effect of zno nanoparticles addition.pdf

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Journal of Materials Science:

Materials in Electronics

ISSN 0957-4522

Volume 24

Number 9

J Mater Sci: Mater Electron (2013)

24:3210-3218

DOI 10.1007/s10854-013-1230-2

Effect of ZnO nanoparticles additionon thermal, microstructure and tensile

roperties of Sn3.5 Ag0.5 Cu (SAC355)solder alloy

A. Fawzy, S. A. Fayek, M. Sobhy,

E. Nassr, M. M. Mousa & G. Saad


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Effect of ZnO nanoparticles addition on thermal, microstructureand tensile properties of Sn3.5 Ag0.5 Cu (SAC355) solder alloy

A. Fawzy S. A. Fayek M. Sobhy

E. Nassr M. M. Mousa G. Saad

Received: 1 February 2013 / Accepted: 5 April 2013 / Published online: 18 April 2013

Springer Science+Business Media New York 2013

Abstract Regarding to the development of SnAgCu

(SAC) lead-free solders for advance electronic compo-nents, the effect of 0.5 wt% nano-sized ZnO particles on

the thermal, microstructure and tensile properties of

Sn3.5 wt% Ag0.5 wt% Cu (SAC355) lead-free solder

alloy is investigated. The results showed that addition

of 0.5 wt% nano-sized ZnO particles into the conventional

lead-free SAC355 solder caused a slight increase of

its liquidus temperature by about 1.1 K. Metallographic

observations of SAC3550.5 wt% ZnO (composite solder)

revealed an obvious refinement in the microstructure

compared with the SAC355 (non-composite) solder. Con-

sequently, addition of nano sized-ZnO particles could

improve the stressstrain characteristics proof stress (ry0.2)

and ultimate strength (rUTS). This was rendered to sup-

pressing effect of ZnO on the coarsening of the intemetallic

compounds (IMCs) Ag3Sn and Cu6Sn5 during the solidi-

fication process in the composite solder and subsequently

dispersion strengthening is considered to be the dominating

mechanism. This will allow the use of SAC355 composite

lead-free solder alloy, to be consistent with the conditions

of usage for conventional SAC solder alloys and to over-

come the serious problem of the excessive growth of IMCs

and the formation of microvoids in the SAC lead-free

solder alloys.

1 Introduction

In electronic industry, solders are important materials

because they provide both the electrical connections and

mechanical response between integrated circuit devices

and the substrate [1]. The strength of solder joints is

affected by thermal stability of the solder alloys. They must

have long-term reliability under extreme conditions; par-

ticularly at relatively high working temperatures. During

the past seven decades, eutectic and near eutectic PbSn

solders have been used as the principal joining materials

because of their low cost, low melting point (183 C), good

solderability and good mechanical properties [25]. PbSn

solders are also highly compatible with electronics assembly

processes and can form stable joints that are usable under a

wide variety of service environments [25]. Because of its

hazardous nature to health and the environment and with

arrival of legislative restriction on the use of lead solders by

European Union, restricting the usage of lead solder alloys

has now become a reality [2].

Motivated by the above reasons, nowadays, with device

miniaturization and high performance demands in Micro-

systems, it has become very significant to improve properties

and reliability of lead-free solder joints [1]. An attractive and

potentially available method of enhancing solder joint is

carried out by adding reinforcements (third and fourth ele-

ments) to solder alloys, to form a composite solder. The

reinforcing particles should have suppressing effect for grain-

boundary sliding, large intermetallic compound formation,

and suppresses also the grain growth, thereby causing the

stress in the solder joints to be distributed uniformly. In this

way the solder joint could provide better reliability with

improved thermal stability of the microstructure [2].

Conventional SnAgCu (SAC) solder alloys are con-

sidered to be one of the best lead-free alloy systems. The

A. Fawzy (&) M. Sobhy E. Nassr M. M. Mousa G. SaadPhysics Department, Faculty of Education, Ain Shams

University, Cairo, Egypt

e-mail: [email protected]

S. A. Fayek

Physics Department, National Centre for Radiation Research

and Technology, Nasr City, Cairo, Egypt

1 3

J Mater Sci: Mater Electron (2013) 24:32103218

DOI 10.1007/s10854-013-1230-2


4/11

main benefits of this alloy system are its relatively low

melting temperature and its superior mechanical properties,

as well as relatively good solderability [6]. However, and

because of their high moduli, SAC solders are not very

satisfactory for certain applications especially in mobile

electronics [7]. In addition, serious problems made the

SAC conventional solder may not guarantee the required

performance at finer degrees of performance due to: (1)higher diffusivity and consequently the excessive growth of

intemetallic compounds (IMCs), (2) formation of microv-

oids and (3) softening nature of the SAC Pb-free solders.

To solve these problems, efforts have been made to

develop solders with low melting point, higher strength and

better microstructure properties.

Recently, addition of nano-sized particles to the con-

ventional SAC solder has been identified as composite

solder alloys. Lead-free SAC composite solders have been

identified as potential materials with higher microstructure

stability and better mechanical properties as compared to

the conventional SAC solders. Mavoori and Jin [8] studiedPb37Sn composite solder with 10 nm Al2O3 and 5 nm

TiO2 reinforcement particles and reported significant

improvements in its mechanical properties. Mechanical

measurements reported significant increases in microhard-

ness, offset yield strength (0.2 %YS), and ultimate tensile

strength (UTS). However, the ductility decreased with

increasing amounts of TiO2 nano-sized particles. Tsao

et al. [8, 9] reported significant improvement in the

mechanical response after adding: (1) nano-sized TiO2particles on Sn35Ag025Cu [9], and (2) nano-sized Al2O3on Sn35Ag5Cu lead-free solders [10]. Shen and Chan

[11] also after studying Sn9Zn composite solder with

ZrO2 nano-sized particles achieved significant improve-

ments in mechanical properties.

It has been found that literature survey revealed that no

studies have been reported so far on lead-free SAC solder

joints containing nano-sized ZnO particles. So, the present

work is devoted for investigating the effect of addition of

nano-sized ZnO particles on thermal, microstructure and

tensile properties of Sn3.5 wt%Ag0.5 wt%Cu (SAC355)

lead free composite solder at various experimental test

conditions for trying to improve its microstructure and

tensile properties.

2 Experimental procedures

A Lead-free solder, Sn3.5 wt%Ag0.5 wt%Cu (SAC355)

solder alloy, is prepared from Sn, Ag and Cu ingots of

99.99 % purity. SAC355 lead free composite solder was

prepared by mechanically mixing 0.5 wt% of nano-sized

ZnO particles into the prepared conventional SAC355 lead

free solder with subsequent remelting in a vacuum furnace

at 603 K for 2 h, followed by casting into a stainless steel

mold and cooled down to room temperature in air.

Ingots of the two alloys in the form of bars were cold

drawn into a 0.8 mm diameter wire. A part of each alloy

was rolled into sheet of 0.4 mm for microstructure inves-

tigations. Specimens with a gauge length of 50 mm were

prepared for tensile testing. Prior to the tensile testing, all

specimens were heat-treated at a temperature of 393 K for2.5 h and then cooled down to room temperature to sta-

bilize microstructure and remove the residual defects pro-

duced during the drawning process. For metallographic

observations, the required sheet specimens were etched in a

solution of 80 % glycerin, 10 % nitric acid and 10 % acetic

acid.

Tensile tests were carried out by straining each speci-

men to fracture. Stressstrain measurements are performed

at different strain rates ranging from 1.7 9 10-4 to

1 9 10-3 s-1 at different testing temperatures ranging

from 298 to 373 K using a computerized tensile testing

machine described elsewhere [12].Melting temperatures of the two solders were analyzed

using differential scanning calorimetry (DSC) at a heating

rate of 10 C/min. Microstructure of the two alloys was

investigated by scanning electron microscopy (SEM).

X-ray diffraction (XRD) and energy dispersive spectros-

copy (EDS) analysis are adopted for determination of the

phases and their elemental composition in both SAC355

solder and SAC355 composite lead-free solder alloys.

3 Results and discussion

3.1 Melting characteristics

Melting temperatures of both SAC355 solder and SAC355

composite solder are determined from the DSC curves

obtained in Fig.1. Figure1a exhibits the DSC curve of the

SAC355 solder while Fig. 1b shows the DSC curve of the

SAC355 composite solder specimen which doped with

0.5 wt% nano-sized ZnO particles. The observed endo-

thermic peaks of the SAC355 solder and SAC355 com-

posite solder were found shifted from 494.18 to 495.26 K.

For each solder, only one peak is observed and melting

temperature of the SAC355 composite solder was found

slightly higher than that of the conventional SAC355 solder

by about 1.1 K. This observation was found similar to

those obtained in other studies on SAC composite solders

[9,10,13,14]. The slightly increase in melting point of the

SAC355 composite solder can be attributed to the effect of

the nano-sized ZnO particles on the rate of solidification.

Such particles may serve as retardation sites for the

solidification process of the IMCs. The reinforcing nano-

sized ZnO particles may also change the surface instability

J Mater Sci: Mater Electron (2013) 24:32103218 3211

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and physical properties of the grain boundary interfacial

characteristics. This result was in good agreement with

those reported for Sn3.5 wt% Ag0.25 wt% Cu rein-

forced with nano-sized TiO2 [9] and SAC355 with nano-

sized Al2O3 particles [10]. X-ray diffraction (XRD), anal-

ysis is performed to emphasize phase composition of the

nano-sized ZnO particles (Fig. 2a). Figure2b represents a

TEM image for the nano-sized ZnO particles used in thisstudy. It showed an average size of nominally polyhedrons

nano-sized ZnO particles with *66 nm diameter.

3.2 Microstructure analysis

Under near equilibrium solidification, X-ray diffraction

investigation of SAC355 solder and SAC355 composite

solder as illustrated by the diffraction patterns shown in

Fig.3a, b exhibited three types of phases;b-Sn, Ag3Sn and

Cu6Sn5 phases. The diffraction patterns of both solders are

found to have nearly the same features. Besides, SEMImages for the microstructure of SAC355 solder alloy and

SAC355 composite solder are shown in Figs4 and 5.

Figure4b showed a significantly decrease in the grain sizes

of the b-Sn matrix of the SAC355 composite solder com-

pared with those in the SAC solder alloy shown in Fig. 4a.

As can be seen from Fig.5a, it is observed that the solid-

ification process exhibited dendritic dark regions and

interdendritic bright regions consisting of needle-like fine

particles besides irregular polygon shapes. EDS analysis,

confirmed that the dark dendrite arms is the b-Sn phase

while the bright interdendritic regions are found to contain

Cu, Sn and Ag. Since the solders used were SAC355, theeutectic mixture contain the needle-like fine Ag3Sn parti-

cles dispersed within the Sn-rich matrix besides irregular

polygons of Cu6Sn5 intermetallic compounds (IMCs). The

presence of these IMCs is confirmed by the XRD shown in

Fig.3.

Addition of nano-sized ZnO particles is found to affect

the microstructure of SAC355 solder. Figure5b revealed

the microstructure of the SAC355 composite solder alloy.

It presents the same features in the microstructure of the

SAC355 solder. It is clear from this figure that b-Sninterdendritic arms (eutectic mixture) as well as the IMC

particles in the composite solder are reduced in size, i.e.

seem to be small and fine compared with those in SAC355

solder alloy. According to the EDS analysis, the eutectic

areas were found to contain Zn, O, Cu, Sn and Ag. Thus, it

can be concluded that the network eutectic areas are

Cu6Sn5 and Ag3Sn besides the ZnO particles. This retar-

dation effect of the ZnO nano-sized particles is similar with

that reported in other studies [911, 1316]. This means

that addition of 0.5 wt% nano-sized ZnO to SAC355 seems

to suppress the formation of the b-Sn dendrites, Ag3Sn

needle-like and the Cu6Sn5 polygon particles yielding auniform dispersion of these IMCs within the Sn-rich mix-

ture producing a fine network like microstructure with the

b-Sn (Fig.5b) which subsequently may affects its physical

and mechanical properties.

3.3 Tensile response

A typical set of representative stressstrain curves of the

SAC355 solder (solid lines) and SAC355 composite solder

(dotted lines) stretched by different strain rates ranging

from 1.7 9 10-4 to 9.2 9 10-2 s-1 at a constant defor-

mation temperature of 298 K are shown in Fig. 6a. Fig-

ure6b shows another set of representative stressstrain

curves of the same solders stretched by a constant strain

rate of 7.4 9 10-4 s-1 at the deformation temperatures

298, 323, 348 and 373 K. From these figures, it is noted

that levels of the stress strain curves shifted towards higher

values by increasing strain rates and/or decreasing the

testing temperatures. Moreover, addition of ZnO is noticed

to increase the level of the stress strain curves at all test

conditions (Strain rate and testing temperature). In details,

stressstrain characteristics namely the ultimate tensile

stress rUTS, the yield stress ry0.2 of both SAC355 solder

and SAC355 composite solder are strongly affected by the

variation of the strain rate, testing temperature and the

existence of the nano-sized ZnO particles. For both solders

at the same testing temperature, increasing strain rate

(Fig.7) gives rise to higher values of rUTS. The proof

stress ry0.2 (not presented here) exhibited the same

behavior as rUTS. At the same strain rate, raising the

testing temperature resulted in a continuous softening; a

decrease in rUTS was observed. This behavior can be

interpreted as follows:

150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300

-60

-40

-20

0

20

160 180 200 220 240 260 280 300

-60

-40

-20

0

20

(a)

(b)

221.18oC (494.18 K)

SAC

222.26oC (495.26 K)

HeatFlo

w(mW/mg)

Temperature (Co)

SAC+ZnO

Fig. 1 DSC curves of a SAC355 and b SAC composite solder

containing ZnO nanoparticles

3212 J Mater Sci: Mater Electron (2013) 24:32103218

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It is well known that for most metals and alloys, solder

alloys experience simultaneous work hardening and

dynamic recovery when they are deformed. The work

hardening and dynamic recovery have opposite effects on

the mechanical properties of the solder alloys, where the

former hardens the material while the latter leads to soft-ening. The stressstrain curves obtained here seems to

present the combined effects of both factors. Figure7

shows the relation between the strain rates and the ultimate

tensile stress (rUTS) for both the SAC355 solder and

SAC355 composite solder at different testing temperatures.

This figure shows that increasing strain rates increased both

the rUTS in both solders. This is because increasing strain

rate is accompanied by an increase in the dislocation

density. As these dislocations move they become mixed. It

is then more difficult for other dislocations to glide through

the material, especially at the lower deformation tempera-

tures. At high testing temperature; dislocation annihilation

seems to occur more rapidly than dislocation generation

during deformation. Therefore, at higher testing tempera-

tures the lower strain rate provided lower strength in thetested solder and dynamic recovery seems to occur,

resulting in the decrease inrUTSas can be concluded from

Fig.7.

It must be noted that the SAC355 composite solder alloy

samples exhibited higher values ofrUTS compared with

those exhibited by the SAC355 solder one at all testing

temperatures. High values of the rUTS in the composite

solder alloy may be attributed to the difference in

the microstructures of these solders (Figs.4, 5). The

(200)

(110)

(201)

(202)

(104)

Intensity(a

rbitraryunits)

2 Theta

(100)

(102)

(101)

(002)

(103)

nano-sized

ZnO

(a)

(b)

Fig. 2 a XRD patterns of the

nano-sized ZnO particles, and

b Bright field TEM of nano-

sized ZnO nanoparticles


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refinement and uniform distribution of the Ag3Sn and

Cu6Sn5 intermetallic compounds seem to provide greater

evidence of dispersion strengthening due to the finer

particle sizes in the composite solder (reinforced) com-

pared to the SAC355 (un-reinforced) solder one. The

relationship between rUTS and the strain rates (_e) for both

alloys can be expressed by the equation [17,18]:

rUTS C _em

where C is a constant, _eis the strain rate and m is the strain

rate sensitivity index. Values of m describe the capacityof the material for necking resistance [18]. These values of

the index m can be obtained from the loglog relation of

the strain rate (_e)and the ultimate tensile stress (rUTS). The

mean value of m for the SAC355 composite solder was

found to be slightly higher than that in the SAC355 solder

indicating higher resistance for necking. This may be

attributed to its finer microstructure which allow for more

interaction between dislocations that created during

deformation and the fine IMC particles beside the nano-

sized ZnO particles.

The similar variation ofry0.2 with testing temperature

and strain rate on can be understood by considering thedeformation as stress assisted and thermally activated

process. Hence, at high strain rates and low testing

(a)

(b)

Fig. 3 XRD patterns of: a SAC355 solder andb SAC355 composite

solder alloys showing the existence of three types of phases

Fig. 4 SEM micrographs for the whole surface of:a SAC355 solder and their corresponding EDS curve and b SAC composite solder and their

EDS curve


1 3


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temperature the yield stress increases due to limited time

for the motion of dislocations as well as the lower ther-

mally activated dislocation motion. This makes it more

difficult for other dislocations to cut through the matrix of

the material. Increasing the deformation temperature will

promote the rearrangement of the dense dislocation net-

works formed by strain hardening into simple and less

mixed networks. This reduces the lattice energy and,therefore, it is not surprising to observe lower values of the

yield stress at higher deformation temperatures, since dis-

locations have more and more freedom (more energetic) to

move and overcome obstacles through matrix. From the

obtained results, values of ry0.2 for the SAC355 solder

alloy exhibited lower than those of the composite solder

one. This is explicable since the microstructure of the

composite solder alloy is characterized by the existence of

finely dispersed particles of the IMCs besides the nano-sized

ZnO particles in the Sn matrix which can act as pinning

centers for the mobile dislocations.

3.4 Role of the nano-sized ZnO particles in the tensile

response of the SAC355 composite solder alloy

It is known that improvement of solder reliability in elec-

tronic components is related to its mechanical strength andthermal properties. So, the effect of microstructure, which

vary with composition, temperature and strain rate during

the life of the component must be predictable. In order to

precisely evaluate the influence of nano-sized ZnO particles

on the microstructure and tensile strength of the solidified

solder we will look back at the SEM micrographs shown in

Figs.4 and 5. As has been discussed before; addition of

small percentage of nano-sized ZnO particles resulted in

refinement of microstructure of the near eutectic SAC355

(b)

(a)

Ag3Sn

Cu6Sn5

- Sn

Fig. 5 SEM micrographs showing the IMCs and the corresponding EDS curves in a SAC355 solder and b SAC355 composite solder alloys


1 3


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10/11

sized ZnO particles, which are mechanically dispersed in

the molten of SAC355 composite solder, are clinging to the

much larger-sized Ag3Sn particles just as spheres cling to a

plane. This can be simplified by treating the Ag3Sn crystal

surface as a plane and the nano-sized ZnO particles as

spheres. Accordingly adsorption of such nano-sized surface

active material can decrease the surface energy of the

Ag3Sn crystal [20,21]. From this standpoint, the obtainedmicrostructure of the SAC355 composite solder can reflect

itself on the tensile response and improvement the tensile

strength of the composite solder. This is because of the: (1)

pinning action of these nano-sized ZnO particles which

subsequently imped sliding of the grain boundaries as can

be deduced from Fig. 4, and (2) the dispersion strengthen-

ing mechanism of the matrix by finely dispersed IMC par-

ticles and the nano-sized ZnO particles [10, 13, 15]. The

above interpretation was confirmed by Yu et al. [21] which

reported finding nano-size Ag3Sn particles on the Cu6Sn5IMC surface in SAC solder; existence of these nano-sized

particles would decrease the interfacial energy and conse-quently suppress the growth of the Cu6Sn5 IMC. Also, Liu

et al. [22] have found that the adsorption of nano-sized

Ag3Sn particles occurs during the solidification of SAC.

This adsorption would decrease the surface energy of the

Cu6Sn5IMC and retard the growth of the whole IMC layer

[21].

From the above analysis, the apparent strengthening

effect of the nano-sized ZnO particles can be attributed to

their suppressing effect on the growth of the overall IMC

particles. This means that these IMCs play the role of a

secondary reinforcing phase and the enhancement of the

tensile strength showed good correlation with the com-

posites microstructure and agree with the theoretical pre-

diction from dispersion strengthening theory [10,14]. This

suggests that addition of nano-sized ZnO particles to the

SAC solder makes composite solder joints to be more

efficient in reducing the growth of the overall IMC parti-

cles at the temperature used in this study.

It is generally known that, additions of second-phase

particles can affect the distance that dislocations move

between obstacles and the forces that cause them to over-

come these obstacles. However, several activated processes

can occur in particle-strengthened materials, such as par-ticle by-pass by dislocation climb, Orowan by-pass, and

attractive interactions between dislocations and particles.

In lead-free solder alloys that strained at temperatures

greater than half of the melting temperature, the strain rate

_e andrUTS may be related to the testing temperature with

the help of the kinetic rate equation [16,23].

_e A rUTS 1=m

exp Q=RT

where A is a constant, R is the universal gas constant, m is the

strain rate sensitivity index and Q is the activation energy.

Figure8 shows the relation between lnrUTSand 1000/T for

both the SAC355 and composite solder alloys. From slopes

of the straight lines obtained in Fig. 8, activation energies of

0.57 eV is obtained for both SAC355 and SAC355 com-

posite solder alloys. This value was found to be close to those

reported for the dislocation motion mechanism in Sn-based

alloys and was found to be in agreement with those obtained

in other researches [2426]. Onthe other hand, the present Q

value did not affect by the existence of the nano-sized ZnO

particles indicating that both solder alloys have the same rate

controlling mechanism.

4 Conclusions

Effect of nano-sized ZnO particles on thermal, micro-

structure and tensile properties of Sn3.5 wt% Ag

0.5 wt% Cu (SAC355) solder alloy was studied. Some

important conclusions are summarized as follows:

1. Addition of nano-sized ZnO particles to SAC355

solder alloy slightly increased the melting temperature.

2. Microstructure investigations revealed that addition of

nano-sized ZnO particles to SAC355 solder inhibited

the growth of the grain size as well as the IMCs Ag3Sn

and Cu6Sn5which subsequently reinforced the strength

of the SAC355 solder.

3. Tensile tests revealed that addition of nano-sized ZnO

particles increased the strength of the SAC355 com-

posite solder.

4. Increasing strain rate resulted in increasing the tensile

strength while increasing the testing temperature

decreased it.

5. Addition of nano-sized ZnO particles will allow the

use of SAC355 composite lead-free solder alloy, to be

0.29 0.30 0.31 0.32 0.33 0.34 0.35 0.36 0.37 0.38

3.3

3.4

3.5

3.6

3.7

3.8

3.9

4.0

4.1

1000/T (K-1)

Ln

UTS

(MPa)

Q= 0.57 eV

(sec-1):

1.7 x10-4

3.8 x10-4

7.4 x10-4

1 x10-3

.

Fig. 8 Relation between LnrUTSand (1000/T (K)) for both SAC355

(solid lines) and SAC355 composite solder alloys (dashed lines)


1 3


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consistent with the conditions of usage for conven-

tional SAC solder alloys and to overcome the serious

problem of the excessive growth of IMCs and the

formation of microvoids in the SAC Pb-free solder

alloys.

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