A Top-Down Approach of Making Sn-3.5Ag Nanosolder Alloy by Swirl Method

doi:10.4236/msa.2011.29175

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Materials Sciences and Applicatio ns, 2011, 2, 1298-1301

doi:10.4236/msa.2011.29175 Published Online September 2011 (http://www.SciRP.org/journal/msa)

A Top-Down Approach of Making Sn-3.5Ag

Nanosolder Alloy by Swirl Method

Hsin Jen Pan1, Chao Yang Lin1*, Jung Hua Chou1, Udit Surya Mohanty2*, Yuan Gee Lee3

1Department of Engineering Science, National Cheng Kung University, Tainan, Taiwan (China); 2Department of Materials Science

and Engineering, National Cheng Kung University, Taiwan (China); 3Department of Automation Engineering and Institute of

Mechatronoptic Systems, Chien-Kuo Institute of Technology, Changhua, Taiwan (China).

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

Received January 18th, 2011; revised February 9th, 2011; accepted March 20th, 2011.

ABSTRACT

The nanoparticles of tin-silver sold er, Sn-3.5Ag, of necklace geometry were made in a swirl batch. It was found that the

addition of the element, Ag, did not vary the microstru cture of the solder matrix, but Ag simp ly diluted into th e Sn ma-

trix randomly. The swirl flow facilitated th e formation of particles with different sizes. It was found that the size distri-

bution of the nanoparticles was strongly related to the height in the swirl batch. In addition, the aggregation of the

nanoparticles was explored and the dispersion of the nanoparticles was achieved by adjusting the pH value of the solu-

tion near the neu tral value.

Keywords: Swirl Flow, Particle Size Distribution, Nanoparticle Aggregation

1. Introduction

Due to the advances in semiconductor manufacturing

technology, the feature size of the complementary

metal-oxide semiconductor (CMOS) structure could ap-

proache nano-scale length. Metal tin is always a major

component of the solder matrix which serves as an inter-

connection between the device and substrate. However, it

cannot withstand the mechanical requirements at ele-

vated temperatures. Traditional lead alloys are being

phased out [1]. Many researchers have been looking for

alternatives to replace lead [2-4]. Eutectic tin-silver,

Sn-3.5Ag, is a popular replacement since the soldering

temperature and surface tension near to that of Sn-37Pb

solder [5]. Metal nanoparticles have recently received

great attention. The nano-metal is not only employed in

photonics, but it also has potential applications in opto-

electronics [6]. Moreover, tin particles also follow Bar-

bier-type allyation reactions and also serve as an abrasive

to antiwear under loading, resulting in reduction of the

particle size to the nanometer scale [7]. Based on these

specific advantages, the size of the Sn-3.5Ag alloy is

scaled down to nanometer scale using the swirl method; a

technique to sieve solid p article size in fluidic bed facili-

ties [8].

2. Experimental Procedure

Metals Sn (99.8%), Ag (99.9%) and paraffin oil (99.8%)

were commercial chemicals. 9.65 grams of Sn powder

was blended with 0.35 grams of Ag powder in a sealed

vacuum glass tube. The tube was heated to 260˚C in an

oven for 2 hours with intermittent shaking. This alloy

was then checked for uniformity and then immersed in

the heating solvent swirl batch of paraffin oil, at 320˚C

for 10 hours. Aliquots were removed and quenched into

liquid nitrogen to prevent aggregation. The viscous solu-

tion, composed of nanoparticles and paraffin, was

blended with chloroform to dilute th e paraffin. The solu-

tion was centrifuged and then was diluted in turn three

times. The nanoparticles formed as black sediments at

the bottom of the tube. The nanoparticles were washed

with water and acetone several times until pH = 7 then

vacuum dried at 40˚C for 12hrs. The upper zone, chloro-

form, was dried out and then the black sediment was

sampled to identify its phase with X-ray diffraction (Ri-

kagu). To prevent the nanoparticles from aggr egation, the

rest of the sediment was dissolved in ammonium hy-

droxide (NH4OH) and dispersed using an ultrasonic

shaker. The alkaline, NH4OH solution was dispensed

onto a glass substrate. After drying, the morphology of

the particle was investigated by a scanning electron mi-

A Top-Down Approach of Making Sn-3.5Ag Nanosolder Alloy by Swirl Method1299

croscope (SEM 2500, Hitachi) and the diffraction pattern

was obtained by a high resolution TEM (JEOL 2100).

3. Results and Discussion

3.1. Morphology Investigation and Phase

Identification

Uniform Sn-3.5Ag nanoparticles were prepared and dis-

persed successfully by the swirl method. However, the

nanoparticles did not disperse individually but joined

together with necklace morphology. The nanoparticles

thus formed showed active surfaces [9].

The trace element, Ag, was resolved in the tin matrix

homogeneously. Figure 1 shows the HRTEM diffraction

pattern of Sn-3.5Ag nanoparticles. This pattern was iden-

tified to be a space group of I41/amd with lattice parame-

ters, a, 5.819 Å, and b, 3.175 Å, respectively. This pat-

tern is consistent with the tetragonal system [10], as that

of the synthesized tin. Thus, the addition of the silver did

not vary the microstructure of the tin matrix.

3.2. Kinetics of the Formation of Sn-3.5Ag

Nanoparticles

The size of the nanoparticles was determined by Oswald

ripening and the rotational speed of the swirl. Figure 2

showed the particle size variation for subsequent sam-

pling time with a constant sampling height. It is noted

that the particle size decreased initially, then reached the

minimum value and finally recoiled appreciably. This

indicated a specific formation procedure as proposed in

the following section .

Figure 1. HRTEM diffraction pattern of Sn-3.5Ag nanosol-

der.

Figure 2. Particle size variation with respect to sampling

time.

Initially, the swirl flow quickly reached steady state.

The bulk homogeneous Sn-Ag alloy was heated and

showed a viscous fluid with a coherence force to main-

tain its spherical-like shape. Because the Sn-Ag alloy did

not resolve in the paraffin solvent, the coherence force

prevailed. When the viscous Sn-Ag alloy was deposited

on the floor of the rotational bath, the immiscible phase,

paraffin and also the Sn-3.5Ag were subjected to fluid

shear stresses. Because the shear force was larger than

the coherence force, the bulk viscous fluid in the bottom

layer was dissevered. This dissevered debris climbed

peripherally up and was dissevered again. Meanwhile the

quantum effect emerged due to the small size of the

Sn-Ag alloy while approaching the nano-scale [11]. Fi-

nally, the shear force could not eclipse the incoherent

force and the alloy vo lume was maintained. The shape of

the nanoparticles could not maintain its sph erical features

when the quantum effect balanced the incoherent force

[12]. The swirl flow facilitated a particle size distribution

with respect to different sampling heights of the swirl

batch as shown in Figure 3. The size of the nanoparticles

decreased with increasing sampling height. A force bal-

ance model was constructed to analyze the particles in

the swirl batch. In the swirl flow, the nanoparticles were

balanced by the flow force and rotated with the same

velocity as that of the fluid. The velocity was a function

of both the radical distance and height of the batch. Ex-

cept at the center of the batch, the nanoparticles experi-

enced a net force perpendicular to the surface of the swirl

flow layer.

3.3. Dispersion of the Aggregated Nanoparticles

The aggregated nanoparticles were dispersed freely by

A Top-Down Approach of Making Sn-3.5Ag Nanosolder Alloy by Swirl Method

1300

Figure 3. The average particle size dipped at different

heights.

adjusting the pH value of the solution. Particle size de-

creased with increasing pH values (Figure 4) up to 8.54,

but then increased appreciably up to pH value of 11.2.

However, the nanoparticles aggregated again, as the so-

lution changed from neutral to alkaline. It has been re-

ported that the metallic nanoparticles possess negative

electrical carriers [13]. In acidic solutions the hydrogen

ions, neutralizes the nanoparticle’s surface. The zeta po-

tential of the nanoaparticles thus approaches zero. An

increase in pH changed the charge of the nanoparticles

from positive to negative. The repulsive force also in-

creased to counteract the attractive force. However, the

nanoparticles became unstable as the negative surface

charge became immoderate. The systematic energy thus

had to be reduced due to the flocculation of the particles

[14]. The particle size was no more spherical but

tetragonal. The non-spherical shape of the particle indi-

cated that some force dominated the lattice assembly.

The shape hence showed a distinct morphology; namely

polyhedral rather than the traditiona l spherical shape.

Figure 4. Particle size distribution v.s pH values in the dis-

persive solution.

4. Conclusions

Tin-silver nanoparticles were prepared successfully by a

swirl method with the trace element, Ag, resolved in the

tin matrix homogeneously. Because the particle size

could approach a steady state value, the kinetics was in-

vestigated in this study. Th e size of the nanoparticles was

determined by the rotational velocity of the swirl and

height off the bottom. To avoid the aggregation of the

nanoparticles, the pH value of the solution was adjusted

and the aggregated particles were dispersed successfully.

5. Acknowledgements

We are grateful for the financial support provided by the

Chien-kuo Techno logy University (C TU-95-RP-AE-007-

007-A).

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