 sample exhibits the best composition forexisting soldering applications.

3. Electrical resistivityFigure 10 shows that the resistivity of the composites

increases continuously with the addition of CeO2 in thematrix. The resistivity of a metal matrix composite isaffected by a number of disturbances in the crystalstructure, such as solute elements, impurities, grainboundaries, dislocations, vacancies, etc. The generalequation for the resistivity of a material is given by theMatthiessen’s rule,[43] which states that the total resis-tivity of a material is the sum of three components: (1)foreign impurities (qi), (2) thermal agitations of metalions of lattice (qt), and (3) the presence of imperfectionsin the crystal, e.g., plastic deformation (qd) etc.

qtotal ¼ qi þ qt þ qd ½4�

For composite solders, the total resistivity valuesare thus expected to increase because of the larger

Fig. 7—Bright-field TEM micrographs of sample D15 showing the (a) CeO2 nanoparticles, and (b) a region of the matrix where few Ag3Sn inter-metallic particles are present.

Fig. 8—Vickers microhardness of the Sn-Ag alloy and compositesprepared with different concentrations of CeO2 in electrolyte.


contributions of qi and qd compared with monolithicsolder samples. qd depends on some factors such as thevolume fraction of pores (Vp), plastic zone (Vpz), andreinforcement (Vr). The effective volume fraction ofscattering centers, (VT) can now be represented as follows:

VT ¼ Vpz þ Vr þ Vp; ½5�

where Vpz, Vr, and Vp represent the volume fraction ofthe plastic zone, reinforcement, and the porosity/pores,respectively.For a particulate reinforcement, the volume fraction

of the deformation region surrounding the reinforce-ment, Vpz, is expressed by

Vpz ¼ ða3 � 1Þ Vr; ½6�

where a is the ratio of the size of the heterogeneousnucleation zone to that of the reinforcement.[44]

Rearranging [5] and [6], we obtain

VT ¼ ða3 � 1Þ Vr þ Vr þ Vp ¼ a3Vr þ Vp ½7�

Fig. 9—DSC curves of the Sn-Ag alloy and composite solder specimens reinforced with different concentrations of CeO2 nanoparticles, (a) D0,(b) D1, (c) D2, (d) D5, (e) D10, (f) D15, (g) D20, (h) D25, and (i) D30.

Fig. 10—Electrical resistivity values of the Sn-Ag alloy and differentcomposites with varying concentrations of CeO2.


The a value is dependent on the type of matrix andalso on the size, shape, and type of the reinforcement,but not on its volume fraction. Thus, according toEq. [7], the effective volume fraction depends on thevolume fraction of reinforcement and pores.

Figure 10 shows the electrical resistivity values of thedifferent samples under investigation. It is observed thatthe resistivities of the composite samples are alwayshigher compared with the monolithic sample, since inmonolithic sample, the resistivity contribution of rein-forcement is zero. The resistivity increase is very slow forthe samples deposited from the electrolyte containing upto 15 g/L CeO2 (i.e., D15). For example, from D0 toD15, the resistivity increases from 12.2 to 13.5 lX cm.This may be because the porosity contribution, Vp, isnot significant enough to cause much disturbance inelectron path. Thus, omitting the porosity (Vp) term inEq. [7], resistivity will increase with only the volumefraction of the CeO2 nanoparticles (Vr). Hence, the totalresistivity increases, but the amount of increase is not sohigh. However, the resistivity increases at a considerable

rate for those samples which are deposited fromelectrolytes containing more than 15 g/L CeO2 andespecially, it is very high for D30. This can be expectedsince the resistivity is also getting affected by thepresence of the significant amount of porosities andcracks in these samples. These porosities and cracks actas additional scattering centers to the path of theelectron motion and increase resistivity. This type ofincrease in resistivity with ceramic reinforcementsobserved is consistent with a number of previousstudies.[45,46] The electrical resistivity values of Sn-Ag/CeO2-based nanocomposite solders measured are quitecomparable with other composites like Sn-Ag/SnO2,Sn-Ag/Y2O3, Sn-0.7Cu/Al2O3 etc.

[47]

4. Wear and friction

a. Surface roughness. For the wear-and-friction study,the composite with maximum hardness, i.e., D15, istaken under investigation in particular and compared

Fig. 11—Roughness profile (3D) of the selected samples: (a) D0 and (b) D15.


with the monolithic Sn-Ag alloy (D0). The surfaceroughness is measured for the samples under consider-ation and is shown in Figure 11.

The 3D roughness profile (Figure 11) shows that forthe composite sample D15, the surface is more unevencompared with the corresponding monolithic sample.The presence of reinforcement phases on the surface willact as a surface projection and thus increases theroughness. This is in conformation with earlier reportson electrodeposited metal matrix composites.[45,46]

b. Wear rate. It is observed that an increase in loadfrom 4 to 10 N causes an increase in the wear rate asexpected (Figure 12). It is also observed that monolithicalloy is having a higher wear rate compared with Sn-Ag/CeO2 composite. This result is in good agreement withthe Archard’s relation,[48] which states that the hardersamples possess higher wear resistance. It has alreadybeen described in Section III–E–1 that the presence ofCeO2 nanoparticles and finer Ag3Sn particles in thecomposite compared with the monolithic alloy increasesthe hardness of the composite significantly. This resultsin higher wear resistance of the composite. It is observedthat the wear rate of D0 increases continuously as theload increases from 4 to 8 N, and it becomes significantas the load increases above 8 N. However, for D15,there is only slight increase in the wear rate as the loadincreases from 4 to 6 N and above that, it remainsalmost constant. For instance, at a load of 8 N, the wearrate of the sample D0 is about 3.07 9 10�4 mm3/Nmwhich increases to 5.19 9 10�4 mm3/Nm at 10 N, whilefor D15 it increases from 1.15 9 10�4 mm3/Nm to1.38 9 10�4 mm3/Nm as the load increases from 8 to10 N.

c. Coefficient of friction (COF). It is observed fromFigure 13 that the COF value of the composite samplesis higher compared with monolithic alloy. This is due tothe higher surface roughness of the composite sampleand also the projected CeO2 nanoparticles present onthe surface, obstructing the sliding motion of the steel

indenter. The measured roughness for D0 and D15 are0.75 and 2.53 lm, respectively. It is also observed thatthe COF rises continuously for both monolithic andcomposite samples as the load increases. This is expectedfor monolithic alloy sample. For composite, this canalso be explained as follows: since the CeO2 is monodi-spersed, i.e., no agglomeration of the composite sampleunder observation, there is a very small possibility ofdetachment of CeO2 particles. Therefore, an increase infriction is observed even at higher loads.[19]

d. Wear track morphologies. Figure 14 shows the SEMmicrographs of wear tracks of samples D0 and D15 atdifferent loads. It is observed that for sample D0, thewidth of the wear tracks increases with an increasingload. A closer look of the wear track of D0 shows thatas the load increases from 4 to 8 N, the width of thetracks increases (Figures 14(a) through (c)). As the loadis increased approaching toward 10 N, the matrix issufficiently strain hardened before the formation ofcracks and ultimately failure occurs (Figures 14(d) and(e)). With the increasing load, the sliding action causesthe repetitive work-hardening, heavy plastic deforma-tion, and shear strains in the worn surfaces giving rise tothe formation of larger-sized cracks. This results in thespalling of the material from the surface layer as shownin Figure 14(e).The width of the tracks of D15 increases with loads.

However, the tracks are much narrower compared withsample D0 as shown in Figure 14. This shows that thepresence of finer Ag3Sn and nano-CeO2 particles in thiscomposite enhances the load-bearing ability of thecomposite resulting in narrow wear tracks. It is alsoobserved that during sliding, the composite sample D15receives minor wear damage showing a mild wear up to8 N. However, with an increase in the load to 10 N, veryfine microcracks originate on the wear surface. Thesemicrocracks may further combine to form a larger crackwith further increase in the load which eventually leadsto failure.

Fig. 12—Wear rate of selected samples as a function of load.Fig. 13—Coefficient of friction of selected samples as a function ofload.


5. Residual stressThe residual stresses of the Sn-Ag alloy and composites

electroplated on Cu substrates are measured and plottedin Figure 15. It is found that with an incorporation ofCeO2 in the composites, the distribution of stress occursin such a way that the compressive stress gets minimized.The monolithic alloy has negative residual stress,i.e., compressive stress, while in case of compositesamples, the stresses are either less negative or positive(Figure 15). The measured residual stress for D0 is�14.5 MPa which decreases to 4.5 MPa for D15.

Nano-CeO2 particles, when incorporated inside thematrix, create local stress fields around themselves. This

stress field distorts the lattice, and this distortionperhaps hinders the free movement of copper atominside tin matrix. Studies have shown that an addition ofhard ceramic particulates in a ductile matrix causes theformation of an annular plastic zone around theparticulates because of high residual stresses aroundthe particulates, which affect the stresses present in thecoatings.[49] It has been reported in the literature thatNi-based coatings reinforced with Al2O3 nanoparticlesexhibit very high tensile stresses compared with mono-lithic matrix.[50] Recently, this kind of result that theaddition of an optimum amount of nanoceramic parti-cles inside the Sn matrix prevents the build up of

Fig. 14—The wear track depth of the selected samples as a function of load.


compressive stresses in the coating has been verified byBhattacharya and his coworkers.[51] The residual com-pressive stress is usually considered as a preliminarycondition for tin whisker growth.[52–54] Hence, it is notdesirable for microelectronic packaging applications.There is a clear indication that composite samples havea better resistance against the whisker growth. It is alsonoticed that for sample D30, the stress completely lies intensile region, which is obvious because of the porousmicrostructure that act as a channel through which thecompressive stresses are dissipated quickly and tensilestress builds up. The CeO2 particles are also present in

the agglomerated form in D30. It has been demon-strated in the past that highly agglomerated ceramicnanoparticles produce a significant increase in theresidual stress (tensile state) of the coating.[55] A detailedcharacterization is needed to confirm this behavior.Line scan analysis is further performed at the sub-

strate and coating interfaces of the monolithic as well asselected composite samples (Figure 16), which gives aclear understanding of the Cu-Sn diffusion in thesesamples. It is observed that the Cu-Sn interdiffusionregion (shown by the vertical lines in Figure 16) is quitewider in case of monolithic sample compared with thecomposite ones. For example, the interdiffusion distancein D0 is ~20 lm which decreases to 4 lm in D15. Alarge interdiffusion region in case of monolithic samplesindicates a severe diffusion of Cu in Sn, while incomposite samples, the nano-CeO2 particles are hinder-ing the diffusion of Cu, and thus their interdiffusionzone gets narrowed. This observation indicates that thepropensity for the growth of Cu-Sn intermetallic will belesser in the composite samples compared with themonolithic sample. The current investigation shows thatthe inclusion of nanoceramic particles can be a betteroption to mitigate the tin whisker growth in lead-freesolders, thereby improving the coating life.

IV. CONCLUSIONS

1. Near-eutectic monolithic Sn-Ag and nano-CeO2-reinforced Sn-Ag composite solder films have beenproduced using pulse electrodeposition techniquefor the first time. The crystallite size of CeO2, ascalculated from the XRD and TEM, lies in therange from 20 to 30 nm.

2. The incorporation of CeO2 particles in the Sn-Agmatrix increases with the increasing CeO2 concen-Fig. 15—Residual stress measurement of the selected samples.

Fig. 16—Line scans analysis of samples: (a) D0 and (b) D15.


tration in the electrolyte up to 15 g/L, and then de-creases because of the agglomeration of CeO2 parti-cles in the bath. The best morphology of thecomposite is realized with dispersing 15 g/L CeO2

in the electrolyte that gives 11.5 wt pct CeO2 in theSn-Ag matrix.

3. The hardness of the Sn-Ag/CeO2 composite withthe maximum amount of monodispersed CeO2 is138 Hv, indicating a tremendous increase in hard-ness over the hardness of the Sn-Ag alloy (18 Hv)deposited under the same condition. The higherhardness of Sn-Ag/CeO2 composite compared withSn-Ag alloy can be attributed to the grain size anddispersion-strengthening effect, and also to therefinement of Ag3Sn intermetallic compound byCeO2 particles.

4. The optimum melting point is obtained with thecomposite prepared from the electrolyte containing15 g/L CeO2, and it is found to be lower than themonolithic alloy by 2.6 K (2.6 �C), which suggestsits possibility to be used without any change in theexisting soldering procedures.

5. There is a rise in the resistivity of the compositematrix over the Sn-Ag alloy. However, it fallswithin the usable limits, as reported for otherSn-based composites being used for electrical con-tact applications.

6. The addition of CeO2 nanoparticles in the Sn-Agmatrix improves the wear and friction resistancewhich ultimately increases the coating life for appli-cation. The residual stress in the Sn-Ag alloy coat-ing is compressive in nature. However, theincorporation of CeO2 nanoparticles in the Sn-Agmatrix reduces the compressive stress, and it be-comes positive, i.e., tensile in the case of D15 (Sn-Ag/CeO2 composite deposited from the electrolytecontaining 15 g/L CeO2) which in turn lowers thechance of Sn whisker formation. Therefore, it issuggested that the properties of Sn-Ag alloy coatingcan be improved by incorporating an optimumamount of reinforcement in the matrix.

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Fabrication of Sn-Ag/CeO2 Electro-Composite Solder by Pulse Electrodeposition

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