Strain Rate Effect on Micromechanical Properties

of SnAgCu Solder Wire

I. Abdullah1, R. Ismail1 and A. Jalar1, Member,IEEE 1Institute of Micro Engineering and Nanoelectronics (IMEN)

Universiti Kebangsaan Malaysia,

43600 Bangi, Selangor, Malaysia.

Email: eizhan@eng.ukm.my

Abstract— Dislocation behavior was occurs when an eutectic

solder alloy of SnAgCu experiencing different strain at room

temperature that require the further analysis in order to relate

the physical and microstructure changes towards the mechanical

performance of lead free solder. In this study, nanoindentation

technique was applied to determine the hardness and modulus on

six variant of strain (0.00015 mms-1, 0.0015 mms-1, 0.015 mms-1,

0.15 mms-1, 1.5 mms-1 and 15 mms-1) after tensile test. The P-h

curves and the micromechanical parameter namely hardness and

residual modulus through nanoindentation test were conducted.

The analysis were obtained strain rate sensitivity (m) and stress

exponent (n) from dwell time in order to determine the

mechanism of grains. The P-h curve result showed the pop-in

event at the ranges of 100 nm to 300 nm. The micromechanical

properties were show the increment of values at high strain rates.

The dominated discontinuity local will occurrence the pop-in

event and will activating dislocation distribution.

Keywords— SnAgCu, nanoindentation, dislocation, P-h curve,

pop-in event, strain rate.

1. INTRODUCTION

Eutectic solder alloys are widely used in the

microelectronics industry [1,2]. Deformation of material is

divide by two namely twinning and dislocation. Dislocation of

grain boundary was occurrence by some load on impact such

strain and compression at constant temperature. However, the

dislocation distribution of grain boundary would be the issue

lack the hardness parameter [3]. In the correlation of

mechanical properties and microstructure, the hardness which

is defined as the indentation load divided by the contact area of

the indentation made, they can be a sensitive enough parameter

to represent the hardening potential of a grain boundary,

however, it need a further research. In many report where an

increase in the measured hardness was observed, the grain

boundary hardening was attribute to segregation of impurity

atoms [4]. For example, doped zone-refined metal such as Sn,

Ag and Pb, tin segregation in alpha Ag-Sn alloy and Cu-Sn.

Wang and Ngan (2004) were solve this issue with proper heat

treatment procedure were followed to achieve enough

segregation of impurities at grain boundary [5].

From modelling or experimental works, nanoindentation

technique is frequently used to measure the properties material

for solder alloys and other material at small volumes. In fact,

Gomez and Basaran (2006) reported the nanoindentation

technique has gained popularity as an experimental tool to

determine the material properties for specimens available in

small volumes. On the result of nanoindentantion on rate

independent materials have shown a strong dependence of

hardness on penetration depth by loading rates [6].

Most of the studies on the lead free deformation

behaviour are the combination of bulk material testing with

strain, strain rate. Therefore the analysis of micromechanical

properties effect of solely the changes strain rates towards the

deformation behaviour of lead free solder is crucially needed.

To clarify the microstructure behaviour, nanoindentation test

was carried in order to determine the micromechanical

properties of Sn-3.0Ag-0.5Cu related to grain boundary with

strain rate effect through tensile test. The deformation

mechanisms was obtained measuring the stress rate sensitivity

and stress exponent from the dwell time analysis results

obtained.

2. SAMPEL PREPARATION AND METHODS

The eutectic of Sn-3.0Ag-0.5Cu (SAC305) solder wire

was provided by local industry, Red Ring Solder (M) Sdn Bhd

was selected in order to determine the micro-mechanical

properties. These specimen were used for the present work.

The selected specimen was cut into 10 cm of length and the

acrylic tapes were attached on the top and the bottom of SAC

solder wire before being used in tensile machine as shown in

Fig 1.

Fig. 1 SAC305 solder wire specimen for tensile test

978-1-4799-5760-6/14/$31.00 ©2014 IEEE

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343

The tensile test were performed on these specimen using

INSTRONTM Microtester machine at room temperature with

six variant of strain rates with the value of 0.00015 mms-1,

0.0015 mms-1, 0.015 mms-1, 0.15 mms-1, 1.5 mms-1 and 15

mms-1. These solder wire with length of 5mm was cut far end

from the rupture area in order to prepared these samples for

nanoindentation test. The nanoindentation samples were

prepared by resin mounting the chosen 5mm length of

SAC305 solder wire. Later, the wet grinding was performed

on the sample using 600, 800 and 1200 grit of abrasive papers

followed by polishing with 3 µm, 1 µm and 0.25 µm of

diamond suspension on silk cloths. In order to determine the

micro-mechanical parameters, the indentation were carried

out by a NanotestTM (Micro Materials) machine with a

Berkovich tip (three-sided pyramidal) of ~50 nm indented

area. The indentation were performed using loading-holding-

unloading cycle’s mode. A constant rate of 0.1 mN/s was

applied to cross section of SAC305 surface until the

maximum load (10 mN) was reached, then held to dwelling in

185 seconds at maximum load until unloading. Four location

on cross section of SAC305 surface were to get the average

values. All of indentation data were analysed based on the

Oliver and Pharr theories [7].

3. RESULTS AND DISCUSSION

The indentation on SAC305 solder wire samples were

carried out after tensile strain rate performed. Fig. 2 depicts the

stress-strain curves on six variant of strain rates. From the

indentation, indentation load (P) and penetration depth (h) are

acquired from indentation penetration into cross section of

SAC305 solder wire surface. The variations of P-h curve as-

received and six variant of strain rates on SAC305 solder wire

as depicted in Fig. 3. The loading of indentation was consist

elastic and plastic deformation. Initial loading during the

indentation has resulted in elastic deformation. When the depth

of penetration increases, the plastic deformation of the material

had started occur. The initial plastic deformation was triggered

by initial changes at elastic region of P-h curve and also stress-

strain curves (in Fig. 2). In addition, the shape of P-h curve is

be related to mechanical properties of this material.

Fig. 2 Stress-strain curves of SAC305 solder wire from strain rate tensile test at room temperature (298.15 K).

According to Fig. 3, the loading section on P-h curve of

indentation for different strain rates have glitches and ladder

shape compared to as-received SAC305 solder wire. This

phenomenon is called a pop-in or displacement burst [4]. The

pop-in was occurrence at initial elastic region that the range

from 100 nm to 300 nm. This occurrence of local discontinuity

was happen during loading on strain rates. The pop-in

phenomenon also related to local discontinuity during

indentation and microstructure change effect from strain rate

[8,9]. The initial pop-in may reveal the shift the perfect elastic

to plastic deformation and would be initiate dislocation among

of grain boundaries of this materials [10]. The pop-in less occur

during low strain rates otherwise it often happens when the

strain rate was increased.

0 200 400 600 800 1000 1200 1400 1600

0

2

4

6

8

Lo

ad

(m

N)

Depth (nm)

As-received

0.00015 mms-1

0.0015 mms-1

0.015 mms-1

0.15 mms-1

1.5 mms-1

15 mms-1

Fig. 3 The variations of P-h curves as-received and six variant of strain rates on SAC305 solder wire

Strain rate also affected to micromechanics properties

through indentation. Fig 4(a) and (b) depict the strain rates

effect to hardness and reduced modulus (elastic). According to

Fig. 4, it was showed that the low strain rate (0.00015 mms-1)

has acquired small values (0.131 GPa and 42.933 GPa) for both

of hardness and reduced modulus compared to high strain rate.

As viewed, the both of values of as-received SAC305 solder

wire have small value rather than six variant of strain rate due

to the grain boundary have not deformed while unloading.

These three of low strain have acquired the closely values of

each other compared to the high strain rates. The significant of

improvement in the hardness of strain rate 0.00015 mms-1 to

15 mms-1 is 41.2 percent. This is due to strain rate has changed

the size and shape of the grains on the surface of this solder

wire material [11]. In this alloy material, the hardness is

strongly depends on the presence of defect in the vicinity of

indentation and the specimen resistance to plastic deformation.

In parallel to hardness changes, the values of reduced modulus

are closely related to the elastic and plastic work that has

changed the microstructure properties in terms of the grains

changes through the dislocations formation [12].

IEEE-ICSE2014 Proc. 2014, Kuala Lumpur, Malaysia

344

To determine the mechanism of deformation of

indentation, the determination of the stress exponent, n was

conducted on indentation in the eutectic of SnAgCu using a

constant load. Goodall and Clyne (2006) has stated that

nanoindentation deformation analysis using constant load

takes the maximum load and the maximum load applied during

nanoindentation to compute the value of n [13]. Mahmudi et al.

2001 was describe the deformation mechanism according to n

value which acquired from researchers as shown in Table

1[14]. m used in the determination of strain hardening occurs

at the eutectic of SnAgCu which m is high value indicates a

high strain hardening and less plastic deformation occurrence

[15,16]. Table 2 shows the strain rate sensitivity, m and stress

exponent, n in SAC305 solder wire for different strain rates at

room temperature. The value of n for strain rate 0.00015 mms-

1 is reduced from 7.5 to 4.9 for strain rate 0.0015 mms-1. Then

the value of n was increased from 5.6 to 6.8 at strain rates of

0.015 mms-1 and 0.15 mms-1. But the value of n has dropped

back on strain rate range of 1.5 mms-1 to 15 mms-1 is from 5.6

to 3.9.

(a)

(b)

Fig. 4 Strain rates effect to hardness (a) and reduced modulus (b) of SAC305 solder wire

Table 1 Description of deformation mechanism according to stress exponent, n

Stress exponent (n) Deformation mechanism

1 diffusion

2 Grain boundary sliding

4 until 6 Dislocation in climb

Over than 8 Dislocation movement / Dislocation in climb

Table 2 n and m value acquired from the indentation at the

cross section of SAC305 solder alloy wire after tensile

test at room temperature (25 ° C)

Strain

Rates

(mms-1)

Stress

Exponent

(n)

Strain rate

sensivity

(m)

0.00015 7.5 0.13

0.0015 4.9 0.21

0.015 5.6 0.18

0.15 6.8 0.15

1.5 5.6 0.18

15 3.9 0.26

According to Table 2, it was showed that the strain rate

effect on the n value which is determine the behaviour of the

SAC305 solder alloy wire microstructure after tensile

conducted. The value of stress exponent, n were show in the

range of 0.00015 mms-1 to 15 mms-1 is between 3 to 8

(according to Table 1). According to the n value, there are a

number of activities in microstructure activities which involves

the grains where the occurrence of initial movement of

dislocation at low strain rates and the dislocation would climb

on a nearby the grains. This caused is a pop-in event in the

SAC305 microstructure. Therefore, this phenomenon related

between microstructural changes and indirectly alter the

properties of hardness and modulus during strains performed.

4. CONCLUSION

The indentation on SAC305 solder wire with six variant

strain rate were performed. The P-h curve were show the initial

local discontinuity was occur at elastic region and the pop-in

event may revealed during indentation penetrate at the range of

100 nm to 300 nm. Therefore, the stress exponent played

important role to clarify to the microstructure behaviour would

started initial the dislocation movement which causes the

occurrence of pop-in phenomenon.

ACKNOWLEDGEMENTS

This work has been sponsored by National University of

Malaysia and under research university grant (UKM-OUP-

NBT-29-143/2011, OUP-2012-120 and DIP-2012-14). The

author would like to thank Ministry of Higher Education for a

scholarship.

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as-received 0.00015 0.0015 0.015 0.15 1.5 15

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