Recent Progress in Rapid Sintering of Nanosilver for [622438]

micromachines
Review
Recent Progress in Rapid Sintering of Nanosilver for
Electronics Applications
Wei Liu1, Rong An1,2,*, Chunqing Wang1,2, Zhen Zheng1, Yanhong Tian1, Ronglin Xu1and
Zhongtao Wang1
1State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001,
China; [anonimizat] (W.L.); [anonimizat] (C.W.); [anonimizat] (Z.Z.);
[anonimizat] (Y.T.); [anonimizat] (R.X.); [anonimizat] (Z.W.)
2Key Laboratory of Micro-Systems and Micro-Structures Manufacturing, Ministry of Education, Harbin
Institute of Technology, Harbin 150080, China
*Correspondence: [anonimizat]; Tel.: +86-451-8641-8725
Received: 22 May 2018; Accepted: 9 July 2018; Published: 10 July 2018
/gid00030/gid00035/gid00032/gid00030/gid00038/gid00001/gid00033/gid00042/gid00045 /gid00001
/gid00048/gid00043/gid00031/gid00028/gid00047/gid00032/gid00046
Abstract: Recently, nanosilver pastes have emerged as one of the most promising high temperature
bonding materials for high frequency and high power applications, which provide an effective
lead-free electronic packaging solution instead of high-lead and gold-based solders. Although
nanosilver pastes can be sintered at lower temperature compared to bulk silver, applications of
nanosilver pastes are limited by long-term sintering time (20–30 min), relative high sintering
temperature (>250C), and applied external pressure, which may damage chips and electronic
components. Therefore, low temperature rapid sintering processes that can obtain excellent nanosilver
joints are anticipated. In this regard, we present a review of recent progress in the rapid sintering
of nanosilver pastes. Preparation of nanosilver particles and pastes, mechanisms of nanopastes
sintering, and different rapid sintering processes are discussed. Emphasis is placed on the properties
of sintered joints obtained by different sintering processes such as electric current assisted sintering,
spark plasma sintering, and laser sintering, etc. Although the research on rapid sintering processes
for nanosilver pastes has made a great breakthrough over the past few decades, investigations on
mechanisms of rapid sintering, and the performance of joints fabricated by pastes with different
compositions and morphologies are still far from enough.
Keywords: nanosilver pastes; rapid sintering; spark plasma sintering; laser sintering; electric current
assisted sintering
1. Introduction
Die-attach materials play a key role in ensuring the performance and reliability of electronic
devices [ 1–4], such as in thermal [ 5–8] and electrical management [ 9,10] for high power devices.
Die-attach materials are generally classified as conductive adhesives, solder alloys, glasses, metal
films, and metal pastes [ 5]. Nowadays, conductive adhesives [ 11–14] and tin (Sn) based solder
alloys [ 15–18] are most commonly used as die-attach materials for level-1 interconnections. However,
these materials are only suitable for low-temperature range applications due to a low value of
performance index, M(0.1–1.8 106W/m, M=K/a, where Kis the thermal conductivity, and ais
the coefficient of thermal expansion) [ 5], and low melting points (<250C) [19]. With the transition
of a microelectronic system towards high power or superpower, high density integrated circuits
and nano-structure interconnections, new die-attach materials and processes, which can realize
low-temperature sintering and high-temperature application, should be developed [ 20,21]. In addition,
a lead-free packaging process for microelectronic components and micro-systems is an inevitable trend
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Micromachines 2018 ,9, 346 2 of 17
in electronics industry [ 22–25]. Nanosilver pastes with high thermal and electrical conductivity, low
sintering temperature [ 9,26–30] and high operating temperature [ 31] have great potential to meet the
requirements of the new generation of electronics [32].
Traditional hot-pressing sintering processes for nanosilver pastes needs to apply external pressure
and complicated temperature profiles, and the processes are usually time-consuming and sometimes
require an inert gas atmosphere [ 33], which severely limit the applications of nanosilver pastes [ 34].
In this regard, many rapid sintering processes have been proposed to overcome the drawbacks of the
hot-pressing sintering processes, such as in-situ formation of nanoparticles and joints, spark plasma
sintering (SPS), laser sintering, and current assisted sintering process. Nanosilver particles can be
directly interconnected by in-situ generation methods. During the process, the nanosilver particles
will in-situ form at the bonding interfaces, and the particles will be relatively less affected by organic
carriers. As a result, sintering temperature and time of the nanoparticles can be lowered obviously.
Mu et al. obtained joints with strength of 60 MPa by using the in-situ generation method, and the
bonding parameters are 5 min at 250C with the pressure of 5 MPa [ 35]. SPS is a rapid sintering
technology developed in recent years. The SPS technology combines the effects of hot-pressing,
resistance heating, and plasma activation. Through a SPS process, joints with shear strength of 50 MPa
can be obtained when the sintering temperature is 200C and the sintering time is as short as 1 min [ 36].
Laser sintering techniques have the characters of high density of energy input and rapid heating.
Sintering of nanosilver pastes can be realized in 10 s by laser irradiation, and shear strength of the
sintered joints can reach 10 MPa [ 37,38]. Current assisted sintering technology can provide enough
heat to achieve the desired sintering temperature in a short sintering time. By using electric current
assisted sintering processes, interconnections can be accomplished within 1.4 s and shear strength of
the joints can reach 90 MPa [ 34]. In this review, the mechanism of nanosilver sintering, synthesis of
nanosilver, and recent progresses in rapid sintering of nanosilver pastes were discussed. Emphasis
was placed on the properties of sintered joints obtained by different sintering processes.
2. Sintering Mechanism of Nanosilver Particles
Nanosilver particles have attracted considerable interest as one of the most promising
interconnecting materials. Therefore, sintering mechanisms of nanosilver particles have become
a hot topic during last few decades [ 20,39–44]. Various sintering models have been developed to
explain the sintering mechanisms [45–47]. The classical sphere-to-sphere model, which has been first
described by Frenkel [ 45], reveals that the sintering process begins with rapid neck formation, and is
followed by neck growth [ 48–50]. In the initial stage, two equal-sized spheres (with radius r) come into
contact as shown in Figure 1, to form a circular neck (with radius x). Subsequently, the neck begins to
grow through different mechanisms of material transportation, which consists of volume diffusion,
grain boundary diffusion, surface diffusion, and viscous flow during the sintering process [45,47].
The sintering equations for different sintering mechanisms can be generally expressed as
follows [47]:
(x/r)n=Bt (1)
where x/ris ratio of the neck radius to the particle radius. Bis a constant which depends on the
particle size, temperature, and geometric and material terms. tis the sintering time and nis a
mechanism-characteristic exponent that is depend on the mass transport process (viscous flow: n= 2;
volume diffusion: n= 4–5; grain boundary diffusion: n= 6; surface diffusion: n= 7).

Micromachines 2018 ,9, 346 3 of 17
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Figure 1. Schematic diagram of the sphere -to-sphere model. Reproduced with permission from [49] .
In order to investigate the dominant sintering mechanism of nanosilver particles, the
relationship between neck diameter and time is established. Figure 2 shows the experimental
logarithm plots of the evolution of the interparticle neck size ratio x/r at different temperatures . The
mechanism -characteristic exponent (the valu es of inverse slope ) at the sintering temperatures of 160,
200, and 250 ° C are 6.7, 8.8, and 8.4, respectively (the mean value is 7.9). These result s indicate that
surface diffusion may be the dominant diffusion mechanism at the sintering temperature range of
160–250 ° C. When the sintering temperatures increase to 300 –350 ° C, volume diffusion is probably
the prevailing diffusion mechanism [51].

Figure 2. Neck growth kinetics during the sintering process of nanosilver particles at different
temperatures. Reproduced with permission from [51] .
Besides growth of the neck, the sintering mechanism also comprises the decomposition of the
organic coating on the silver particles. Fourier -transform infrared spectroscopy (FTIR) analysis was
performed to investigate the change of organic residues in nanosilver pastes during sintering
processes [51]. The organic materials coated on silver particles play an important role in affect ing the
sintering mechanisms. By taking the Polyvinylpyrrolidone (PVP) as an example, as the sintering
temperature is below 250 ° C, the PVP still coat s on the silver particles , and the surface diffusion is the
dominant diffusion mechanism. When the temperature is increase d above 300 ° C, the PVP is
destroyed , and the main sintering mechanism changes to volume diffusion. This indicates that the
sintering mechanisms may be related with the decomposition of organic components in the
nanosilver pastes. When alkylamine is utilized as a dispertant, the alkylamine will evaporate from
130 °C, thereby f acilitating a low temperature sintering process of nanosilver particles [52].
Based on the classical sphere -to-sphere model, Yan et al. have revealed the relationship between
the strength of joint s and the neck growth of silver particles [51]. Basically, t he strength of joint s
Figure 1. Schematic diagram of the sphere-to-sphere model. Reproduced with permission from [49].
In order to investigate the dominant sintering mechanism of nanosilver particles, the relationship
between neck diameter and time is established. Figure 2 shows the experimental logarithm plots of the
evolution of the interparticle neck size ratio x/rat different temperatures. The mechanism-characteristic
exponent (the values of inverse slope) at the sintering temperatures of 160, 200, and 250C are 6.7,
8.8, and 8.4, respectively (the mean value is 7.9). These results indicate that surface diffusion may
be the dominant diffusion mechanism at the sintering temperature range of 160–250C. When the
sintering temperatures increase to 300–350C, volume diffusion is probably the prevailing diffusion
mechanism [51].
Micromachines 2018 , 9, x FOR PEER REVIEW 3 of 17

Figure 1. Schematic diagram of the sphere -to-sphere model. Reproduced with permission from [49] .
In order to investigate the dominant sintering mechanism of nanosilver particles, the
relationship between neck diameter and time is established. Figure 2 shows the experimental
logarithm plots of the evolution of the interparticle neck size ratio x/r at different temperatures . The
mechanism -characteristic exponent (the valu es of inverse slope ) at the sintering temperatures of 160,
200, and 250 ° C are 6.7, 8.8, and 8.4, respectively (the mean value is 7.9). These result s indicate that
surface diffusion may be the dominant diffusion mechanism at the sintering temperature range of
160–250 ° C. When the sintering temperatures increase to 300 –350 ° C, volume diffusion is probably
the prevailing diffusion mechanism [51].

Figure 2. Neck growth kinetics during the sintering process of nanosilver particles at different
temperatures. Reproduced with permission from [51] .
Besides growth of the neck, the sintering mechanism also comprises the decomposition of the
organic coating on the silver particles. Fourier -transform infrared spectroscopy (FTIR) analysis was
performed to investigate the change of organic residues in nanosilver pastes during sintering
processes [51]. The organic materials coated on silver particles play an important role in affect ing the
sintering mechanisms. By taking the Polyvinylpyrrolidone (PVP) as an example, as the sintering
temperature is below 250 ° C, the PVP still coat s on the silver particles , and the surface diffusion is the
dominant diffusion mechanism. When the temperature is increase d above 300 ° C, the PVP is
destroyed , and the main sintering mechanism changes to volume diffusion. This indicates that the
sintering mechanisms may be related with the decomposition of organic components in the
nanosilver pastes. When alkylamine is utilized as a dispertant, the alkylamine will evaporate from
130 °C, thereby f acilitating a low temperature sintering process of nanosilver particles [52].
Based on the classical sphere -to-sphere model, Yan et al. have revealed the relationship between
the strength of joint s and the neck growth of silver particles [51]. Basically, t he strength of joint s
Figure 2. Neck growth kinetics during the sintering process of nanosilver particles at different
temperatures. Reproduced with permission from [51].
Besides growth of the neck, the sintering mechanism also comprises the decomposition of the
organic coating on the silver particles. Fourier-transform infrared spectroscopy (FTIR) analysis
was performed to investigate the change of organic residues in nanosilver pastes during sintering
processes [ 51]. The organic materials coated on silver particles play an important role in affecting
the sintering mechanisms. By taking the Polyvinylpyrrolidone (PVP) as an example, as the sintering
temperature is below 250C, the PVP still coats on the silver particles, and the surface diffusion is
the dominant diffusion mechanism. When the temperature is increased above 300C, the PVP is
destroyed, and the main sintering mechanism changes to volume diffusion. This indicates that the
sintering mechanisms may be related with the decomposition of organic components in the nanosilver
pastes. When alkylamine is utilized as a dispertant, the alkylamine will evaporate from 130C, thereby
facilitating a low temperature sintering process of nanosilver particles [52].
Based on the classical sphere-to-sphere model, Yan et al. have revealed the relationship between
the strength of joints and the neck growth of silver particles [ 51]. Basically, the strength of joints

Micromachines 2018 ,9, 346 4 of 17
depends on the inherent strength of the material toand percent of the bonding interface, which should
be proportional to the ratio of the effective bond area between the adjacent particles. The bonding area
between the two contacting particles ( s) is calculated as follows:
s=x2(2)
where xis the neck radius and the area of the sphere section of the initial particles ( S) is given as
follows:
S=r2(3)
where ris the initial radius. Therefore, the ratio of effective bond area ( R) in each particle is expressed
as follows:
R=s
S=x2
r2= (x
r)2
(4)
According to the transverse rupture strength model [ 53], the joint strength is also related to the
fractional density Vs, the effective number of bond Nc/and the stress concentration factor K. Thus,
it is suggested that the shear strength ( t) is expressed as follows:
t=VS(NC
K)t0R=VS(NC
K)t0(x
r)2
(5)
According to the model, the strength of the sintered nanosilver joints is proportional to the ratio of
effective bond areas between the adjacent particles. The ratio of effective bond areas usually increases
by elevating the sintering temperature and pressure, which will help to improve the strength of the
joint. Yan et al. have performed shear tests of joints sintered at different temperatures. The results
confirmed that the strength of the sintered nanosilver joints is proportional to the ratio of effective
bond areas between the adjacent particles [51].
3. Preparation of Nanosilver Particles and Pastes
According to the reaction conditions, preparation methods of nanosilver particles can be divided
into chemical reduction methods [ 54–57], micro emulsion methods [ 58–60], template methods [61–63] ,
electrochemical methods [ 64–68], light induced or photocatalytic reduction methods [ 69–71],
microwave or ultrasonic assisted methods [ 72–77], radiation reduction methods [ 78–80], and so
on. Among them, the chemical reduction method is simple, fast, and more commonly used in
preparation of nanosilver particles [ 81,82]. Therefore, nanosilver particles (less than 20 nm) are
traditionally precipitated from the silver salt solution by chemical reduction. Briefly, reducing agents
such as Ascorbic Acid [ 83], Monohydrate Hydrazine [ 84], Sodium Citrate [ 85], Dehydrate Sodium
Citrate [ 21,86], Polyvinyl Pyrrolidone [ 87–89], Ethylene Diamine Tetraacetic Acid [ 90–92], Sodium
Sulfite [ 54,93] or Sodium Borohydride [ 57,94] are added to the silver salt solution, for instance, Silver
Nitrate [ 95,96], Silver Chloride [ 97,98] or Silver Ammonia Solution [ 99,100], and then the chemical
reduction reaction will occur in a polar solvent such as Ethanol [ 101,102], Methanol [ 103,104] or
Tetrahydrofuran [ 105,106]. Finally, the nanoparticles and the solution are separated by the centrifugal
method [ 107]. Research shows that ethanol that is low-cost, environmentally friendly, and easy to
volatilize is favorable to form small and uniform spherical nanosilver particles. Once high-quality
nanosilver particles with uniform morphology and good dispersion are obtained, nanosilver pastes
can be prepared. Generally, there are approaches to prepare the nanosilver pastes preparation. One is
adding the dispersant, organic carrier, and diluent to an organic solvent, such as acetone or ethanol,
and then adding commercial nanosilver particles into them. The mixture should be dispersed evenly
by mechanical or ultrasonic assisted mixing. Finally, the organic solvent is evaporated by vacuum
heating [ 41,108]. In general, the nanosilver pastes obtained by this method require higher sintering
temperature, longer sintering time, and also need to apply high pressure during the sintering process.
This is because the sintering of nanosilver particles depend on the thermal decomposition of organic

Micromachines 2018 ,9, 346 5 of 17
carriers. However, the organic carriers are usually a long chain polymerization whose thermal
decomposition temperature is above 250C [109]. Notably, Lee et al. found that besides the dispersant,
negative pressure aging can also effectively solve the aggregation of nanoparticles, which will promote
the sintering process of nanoparticles [110].
Another method is centrifugal separation [ 111,112]. First, silver nanoparticles are repeatedly
washed to remove impurities. Afterwards, flocculant is added to destroy the balance of a solution,
and then nanosilver particles precipitate. After centrifugation, a high concentration nanosilver pastes
are obtained. The nanosilver pastes usually have lower sintering temperature as compared with the
nanosilver pastes with an organic carrier.
4. Rapid Sintering Processes of Nanoparticles
Lu et al. [ 41,113,114] are pioneers who have carried out research in the area of nanoparticle
sintering. They have utilized commercial silver particles with the average diameter of 30 nm to prepare
nanosilver pastes and then realized the interconnection between the SiC chip at 234C for about 60 min
with a certain applied external pressure. Shear strength of the joints was 17–40 MPa. In order to achieve
good sintering properties of joints, a long sintering time, high sintering temperature, and external
pressure are usually applied on the samples, which may hinder the application of the nanosilver
pastes. Therefore, new processes need to be developed to shorten the sintering time, simplify the
sintering process, and improve the sintering properties of the joints. Recently, extensive studies on
the rapid sintering processes for nanosilver have been carried out [ 34,115–117]. Processes such as
discharge plasma assisted sintering, laser sintering, and current assisted sintering cannot only enhance
the efficiency of sintering, but also improve the properties of the sintered joints. The related research is
shown below.
4.1. In-Situ Formation of Nanoparticles and Joints
Recently, Hirose et al. and Toshiaki et al. [ 118,119] have proposed a novel metal-to-metal bonding
process through the in-situ formation of silver nanoparticles with Ag 2O micro-particles. During
the bonding process, in-situ formation of silver nanoparticles has been achieved through a reaction
between the Ag 2O particles and triethylene glycol (TEG). The silver nanoparticles are relatively less
affected by organic carriers. As a result, the sintering temperature of the nanoparticles can be lowered
obviously to about 200C. Moreover, the cost of micron-sized Ag 2O particles is relatively lower than
commercial Ag nanoparticles. In a word, this process can both reduce cost and decrease the sintering
temperature of the silver nanoparticles [ 35,118,120,121]. To retard migration of the Ag ion in the joints,
Cu particles or Ag coated Cu particles were added into the mixed pastes [ 122–125]. Micron-sized
Ag2O pastes have been successfully used in a low-temperature sintering process for the connection
between silver plated copper blocks, and the sintering time can be controlled within 1 min as shown in
Figure 3 [118].
Micromachines 2018 , 9, x FOR PEER REVIEW 5 of 17
carriers. However, the organic carriers are usually a long chain polymerization whose thermal
decompo sition temperature is above 250 ° C [10 9]. Notably, Lee et al. found that besides the
dispersant, negative pressure aging can also effectively solve the aggregation of nanoparticles , which
will promote the sintering process of nanoparticles [1 10].
Another method is centrifugal separation [11 1,112]. First, silver nanoparticles are repeatedly
washed to remove impurities. Afterwards, flocculant is added to destroy the balance of a solution,
and then nanosilver particles precipitate. After centrifugation,a high concentration nanosilver
pastes are obtained. The nanosilver pastes usually have lower sintering temperature as compared
with the nanosilver pastes with an organic carrier.
4. Rapid Sintering Processes of Nanoparticles
Lu et al. [41,113,114] are pioneers who have carried out research in the area of nanoparticle sintering .
They have utilized commercial silver particles with the average diameter of 30 nm to prepare
nanosilver pastes and then realized the interconnection between the SiC chip at 234 ° C for abo ut 60
min with a certain applied external pressure . Shear strength of the joints was 17–40 MPa. In order to
achieve good sintering properties of joints, a long sintering time, high sintering temperature, and
external pressure are usually applied on the samples, which may hinder the application of the
nanosilver pastes. Therefore, new processes need to be developed to shorten the sintering time,
simplify the sintering process, and improve the sintering properties of the joints. Recently, extensive
studies on the rapid sintering process es for nanosilver have been carried out [34,115 –117]. Processes
such as discharge plasma assisted sintering, laser sintering, and current assisted sintering cannot only
enhance the efficiency of sintering, but also improve th e properties of the sintered joints . The related
research is shown below.
4.1. In -Situ Formation of Nanoparticles and Joints
Recently, Hirose et al. and Toshiaki et al. [118,119] have proposed a novel metal -to-metal
bonding process through the in -situ form ation of silver nanoparticles with Ag 2O micro -particles.
During the bonding process, in-situ formation of silver nanoparticles has been achieved through a
reaction between the Ag 2O particles and t riethylene glycol (TEG) . The silver nanoparticles are
relati vely less affected by organic carriers . As a result, the sintering temperature of the nanoparticles
can be lowered obviously to about 200 °C. Moreover, the cost of micron -sized Ag 2O particles is
relatively low er than commercial Ag nanoparticles. In a word, this process can both reduce cost and
decrease the sintering temperature of the silver nanoparticles [35,118,120,121]. To retard migration of
the Ag ion in the joints, Cu particles or Ag coated Cu particles were added into the mixed pastes [122 –
125]. Micr on-sized Ag 2O pastes have been successfully used in a low -temperature sintering process
for the connection between silver plated copper blocks, and the sintering time can be controlled
within 1 min as shown in Fig ure 3 [118].

Figure 3. Relationship between bonding parameters and tensile strength of the joints : (a) Bonding
temperature; ( b) Holding time at 250 ° C. Reproduced with permission from [ 118].

(a) (b)
Figure 3. Relationship between bonding parameters and tensile strength of the joints: ( a) Bonding
temperature; ( b) Holding time at 250C. Reproduced with permission from [118].

Micromachines 2018 ,9, 346 6 of 17
4.2. Spark Plasma Sintering
Spark plasma sintering (SPS) is a rapid sintering technology developed in recent years [ 126–128],
and the technology has many extraordinary advantages such as fast heating speed (up to 500C/min),
and short sintering time (30–300 s) [ 129–133]. In addition, pressure is usually applied in the SPS process
to help to form a better contact between nanoparticles, thereby accelerating grain boundary diffusion,
lattice diffusion, and viscous flow during the sintering process [ 134]. All of the mechanisms could help
to control the microstructure and achieve a higher density of the sintered materials. Furthermore, SPS
also has the advantages of simple operation, high reproducibility, space saving, energy saving, and
low cost [131,135].
Alayli et al. [ 36] used nanosilver particles and the SPS process to bond power semiconductor
chips with metallized substrates. Electrical and thermal properties of the samples were both better
than those sintered by conventional hot pressing processes. As shown in Figure 4, the shear strength
of the joints reached 100 MPa with the sintering parameters of 300C, 1 min, and 3 MPa. When
the sintering temperature was reduced to the range of 150–200C, shear strength of the joints was
also as high as 30–50 MPa. Munir et al. [ 131] systematically summarized the influence of different
parameters of SPS on properties of sintered samples. It was found that the heating rate (50–700C/min)
had little effect on the density of the sintered samples at the same sintered temperature and time.
However, the heating rate could influence the size of the sintered nanoparticles. By increasing the
sintering pressure, sintering temperature could be decreased, and grain growth of the joints was also
restricted. Santanach et al. [ 136] considered that the density of the sintered samples could be increased
through prolonging the sintering time. Ng et al. [ 137] believed that sintering temperature could also
affect density of the joints. Relative density of the samples almost reached 100% when the sintering
temperature was increased to 300C, as shown in Figure 5.
Micromachines 2018 , 9, x FOR PEER REVIEW 6 of 17
4.2. Spark Plasma Sintering
Spark plasma sintering (SPS) is a rapid sintering technol ogy developed in recent years [126–128],
and t he technology has many extraordinary advantages such as fast heating speed ( up to 500 ° C/min) ,
and short sintering time (30–300 s) [129 –133]. In addition, pressure is usually applied in the SPS
process to help to form a better contact between nano particles, thereby accelerating grain boundary
diffusion, lattice diffusion, and viscous flow during the sintering process [134]. All of the mechanisms
could help to control the microstructure and achieve a higher densi ty of the sintered materials.
Furthermore, SPS also has the advantages of simple operation, high reproducibility, space saving,
energy saving, and low cost [131,135].
Alayli et al. [36] used nanosilver particles and the SPS process to bond power semiconduc tor
chips with metallized substrates . Electrical and thermal properties of the samples were both better
than those sintered by conventional hot pressing processes. As shown in Fig ure 4 , the shear strength
of the joints reached 100 MPa with the sintering pa rameters of 300 ° C , 1 min, and 3 MPa. W hen the
sintering temperature was reduced to the range of 150–200 ° C, shear strength of the joints was also
as high as 30–50 MPa. Munir et al. [13 1] systematically summarized the influence of different
parameters of S PS on properties of sintered samples . It was found that the heating rate (50 –700 ° C/min)
had little effect on the density of the sintered samples at the same sintered temperature and time .
However, the heating rate could influence the size of the sintered nanoparticles. By increasing the
sintering pressure, sintering temperature could be decreased, and grain growth of the joints was also
restricted. Santanach et al. [13 6] considered that the density of the sintered samples could be
increased through prolonging the sintering time. Ng et al. [13 7] believed that sintering temperature
could also affect density of the joints. Relative density of the samples almost reached 100% when the
sintering temperature was increased to 300 ° C, as sh own in Fig ure 5 .

Figure 4. Compression tests on silver samples sintered by spark plasma sintering (SPS) at a low
pressure (3 MPa), for a short dwell time (1 min), at a 300 ° C, b 200 ° C and c 150 ° C. Reproduced with
permission from [36].

Figure 5. Relative density as a function of variable : (a) SPS temperature ; (b) hold time. Reproduced
with permission from [137].
Figure 4. Compression tests on silver samples sintered by spark plasma sintering (SPS) at a low
pressure (3 MPa), for a short dwell time (1 min), at a 300C, b 200C and c 150C. Reproduced with
permission from [36].
Micromachines 2018 , 9, x FOR PEER REVIEW 6 of 17
4.2. Spark Plasma Sintering
Spark plasma sintering (SPS) is a rapid sintering technol ogy developed in recent years [126–128],
and t he technology has many extraordinary advantages such as fast heating speed ( up to 500 ° C/min) ,
and short sintering time (30–300 s) [129 –133]. In addition, pressure is usually applied in the SPS
process to help to form a better contact between nano particles, thereby accelerating grain boundary
diffusion, lattice diffusion, and viscous flow during the sintering process [134]. All of the mechanisms
could help to control the microstructure and achieve a higher densi ty of the sintered materials.
Furthermore, SPS also has the advantages of simple operation, high reproducibility, space saving,
energy saving, and low cost [131,135].
Alayli et al. [36] used nanosilver particles and the SPS process to bond power semiconduc tor
chips with metallized substrates . Electrical and thermal properties of the samples were both better
than those sintered by conventional hot pressing processes. As shown in Fig ure 4 , the shear strength
of the joints reached 100 MPa with the sintering pa rameters of 300 ° C , 1 min, and 3 MPa. W hen the
sintering temperature was reduced to the range of 150–200 ° C, shear strength of the joints was also
as high as 30–50 MPa. Munir et al. [13 1] systematically summarized the influence of different
parameters of S PS on properties of sintered samples . It was found that the heating rate (50 –700 ° C/min)
had little effect on the density of the sintered samples at the same sintered temperature and time .
However, the heating rate could influence the size of the sintered nanoparticles. By increasing the
sintering pressure, sintering temperature could be decreased, and grain growth of the joints was also
restricted. Santanach et al. [13 6] considered that the density of the sintered samples could be
increased through prolonging the sintering time. Ng et al. [13 7] believed that sintering temperature
could also affect density of the joints. Relative density of the samples almost reached 100% when the
sintering temperature was increased to 300 ° C, as sh own in Fig ure 5 .

Figure 4. Compression tests on silver samples sintered by spark plasma sintering (SPS) at a low
pressure (3 MPa), for a short dwell time (1 min), at a 300 ° C, b 200 ° C and c 150 ° C. Reproduced with
permission from [36].

Figure 5. Relative density as a function of variable : (a) SPS temperature ; (b) hold time. Reproduced
with permission from [137].
Figure 5. Relative density as a function of variable: ( a) SPS temperature; ( b) hold time. Reproduced
with permission from [137].

Micromachines 2018 ,9, 346 7 of 17
4.3. Laser Sintering
Laser sintering techniques can realize fast sintering of joints with excellent properties as compared
with conventional hot-pressing sintering [ 22,34,138,139]. At present, laser sintering techniques have
been widely used in sintering processes of metal, ceramic, and composite materials [140].
Yu et al. [ 38] realized the bonding of a high power light-emitting diode (LED) chip
(60 mil 60 mil ) with silver nanoparticles through a laser sintering process. An infrared radiation laser
(30 W, dspot= 600 m,l= 980 nm) was utilized in the study. The whole laser sintering process was 10 s
after drying the organic solvent on a hot plate (230C, 1 min). Shear strength of the laser sintered joints
could reach 9 MPa, which was higher than those fabricated by hot-pressing sintering in a convection
oven (250C, 3 h). In addition, the LED devices showed very good performance in luminous efficiency
and reliability. Liu et al. [ 141] have realized laser sintering dieattach processes using nanosilver pastes
within 1 min. Better shear strength was obtained with increasing laser power, irradiation time, and
load. Moreover, the shear strength of joints irradiated by 2–5 min of laser beam was comparable to that
of the joints sintered by the hotplate for 80 min. Qin et al. [ 142] used a continuous wave diode pumped
solid state (CWDPSS) laser to sinter thin films composed of Ag nanoparticles. The laser sintering
process obtained a unique transparent conductive network structure due to the rapid heating and
cooling process, whereas conventional heat treatment only formed isolated silver grains during the
slow heating process, as shown in Figure 6. Liu et al. [ 143] successfully synthesized and transferred
a transparent conductive silver film via the laser sintering process. Kunsik et al. [ 144] have realized
laser sintering of nanosilver ink through a digital micro mirror (DMD) with high efficiency instead of
the traditional printing and scanning process. Habeom Lee et al. have realized fast laser sintering of
silver nanoparticle ink on plastic substrates with good properties. The laser scanning speed is 5 mm/s.
In the study, the focusing lens of laser system was modified as a micro lens array or a cylindrical
lens to generate multiple beamlets or an extended focal line. The modified optical settings are found
to be advantageous for the creation of repetitive conducting patterns or areal sintering of the silver
nanoparticle ink layer [145].
Micromachines 2018 , 9, x FOR PEER REVIEW 7 of 17
4.3. Laser Sintering
Laser sintering techniques can realize fast sintering of joints with excellent properties as compared
with conventional hot -pressing sintering [ 22,34,138 ,139]. At present, laser sintering techniques ha ve
been widely used in sintering processes of metal, ceramic, and composite material s [140].
Yu et al. [38] realized the bonding of a high power light -emitting di ode (LED) chip
(60 mil × 60 mil) with silver nanoparticles through a laser sintering process. A n infrared radiation
laser (30 W, dspot = 600 μm, λ = 980 nm ) was utilized in the study. The whole laser sintering process
was 10 s after drying the organic sol vent on a hot plate (230 ° C , 1 min) . Shear strength of the laser
sintered joints could reach 9 MPa, which was higher than those fabricated by hot-press ing sintering
in a convection oven (250 ° C , 3 h). In addition, the LED devices show ed very good performan ce in
luminous efficiency and reliability. Liu et al. [141] have realized laser sintering dieattach processes
using nanosilver pastes within 1 min. Better shear strength was obtained with increasing laser power,
irradiation time, and load. Moreover, the shear strength of joints irradiated by 2 –5 min of laser beam
was comparable to that of the joints sintered by the hotplate for 8 0 min. Qin et al. [142] used a continuous
wave diode pumped solid state (CWDPSS) laser to sinter thin films composed of Ag nanoparticles.
The laser sintering process obtained a unique transparent conductive network structure due to the
rapid heating and co oling process , whereas conventional heat treatment only formed isolated silver
grains during the slow heating process, as shown in Fig ure 6 . Liu et al. [143] successfully synthesized
and transferred a transparent conductive silver film via the laser sinter ing process . Kunsik et al. [144]
have realized laser sintering of nanosilver ink through a digital micro mirror (DMD) with high
efficiency instead of the traditional printing and scanning process. Habeom Lee et al. have realized
fast laser sintering of silver nanoparticle ink on plastic substrates with good properties. The laser
scanning speed is 5 mm/s. In the study, the focusing lens of laser system was modified as a micro lens
array or a cylindrical lens to generate multiple beamlets or an extended focal line. The modified
optical settings are found to be advantageous for the creation of repetitive conducting patterns or
areal sintering of the silver nanoparticle ink layer [145].

Figure 6. Scanning electron microscope (SEM) micrographs of nanosilver films : (a) heat treatment in
air; (b) laser sintering. Reproduced with permission from [14 2].
Yu et al. [146–149] compared the effects of laser type, wavelength, and power on the electrical
prop erties and surface morpholog ies of sintered nano thin film. The result s showed that the
picosecond pulse d laser did less damage to the substrate as compared with the nanosecond pulse d
laser and continuous laser. In addition, resistivity of sintered nano th in film decreased gradually , and
particle size became larger with the increase of the laser power. Cheng et al. [37] simulated the
ultrafast melting and re -solidification process of nanoparticles through a one -dimensional , two-
temperature model. The results obtained from the model were in good agreement with the
experimental data. Huang et al. [150] studied the effects of d ifferent particle size and laser frequency
on the phase change s of the particles, including melting, vaporization, and re -solidifi cation. Choi et
al. [151] measured the in -situ electrical resistance of laser sintered inkjet -printed ink to study its
thermal conductivity with the Wiedemann –Franz law. It was found that thermal conductivity of the
sintered inkjet -printed ink would increa se with the increase of laser input energy . Moreover, the
thermal conductivity was also related with surface morpholog ies of the aggregated nanoparticles.
(a) (b)
Figure 6. Scanning electron microscope (SEM) micrographs of nanosilver films: ( a) heat treatment in
air; ( b) laser sintering. Reproduced with permission from [142].
Yu et al. [ 146–149] compared the effects of laser type, wavelength, and power on the electrical
properties and surface morphologies of sintered nano thin film. The results showed that the picosecond
pulsed laser did less damage to the substrate as compared with the nanosecond pulsed laser and
continuous laser. In addition, resistivity of sintered nano thin film decreased gradually, and particle
size became larger with the increase of the laser power. Cheng et al. [ 37] simulated the ultrafast
melting and re-solidification process of nanoparticles through a one-dimensional, two-temperature
model. The results obtained from the model were in good agreement with the experimental data.
Huang et al. [ 150] studied the effects of different particle size and laser frequency on the phase changes
of the particles, including melting, vaporization, and re-solidification. Choi et al. [ 151] measured the
in-situ electrical resistance of laser sintered inkjet-printed ink to study its thermal conductivity with
the Wiedemann–Franz law. It was found that thermal conductivity of the sintered inkjet-printed ink

Micromachines 2018 ,9, 346 8 of 17
would increase with the increase of laser input energy. Moreover, the thermal conductivity was also
related with surface morphologies of the aggregated nanoparticles.
Up to date, different types of laser have been used in the sintering process of silver nanoparticles.
However, the mechanism of laser sintering still requires further study.
4.4. Current Assisted Sintering
Current assisted sintering technology can provide enough heat to achieve the desired sintering
temperature in a short sintering time [ 152–157], which will restrain the coarsening of nanoparticles
during the sintering process and then the fine microstructures of the joints, thereby making the joints
possess good mechanical properties [ 158]. The shear strength of the joints fabricated by the current
assisted sintering process could reach about 90 MPa with the parameters of 8.25 kA of current density
for 1400 ms [ 159]. Mei et al. [ 114,160] used this technology to interconnect copper substrates with silver
nanoparticles. Shear strength of the joints could reach 40 MPa within 1 s current assisted sintering.
Moreover, the joints had better mechanical fatigue performance than those fabricated by traditional
hot-pressing sintering methods [ 161]. Figure 7 shows that the shear strength of the current assisted
sintered joints would increase when the current and sintering time was increased, and the maximum
strength could reach 96.7 MPa. Figure 8 shows fracture surfaces of the joints. Microstructures of the
joints became denser when the current was increased. Li et al. [ 34] found that the shear strength of
the sintered joints was closely related to the peak temperature of the sintering process. Xie et al. [ 162]
achieved robust bonding of large chips (>100 mm2) with nanosilver by current assisted sintering
within 10 s. Moreover, thermal resistance and density of the joints could reach 0.18C/W and 89.6%,
respectively. Transmission electron microscopy (TEM) results indicated that the better performances
of the chip and joints were attributed to the high density of twins in the joints formed in the current
assisted sintering process. Mei et al. [ 163] realized a current assisted sintering of nanosilver paste
within 1200 ms, and the strength of the joints could reach 50 MPa. The current assisted sintering
process could be divided into three stages: rearrangement of adjacent nanosilver particles, liquid phase
assisted densification, and densification by plastic deformation and elimination of crystal defects.
Micromachines 2018 , 9, x FOR PEER REVIEW 8 of 17
Up to date , different types of laser have been used in the sintering process of silver nanoparticles .
However, the mechanism of laser sintering still requires further study .
4.4. Current Assisted Sintering
Current assisted sintering technology can provide enough heat to achieve the desired sintering
temperature in a short sintering time [15 2–157], which will restrain the coarsening of nano particles
during the sintering process and then the fine microstructures of the joints, thereby making the joints
possess good mechanical properties [15 8]. The shear strength of the joints fabricated by the c urren t
assisted sintering process could reach about 90 MPa with the parameters of 8.25 kA of current density
for 1400 ms [1 59]. Mei et al. [114,16 0] used this technology to interconnect copper substrates with
silver nano particles. Shear strength of the joints could reach 40 MPa within 1 s current assisted
sintering . Moreover, the joints ha d better mechanical fatigue performance than those fabricated by
traditional hot -pressing sinter ing methods [161]. Figure 7 shows that the shear strength of the current
assiste d sintered joints would increase when the current and sintering time was increas ed, and the
maximum strength could reach 96.7 MPa. Figure 8 shows fracture surface s of the joints.
Microstructures of the joints became denser when the current was increase d. Li et al. [34] found that
the shear strength of the sintered joints was closely related to the peak temperature of the sintering
process . Xie et al. [16 2] achieved robust bonding of large chips (>100 mm2) with nanosilver by c urrent
assisted sinterin g withi n 10 s. Moreover, thermal resistance and density of the joints could reach 0.18
°C/W and 89.6%, respectively. Transmission electron microscopy (TEM) results indicated that the
better performances of the chip and joints were attributed to the high density o f twins in the joints
formed in the c urrent assisted sinterin g process. Mei et al. [16 3] realized a c urrent assisted sintering
of nanosilver paste within 1200 ms, and the strength of the joints could reach 50 MPa. The c urrent
assisted sintering process cou ld be divided into three stages: rearrangement of adjacent nanosilver
particles, liquid phase assisted densification, and densification by plastic deformation and
elimination of crystal defects.
Urbański et al. [164] also employed high frequency and high v oltage (HFHV) electric energy to
sinter nanoparticles, and applied the process to print a conductive pathway with nanosilver ink.

Figure 7. Comparison of shear strength : (a) Alternating Current (AC); (b) sintering time. Reproduced
with permission from [1 61].
(a) (b)
Figure 7. Comparison of shear strength: ( a) Alternating Current (AC); ( b) sintering time. Reproduced
with permission from [161].

Micromachines 2018 ,9, 346 9 of 17
Micromachines 2018 , 9, x FOR PEER REVIEW 9 of 17

Figure 8. Fracture surface s of samples sintered with different current : (a) 5.50 kA ; (b) 6.50 kA ; (c) 7.00
kA; (d) 8.25 kA. Reproduced with permission from [1 61].
The reliability of sintered joints prepared by electric current assisted sintering and hot -press ing
sintering were evaluate d by cyclic shear test, respectively. The joints fabricated by electric current
assisted sintering are more reliable than those by hot-pressing sintering [34].
5. Conclusions and Future Prospects
This review has summarize d recent progress in the rapid sintering of nanosilver for electronics
application . Emphasis is placed upon i n-situ formation of nanoparticles and joints , spark plasma
assisted sintering, laser sintering and electr ic current assisted sintering. Shear strength and
microstructure s of sintered joints are also discussed in terms of key process parameters , such as
sintering temperature, time, current, et al . Table 1 shows the comparison of the sintering processes.
The cu rrent assisted sintering could obtain relatively high shear strength in the shortest sintering
time. The process of in-situ formation of nanoparticles and joints is economic because Ag 2O is used
as the raw material rather than the nanosilver . Spark plasma assisted sintering can obtain joints with
high density. Laser sintering has the potential in precise selective sintering, and the process is often
used to sinter nanosilver ink s to form conductive network s. Current assisted sintering is usually used
for c onnection between dissimilar materials . Moreover, the joints will have excellent shear
performance and anti -fatigue propert ies.
Table 1. Comparison of different rapid sintering methods.
Sintering Method Sintering Time Shear Strength Cost Ref.
Hot-pressing 30–90 min 30–84 MPa Low [22,16 5–167]
In-situ formation 3–5 min 50–70 MPa Low [35,11 8,120,121]
Spark Plasma 30–300 s 30–100 MPa Medium [36,12 9–132]
Laser 1–15 s 8–10 MPa High [38,16 8–170]
Current 0.1–1 s 40–97 MPa Medium [34,11 5,159–161]
Although the rapid sintering process es have many advantages as compared with conventional
hot-pressing sintering processes , there are still a lot of challenges in the application s of the processes
to electronic packaging . To promote the application of the rapid sinteri ng processes , future work
should focus on the following points:
(1) In some rapid sintering processes, the sintering time may be less than 1 s. T he sintering
mechanism of the processes may be different from that of traditional hot -press ing sintering. The
sphere -to-sphere model may not be proper to explain the sintering behavior in the rapid
sintering processes. Therefore, more work needs to be done to explore the mechanism of rapid
sintering .
Figure 8. Fracture surfaces of samples sintered with different current: ( a) 5.50 kA; ( b) 6.50 kA;
(c) 7.00 kA; ( d) 8.25 kA. Reproduced with permission from [161].
Urba´ nski et al. [ 164] also employed high frequency and high voltage (HFHV) electric energy to
sinter nanoparticles, and applied the process to print a conductive pathway with nanosilver ink.
The reliability of sintered joints prepared by electric current assisted sintering and hot-pressing
sintering were evaluated by cyclic shear test, respectively. The joints fabricated by electric current
assisted sintering are more reliable than those by hot-pressing sintering [34].
5. Conclusions and Future Prospects
This review has summarized recent progress in the rapid sintering of nanosilver for electronics
application. Emphasis is placed upon in-situ formation of nanoparticles and joints, spark plasma
assisted sintering, laser sintering and electric current assisted sintering. Shear strength and
microstructures of sintered joints are also discussed in terms of key process parameters, such as
sintering temperature, time, current, et al. Table 1 shows the comparison of the sintering processes.
The current assisted sintering could obtain relatively high shear strength in the shortest sintering time.
The process of in-situ formation of nanoparticles and joints is economic because Ag 2O is used as the
raw material rather than the nanosilver. Spark plasma assisted sintering can obtain joints with high
density. Laser sintering has the potential in precise selective sintering, and the process is often used
to sinter nanosilver inks to form conductive networks. Current assisted sintering is usually used for
connection between dissimilar materials. Moreover, the joints will have excellent shear performance
and anti-fatigue properties.
Table 1. Comparison of different rapid sintering methods.
Sintering Method Sintering Time Shear Strength Cost Ref.
Hot-pressing 30–90 min 30–84 MPa Low [22,165–167]
In-situ formation 3–5 min 50–70 MPa Low [35,118,120,121]
Spark Plasma 30–300 s 30–100 MPa Medium [36,129–132]
Laser 1–15 s 8–10 MPa High [38,168–170]
Current 0.1–1 s 40–97 MPa Medium [34,115,159–161]
Although the rapid sintering processes have many advantages as compared with conventional
hot-pressing sintering processes, there are still a lot of challenges in the applications of the processes to
electronic packaging. To promote the application of the rapid sintering processes, future work should
focus on the following points:
(1) In some rapid sintering processes, the sintering time may be less than 1 s. The sintering
mechanism of the processes may be different from that of traditional hot-pressing sintering. The

Micromachines 2018 ,9, 346 10 of 17
sphere-to-sphere model may not be proper to explain the sintering behavior in the rapid sintering
processes. Therefore, more work needs to be done to explore the mechanism of rapid sintering.
(2) In some rapid sintering processes, such as in-situ formation of nanoparticles and joints, high
external pressure still needs to be applied on chips. The pressure may damage the chip during
the sintering processes. Future work needs to focus on reducing the pressure applied on chips
during the rapid sintering processes.
(3) Generally, binder and dispersants in the nanosilver pastes can prevent the undesirable premature
coalescence or agglomeration of nanosilver particles, and the metastable structure will be retained
until the organic carriers have been burned out at relatively higher temperatures. It is necessary
to study the burnout characteristics of the different organics systems and design nanosilver pastes
with a proper processing temperature.
(4) The bonding between different nanopaticles and metal films is a complicated process, which is
related with physical, mechanical, electrostatic, diffusion, and chemical characters of the materials.
Sintering parameters, such as sintering temperature, sintering time, pressure, and atmosphere
will affect the bonding process and qualities of the interfaces of the materials. Future work needs
to focus on the interfacial reactions and behaviors between nanoparticles and metal films during
the rapid sintering processes.
(5) Currently, studies on sintering mechanisms of nanosilver particles are usually based on
the spherical particle models. However, beside nanoparticles with spherical morphology,
nanomaterials with other morphologies such as wires, belts, disks, and flakes are also widely
mixed in pastes. The sintering mechanism of nanomaterials with different morphologies during
rapid sintering processes still needs great effort.
Funding: This research was funded by National Natural Science Foundation of China grant number 51375003.
Conflicts of Interest: The authors declare no conflict of interest.
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