Synthesis of Sn-3.5Ag Alloy Nanosolder by Chemical Reduction Method

doi:10.4236/msa.2011.210199

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Materials Sciences and Applicatio ns, 2011, 2, 1480-1484

doi:10.4236/msa.2011.210199 Published Online October 2011 (http://www.SciRP.org/journal/msa)

Synthesis of Sn-3.5Ag Alloy Nanosolder by

Chemical Reduction Method

Hsin Jen Pan1, Chao Yang Lin1*, Udit Surya Mohanty2*, Jung Hua Chou1

1Department of Engineering Science, National Cheng Kung University, Tainan, Chinese Taipei; 2Department of Materials Science

and Engineering, National Cheng Kung University, Chinese Taipei.

Email: *[email protected], *[email protected]

Received June 6th, 2011; revised August 22nd, 2011; accepted September 20th, 2011.

ABSTRACT

The synthesis of Sn-3.5Ag alloy nanosolder was investigated by chemical reduction method. In this method, chemical

precipitation was achieved by using sodium NaBH4 as a reducing agent and PVP (poly-m-vinyl 2-pyrrolidone) as a

stabilizer. The exp erimental results obtained with differen t amounts of NaBH4 and PVP were compared. X-ra y diffrac-

tion (XRD) patterns revealed that Ag3Sn was formed due to the successful alloying process. Scanning Electron Micros-

copy (SEM) and Transmission Electron Microscopy (TEM) demonstrated a change in the morphology of Sn-3.5Ag alloy

nanosolder with increase in the PVP content in the bath. The size of the nanoparticles ranged from 300 to 700 nm. The

nanosolder/nanoparticles were thus synthesized successfully under controlled and optimized chemical reduction proc-

ess.

Keywords: Alloy, Agglomeration, Nanoparticles, Chemical Reduction, Morpho logy, X-Ray Diffraction

1. Introduction

The electronics industry has grown rapidly since the

1960s and the corresponding process technology for in-

tegrated circuits (IC) has also become more complicated

[1]. One of the most important challenges faced by the

electronics industry is the technology of IC devices

packaging, with regards to the electrical connectivity and

environmental protection. Presently, one of the most im-

portant package materials is the development of solder

alloy. The cost of the solder needs to be low and the per-

formance needs to be higher [2] for better reliability.

SnPb solders have long been used as interconnect mate-

rials in microelectronic packaging. Due to the health

threat of lead to human beings, the use of lead-free in-

terconnect materials is imperative. In the last few years

tin/silver (96.5Sn3.5Ag) has been one of the promising

alternatives for Sn/Pb solders [3]. However, the melting

point (Tm) of 96.5Sn3.5Ag alloy (222˚C) is more than

30˚C) higher than that of eutectic Sn/Pb solders (183˚C).

The high Tm requires a higher reflow temperature in the

electronics manufacturing process. This high process

temperature in electronics assembly has adverse effects

not only on energy consumption, but also on substrate

warpage, thermal stress and popcorn cracking in molded

components, resulting in poor reliability of the devices.

As such, studies on lowering the processing temperature

of the lead-free metals are our main interest. The melting

point can be dramatically decreased when the size of

substances is reduced to nanometer size [4]. At present,

the size dependent melting behavior of metal nanoparti-

cles has been found both theoretically and experimentally

[5-10]. The high ratio of surface area to volume of nano

particles has been known as one of the driving forces for

the size dependent melting point depression. Duh et al.

[11] has demonstrated successfully the synthesis of

Sn-3.5Ag-xCu (x = 0.2, 0.5, 1.0) nanoparticles by che-

mical precipitation with NaBH4 for lead-free solder ap-

plications. They have successfully shown the melting by

differential scanning calorimetry (DSC) of their synthe-

sized SnAgCu alloy nanoparticles. However, no obvious

melting point depression behavior was observed, which

might be due to the surface oxidation or heavy agglom-

eration of their synthesized nanoparticles. The solder

alloy powders have been synthesized by various methods

such as solid state melting process [12], electrolytic

deposition [13] and chemical reduction [14]. The solid

state melting process is a commonly used technique but

the limitation is the minimum powder size. The mini-

mum size obtained is about 100nm even when treated by

Synthesis of Sn-3.5Ag Alloy nanosolder by Chemical Reduction Method1481

advanced grinding techniques [15]. Nevertheless, che-

mical reduction technique can produce high quality mi-

cro and nano scale powders, especially gold and silver

nano particles [16,17]. The advantages of the reduction

process are its lower cost, greater convenience and capa-

bility of producing smaller powders. Chemical methods

have emerged to be indispensable for synthesizing nano-

crystals of various types of materials. These methods are

generally carried out under mild conditions and are rela-

tively straightforward. Nanodimensional materials in the

form of embedded solids, liquids, and foams have also

been prepared by chemical means and such materials

have been in use for some time [18-20]. El Sayed and

co-workers [21] used simultaneous reduction of silver

and gold salt to form Ag/Au alloy nanoparticle with size

of 1 nm and less than 10 nm, respectively. The synthesis

of Sn-Sb microcrystalline powders with a particle size of

<300 nm has also been reported [22] by using NaBH4 as

a reducing agent.

The present paper investigates the synthesis of Sn-3.5

Ag alloy nanosolder/nanoparticles by chemical reduction

techniques in presence of stabilizer PVP.

2. Experimental

The stoichiometric amounts of the salts i.e. SnSO4 and

AgNO3 were procured from (Shimakyu’s Pure Chemi-

cals, Japan) and were dissolved in aqueous solution as

the metal precursors. NaBH4 and poly (m-vinyl 2-pyr-

rolidone) (PVP) were obtained from Pancreac Sintesis

and from Alfa Aesar respectively. Solutions of the metal

precursors were rapidly added to NaBH4, PVP, and

NaOH (from Wako Pure Chemical Industries) solutions

under rapid stirring for 3 - 4 h. The stirring time was

varied from12 h to 48 h to ensure complete reduction.

After stirring the black precipitates obtained were washed

several times with distilled water and filtered until the

filtrate pH was the same as that of distilled water and

filtered. The precipitates or powders were then dried in

an oven maintained at 40˚C for 12 h. The morphologies

of the synthesized nanosolders/ nanoparticles were inves-

tigated by using TEM (Transmission electron micros-

copy, JEOL JEM-2100, Japan) and SEM (scanning elec-

tron microscopy) (JEOL, JSM-7000, Japan) techniques.

For TEM analysis, few drops of SnAg nano-particle was

dispersed in few milliliters of isopropanol in an ultra-

sonic bath and a drop of this dispersion was placed on a

carbon film supported by copper grid. The nanosolders

obtained were characterized with an X-ray diffractometer

(XRD, Rigaku, DMAX-200/PC).

3. Results and Discussion

To understand the effect of reducing agent (NaBH4) and

stabilizer (PVP) on the chemical reduction process, the

amount of NaBH4 and PVP was varied. The experimental

results obtained by using different amounts of reducing

agent and stabilizer for the synthesis of nano scale

Sn-3.5Ag alloy solder revealed that increased addition of

NaBH4 to the bath (0.01 - 1 g) transformed the color of

the precipitate from grey to black. It might be attributed

to the fact that that the reduction reaction was incomplete

because of the insufficient amount of reducing agent. For

the reduction reaction to be completed, it is essential to

keep the reducing agent amount of at least 0.1 g in this

system.

Because chemical reduction technique is employed in

this work, hence any chemical reaction resulting in a

solvent would consist of three steps: seeding, particle

growth, and growth termination by capping. An impor-

tant process that occurs during the growth of a colloid is

Ostwald ripening. Ostwald ripening is a growth mecha-

nism whereby smaller particles dissolve releasing mono-

mers or ions for consumption by larger particles, the

driving force being the lower solubility of larger particles.

The seeding, nucleation, and termination steps are often

not separable and one, therefore, starts with a mixture of

the nanocrystal constituents, capping agents, and the

solvent. The relative rates of the steps can be altered by

changing parameters such as concentration and tempera-

ture. One of the important factors that determine the

quality of a synthetic procedure is the monodispersity of

the nanocrystals obtained. It is desirable to have nano-

particles of nearly the same size, in order to be able to

relate the size and the property under study. Hence, the

narrower the size distribution, the more attractive is the

synthetic procedure. Steric stabilization of hydrosols can

be brought about by the use of polymers as stabilizing

agents. Natural polymers such as starch and cellulose,

synthetic polymers, such as polyvinyl pyrrolidone (PVP),

polyvinyl alcohol, and polymethyl vinylether are used as

stabilizing agents.

As nano-particles tend to agglomerate together in the

SEM images, thus PVP plays an important role in the

successful synthesis of the Sn-3.5Ag alloy nanoparticles.

SEM image (Figure 1) for Sn-3.5Ag solder alloy ob-

tained without PVP demonstrated that most of the Sn-Ag

nanoparticles aggregated into a cluster. Increased addi-

tion of PVP (0.2 g) resulted in stronger aggregation of

primary particles. Nevertheless, further increase in the

PVP content to 2 g significantly decreased the agglome-

ration of nanoparticles and greater dispersion of nano-

particles was noticed (Figure 2). The dispersion of the

nanoparticles in the presence of PVP could be attributed

to the fact that the protective polymer PVP adsorbed on

the nanoparticles and exhibited protective function by

steric stabilization [23]. The other part of the protective

polymer dissolved in the free-state in the suspension of

Synthesis of Sn-3.5Ag Alloy nanosolder by Chemical Reduction Method

1482

Figure 1. SEM micrograph of Sn-3.5Ag nanoparticles syn-

thesized without PVP.

Figure 2. High Magnification SE micrograph of Sn-3.5Ag

alloy nanoparticles synthesized wi th 2 g of PVP.

the alloy nanoparticles as a free polymer. Thus it can be

established from the present study that PVP molecules

adsorbed on the surface of Sn-Ag nanoparticles and pro-

tected against coagulation and precipitation of alloy

nanoparticles. It has also been reported [24] that PVP has

been used to improve the dispersability of ITO nanopar-

ticles in an aqueous solution. The morphology of Sn-

3.5Ag solder alloy with 2 g of PVP in the solution was

investigated by TEM. The micrograph (Figure 3) re-

vealed a large number of primary particles aggregated

strongly to form secondary nanoparticles of larger size.

However, discrepancies were seen between SEM and

TEM micrographs. The discrepancies might be attributed

to the nucleation and particle growth in the related proc-

ess. As reported by several authors [25,26] the formation

of metal atoms after mixing two solutions under rapid

Figure 3. TEM image of Sn-3.5Ag alloy nanoparticles with 2

g of PVP.

stirring resulted from the transfer of electrons from the

reducing agent NaBH4 to the metallic ions. The reduction

by borohydride has been in existence for a number of

years [27,28]. The basic reaction involves the hydrolysis

of the borohydride accompanied by the evolution of hy-

drogen.



BH2H OBO4H (1)

Homiyama and coworkers [29,30] made Cu sols by the

borohydride reduction of Cu (II) salts. Green and O’Brien

[31] prepared Cr and Ni nanoparticles by carrying out the

reduction with Li or Na borohydride at high temperatures

in coordinating solvents. In the present study a stoichio-

metric reaction occurs between sodium borohydride and

metal precursors was presented in the following reactions

below:

BH4− +8OH− +4Sn2+→ B(OH)4− + 4H2O + 4Sn (2)

BH4− +8OH− +8Ag+→ B(OH)4− +4H2O + 8Ag (3)

The transfer of electrons from the reducing agent to

the metal precursors was dependent on the standard re-

dox potential [32] of the two species investigated in our

study of the saturation concentration and formed larger

nanosize primary particles [33]. As more metal atoms

were generated in the system, the primary particles con-

tinued to grow by diffusion to form larger crystalline

particles or aggregated to form polycrystalline particles.

The XRD pattern for Sn-3.5Ag alloy (Figure 4) ex-

hibited prominent peaks at scattering angles of 30.9, 32.2,

44.1, 45.25 and 55.2 which correspond to scattering from

(200) (101) (220) (211) and (301) crystal planes respec-

tively of body centered tetragonal phase of Sn [34]. On

the other hand, due to successful alloying of Sn and Ag,

Synthesis of Sn-3.5Ag Alloy nanosolder by Chemical Reduction Method1483

XRD (Sn-3.5Ag)

200

400

600

800

1000

30 32 34 36 38 40 42 44 46 48 50

2θ

Intensity

Sn(101

)

Ag3Sn (201)

Ag3Sn (020)

Ag3Sn (211)

Sn (220

)

Sn (211

)

Sn (200

)

Figure 4. XRD analysis results for Sn-3.5Ag alloy nanopar-

ticles with 2 g PVP.

Ag3Sn peaks were observed in the XRD spectrum. Few

unwanted reflections were also observed, indicating

some impurities, produced during the reduction reaction.

4. Conclusions

With well-controlled process parameters, such as the

amount of reducing agent and stabilizer, the Sn-3.5Ag

alloy nanoparticle could be synthesized successfully by

chemical reduction technique. The advantages of chemi-

cal reduction reaction are that not only two-component

alloy compounds, but also multi-component alloy nano-

particles can be developed by this process.

5. Acknowledgements

We are grateful to the Centre of Micro Nanoscience and

Technology for providing us the instrumental facilities.

REFERENCES

[1] S. M. Sze, “VLSI Technology,” McGraw-Hill, Boston,

1988.

[2] K. M. Monahan, “Enabling Double Patterning at the 32

nm Node,” IEEE International Symposium on Semicon-

ductor Manufacturing, Tokyo, 25-27 September 2006, pp.

126-129. doi:10.1109/ISSM.2006.4493040

[3] I. Artaki, A. M. Jackson and P. T. Vianco, “Evaluation of

Lead-Free Solder Joints in Electronic Assemblies,” Jour-

nal of Electronic Materials, Vol. 23, No. 8, 1994, pp.

757-764. doi:10.1007/BF02651370

[4] P. Pawlow, “Über Die Abhängigkeit des Schmelzpunktes

von der Berflächenenergie Eines Festen Körpers,” Zeits-

chrift für Physikalische Chemie, Vol. 65, 1909, pp. 545-

548.

[5] S. L. Lai, J. Y. Guo, V. Petrova, G. Ramanath and L. H.

Allen. “Size-Dependent Melting Properties of Small Tin

Particles: Nanocalorimetric Measurements,” Physical Re-

view Letters, Vol. 77, No. 1, 1996, pp. 99-102.

doi:10.1103/PhysRevLett.77.99

[6] T. Bachels, H. J. Guntherodt and R. Schafer, “Melting of

Isolated Tin Nanoparticles,” Physical Review Letters, Vol.

85, No. 6, 2000, pp. 1250-1253.

doi:10.1103/PhysRevLett.85.1250

[7] R. Kofman, P. Cheyssac and F. Celestini, “Comment on

Melting of Isolated Tin Nanoparticles,” Physical Review

Letters, Vol. 86, No. 7, 2001, p. 1388.

doi:10.1103/PhysRevLett.86.1388

[8] M. Schmidt, R. Kusche, B. Issendroff and H. Haberland,

“Irregular Variations in the Melting Point of Size-Selected

Atomic Clusters,” Nature, Vol. 393, No. 6682, 1998, pp.

238-240. doi:10.1038/30415

[9] S. J. Zhao, S. Q.Wang, D. Y. Cheng and H. Q. Ye, “Three

Distinctive Melting Mechanisms in Isolated Nanoparti-

cles,” Journal of Physical Chemistry B, Vol. 105, No. 51,

2001, pp. 12857-12860. doi:10.1021/jp012638i

[10] F. Baletto, A. Rapallo, G. Rossi and R. Ferrando, “Dy-

namical Effects in the formation of Magic Cluster Struc-

tures,” Physical Review B, Vol. 69, No. 23, 2004, pp.

235421-235426. doi:10.1103/PhysRevB.69.235421

[11] L. Y. Hsiao and J. G. Duh, “Synthesis and Characterisa-

tion of Lead-Free solders by Chemical Reduction Method,”

Journal of Electrochemical Society, Vol. 152, No. 9, 2005,

pp. J105-J109. doi:10.1149/1.1954928

[12] H. B. Bakoglu, “Circuits, Interconnections and Packaging

for VLSI, Chapter 2,” Addison Wesley, Boston, 1990.

[13] Y. Gaoa, C. Zou, B. Yang, Q. Zhai, J. Liu, E. Zhuravlevd

and C. Schick, “Nanoparticles of SnAgCu Lead-Free

Solder Alloy with an Equivalent Melting Temperature of

SnPb Solder Alloy,” Journal of Alloy and Compounds,

Vol. 484, No. 1-2, 2009, pp. 777-781.

doi:10.1016/j.jallcom.2009.05.042

[14] J. Stevanovic, V. Cosovic, J. S. Trosic, B. Jordovic and O.

Pesic, Arch Materials Sci, Vol. 28, No. 1, 2007, p. 155.

[15] K. S. Chou and C Y. Ren, “Synthesis of Nanosized Silver

Particles by Chemical Reduction Method,” Materials

Chemistry and Physics, Vol. 64, No. 3, 2000, pp. 241-246.

doi:10.1016/S0254-0584(00)00223-6

[16] Y. W. Yen and S. W. Chen, “Phase Equilibria of the Ag-

Sn-Cu Ternary System,” Journal of Materials Research, Vol.

19, No. 8, 2004, pp. 2298-2305.

doi:10.1557/JMR.2004.0296

[17] T. C. Huang, M. C. Wei and H. I. Chen, “Preparation of

Hydrogen-Permselective Palladium-Silver Alloy Com-

posite Membranes by Electroless Co-Deposition,” Sepa-

ration and Purification Technology, Vol. 32, No. 1-3,

2003, pp. 239-245. doi:10.1016/S1383-5866(03)00063-7

[18] M. C. Daniel and D. Astruc, “Gold Nanoparticles: As-

sembly, Supramolecular Chemistry, Quantum-Size-Re-

lated Properties, and Applications toward Biology, Ca-

talysis, and Nanotechnology,” Chemical Reviews, Vol.

104, No. 1, 2004. pp. 293-346.

doi:10.1021/cr030698+

[19] C. Burda, X. Chen, R. Narayanan and M. A. El-Sayed,

“The Chemistry and Properties of Nanocrystals of Dif-

ferent Shapes,” Chemical Reviews, Vol. 105, No. 4, 2005,

pp. 1025-1102. doi:10.1021/cr030063a

[20] B. L. Cushing, V. L. Kolesnichenko and C. J. O’Connor,

Synthesis of Sn-3.5Ag Alloy nanosolder by Chemical Reduction Method

1484

“Recent Advances in the Liquid-Phase Syntheses of In-

organic Nanparticles,” Chemi cal Reviews, Vol. 104, No. 2,

2004, pp. 3893-3946. doi:10.1021/cr030027b

[21] S. Link, Z. L. Wang and M. A. El Sayed, “Alloy Forma-

tion of Gold-Silver Nanoparticles and the Dependence of

the Plasmon Absorption on Their Composition,” Journal

of Physical Chemistry B, Vol. 103, No. 18, 1999, pp.

3529-3533. doi:10.1021/jp990387w

[22] M. A. Yang, M. A. Winter and J. O. Besenhard, “Small

Particle Size Multiphase Li-Alloy Anodes for Lithium-

Ion-Batteries,” Soli d State Ioni cs, Vol. 90, No. 1-4, 1996,

pp. 281-287. doi:10.1016/S0167-2738(96)00389-X

[23] B. Yin, H. Ma, S. Wang and S. Chen, “Electrochemical

Synthesis of Silver Nanoparticles under Protection of

Poly(N-Vinylpyrrolidone),” Journal of Physical Chemis-

try B, Vol. 107, No. 34, 2003, pp. 8898-8904.

doi:10.1021/jp0349031

[24] J. E. Song, D. K. Lee, Y. H. Kim and Y. S. Kang, “Pre-

paration of Water Dispersed Indium Tin Oxide Sol So-

lution,” Molecular Crystals and Liquid Crystals, Vol. 444,

No. 1. 2006, pp. 247-255.

doi:10.1080/15421400500365078

[25] A. Corrias, G. Ennas, G. Licheri, G. Marongiu and G.

Paschina, “Amorphous Metallic Alloy Powders Prepared

by Chemical Reduction of Metal Ions with Potassium

Borohydride in Aqueous Solution,” Chemistry of Materi-

als, Vol. 2, No. 4, 1990, pp. 363-366.

doi:10.1021/cm00010a010

[26] D. Zeng and M. J. Hampden-Smith, “Synthesis and

Characterization of Nanophase Group 6 Metal (M) and

Metal Carbide (M2C) Powders by chemical Reduction

Methods,” Chemistry of Materials, Vol. 5, No. 5, 1993,

pp. 681-689. doi:10.1021/cm00029a018

[27] H. I. Schlesinger, H. C. Brown, A. E. Finholt, J. R. Gil-

breath, H. R. Kockstra and E. K. Hyde, “Sodium Boro-

hydride, Its Hydrolysis and Its Use as a Reducing Agent

and in the Generation of Hydrogen,” Journal of American

Chemical Society, Vol. 75, No. 1, 1953, pp. 215-219.

doi:10.1021/ja01097a057

[28] H. C. Brown and C. A. Brown, “New Highly Active Metal

Catalysts for the Hydrolysis of Borohydride,” Journal of

American Chemical Society, Vol. 84, No. 8, 1962, pp. 1493-

1494. doi:10.1021/ja00867a034

[29] H. Hirai, H. Wakabayashi and M. Komiyama, “Poly-

mer-Protected Copper Colloids as Catalysts for Selective

Hydration of Acrylonitrile,” Chemistry Letters, Vol. 12,

No. 7, 1983, pp. 1047-1050. doi:10.1246/cl.1983.1047

[30] H. Hirai, H. Wakabayashi and M. Komiyama, “Prepara-

tion of Polymer Protected Colloidal Dispersion of Cop-

per,” Bulletin of the Chemical Society of Japan, Vol. 59,

No. 2, 1986, pp. 367-372. doi:10.1246/bcsj.59.367

[31] M. Green and P. O. Brien, “The Preparation of Organi-

cally Functionalised Chromium and Nickel Nano Par-

ticles,” Chemical Communications, Vol. 29, No. 19, 2001,

pp. 1912-1913. doi:10.1039/b107108b

[32] M. Pourbaix, “Atlas of Electrochemical Equilibria in

Aqueous Solutions,” Oxford University Press, New York,

1974.

[33] Y. Zhao, Z. Zhang and H. Dang, “Preparation of Tin

Nanoparticles by Solution Dispersion,” Materials Science

and Engineering A, Vol. 359, No. 1-2, 2003, pp. 405-407.

doi:10.1016/S0921-5093(03)00395-2

[34] D. V. Goia, “Preparation and Formation Mechanisms of

Uniform Metallic Particles in Homogeneous Solutions,”

Journal of Materials Chemistry, Vol. 14, No. 4, 2004, pp.

451-458.