AuSn20 Eutectic Electrodeposition through Alternative Complexingof Pyrophosphoric Acid: Insights from Electrochemical and DFTMethodsGuofeng Cui,*,† Shaofang Liu,† Jie Zhao,‡ Edward F. Holby,§ Qing Li,§ and Gang Wu§

†Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province, School of Chemistry and ChemicalEngineering, Sun-Yat Sen University, Guangzhou, 510275, China‡School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China§Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States

ABSTRACT: Eutectic AuSn20 solder is an important material for electronicpackaging technology due to its superior mechanical and thermal conductiveproperties. In this work, AuSn20 alloy films are prepared via the electrodepositionmethod for the first time. The electrodeposition is cost-effective with improved controlover the alloy content when compared to traditional powdered metallurgy methods.Pyrophosphoric acid was found to be an effective complexing agent to minimize thedifference of the deposition potentials between Au and Sn, making the codeposition ofAuSn alloys possible. Importantly, electrochemical characterization was combined withdensity functional theory (DFT) calculations to provide insight into the mechanism ofthe alloy codeposition when pyrophosphoric acid was used as the complexing agent. Inparticular, natural bond orbital (NBO) charge distribution and the lowest unoccupiedmolecular orbital (LUMO) characteristics of [P2O7]

4−-Sn(II) and [P2O7]4−-Au(I)

complexes are calculated, suggesting that [P2O7]4− is able to coordinate more strongly

with Sn(II) than Au(I). As a result, it can thus shift the deposition potentials of Au(I)and Sn(II) much closer. As the DFT predicted, the role of pyrophosphoric acid as a complexing agent has been experimentallyverified, making codeposition of Au and Sn realistic. The structures of the obtained AuSn20 films are determined using scanningelectron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX) and found to be consistent with AuSn/Au5Sneutectic as predicted by the Au−Sn phase diagram. Additionally, the measured melting point is in good agreement with thetheoretically determined one. Relevant tests demonstrated in this work indicate that the newly developed electrodepositedAuSn20 alloy coatings are suitable for microelectronic soldering applications.

■ INTRODUCTION

AuSn20 eutectic solder is extensively employed in theelectronic packaging of optoelectronic and high-poweredelectronic components1−3 due to its superior mechanical andthermal conduction properties.4 In addition, it plays a key rolein some special technologies, including gas leak sealingtreatments, high-powered laser diode packaging,5 and non-soldering flux packaging.6

The AuSn20 eutectic consists of AuSn and Au5Sn phases7

with a Sn content of 20 wt %.8 AuSn20 solder is commonlyprepared by powder compacting9 or sputtering10 methods dueto the incompatibility of Au and Sn. However, the AuSn20solder fabricated using these techniques cannot meet thedemand for microscaled solder manufacturing due tolimitations of thickness and pattern formation. Thus, selectiveelectroplating of AuSn20 solder would be an attractive wayovercoming such limitations. Such an electrodepositiontechnique, however, has yet to be fully developed forAuSn2011,12 due to the large difference in electrodepositionpotentials between Au and Sn. In principle, codeposition ofmetals with different deposition potentials requires the use of

complexing agents. Thus, proper choice of the effectivecomplex agent becomes a key to realizing the Au and Sncodeposition via an electrochemical method. In this work, wefound that the Sn deposition potential is drastically reducedafter adding pyrophosphoric acid into Sn(II) solution. On theother hand, pyrophosphoric acid has a less significant effect onthe deposition potential of Au(I). This suggests thatpyrophosphoric acid should enable codepositing these twometal ions simultaneously. As a result, AuSn20 eutectic filmscan be successfully electrodeposited from an aqueous solutionin the presence of the pyrophosphoric acid, and the eutecticcharacteristics of the electrodeposited AuSn20 films areconfirmed by scanning electron microscopy (SEM), energydispersive X-ray spectrometry (EDX), and melting pointmeasurements. Importantly, the complexing behavior ofpyrophosphoric acid with Sn(II)/Au(I) is analyzed usingelectrochemical characterization and density functional theory(DFT). This theoretical study clearly elucidates the mechanism

Received: April 21, 2013Published: September 17, 2013

Article

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© 2013 American Chemical Society 21228 dx.doi.org/10.1021/jp408721e | J. Phys. Chem. C 2013, 117, 21228−21233

that the deposition potential of Sn(II) is more strongly affectedby pyrophosphoric acid than Au(I), thereby resulting in acodeposition of Sn(II) and Au(I).

■ EXPERIMENTAL SECTION

Preparation of AuSn20 Films by Electrodeposition.Al2O3 ceramic is used as a substrate for the electrodepositionprocess. These substrates are metalized with a tungsten andtitanium alloy using magnetron sputtering. An Au film is thenelectroplated on the surface followed by ethanol and acetonecleaning. Finally, the AuSn20 alloy is electrodeposited on thepretreated substrates using an electrolyte containing 5 g/L[KAu(CN)2], 7 g/L SnSO4, and 60 g/L potassiumpyrophosphate (K4P2O7). All chemical agents used areanalytically pure. All solutions are prepared with deionizedwater with an electric conductivity over 18.0 MΩ. Thetemperature of the electrolyte is controlled at 50 ± 1°C. Allelectrolytes are purged with nitrogen before using. The pHvalue of electrolyte is adjusted to 6.0 using diluted H2SO4solution. The current density used for the AuSn20 electro-deposition is 0.6 A/dm2.Electrochemical Measurements. Cathodic polarization

measurements are performed on an electrochemical work-station (model ref. 600, Gamry Inc., Warminster, PA) in aconventional three-compartment Pyrex cell. A defined samplearea of 1.0 cm2 is exposed to the electrolyte. A Luggin capillaryis placed near the working electrode to minimize the ohmicdrop. A saturated mercurous sulfate electrode (MSE) and aplatinum sheet are used as the reference and the counterelectrodes, respectively. All electrode potentials are with respectto the MSE in this work unless stated otherwise. In order tostudy the complexing characteristics of pyrophosphoric acid,solutions containing 5 g/L [KAu(CN)2] and 7 g/L SnSO4 withor without adding 60 g/L K4P2O7 are prepared for acomparison to study the role of the complexing agent.Characterization of AuSn20 Alloy Films. Field-emission

scanning electron microscope (FE-SEM) images are obtainedusing a JSM-6330F scanning electron microscope (JEOL Ltd.,Tokyo, Japan). The elemental composition of different areas inthe eutectic is determined by an energy-dispersive X-ray

spectrometer (EDX) on a Link-ISIS-300 (Oxford Corp.,Beverly, MA). The melting point of AuSn20 eutectic samplesis measured by differential scanning calorimetry (DSC) on aNetzsch STA 449C instrument under flowing N2. The rate ofincreasing temperature is 10 °C/min.

DFT Calculations. The computations in this work arecarried out using the Gaussian 03 code,13 employing the hybridBecke exchange and Lee, Yang, and Parr correlation(B3LYP)14−16 exchange and correlation functional. Forelements H, O, C, and P, 6-311G(d,p) basis sets are used inthis work. The LANL2DZ basis sets were adopted by Hay andWadt for modeling Sn atoms17−19 and by Homma et al.20−24

and Bozzini et al.25 for studying electroless and electro-deposition processes. Thus, we utilize these basis sets for Snand Au. The molecular geometry and natural bond orbital(NBO) of all species were calculated in order to study thecomplexing behavior of K4P2O7 with Sn(II) and Au(I).

■ RESULTS AND DISCUSSIONAccording to the Au−Sn thermodynamic phase diagram,7 theAuSn20 eutectic occurs at 20.0 wt % or 29.0 at. % Sn26 andconsists of ζ phase (Au5Sn) and δ phase (AuSn). The latticestructures of ζ phase (Au5Sn) and δ phase (AuSn) are HCPand NiAs structure, respectively, as shown in Figure 1.Although the melting points of Au5Sn and AuSn are 190.0and 419.3 °C, respectively, the melting point of AuSn20 is278.0 °C, as indicated by the blue area in Figure 1.The standard deposition potentials of Au(I) to Au(0) and

Sn(II) to Sn(0) are 1.692 V and −0.136 V vs normal hydrogenelectrode (NHE), respectively, as shown in eqs 1 and 2. E0 isthe standard potential.

+ → =EAu(I) e Au ( 1.692 V)0(1)

+ → = −ESn(II) 2e Sn ( 0.136 V)0(2)

To realize electrochemical codepoistion of Au and Sn, thedepositon potentials of both metals should be relatively close,which is not the case for Au and Sn at their standard conditions.Considering a Au(I) precursor of [KAu(CN)2] in solution at aconcentration of 5 g/L, the real deposition potential can be

Figure 1. Au and Sn alloy phase diagram7 and lattice structure of Au5Sn and AuSn. Au and Sn atoms are represented by yellow and gray spheres,respectively.

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calculated based on the Nernst equation (eq 3) using a stabilityconstant of complex [Au(CN)2)]

− of 2 × 1038.27,28

= + × = −E ERTnF

C Vln 0.7640(3)

Likewise, the reduction potential of Sn is calculated to be−0.184 V vs NHE for a SnSO4 solution at a concentration of 7g/L.

= + × = −E ERTnF

C Vln 0.1840(4)

The potential differences of MSE and NHE are 0.616 V.According to Nernst calculation results in eqs 3 and 4, thedeposition potentials of Au and Sn are −1.380 and −0.800 V vsMSE, respectively. During the metal electrodeposition process,a higher cathodic overpotential is beneficial for forming finegrain particles. Therefore, if these solutions and concentrationsare to be used for electrochemical codeposition, it is necessaryto shift the deposition potential of Sn to a negative directionand lower than that of Au by using a proper complexing agent.

Figure 2. Cyclic voltammograms of electrolytes with differentconcentrations of Sn(II) (a) and Au(I) (b).

Figure 3. Cyclic voltammetry curve of electrolyte containing 7g/LSn(II) in different concentrations of pyrophosphate.

Figure 4. Polarization curves of 7 g/L Sn(II) with 60 g/L K2P2O7(black line), 5 g/L Au(I) with 60 g/L K2P2O7 (red line), and theirmixed solution (blue line).

Figure 5. NBO charge distribution of [P2O7]4− with Sn(II) (a) and

Au(I) (b).

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In fact, the shifting potential of Sn is more desirable, comparedto shifting the potential of Au to a positive direction, becausethe former can result in a larger overpotential and lead to fine-grained particles.In the absence of a complexing agent, as shown in Figure 2a,

the measured reduction potential of Sn is −0.645 Vindependent of the concentration of Sn(II) ranging from 1 to5 g/L, while the corresponding reduction peak of currentdensity increases from −2.620 to −6.270 mA/dm2. It is worthnoting that the measured reduction potential of Sn is slightlyhigher than the value predicted by the Nernst equation,probably due to the nonideal behavior in the real electrolyte. Inthe case of Au(I), the measured reduction potential of Au isabout −1.121 V (Figure 2b), which is also slightly higher thanthe theoretical value (−1.380 V). Compared to Au(I), thereduction potential of Sn(II) is found to be 0.647 V higher inthe solutions before adding complex agent.In Figure 3, the reduction potential of Sn shifts negatively

from −0.645 V (Figure 2) to −0.850 V when the addition ofpyrophosphate acid is up to 60 g/L. No significant shift ofpotential can be observed when further increasing theconcentrations of pyrophosphate. This is probably due tothat the Sn(II) ions are fully complexed by [P2O7]

4− at thisconcentration. Thus, it was found that pyrophosphate candrastically decrease the deposition potential of Sn(II) and makeit possible to electrochemical codeposition of Sn(II) and Au(I)simultaneously.The polarization curves shown in Figure 4 demonstrate that

the as-prepared solution electrodeposition potentials of Sn(II)and Au(I) with 60 g/L K2P2O7 are −0.896 and −1.066 V,respectively. After they were mixed together, the codepositionpotential moves to −1.045 V. This indicates that K2P2O7 is

more easily to complex with Sn(II), compared to Au(I),because the potential shifts more negatively for Sn(II).Therefore, the addition of pyrophosphate is critical to realizethe negative shifts of Sn(II) reduction potentials.In order to explain the difference in shifts of deposition

potential between Sn(II) and Au(I) resulting from pyrophsos-phate, DFT calculation was used to provide more insight into

Figure 6. LUMO molecular orbital of [P2O7]4− with Sn(II) (a) and

Au(I) (b).

Figure 7. SEM images of AuSn20 eutectic film (a, b), the ζ phase(Au5Sn) and δ phase (AuSn) are analyzed by EDX (c).

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the codeposition mechanism. Figure 5a shows the naturalcharge distribution and the structure for [P2O7]

4− coordinatedwith Sn(II). This Sn2P2O7 structure has been previouslyreported by Buchner.29 According to our interpretation ofNBO charge distribution calculations, end position O atoms aremore negatively charged with charge values ranging from−1.212 to −1.244. The charge of Sn(II) atoms are 1.529. Thecharge differences between O and Sn(II) are 2.741 and 2.773.In the case of Au(I) complexed with [P2O7]

4−, the charge ofthe end position O atoms is −1.141 and −1.138. The charge ofAu(I) is 0.537 as shown in Figure 5b. Thus, the differences ofcharge between O and Au(I) in this complex are 1.678 and1.675, which are significantly less than those calculated from theO/Sn(II) complexed system. Apparently, the electrostaticinteraction between oxygen atoms and Sn(II) is much stronger,compared to that between oxygen atoms and Au(I) in thecomplexing environment.Figure 6 shows the calculated lowest unoccupied molecular

orbital (LUMO) structure for the [P2O7]4− complexing with

Sn(II) or Au(I) ions. During the electrodeposition, the metalliccomplex must enter the electrical double layer and obtainelectrons from the cathode. Consequently, the obtainedelectrons will enter the LUMO, and thus the metallic cationswill be reduced to metallic atoms. As for the LUMO of thecalculated [P2O7]

4−/Sn(II) complex, there exists a significantoverlap between the orbitals of Sn(II) and end position oxygenatoms (in the area marked by the black dotted line in Figure5a). Such overlap leads to a deformation of the Sn(II) orbital.

However, there is no evident orbital overlap observed in theLUMO calculations for the [P2O7]

4−/Au(I) complex (Figure6b). As a result, the DFT computation indicates that interactionbetween [P2O7]

4− and Sn(II) is much stronger than thatbetween [P2O7]

4− and Au(I). These findings are consistentwith the experimentally observed shifts in deposition potential.In order to study the morphology of the prepared AuSn20

films, the electrodeposited Au−Sn alloys are analyzed by SEMas displayed in Figure 7a and b. Reflective gray and dark grayareas are observed on the alloy surface, indicative of two phasesbeing present in the sample. To identify the composition, bothphases are analyzed using EDX (Figure 7c). According to theelemental analysis, the dark gray and reflective gray areas are δphase (AuSn) and ζ phase (Au5Sn), respectively.Finally, AuSn20 solder bump arrays were deposited on Al2O3

ceramic via the developed electrdeposition methods, asillustrated in Figure 8a. Importantly, the edges of the solderbumps are very even, suggesting a superior platform forfabricating standard solder bumps, as evidenced by Figure 8band c. In addition, differential scanning calorimetry (DSC)measurements show that there is a single absorption peak at280.9 °C (Figure 8d). This indicates a melting point of Au−Snalloy solder bumps prepared through electrodeposition at 280.9°C, in good agreement with that of the AuSn20 eutectic.

■ CONCLUSION

Eutectic AuSn20 films were electrochemically deposited onAl2O3 ceramic surfaces by using a pyrophosphoric acid

Figure 8. Image of electroplated AuSn20 solder bump arrays on Al2O3 ceramic (a and b), SEM image of the border of solder bumps (c), and DSCanalysis of solder bumps (d).

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complexing agent. The electrodeposition potential of Sn(II)shifts negatively and approaches that of Au(I) in the presenceof pyrophosphoric acid. To provide some insights into themechanisms, the coordination behavior between [P2O7]

4− andcations Sn(II) or Au(I) is investigated by NBO chargedistribution and LUMO characteristics. These results indicatethat [P2O7]

4− strongly complexes with Sn(II), relative to Au(I),thereby altering the deposition potential. Using this technique,AuSn20 eutectic solder bump arrays were, for the first time,successfully fabricated via an electrochemical codepositionmethod.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: cuigf@mail.sysu.edu.cn.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSG.F.C. gratefully acknowledges the financial support byNational Natural Science Foundation of China (51271205,50801070), “The Fundamental Research Funds for the CentralUniversities” (11lgpy08), “Guangzhou Pearl Technology theNova Special Project” (2012J2200058), “Research andApplication of Key Technologies Oriented the IndustrialDevelopment” (90035-3283309), “Plan of Science andTechnology Project” by the DaYa Gulf district in Huizhoucity (31000-4207387), and the Innovative Laboratory Fund bySun Yat-Sen University.

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