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.
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.
4
2

22
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)
0
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.
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