F. A. M. YUSOFF et al.: TEMPERATURE-INDUCED THICKNESS REDUCTION OF MICROMECHANICAL PROPERTIES ...
373–379
TEMPERATURE-INDUCED THICKNESS REDUCTION OF
MICROMECHANICAL PROPERTIES OF Sn-0.7Cu SOLDER ALLOY
TEMPERATURNO INDUCIRANO ZMANJ[ANJE DEBELINE IN
MIKROMEHANSKIH LASTNOSTI ZLITINE ZA SPAJKANJE
Sn-0,7Cu
Fateh Amera Mohd Yusoff
1
, Maria Abu Bakar
2*
, Azman Jalar
1,2
1
Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
2
Department of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
Prejem rokopisa – received: 2022-02-04; sprejem za objavo – accepted for publication: 2022-05-16
doi:10.17222/mit.2022.392
Solders are used in electronic packaging for metallurgical interconnections. Thermomechanical methods are used to modify the
properties of a material. Cubic Sn-0.7Cu solder alloy was subjected to heat treatment at 30–150 °C for 20 min, followed by 80
% compression. The control samples used in this study were only subjected to heat treatment. This study used the
nanoindentation approach to investigate the reductions in the modulus and hardness of the lead-free Sn-0.7Cu solder alloy after
thermomechanical treatment. Samples with 80 % compression showed slight changes in the reduced modulus (approximately
24 %) and hardness (approximately 14 %) after thermomechanical treatment. In contrast, the solder alloy that underwent heat
treatment alone (the control sample) showed shifts in the hardness and reduced modulus of approximately 54 % and 66 %, re-
spectively. The production of new recrystallized grains resulted in smaller changes in the micromechanical properties. These
findings demonstrated that thermomechanical treatment can both modify and stabilize the properties of the Sn-0.7Cu solder al-
loy, such as micromechanical properties.
Keywords: micromechanical properties, solder alloy, nanoindentation, thermomechanical treatment
Spajkanje se uporablja pri elektronskem pakiranju medsebojnih metalur{kih povezav. Izbrane termomehanske metode se
pogosto uporabljajo za modifikacijo lastnosti materiala. Predmet raziskave je bila zlitina za spajkanje Sn-0,7Cu v obliki kvadra.
Spajko so toplotno obdelovali 20 min v temperaturnem obmo~ju med 30 °C in 150 °C v korakih po 30 °C, nato je sledila njena
80 % tla~na deformacija. V raziskavi so uporabili le toplotno obdelane kontrolne vzorce. Za ugotavljanje u~inka toplotne
obdelave so uporabili metodo nanoindentacije in ugotavljali zmanj{anja modula in trdote spajke Sn-0,7Cu, ki ni vsebovala
ekolo{ko spornega svinca (Pb). Vzorci, ki so bili 80 % tla~no deformirani so po termomehanski obdelavi kazali rahlo
spremembo oziroma zmanj{anje modula elasti~nosti (pribli`no 24 %) in trdote (pribli`no 14 %). V nasprotju s kontrolnim
vzorcem, ki ni bil toplotno obdelan se je pokazal obrat v smeri zmanj{anja modula elasti~nosti in trdote za pribli`no 54 % oz.
66 %. Nastanek novih rekristaliziranih zrn je povzro~il majhno spremembo mikromehanskih lastnosti. Ugotovitve in raziskave
avtorjev tega ~lanka so pokazale, da termomehanska obdelava lahko spremeni in stabilizira mikromehanske lastnosti spajke
Sn-0,7Cu.
Klju~ne besede: mikromehanske lastnosti, zlitina za spajkanje, nanoindentacija, termomehanska obdelava
1 INTRODUCTION
Temperature is one of the factors that affect the me-
chanical properties of a material by altering its micro-
structure. Heat treatment, such as aging at relatively high
temperatures, is well appreciated as an effective tech-
nique for changing the microstructures of materials, re-
sulting in changes in their mechanical performances.
1
Thermomechanical treatment is similar to heat treatment,
but it includes a mechanical action such as compression
to change the microstructure and characteristics of the
material.
2
This treatment is commonly utilized for the
structural materials used in the automotive and construc-
tion industries, which must exhibit high mechanical
properties and long-term reliability.
3
The microstructure
of the affected material is regulated by simultaneously
varying the temperature and applying a mechanical load
to the material. This can produce microstructural
changes such as grain-size refinement and an increased
dislocation density. It has been proven that refining its
grains improves a material’s mechanical properties, in-
cluding its hardness and tensile strength. Li et al. re-
ported that the application of hot rolling to Nb-Ti steel
led to a higher tensile strength than that without hot roll-
ing.
4
However, thermomechanical treatment at specific
temperatures and forms of stress such as compression,
tension or shear can cause a material to deform. Com-
pression loads cause a reduction in the material thickness
and have varying effects on the mechanical characteris-
tics. For example, a study was conducted on the wire for-
mation using thermomechanical treatment with mechani-
cal processes at different reduction percentages.
3
The
results revealed that the hardness increased at a higher
proportion of reduction, which was influenced by a grain
refinement and more efficient dislocation density. As a
result, it is critical to understand the relationships be-
Materiali in tehnologije / Materials and technology 56 (2022) 4, 373–379 373
UDK 679.7.027.3:669.65 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 56(4)373(2022)
*Corresponding author's e-mail:
maria@ukm.edu.my

tween various processes and material properties because
they are linked.
In electronic packaging, solder alloys are frequently
employed as the connecting materials. Because of their
beneficial features, lead solder alloys such as Sn-Pb used
to be particularly popular. However, for environmental
and human health reasons, the use of Sn-Pb in electronic
packaging is no longer permitted because of the toxicity
of lead.
5
Nowadays, lead-free solder alloys such as
Sn-Cu, Sn-Bi, Sn-In and Sn-Zn are potential candidates
to replace lead solder alloys.
6
Many studies investigated
the potentials of lead-free solder alloys in terms of their
mechanical properties and reliability. The properties of
solder materials continue to improve over time to meet
the challenges posed by continuous advances in the elec-
tronic packaging technology, which aims at miniaturiza-
tion and multi-functionality to ensure the solder joint re-
liability.
7
These developments mean that solder joints are
not only responsible for ensuring effective electrical-cur-
rent connections with sufficient conductivity but also
need to a have high mechanical strength to ensure good
performance in the long term. For these studies, changes
in the solder alloy properties depend on the process and
the micromechanical changes that occur as a result of the
process. Therefore, it is vital to study the potentials of
the lead-free solder alloys subjected to a thermomecha-
nical treatment to better understand the relationship be-
tween their properties and the process.
Small-scale mechanical property characterization of
solder materials is necessary because of the miniaturiza-
tion of solder joints and understanding of the reliability
of electronic devices. Microhardness tests, shear tests,
impact tests, Vickers tests, bending tests and tensile tests
were all used in the past to assess the mechanical proper-
ties of solder alloys.
8
Giuranno et al. studied the mechan-
ical properties of SAC solder alloys using the Vickers
method.
9
Indentations were made in the region of the sol-
der alloy and substrate. However, this conventional
method can only determine the mechanical properties of
bulk materials. In contrast, the nanoindentation method
can determine the mechanical properties locally.
10
Nanoindentation is a widely used method to characterize
the mechanical properties of small structures without
damaging the sample. This method also makes it possi-
ble to control the load, depth and exact test position. The
mechanical properties and deformations can be obtained
from a load versus depth curve. For example, the me-
chanical properties of intermetallic compounds (IMC) at
the interface of Sn-3.0Ag-0.5Cu/Cu solder joints were
investigated using the nanoindentation method.
11
The re-
sults showed that the hardness, elastic modulus and creep
properties of Cu
3
Sn and Cu
6
Sn
5
could be determined.
Therefore, this study investigated the effect of the
thermomechanical treatment temperature on the micro-
structure and micromechanical properties of Sn-Cu sol-
der alloys via nanoindentation.
2 EXPERIMENTAL PART
Cube-shaped Sn-0.7Cu samples, with dimensions of
(6×6×10)mm(length × width × height), were individ-
ually subjected to heat treatment in an oven at (30, 60,
90, 120 and 150) °C for 20 min. Subsequently, the sam-
ples were compressed with a push-pull gauge until the
thickness was reduced by 80 % of the original thickness,
from 10 mm to 2 mm (Figure 1), and then quickly
quenched in a water medium. Samples that were not ex-
posed to compression tests (heat-treated) were used as
control samples. Cross-sections of the Sn-0.7Cu alloy
samples were obtained using a metallographic technique.
The samples were cold-mounted in epoxy resin and
cured for 3–4 h at room temperature. The cold mounting
with a sample was removed from the mold container and
subjected to a grinding process using silicon carbide
(SiC) paper in grits of 800, 1000, 1200, 2000, and 4000.
Consequently, the samples were polished using a pol-
ishing cloth with 6-μm and 1-μm diamond sprays. The
microstructure of each sample was captured using an in-
finite focus microscope (IFM) at a magnification of 50×.
Nanoindentation tests were performed to determine the
hardness and reduced modulus. The maximum load was
10 mN, the loading and unloading rate was 0.5 mN/s,
and the dwell time was 180 s. Each sample underwent
five nanoindentation tests and the average result was cal-
F. A. M. YUSOFF et al.: TEMPERATURE-INDUCED THICKNESS REDUCTION OF MICROMECHANICAL PROPERTIES ...
374 Materiali in tehnologije / Materials and technology 56 (2022) 4, 373–379
Figure 2: Load, P, against depth, h, during the nanoindentation test
Figure 1: Sn-0.7Cu solder alloy before (left) and after compression
(right)

culated. This test recorded the loading and unloading
curves when the load (P) was plotted against the indenta-
tion depth (h)( Figure 2). During the nanoindentation
test, hardness properties were obtained from the P-h pro-
file using the Oliver-Pharr method, as shown in Equation
(1):
H
P
A
=
max
c
(1)
where H is the hardness of the material, P
max
(MPa) is
the maximum load applied to the material, and A
c
is the
contact area. In addition to the hardness properties, the
reduced modulus (E
r
) could also be obtained from the
curve of P against h. E
r
was calculated using Equa-
tion (2):
1 1 1
E
v
E
v
E
i
i r
s
2
s
2
=
−
−
− ()()
(2)
where E
s
and v
s
are Young’s modulus and Poisson’s ra-
tio of the sample, respectively; E
i
and v
i
are Young’s
modulus and Poisson’s ratio for the indentation, respec-
tively.
3 RESULTS
Figure 3 depicts the load-depth (P-h) profiles ob-
tained with the nanoindentation tests of the Sn-0.7Cu
solder alloy after it was subjected to different tempera-
tures and treatments. During the micromechanical test, a
load was applied using an indenter, which was referred
to as the loading, causing the tip of the indenter to pene-
trate the surface and enter the structure of the solder al-
loy. During the load increment, the penetration depth
was calculated until a maximum load of 10 mN was
reached. At 10 mN, the indenter was left static for 180 s,
which was called the dwell time. After 180 s, the in-
denter started to move out of the solder alloy structure
(unloading), causing a decrease in the penetration depth.
The variation in the penetration depth seen at different
temperatures was caused by micromechanical property
differences.
Figure 4 shows that the hardness decreased with the
increasing temperature. The hardness of the heat-treated
solder alloy (the control sample) decreased from
320 MPa at 30 °C to 147 MPa at 150 °C. The thermo-
mechanically treated solder alloy also showed the same
hardness trend up to 90 °C, with a slight increase to
167 MPa at 150 °C. In general, the thermomechanically
treated samples showed almost the same hardness trend
as the control samples. This is known as softening, and it
confirmed the hypothesis that the hardness of a metal
material decreases as the temperature increases during
heat treatment.
12–14
Several studies were conducted to
evaluate this process and relate different temperatures to
mechanical properties of solder alloys. Abdullah et al.
performed tensile and nanoindentation tests with a
Sn-3.0Ag-0.5Cu (SAC305) solder wire at a temperature
range of 25–200 °C. They found that the yield strength,
ultimate tensile strength and hardness decreased with an
increase in the temperature at a given strain rate. They
F. A. M. YUSOFF et al.: TEMPERATURE-INDUCED THICKNESS REDUCTION OF MICROMECHANICAL PROPERTIES ...
Materiali in tehnologije / Materials and technology 56 (2022) 4, 373–379 375
Figure 3: Load-depth graphs of Sn-0.7Cu solder alloy after: a) heat
treatment, b) thermomechanical treatment
Figure 4: Hardness versus temperature for the control and thermo-
mechanically treated solder-alloy samples

also stated that the reduction in the hardness was mainly
caused by recrystallization due to the application of ele-
vated temperatures.
15
Table 1 lists the results of the stud-
ies on the mechanical properties of alloys at various tem-
peratures.
1,16,17
According to the table, the tensile strength
and ultimate tensile strength decreased as the tempera-
ture increased. These results show that our research was
consistent with earlier alloy studies.
Table 1: Studies on mechanical properties of alloys at different tem-
peratures
Authors
Temperature
(°C)
Tensile
strength (MPa)
Ultimate ten-
sile strength
(MPa)
Tang, Long,
and Yang
1
year 2000
No annealing 36.0
–
75 35.0
150 27.0
210 27.3
230 18.9
Filizzolab et
al.
16
year 2001
50 29.0
–
100 30.0
150 28.7
200 28.0
250 20.0
300 22.0
Tang et al.
17
year 2020
100
–
479.2
175 461.6
200 457.8
225 451.3
250 444.3
300 422.6
350 379.4
400 376.7
However, a difference between the Sn-0.7Cu sol-
der-alloy samples treated at 30 °C and 150 °C could be
clearly observed, as shown in Figure 5. The thermo-
mechanically treated samples showed an approximately
14 % difference between their hardness values, indicat-
ing a greater stability than that of the control samples,
which showed an almost four times higher difference of
54 %.
Hardness is a measure of a material’s resistance to
plastic deformation.
18
The definition of hardness in a
nanoindentation test is the resistance of a sample to the
indenter as it penetrates the sample surface when a load
is applied. This indicates that a decrease in the indenta-
tion depth signifies an increase in the localized hardness.
The surface-modified layer of 18CrNiMo7-6 steel after
case hardening was investigated with nanoindentation,
and it was found that an increase in the hardness affected
the nanoindentation depth.
19
The maximum depth results
(Figure 6) in this study were consistent with the local-
ized hardness properties as an increase in the hardness
made it more difficult for the indenter tip to penetrate the
surface of a sample. For instance, for a sample with 80 %
compression and a hardness of 194 MPa, a maximum
depth of 1451 nm was achieved, while a hardness value
of 167 MPa resulted in a maximum depth of 1558 nm.
This was due to the occurrence of softening caused by
microstructural changes influenced by thermodynamic
conditions such as temperature.
18
Figure 7 shows that the reduced modulus of
Sn-0.7Cu decreased with the increasing temperature. The
reduced modulus of the control sample showed more sig-
F. A. M. YUSOFF et al.: TEMPERATURE-INDUCED THICKNESS REDUCTION OF MICROMECHANICAL PROPERTIES ...
376 Materiali in tehnologije / Materials and technology 56 (2022) 4, 373–379
Figure 7: Reduced modulus against temperature for the Sn-0.7Cu al-
loy at different conditions
Figure 5: Comparison of the hardness values for Sn-0.7Cu solder-al-
loy samples treated at 30 °C and 150 °C
Figure 6: Maximum depth for the Sn-0.7Cu solder-alloy sample after
heat treatment and thermomechanical treatment

nificant changes than the samples with 80 % compres-
sion. The reduced modulus of the control sample was
149 GPa at 30 °C and it decreased to 51 GPa at 150 °C,
which was a change of approximately 66 %. The reduced
modulus of the Sn-0.7Cu samples with 80 % compres-
sion showed smaller changes of approximately 24 %, de-
creasing from 103 GPa at 30 °C to 78 GPa at 150 °C.
According to one study, the reduced modulus is related
to the intrinsic properties rather than the microstruc-
ture.
10
These significant changes of the control sample
could have been due to abrupt changes in the intrinsic
properties or crystallographic orientation after the ther-
mal treatment. This conclusion is supported by a study
on high-entropy alloys (HEAs), which form FCC phases
when the reduced modulus drops sharply.
20
The small
changes in the reduced modulus values for the Sn-0.7Cu
solder alloy with 80 % compression indicated little or no
change in the intrinsic properties.
4 DISCUSSION
The effect of microstructural evolution on the micro-
mechanical properties of the Sn-0.7Cu solder alloy was
examined through a microstructural examination. Differ-
ent trends for the hardness values of the control samples
and the thermomechanically treated samples were due to
the formation recrystallization (RX) of the grains. Fig-
ures 8 and 9 show IFM images of the samples after the
heat and thermomechanical treatments at 30 °C and
120 °C, respectively. The microstructure of the solder al-
loy at 120 °C was selected as the starting point for hard-
ening. The bright-field image shows  -Sn (tin-rich)
phase grains, while the dark-field image shows Cu
6
Sn
5
(eutectic) phase grains.
21
Before the heat treatment, the
microstructure of the Sn-0.7Cu solder alloy had distinct
 -Sn grains with large gaps between them (Figure 8a).
A further increase in the temperature during the heat
treatment resulted in an increase in the number of  -Sn
grains (Figure 8b).
In general, during thermal activation such as heat
treatment, the material under consideration tends to
transform to a lower-energy state through a sequence of
microstructural changes. There are three phases in this
process: recovery, recrystallization and grain growth. Be-
fore the heat treatment, the Sn-0.7Cu samples contained
defects such as dislocations, which were sparsely distrib-
uted in the microstructure. When the control sample was
heat-treated at a higher temperature (120 °C), rapid re-
covery occurred. The extinction and rearrangement of
dislocations occurred, some of them merged, leading to a
F. A. M. YUSOFF et al.: TEMPERATURE-INDUCED THICKNESS REDUCTION OF MICROMECHANICAL PROPERTIES ...
Materiali in tehnologije / Materials and technology 56 (2022) 4, 373–379 377
Figure 9: IFM images of Sn-0.7Cu after thermomechanical treatment at: a) 30 °C, b) 120 °C
Figure 8: IFM images of Sn-0.7Cu after heat treatment at: a) 30 °C, b) 120 °C

decrease in the dislocation density. After the recovery
was complete, nucleation occurred at the grain bound-
aries, and the grains began to grow to form a new grain
structure. As the grains grew, the dislocations at the
boundaries of the newly formed grains were obliterated.
Consequently, the hardness of the Sn-0.7Cu sample de-
creased significantly, to 161 MPa at 120 °C.
Thermomechanical treatment with 80 % compression
caused the  -Sn phase and Cu
6
Sn
5
particles to be
strongly deformed perpendicular to the compression di-
rection, forming elongated  -Sn and Cu
6
Sn
5
, as shown in
Figure 9. At 80 % compression, a new dislocation
formed, which was trapped by the existing dislocations
in the sample. Consequently, the sample became thermo-
dynamically unstable. Cu
6
Sn
5
particles, sparsely distrib-
uted in the microstructure, acted as the barriers to the
movement of mobile dislocations, resulting in the accu-
mulation of dislocations and a high dislocation density.
22
The thermomechanical treatment at the low temperature
(30 °C) caused the highly accumulated dislocations to
undergo cross-slip or rearrangement, resulting in the
early formation of low-angle grain boundaries (LAGBs)
or subgrains, as shown in Figure 9a. When the tempera-
ture was further increased to 120 °C during the thermo-
mechanical treatment, these subgrain boundaries were
retarded by the Cu
6
Sn
5
particles, and dislocations were
continuously included in these boundaries. Finally, the
LAGBs transformed into high-angle grain boundaries
(HAGBs), i.e., new recrystallized grains were formed
(Figure 9b). The new recrystallized grains resulted in a
more refined microstructure, increasing the hardness of
the Sn-0.7Cu solder alloy to 166 MPa. In their work on
SAC305, Long et al. claimed that the rapid release of
heat during the loading or compression process aided the
refinement of grains.
23
The Sn-0.7Cu solder alloy subjected to thermo-
mechanical treatment with 80 % compression displayed
the best stability among the samples, according to a
micromechanical investigation utilizing the nanoinden-
tation method. Consequently, the thermomechanical link
with the micromechanical properties of the solder alloy
is genuine, which is particularly beneficial for predicting
its quality.
5 CONCLUSIONS
The relationship between the thermomechanical
treatment and mechanical properties of the Sn-0.7Cu sol-
der alloy was successfully investigated using the nano-
indentation approach. The results showed that the hard-
ness and reduced modulus of the control sample
decreased by approximately 54 % and 66 %, respec-
tively. The solder alloy with 80 % compression showed
minor changes in the localized hardness and reduced
modulus of approximately 14 % and 24 %, respectively.
These smaller changes in the micromechanical properties
were associated with the formation of new recrystallized
grains. These results suggest that thermomechanical
treatment can alter the microstructure and micro-
mechanical properties of Sn-0.7Cu alloys and help pre-
dict the potential and performance of solder alloy materi-
als in the field of electronic packaging.
Acknowledgment
The authors would like to acknowledge the financial
support of the Ministry of Higher Education, Malaysia
(grant number FRGS/1/2019/STG07/UKM/03/1) and the
National University of Malaysia (UKM) for the research
facilities support.
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