The adsorption of Ag3Sn nano-particles on Cu-Sn intermetallic compounds of Sn-3Ag-0.5Cu/Cu during soldering

Article abstract of DOI:10.1016/j.jallcom.2009.11.131

The interfacial reactions between Sn-3Ag-0.5Cu solder and Cu substrate at 250 °C and 300 °C from 30 s up to 1800 s were investigated. The average size of the Ag₃Sn nano-particles adsorbed on the Cu₆Sn₅ intermetallic compounds as a function of soldering temperature and time were studied. The results show that the adsorption of Ag₃Sn particles occurs during solidification process and the number of Ag₃Sn particles increases with the morphology changing of Cu₆Sn₅ intermetallic compounds. When soldering at a fixed temperature, the average sizes of the Ag₃Sn nano-particles are almost the same with the different soldering durations. However, the Ag₃Sn nano-particles observed on intermetallic compounds are smaller at 300 °C than those at 250 °C. The Ag₃Sn particles arrange in line in the same plane and the arrangements of Ag₃Sn lines order two perpendicular orientations, but there are no relationship with the Cu₆Sn₅ surface orientations.

Full text of DOI:10.1016/j.jallcom.2009.11.131

Journal of Alloys and Compounds 492 (2010) 433–438
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
The adsorption of Ag3Sn nano-particles on Cu–Sn intermetallic compounds of
Sn–3Ag–0.5Cu/Cu during soldering
Xiaoying Liua, Mingliang Huanga, Yanhui Zhaoa, C.M.L. Wub, Lai Wanga,∗
a Department of Materials Engineering, Dalian University of Technology, Dalian, Liaoning 116024, China
b Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
The interfacial reactions between Sn–3Ag–0.5Cu solder and Cu substrate at 250 ◦C and 300 ◦C from 30 s
up to 1800 s were investigated. The average size of the Ag3Sn nano-particles adsorbed on the Cu6Sn5
intermetallic compounds as a function of soldering temperature and time were studied. The results
show that the adsorption of Ag3Sn particles occurs during solidification process and the number of Ag3Sn
particles increases with the morphology changing of Cu6Sn5 intermetallic compounds. When soldering
at a fixed temperature, the average sizes of the Ag3Sn nano-particles are almost the same with the
different soldering durations. However, the Ag3Sn nano-particles observed on intermetallic compounds
are smaller at 300 ◦C than those at 250 ◦C. The Ag3Sn particles arrange in line in the same plane and the
arrangements of Ag3Sn lines order two perpendicular orientations, but there are no relationship with the
Cu6Sn5 surface orientations.
Article history:
Received 5 October 2009
Received in revised form
18 November 2009
Accepted 18 November 2009
Available online 22 November 2009
Keywords:
Sn–3Ag–0.5Cu
Cu6Sn5
Ag3Sn
© 2009 Elsevier B.V. All rights reserved.
Intermetallic compound (IMC)
Adsorption
1. Introduction
solder would strengthen the solder matrix [12,13]. Besides these
bulk Ag3Sn compounds, researchers also found some nano-sized
Currently there is an increasing trend to use lead-free solders in
microelectronic devices due to environmental issues [1]. In order
to meet the required mechanical properties, adequate wettabili-
ties as well as the comparable melting temperature, eutectic Sn–Ag
and Sn–Ag–Cu lead-free solders have been considered to replace
lead–tin ones [2–4]. The microstructures of these lead-free sol-
ders are different from that of eutectic Sn–Pb. Some dispersion
phases, such as Ag3Sn and Cu6Sn5, distribute uniformly in the sol-
der matrix. A survey of the literature showed that three types of
Ag3Sn compounds during solidification at different cooling rates
were found to be particle-like, needle-like and plate-like [5]. Dif-
ferent cooling rates and Ag contents resulted in different types
of Ag3Sn. Especially, plate-like Ag3Sn has been widely studied
in recent investigations. The plate-like Ag3Sn could influence the
growth and morphology of Cu6Sn5 IMC and the stability of the sol-
der joint [6–8]. Large Ag3Sn plates could adversely affect the plastic
deformation properties of the solder [9] and caused plastic-strain
localization at the boundary between the Ag3Sn plates and the
bounding ␤-Sn phase [10]. In practice, many researchers believed
that large Ag3Sn platelets exhibited a mixture of both ductile and
brittle fractures [9,11], while fine Ag3Sn particles formed in the
Ag3Sn particles adsorbed on Sn–Ag–Cu/Cu or Ni interface after sol-
dering and aging. Yu et al. [14] has reported that nano-sized Ag3Sn
particles were present on the surface of IMCs when Ni and Cu sub-
strates were dipped into the molten Sn–3.5Ag–0.7Cu and Sn–3.5Ag
solders. The existence of these particles would decrease the inter-
facial energy and suppress the growth of the IMC layer. Wu and
Huang [15] have found that tiny Ag3Sn particles were embedded
in the Ni3Sn4 or Cu6Sn5 grains during aging. Qi et al. [16] have
investigated the different formation mechanisms of Ag3Sn both in
the solder and near the interface under thermal shearing cycling
condition. They found that the Ag3Sn particles formed first on ␩-
Cu6Sn5 substrate and the coarsening of Ag3Sn would inhibit the
further growth of Cu6Sn5 in thermal cycling. So the shape and dis-
tribution of Ag3Sn phase in the solder joints are critical issues for
the reliability of solder joints. However, the formation mechanism
and influencing factors for nano-sized Ag3Sn particles are unclear.
According to this issue, the adsorption of Ag3Sn nano-particles on
Cu–Sn IMCs at different soldering temperatures, soldering time and
cooling rate are studied in the present work.
2. Experimental procedures
Commercial Sn–3Ag–0.5Cu powders with the size of 30–45 ␮m were mixed
with RMA (Rosin Mildly Active) flux to form solder paste. For the substrate, 99.9%
pure polycrystalline Cu with the size of 8 mm × 8 mm × 0.1 mm and single crystal
Cu disk of Ø8 mm × 0.1 mm were used. After polishing, the coupons were degreased
∗
Corresponding author. Tel.: +86 411 84707636; fax: +86 411 84709284.
E-mail address: wangl@dlut.edu.cn (L. Wang).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.11.131

434
X. Liu et al. / Journal of Alloys and Compounds 492 (2010) 433–438
The soldering durations were 30 s, 60 s, 120 s, 300 s, 600 s and 1800 s, respectively.
In order to observe the morphologies of IMCs, the bulk solder was removed by deep
etching in 10 vol.% HNO3 disolution with ultrasonic wave. The morphologies of the
solder joints as well as the IMCs interface were studied using a scanning electron
microscope (SEM). Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD)
were also used to analyze the composition of selected area.
3. Results
3.1. The adsorption of Ag3Sn particles
The three-dimensional structure of the interfacial ␩-Cu6Sn5 was
observed by selectively etching the bulk solder. Fig. 2 shows the
SEM top views of the IMCs on Sn–3Ag–0.5Cu/polycrystalline Cu
interface soldered for different times at 250 ◦C. It is clear that
the scallop-shaped ␩-Cu6Sn5 grains with the size of 1.5 ␮m were
formed at the interface soldered for 30 s (Fig. 2A). Meanwhile some
light-colored nano-particles which were estimated to be Ag3Sn
(shown in Fig. 3) with the size of 200 nm were found around the
IMCs. With the increasing soldering time, the bigger Cu6Sn5 com-
pounds incorporated the smaller ones sideward to form bigger
scallop-shaped Cu6Sn5 compounds in a ripening process and the
Fig. 1. Temperature profiles for a solder bump during reflow. The cooling rate is
about 4 K s−1
.
in a 10 vol.% water solution of HNO3, followed by cleaning in ethanol and drying in
air. Patches of solder paste, with a diameter of 5 mm and a height of 1 mm, were
laid on the Cu substrates, put into the oven with the soldering profile shown in
Fig. 1 and then cooled in air. The soldering temperatures were 250 ◦C and 300 ◦C.
Fig. 2. The SEM images of the IMC of Sn–3Ag–0.5Cu/polycrystalline Cu soldering at 250 ◦C: (A) 30 s, (B) 60 s, (C) 120 s, (D) 300 s, (E) 600 s and (F) 1800 s.

X. Liu et al. / Journal of Alloys and Compounds 492 (2010) 433–438
435
morphologies of Cu6Sn5 compounds became flat. Compared with
the samples soldered for 30 s, it can be seen that more Ag3Sn parti-
cles adsorbed on the surface of IMCs soldered for 5 min. However,
the Ag3Sn phases did not have obvious changes in size with the
increasing soldering time.
Fig. 4 shows the morphology of IMCs on Sn–3Ag–0.5Cu/
polycrystalline Cu interface soldered at 300 ◦C for different times.
From SEM and XRD analysis, the compositions of IMC grains solder-
ing for different times were the same while the IMCs morphologies
changed from facet to stick-shape. The elongated stick-shaped IMCs
were found at the interface and the length of these IMCs was about
30 ␮m soldered for 30 s (Fig. 4A). It is well known that in the liquid
tin–solid copper system, more complex growth mechanisms gov-
ern the formation and morphological evolution of the interfacial
layers, giving a scalloped interface between the ␩ phase and liq-
uid tin. However, Gagliano and Fine [17] had reported that Cu6Sn5
whiskers were also observed in the solidified tin and noted to
emanate from the top of the ␩ scallops. The same phenomenon
was also observed in the present work. After soldering for 5 min,
the longer Cu6Sn5 column broke off or dissolved into the matrix
and Cu6Sn5 grains tended to form the scallop-type morphologies.
Fig. 3. XRD patterns of interfacial IMC at Sn–3Ag–0.5Cu/polycrystalline Cu joints
reflowed at 250 ◦C for 30 s.
Fig. 4. The SEM images of the IMC of Sn–3Ag–0.5Cu/polycrystalline Cu soldering at 300 ◦C: (A) 30 s, (B) 60 s, (C) 120 s, (D) 300 s, (E) 600 s and (F) 1800 s.

436
X. Liu et al. / Journal of Alloys and Compounds 492 (2010) 433–438
Fig. 6. The morphology of IMC cooling at different rate: (A) cooled in air and (B)
cooled in water.
Fig. 5. Data plots of average diameter vs. time in reaction at different temperatures:
(A) Cu6Sn5 grains and (B) Ag3Sn nano-particles.
along two perpendicular directions. In order to make sure whether
there was a relationship between the orientation of Cu6Sn5 com-
pounds and the arrangement of Ag3Sn nano-particles, the (0 0 1) Cu
single crystal was used as substrate. In the previous literature [19],
Cui had found that the scallop-type Cu6Sn5 formed on (0 0 1) single
crystal Cu at 250 ◦C, but the prism-type Cu6Sn5 generated at 300 ◦C.
So the temperature in the present work was 300 ◦C. The SEM top
view images of Cu6Sn5 compounds soldering at 300 ◦C for 5 min
were shown in Fig. 7. It is very interesting to see that the Ag3Sn
particles are arranged in a line. The plane of area 4 was parallel
to that of areas 1 and 2. The arrangement of Ag3Sn nano-particles
in area 2 was elongated along two perpendicular directions, but
it was not parallel to the arrangement in area 1. The arrangement
directions of Ag3Sn nano-particles in area 4 were parallel to that
in area 5. It indicated that the orientation of Cu6Sn5 surface had no
effect on the arrangement of Ag3Sn nano-particles. From the above
experiments, it became obvious that the arrangement directions
of Ag3Sn nano-particles were not controlled by the orientation of
Cu6Sn5 surface. The ordered structures of Ag3Sn had been formed
in liquid solder and the formation mechanism of orientation was
controlled by the growth orientation of Ag3Sn compounds in solder
matrix.
Compared with that soldering at 250 ◦C, the size of Ag3Sn particles
soldering at 300 ◦C was smaller and the number was increased. The
average sizes of Cu6Sn5 and Ag3Sn compounds were shown in Fig. 5.
3.2. The effect of cooling rate on the size of Ag3Sn nano-particles
From the experiment, it is obvious that when soldering for dif-
ferent times at a fixed temperature, increasing soldering time had
only little effect on the size of the Ag3Sn particles. In order to verify
this conclusion, the different cooling rate was used in subsequent
experiment. The soldering process of Sn–Ag–Cu alloy had been
reported in Section 2 except that the solder was cooled in water
after melting in oven. The primary cooling rate was about 30 ◦C/s,
and the melting temperature profile is the same as that shown in
Fig. 1. Fig. 6 shows the top images of Cu6Sn5 grains and Ag3Sn par-
ticles adsorption at a rapid cooling rate. The size of Ag3Sn particles
was smaller than that cooled in air. It indicated that the forma-
tion of Ag3Sn particles took place in the solidification process. It
is well known that the undercooling increased with the cooling
rate. As a result, a high nucleation rate was obtained from a high
undercooling rate.
4. Discussion
3.3. The adsorption of Ag3Sn nano-particles on (0 0 1) Cu single
crystal
In the Sn–3Ag–0.5Cu solder, precipitate Ag3Sn phase existed.
Bian et al. [20] had investigated the liquid structure of Cu–Al, Al–Ni,
etc. and reported that if IMC formed in the alloys, there would exist
Zou et al. [18] had found that the regular prism-type Cu6Sn5
grains formed on (0 0 1) Cu single crystals were elongated either

X. Liu et al. / Journal of Alloys and Compounds 492 (2010) 433–438
437
But in order to have the largest negative free energy change in a
short time, the rate of free energy change becomes more impor-
tant and the high rate of free energy change is favorable [23]. Tu
et al. has regarded that which reaction path can give the largest
rate of free energy change may depend on the morphology and
microstructure of the product phase. For example, the scallop-type
IMC had a high growth rate at the interface of Sn–Ag–Cu/Cu [24].
As shown in Figs. 2 and 4, the morphologies of the Cu6Sn5 change
obviously.
The average sizes of the Ag3Sn particles on the surface of the
␩-Cu6Sn5 were the same when soldering from 30 s to 1800 s at a
certain temperature. The effects of soldering time and tempera-
ture on the average sizes of Ag3Sn particles are quite intricate. The
change of average size and number of the Ag3Sn nano-particles
is caused by a number of factors. First, it has been found that the
presence and the disappearance of the MRO structures in molten
metals were a function of temperature and was corresponded to
the tendency towards chemical compound formation [25]. If IMCs
were formed in the alloy, the distribution of the atoms was not ran-
dom in microstructure in liquid state. The IMC structure in liquid
state can be kept in the solid state if the liquid temperature has
not exceeded the melting temperature too much. When soldering
at 250 ◦C, the soldering temperature is only 33 ◦C above the melt-
ing temperature and the affinity between Sn and Ag is higher. The
ordered groups are more dominant. This explains why soldering
at a fixed temperature, the size of adsorbed Ag3Sn particles are the
same. However, when the soldering temperature is 300 ◦C, which is
83 ◦Cabove the melting temperature, Ag3Sn structure in liquid state
is liberated and ordered Ag3Sn groups are less prevalent. Thus, the
sizes of adsorbed Ag3Sn particles are smaller than those at 250 ◦C.
Secondly, the size of the Cu6Sn5 IMCs plays an important role in
adsorbing the Ag3Sn particles. The bigger the Cu6Sn5 IMC, the larger
the interfacial energy. So in order to decrease the surface energy of
the compound, more Ag3Sn particles are to be adsorbed on the sur-
face. Based on above analysis, the number of Ag3Sn nano-particles
increases with increasing soldering time. Due to the cooling rate for
the same substrate is nearly the same with the different soldering
time, the Ag3Sn phase will have the same size. This reason explains
why the soldering time had little effect on the Ag3Sn particle size.
From Fig. 2(E) and (F) and Fig. 4(E) and (F), it can be seen that the
Ag3Sn particles arranged in lines in the same plane and the arrange-
ment of Ag3Sn particles ordered two perpendicular orientations.
This phenomenon is difficult to explain. Zou et al. [26] has reported
the regular Ag3Sn compounds with parallel edges formed between
molten Sn and (0 0 1) Ag single crystals went along two perpen-
dicular directions. Moreover the three-dimensional morphology of
Ag3Sn compounds in solder matrix still has the same phenomenon
[5]. In the present study, the arrangement of Ag3Sn nano-particles
may have the relationship with the Ag3Sn compounds in solder
matrix. They gather with the same growth orientation of Ag3Sn
compounds in liquid solder and are “captured” by IMCs during
cooling process.
Fig. 7. SEM top view images of Cu6Sn5 compounds and Ag3Sn nano-particles on
(0 0 1) Cu single crystal substrate.
not only short-range order (SRO) structure but also medium-range
order (MRO) in the liquid metal under certain condition. Because
the higher affinity between Sn and Ag as well as the lower tem-
perature there maybe not only SRO Ag3Sn groups but also MRO
Ag3Sn groups in the liquid solder, Ag atoms first react with Sn to
form Ag3Sn phase in liquid solder. During solidification process,
the Ag3Sn phase precipitate near the IMCs and are likely to be
“captured” by the IMCs.
The interface free energy of crystal G can be given by:
G = ꢀA
(1)
where ꢀ is the surface energy and equals to the surface tension
in numerical value; A is the surface area of the grain or the plane.
Due to the same surface tensions between Cu6Sn5 grains and liq-
uid solder for these joints, based on Eq. (1), the larger Cu6Sn5 grains
have higher interface free energies. So it is easy to absorb the Ag3Sn
nano-particles on the large and smooth Cu6Sn5 grains. The adsorp-
tion will decrease the surface energy of the Cu6Sn5 compounds and
retard the growth of the whole IMC layer [14].
The Gibbs absorption equation is expressed as [21]:
ꢀ
ꢁ
x
dꢀ
ꢁ = −
(2)
RT dx
T
where ꢁ is the differential concentration at unit interface between
the solute at surface and that inside the adjacent solution; x is the
gram-atom fraction; R is the general gas constant; T is the abso-
lute temperature and (dꢀ/dx)T gives the change of surface tension
with the concentration at a given temperature. If the solute reduces
the surface energy of the interface, its concentration at the inter-
face will be much higher than that inside the solution. In other
words, adsorption at the interface is likely to occur at this condi-
tion. It is well known that the larger the surface tension is, the faster
the plane grow and the more the amount of surface-active mate-
rials adsorb [22]. The Ag3Sn nano-particles play the role of surface
energy reducer for the growth of interfacial Cu6Sn5 grains during
the solidification.
5. Conclusions
The formation of Ag3Sn nano-particles and the theory of surface-
active material adsorption are used to explain the effect of the
surface energy decreasing of IMCs [14]. Although it is quite difficult
to calculate the interfacial energy, it is obvious that the adsorption
will decrease the surface energy of the compounds. Furthermore,
it is no doubt that the existence of them would restrain the growth
of the whole IMCs layer during aging process.
The morphologies of the Cu6Sn5 IMCs at different soldering
temperature are distinct from each other. The interfacial reac-
tions forming IMC layers are governed by release of free energy.
The reactions between Sn–3Ag–0.5Cu/polycrystalline or (0 0 1)
single crystal Cu substrate at 250 ◦C and 300 ◦C were investigated
in the present study. Adsorption of Ag3Sn particles was found
on the Cu6Sn5 IMCs surface during the reflowing process. The
results showed that the morphologies of the Cu6Sn5 IMCs were
changed with different soldering temperature and time. The sizes
of adsorbed Ag3Sn nano-particles were almost the same with the
different soldering durations, but the particles were smaller at high
temperature (300 ◦C) than those at low temperature (250 ◦C), and
they were also smaller when specimen were cooled in water com-

438
X. Liu et al. / Journal of Alloys and Compounds 492 (2010) 433–438
pared to those cooled in air. The Ag3Sn particles are arranged in
line in the same plane and the arrangements of Ag3Sn lines order
two perpendicular orientations, but there are no relationship with
the Cu6Sn5 surface orientation. The arrangement of Ag3Sn particles
may be affected by the growth orientation of Ag3Sn compounds
in liquid solder and the particles were “captured” by IMCs during
cooling process.
[4] C.Y. Lin, J.H. Chou, Y.G. Lee, U.S. Monhanty, J. Alloys Compd. 470 (2009) 328–331.
[5] H.T. Lee, Y.F. Chen, T.F. Hong, K.T. Shih, 2009 International Conference on Elec-
tronic Packaging Technology & High Density Packaging, 2009, p. 646.
[6] K.S. Kim, S.H. Huh, K. Sugannuma, J. Alloys Compd. 352 (2003) 226–236.
[7] H.W. Tseng, C.Y. Liu, Mater. Lett. 62 (2008) 3887–3889.
[8] F.L. Zhu, H.H. Zhang, R.F. Guan, S. Liu, J. Alloys Compd. 438 (2007) 100–105.
[9] K.S. Kim, S.H. Huh, K. Suganuma, Mater. Sci. Eng. A: Struct. 333 (2002) 106–114.
[10] D.R. Frear, J.W. Jang, J.K. Lin, C. Zhang, JOM 53 (2001) 28–33.
[11] J. Zhao, Y. Miyashita, Y. Mutoh, Int. J. Fatigue 23 (2001) 723–731.
[12] K. Zeng, K.N. Tu, Mater. Sci. Eng. R 38 (2002) 55–105.
[13] F. Guo, S. Choi, K.N. Subramanian, T.R. Bieler, J.P. Lucas, A. Achari, M. Paruchuri,
Mater. Sci. Eng. A: Struct. 351 (2003) 190–199.
Acknowledgements
[14] D.Q. Yu, L. Wang, C.M.L. Wu, C.M.T. Law, J. Alloys Compd. 389 (2005) 153–158.
[15] C.M.L. Wu, M.L. Huang, IEEE Trans. Adv. Packag. 28 (2005) 128–133.
[16] L.H. Qi, J.H. Huang, X.K. Zhao, H. Zhang, J. Alloys Compd. 469 (2009) 102–107.
[17] R.A. Gagliano, M.E. Fine, JOM 53 (2001) 33–38.
[18] H.F. Zou, H.J. Yang, Z.F. Zhang, Acta Mater. 56 (2008) 2649–2662.
[19] Y.P. Cui, M.L. Huang, 2009 International Conference on Electronic Packaging
Technology & High Density Packaging, 2009, p. 593.
This work is supported by National Key Technologies R&D Pro-
gram (2006BAE03B02-2), NSFC key program (U0734006), and Key
Laboratory Program in Liaoning Province (20060133). The authors
would like to thank Prof. Lawrence Wu at City University of Hong
Kong for his cooperation in this study.
[20] X.F. Bian, P.X. Min, X.B. Qin, M.H. Jiang, Sci. China (Series E) 32 (2002) 145–151.
[21] J. Shen, Y.C. Chen, J. Alloys Compd. 477 (2009) 909–914.
[22] J. Wang, L.G. Zhang, H.S. Liu, L.B. Liu, Z.P. Jin, J. Alloys Compd. 455 (2008)
159–163.
References
[23] C.M.L. Wu, D.Q. Yu, C.M.T. Law, Mater. Sci. Eng. R 44 (2004) 1–44.
[24] K.N. Tu, K. Zeng, Mater. Sci. Eng. R 34 (2001) 1–58.
[25] I.E. Anderson, B.A. Cook, J. Harringa, R.L. Terpatra, J. Electron. Mater. 31 (2002)
1166–1174.
[1] F. Wang, M. O’Keefe, B. Brinkmeyer, J. Alloys Compd. 477 (2009) 267–273.
[2] U. Böyük, N. Marassh, J. Alloys Compd. 485 (2009) 264–269.
[3] C.L. Wei, C.R. Kao, Proceedings of the 4th International Symposium on Elec-
tronic Materials and Packaging Conference, IEEE Press, Taiwan, China, 2002,
pp. 330–334.
[26] H.F. Zou, H.J. Yang, J. Tan, Z.F. Zhang, J. Mater. Res. 24 (2009) 2141–2144.