E ﬀ ect of Nanoparticles Addition on the Microstructure and Properties of Lead-Free Solders: A Review

: With the development of microelectronic packaging and increasingly speciﬁc service environment of solder joints, much stricter requirements have been placed on the properties of lead-free solders. On account of small size e ﬀ ect and high surface energy, nanoparticles have been widely used to improve the microstructure and properties of lead-free solders. Therefore, the composite solders bearing nanoparticles have recently attracted wide attention. This article reviewed the recent research on SnAgCu, SnBi, and SnZn composite solder alloys and introduced the e ﬀ ect of nanoparticles on their microstructure, mechanical properties, wettability, and reliability. The mechanism of nanoparticles strengthening was analyzed and summarized. In addition, the shortcomings and future development trends of nanoparticle-reinforced lead-free solders were discussed, which is expected to provide some theoretical reference for the application of these composite solder in 3D IC package.


Introduction
Under the drive of device miniaturization, high-density packaging technologies, such as ball grid array (BGA), chip scale package (CSP), and wafer level packaging (WLP), have been applied in the advanced electronical equipment, which decreases the joint size and calls for excellent property of packaging materials [1]. To meet the actual requirement of electronic industries, packaging materials for electronic devices are required to have not only perfect electrical conductivity, wettability, and mechanical properties, but also low cost and high reliability. Traditional SnPb solders were widely used in electronic packaging because of low cost and excellent performance. However, due to the toxicity of lead, most countries have implemented a series of laws such as waste electrical and electronic equipment (WEEE) and restriction of hazardous substances (RoHS), to restrict the use of lead in various electronic and electrical products [2].
Electrically conductive adhesives (ECAs) and lead-free solders were considered as promising alternatives of SnPb solders. ECAs, mainly composed of matrix polymer and conductive fillers, offer numerous advantages such as environmental friendliness, lower processing temperature, and fine pitch capability [3]. However, the application of ECAs is limited due to its lower thermal conductivity TiO 2 nanoparticles could effectively suppress the growth of the IMC layer by preventing the diffusion of Cu and Sn atoms.

Microstructure
Adding trace La2O3 nanoparticles to SAC-305 solder could provide a large number of nucleation sites to refine the microstructure of the composite solder. However, when the content exceeded 0.1 wt.%, the nanoparticles would aggregate together, resulting in the formation of cracks and voids, as shown in Figure 1 [23]. In the reflow process, a brittle interfacial layer is rapidly formed at the interface between the molten solder and the substrate; meanwhile, the overgrowth of intermetallic compounds (IMCs) will result in lower connecting strength of solder joints [24,25]. After multiple reflow, the coarsening of the Cu6Sn5 layer occurs and the Cu3Sn layer is formed at the interface between the Cu6Sn5 layer and the substrate, which decreases the reliability of the joints. Chan et al. [26] found that adding Zn nanoparticles could decrease the growth rate of Cu6Sn5 and suppress the formation of Cu3Sn. Research [27] found that the grain size of Cu6Sn5 IMC increased with the increasing reflow time, and TiO2 nanoparticles could effectively suppress the growth of the IMC layer by preventing the diffusion of Cu and Sn atoms.
Considering the effect of secondary phase particles on the migration of grain boundary during grain growth, the total restraining force acting on the boundary can be expressed as follows: b 3fγ = 2r F (1) where f is the volume fraction of secondary phase particles at grain boundary, γb is the interface energy per unit area of grain boundary, and the r is the radius of secondary phase particles. It is obvious that F is inversely proportional to r. Therefore, the γ-Fe2O3 nanoparticle with smaller size have a better refining effect on β-Sn phase in SAC105-Fe2O3 composite solder, as shown in Figure 2 [28]. On account of high surface energy, the doped CNTs were easily adsorbed on the surface of β-Sn and IMCs, which significantly refined the microstructure of the composite solder by inhibiting the migration of grain boundaries [29]. (f) high-resolution image of (e) [23].
Considering the effect of secondary phase particles on the migration of grain boundary during grain growth, the total restraining force acting on the boundary can be expressed as follows: where f is the volume fraction of secondary phase particles at grain boundary, γ b is the interface energy per unit area of grain boundary, and the r is the radius of secondary phase particles. It is obvious that F is inversely proportional to r. Therefore, the γ-Fe 2 O 3 nanoparticle with smaller size have a better refining effect on β-Sn phase in SAC105-Fe 2 O 3 composite solder, as shown in Figure 2 [28]. On account of high surface energy, the doped CNTs were easily adsorbed on the surface of β-Sn and IMCs, which significantly refined the microstructure of the composite solder by inhibiting the migration of grain boundaries [29].
energy per unit area of grain boundary, and the r is the radius of secondary phase particles. It is obvious that F is inversely proportional to r. Therefore, the γ-Fe2O3 nanoparticle with smaller size have a better refining effect on β-Sn phase in SAC105-Fe2O3 composite solder, as shown in Figure 2 [28]. On account of high surface energy, the doped CNTs were easily adsorbed on the surface of β-Sn and IMCs, which significantly refined the microstructure of the composite solder by inhibiting the migration of grain boundaries [29].

Mechanical Properties
During operation, solder joints are subjected to the thermal-mechanical stress because of the temperature change and mismatch in the CTEs (coefficients of thermal expansion) of different materials. Therefore, high enough strength is required for the joints due to the accumulation of residual stress during operation caused by CETs mismatch [29]. The addition of minor Al nanoparticles into the SnAgCu solder could obviously refine Cu6Sn5, Ag3Sn IMCs, and eutectic microstructure, thus enhancing the mechanical properties of the solder. However, the mechanical properties of the solder would decrease once the content of Al nanoparticles exceeded 0.1 wt.%, as shown in Figure 3 [30]. Tang [31] found the fracture of some Sn0.3Ag0.7Cu as flowed solder joints presented brittle characteristic and some microcracks occurred. Owing to the growth of IMCs during reflow, a large stress was accumulated inside or around the IMC, which would result in local stress concentration and strength reduction in the tensile test. Since Mn nanoparticles addition could reduce the IMC thickness and reduce the accumulation of stress during reflow, the UTS of Mn-containing solder joints was improved.  Compared with the original solder, the strength and hardness of the SnAgCu-nano TiO2 composite solder were significantly improved because TiO2 particles could pin the movement of dislocations and grain boundaries. However, it is worth noting that the plasticity of the composite solder decreased to a certain extent and many holes were found at the Ag3Sn grain boundaries [32,33].

Mechanical Properties
During operation, solder joints are subjected to the thermal-mechanical stress because of the temperature change and mismatch in the CTEs (coefficients of thermal expansion) of different materials. Therefore, high enough strength is required for the joints due to the accumulation of residual stress during operation caused by CETs mismatch [29]. The addition of minor Al nanoparticles into the SnAgCu solder could obviously refine Cu 6 Sn 5 , Ag 3 Sn IMCs, and eutectic microstructure, thus enhancing the mechanical properties of the solder. However, the mechanical properties of the solder would decrease once the content of Al nanoparticles exceeded 0.1 wt.%, as shown in Figure 3 [30]. Tang [31] found the fracture of some Sn0.3Ag0.7Cu as flowed solder joints presented brittle characteristic and some microcracks occurred. Owing to the growth of IMCs during reflow, a large stress was accumulated inside or around the IMC, which would result in local stress concentration and strength reduction in the tensile test. Since Mn nanoparticles addition could reduce the IMC thickness and reduce the accumulation of stress during reflow, the UTS of Mn-containing solder joints was improved.

Mechanical Properties
During operation, solder joints are subjected to the thermal-mechanical stress because of the temperature change and mismatch in the CTEs (coefficients of thermal expansion) of different materials. Therefore, high enough strength is required for the joints due to the accumulation of residual stress during operation caused by CETs mismatch [29]. The addition of minor Al nanoparticles into the SnAgCu solder could obviously refine Cu6Sn5, Ag3Sn IMCs, and eutectic microstructure, thus enhancing the mechanical properties of the solder. However, the mechanical properties of the solder would decrease once the content of Al nanoparticles exceeded 0.1 wt.%, as shown in Figure 3 [30]. Tang [31] found the fracture of some Sn0.3Ag0.7Cu as flowed solder joints presented brittle characteristic and some microcracks occurred. Owing to the growth of IMCs during reflow, a large stress was accumulated inside or around the IMC, which would result in local stress concentration and strength reduction in the tensile test. Since Mn nanoparticles addition could reduce the IMC thickness and reduce the accumulation of stress during reflow, the UTS of Mn-containing solder joints was improved.  Compared with the original solder, the strength and hardness of the SnAgCu-nano TiO2 composite solder were significantly improved because TiO2 particles could pin the movement of dislocations and grain boundaries. However, it is worth noting that the plasticity of the composite solder decreased to a certain extent and many holes were found at the Ag3Sn grain boundaries [32,33]. Compared with the original solder, the strength and hardness of the SnAgCu-nano TiO 2 composite solder were significantly improved because TiO 2 particles could pin the movement of dislocations and grain boundaries. However, it is worth noting that the plasticity of the composite solder decreased to a certain extent and many holes were found at the Ag 3 Sn grain boundaries [32,33]. The mechanical properties of SnAgCu-nano La 2 O 3 composite solder were greatly improved because the addition of La 2 O 3 nanoparticles refined the microstructure and increased dislocation density by secondary phase strengthening [23].
Ag-GNSs nanoparticles were added into SnAgCu solder and the research result shows that compared with GNSs nanoparticles, the strengthening effect of Ag-GNSs nanoparticles is more obvious because Ag can react with Sn matrix to form Ag3Sn, which makes the distribution of Ag-GNSs in the matrix more uniform and stable [34].

Wettability
Due to the high melting point, Ni nanoparticles addition can form a frame construction in the solder joint, thereby promoting the spreading of the molten SnAgCu solder [35,36]. During the wetting process, Ag nanoparticles in the composite flux gathered at the interface between the molten SnAgCu solder and the Cu substrate, reducing the surface energy of the system and greatly increasing the wetting force [37].
Absorption of Fe 2 O 3 nanoparticles on the surface of the Cu substrate decreased the surface tension of molten SnAgCu-nano Fe 2 O 3 composite solder and thus the wettability of the solder was improved [38]. Figure 4 shows the effect of α-Al 2 O 3 nanoparticles on the spreading area of Sn-0.3Ag-0.7 Cu solder; the maximum spreading area of the composite solder is 79 mm 2 with 0.12 wt.% Al 2 O 3 nanoparticles addition [39]. The wettability of the composite solder increases because α-Al 2 O 3 nanoparticles are easily adsorbed at the surface of Cu substrate and decrease the surface tension of the solder. According to the Young equation, the contact angle increases when the surface tension is reduced, resulting in a bigger spreading area; the schematic diagrams of the wetting and spreading process is shown in Figure 5. obvious because Ag can react with Sn matrix to form Ag3Sn, which makes the distribution of Ag-GNSs in the matrix more uniform and stable [34].

Wettability
Due to the high melting point, Ni nanoparticles addition can form a frame construction in the solder joint, thereby promoting the spreading of the molten SnAgCu solder [35,36]. During the wetting process, Ag nanoparticles in the composite flux gathered at the interface between the molten SnAgCu solder and the Cu substrate, reducing the surface energy of the system and greatly increasing the wetting force [37].
Absorption of Fe2O3 nanoparticles on the surface of the Cu substrate decreased the surface tension of molten SnAgCu-nano Fe2O3 composite solder and thus the wettability of the solder was improved [38]. Figure 4 shows the effect of α-Al2O3 nanoparticles on the spreading area of Sn-0.3Ag-0.7 Cu solder; the maximum spreading area of the composite solder is 79 mm 2 with 0.12 wt.% Al2O3 nanoparticles addition [39]. The wettability of the composite solder increases because α-Al2O3 nanoparticles are easily adsorbed at the surface of Cu substrate and decrease the surface tension of the solder. According to the Young equation, the contact angle increases when the surface tension is reduced, resulting in a bigger spreading area; the schematic diagrams of the wetting and spreading process is shown in Figure 5.
Appropriate content of GNSs can significantly increase the spreading area of SnAgCu solder; however, when the content exceeds 0.1 wt.%, GNSs tend to agglomerate and float onto the top of the molten solder and, hence, the wettability of the composite solder decreases [40].  Appropriate content of GNSs can significantly increase the spreading area of SnAgCu solder; however, when the content exceeds 0.1 wt.%, GNSs tend to agglomerate and float onto the top of the molten solder and, hence, the wettability of the composite solder decreases [40].

Reliability
After high-temperature storage, the coarsening IMC layer of solders joint could be obtained, which has been proven to be harmful to solder joints [41][42][43]. Competitive growth of Cu 6 Sn 5 and Cu 3 Sn layers reduces the mechanical properties of solder joints and increases the risk of fracture [44]. Zhao et al. [45] found the absorption of TiO 2 nanoparticles on the surface of interfacial IMCs could hinder the diffusion and reaction between Cu and Sn, so the formation and growth of IMCs was inhibited. With 0.5 wt.% TiO 2 nanoparticles addition, the activation energies of overall IMC layer growth increased from 42.48 KJ/mol to 60.31 KJ/mol, which meant more energy was needed for elements diffusion and IMCs growth [46]. TiC nanoparticles can also lower the growth rate of IMCs layer during aging. One reason is that the particles can be adsorbed on the surface of IMCs and the other is that the gathering of TiC particles at the interface decreases the concentration gradient of Sn. Consequently, appropriate content of TiC nanoparticles effectively inhibited the growth of Cu 6 Sn 5 and Cu 3 Sn layers during aging, as shown in Figure 6 [47].

Reliability
After high-temperature storage, the coarsening IMC layer of solders joint could be obtained, which has been proven to be harmful to solder joints [41][42][43]. Competitive growth of Cu6Sn5 and Cu3Sn layers reduces the mechanical properties of solder joints and increases the risk of fracture [44]. Zhao et. al [45] found the absorption of TiO2 nanoparticles on the surface of interfacial IMCs could hinder the diffusion and reaction between Cu and Sn, so the formation and growth of IMCs was inhibited. With 0.5 wt.% TiO2 nanoparticles addition, the activation energies of overall IMC layer growth increased from 42.48 KJ/mol to 60.31 KJ/mol, which meant more energy was needed for elements diffusion and IMCs growth [46]. TiC nanoparticles can also lower the growth rate of IMCs layer during aging. One reason is that the particles can be adsorbed on the surface of IMCs and the other is that the gathering of TiC particles at the interface decreases the concentration gradient of Sn. Consequently, appropriate content of TiC nanoparticles effectively inhibited the growth of Cu6Sn5 and Cu3Sn layers during aging, as shown in Figure 6  The increasing current density in IC brings a deteriorative electromigration effect and the improvement of EM resistance of solder joints gradually becomes a research focus in reliability research [48,49]. CNTs will entangle with each other after adding it in the strengthening phase in lead-free solder. Because of the excellent electrical conductivity of CNTs, the conductivity channel formed by CNTs greatly decreases the current intensity of the solder matrix, which increases the service life of the solder joints [29,50,51]. After SAC-CNTs/Cu solder joints underwent aging at 100 °C and a current density of 1.2×10 4 A/cm 2 for 336 h, a Cu3Sn layer formed between the Cu6Sn5 layer and the Cu substrate. The diffusion of Cu atoms from the solder matrix to the interface was impeded due to the pinning effect of CNTs, therefore the thickness of Cu6Sn5 and Cu3Sn layers at both the anode side and cathode side decreased with the increase of CNTs content [52]. The increasing current density in IC brings a deteriorative electromigration effect and the improvement of EM resistance of solder joints gradually becomes a research focus in reliability research [48,49]. CNTs will entangle with each other after adding it in the strengthening phase in lead-free solder. Because of the excellent electrical conductivity of CNTs, the conductivity channel formed by CNTs greatly decreases the current intensity of the solder matrix, which increases the service life of the solder joints [29,50,51]. After SAC-CNTs/Cu solder joints underwent aging at 100 • C and a current density of 1.2 × 10 4 A/cm 2 for 336 h, a Cu 3 Sn layer formed between the Cu 6 Sn 5 layer and the Cu substrate. The diffusion of Cu atoms from the solder matrix to the interface was impeded due to the pinning effect of CNTs, therefore the thickness of Cu 6 Sn 5 and Cu 3 Sn layers at both the anode side and cathode side decreased with the increase of CNTs content [52].
Since lead-free solders are usually exposed to corrosion environments, it is important to improve corrosion resistance of the solder for improving reliability and mechanical properties of solder joints [53]. Han [54] found that GNSs could significantly improve corrosion resistance of SAC solder and the optimum content was 0.03 wt.%. Uniformly distributed GNSs increased the tortuosity of oxygen diffusion pathway and inhibited the diffusion of oxygen in the solder, as illustrated in Figure 7. Furthermore, the addition of GNSs refined the microstructure of the solder and the fine grains formed the initial passivation film to inhibit further corrosion.

Microstructure
After adding Ni nanoparticles into SnBiAg solder, the Bi-rich phase and Ag3Sn IMC was refined because the formation of Ni-Sn IMCs could act as nucleation sites and inhibit the grain growth [55]. With the addition of 0.4 wt.% Ag nanoparticles, a mass of dispersed nano-Ag3Sn particles formed, which inhibited the grain growth and provided a refined microstructure [56]. It is worth noting that refinement effect does not always increase with the decrease of nanoparticles dimension. Because of small size and high surface energy, Ag nanoparticles with a size of 31 nm have a stronger tendency to agglomerate to larger particles and cannot disperse in the solder matrix in comparison with those of 76 nm and 133 nm [57]. As a result, SnBi solder-doped 31 nm Ag nanoparticles presents larger grain size and interphase spacing, as shown in Figure 8.

Microstructure
After adding Ni nanoparticles into SnBiAg solder, the Bi-rich phase and Ag 3 Sn IMC was refined because the formation of Ni-Sn IMCs could act as nucleation sites and inhibit the grain growth [55]. With the addition of 0.4 wt.% Ag nanoparticles, a mass of dispersed nano-Ag 3 Sn particles formed, which inhibited the grain growth and provided a refined microstructure [56]. It is worth noting that refinement effect does not always increase with the decrease of nanoparticles dimension. Because of small size and high surface energy, Ag nanoparticles with a size of 31 nm have a stronger tendency to agglomerate to larger particles and cannot disperse in the solder matrix in comparison with those of 76 nm and 133 nm [57]. As a result, SnBi solder-doped 31 nm Ag nanoparticles presents larger grain size and interphase spacing, as shown in Figure 8.
Liu et al. [58] has reported the influence of Cu nanoparticles on the microstructure of SnBi nano-composite solder. The formation of Cu 6 Sn 5 IMC phase was detected by analyzing XRD patterns of SnBi-3Cu solder. During solidification, the Cu 6 Sn 5 nanoparticles served as nucleation sites and greatly increased the nucleation rate, which led to the grain refinement of the solder bulk. Wu et al. [59] found that with 0.05 wt.% Cu 6 Sn 5 nanoparticles addition, the grain refinement of SnBi-nano Cu 6 Sn 5 composite solder reached the maximum. Smaller amount of Cu 6 Sn 5 nanoparticles had no obvious effect on the nucleation rate, however when the content was excessive, the agglomeration of Cu 6 Sn 5 nanoparticles would weaken its refinement effect. which inhibited the grain growth and provided a refined microstructure [56]. It is worth noting that refinement effect does not always increase with the decrease of nanoparticles dimension. Because of small size and high surface energy, Ag nanoparticles with a size of 31 nm have a stronger tendency to agglomerate to larger particles and cannot disperse in the solder matrix in comparison with those of 76 nm and 133 nm [57]. As a result, SnBi solder-doped 31 nm Ag nanoparticles presents larger grain size and interphase spacing, as shown in Figure 8.  Ni can react with SnBi solder matrix to form some stable intermetallic compounds such as Ni 3 Sn 4 and Ni 3 Sn, thereby increasing the bonding force between Ni-CNTs and the matrix. As a result, Ni-CNTs distribute more uniformly and become harder to agglomerate in comparison with pure CNTs [60,61]. Therefore, Ni-CNTs can effectively suppress the growth of interfacial IMCs because of their higher stability in the solder joints during soldering.

Mechanical Properties
The formation of IMCs layer is the basis of fine bonding between the solder and substrate, but the excessive growth of IMCs layer will reduce the reliability of interconnection and the lifetime of electronic devices. Research [57] found Ag nanoparticles addition could suppress the growth of Cu-Sn IMC and improve the shear strength of the solder joints by 18.9% after 180 min liquid reaction at 220 • C. The improvement behavior can be explained by refinement strengthening, dispersion strengthening, and absorption theory. Under external force, the deformation of the composite solder is dispersed to smaller grains with different orientation, which suppressed the formation of cracks and increased the ductility of the solder.
Wu et al. [62] reported that 0.05 wt.% Ni-CNTs could effectively increase the ultimate tensile strength (UTS) of SnBi solder bulks and joints, as shown in Figure 9. What is more, the fracture surface of SnBi-CNTs solder appeared more regular than the plain SnBi solder and CNTs were detected pining at the grain boundaries in high magnification, which demonstrated that the grain growth was suppressed, and the microstructure was refined by Ni-CNTs addition. However, excessive CNTs could gradually translate the fracture modes from ductile fracture to brittle fracture and decrease the UTS and elongation of the composite solder at the same time.
strength (UTS) of SnBi solder bulks and joints, as shown in Figure 9. What is more, the fracture surface of SnBi-CNTs solder appeared more regular than the plain SnBi solder and CNTs were detected pining at the grain boundaries in high magnification, which demonstrated that the grain growth was suppressed, and the microstructure was refined by Ni-CNTs addition. However, excessive CNTs could gradually translate the fracture modes from ductile fracture to brittle fracture and decrease the UTS and elongation of the composite solder at the same time. It was found that the effect of GNSs on the fracture surface of SnBi solder was similar to that of CNTs [63]. The phenomenon that GNSs embedded in the fractured solder bulk was investigated, meaning strong bonding was formed between GNSs and the matrix. When the solder was under tensile stress, GNSs would be pulled out from the matrix to reduce the stress concentration that appeared in the crack tip. However, due to the restacking behavior of GNSs, the elongation of the composite solders decreased when the content exceeded 0.01%. Wu et al. [64] used finite element modeling (FEM) method to analyze the stress distribution at the interface of GNSs reinforced Sn58Bi0.7Zn solder joints (after aging for 120 min) in the tensile test. The result showed that the GNSs addition changed the maximum stress position from the Cu/IMC interface in undoped joints to the IMC/solder interface in GNSs-doped joints, which indicated that the composite solder joints have better tensile properties.

Wettability
Adding BaTiO3 nanoparticles can reduce the surface tension between the liquid solder and increase the fluidity of the molten solder, thereby increasing the spreading coefficient of SnBi solder greatly [65]. Compared with original SnBi solder, the spread area of SnBi-1Y2O3 composite solder on the Cu substrate increased by 20% under the same experimental conditions, as shown in Figure 10 [66]. When the addition of Y2O3 exceeded 1 wt.%, the increasing viscosity of the molten solder inhibited its flow and, hence, the wettability decreased. It was found that the effect of GNSs on the fracture surface of SnBi solder was similar to that of CNTs [63]. The phenomenon that GNSs embedded in the fractured solder bulk was investigated, meaning strong bonding was formed between GNSs and the matrix. When the solder was under tensile stress, GNSs would be pulled out from the matrix to reduce the stress concentration that appeared in the crack tip. However, due to the restacking behavior of GNSs, the elongation of the composite solders decreased when the content exceeded 0.01%. Wu et al. [64] used finite element modeling (FEM) method to analyze the stress distribution at the interface of GNSs reinforced Sn58Bi0.7Zn solder joints (after aging for 120 min) in the tensile test. The result showed that the GNSs addition changed the maximum stress position from the Cu/IMC interface in undoped joints to the IMC/solder interface in GNSs-doped joints, which indicated that the composite solder joints have better tensile properties.

Wettability
Adding BaTiO 3 nanoparticles can reduce the surface tension between the liquid solder and increase the fluidity of the molten solder, thereby increasing the spreading coefficient of SnBi solder greatly [65]. Compared with original SnBi solder, the spread area of SnBi-1Y 2 O 3 composite solder on the Cu substrate increased by 20% under the same experimental conditions, as shown in Figure 10 [66]. When the addition of Y 2 O 3 exceeded 1 wt.%, the increasing viscosity of the molten solder inhibited its flow and, hence, the wettability decreased.
IMC/solder interface in GNSs-doped joints, which indicated that the composite solder joints have better tensile properties.

Wettability
Adding BaTiO3 nanoparticles can reduce the surface tension between the liquid solder and increase the fluidity of the molten solder, thereby increasing the spreading coefficient of SnBi solder greatly [65]. Compared with original SnBi solder, the spread area of SnBi-1Y2O3 composite solder on the Cu substrate increased by 20% under the same experimental conditions, as shown in Figure 10 [66]. When the addition of Y2O3 exceeded 1 wt.%, the increasing viscosity of the molten solder inhibited its flow and, hence, the wettability decreased.  With the addition of 0.5 wt.% Ni nanoparticles, the contact angle of molten SnBiAg-Ni composite solder on the Cu substrate decreased from 33.1 • to 23.4 • [55]. Due to extremely high surface activity, Ni nanoparticles tended to gather at the molten SnBi/Cu surface, which lowered the surface tension between the molten solder and the substrate and significantly promoted the spreading process. Therefore, the wettability of SnBi solder bearing Ni nanoparticles was improved.
Sun et al. [67] reported that nano-Cu particles could react with Sn to form stable Cu 6 Sn 5 IMC during reflow, which inhibited the spreading of the molten solder. Therefore, the spread ratio (SR) value of SnBi solder bearing Cu nanoparticles decreased with the increasing content of Cu nanoparticles.

Reliability
The reliability of SnBi solder joint is sensitive to the high-density current, which is much more severe in the increasingly finer pitch of advanced electronical devices [68]. Compared with other Sn-based solders, the Cu 6 Sn 5 layer at the SnBi/Cu interface grows faster and the probability of cracks formation increases under high-density current. The current also causes the migration and segregation of Sn and Bi atoms in the solder, which results in the formation of Sn-rich layer and brittle Bi-rich layer on the anode side and cathode side, respectively [69,70]. Numerous studies have shown that adding nanoparticles can significantly improve the electromigration resistance of SnBi solders.
Chan et al. [71] reported that nano-Ag particles could react with Sn to form fine and dispersed Ag 3 Sn particles, which refined the microstructure of the composite solder and hindered the migration of Bi atoms. After adding 2 wt.% Ag nanoparticles, Bi atomic flux of Sn58Bi solder decreased from 7.43 × 10 12 atoms/cm 3 to 4.77 × 10 12 atoms/cm 3 under the current density of 5 × 10 3 A/cm 2 at 303 K, which meant SnBi-Ag solder joints have stronger EM resistance. Nano-Al 2 O 3 particles tended to gather at the surface of interfacial IMCs layer and inhibited the atomic migration in the solder joints under the electro-thermal treatment, and thus, the EM resistance of the composite solder is significantly improved [72].
The addition of nanoparticles also has an important influence of the corrosion resistance. The corrosion rate can be calculated as: where M and M t represent the mass of sample before and after corrosion, S is the surface area of sample, T is the corrosion time, and ρ is the density of sample [73]. Figure 11 [59] illustrates that 0.05 wt.% and 0.1 wt.% Cu 6 Sn 5 nanoparticles obviously suppress the growth of corrosion rate, especially in the first 10 days. The grain-refining effect of Cu 6 Sn 5 nanoparticles results in more boundaries, which can serve as corrosion barriers, thus improving the corrosion resistance of the composite solder. A similar phenomenon was detected in the corrosion process of the SnBi-GNSs composite solder [63]. After corrosion of 400 h, Bi dendrites formed and less ditches existed at the gapping space of surface microstructure with the addition of 0.05 wt.% GNSs. Besides, the corrosion rate of the SnBi-0.05GNSs solder almost remained stable at 0.45 mm/y.

Microstructure
The matrix microstructure of the SnZn solder mainly consists of β-Sn phase and needle-shaped α-Zn phase, in which the coarsening α-Zn phase greatly decreases the mechanical properties of SnZn solder [74]. Besides, the existence of Zn-rich phase greatly reduces the oxidation resistance of the SnZn solder, which limits the practical use of the SnZn solder. Different from other Sn-based solders, the interfacial IMCs layer of the SnZn/Cu solder joint is mainly Cu5Zn8, which increases the risk of fracture of solder joints during service due to its poor reliability [75].
It is generally known that soldering process parameters have a great influence on the microstructure and properties of solder joints. Zhang et al. [76] found the thickness of Cu5Zn8 layer increased with the reflow time and temperature, and the addition of Ni and Ag nanoparticles could effectively suppress the IMC growth behavior. Besides, with the addition of Ni nanoparticles, the grain growth of needle-shaped α-Zn phase could also be inhibited because Zn-Ni IMCs and Ni nanoparticles increased the heterogeneous nucleation sites, as shown in Figure 12 [77]. Due to high surface energy and large special surface area, Al2O3 nanoparticles can also provide nucleation sites during the solidification process of the solder. So, it can not only inhibit the coarsening of dendrite Sn-Zn eutectic structure, but also refine the microstructure of the composite solder [78].

Microstructure
The matrix microstructure of the SnZn solder mainly consists of β-Sn phase and needle-shaped α-Zn phase, in which the coarsening α-Zn phase greatly decreases the mechanical properties of SnZn solder [74]. Besides, the existence of Zn-rich phase greatly reduces the oxidation resistance of the SnZn solder, which limits the practical use of the SnZn solder. Different from other Sn-based solders, the interfacial IMCs layer of the SnZn/Cu solder joint is mainly Cu 5 Zn 8 , which increases the risk of fracture of solder joints during service due to its poor reliability [75].
It is generally known that soldering process parameters have a great influence on the microstructure and properties of solder joints. Zhang et al. [76] found the thickness of Cu 5 Zn 8 layer increased with the reflow time and temperature, and the addition of Ni and Ag nanoparticles could effectively suppress the IMC growth behavior. Besides, with the addition of Ni nanoparticles, the grain growth of needle-shaped α-Zn phase could also be inhibited because Zn-Ni IMCs and Ni nanoparticles increased the heterogeneous nucleation sites, as shown in Figure 12 [77]. Due to high surface energy and large special surface area, Al 2 O 3 nanoparticles can also provide nucleation sites during the solidification process of the solder. So, it can not only inhibit the coarsening of dendrite Sn-Zn eutectic structure, but also refine the microstructure of the composite solder [78].
The gathering of ZrO 2 nanoparticles at the phase boundaries decreased surface energy of the boundaries and inhibited the growth of Zn-rich phase [79]. Therefore, the average grain size and spacing of Zn-rich phase were significantly reduced by the addition of ZrO 2 nanoparticles, as shown in Table 1. It was also reported that the addition of ZrO 2 changed the thickness and composition of interfacial IMCs in SnZn-ZrO 2 /Cu solder joints. Due to the refining effect of ZrO 2 nanoparticles, less Zn atoms could migrate from the solder bulk to the interface to form Cu 5 Zn 8 layer while the migration of Cu atoms became easier, resulting in the increased thickness of Cu 6 Sn 5 layer. A small addition of ZnO nanoparticles can significantly decrease the sizes of the β-Sn dendrites [80]. The refinement can be attributed to the absorption of ZnO on the grain surface, which reduces the growth rate of β-Sn grains.
It is generally known that soldering process parameters have a great influence on the microstructure and properties of solder joints. Zhang et al. [76] found the thickness of Cu5Zn8 layer increased with the reflow time and temperature, and the addition of Ni and Ag nanoparticles could effectively suppress the IMC growth behavior. Besides, with the addition of Ni nanoparticles, the grain growth of needle-shaped α-Zn phase could also be inhibited because Zn-Ni IMCs and Ni nanoparticles increased the heterogeneous nucleation sites, as shown in Figure 12 [77]. Due to high surface energy and large special surface area, Al2O3 nanoparticles can also provide nucleation sites during the solidification process of the solder. So, it can not only inhibit the coarsening of dendrite Sn-Zn eutectic structure, but also refine the microstructure of the composite solder [78].

Mechanical Properties
The addition of 1.0 wt.% Sb nanoparticles greatly increased the tensile strength and decreased the elongation of the composite solder, as shown in Figure 13 [81]. The formation of ε-Sb 3 Zn 4 not only reduced the amount of Zn atoms to form Zn-rich phase, but also acted as nucleation sites, making the distribution of Zn-rich phase more uniform. Thus, the tensile properties of the SnZn-nano Sb 3 Zn 4 composite solder were improved. However, the microstructure of the solder matrix deteriorated when excessive Sb was added, resulting in the decrease of the tensile strength. It was reported that Al 2 O 3 nanoparticles have a similar effect on the mechanical properties of SnZn solders [78].
Zhang et al. [82] found Ag nanoparticles could react with Sn atoms to form spherical-shaped Ag 3 Sn IMC phase, which refined the solder matrix and improved the mechanical properties. SnZn solder joints showed a typical brittle fracture, while SnZn-Ag solder joints presented dimple failure morphologies because of the refining effect of Ag nanoparticles. Due to the pinning effect and dispersion strengthening of ZnO, the sliding of grain boundaries was retarded and the tensile creep properties of the composite were improved [80]. the elongation of the composite solder, as shown in Figure 13 [81]. The formation of ε-Sb3Zn4 not only reduced the amount of Zn atoms to form Zn-rich phase, but also acted as nucleation sites, making the distribution of Zn-rich phase more uniform. Thus, the tensile properties of the SnZn-nano Sb3Zn4 composite solder were improved. However, the microstructure of the solder matrix deteriorated when excessive Sb was added, resulting in the decrease of the tensile strength. It was reported that Al2O3 nanoparticles have a similar effect on the mechanical properties of SnZn solders [78]. Zhang et al. [82] found Ag nanoparticles could react with Sn atoms to form spherical-shaped Ag3Sn IMC phase, which refined the solder matrix and improved the mechanical properties. SnZn solder joints showed a typical brittle fracture, while SnZn-Ag solder joints presented dimple failure In summary, Table 2 lists the mechanical properties of some lead-free solders bearing nanoparticles. As seen from the table, SnBi-based solders present higher strength and carbon nanomaterials (CNTs, GNSs) have great influence of the composite solders. In general, the addition of nanoparticles improves the tensile strength of the solders, but simultaneously reduces the elongation due to their strong restriction effect on the movements of dislocation. Therefore, the optimum content should be further determined by comprehensive comparison of mechanical properties and other performance such as wettability and reliability.

Wettability
The SnZn solder has a poor wettability due to its poor oxidation resistance, which decreases the reliability of the solder joints. Figure 14 [88] shows that the addition of ZrC nanoparticles has a significant impact on the wettability of the SnZn solder. Compared with the plain SnZn solder, the spreading area of the SnZn-0.06 ZrC composite solder could be increased from 143.64 mm 2 to 190.92 mm 2 . However, when the content is excessive, ZrC nanoparticles would agglomerate at the surface of the molten solder, which inhibited the wetting and spreading of the liquid solder on Cu substrate. Generally, the larger the width of wetting ring is, the better wettability the solder has. Figure 15 illustrates that with the addition of Al2O3 nanoparticles, the spreading area and the width of precursor film increased [90]. Therefore, the wettability was improved by adding small amount of Al2O3 nanoparticles.

Reliability
During aging and reflow, the thickness of brittle Cu5Zn8 interfacial layer increases with time, which brings a great threat to the reliability of SnZn solders. Some research reported that RE addition could suppress the coarsening of Cu5Zn8 IMC, but it would also result in spontaneous growth of Sn whiskers and finally lead to the potential failure of electronics devices [91]. Research [92] shows that Ni nanoparticles can effectively increase the shear load of the solder joints due its grain refinement.
The practical use of SnZn solders is limited because the existence of Zn-rich phase greatly reduces the oxidation resistance of the SnZn solder [93]. Under high temperature and humidity conditions, water vapor and oxygen oxidized both the surface microstructure and Zn-rich phase in the solder matrix, resulting in volume expansion and the increase of internal stress, which easily induces cracks along Sn grain boundaries. Chan et al. [92] found the addition of Ni nanoparticles could aid the formation of IMCs in the grain boundaries, so the penetration of H2O and O2 was inhibited as well as the formation of ZnO. It is also reported that Ag nanoparticles increased the thermal and humidity stability of the SnZn-based solder alloy [76]. As shown in Figure 16, the oxidation of Zn-rich phase was significantly inhibited with the addition of Ni and Ag nanoparticles. Generally, the larger the width of wetting ring is, the better wettability the solder has. Figure 15 illustrates that with the addition of Al 2 O 3 nanoparticles, the spreading area and the width of precursor film increased [89]. Therefore, the wettability was improved by adding small amount of Al 2 O 3 nanoparticles. Generally, the larger the width of wetting ring is, the better wettability the solder has. Figure 15 illustrates that with the addition of Al2O3 nanoparticles, the spreading area and the width of precursor film increased [90]. Therefore, the wettability was improved by adding small amount of Al2O3 nanoparticles.

Reliability
During aging and reflow, the thickness of brittle Cu5Zn8 interfacial layer increases with time, which brings a great threat to the reliability of SnZn solders. Some research reported that RE addition could suppress the coarsening of Cu5Zn8 IMC, but it would also result in spontaneous growth of Sn whiskers and finally lead to the potential failure of electronics devices [91]. Research [92] shows that Ni nanoparticles can effectively increase the shear load of the solder joints due its grain refinement.
The practical use of SnZn solders is limited because the existence of Zn-rich phase greatly reduces the oxidation resistance of the SnZn solder [93]. Under high temperature and humidity conditions, water vapor and oxygen oxidized both the surface microstructure and Zn-rich phase in the solder matrix, resulting in volume expansion and the increase of internal stress, which easily induces cracks along Sn grain boundaries. Chan et al. [92] found the addition of Ni nanoparticles could aid the formation of IMCs in the grain boundaries, so the penetration of H2O and O2 was inhibited as well as the formation of ZnO. It is also reported that Ag nanoparticles increased the thermal and humidity stability of the SnZn-based solder alloy [76]. As shown in Figure 16, the oxidation of Zn-rich phase was significantly inhibited with the addition of Ni and Ag nanoparticles.

Reliability
During aging and reflow, the thickness of brittle Cu 5 Zn 8 interfacial layer increases with time, which brings a great threat to the reliability of SnZn solders. Some research reported that RE addition could suppress the coarsening of Cu 5 Zn 8 IMC, but it would also result in spontaneous growth of Sn whiskers and finally lead to the potential failure of electronics devices [90]. Research [91] shows that Ni nanoparticles can effectively increase the shear load of the solder joints due its grain refinement.
The practical use of SnZn solders is limited because the existence of Zn-rich phase greatly reduces the oxidation resistance of the SnZn solder [92]. Under high temperature and humidity conditions, water vapor and oxygen oxidized both the surface microstructure and Zn-rich phase in the solder matrix, resulting in volume expansion and the increase of internal stress, which easily induces cracks along Sn grain boundaries. Chan et al. [91] found the addition of Ni nanoparticles could aid the formation of IMCs in the grain boundaries, so the penetration of H 2 O and O 2 was inhibited as well as the formation of ZnO. It is also reported that Ag nanoparticles increased the thermal and humidity stability of the SnZn-based solder alloy [76]. As shown in Figure 16, the oxidation of Zn-rich phase was significantly inhibited with the addition of Ni and Ag nanoparticles.

Conclusions
Although lots of research results have been achieved in nanoparticle-reinforced lead-free solder, there are still several shortages existing in the application of the composite solder. Firstly, nanoparticle-reinforced lead-free solders remain in the experimental research stage and hardly replace the SnPb solder to meet the requirement of electronic package industries. Secondly, compared with microalloying, the amount of research on nanoparticle-reinforced lead-free solders is still relatively less and the influence mechanism of nanoparticles on the reliability of the composite solders in various service environments is insufficient. What is more, the preparation process of nanoparticle-reinforced composite solder is complex and the cost is high, which limits its industrial application. To further improve the properties of nanoparticle-reinforced lead-free solders, future development trends can be carried out in the following aspects: 1. With the development of electronic devices towards miniaturization and high performance, stricter requirements have been put forward for the reliability of lead-free solders. The current research mainly focuses on the effect of thermal stress and current stress on the performance of the solders. However, more and more electronic devices are applied to harsh environments such as cryogenic temperatures [94,95] and irradiation [96]. Therefore, research on the reliability of the solder bearing nanoparticles in these harsh environments will become a hot spot in this subject. 2. It has been proven that the coupling effects of RE and nanoparticles can further refine the microstructure and improve the mechanical properties of lead-free solders [97]. Thus, the combined addition of RE and nanoparticles may become a new reinforcement method to improve the performance of the solders. 3. Due to small tendency of agglomeration and fine physical and chemical properties, carbonbased nanomaterials can effectively improve the microstructure of lead-free solders and improve the properties [98,99]. Consequently, the development of new carbon-based

Conclusions
Although lots of research results have been achieved in nanoparticle-reinforced lead-free solder, there are still several shortages existing in the application of the composite solder. Firstly, nanoparticle-reinforced lead-free solders remain in the experimental research stage and hardly replace the SnPb solder to meet the requirement of electronic package industries. Secondly, compared with microalloying, the amount of research on nanoparticle-reinforced lead-free solders is still relatively less and the influence mechanism of nanoparticles on the reliability of the composite solders in various service environments is insufficient. What is more, the preparation process of nanoparticle-reinforced composite solder is complex and the cost is high, which limits its industrial application. To further improve the properties of nanoparticle-reinforced lead-free solders, future development trends can be carried out in the following aspects:

1.
With the development of electronic devices towards miniaturization and high performance, stricter requirements have been put forward for the reliability of lead-free solders. The current research mainly focuses on the effect of thermal stress and current stress on the performance of the solders. However, more and more electronic devices are applied to harsh environments such as cryogenic temperatures [93,94] and irradiation [95]. Therefore, research on the reliability of the solder bearing nanoparticles in these harsh environments will become a hot spot in this subject.

2.
It has been proven that the coupling effects of RE and nanoparticles can further refine the microstructure and improve the mechanical properties of lead-free solders [96]. Thus, the combined addition of RE and nanoparticles may become a new reinforcement method to improve the performance of the solders.

3.
Due to small tendency of agglomeration and fine physical and chemical properties, carbon-based nanomaterials can effectively improve the microstructure of lead-free solders and improve the