Q. WANG et al.: EFFECT OF A NANO-ZnO ADDITION ON THE WETTABILITY AND INTERFACIAL STRUCTURE ...
79–83
EFFECT OF A NANO-ZnO ADDITION ON THE WETTABILITY
AND INTERFACIAL STRUCTURE OF Sn-BASED Pb-FREE
SOLDERS ON ALUMINUM
VPLIV DODATKA NANO-ZnO NA OMO^LJIVOST IN
STRUKTURO NA FAZNI MEJI PRI SPAJKANJU ALUMINIJA
S SPAJKAMI NA OSNOVI Sn BREZ DODATKA [KODLJIVEGA
SVINCA
Qingfeng Wang
1
, Yu Ding
2
, Fengjiang Wang
2*
1
School of Naval Architecture & Ocean Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, P. R. China
2
Provincial Key Laboratory of Advanced Welding Technology, Jiangsu University of Science and Technology, Zhenjiang 212003, P. R. China
Prejem rokopisa – received: 2019-06-16; sprejem za objavo – accepted for publication: 2019-08-18
doi:10.17222/mit.2019.131
Aluminum soldering with a low-temperature Pb-free solder is very important in electronics and radiator applications. In this
paper, a nano-ZnO addition was introduced into the interfacial reaction between a Sn-3.5Ag or Sn-9Zn solder and aluminum to
improve the wettability, interfacial structure and mechanical properties of joints. A spreading test showed that the Sn-3.5Ag
solder had higher wettability than Sn-9Zn. With the nano-ZnO addition, the wettability of these two solders was obviously
improved. An analysis of the interfacial structure showed that the bonding between Sn-Zn and Al was realized with a Zn-Al
solid solution, while that between the Sn-3.5Ag solder and Al was realized with an Ag 2Al intermetallic layer. The introduction
of nano-ZnO obviously accelerated the formation of Ag 2Al of Sn-3.5Ag on Al. Finally, a shear test showed that the nano-ZnO
addition helped increase the shear strength of Al solder joints.
Keywords: aluminum soldering, wettability, interfacial reaction, shear strength, nano-ZnO
Za spajkanje aluminija, na primer radiatorjev in elektronskih sestavnih delov so zelo pomembne spajke brez dodatka {kodljivega
svinca. V ~lanku avtorji opisujejo vpliv dodatka nano-ZnO na reakcijo, ki poteka na fazni meji med spajko Sn-3,5Ag oz. spajko
Sn-9Zn in aluminijem. Ta je bil dodan za izbolj{anje omo~ljivosti, medfazne strukture in mehanskih lastnosti spoja. Test
porazdelitve spajk je pokazal, da ima spajka Sn-3,5Ag bolj{o omo~ljivost kot spajka Sn-9Zn. Dodatek nano-ZnO je, dokazano v
primeru obeh spajk, izbolj{al njuno omo~ljivost. Analiza strukture na fazni meji je pokazala, da je pri{lo v primeru spajkanja s
spajko Sn-9Zn do nastanka mo~ne vezi med Sn-Zn in Al z nastankom trdne raztopine Zn-Al, medtem ko je med spajko
Sn-3,5Ag in Al nastala intermetalna plast Ag 2Al. Uvedba nano-ZnO je o~itno pospe{ila tvorbo Ag 2Al iz Sn-3,5Ag na Al.
Nazadnje je stri`ni preizkus pokazal {e, da dodatek nano-ZnO pomaga izbolj{ati stri`no trdnost spajkanega spoja Al.
Klju~ne besede: spajkanje, omo~ljivost, reakcije na fazni meji, stri`na trdnost, nano-ZnO
1 INTRODUCTION
In the electronics industry, copper and aluminum are
the most commonly used materials for heat sinks due to
their excellent thermal conductivities. Aluminum,
especially, exhibits a low density at a low cost and a high
corrosion resistance, and it often replaces copper. To
enhance the heat transfer across diverse media, the recent
heat-sink-construction development has aimed to form
true metallurgical bonds between aluminum-based pro-
ducts using the low-temperature soldering method with
Sn-based solders because it causes less heat distortion.
Currently, the most recommended Sn-based solder is
the Sn-Zn Pb-free series.
1–6
X. Chen et al.
7–8
observed the
interfacial structure of Sn-9Zn/Al and found that the
metallurgical bonding between them was carried out due
to the dissolution of Al into the solder.To improve the
Sn-9Zn/Al joint performance, M. L. Huang et al.
3–5,
incorporated small amounts of Ag, Al or Ni into a Sn-Zn
solder to obtain higher shear strength and corrosion
resistance of the joints. Besides the Sn-Zn solder, Y . Yao
et al.
10
tried to use a Sn-3.5Ag solder, and found that the
bonding between the solder and Al was realized due to
the formation of Ag-Al intermetallic compounds
(IMCs).On the other hand, due to the poor wettability of
Sn-based solders on Al during soldering,
2,6
ultrasonic
assisting has also been introduced into Al-soldering in
recent years, which helps increase the joint strength and
expand the soldered Al-alloys from the 1000 series to
5000–7000 series.
11–16
Although Al-soldering was first illustrated by Spra-
ragen in 1940,
17
there are still some issues concerning
the wettability arising from the existence of an alumina
oxide film, deterioration of the joint strength arising
from the formation of brittle intermetallic compounds
(IMCs) at the interface and poor corrosion resistance
arising from a large difference in the electrode potential
between Al and solders.
18–20
In this paper, to improve the
Materiali in tehnologije / Materials and technology 54 (2020) 1, 79–83 79
UDK 620.1:669.1:669.48:669.71:621.791/.792 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(1)79(2020)
*Corresponding author's e-mail:
fjwang@just.edu.cn (Fengjiang Wang)

performance of a solder on Al, nano-ZnO powders were
introduced into the interfacial reaction between solders
and Al, and the wettability and interfacial structures
were investigated.
2 EXPERIMENTAL PART
The selected solders were Sn-9Zn and Sn-3.5Ag
Pb-free solders. They were fabricated from pure Sn, Zn
and Ag with a purity of 99.9 w/%. Appropriate amounts
of the raw materials were encapsulated into a quartz tube
with an evacuation system, melted at 600 °C for6ha n d
then cast into a steel mold to obtain these two solders.
The used substrate was 1060 pure Al. To effectively
clean the oxide film on Al, an alkaline-paste flux was
developed, based on references, into MSDS sheets of
commercial-aluminum solder fluxes. The main compo-
nents were N-(2-Hydroxyethyl) ethylenediamine (CAS:
111-41-1), triethanolamine (CAS: 102-71-6) and
ammonium tetrafluoroborate (CAS: 13826-83-0) with
PEG-6000 as the thickening agent. Then, about 5 w/% of
nano-ZnO powders with sizes of 1–100 nm were fully
mixed into the paste flux to complete the addition of
nano-ZnO.
Wetting was finished with a spreading test following
the standard of JIS Z 3198. The dimensions of Al-pads
were (40 × 40 × 0.5) mm. Chemical cleaning was used
for pad cleansing, which used NaOH solvents and acids
to remove contaminant residues and oxide films, res-
pectively. It was then followed by a DI-water rinse and
drying. The mass of the solder used for the wetting test
was 0.3 g. The weighted solder was melted and solidified
into a solder ball in a molten rosin, followed by ultra-
sonic cleaning with acetone. The solder ball covered
with the flux was then wetted on an Al-pad at a tempera-
ture of 250 °C for 30 s. The spreading area was
measured to evaluate the wettability of the solder on Al.
After the wetting test, the wetted samples were
cross-sectioned, mechanically grinded with waterproof
abrasive paper, polished with a 0.05-μm colloidal-silica
solution and then etched with a 3-% HNO
3
solution.
Interfacial structures were observed with an optical
microscope (OM) and a scanning electron microscope
(SEM). Energy dispersive X-ray spectroscopy (EDS)
was used for a compositional analysis.
To evaluate the effect of the nano-ZnO addition on
the mechanical properties of the solder, lap-solder joints
were prepared with two Al pieces with a size of (40 × 40
× 0.5) mm. The thickness of the solder was 0.5 mm. A
shear test was then carried out on the joints with a speed
of 0.3 mm/min.
3 RESULTS
Figure 1 presents the morphologies of the Sn-9Zn
and Sn-3.5Ag solders on Al-pads after the wetting tests
using the flux without or with the addition of nano-ZnO.
The average spreading area of the solders on the Al-pads
was then calculated and the results are shown in
Figure 2. Without the addition of nano-ZnO, the wetting
area of the Sn-9Zn and Sn-3.5Ag solders on 1060 Al was
about 60 mm
2
and 80 mm
2
, respectively. The wettability
of the Sn-3.5Ag solder was a little higher than that of the
Sn-9Zn solder during aluminum soldering. After the
addition of nano-ZnO, the wetting areas for both the
Sn-3.5Ag and Sn-9Zn solders increased to about
150 mm
2
. Therefore, the nano-ZnO addition obviously
promoted the wettability of the solder on aluminum. In
Figure 1, we also clearly see a wetting ring of a bright
color at the outer edge of the solder. M N. Popescu et
al.
21
defined it as the precursor film, which emanated
from the three-phase contact-line region. E. B. Webb et
al.
22
suggested that the precursor film provided for a
better wettability. From the morphology of the precursor
film in Figure 1, it can be concluded that the addition of
nano-ZnO results in a strong affinity of the solder with
the aluminum substrate, promoting the wettability of the
solder on an aluminum substrate.
From the binary-phase diagrams of Sn-Al, Zn-Al and
Ag-Al, it is clear that the Al atoms mainly interact with
the Ag or Zn atoms, but not with the Sn atoms. In the
Zn-Al binary system, as shown in Figure 3a, mutual dis-
solubility between Al and Zn produces a solid solution
with a wide range of the solution content of Zn in Al. In
the Ag-Al binary system, as shown in Figure 3b,A l
reacts with Ag to produce different kinds of intermetallic
compounds (IMCs). They produce different types of
metallurgical bonding at the interfaces of Sn-9Zn/Al and
Sn-3.5Ag/Al. To investigate the interfacial structures, the
wetted samples were cross-sectioned and polished.
Figure 4 shows interfacial microstructures obtained with
optical observation, in which the upper and bottom areas
Q. WANG et al.: EFFECT OF A NANO-ZnO ADDITION ON THE WETTABILITY AND INTERFACIAL STRUCTURE ...
80 Materiali in tehnologije / Materials and technology 54 (2020) 1, 79–83
Figure 1: Photographs of wetted samples on Al: a) Sn-9Zn without
ZnO, b) Sn-9Zn with ZnO, c) Sn-3.5Ag without ZnO and d) Sn-3.5Ag
with ZnO

are the solder and the aluminum substrate, respectively.
As observed in Figure 4a, the bonding between the
Sn-9Zn solder and aluminum was realized with a thin
layer. With the nano-ZnO addition, the thickness of this
interfacial bonding layer was thickened, as shown in
Figure 4b. Meanwhile, many rod-like phases of a dark
color are observed in the Sn-9Zn solder matrix. At the
Q. WANG et al.: EFFECT OF A NANO-ZnO ADDITION ON THE WETTABILITY AND INTERFACIAL STRUCTURE ...
Materiali in tehnologije / Materials and technology 54 (2020) 1, 79–83 81
Figure 3: Phase diagrams of:a) the Zn-Al and b) Ag-Al binary system
Figure 4: OM interfacial structures of wetted samples: a) Sn-9Zn
without ZnO, b) Sn-9Zn with ZnO, c) Sn-3.5Ag without ZnO and d)
Sn-3.5Ag with ZnO
Figure 2: Spreading areas of solders on Al
Figure 5: Elemental mapping at the interface of the wetted samples: a) Sn-9Zn without ZnO, b) Sn-9Zn with
ZnO, c) Sn-3.5Ag without ZnO and d) Sn-3.5Ag with ZnO

interface of the Sn-3.5Ag/Al couple, an interfacial bond-
ing area with a thinner thickness of about 2 μm was also
produced, as shown in Figure 4c. With the nano-ZnO
introduced into the reaction between the Sn-3.5Ag solder
and Al (Figure 4d), the interfacial bonding phase bet-
ween the solder and Al was obviously thickened to a
thickness of about 20 μm.
To further confirm the compositions of these inter-
facial phases, SEM observation and EDS elemental
mapping were used, and the results are shown in
Figure 5. At the interface of the Sn-9Zn solder and Cu,
the bonding between them was mainly realized with Al
and Zn, and it was not influenced by the addition of
nano-ZnO. It is clear from the EDS compositional
analysis that it was composed of Zn, Al and a small
amount of Sn, forming the Zn-Al solid solution from the
Zn-Al phase diagram. At the interface of the Sn-3.5Ag
solder and aluminum without the addition of nano-ZnO,
the interfacial phase included Ag, Al and Sn, as shown in
Figure 5c. With the nano-ZnO addition, it seems that the
addition of ZnO obviously promoted the reaction bet-
ween Sn-3.5Ag and Al, as shown in Figure 5d. The
phase at the interface was mainly composed of Ag and
Al. Moreover, Zn was also accumulated at the interface
of Ag and Al. According to the Ag-Al phase diagram, it
belonged to an IMC phase. As there were three IMC
phases in the Ag-Al system, the μ,  and  phases, we
ultrasonically cleaned the solder residue on the Al
substrate on the Sn-3.5Ag/Al wetted sample and then
used X-ray diffraction (XRD) to detect the composition
of this interfacial layer. The result is shown in Figure 6
and it can be confirmed that the IMC layer between the
Sn-Ag solder and Al was the Ag
2
Al intermetallic
compound. Therefore, as in the case of the addition of
nanoparticles to another solder,
23
nano-ZnO was accumu-
lated at the interface during the reaction between the
solder and the Al substrate, altering the composition of
the interfacial IMC.
Figure 7 shows the shear strength of Al/solder/Al lap
joints with the effect of the nano-ZnO addition. The
results show that the Sn-3.5Ag solder had a higher
strength than the Sn-9Zn solder. Moreover, the addition
of nano-ZnO allowed an increase in the strength of the
solder joints on Al. This can be attributed to the for-
mation of a thicker IMC at the interface and also the
higher strength of the Sn-3.5Ag solder.
4 DISCUSSION
In the Sn-3.5Ag solder, Ag commonly reacted with
Sn to produce the Ag
3
Sn IMC phase, which was uni-
formly distributed in the solder matrix. However, during
the Sn-Ag soldering on Al, Ag accumulated at the
interface and reacted with Al to form the Ag
2
Al IMC
phase. This can be attributed to the fact that Ag-Al had a
stronger chemical affinity than Ag-Sn. The chemical
affinity between two elements can be calculated with the
chemical bond parameter function. The function between
atoms A and B can be described as follows in Equation
(1):
24
[]  = (/), , (/), Zr X Zr X
AA BB
(1)
Where ) is the chemical bond parameter function,
Z/r is the ratio of electric charge to ionic radius and X is
the negative electricity of an atom. Accordingly, the
chemical affinity between two atoms (f) is:
f
Zr
Zr
X  (/)
(/)
A
B
+Δ (2)
The chemical affinities between atoms that occurred
in the soldering with Al can be calculated, and the results
are listed in Table 1. In the Sn-Zn/Al system, Al-Zn
exhibited a higher bonding strength than Sn-Zn, and the
interfacial phase was mainly composed of the Al-Zn
solid solution. In the Sn-3.5Ag/Al system, Al-Ag had the
maximum value of chemical affinity and was the prin-
cipal product at the interface. With the ZnO introduced
into the interfacial reaction, Zn/Ag also had a higher
affinity and, therefore, Zn was accompanied by Ag to
precipitate at the interface.
Q. WANG et al.: EFFECT OF A NANO-ZnO ADDITION ON THE WETTABILITY AND INTERFACIAL STRUCTURE ...
82 Materiali in tehnologije / Materials and technology 54 (2020) 1, 79–83
Figure 7: Shear strength of the solder joints
Figure 6: XRD result for the interface of Sn-3.5Ag/Al

Table 1: Calculated values of chemical affinities between atoms
Metallic
couple
Al/Ag Al/Zn Sn/Ag Zn/Ag Zn/Sn
(/)
(/)
Zr
Zr
A
B
6/0.79 6/2.70 1.82/0.79 2.7/0.79 2.7/1.82
 X 0.4 0.1 0.1 0.3 0.2
f 7.995 2.322 2.403 3.718 1.68
5 CONCLUSIONS
In this paper, the wettability and interfacial structures
of the Sn-9Zn and Sn-3.5Ag Pb-free solders on alumi-
num were studied. To improve the performance of solder
joints, a nano-ZnO addition was introduced into the
interfacial reaction. The following results can be drawn:
The spreading test showed that the Sn-3.5Ag solder
had a higher wettability than the Sn-9Zn solder on alu-
minum. The addition of nano-ZnO can obviously pro-
mote the wettability of both the Sn-9Zn and Sn-3.5Ag
solders.
Reaction products between the solder and Al were
determined by the solder-alloy composition and the
chemical affinity between the atoms. The Zn-Al solid
solution and Ag
2
Al intermetallic phase were obtained
with the Sn-9Zn solder and Sn-3.5Ag solder, respect-
ively. The addition of nano-ZnO accelerated the growth
of the Ag
2
Al layer at the interface of Sn-3.5Ag/Al.
The shear test showed that the Sn-3.5Ag/Al solder
joint had a higher shear strength than the Sn-9Zn/Al
joint. The nano-ZnO addition also helped improve the
shear strength of solder joints.
Acknowledgment
The authors would like to acknowledge the support
provided by the National Natural Science Foundation of
China (Grant No. 51875269).
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