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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 o C, 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 o C 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.
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70 The Open Surface Science Journal, 2011, 3, 70-75
1876-5319/11 2011 Bentham Open
Open Access
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 [3].
The development of nanojoining gives us a potential
method to find suitable Pb-free packaging materials and the
relative processing technology [4]. 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;
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 [12]. 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
[13]. 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 [14]. 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 [9].
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 [13] 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.
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
3.1. 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 [15]. 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
a b
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 [16]. 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 [15]. From the above
analysis, it is known that the bonding temperature should be
higher than 150 oC to make sure the sintering-bonding
process occurs.
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) [17]. 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 [18].
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 [13]. 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.
a c
50 100 150 200 250 300
Heat Flow(W/g)
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 [9]. 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).
a b
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
organic remains.
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
[23]. 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 [24]. 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 [25]. 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
Shear Strength/(MPa)
Bonding Temperature/(oC)
--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
bonded area.
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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... Although lap joint is strictly not a pure shear joint, it is the most common joint used and reported in the literature for creep studies because of its simplicity and relevance in the real application. Similar to other [55,[63][64][65] structural materials, the creep deformation process of sintered Ag is also consisted of three stages, as shown in Fig. 7 [68]. In the transient creep stage, the shear strain rate decreased rapidly over time. ...
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Due to the miniaturization development in the electronic packaging industry and the significant thermal management requirement for high-power electronic devices, the sintered Ag has become one of the promising die-attach materials. Nevertheless, the mechanical reliability of the sintered Ag is still undergoing intensive examinations and investigations by both academy and industry. In this paper, the research progress is reviewed by focusing on the tensile, creep, and fatigue properties of sintered Ag in recent years to facilitate finite element (FE) simulations of mechanical reliability. The purpose is to obtain the mechanical reliability of the sintered Ag at a low cost by combining FE simulation. Firstly, to understand the constitutive behavior and quantify the mechanical properties as the basis of FE analysis, the stress-strain curves of the sintered Ag are adopted from tensile tests subjected to varying strain rates and temperatures. As the high temperature is the most influential factor in the service condition of die-attach materials, the relationship between constitutive parameters and temperatures is summarized. To quantify the creep behavior of packaging structures in the long-term service state, the constitutive models for creep under shear strain are addressed and the steady-state creep strain rate is emphasized. The influence of temperature and applied shear stress on creep strain rate is revealed. Regarding the effect of sintering condition on creep deformation, it is further explained that a higher applied pressure during the sintering process improves the initial shear strength of Ag lap joints and thus enhance the creep resistance against shear deformation. Finally, the fatigue behavior with damage accumulation under cyclic shear loading is reviewed by focusing on the evolutions of ratcheting response and hysteresis loop. To complement the FE predictions of mechanical reliability, the empirical damage models and fatigue life models are discussed to achieve a concise understanding
... Many experimental methods have been used to measure the melting point of nanoparticles, such as transmission electron microscopy (TEM) [2], differential scanning calorimetry (DSC) [3], nanometer scale calorimetry [4], differential thermal analysis (DTA) coupled to thermal gravimetric analysis (TGA) techniques [5], etc. It has been found that the melting points of metals with different shapes [1][2][3][4][5][6][7][8][9][10] and semiconductors [11] decrease as their thickness decreases. The first theoretical description of the size-dependent melting point of nanoparticles was in 1909 by the relation known as Gibbs-Thompson relation [12], which takes the following form: ...
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A model is proposed to calculate the melting points of nanoparticles based on the Len-nard-Jones (L-J) potential function. The effects of the size, the shape, and the atomic volume and surface packing of the nanoparticles are considered in the model. The model, based on the L-J potential function for spherical nanoparticles, agrees with the experimental values of gold (Au) and lead (Pb) nanoparticles. The model, based on the L-J potential function, is consistent with Qi and Wang's model that predicts the Gibbs-Thompson relation. Moreover, the model based on the non-integer L-J potential function can be used to predict the melting points of nanoparticles.
... Nano-silver materials have low sintering temperatures, and the sintered structures take advantage of the high melting point and excellent electrical and thermal conductivities. To date, extensive studies on nano-silver sintering have been carried out [7][8][9][10]. Joints sintered by silver nanoparticles (NPs) have been reported with excellent thermal and electrical properties compared to the commonly used Pb-free joints [11][12][13]. ...
In high power electronics packaging, sintered silver nanoparticle joints suffer from thermal-humidity- electrical-chemical joint driven corrosion in extreme environments. In this paper, we conducted aging tests on sintered silver nanoparticles under high-temperature, high-humidity, and high-sulphur conditions. The results show that: (1) the sample under the dry high-sulphur conditions at a high temperature exhibited the highest degree of sulphidation; (2) Reactive force field (ReaxFF) molecular dynamics (MD) simulations of sintered silver nanoparticle sulphidation revealed the sulphidation layer was formed by silver atoms upward migration. This work paves the way for further investigation on sintered silver nanoparticles corrosion considering multi-physics coupling effects.
... The bondability of robust joints is determined by uniformities of deposition layers during bonding with Ag NPs. The well-known "coffee ring effect" [54,55] influences the bonding of metal NPs at low temperatures. This effect occurs when the suspension particles are pushed to the wetting edge due to the different evaporation rates of colloid solutions. ...
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Metal nanoparticles (NPs) have attracted growing attention in recent years for electronic packaging applications. Ag NPs have emerged as a promising low-temperature bonding material owing to their unique characteristics. In this study, we mainly review our research progress on the interconnection of using polyol-based Ag NPs for electronic packaging. The synthesis, sintering-bonding process, bonding mechanism, and high-temperature joint properties of Ag NP pastes are investigated. The paste containing a high concentration of Ag NPs was prepared based on the polyol method and concentration. A nanoscale layer of organic components coated on the NPs prevents the coalescence of Ag NPs. The effects of organic components on the bondability of the Ag NP paste were studied. Compared to the aqueous-based Ag NP paste, the polyol-based Ag NP with the reduction of organic component can improve the bondability, and the coffee ring effect was successfully depressed due to the increased Marangoni flow. The sintering behaviors of Ag NPs during the bonding process were investigated using the classical sphere-to-sphere approach. The mechanical property of joints using this Ag paste was better than that using Pb95Sn5 solders after storage at high temperatures. The sintering–bonding technology using polyol-based Ag NPs was helpful to the low-temperature interconnection for electronic packaging applications.
... Moreover, nanometer-sized metal particles are used in the spatial investigation of different bio-molecules, like some nucleic acid, metabolite, lipid, fatty acid, peptide, glycosphingolipid, and various drug molecules, to see these molecules with high spatial resolution and sensitivity [7]. Additionally, due to unique characteristics of nanometer-sized metal particles make them well suited for appli-cations such as catalysis [8], sensors [9], solar cells [10], tracesubstance detection [11], paint [12], quantum computers [13], quantum lasers [14], energy and environment [15], textile industries [16], optical limiting devices [17], energy storage [18], single electron transistors [19], food packaging [20], whereas their tendency to unite into a solid makes them an important substance in electronic applications at lower temperature [21], drug delivery, photo-imaging, wound healing and antimicrobial and anticancer agents [22,23]. Applications of metal nanoparticles are shown in Fig. 1. ...
Biogenic synthesis of metal/metallic nanoparticles (MNPs) by reducing metal ions using secondary metabolites of plant essential oils is a single step and an eco-friendly approach. This biogenic synthesis of metal nanoparticles is somewhat fast, conducted readily at low temperatures and pressures, and a quite easy process. Metal nanoparticle synthesis using essential oils is environmentally-friendly or bio-safe. Essential oils (EOs) are complex lipids and contain a range of volatile and organic bioactive compounds, having huge applications in the medical, cosmetics and food industries. In this review, we have discussed the current developments on preparation of metal nanoparticles using essential oils and their biological properties in various applications. The various methods of nanoparticle characterizations are analyzed and potential applications of these nanoparticles are reviewed.
... The finding that decreases in particle size decrease the melting point of the particles and promote diffusion and sintering provides an idea for finding suitable die-attach approaches [3,4]: sintering of metallic nanoparticles. Sintered joints can be prepared at high temperatures by low-temperature processes and are the replacement of solder joints because the single-metal system prevents the formation of brittle intermetallic components [5][6][7][8][9]. Among metallic nanoparticles, silver (Ag) nanoparticles have attracted considerable interest, due to its excellent thermal and electrical conductivity, reliable mechanical performance, and outstanding oxidation resistance [10]. ...
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Silver (Ag) microparticle sintering bonding is a promising die-attach method for power device packaging. In this study, an ultrasonic-assisted bonding method that bonds chestnut-burr-like Ag microparticles rapidly at low temperatures is reported. Robust joints with an average shear strength of 36.2 MPa were achieved under ~ 240 °C (actual) 7 MPa in 300 s. Based on characterization of sintered microstructures obtained with different ultrasonic time and power, effects of the ultrasonic vibration were studied. Two unique microstructures, microbridges and dense layers, were generated with the ultrasonic vibration. The former achieved microparticle sintering, and the latter changed fracture mode of the joints from brittle interfacial debonding to ductile fracture. The results indicate the microbridges and dense layers enhanced the joints within a certain range and are generated due to crystallization driven by localized plastic deformation and localized high temperatures.
... Ag-Sn bimetal nanoparticles were prepared by 2 Steps: The Ag nanoparticles were synthesized by using a chemical reduction action in aqueous solution [19], and Ag-Sn bimetal nanoparticles were fabricated by electroless plating the Sn on Ag nanoparticles [20]. ...
In this paper, high strength Cu-Cu interconnections were achieved by sintering the paste of Ag-Sn bimetallic nanoparticles at low temperature. Compared with nano-Ag paste, the outer Sn coatings of the nano-Ag particles were found to be favorable for the densification of the bondline. The microstructures of Ag-Sn bimetallic nanoparticles and the bondlines under different sintereing conditions were studied in detail by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The Ag-Sn bimetallic nanoparticles bondline exhibit a high shear strength of 35.3 MPa and a low resistivity of 9.5 μΩ·cm, when sintered at 260 °C for 20 min under a pressure of 0.5 MPa. The electrochemical migration time of this sintered Ag-Sn bimetallic nanoparticles was prolonged to be ten times of that of sintered nano-Ag. This bonding technology based on Ag-Sn bimetallic nanoparticles was a promising die attach method for high temperature power device packaging.
... The measured values of shear strength of the joints are presented in Table 1. The shear strength of the Cu/Ag/Cu joints obtained in the experiments with and without a die is within the range of shear strength values achieved in joints formed with the use of silver nanoparticle pastes and silver nanopowders [14]. Joints formed in the die-assisted experiments showed a slightly higher shear strength compared with joints formed by forcing electric current directly through the stack. ...
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Silver oxalate Ag2C2O4 is an interesting compound from both fundamental and practical viewpoints. Decomposition of Ag2C2O4 results in the formation of metallic silver, whose size and morphology can be tailored for different applications. In this work, we focused on the morphological features of silver that forms upon decomposition of Ag2C2O4 in a spark plasma sintering (SPS) apparatus. While crystals heated in a conventional furnace decompose to produce silver in the form of a pseudomorph, decomposition induced by heating in a SPS chamber at a rate of 30 ºC·min-1 via a pressureless process results in the formation of foam-like porous silver. Decomposition of Ag2C2O4 was also carried out under pressure by placing the powder between copper plates and heating the assembly in a SPS die. A feasibility study has shown that Ag2C2O4 powders can be used for joining copper plates in a fast process enabled by the SPS method. Joints formed by the SPS method at 300ºC and 13 MPa were formed by silver with 500-nm crystallites; the shear strength of the joints was 45 MPa.
... Additionally, the single material system enables downscaling to form high-density fine-pitch interconnect [6]. Ag nanoparticle pastes have been developed for Ag has reputable conductivity and mechanical properties [7][8][9][10][11]. Ziyu Liu et al. revealed the thermal stability and physical mechanism of bonding using Ag nanostructures as intermediate. ...
Conference Paper
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Many power semiconductor devices now require high tolerance of current density and reliability at high temperature, therefore Cu-Cu bonding using an insert material has raised the level of concerns for its great thermal stability and conductivity. In this study, a low-pressure bonding process was developed to achieve a Cu-Cu bonding using preoxidized Cu microparticles under formic acid atmosphere. The Cu microparticles were preoxidized to generate oxide films and Cu oxide nanostructures, which were then reduced and bonded at 300 °C under formic acid atmosphere to achieve a Cu-Cu bonding. Shear strength of the Cu-Cu bondings were tested to optimize the parameters of bonding process. Fracture surfaces of the Cu-Cu bonding, as well as cross-sectional microstructures, were observed by scanning electrical microscope (SEM) and components were identified by X-ray diffraction (XRD) to investigate the bonding mechanism. The findings reveal that the oxide films and the nanostructures play key roles in this reduction bonding process, which is a promising method to obtain a Cu-Cu bonding satisfying the requirements of power device packaging.
Sintering Ag particles for connection of power devices is a hot topic due to the superior thermal and electrical Properties of Ag to conventional solder paste. However, there are still some dilemmas for the Sintering Ag particles such as high Nano/ultrafine grained structure Ag particles, requirements of Pressure and surface metallization. In this work, we realized a robust die Direct bonding aluminum (DBA) substrate with Sintering micron Ag flake particles. The bonding strength of die attachment can reach about 35 MPa under pressureless, atmospheric, and 200 °C Sintering condition. By analysis the structure of sintered Ag particles, we found the micron flakes can be sintered into a uniform porous structure as low as 200 °C. The sintered die attachment structure shows a imitate attachment Direct bonding aluminum (DBA)Transmission Electron Microscopy (TEM) observations of bonding Interface. The High-temperature storage results indicate this die attachment has excellent thermal reliability that no significant degradation occurred even after 1000 h Aging at 250 °C.
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Many important advances in technology have been associated with nanotechnology and the miniaturization of components, devices and systems. Microjoining has been closely associated with the evolution of microelectronic packaging, but actually covers a much broader area, and is essential for manufacturing many electronic, precision and medical products. Part one reviews the basics of microjoining, including solid-state bonding and fusion microwelding. Part two covers microjoining and nanojoining processes, such as bonding mechanisms and metallurgy, process development and optimization, thermal stresses and distortion, positioning and fixturing, sensing, and numerical modelling. Part three discusses microjoining of materials such as plastics, ceramics, metals and advanced materials such as shape memory alloys and nanomaterials. The book also discusses applications of microjoining such as joining superconductors, the manufacture of medical devices and the sealing of solid oxide fuel cells. This book provides a comprehensive overview of the fundamental aspects of microjoining processes and techniques. It is a valuable reference for production engineers, designers and researchers using or studying microjoining technologies in such industries as microelectronics and biomedical engineering. Reviews the basics of nanojoining including solid-state bonding and fusion microwelding. Covers microjoining and nanojoining processes such as bonding mechanisms and metallurgy, sensing and numerical modelling. Examines applications of microjoining such as the manufacturing of medical devices, and the sealing of solid oxide fuel cells.
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We have proposed a novel bonding process using silver nanoparticles. which can be alternative to lead-rich high melting point solders. The bonding mechanism of silver metallo-organic nanoparticles to bulk materials (gold and copper) is discussed based on the observations of the bonded interface using Transmission Electron Microscope (TEM). At the interface of sintered silver and bulk gold. the crystal orientation of silver corresponded to that of gold. It is thought that the epitaxial layer of silver formed through silver nanoparticles being oriented in the direction of the gold crystal. At the interface of sintered silver and bulk copper, no epitaxial layer of silver on the copper crystal formed. Though the appearance of the crystal structure of silver/copper interface is different from that of the silver/gold interface. copper as well as gold are coherent with silver. and have been Successfully bonded using the silver nanoparticles.
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The metal-to-metal bonding has been successfully achieved via the bonding process using Ag metallo-organic nanoparticles at a bonding temperature of around 300-, which can be alternative to the current microsoldering in electronics assembly using high-temperature solders. However, further reduction of bonding temperature and/or bonding pressure is needed. In the present research, a novel bonding process through in-situ formation of Ag nanoparticles instead of the filler material of the Ag metallo-organic nanoparticles has been developed. The Ag nanoparticles can form by the reduction of Ag2O particles. In this study, the Ag2O particles were mixed with triethylene glycol as a reducing agent to form a paste for bonding. The Au coated cylindrical specimens were bonded using the paste. The Ag nanoparticles formed at around 130 to 160 through the reduction process of Ag2O particles with triethylene glycol. The Ag nanoparticles were immediately sintered each other due to a great surface energy per volume. A transmission electron microscope observation revealed that the sintered Ag metallurgically bonded to the Au substrate at around 160 and a dense Ag layer formed after further heating. The tensile strength of the joint bonded at 250 under a bonding pressure of 5MPa was around 60MPa
Ag nanoparticle paste with size distribution of 20-80 nm was prepared by chemical reduction reaction. Each nanoparticle was covered with a thin organic shell which can prevent its aggregation. Micro-structural evolution was observed using scanning electron microscope (SEM). The results suggested that the sintered Ag layer was a connected porous-structure after sintered for 30min at 200°C. At sintering temperature above 250°C, some Ag grains grew up obviously. The sintering-bonding with Ag nanoparticle paste of silver plated pure coppers was performed at 250°C under 10MPa. The joint had a shear strength of 39 MPa. The microscopic analysis of sintered Ag layers from the sheared fracture appearance of joints showed that sintered Ag layer had a dense structure. There was no obvious trace of plastic distortion at the low magnification SEM images of fracture-appearance of joints. However, at high magnification SEM images displayed the dimple structure feature which is typical microstructure at fracture surface of ductile material.
Solid state sintering of Ag nanoparticles was used to bond Cu wires to Cu foils at temperatures less than 250°C. The Ag nanoparticles are coated with an organic shell to prevent sintering at room temperature. After annealing the nanoparticles at 200°C, the decomposition of the organic shell was confirmed using TGA and Raman spectroscopy. The joint strength was measured by tensile shear tests, which shows that the joint strength increases as the bonding temperature increases. Metallic bond between Ag nanoparticles and Cu was achieved with no contamination. Bonds formed by our method, was confirmed to withstand temperatures higher than the bonding temperatures.
We propose a novel bonding process using Ag metallo-organic nanoparticles as a new application of nanotechnologies. The average size of the Ag nanoparticles is around 11 nm, and each particle is covered with an organic shell. Therefore, it has the outstanding feature that each nanoparticle exists independently. However, removal of the organic shell is necessary to bring out characteristics of the nanoparticle. Its decomposition temperature measured by thermal analysis is 573 K or less. In addition, it revealed that the thermal characteristic of the organic shell differed completely from Myristyl alcohol, from which the organic shell was derived. At a low bonding temperature of 573 K at a bonding pressure of 1 or 5 MPa, Cu-to-Cu joining using the Ag nanoparticles was achieved. The shear strength of the joints was 25–40 MPa, which was significantly higher than that made using Ag fine particles of 100 nm in size. That is because the reduction of the particle size to a nano-order improved the sintering of Ag particles and the bondability to Cu. Transmission electron microscope observations revealed that metallurgical bonding could be realized at the interface between the Cu and the Ag layer sintered with Ag nanoparticles. This bonding is suggested to originate from the large surface energy contribution caused by the nano-size particles.
We investigated a new bonding technique utilizing nano-scaled particles for use in high-temperature environments. The results of our investigations revealed that the method could be used to form bonds by simultaneously applying heat and pressure. Moreover, compared to a conventional Pb-5Sn-solder bond, a nanoparticle-based bond suffered no degradation in bonding strength over an elevated-temperature holding period of 1000 h at 250 °C, and its discharge characteristics were improved (i.e., increased) threefold. It is possible to extend this bonding technique to mounting components in devices that operate in high-temperature environments, e.g., it can be used to mount components such as silicon carbide (SiC) devices, which are expected to be applied in environments with temperatures exceeding 250 °C.
Multi-level packing of spherical ceramic particles is simulated in three dimensions. The packing process is composed of two steps: particles are packed to make clusters and clusters are packed to make aggregates. Cluster rearrangement is simulated by randomly rolling each cluster over its neighbour's surface. The degree of cluster rearrangement during the cluster condensation process controls agglomerate compactness. This approach was possible using the simple assumption that clusters are spheres with radii equal to the mean interparticle distance in the clusters. The clusters which are rearranged have higher packing density than the clusters which are not rearranged. The effect of cluster rearrangement during condensation on the total compactness of agglomerates is found to be comparable to the effect of particle rearrangement during condensation.
We report a unified model, free of any adjustable parameter, for size-dependence of intrinsic diffusion activation energy of elements in crystals. It is found that as the size of the nanocrystals decreases, the diffusion activation energy of atoms decreases and the corresponding diffusion coefficient strongly increases due to the Arrhenius relationship between them, which leads to evident diffusion at the room temperature. The model prediction is in agreement with the experimental diffusion results of N into bcc Fe and Ag into Au nanoparticles.
The melting behavior of 0.1–10-nm-thick discontinuous indium films formed by evaporation on amorphous silicon nitride is investigated by an ultrasensitive thin-film scanning calorimetry technique. The films consist of ensembles of nanostructures for which the size dependence of the melting temperature and latent heat of fusion are determined. The relationship between the nanostructure radius and the corresponding melting point and latent heat is deduced solely from experimental results i.e., with no assumed model by comparing the calorimetric measurements to the particle size distributions obtained by transmission electron microscopy. It is shown that the melting point of the investigated indium nanostructures decreases as much as 110 K for particles with a radius of 2 nm. The experimental results are discussed in terms of existing melting point depression models. Excellent agreement with the homogeneous melting model is observed.