Effect of Nanoparticles Addition on the Microstructure and Properties Of

applied sciences

Review Eﬀect of Nanoparticles Addition on the Microstructure and Properties of Lead-Free Solders: A Review

Peng Zhang 1, Songbai Xue 1,* , Jianhao Wang 1 , Peng Xue 2, Sujuan Zhong 3 and Weimin Long 3 1 College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China; [email protected] (P.Z.); [email protected] (J.W.) 2 School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210096, China; [email protected] 3 State Key Laboratory of Advanced Brazing Filler Metals and Technology, Zhengzhou Research Institute of Mechanical Engineering Co., Ltd., Zhengzhou 450002, China; [email protected] (S.Z.); [email protected] (W.L.) * Correspondence: [email protected]; Tel.: +86-025-8489-6070

Received: 12 April 2019; Accepted: 13 May 2019; Published: 17 May 2019

Abstract: 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 effect 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 effect 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.

Keywords: nanoparticle; composite lead-free solder; microstructure; mechanical properties; wettability; reliability

1. 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

Appl. Sci. 2019, 9, 2044; doi:10.3390/app9102044 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 2044 2 of 20 and electrical conductivity, poor mechanical properties, and low reliability [4]. Especially, ECAs can hardly meet the requirements of three-dimension integrated circuits (3D ICs) packaging technology, which easily produces high current density and Joule heating. Therefore, compared with ECAs, the development of lead-free solder alloys with better reliability, thermal, and electrical properties to replace SnPb solders has always been one of research focuses in the field of microelectronic joining. Although the application of SnPb solder is limited, its excellent performance makes it a reference standard for the development of new lead-free solders [5]. Nowadays, several Sn-based lead-free solder alloys have been synthesized, such as SnAgCu, SnZn, and SnBi solder alloys. However, these lead-free solders still have some shortages such as high melting point, poor wettability and reliability, and high cost, which fail to meet the actual production needs. As a result, it is urgent that an effective way to improve the performance of the solder alloys is found. Currently, researchers mainly improve the performance of lead-free solders by microalloying and nanoparticles strengthening. Microalloying is a method of improving the properties of the solder by adding trace alloying elements to change the solder composition. The alloying elements can be mainly divided into rare earth (RE) elements (Ce, La, Pr, Nd, etc.) [6,7] and other metal elements (Ga, In, Mg, Ni, Ag, etc.) [8–11]. However, there are still some shortcomings in this method, which limits the widely application of the lead-free solders containing minor alloying elements. For example, adding excessive RE elements will result in the formation of Sn whisker on the surface of solder joints, causing premature failure of electronic device [12,13]. Due to excellent physicochemical properties and size effects, nanoparticles play a significant role in refining grains, increasing mechanical properties, and improving wettability of lead-free solder [14–19]. Therefore, more and more researchers are focused on the preparation and performance characterization of the composite solders reinforced by various nanoparticles. Up to now, types of nanoparticles studied extensively mainly includes metals (Cu, Ni, Mn, Co, etc.), oxides (La2O3, Fe2O3, TiO2, etc.), ceramic (SiC, TiC, etc.), and carbon materials (carbon nanotubes (CNTs), graphene nanosheets (GNSs), etc.). Although some research results have been achieved, there are still some deficiencies in the strengthening effect of nanoparticles. For example, the effect of nanoparticles on the melting characteristics of lead-free solder is not obvious [20–22]. Besides, there is no practical application of the composite solders reinforced by nanoparticles in electronic industries because of its high cost and complex preparation. Therefore, in order to comply with the trend of green development and meet the requirements of the electronics industry for packaging materials, the influence of nanoparticles doping on the microstructure and properties of lead-free solders was analyzed comprehensively, which is expected to provide some theoretical reference for the development of high-performance nanoparticle-reinforced lead-free solders.

2. SnAgCu-Based Solders

2.1. 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 Figure1[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 Appl. Sci. 2019, 9, x FOR PEER REVIEW 2 of 20 hardly meet the requirements of three-dimension integrated circuits (3D ICs) packaging technology, which easily produces high current density and Joule heating. Therefore, compared with ECAs, the development of lead-free solder alloys with better reliability, thermal, and electrical properties to replace SnPb solders has always been one of research focuses in the field of microelectronic joining. Although the application of SnPb solder is limited, its excellent performance makes it a reference standard for the development of new lead-free solders [5]. Nowadays, several Sn-based lead-free solder alloys have been synthesized, such as SnAgCu, SnZn, and SnBi solder alloys. However, these lead-free solders still have some shortages such as high melting point, poor wettability and reliability, and high cost, which fail to meet the actual production needs. As a result, it is urgent that an effective way to improve the performance of the solder alloys is found. Currently, researchers mainly improve the performance of lead-free solders by microalloying and nanoparticles strengthening. Microalloying is a method of improving the properties of the solder by adding trace alloying elements to change the solder composition. The alloying elements can be mainly divided into rare earth (RE) elements (Ce, La, Pr, Nd, etc.) [6,7] and other metal elements (Ga, In, Mg, Ni, Ag, etc.) [8-11]. However, there are still some shortcomings in this method, which limits the widely application of the lead-free solders containing minor alloying elements. For example, adding excessive RE elements will result in the formation of Sn whisker on the surface of solder joints, causing premature failure of electronic device [12,13]. Due to excellent physicochemical properties and size effects, nanoparticles play a significant role in refining grains, increasing mechanical properties, and improving wettability of lead-free solder [14-19]. Therefore, more and more researchers are focused on the preparation and performance characterization of the composite solders reinforced by various nanoparticles. Up to now, types of nanoparticles studied extensively mainly includes metals (Cu, Ni, Mn, Co, etc.), oxides (La2O3, Fe2O3, TiO2, etc.), ceramic (SiC, TiC, etc.), and carbon materials (carbon nanotubes (CNTs), graphene nanosheets (GNSs), etc.). Although some research results have been achieved, there are still some deficiencies in the strengthening effect of nanoparticles. For example, the effect of nanoparticles on the melting characteristics of lead-free solder is not obvious [20-22]. Besides, there is no practical application of the composite solders reinforced by nanoparticles in electronic industries because of its high cost and complex preparation. Therefore, in order to comply with the trend of green development and meet the requirements of the electronics industry for packaging materials, the influence of nanoparticles doping on the microstructure and properties of lead-free solders was analyzed comprehensively, which is expected to provide some theoretical reference for the development of high-performance nanoparticle-reinforced lead-free solders.

2. SnAgCu-Based Solders

2.1. Microstructure Appl. Sci. 2019, 9, 2044 3 of 20 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.%,TiO2 nanoparticles the nanoparticles could would eﬀectively aggregate suppress together, the growth resulting of the in IMCthe formation layer by preventing of cracks and the voids, diﬀusion as shownof Cu and in Figure Sn atoms. 1 [23].

Appl. Sci. 2019, 9, x FOR PEER REVIEW( a) (b) 3 of 20

(e) (f)

Figure 1. FieldField emission scanning electron microscope (FESEM)(FESEM) imageimage ofof SnAgCu–nanoSnAgCu–nano LaLa22OO33 composite solder: ( (aa)) 0% 0% La La22OO3;3 ;((bb) )0.01% 0.01% La La2O2O3; 3(;(c)c 0.03%) 0.03% La La2O23O; 3(d;()d 0.05%) 0.05% La La2O23O; (3e;() 0.1%e) 0.1% La2 LaO32;O (f3); (high-resolutionf) high-resolution image image of (e) of (e)[23]. [23 ].

InConsidering the reflowthe process, effect ofa brittle secondary interfacial phase laye particlesr is rapidly on the formed migration at ofthe grain interface boundary between during the moltengrain growth, solder theand totalthe substrate; restraining meanwhile, force acting the on ov theergrowth boundary of intermetallic can be expressed compounds as follows: (IMCs) will result in lower connecting strength of solder joints [24,25]. After multiple reflow, the coarsening of 3 f γb the Cu6Sn5 layer occurs and the Cu3Sn layer isF formed= at the interface between the Cu6Sn5 layer and(1) 2r the substrate, which decreases the reliability of the joints. Chan et al. [26] found that adding Zn wherenanoparticlesf is the volumecould decrease fraction of the secondary growth phaserate of particles Cu6Sn at5 and grain suppress boundary, theγb isformation the interface of Cu energy3Sn. perResearch unit area[27] offound grain that boundary, the grain and size the of Cur is6Sn the5 IMC radius increased of secondary with the phase increasing particles. reflow It is time, obvious and TiOthat2 Fnanoparticles is inversely could proportional effectively to r suppress. Therefore, the growth the γ-Fe of2O the3 nanoparticle IMC layer by with preventing smaller the size diffusion have a ofbetter Cu and refining Sn atoms. effect on β-Sn phase in SAC105–Fe2O3 composite solder, as shown in Figure2[ 28]. On accountConsidering of high the surfaceeffect of energy, secondary the dopedphase partic CNTsles were on the easily migration adsorbed of grain on the boundary surface ofduringβ-Sn grainand IMCs, growth, which the total significantly restraining refined force the acting microstructure on the boundary of the can composite be expressed solder as by follows: inhibiting the migration of grain boundaries [29]. 3fγ F = b (1) 2r 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].

(a) (b) Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 20

(e) (f)

Figure 1. Field emission scanning electron microscope (FESEM) image of SnAgCu–nano La2O3

composite solder: (a) 0% La2O3; (b) 0.01% La2O3; (c) 0.03% La2O3; (d) 0.05% La2O3; (e) 0.1% La2O3; (f) high-resolution image of (e) [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: 3fγ F = b (1) 2r

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- Appl.Sn Sci.and2019 IMCs,, 9, 2044 which significantly refined the microstructure of the composite solder by inhibiting4 of 20 the migration of grain boundaries [29].

Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 20 Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 20 (a) (b)

(c) (d) (c) (d) Figure 2. Microstructure of the solders: (a) SAC105; (b) SAC105-nano Fe2O3 (20 nm); (c) SAC105-nano FigureFigure 2. Microstructure2. Microstructure of theof the solders: solders: (a) ( SAC105;a) SAC105; (b )(b SAC105-nano) SAC105-nano Fe 2FeO32O(203 (20 nm); nm); (c) (c SAC105-nano) SAC105-nano Fe2O3 (50 nm); (d) SAC105-nano Fe2O3 (200 nm) [28]. Fe2FeO23O(503 (50 nm); nm); (d ()d SAC105-nano) SAC105-nano Fe Fe2O2O3 (2003 (200 nm) nm) [28 [28].]. 2.2. Mechanical Properties 2.2.2.2. Mechanical Mechanical Properties Properties During operation, solder joints are subjected to the thermal-mechanical stress because of the DuringDuring operation, operation, solder solder joints joints are are subjected subjected to to the the thermal-mechanical thermal-mechanical stress stress because because of of the the temperature change and mismatch in the CTEs (coefficients of thermal expansion) of different temperaturetemperature change change and and mismatch mismatch in the in CTEs the (coeCTEsfficients (coefficients of thermal of expansion)thermal expansion) of different of materials. different materials. Therefore, high enough strength is required for the joints due to the accumulation of Therefore,materials. high Therefore, enough strengthhigh enough is required strength for is the re jointsquired due for tothe the joints accumulation due to the of residualaccumulation stress of residual stress during operation caused by CETs mismatch [29]. The addition of minor Al duringresidual operation stress causedduring byoperation CETs mismatch caused [29by]. CETs The additionmismatch of minor[29]. The Al nanoparticlesaddition of intominor the Al nanoparticles into the SnAgCu solder could obviously refine Cu6Sn5, Ag3Sn IMCs, and eutectic SnAgCunanoparticles solder could into obviouslythe SnAgCu refine so Culder6Sn could5, Ag3 Snobviously IMCs, and refine eutectic Cu microstructure,6Sn5, Ag3Sn IMCs, thus and enhancing eutectic microstructure, thus enhancing the mechanical properties of the solder. However, the mechanical themicrostructure, mechanical properties thus enhancing of the solder.the mechanical However, prop theerties mechanical of the propertiessolder. However, of the solderthe mechanical would properties of the solder would decrease once the content of Al nanoparticles exceeded 0.1 wt.%, as decreaseproperties once of the the content solder ofwould Al nanoparticles decrease once exceeded the content 0.1 wt.%, of Al as nanoparticles shown in Figure exceeded3[ 30]. 0.1 Tang wt.%, [31] as shown in Figure 3 [30]. Tang [31] found the fracture of some Sn0.3Ag0.7Cu as flowed solder joints foundshown the in fracture Figure of3 some[30]. Tang Sn0.3Ag0.7Cu [31] found as the flowed fracture solder of some joints Sn0.3Ag0.7Cu presented brittle as flowed characteristic solder andjoints presented brittle characteristic and some microcracks occurred. Owing to the growth of IMCs during somepresented microcracks brittle occurred. characteristic Owing and to thesome growth microcrack of IMCss occurred. during reflow, Owing a large to the stress growth was of accumulated IMCs during reflow, a large stress was accumulated inside or around the IMC, which would result in local stress insidereflow, or arounda large stress the IMC, was which accumulated would resultinside inor localaround stress the concentrationIMC, which would and strength result in reduction local stress concentration and strength reduction in the tensile test. Since Mn nanoparticles addition could reduce inconcentration the tensile test. and Since strength Mn nanoparticles reduction in the addition tensile couldtest. Since reduce Mn thenanoparticles IMC thickness addition and reducecould reduce the the IMC thickness and reduce the accumulation of stress during reflow, the UTS of Mn-containing accumulationthe IMC thickness of stress and during reduce reflow, the accumulation the UTS of Mn-containing of stress during solder reflow, joints the was UTS improved. of Mn-containing solder joints was improved. solder joints was improved.

(a) (b) (a) (b) Figure 3. TheThe mechanical mechanical properties properties of SAC105–xAl SAC105–xAl solder joints: ( (aa)) Tensile Tensile force; ( b) shear shear force force [30]. [30]. Figure 3. The mechanical properties of SAC105–xAl solder joints: (a) Tensile force; (b) shear force [30].

Compared with the original solder, the strength and hardness of the SnAgCu–nano TiO2 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 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 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]. solder decreased to a certain extent and many holes were found at the Ag3Sn grain boundaries [32,33]. The mechanical properties of SnAgCu–nano La2O3 composite solder were greatly improved because The mechanical properties of SnAgCu–nano La2O3 composite solder were greatly improved because the addition of La2O3 nanoparticles refined the microstructure and increased dislocation density by the addition of La2O3 nanoparticles refined the microstructure and increased dislocation density by secondary phase strengthening [23]. secondary phase strengthening [23]. Ag-GNSs nanoparticles were added into SnAgCu solder and the research result shows that 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 compared with GNSs nanoparticles, the strengthening effect of Ag-GNSs nanoparticles is more Appl. Sci. 2019, 9, 2044 5 of 20

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]. The mechanical propertiesAppl. Sci. 2019 of, 9 SnAgCu–nano, x FOR PEER REVIEW La2O 3 composite solder were greatly improved because the addition5 of of20 La2O3 nanoparticles refined the microstructure and increased dislocation density by secondary phase strengtheningobvious because [23 ].Ag can react with Sn matrix to form Ag3Sn, which makes the distribution of Ag- GNSsAg-GNSs in the matrix nanoparticles more uniform were and added stable into [34]. SnAgCu solder and the research result shows that compared with GNSs nanoparticles, the strengthening effect of Ag-GNSs nanoparticles is more obvious because2.3. Wettability Ag can react with Sn matrix to form Ag3Sn, which makes the distribution of Ag-GNSs in the matrixDue more to uniformthe high andmelting stable point, [34]. 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 2.3. Wettability wetting process, Ag nanoparticles in the composite flux gathered at the interface between the molten SnAgCuDue tosolder the high and meltingthe Cu point,substrate, Ni nanoparticles reducing the addition surface can energy form aof frame the system construction and greatly in the solderincreasing joint, the thereby wetting promoting force [37]. the spreading of the molten SnAgCu solder [35,36]. During the wetting process,Absorption Ag nanoparticles of Fe2O3 in nanoparticles the composite on flux the gathered surface atof the the interface Cu substrate between decreased the molten the SnAgCu surface soldertension and of molten the Cu SnAgCu–nano substrate, reducing Fe2O3 the composite surface energysolder and of the thus system the wettability and greatly of increasingthe solder was the wettingimproved force [38]. [37 Figure]. 4 shows the effect of α-Al2O3 nanoparticles on the spreading area of Sn–0.3Ag– 0.7 CuAbsorption solder; the of maximum Fe2O3 nanoparticles spreading ar onea theof the surface composite of the solder Cu substrate is 79 mm decreased2 with 0.12 thewt.% surface Al2O3 tensionnanoparticles of molten addition SnAgCu–nano [39]. The Fewettability2O3 composite of the soldercomposite and thussolder the increases wettability because of the α solder-Al2O3 wasnanoparticles improved are [38 easily]. Figure adsorbed4 shows at thethe esurfaceffect of ofα Cu-Al substrate2O3 nanoparticles and decrease on the the spreadingsurface tension area ofof Sn–0.3Ag–0.7the solder. According Cu solder; to the the maximum Young equation, spreading the area contact of the angle composite increases solder when is 79 the mm surface2 with 0.12tension wt.% is Alreduced,2O3 nanoparticles resulting in addition a bigger [39 spreading]. The wettability area; the ofschematic the composite diagrams solder of increasesthe wetting because and spreadingα-Al2O3 nanoparticlesprocess is shown are easilyin Figure adsorbed 5. at the surface of Cu substrate and decrease the surface tension of the solder.Appropriate According content to the of Young GNSs equation,can significantly the contact increase angle the increases spreading when area the of surface SnAgCu tension solder; is reduced,however, resulting when the in content a bigger exceeds spreading 0.1 wt.%, area; GNSs the schematic tend to agglomerat diagrams ofe and the wettingfloat onto and the spreading top of the processmolten issolder shown and, in Figurehence, 5the. wettability of the composite solder decreases [40].

FigureFigure 4. 4.Spreading Spreading areas areas of of SnAgCu–nano SnAgCu–nano Al Al2O2O33 composite soldersolder [[39].39].

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].

(a)

(b) Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 20

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].

2.3. 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 mm2 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].

Appl. Sci. 2019, 9, 2044 Figure 4. Spreading areas of SnAgCu–nano Al2O3 composite solder [39]. 6 of 20

(a)

Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 20 (b)

Figure 5. Schematic diagram of the effect of Al2O3 nanoparticles on the wetting and spreading: (a) Figure 5. Schematic diagram of the eﬀect of Al O nanoparticles on the wetting and spreading: Contribution to wetting; (b) contribution to spreading2 3 [39]. (a) Contribution to wetting; (b) contribution to spreading [39].

2.4. 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 [41-43]. Competitive growth of Cu6Sn5 and which has been proven to be harmful to solder joints [41–43]. Competitive growth of Cu6Sn5 and Cu3Sn layers reduces the mechanical properties of solder joints and increases the risk of fracture [44]. 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 Zhao et al. [45] found the absorption of TiO2 nanoparticles on the surface of interfacial IMCs could hinder the didiffusionffusion and reaction between Cu and Sn, so the formationformation and growth of IMCs was inhibited. With 0.5 wt.% TiO2 nanoparticles addition, the activation energies of overall IMC layer inhibited. With 0.5 wt.% TiO2 nanoparticles addition, the activation energies of overall IMC layer growth increasedincreased from from 42.48 42.48 KJ /KJ/molmol to 60.31 to 60.31 KJ/mol, KJ/mol, which which meant meant more energy more wasenergy needed was forneeded elements for dielementsffusion anddiffusion IMCs and growth IMCs [46 growth]. TiC nanoparticles [46]. TiC nanoparticles can also lower can the also growth lower ratethe ofgrowth IMCs rate layer of during IMCs aging.layer during One reason aging. is One that reason the particles is that can the be particles adsorbed can on be the adsorbed surface ofon IMCs the surface and the of other IMCs is and that the gatheringother is that of the TiC gathering particles atof theTiC interfaceparticles decreasesat the interface the concentration decreases the gradient concentration of Sn. gradient Consequently, of Sn. Consequently, appropriate content of TiC nanoparticles effectively inhibited the growth of Cu6Sn5 appropriate content of TiC nanoparticles effectively inhibited the growth of Cu6Sn5 and Cu3Sn layers duringand Cu aging,3Sn layers as shown during in aging, Figure as6 [shown47]. in Figure 6 [47].

(a) (b)

Figure 6. EffectEﬀect of of TiC TiC nanoparticles nanoparticles content content on on the the thickness thickness of of IMCs IMCs layers layers during during aging aging (a) (Cua) Cu6Sn65Sn; (b5); (Cub)3 CuSn 3[47].Sn [ 47].

The increasing current density in IC bringsbrings aa deteriorativedeteriorative electromigration eeffectﬀect and thethe improvementimprovement of EMEM resistanceresistance of soldersolder jointsjoints gradually becomes a research focus in reliability research [[48,49].48,49]. CNTs CNTs will entangle with each other after adding it in thethe strengtheningstrengthening phasephase inin lead-freelead-free solder.solder. Because of the excellent electrical conductivity of CNTs, thethe conductivityconductivity channelchannel formed byby CNTs CNTs greatly greatly decreases decreases the the current current intensity intensity of the of solder the solder matrix, matrix, which which increases increases the service 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 1.2×104 A/cm2 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]. 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. Appl. Sci. 2019, 9, 2044 7 of 20

life of the solder joints [29,50,51]. After SAC-CNTs/Cu solder joints underwent aging at 100 ◦C and a current density of 1.2 104 A/cm2 for 336 h, a Cu Sn layer formed between the Cu Sn layer and the × 3 6 5 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]. 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 Figure7. Furthermore, the addition of GNSs refined the microstructure of the solder and the fine grains formed theAppl. initial Sci. 2019 passivation, 9, x FOR PEER film REVIEW to inhibit further corrosion. 7 of 20

Figure 7. Schematic representation of the inhibition e ffectﬀect of graphene nanosheets (GNSs) on oxygen didiffusionﬀusion [[54].54].

3. SnBi-Based Solders

3.1. Microstructure After adding Ni nanoparticles into SnBiAg solder, the Bi-rich phase and Ag Sn IMC was refined After adding Ni nanoparticles into SnBiAg solder, the Bi-rich phase and Ag33Sn IMC was refined because the formation of Ni-Sn IMCs could act as nu nucleationcleation sites sites and and inhibit inhibit the the grain grain growth growth [55]. [55]. With the addition of 0.4 wt.% Ag nanoparticles, a mass of dispersed nano-Ag Sn particles formed, 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 refinedrefined microstructure [[56].56]. It is worth noting that refinementrefinement eeffectffect does notnot alwaysalways increase withwith thethe decreasedecrease ofof nanoparticlesnanoparticles dimension.dimension. Because of small size andand highhigh surfacesurface energy,energy, Ag Ag nanoparticles nanoparticles with with a a size size of of 31 31 nm nm have have a strongera stronger tendency tendency to agglomerateto agglomerate to largerto larger particles particles and and cannot cannot disperse disperse in thein the solder solder matrix matrix in comparisonin comparison with with those those of 76of 76 nm nm and and 133 133 nm nm [57]. [57]. As aAs result, a result, SnBi SnBi solder-doped solder-doped 31 nm 31 Ag nm nanoparticles Ag nanoparticles presents presents larger larger grain sizegrain and size interphase and interphase spacing, spacing, as shown as shown in Figure in Figure8. 8. Liu et al. [58] has reported the influence of Cu nanoparticles on the microstructure of SnBi nano-composite solder. The formation of Cu6Sn5 IMC phase was detected by analyzing XRD patterns of SnBi-3Cu solder. During solidification, the Cu6Sn5 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.% Cu6Sn5 nanoparticles addition, the grain refinement of SnBi–nano Cu6Sn5 composite solder reached the maximum. Smaller amount of Cu6Sn5 nanoparticles had no obvious effect on the nucleation rate, however when the content was excessive, the agglomeration of Cu6Sn5 nanoparticles would weaken its refinement effect.

(a) (b)

Figure 8. SEM morphology of as-prepared solder: (a) Pure SnBi solder; (b) SnBi solder-doped 31 nm Ag nanoparticles; (c) SnBi solder-doped 76 nm Ag nanoparticles; (d) SnBi solder-doped 133 nm Ag nanoparticles [57]. Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 20

Figure 7. Schematic representation of the inhibition effect of graphene nanosheets (GNSs) on oxygen diffusion [54].

3. SnBi-Based Solders

3.1. 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 Appl.of 76 Sci. nm2019 and, 9, 2044133 nm [57]. As a result, SnBi solder-doped 31 nm Ag nanoparticles presents larger8 of 20 grain size and interphase spacing, as shown in Figure 8.

(a) (b)

Figure 8. SEM morphology of as as-prepared-prepared solder: ( a)) Pure SnBi solder; ( b) SnBi solder-doped 31 nm Ag nanoparticles; (c) SnBi solder-doped 76 nm Ag nanoparticles; (d) SnBi solder-doped 133 nm Ag nanoparticles [[57].57].

Ni can react with SnBi solder matrix to form some stable intermetallic compounds such as Ni3Sn4 and Ni3Sn, 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.

3.2. 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 Figure9. 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. Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 20

Liu et al. [58] has reported the influence of Cu nanoparticles on the microstructure of SnBi nano- composite solder. The formation of Cu6Sn5 IMC phase was detected by analyzing XRD patterns of SnBi-3Cu solder. During solidification, the Cu6Sn5 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.% Cu6Sn5 nanoparticles addition, the grain refinement of SnBi–nano Cu6Sn5 composite solder reached the maximum. Smaller amount of Cu6Sn5 nanoparticles had no obvious effect on the nucleation rate, however when the content was excessive, the agglomeration of Cu6Sn5 nanoparticles would weaken its refinement effect. Ni can react with SnBi solder matrix to form some stable intermetallic compounds such as Ni3Sn4 and Ni3Sn, 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.

3.2. 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 Appl.could Sci. gradually2019, 9, 2044 translate the fracture modes from ductile fracture to brittle fracture and decrease9 of the 20 UTS and elongation of the composite solder at the same time.

(a) (b)

Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 20 (c) (d)

(e) (f)

Figure 9. EffectEﬀect of of carbon carbon nanotubes nanotubes (CNT) (CNT) on th thee tensile tensile properties of SnBi solder: ( (aa)) ultimate ultimate tensile tensile strength (UTS) of SnBi-xCNT sold solderer bulks and solder joints; ( b)) tensile tensile stress–displacement stress–displacement curves of the SnBi-xCNT solder bulks (x = 0,0, 0.05, 0.05, 0.1, 0.1, 0.2); 0.2); ( (cc––ff)) fracture fracture surfaces surfaces of of the SnBi-xCNT solder slab (x = 0,0, 0.05, 0.05, 0.1, 0.1, 0.2) 0.2) [62]. [62].

It was was found found that that the the effect eﬀect of of GNSs GNSs on on the the fractu fracturere surface surface of of SnBi SnBi solder solder was was similar similar to tothat that of CNTsof CNTs [63]. [63 The]. The phenomenon phenomenon that that GNSs GNSs embedded embedded in in the the fractured fractured solder solder bulk bulk was was investigated, investigated, meaning strongstrong bondingbonding was was formed formed between between GNSs GNSs and and the matrix.the matrix. When When the solder the solder was under was tensileunder tensilestress, GNSsstress, wouldGNSs bewould pulled be outpulled from out the from matrix the to matrix reduce to the reduce stress the concentration stress concentration that appeared that appearedin the crack in tip.the However,crack tip. However, due to the due restacking to the re behaviorstacking of behavior GNSs, the of GNSs elongation, the elongation of the composite of the compositesolders decreased solders when decreased the content when exceededthe content 0.01%. exceeded Wu et al.0.01%. [64] usedWu et ﬁnite al. [6 element4] used modeling finite element (FEM) modelingmethod to (FEM) analyze method the stress to distribution analyze the at stress the interface distribution of GNSs at reinforcedthe interface Sn58Bi0.7Zn of GNSs solder reinforced joints Sn58Bi0.7Zn(after aging forsolder 120 joints min) in(after the aging tensile for test. 120 Themin) result in the showed tensile test. that The the result GNSs showed addition that changed the GNSs the additionmaximum changed stress position the maximum from the stress Cu/IMC position interface from in the undoped Cu/IMC joints interface to the in IMC undoped/solder joints interface to the in IMC/solderGNSs-doped interface joints, which in GNSs-doped indicated that joints, the compositewhich indicated solder that joints the have composite better tensile solder properties. joints have better tensile properties. 3.3. Wettability

3.3. WettabilityAdding 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 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.

Figure 10. The effect of Y2O3 content on the spread area of SnBi–nano Y2O3 composite solder on the Cu substrate [66]. Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 20

(e) (f)

Figure 9. Effect of carbon nanotubes (CNT) on the tensile properties of SnBi solder: (a) ultimate tensile strength (UTS) of SnBi-xCNT solder bulks and solder joints; (b) tensile stress–displacement curves of the SnBi-xCNT solder bulks (x = 0, 0.05, 0.1, 0.2); (c–f) fracture surfaces of the SnBi-xCNT solder slab (x = 0, 0.05, 0.1, 0.2) [62].

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.

3.3. Wettability Appl. Sci. 2019, 9, 2044 10 of 20 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 greatly [65]. Compared with original SnBi solder, the spread area of SnBi–1Y2O3 composite solder on thethe CuCu substratesubstrate increased increased by by 20% 20% under under the the same same experimental experimental conditions, conditions, as shown as shown in Figure in Figure 10[ 66 10]. [66]. When the addition of Y2O3 exceeded 1 wt.%, the increasing viscosity of the molten solder When the addition of Y2O3 exceeded 1 wt.%, the increasing viscosity of the molten solder inhibited its ﬂowinhibited and, its hence, flow the and, wettability hence, the decreased. wettability decreased.

Figure 10. The effect of Y2O3 content on the spread area of SnBi–nano Y2O3 composite solder on the Figure 10. The effect of Y2O3 content on the spread area of SnBi–nano Y2O3 composite solder on the Cu substrate [66]. Cu substrate [66]. 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 Cu6Sn5 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.

3.4. 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 Cu6Sn5 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 Ag3Sn 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 1012 atoms/cm3 to 4.77 1012 atoms/cm3 under the current density of 5 103 A/cm2 at 303 K, × × × which meant SnBi-Ag solder joints have stronger EM resistance. Nano-Al2O3 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:

7 8.76 10 (M Mt) R = × × − (2) STρ Appl. Sci. 2019, 9, 2044 11 of 20

where M and Mt 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.% Cu6Sn5 nanoparticles obviously suppress the growth of corrosion rate, especially in the first 10 days. The grain-refining effect of Cu6Sn5 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 solderAppl. Sci. almost 2019, 9, remainedx FOR PEER stableREVIEW at 0.45 mm/y. 11 of 20

FigureFigure 11.11. CorrosionCorrosion raterate ofof SnBi–xnanoSnBi–xnano CuCu66Sn55 composite solder: x x = 0%,0%, 0.03%, 0.03%, 0.05%, 0.05%, 0.1% 0.1% [59]. [59].

4. SnZn-Based Solders

4.1. Microstructure 4.1. Microstructure β The matrix microstructure of the SnZn solder mainly consists of β-Sn phase and needle-shaped α α α-Zn phase, in which the coarsening α-Zn phase phase greatly greatly decreases the mechanical properties of SnZn solder [[74].74]. Besides, Besides, the the existence existence of of Zn-rich Zn-rich phas phasee greatly reduces the oxidation resistance of the SnZn solder, which limits the practical use of the Sn SnZnZn solder. solder. Different Different from other Sn-based solders, solders, the interfacial IMCs layer of the SnZn/Cu solder joint is mainly Cu5Zn8, which increases the risk of the interfacial IMCs layer of the SnZn/Cu solder joint is mainly Cu5Zn8, which increases the risk of fracture of solder joints duringduring serviceservice duedue toto itsits poorpoor reliabilityreliability [[75].75]. It isis generally generally known known that solderingthat soldering process process parameters parameters have a great have influence a great on theinfluence microstructure on the and properties of solder joints. Zhang et al. [76] found the thickness of Cu Zn layer increased with microstructure and properties of solder joints. Zhang et al. [76] found the 5thickness8 of Cu5Zn8 layer theincreased reflow with time the and reflow temperature, time and andtemperature, the addition and the of Ni addition and Ag of nanoparticlesNi and Ag nanoparticles could effectively could suppresseffectively the suppress IMC growth the IMC behavior. growth Besides, behavior. with Beside the additions, with of the Ni addition nanoparticles, of Ni thenanoparticles, grain growth the of α needle-shapedgrain growth of-Zn needle-shaped phase could alsoα-Zn be phase inhibited could because also be Zn-Ni inhibited IMCs andbecause Ni nanoparticles Zn-Ni IMCs increased and Ni thenanoparticles heterogeneous increased nucleation the heteroge sites, asneous shown nucleation in Figure sites,12[ 77 as]. shown Due to in high Figure surface 12 [77]. energy Due and to largehigh special surface area, Al O nanoparticles can also provide nucleation sites during the solidification surface energy and large2 special3 surface area, Al2O3 nanoparticles can also provide nucleation sites processduring the of thesolidification solder. So, process it can not of onlythe solder. inhibit So, the it coarsening can not only of dendriteinhibit the Sn-Zn coarsening eutectic of structure, dendrite butSn-Zn also eutectic refine thestructure, microstructure but also refine of the compositethe microstructure solder [78 of]. the composite solder [78]. The gathering of ZrO2 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 ZrO2 nanoparticles, as shown in Table1. It was also reported that the addition of ZrO 2 changed the thickness and composition of interfacial IMCs in SnZn-ZrO2/Cu solder joints. Due to the refining effect of ZrO2 nanoparticles, less Zn atoms could migrate from the solder bulk to the interface to form Cu5Zn8 layer while the migration of Cu atoms became easier, resulting in the increased thickness of Cu6Sn5 layer. A small addition of

(a) (b)

Figure 11. Corrosion rate of SnBi–xnano Cu6Sn5 composite solder: x = 0%, 0.03%, 0.05%, 0.1% [59].

4. SnZn-Based Solders

4.1. 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 Appl.grain Sci. growth2019, 9, 2044of needle-shaped α-Zn phase could also be inhibited because Zn-Ni IMCs and12 of Ni 20 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 β duringZnO nanoparticles the solidification can signiﬁcantly process of decreasethe solder. the So, sizes it ofcan the not-Sn only dendrites inhibit the [80]. coarsening The reﬁnement of dendrite can be β Sn-Znattributed eutectic to the structure, absorption but of also ZnO refine on the the grain microstructure surface, which of the reduces composite the growth solder rate [78]. of -Sn grains.

(a) (b)

Figure 12. Microstructure of: (a) SnZnBi solders after being reflowed for 5 min; (b) SnZnBi–0.5Ni composite solders after being reflowed for 30 min; (c) SnZnBi solders after being reflowed for 30 min; (d) SnZnBi–0.5Ni composite solders after being reflowed for 30 min [77].

Table 1. Grain size and spacing of Zn-rich phase in the SnZnBi–xZrO2 solder.

Maximal Size Minimal Size Average Average Spacing Solder (µm) (µm) Size (µm) (µm) SnZnBi 10.5 1.2 4.6 0.09 2.2 0.1 ± ± SnZnBi–0.25ZrO 2.5 0.5 1.5 0.1 1.3 0.04 2 ± ± SnZnBi–0.5ZrO 6.6 0.4 2.8 0.3 1.1 0.06 2 ± ± SnZnBi–1ZrO 8.3 0.6 3.4 0.5 2.1 0.08 2 ± ±

4.2. 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-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 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]. Appl. Sci. 2019, 9, x FOR PEER REVIEW 12 of 20

Figure 12. Microstructure of: (a) SnZnBi solders after being reflowed for 5 min; (b) SnZnBi–0.5Ni composite solders after being reflowed for 30 min; (c) SnZnBi solders after being reflowed for 30 min; (d) SnZnBi–0.5Ni composite solders after being reflowed for 30 min [77].

The gathering of ZrO2 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 ZrO2 nanoparticles, as shown in Table 1. It was also reported that the addition of ZrO2 changed the thickness and composition of interfacial IMCs in SnZn-ZrO2/Cu solder joints. Due to the refining effect of ZrO2 nanoparticles, less Zn atoms could migrate from the solder bulk to the interface to form Cu5Zn8 layer while the migration of Cu atoms became easier, resulting in the increased thickness of Cu6Sn5 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.

Table 1. Grain size and spacing of Zn-rich phase in the SnZnBi–xZrO2 solder.

Maximal Size Minimal Size Average Size Average Spacing Solder (μm) (μm) (μm) (μm) SnZnBi 10.5 1.2 4.6 ± 0.09 2.2 ± 0.1 SnZnBi– 2.5 0.5 1.5 ± 0.1 1.3 ± 0.04 0.25ZrO2 SnZnBi– 6.6 0.4 2.8 ± 0.3 1.1 ± 0.06 0.5ZrO2 SnZnBi–1ZrO2 8.3 0.6 3.4 ± 0.5 2.1 ± 0.08

Figure 13. EffectEﬀect of of Sb Sb nanoparticles nanoparticles on on the the tensile tensile prop propertieserties of of the the Sn–9Zn–xSb Sn–9Zn–xSb composite composite solders solders (x (x= 0,= 0.5,0, 0.5, 1.0, 1.0, 1.5) 1.5) [81]. [81 ].

InZhang summary, et al. Table[82] found2 lists theAg mechanical nanoparticles properties could react of some with lead-free Sn atoms solders to form bearing spherical-shaped nanoparticles. AsAg3 seenSn IMC from phase, the which table, SnBi-basedrefined the solder solders matrix present and higher improved strength the mechanical and carbon properties. nanomaterials SnZn (CNTs,solder joints GNSs) showed have great a typical inﬂuence brittle of fracture, the composite while solders. SnZn-Ag In solder general, joints the additionpresented of dimple nanoparticles failure 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.

Table 2. The mechanical properties of nanoparticle-reinforced lead-free solders.

Solders UTS (MPa) Elongation (%) Microhardness References Sn3.0Ag0.5Cu 46.0 29.5 / [83] Sn0.3Ag-0.5Cu/0.1Mn 56.5 // [31] Sn3.5Ag0.25Cu/1TiO2 70.1 25.2 18.5 HV [32] Sn3.0Ag0.5Cu/0.05La2O3 79.0 14.1 13 HV [23] Sn3.0Ag0.5Cu/0.03GNSs 50.3 23.5 / [83] 3Sn3.0Ag0.5Cu/0.1GNSs 50.7 21.5 / [83] Sn3.0Ag0.5Cu/0.2Ni-GNSs 58.4 / 14.6 HV [84] Sn3.0Ag0.5Cu/0.05Ag-GNSs 50.1 14.7 / [85] Sn3.8Ag0.7Cu/1CNTs 56.7 24.4 / [86] Sn58Bi 81 21 0.30 GPa [63] Sn58Bi/0.01GNSs 84 32 0.26 GPa [63] Sn58Bi/0.1GNSs 93 17 0.41 GPa [63] Sn58Bi/0.03CNTs 94.2 21.7 / [60] Sn9Zn 41.0 43.4 13.6 BHN [87] Sn9Zn/1.0Ag 43.6 19.4 16 BHN [87] Sn9Zn/1.0Sb 53.6 21.6 15.9 BHN [81] Sn9Zn/1.0Al2O3 43.4 44.9 27 HV [78]

4.3. 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 mm2 to 190.92 mm2. 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. Appl. Sci. 2019, 9, 2044 14 of 20 Appl. Sci. 2019, 9, x FOR PEER REVIEW 14 of 20 Appl. Sci. 2019, 9, x FOR PEER REVIEW 14 of 20

FigureFigure 14. 14. Spreading Spreading area area of SnZn–xZrCof SnZn–xZrC (x =(x 0, = 0.03,0, 0.03, 0.06, 0.06, 0.09, 0.09, 0.12) 0.12) composite composite solders solders [89]. [88 [89].].

Generally,Generally, thethe the largerlarger larger thethe the widthwidth width ofof wettingofwetting wetting ringring ring is,is, theis,the the betterbetter better wettabilitywettability wettability thethe the soldersolder solder has.has. has. Figure Figure 15 15 illustrates that that with with the the addition addition of Al of2O Al3 nanoparticles,O nanoparticles, the spreading the spreading area and area the width and the of precursor width of illustrates that with the addition of Al2O23 nanoparticles,3 the spreading area and the width of precursor precursorfilm increased ﬁlm increased[90]. Therefore, [89]. Therefore, the wettabilit the wettabilityy was improved was improved by adding by addingsmall amount small amount of Al2O of3 film increased [90]. Therefore, the wettability was improved by adding small amount of Al2O3 Alnanoparticles.2nanoparticles.O3 nanoparticles.

(a)( a) (b)( b) (c)( c)

Figure 15. Macro morphology of SnZn–Al2O3 composite solders after wetting on the 6061-aluminum FigureFigure 15. 15.Macro Macro morphology morphology of SnZn–Alof SnZn–Al2O32Ocomposite3 composite solders solders after after wetting wetting on on the the 6061-aluminum 6061-aluminum alloy: (a) 0% Al2O3; (b) 0.5% Al2O3; (c) 1.0% Al2O3 [90]. alloy:alloy: (a) ( 0%a) 0% Al 2AlO23O;(3b; ()b 0.5%) 0.5% Al Al2O23O;(3;c ()c 1.0%) 1.0% Al Al2O2O3 3[ 89[90].].

4.4.4.4. Reliability Reliability

During aging and reflow,reflow, thethe thicknessthickness ofof brittlebrittle CuCu5Zn8 interfacialinterfacial layer layer increases increases with with time, time, During aging and reflow, the thickness of brittle Cu5 5Zn8 8 interfacial layer increases with time, whichwhich brings brings a great a great threat threat to theto the reliability reliability of SnZnof SnZn solders. solders. Some Some research research reported reported that that RE RE addition addition could suppress the coarsening of CuCu5Zn8 IMC,IMC, but but it it would would also also result result in spontaneous growth of Sn could suppress the coarsening of Cu5 5Zn8 8 IMC, but it would also result in spontaneous growth of Sn whiskerswhiskers and and finallyfinally finally lead lead to theto the potential potential failure failure of electronicsof electronics devicesdevices devices [[91].90 [91].]. Research Research [[92]91 [92]] shows shows that that NiNi nanoparticles nanoparticles can can eeffectivelyff ectivelyeffectively increaseincrease increase thethe the shearshear shear loadload load ofof theofthe the soldersolder solder jointsjoints joints duedue due itsits grainitsgrain grain refinement.refinement. refinement. TheThe practicalpractical practical use use use of of SnZn ofSnZn SnZn solders solders solders is limited is islimited limited because because because the existence the the existence existence of Zn-rich of ofZn-rich phase Zn-rich greatlyphase phase reducesgreatly greatly thereducesreduces oxidation the the oxidation resistance oxidation resistance of resistance the SnZn of of solderthe the SnZn SnZn [92 ].solder Undersolder [93]. high[93]. Under temperatureUnder high high temperature andtemperature humidity and and conditions, humidity humidity waterconditions,conditions, vapor water and water oxygenvapor vapor and oxidized and oxygen oxygen both oxidized oxidized the surface both both the microstructure the surface surface microstructure microstructure and Zn-rich andphase and Zn-rich Zn-rich in the phase solderphase in in matrix,thethe solder solder resulting matrix, matrix, in resulting volume resulting expansion in involume volume and expansion expansion the increase and and ofthe internalthe increase increase stress, of ofinternal which internaleasily stress, stress, induces which which crackseasily easily alonginducesinduces Sn cracks grain cracks along boundaries. along Sn Sn grain grain Chan boundaries. boundaries. et al. [91 Chan] foundChan et theetal. al.[92] addition [92] found found of the Ni the addition nanoparticles addition of ofNi Ni couldnanoparticles nanoparticles aid the could aid the formation of IMCs in the grain boundaries, so the penetration of H2O and O2 was formationcould aid of IMCsthe formation in the grain of boundaries,IMCs in the so grain the penetrationboundaries, of so H 2theO andpenetration O2 was inhibited of H2O and as well O2 aswas theinhibitedinhibited formation as aswell ofwell ZnO.as asthe Itthe formation is alsoformation reported of ofZnO. ZnO. that It Ag isIt also nanoparticlesis also reported reported increasedthat that Ag Ag nanoparticles thenanoparticles thermal andincreased increased humidity the the stabilitythermalthermal ofand and the humidity SnZn-basedhumidity stability stability solder of alloy ofthe the SnZn-based [76 SnZn-based]. As shown solder solder in Figure alloy alloy 16[76]. ,[76]. the As oxidationAs shown shown in of inFigure Zn-rich Figure 16, phase 16, the the wasoxidationoxidation significantly of ofZn-rich Zn-rich inhibited phase phase withwas was significantly the significantly addition inhibited of inhibited Ni and with Ag with nanoparticles. the the addition addition of ofNi Ni and and Ag Ag nanoparticles. nanoparticles. Appl. Sci. 20192019,, 9,, x 2044 FOR PEER REVIEW 1515 of of 20

(a) (b)

Figure 16. MicrostructureMicrostructure of of (a ()a )SnZnBi, SnZnBi, (b ()b SnZnBi–0.5Ag,) SnZnBi–0.5Ag, and and (c) ( cSnZnBi–0.5Ni) SnZnBi–0.5Ni solder solder alloys alloys at 85 at

°C/85%85 ◦C/85% relative relative humidity humidity (RH) (RH) after after exposure exposure for 250 for h 250 and h (d and) energy (d) energy dispersive dispersive spectrometer spectrometer (EDS) analysis(EDS) analysis of region of regionP in (a) P [76]. in (a) [76].

5. Conclusions Conclusions Although lots lots of of research research results results have have been been achieved achieved in nanoparticle-reinforced in nanoparticle-reinforced lead-free lead-free solder, theresolder, are there still areseveral still severalshortages shortages existing existingin the application in the application of the composite of the composite solder. Firstly, solder. nanoparticle-reinforcedFirstly, nanoparticle-reinforced lead-free lead-free solders solders remain remain in the in theexperimental experimental research research stage stage and and hardly hardly replace the SnPb solder solder to to meet meet the the requirement requirement of of electronic electronic package package industries. industries. Secondly, Secondly, compared compared with microalloying,microalloying, the the amount amount of researchof research on nanoparticle-reinforced on nanoparticle-reinforced lead-free lead-free solders issolders still relatively is still relativelyless and the less influence and the mechanism influence ofmechanism nanoparticles of onnanoparticles the reliability on of the the reliability composite of solders the composite in various soldersservice environmentsin various service is insu environmentsfficient. What is more,insufficient. the preparation What is more, process the of preparation nanoparticle-reinforced process of nanoparticle-reinforcedcomposite solder is complex composite and the solder cost is is complex high, which and limitsthe cost its is industrial high, which application. limits its Toindustrial further application.improve the To properties further ofimprove nanoparticle-reinforced the properties of lead-free nanoparticle-reinforced solders, future development lead-free solders, trends future can be developmentcarried out in trends the following can be carried aspects: out in the following aspects: 1. WithWith the the development development of of electronic electronic devices to towardswards miniaturization and and high performance, stricterstricter requirements requirements have have been been put put forward forward for for the reliability of lead-free solders. The current researchresearch mainly mainly focuses focuses on on the the effect effect of of thermal thermal stress stress and current stress on the performance of thethe solders. solders. However, However, more more and and more more electronic electronic devices devices are are applied applied to to harsh harsh environments environments such such asas cryogenic cryogenic temperatures [[94,95]93,94] andand irradiation irradiation [95 [96].]. Therefore, Therefore, research research on on the the reliability reliability of the of thesolder solder bearing bearing nanoparticles nanoparticles in these in these harsh harsh environments environments will become will become a hot spot a hot in spot this subject.in this 2. subject.It has been proven that the coupling effects of RE and nanoparticles can further refine the 2. Itmicrostructure has been proven and improve that the the coupling mechanical effects properties of RE ofand lead-free nanoparticles solders [ 96can]. Thus,further the refine combined the microstructureaddition of RE and and improve nanoparticles the mechanical may become pr aoperties new reinforcement of lead-free methodsolders [97]. to improve Thus, the combinedperformance addition of the solders.of RE and nanoparticles may become a new reinforcement method to 3. improveDue to small the performance tendency of agglomerationof the solders. and fine physical and chemical properties, carbon-based 3. Duenanomaterials to small tendency can effectively of agglomeration improve the and microstructure fine physical of lead-freeand chemical solders properties, and improve carbon- the based nanomaterials can effectively improve the microstructure of lead-free solders and improve the properties [98,99]. Consequently, the development of new carbon-based Appl. Sci. 2019, 9, 2044 16 of 20

properties [97,98]. Consequently, the development of new carbon-based nanomaterial-reinforced solders with better performance may become a new direction for the future research. 4. At present, lead-free solders bearing nanoparticles are mainly fabricated by mechanical mixing of the solder particles and nanoparticles. It is claimed that the nanoparticle-reinforced lead-free solders can also be directly prepared by adding nanoparticles though ﬂux doping [37,99] and in-situ formation [100]. Therefore, exploring new preparation methods with simpler processes and low cost attracts wide attention from researchers.

Author Contributions: S.X. conceived the idea; P.Z. and J.W. wrote the paper; P.X., S.Z., and W.L. advised the paper. All authors reviewed the paper. Funding: This work was funded by the National Natural Science Foundation of China, grant No.51675269 and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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