[Research Paper] 대한금속･재료학회지 (Korean J. Met. Mater.), Vol. 55, No. 1 (2017), pp.72~76DOI: 10.3365/KJMM.2017.55.1.72

72

Structural Analysis of Three-Component Nanoparticles of Sn-58Bi and Cu Wires Prepared by Pulsed Wire Discharge

Jong Hwan Kim1, Dae Sung Kim1, Hisayuki Suematsu2, Kenta Tanaka2, and Bong Ki Ryu1,*

1Department of Materials Science and Engineering, Pusan National University, Pusan 46241, Republic of Korea2Extreme Energy-Density Research Institute, Nagaoka University of Technology, Niigata 940-2188, Japan

Abstract: We used Sn-58Bi and Cu wires to investigate the effects of variable conditions (such as pressure and wire diameter) on the formation of three-component nanoparticles. In the synthesis of the three-component nanoparticles, pulsed wire discharge was used to sublimate the wires. In this system, the K factor is described as Ec/Es, where Ec and Es are respectively the applied energy and the sublimation energy of the system. Experiments were conducted in a N2 atmosphere using the following parameters: voltage of 6 kV, pressure of 50–100 kPa, and Cu wire diameters of 0.1 and 0.2 mm. X-ray diffraction and field emission scanning electron microscopy were employed for structural analysis, particle size distribution analysis, collection rate, and composition studies of the nanoparticles.

†(Received May 4, 2016; Accepted July 12, 2016)

Keywords: alloy wire, nanopowder, pulsed wire discharge

1. INTRODUCTION

With the continuing trend of miniaturization and weight

reduction of electronic products, modern electronic packaging

technology has received increased research attention [1,2].

Traditionally, Pb solders have been employed in chip

packaging, but due to the global environmental issues

associated with the toxicity of Pb, its use has been

increasingly discouraged. Instead, Pb-free solders such as

Sn-Ag-Cu or Sn-Cu solder systems have been widely studied

as alternative electronics packaging materials [3–9]. The

Sn-3.5Ag-0.5Cu composition, which is often used in current

industrial processes, has a melting point of 217 ℃. In order to

ensure sufficient wettability, reaction characteristics, and

reliable yield, it is necessary to apply a temperature profile of

250 ℃ or more when examining such solders [10,11].

However, at temperatures above 250 ℃, the substrate

materials deteriorate and an excessive intermetallic

compound layer can occur at the solder joint, which adversely

affects mechanical reliability. Therefore, a low melting point

is necessary for the research and development of such solders.

*Corresponding Author: Bong Ki Ryu[Tel: +82-51-510-2384, E-mail: bkryu@pusan.ac.kr]Copyright ⓒ The Korean Institute of Metals and Materials

With Sn-58Bi solders, it is possible to significantly lower

the processing temperature because it has a low melting point

(139 ℃) compared to Sn-Ag-Cu based solders. Furthermore,

recent studies of Sn-58Bi solders have revealed their

relatively high strength values compared to Sn-Ag-Cu and

Sn-Pb solders [12,13]. In previous reports, Sn-58Bi solder

alloys undergoing tensile tests have shown relatively high

intensity values compared to other solder alloys; this indicates

that brittleness is high, and fracturing has been observed

during plastic deformation or elastic deformation [14,15]. The

plastic deformation of Sn-58Bi solder can be improved by

reducing the deformation rate. When the solder joints are only

under the influence of stresses originating from differences in

the thermal expansion coefficients at the solder joint, they

exhibit good reliability, but they exhibit poor reliability when

subjected to sudden physical impacts. In addition, the melting

point of Sn-58Bi is 139 ℃, at which temperature the

reliability of the solder joint is rapidly reduced. Therefore,

changes in the physical properties of Sn-Bi solders are very

important [16-18].

Various methods for improving the physical properties of

solder material have been studied. One such method involves

the synthesis of nanoparticles using pulsed wire discharges

(PWD) [19–21]. When the solder material is nano-sized, the

Jong Hwan Kim, Dae Sung Kim, Hisayuki Suematsu, Kenta Tanaka and Bong Ki Ryu 73

Table 1. Experimental parameters used in this study.Wire Sn-58Bi 0.1 mm Cu 0.2 mm Cu

Diameter, d/mm 0.5 0.1 0.2Length of wire, Ls/mm 32Sublimation energy of the wire, Es/J 89.74 13.93 55.72Capacitance of capacitor, C/μF 30Charging Voltage, Vc/kV 6Charging energy Ec/J 540Relative energy, K (=Ec/Es) - 5.21 3.71Atmosphere gas N2Pressure of atmosphere gas, Pa/kPa 50, 100

Fig. 1. Schematic diagram of the pulsed wire discharge setup.

Fig. 2. XRD patterns of Sn-58Bi nanoparticles with 0.1 mm Cu (left) and 0.2 mm Cu (right). (a) 50 kPa and (b) 100 kPa.

surface energy at the interface increases, allowing the

sintering temperature to be reduced. Furthermore, by using

nanomaterials this method makes it possible to improve the

solder’s physical properties without including a third element

or requiring an increase in the intermetallic compound (IMC)

growth [22,23].

The solubility of Cu is extremely low in Sn-58Bi, but in

the case of eutectic Sn-3.5Ag, the solubility of Cu increases,

and at a high reflow temperature the Sn reacts very quickly

with Cu to form a large amount of IMC. However, the

amount of added Cu determines the type and the growth rate

of the IMC [24]. Adding Cu to Sn-58Bi solders can improve

the mechanical properties of those solder joints. In addition,

the strength of the Sn-58Bi solder joints with added Cu is

comparable to the strength of Sn-Pb joints, owing to the

structural reinforcement provided by the IMCs.

A certain amount of Cu can form Cu6Sn5, but as mentioned

elsewhere, the solubility of Cu is not suitable for this purpose.

Due to the fact that the solder material produced by PWD is

nanoscale in size (which overcomes several material

limitations), it is possible to synthesize a variety of

component systems in this fashion. In this study, we have

attempted to overcome the limitations of Sn-58Bi solder

materials by using nanoparticles and experiments were also

performed to synthesize Cu wires with Sn-58Bi.

2. EXPERIMENTAL PROCEDURES

Figure 1 shows a detailed view of a PWD device, where

reactions were carried out with wires on both ends of the

electrodes under an applied voltage. Experiments were

carried out by installing wires of Sn-58Bi (0.5 mm) and Cu

(0.1 and 0.2 mm; Nilaco, 99.9%), with lengths of 32 mm. The

produced nanoparticles were gathered by a filter installed in

the device. After the wires were set, the internal pressure in

the device was maintained at a vacuum pressure < 50 Pa in

order to prevent oxidation. To synthesize the nanoparticles,

N2 gas was used to fill the chamber, at two different pressures

of 50 and 100 kPa. The capacitance (C) and voltage (V) used

to synthesize the two types of wire were 30 μF and 6 kV

respectively, and the stored energy, Ec was determined using

Equation (1).

(1)

The experimental conditions are shown in Table 1. The

sublimation energy required for 0.5 mm Sn-58Bi and 0.1 mm

and 0.2 mm Cu wires was calculated using the heat capacity

and the melting point of each material, and the value of

was determined accordingly. In order to determine the

74 대한금속･재료학회지 제55권 제1호 (2017년 1월)

Fig. 3. Liquidus projection with isotherms of the Bi-Cu-Sn system (left) and Sn-rich region (right).

Table 2. Physical properties of each wireDensity (g/cm3)

Volume (mm3) Weight (g) Cu amounts in

Sn-58Bi (%)Sn-58Bi 7.53 6.28 0.0473 -

Cu (0.1 mm)8.96

0.251 0.00225 4.5Cu (0.2 mm) 1.005 0.00900 15.9

Fig. 4. Particle size analyses corresponding to (a) Cu 0.1 mm, 50 kPa, (b) Cu 0.1 mm, 100 kPa, (c) Cu 0.2 mm, 50 kPa, and (d) Cu 0.2 mm, 100 kPa.

particle size, field emission scanning electron microscopy

(FESEM; Zeiss SUPRA25) and particle size analysis

(Zetasizer Nano S90, Malvern) were employed. X-ray

diffraction studies (XRD; Rigaku RINT 2500HF) were

carried out to analyze the structures of the nanoparticles.

3. RESULTS AND DISCUSSION

Figure 2 shows the XRD patterns of the synthesized

nanoparticles. In the current experiments, the ratios of Cu in

the Sn-58Bi wires for the 0.1 and 0.2 mm Cu wires were 4.5

and 15.9 wt%, respectively (Table 2). The XRD patterns of

all the specimens showed standard Sn and Bi peaks.

However, while small peaks corresponding to copper-tin

compounds (Cu6.26Sn5; DB: 00-004-0673, Cu5.6Sn; DB:

00-031-0487) were observed in the XRD pattern of the

nanoparticles with 0.2 mm Cu wire, they were not observed

in the case of the 0.1 mm Cu wire.

When a large amount of energy is momentarily applied to a

Cu wire, it reacts favorably with Sn to form a Cu6Sn5 phase.

However, in order to make this process feasible, a certain

amount of Cu is necessary to precipitate Cu6Sn5, as shown in

Fig. 3 [25-27]. However, the synthesized sample composition

in this study was above the solubility line of Cu6Sn5,

indicating that a different behavior is involved when using

PWD. One possible explanation for this phenomenon may be

the high energy momentarily applied to sublimate the solid.

The weight ratio in the case of 0.2 mm Cu wire was 15.9

wt%, but the peaks of the Cu3Sn and Cu phases do not appear

in the XRD data.

Another possibility is the shift of the liquidus curve.

According to Fig. 3, it is possible to synthesis Cu6Sn5 with

only a small amount of Cu. Commonly, when a liquid is

cooled down there are many possible reaction sites, but in

PWD, there are fewer reaction sites, because PWD applies the

electric energy for only a very short time. Therefore, when

there is Cu, there are fewer sites to react with Sn to form

IMC. This confirms that the instantaneous energy provided to

the Cu wire aids in the reaction with Sn-58Bi, resulting in

nanosized IMCs.

The size analysis of the synthesized nanoparticles is shown

in Fig. 4. Smaller particles were formed at 50 kPa, and the

size distribution of the particles became more uniform at 100

kPa. These results are consistent with the results published by

Suematsu [28]. The addition of the Cu wire does not have a

significant effect on the particle size distribution. As shown in

the Sn-Bi-Cu phase diagram in Fig. 3, a weight percentage of

15 or more is required for the formation of copper-tin

compounds when using a 0.2 mm Cu wire. Figures 5 and 6

Jong Hwan Kim, Dae Sung Kim, Hisayuki Suematsu, Kenta Tanaka and Bong Ki Ryu 75

Fig. 5. FESEM images of Sn-58Bi and Cu nanoparticles for (a) Cu 0.1 mm, 50 kPa, (b) Cu 0.1 mm, 100 kPa, (c) Cu 0.2 mm, 50 kPa, and (d) Cu 0.2 mm, 100 kPa.

Fig. 6. Composition analysis of Sn-58Bi and Cu nanoparticles by EDX for (a) Cu 0.1 mm, 50 kPa, (b) Cu 0.1 mm, 100 kPa, (c) Cu 0.2 mm, 50 kPa and (d) Cu 0.2 mm, 100 kPa.

show SEM images and the energy dispersive X-ray analysis

of the nanoparticles, respectively, which further confirm the

size and distribution of these particles and allow us to verify

our initial synthesis plans.

The sublimation energy of each substance is calculated

from the integral value of temperature vs. heat capacity, using

the Shomate Equation [29]

＊×＊＊

(2)

where Cp is the heat capacity, t is the temperature (K)/1000

and A, B, C, D and E are constants. The calculated values of

the sublimation energies of each element are as follows: Sn =

23.64 J/mm3, Bi = 11.11 J/mm3 and Cu = 55.43 J/mm3. The

calculated sublimation energy values were all slightly smaller

than the applied energy, which allowed the Sn-58Bi and Cu

wires to be easily sublimated. Although the expected Cu

contents were 4.5 and 15.9 wt%, the real content of Cu was

2.54–10.64 wt%. This was caused by the differences between

the sublimation energies of each wire (14.29 J/mm3 for

Sn-58Bi wire and 55.43 J/mm3 for the Cu wire). These

sublimation energies also depend on the volume of each wire,

which were 6.28 mm3 for the Sn-58Bi 0.2512 mm3 for the 0.1

mm Cu wire and 1.0048 mm3 for the 0.2 mm Cu wire.

These data demonstrate that when the Cu wire contents are

reduced, most of the energy becomes concentrated on the

Sn-58Bi wire, which has a low sublimation energy.

Regardless of the Cu6Sn5 and Cu3Sn phases present, the Cu

compound was anteriorly sublimated, and Sn reacted with a

small amount of sublimated Cu when using the 0.1 mm Cu

wire. When the 0.2 mm Cu wire was used, the Cu contents

increased sharply, decreasing the energy absorbed by the

Sn-Bi wire, resulting in IMC formation.

4. CONCLUSIONS

In this paper, three-component nanoparticles were

synthesized using Sn-58Bi and Cu wires. We controlled the

Cu wire contents to determine the solubility of Cu with the

Sn-58Bi wire using PWD. Theoretically, in the Sn-Bi-Cu

phase diagram, a small amount of Cu reacts with Sn to form

the IMC. When a lesser amount of Cu wire was sublimated

with the Sn-58Bi wire, most of the energy was absorbed by

the Sn-58Bi wire and not enough energy was available to

sublimate Cu to react with Sn and form the IMC. When the

Cu content exceeds a certain amount, it absorbs a sufficient

amount of energy to react with Sn to form a Cu IMC and Cu

precipitates.

76 대한금속･재료학회지 제55권 제1호 (2017년 1월)

ACKNOWLEDGEMENT

This work was supported by a 2-Year Research Grant of

Pusan National University.

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