70 The Open Surface Science Journal, 2011, 3, 70-75
1876-5319/11 2011 Bentham Open
Low Temperature Bonding of Cu Metal through Sintering of Ag
Nanoparticles for High Temperature Electronic Application
Guisheng Zou*,1, Jianfeng Yan1, Fengwen Mu1, Aiping Wu1, Jialie Ren1, Anming Hu2 and
Y. Norman Zhou2
1Department of Mechanical Engineering & Key Laboratory for Advanced Materials Processing Technology, Ministry of
Education of P. R. China, Tsinghua University, Beijing, 100084, P.R. China
2Centre for Advanced Materials Joining, Department of Mechanical and Mechatronics Engineering, University of
Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada
Abstract: Lead-based solders bring pollution to the environment and result in health threat to humans. The preparation
and application of metallic nanoparticles provide a potential method to develop Pb-free bonding materials. In this article,
bonding of Ag-coated Cu bulks was realized through low temperature sintering by directly using the chemically-reduced
Ag nanoparticle paste and baked nanoparticle powders at 60 oC, respectively. The results indicate that the capillary flow
of paste caused a ring-like deposit on substrate coating, while this phenomenon disappeared when using the powders.
Increasing bonding temperature facilitated the sintering, and shear strengths of 20 MPa and 84 MPa was obtained at
bonding temperature of 250 oC for 30 min under 20 MPa when using the Ag paste and powders, respectively. High joint
strength value of bonded region using paste is due to the high effective bonding pressure and small effective bonded area.
Finally, for the extensive application in packaging industry especially for high temperature electronics, challenges such as
the improvement of screen and stencil printing ability of paste, avoiding bonding pressure and lowering the cost were
pointed out based on our and other researcher’s achievements. Cu nanoparticle or Cu-containing nanoparticle mixture
pastes are promising but the problems of oxidation and bonding stability must be resolved urgently.
Keywords: Ag nanoparticle paste, Ag nanoparticle powder, lead-free bonding materials, electronic packaging.
Lead-based solders are used widely as electronic
packaging materials. Howerver, Pb and its compounds result
in serious health threat to humans when deposing the Pb-
containing electonic devices in the environment. Emerging
regulations have targeted the elimination of Pb usage in
electronic assemblies in many countries [1, 2]. The
development of Pb-free solders has become imperative. For
a practical solder a low melting temperature, a high
wettability and mechanical integrity, an easy fabrication an d
an afforable cost are required. Although some Pb-free
solders are commercially available, such as Sn-Ag,Sn-Cu,
Sn-Ag-Cu, Sn-Bi alloy systems, none of them matches high
performance standards .
The development of nanojoining gives us a potential
method to find suitable Pb-free packaging materials and the
relative processing technology . It is well known that, the
decrease of size of the particles results in an increase of the
diffusion coefficient and a decrease of melting temperature
due to the large surface energy and high surface area to
volume ratio [5-8]. The size effect can be applied to prepare
nanoparticles for low temperature Pb-free interconnect in
electronics packaging and assembly. Hirose’s reseach group
*Address correspondence to this author at the Department of Mechanical
Engineering, Tsinghua University, Room 106, Welding Building, Beijing,
100084, P.R. China; Tel: +86-10-62794670; Fax: +86-10-62773637;
E-mails: email@example.com; firstname.lastname@example.org
has reported a novel bonding process of non-coated or Ni/Ag
(or Ni/Au) coated Cu bulks at temperature range of 200-400
oC for 3 min under a pressure of 1-10 MPa by using Ag
metallo-organic nanoparticle pastes. In their study, pastes
were prepared by mixing different organic solvents and the
chemically-reduced Ag nanoparticles with an organic shell
[9-11]. Based on the sintering-bonding research of small area
SiC chips without bonding pressure, Lu’s group further
investigated the bonding of large-area (>100mm2) chips at
around 275 oC under 1-5 MPa using Ag paste, which was
prepared by mixing various organic components (including
dispersant/surfactant, binder and thinner) and chemically-
reduced Ag nanoparticles . Recently, Hani et al. have
realized the bonding of fine Cu wire to Cu pellets at
temperatures below 160 oC under 5 MPa pressure for 30 min
directly using a chemically-reduced Ag nanoparticle paste
. The further reseach results of Hu and Hani et al.
comfirmed that sintered network of Ag nanoparticle paste at
only 100oC can work as bonding structures for fine Cu wire
to Cu pad applied for polymeric flexible electronics
packaging . It is worth to note that sintered joints with
Ag nanopastes can be operated at much higher surrounding
temperature compared to the soldering temperature. The
melting point of the sintered Ag layer close to 960 oC, a
melting point of Ag bulks, is much higher than the melting
point of the individual Ag nanoparticle in a paste .
In this work, two styles of Ag nanoparticle materials
were used for sintering bonding: one is original chemically-
reduced Ag nanoparticle paste similar to that used in
Low Temperature Bonding of Cu Metal through Sintering of Ag Nanoparticles The Open Surface Science Journal, 2011, Volume 3 71
reference  and another is Ag nanoparticle powders by
baking the paste at 60 oC. We compared their characteristics
and bondability. The morphology and size distribution of
nanoparticles were characterized by scanning electron
microscopy (SEM) and transmission electron microscopy
(TEM). Thermal characteristics of Ag nanoparticles were
detected using differential scanning calorimetry (DSC). The
bondability of Ag nanoparticle paste and nanoparticle
powder was evaluated by bonding Ag-coated Cu bulks. The
bonding strengths were analyzed by shearing tests. The
morphology and microstructures at the interface of the
fractured joints were also observed.
2. MATERIALS AND METHODOLOGY
Ag nanoparticles used in this study were prepared using a
chemical reduction action in aqueous solution. Silver nitrate
(AgNO3) and sodium citrate dihydrate (Na3C6H5O72H2O),
both in analytical grade, were used. Silver nitrate dissolved
in deionized water was used as precursors. After the solution
was heated to 80 oC, sodium citrate dihydrate solution, which
served as a reducing agent, was added slowly while
vigorously stirring under an ambient atmosphere.
Subsequently the mixed solution was heated to 90 oC, and
kept for 1 hour with magnetic stirring. As reduction action
occurred, th e color of the suspension solution turned from
colorless to yellow first, and then to grayish green, which
indicated the formation of Ag nanoparticles. Then the
suspension solution was cooled to room temperature. The
concentration of Ag nanoparticles was increased by
centrifugation. The Ag nanoparticle paste was extracted
using a syringe after removing the top clear water. Based on
this, Ag nanoparticle powder was prepared by baking the
concentrated paste at 60 oC. The nanoparticle powder
accumulated at the bottom of the glassware baker was
collected using a clean spade.
In order to determine the size distribution of Ag
nanoparticles, an appropriate concentrated Ag nanoparticle
suspension solution (called paste) was detected using a
malvern instrument zetasizer. Then the size and shape of the
Ag nanoparticles were analyzed by SEM and TEM. Samples
for SEM were prepared by dropping Ag nanoparticle paste
onto silicon substrates. The TEM specimens were prepared
by dripping a few drops of Ag nanoparticle paste onto a
carbon-coated copper grid and drying them in air. The
thermal property of these silver powders was characterized
by DSC methods with heating rate of 10 oC/min from 25 oC
to 300 oC in the N2 atmosphere (purity of N2 is 99.9%, 40
Ag-coated Cu metals were bonded using Ag nanoparticle
paste and powders. The specimens used for bonding
experiments consist of two Cu cylinders (Fig. 1). Ag
nanoparticle paste was supplied on the faying surfaces of
bonding specimens drop by drop. Put the smaller specimen
on the top of bigger specimen when enough Ag nanoparticle
paste was gathered. Similarly, a thin layer of Ag nanoparticle
powders was applied by sandwiched between two Cu metals.
Afterwards, these prepared specimens were heated up to the
bonding temperature in air under a bonding pressure
(bonding temperature: from 150 oC to 250oC; bonding time:
30mins). The bonding pressure (Pb) is calculated from the
equation: Pb=F/S, where F is the maximum applied force on
the specimens, and S is the lower surface area of the small
Cu metal cylinder. So the nominal bonding pressure is
20MPa (Calculated bonding pressure). The bonding strength
was evaluated by measuring the shear strength measured
using a thermal-mechanical simulator Gleeble 1500D with a
cross-head displacement speed of 5 mm/min at room
Fig. (1). Schematic illustration of the bonding specimens of big
cylinder (=10 mm, h=5 mm) and small cylinder (=6 mm, h=5
3. RESULTS AND DISCUSSION
3.1. Characteristics of Ag Nanoparticles
As shown in SEM image (Fig. 2a), most of Ag
nanoparticles are spherically shaped particles. Some
nanorods are included. The size distribution of these
nanoparticles is from 20 nm to 80 nm . This size
distribution was also confirmed by an analysis on a malvern
instrument zetasizer, in which effective diameter of
nanoparticles is 45.4 nm. TEM image (Fig. 2b) displays
these nanoparticles are covered by thin organic shells
inferred as citrate function group, and these organic shells
can prevent the sintering of nanoparticles during storage.
Fig. (2). Images of the synthesized silver nanoparticles: a-SEM and
The morphology of Ag nanoparticle powders is
confirmed by SEM image (Fig. 3a, b) and TEM image (Fig.
3c). These powders are actually many microscale plates (Fig.
3a). The micron-sized plates consist of nanoscale particles
with diameter of about 40 nm (Fig. 3b). Each nanoparticle
still has an organic shell when observed using TEM, while
no sintering of Ag nanoparticles occurs (Fig. 3c). The
formation of microscale plates is thus due to the
agglomeration of adjacent nanoparticles. It should be note
72 The Open Surface Science Journal, 2011, Volume 3 Zou et al.
that there are some differences between agglomeration and
sintering. Agglomeration is a procedure in which powder
compacts are attracted together by weak forces (Van der
Waals/electrostatic forces, etc). While the sintering is a
procedure that the material is bonded together by solid necks
of significant strength such as metallic force . It is
reasonable to reduce that these Ag nanoparticles are
agglometed together due to weak bonding, since organic
shells still cover on the surfaces. This indicates that the
nanoparticle powder can be used as a bonding material.
Ag nanoparticle powders were analyzed using DSC in a
temperature of 50 o
C to 300 oC in a flowing nitrogen
atmosphere (Fig. 4). When the temperature was increased
above 150 oC, some organic component covered on the
nanoparticles may oxidize or burn out by reacting with the
surface adsorbed oxygen or traced oxygen in N2 flow. And
the exothermic peak may be caused by this chemical
reaction. After the burn out of the organic components, the
shell of nanoparticles disappears. The nanoparticle starts to
aggregate, and therefore sintering of nanoparticle will occur.
Our recent work confirmed that the sintering of Ag
nanoparticles will start at 150 oC . From the above
analysis, it is known that the bonding temperature should be
higher than 150 oC to make sure the sintering-bonding
3.2. Morphology and Microstructure of the Joints
A ring-lik e structure appears at the surface of the joint
bonded using Ag nanoparticle paste (Fig. 5a). This ring-like
shape region, which has a brighter color, is the actual contact
area between Ag nanoparticle paste and the two Cu pellets
(Fig. 5a). At higher magnification, the microstructure of this
ring like shape region displays a dimple structure, which
suggests ductile behavior in sintered Ag nanoparticles (Fig.
5b) . This ring like shape is formed during the Ag
nanoparticle paste coating process. When the Ag
nanoparticle paste is dripped on the Cu metal surface,
simultaneous evaporation of the water occurs. At the
perimeter, all the liquid is removed and the drop shrinks. But
the radius of the drop cannot shrink, as its contact line is
pinned. To prevent the shrinkage, liquid must flow outwards.
The liquid evaporating from the edge is replenished by the
liquid from the interior. The resulting outward flow can carry
most of the dispersed Ag nanoparticles to the edge. The Ag
nanoparticles will gather at the edge of the drop and leave a
dense ring-like deposit along the perimeter .
Fig. (4). DSC curves of the as-prepared Ag nanoparticle powder.
Hani et al. reported that this coffee ring effect can be
suppressed by muti-deposition of the nanoparticles . In
that study, Ag nanoparticle paste was used to bond Cu wires
to Cu foils, and the influence of coffee ring effect was not
obvious. In this study, the coffee ring effect can not be well
suppressed by multi-deposition process.
A dense Ag sintered layer forms between the two Cu
pellets after bonding when using Ag nanoparticle powder
under a pressure of 20 MPa (Fig. 6a). The coffee ring effect
totally disappears, because these Ag nanoparticles are placed
in solid forms. It should be noted that the agglomeration of
the Ag nanoparticles will reduce the surface energy to some
extent. In this case each nanoparticle still has an organic
Fig. (3). SEM (a, b) and TEM (c) image of Ag nanoparticle powder.
50 100 150 200 250 300
Low Temperature Bonding of Cu Metal through Sintering of Ag Nanoparticles The Open Surface Science Journal, 2011, Volume 3 73
shell and no sintering of Ag nanoparticles occurs. So the
nanoparticle powders can be used as bonding materials.
Some dimple structures appear when observing the interface
microstructure of the joint at higher magnification. The
dimple surface structure indicates the formation of a good
interface (Fig. 6b). Ag nanoparticle powders can be used as
lead-free materials to bond Cu metal bulks at low
temperatures, and this proves potential useful for application
in electronic packaging industry. Nevertheless, the bonding
pressure using Ag nanoparticle paste or powders is still too
high. A further study to decrease of bonding pressure is
necessary in the future.
3.3. Mechanical Property of the Joints
The shear strength of the joints was measured when using
Ag nanoparticle paste and powders at different temperatures
(Fig. 7). Th e joint strengths in crease with the rising of
bonding temperature. The shear strength of bonding using
Ag nanoparticle powder increases to 19.6 MPa at 250 oC.
Similarly, the shear strength of the joint is increased to 84.2
MPa at bonding temperature of 250 oC using Ag nanoparticle
paste. A higher bonding temperature is beneficial to joint
formation. Higher temperatures favor the sintering process of
Ag nanoparticles, and allow for better diffusion and contact
between the Ag nanoparticles and base metal [15, 19]. The
shear strength using Ag nanoparticle paste is higher than the
reported result . The Ag nanoparticles used there are Ag
metallo-organic nanoparticles. In this study, the current Ag
nanoparticle paste is directly condensed from Ag
nanoparticle solution through centrifugation (almost pure
nanoparticles). The differences of particle size of Ag
nanoparticles and the variable organic content between the
present work and other reports may lead to different joint
strengths. Besides that, a higher bonding pressure also
enhances the joint strength.
It can be seen that joint strength using Ag nanoparticle
paste is higher than that using Ag nanoparticle powder under
Fig. (5). SEM image of interface microstructure of joint using Ag nanoparticle paste: a-lower magnification; b-higher magnification
(bonding temperature: 200 oC; calculated bonding pressure is 20 MPa; bonding time: 30 mins).
Fig. (6). SEM image of interface microstructure of joint using Ag nanoparticle powder: a-lower magnification b-higher magnification
(bonding temperature: 200 oC; calculated bonding pressure:20 MPa; bonding time:30 mins).
74 The Open Surface Science Journal, 2011, Volume 3 Zou et al.
the same conditions. This is because the actual bonding area
was smaller than the lower surface of the Cu metal cylinder
due to the coffee ring effect. This coffee ring area can be
exactly determin ed. By using the coffee ring area, the
estimated bonding pressure is about 80 MPa. Thus, the
difference of the shear strength using Ag nanoparticle paste
and Ag nanoparticle powder mainly comes from the different
processing pressure applied for bonding.
Fig. (7). Shear strength of joints which were bonded using Ag
nanoparticle paste and powder at different temperatures.
3.4. Bondability and Challenges with Nanoparticle Paste
In general, as an ideal substitute for Pb-based solders,
metallic nanoparticle paste must possess the features,
including a good sintering-ability at relatively low
processing temperatures, good flowability and viscosity for
screen and stencil printing. Our results demonstrated that
both the original chemically-reduced Ag nanoparticle paste
without any adding of other organic components and the
baked Ag powders at 60 oC can be used directly to bond the
Cu substrate at low temperature, but an appropriate pressure
must be provided in order to improve the bonding strength.
Recently, Hu et a l. reported that condensed Ag paste can
bond Cu wires to Cu electrodes at a temperature as low as
100oC, very promising for polymeric flexible electronics [13,
14]. Hirose’s group [9-11] proved that the good flowability
on Cu and Ag or Au coating was achieved when appropriate
solvents were added into the chemically-reduced Ag
nanoparticles. High quality joints were achieved when the
bonding area is smaller than the cylinder head surface of 5
mm in diameter. However, the bonding pressure generally at
5 MPa is still needed. Furthermore, up to today, few
experiments are dedicated to explore whether organic
components will inevitably remain in the sintered Ag layer
when the bonded area becomes larger such as 100~200 mm2
or more. In order to improve the bondability, Lu’s group
added three kinds of organic components into the
commercial Ag nanoparticles, such as fish oil as dispersant,
polyvinyl alcohol as binder and polymer with short
hydrocarbon chains as thinner [12, 20]. The investigation
indicated that the small area attachment of SiC chips to
substrates with Ag, Au and copper can be metallized at about
275 oC without pressure. Meanwhile, for a large-area
(>100mm2) chip attachment a designed multi-step heating
profile a pressure (about 5 MPa) is still needed to reduce the
Besides, Maruyama and Ogura et al. [21, 22] developed a
special paste to replace high-temperature Pb-rich solder,
which contains a small amount of solvent, but primarily
consists of micro-scale Ag powders with a diameter of 300
nm and alkoxide passivated Ag nanoparticles with an
average diameter of 5 nm. Using this formula, the
connection of Au-coated Si diode chips to Cu bases at 350
°C in N2 without external pressure was realized, obtaining a
diode package with electrical and thermal properties similar
to those of Pb-5Sn soldered joints. However, the shear
strengths about 12-14 MPa of joints of Cu cylinder with 5
mm in diameter to Cu plate bonded in N2, only about half of
the strength when soldered in air, and one third when
soldered under a pressure of 1 MPa. Furthermore, although
the pressureless sintering packaging has been achieved, the
relatively high sintering temperature of 350 °C, low joint
strength and N2 ambient requiremen t are still issues for
application. Further research for pressureless sintering
packaging in air at relatively low bonding temperatures such
as lower than 250 °C is imperative. The optimization of the
particle size, (dispersant/surfactant, binder and thinner) and
their amounts may be the key parameters for investigation.
Another challenge facing the extensive electronics
packaging and assembly applications involves the lack of Ag
metal and the relatively high cost of Ag nanoparticle and Ag
nanoparticle paste preparation. The synthesis of Ag
nanoparticle has been simplified by Hirose’s research group
. They exploited a novel bonding process through in situ
quick formation of Ag nanoparticles at only around 130 °C-
160 °C by the chemical reduction action of Ag2O micro-
particles with triethylene glycol as a reducing agent. Using
the paste mixture of Ag2O microparticles and triethylene
glycol, joints with a 60 MPa tensile strength of Au-coated
Cu cylindrical specimens and a dense sintered Ag layer were
obtained with the bonding conditions of 250 °C temperature
for only 5 min under a pressure of 5 MPa.
In the meantime, Morisada et al . developed another
sintering-bonding technology to reduce the cost and the Ag
ion migration, which would cause short circuit . In their
experiments, a special paste by mixing the 7.9 nm
nanoparticles with 72.0 mass %Ag, or 498.0 nm
nanoparticles with 98.4 mass %Cu and terpineol was used to
bond Cu to Cu cylinders with 5 mm in diameter. During the
paste preparation, a constant mass ratio of 3:7 was used for
the terpineol: metal. The results indicated that it was difficult
to obtain strong joints using Cu nanoparticles (actually the
average size in microscale), while the addition of Ag
nanoparticles into Cu nanoparticles significantly increased
the joint strength. A shear strength of 50 MPa and a four
time higher ionic migration resistance compared with
counter-electrodes made only of Ag nanoparticles were
obtained under the conditions of Cu50%-Ag50% metal ratio,
and 350 °C bonding temperature for 5 min under 10 MPa.
Particularly, the research group in University of Waterloo
recently realized the sintering-bonding of 500 m diameter
Cu wire to Cu pad by directly utilizing the original
chemically-reduced Cu nanoparticle paste . The joint
with a shear strength of about 14 MPa was obtained at only
200 °C under a pressure of 5 MPa. However, the oxidation
140 160 180 200 220 240 260
●--Ag nanoparticle paste
■--Ag nanoparticle powder
Low Temperature Bonding of Cu Metal through Sintering of Ag Nanoparticles The Open Surface Science Journal, 2011, Volume 3 75
and thus the bonding stability of Cu nanoparticles are not
well studied. Further systematical and profound
investigations for this are desired.
Both original chemically-reduced Ag nanoparticle paste
and the baked Ag powders at 60 o
C can be used directly to
bond the Ag-coated Cu substrates at low temperature and an
appropriate pressure. A ring-like deposit was formed when
dripping the paste on the substrate coating due to the coffee
ring effect, while no ring-like deposit appeared when using
the powders. High temperature was beneficial to sintering-
bonding, and shear strengths of 20 MPa and 84MPa were
obtained at bonding temperature of 250 oC for 30 min under
20 MPa when using the Ag paste and powders, respectively.
High joint strength value when using paste is attributed to
the high effective bonding pressure and small effective
Challenges in nanoparticle paste sintering-bonding for
packaging applications are as follows: 1) Improving the
particle dispersion, flowability and viscosity for screen and
stencil printing, 2) Greatly reducing or avoiding bonding
pressure, 3) Lowering the sintering temperature and cost.
Although the problems of oxidation and bonding stability
urgently need to be resolved, Cu nanoparticle or Cu-
containing nanoparticle mixture paste are relatively
This research was supported by the National Natural
Science Foundation of China (Grant No. 51075232) and by
Tsinghua University Initiative Scientific Research Program
(Grant No.2010THZ 02-1).
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