**3.3. Thermal behavior (melting point and melting range) according to surface oxidation and intermetallic compound formation**

Near-future generations of electronics are expected to be flexible, bendable, and wearable [1]. The use of flexible devices requires the development of a novel solder that can be reflowed at a low temperature to avoid thermal damage to these flexible devices, which usually have temperature-sensitive components [3, 4]. In addition, the melting point of a solder alloy should be the first priority for consideration when it comes to the manufacturing process [3, 4]. Meanwhile, the eutectic point (usually, the low melting point) in a binary phase diagram is where a liquid phase and two solid phases can coexist at equilibrium. Thus, a large number of low melting point solders with eutectic compositions are mostly used for flip chip solder joint applications between microchips and substrates.

**Figure 12** shows the solidus and liquidus temperatures, and the differences between both temperatures, referred to as the melting range of Sn–58Bi–xNi (x = 0.05, 0.1, 0.5 and 1.0 wt.%) solder alloys. The addition of Ni apparently lowered the solidus and liquidus temperatures [29]. This phenomenon was attributable to the fact that the addition of small amounts of Ni altered the composition of the alloy to resemble the eutectic composition of the ternary Sn–Bi–Ni alloy system [29].

Low Melting Temperature Solder Materials for Use in Flexible Microelectronic Packaging... http://dx.doi.org/10.5772/intechopen.70272 21

**Figure 12.** Solidus and liquidus temperatures of Sn–58Bi–xNi [29].

the solder constituent electrolytes [32]. This is mainly due to the increased hydrogen evolution caused by lower ion concentration in the vicinity of the nucleation/deposition sites [32]. On the other hand, the addition of MWCNTs into the solder alloy reduces the current efficiency because pristine MWCNTs are trapped in the deposited composites [32]. Due to the bridging effects of

trapped MWCNTs, the Sn–Bi–CNT composite is denser than the pure Sn–Bi alloy [32].

**Figure 11.** Current efficiency change of the Sn–Bi solder under CNT load [32].

**and intermetallic compound formation**

20 Recent Progress in Soldering Materials

between microchips and substrates.

**3.3. Thermal behavior (melting point and melting range) according to surface oxidation** 

Near-future generations of electronics are expected to be flexible, bendable, and wearable [1]. The use of flexible devices requires the development of a novel solder that can be reflowed at a low temperature to avoid thermal damage to these flexible devices, which usually have temperature-sensitive components [3, 4]. In addition, the melting point of a solder alloy should be the first priority for consideration when it comes to the manufacturing process [3, 4]. Meanwhile, the eutectic point (usually, the low melting point) in a binary phase diagram is where a liquid phase and two solid phases can coexist at equilibrium. Thus, a large number of low melting point solders with eutectic compositions are mostly used for flip chip solder joint applications

**Figure 12** shows the solidus and liquidus temperatures, and the differences between both temperatures, referred to as the melting range of Sn–58Bi–xNi (x = 0.05, 0.1, 0.5 and 1.0 wt.%) solder alloys. The addition of Ni apparently lowered the solidus and liquidus temperatures [29]. This phenomenon was attributable to the fact that the addition of small amounts of Ni altered the composition

of the alloy to resemble the eutectic composition of the ternary Sn–Bi–Ni alloy system [29].

Using the differential scanning calorimetry (DSC), when the content of In was increased to 23.8 wt.%. Kim et al. determined that the prominent endothermic peak of the Bi–Sn–In powders shifted to 82.0°C from that of the Bi–Sn powders, which have a peak at 139.6°C [3, 4]. The continuous addition of 4.8 wt.% Ga shifted the peak even more to 60.3°C. Meanwhile, there was a slight broadening in the solidus line of the melting peak of the Bi–Sn–In solder powders mainly due to the formation of an In-rich phase [3]. Furthermore, the formation of new Ga0.9In0.1, BiIn, and In0.2Sn0.8 IMCs according to the addition of 4.8 wt.% Ga to the Bi–Sn–In solder alloy system also influenced the melting range broadening [3]. Kim et al. also show that ternary Bi–Sn–In nanoparticles, with a 71.1°C melting temperature, entered among the intervals of the higher melting temperature (79.4°C) micropowders and then reflowed at 110°C on a flexible polyethylene terephthalate (PET) substrate [4].

The fundamental thermal properties of Sn–58Bi–xZn (x = 0 and 0.7 wt.%) solder alloys were also analyzed by DSC, with results as shown in **Figure 13**. The results indicate that the solidus temperature of solder alloys slightly decreased with the addition of Zn content [33]. The reduction in solidus temperature of solders can probably be attributed to the increase in the surface instability due to the higher surface energy induced by the addition of Zn [33].

As can be seen in **Figure 14a**, the eutectic solder (In–Sn) had a low melting point of 118.5°C and a narrow melting range. The DSC curves of the hypo-eutectic Sn–70In and eutectic Sn–Bi solders were also presented in **Figure 14b** and **c**, respectively. In addition, the Bi53–Sn26–Cd21 solder presented in **Figure 14d** had the lowest melting temperature and a narrow melting range; thus, this solder had more active phase transformation than the others. The solidus temperatures, liquidus temperatures, and mushy temperature zones of Sn–58Bi, Sn–40Bi–0.1Cu, and Sn–40Bi–2Zn–0.1Cu solder alloys are collected in **Table 2**. The melting peak of the eutectic Sn–Bi solder decreased from 139.0 to 132.2°C according to the addition of a small amount of Cu; however, the addition of 2 wt.% Zn into the Cu-containing solder imparted a slight increase in the melting point (136.3°C) of

**Figure 13.** DSC curves of Sn–58Bi–xZn (x = 0 and 0.7) solders [33].

the Sn–40Bi–2Zn–0.1Cu solder [17]. Thus, the addition of Cu decreased the melting point of the Sn–Bi-based solder, while the addition of Zn provided the reverse effect (melting temperature increase) [17]. Moreover, Cu addition decreased the melting range of the Sn–Bi-based solder from 27.2 to 22.0°C, while the addition of Zn to Sn–40Bi–2Zn– 0.1Cu increased the melting range slightly to 23.1°C [17]. The thermal conductivity of the Sn–40Bi–2Zn–0.1Cu solder of 24.51 W/(m·K) was the highest, while the Sn–40Bi–0.1Cu solder took second place with a value of 20.48 W/(m·K). Zn and Cu additions obviously improved the thermal conductivity of the Sn–Bi-based solder alloy [17]. Aksoz et al. reported that the thermal conductivity of pure Zn is 116 W/(m·K), which is higher than those of pure Bi (8 W/(m·K)) and pure Sn (67 W/(m·K)) [34]. This is the reason that the Sn–40Bi–2Zn–0.1Cu solder has the highest thermal conductivity [34]. The temperatures of the endothermic peaks of the nine Sn–Bi–Sb alloys are shown in **Table 3**. All the main peaks appear at around 147°C. The melting range of all the Sn–Bi–Sb alloys is larger than that of the eutectic alloy [18]. Side peaks are observed in many DSC profiles of the Sn–Bi– Sb alloys [18]. As the Bi content is reduced, the melting range obviously becomes large. Meanwhile, the melting range and the liquidus temperature reached maximum values for the composition of Sn–48Bi–1.8Sb and then started to drop when Sb content changed [18]. The melting range may be attributed to the fact that the proportion of the eutectic structure will change when Bi or Sb content changes [18]. For the liquidus temperature, it was found that the primary phase changes to the β-Sn phase, when the Sb content is more than 1.8% [18]. The presence of second phase implies that the remaining primary phase continues to melt after quasi-peritectic reaction [18].

Low Melting Temperature Solder Materials for Use in Flexible Microelectronic Packaging... http://dx.doi.org/10.5772/intechopen.70272 23

**Figure 14.** DSC curves of (a) In–50Sn, (b) In–30Sn, (c) Bi–42Sn, and (d) Bi–26Sn–21Cd solders [16].

the Sn–40Bi–2Zn–0.1Cu solder [17]. Thus, the addition of Cu decreased the melting point of the Sn–Bi-based solder, while the addition of Zn provided the reverse effect (melting temperature increase) [17]. Moreover, Cu addition decreased the melting range of the Sn–Bi-based solder from 27.2 to 22.0°C, while the addition of Zn to Sn–40Bi–2Zn– 0.1Cu increased the melting range slightly to 23.1°C [17]. The thermal conductivity of the Sn–40Bi–2Zn–0.1Cu solder of 24.51 W/(m·K) was the highest, while the Sn–40Bi–0.1Cu solder took second place with a value of 20.48 W/(m·K). Zn and Cu additions obviously improved the thermal conductivity of the Sn–Bi-based solder alloy [17]. Aksoz et al. reported that the thermal conductivity of pure Zn is 116 W/(m·K), which is higher than those of pure Bi (8 W/(m·K)) and pure Sn (67 W/(m·K)) [34]. This is the reason that the Sn–40Bi–2Zn–0.1Cu solder has the highest thermal conductivity [34]. The temperatures of the endothermic peaks of the nine Sn–Bi–Sb alloys are shown in **Table 3**. All the main peaks appear at around 147°C. The melting range of all the Sn–Bi–Sb alloys is larger than that of the eutectic alloy [18]. Side peaks are observed in many DSC profiles of the Sn–Bi– Sb alloys [18]. As the Bi content is reduced, the melting range obviously becomes large. Meanwhile, the melting range and the liquidus temperature reached maximum values for the composition of Sn–48Bi–1.8Sb and then started to drop when Sb content changed [18]. The melting range may be attributed to the fact that the proportion of the eutectic structure will change when Bi or Sb content changes [18]. For the liquidus temperature, it was found that the primary phase changes to the β-Sn phase, when the Sb content is more than 1.8% [18]. The presence of second phase implies that the remaining primary phase

continues to melt after quasi-peritectic reaction [18].

**Figure 13.** DSC curves of Sn–58Bi–xZn (x = 0 and 0.7) solders [33].

22 Recent Progress in Soldering Materials


**Table 2.** Solidus temperatures, liquids temperatures, melting ranges, and mean thermal conductivity of the solder alloys [17].

**Figure 15** shows the variation of the melting point of composite solder alloys with different dopant contents (both CNTs and Ni–CNTs) compared to the melting point of the Sn–57.6Bi–0.4Ag solder (about 140°C). It can be found that all the melting points were within the range of 139.3– 139.6°C [35]. It has been reported that both CNTs and Ni–CNTs have an effect of reducing the melting point of solder alloys [35]. In particular, with the combined effect of CNTs and Ni–CNTs doped Sn–57.6Bi–0.4Ag solder alloys showed relatively lower melting points than those of CNTs doped solder alloys [35]. However, such small additions of CNTs and Ni–CNTs cannot have a significant influence on the melting point of Sn–57.6Bi–0.4Ag solder alloy [35].


**Table 3.** Thermal behaviors of standard Sn–58Bi alloy, Sn–52Bi–1.8Sb alloys, and Sn–48Bi–xSn alloys [18].

**Figure 16** shows the DSC endothermic peaks of Sn–Bi nanocomposites reinforced with 0.02 or 0.05 g of reduced graphene nanosheets. While a large endothermic peak corresponding to the melting reaction in the range of 139.0°C of Sn–Bi solder has been observed, it was found that the melting point of Sn–Bi nanocomposites reinforced with reduced graphene nanosheets was about 139.0°C, which indicates that there was no significant effect on the thermal behavior of the nanocomposite solder, despite of the addition of reduced graphene nanosheets [11].

#### **3.4. Mechanical properties**

The durability and reliability of electronic products, as related to the mechanical properties of the solder joints, have become very important [13, 36–41]. This is especially true for portable, wearable devices, which frequently experience mechanical shock loadings caused by external forces [4]. Particularly for drop tests, during which the strain rate is very high, high mechanical shock resistance of solders is needed for these materials to fulfill their roles of structural materials. In addition, low melting point solders experience significantly high stresses during the reflow process owing to thermal gradient difference [3, 4]. Thus, there has been continuous interest in better understanding of the mechanical properties and in inventing high durability and reliability low melting point solders. One frequently utilized way to influence the mechanical properties of low melting point solder joints in a given system is to either alloy the materials or add small or large amounts of additional elements. In particular, any metal oxides or impurities may have marked effects on the mechanical properties of low melting point solders. Additional elements can fundamentally influence the mechanical properties of low melting point solders. First, additional elements can have an influence on the mechanical properties of the interfacial reactions between the solder and the substrate. Second, additives can positively change the mechanical properties of low melting point solders. Third, they can impart negative side effects, which result in a sacrifice of other mechanical properties of low Low Melting Temperature Solder Materials for Use in Flexible Microelectronic Packaging... http://dx.doi.org/10.5772/intechopen.70272 25

**Figure 16** shows the DSC endothermic peaks of Sn–Bi nanocomposites reinforced with 0.02 or 0.05 g of reduced graphene nanosheets. While a large endothermic peak corresponding to the melting reaction in the range of 139.0°C of Sn–Bi solder has been observed, it was found that the melting point of Sn–Bi nanocomposites reinforced with reduced graphene nanosheets was about 139.0°C, which indicates that there was no significant effect on the thermal behavior of the nanocomposite solder, despite of the addition of reduced graphene nanosheets [11].

**Table 3.** Thermal behaviors of standard Sn–58Bi alloy, Sn–52Bi–1.8Sb alloys, and Sn–48Bi–xSn alloys [18].

The durability and reliability of electronic products, as related to the mechanical properties of the solder joints, have become very important [13, 36–41]. This is especially true for portable, wearable devices, which frequently experience mechanical shock loadings caused by external forces [4]. Particularly for drop tests, during which the strain rate is very high, high mechanical shock resistance of solders is needed for these materials to fulfill their roles of structural materials. In addition, low melting point solders experience significantly high stresses during the reflow process owing to thermal gradient difference [3, 4]. Thus, there has been continuous interest in better understanding of the mechanical properties and in inventing high durability and reliability low melting point solders. One frequently utilized way to influence the mechanical properties of low melting point solder joints in a given system is to either alloy the materials or add small or large amounts of additional elements. In particular, any metal oxides or impurities may have marked effects on the mechanical properties of low melting point solders. Additional elements can fundamentally influence the mechanical properties of low melting point solders. First, additional elements can have an influence on the mechanical properties of the interfacial reactions between the solder and the substrate. Second, additives can positively change the mechanical properties of low melting point solders. Third, they can impart negative side effects, which result in a sacrifice of other mechanical properties of low

**3.4. Mechanical properties**

**Composition (wt.%)**

**Main peak temperature (°C)**

24 Recent Progress in Soldering Materials

**Sec peak temperature (°C)**

Sn–58Bi 143.1 139.4 148.0 8.6 Sn–52Bi–1.8Sb 147.7 140.6 152.0 11.4 Sn–48Bi–1.8Sb 146.5 163.0 140.9 172.7 31.8 Sn–44Bi–1.8Sb 146.9 169.0 141.9 180.5 38.6 Sn–48Bi–1.0Sb 144.7 162.0 140.6 168.7 28.1 Sn–48Bi–1.4Sb 146.8 163.3 141.2 170.4 29.2 Sn–48Bi–1.8Sb 146.5 163.0 140.9 172.7 31.8 Sn–48Bi–2.0Sb 147.6 164.4 142.3 169.7 27.4 Sn–48Bi–2.4Sb 148.5 163.3 142.8 169.3 26.5 Sn–48Bi–2.8Sb 148.0 162.6 143.6 168.4 24.8

**Solid temperature** 

**Liquid temperature (°C)** **Melting range** 

**(°C)**

**(°C)**

**Figure 15.** Plot of the variation of melting point of solder alloys with different amounts of CNTs or NI–CNTs [35].

melting point solders. In this section, therefore, we report on a number of investigations about the effects of different alloying elements, as well as the effects of metal oxides or impurities, in low melting point solders.

**Figure 16.** DSC of Sn–bi/RGOS nanocomposites with Sn content of 36.0 wt.% [11].

After the addition of 0.05 wt.% Cu6 Sn5 nanoparticles in Sn–Bi solder, the tensile properties of the solder underwent brittleness caused by a change in ductility [36]. However, nanoindentation testing revealed that the creep resistance of the Sn–Bi–Cu6 Sn5 solder is enhanced through the creep mechanism transformation [36]. In corrosion experiments, samples with Cu6 Sn5 nanoparticles exhibit a lower corrosion rate [36].

Adding different sized Ag nanoparticles to a eutectic Sn–Bi alloy system refined the grain (microstructure), suppressed the growth and expansion of the interfacial IMCs, and increased the shear strength of the solder joint [38]. To be specific, the reinforcement with 76 nm Ag nanoparticles refined the microstructure by 49.1% and enhanced the microhardness by 12.2% compared to the standard Sn–Bi solder because the extent of the formation of the Cu–Sn IMC decreased from 0.394 to 0.339, suppressing the IMC thickness by 39.7% and improving the shear strength by 18.9% after reflowing at 220°C for 180 min. However, after the addition of both larger (133 nm) and smaller (31 nm) Ag nanoparticles, such thermomechanical properties improvements were lower than those of the solder having 76 nm Ag nanoparticles [38]. These improvements might have been due to refinement and dispersion strengthening and adsorption [38]. In fact, although the solder with smaller Ag nanoparticles should have had higher property improvements than those with larger ones, the agglomeration of the smaller sized Ag nanoparticles deteriorated the overall solder's properties and reduced the practical improvements [38]. Overall, an optimal particle size was proposed to balance the theoretical improvement and the agglomeration weakening; this size generated the best real improvement [38].

The reinforcement effects of the Al<sup>2</sup> O3 nanoparticles in Sn–58Bi solder were investigated from the aspects of electromigration, shear strength, and microhardness [39]. The experimental results show that the Al2 O3 nanoparticles significantly improved the mechanical performances of the solder joints. The addition of Al2 O3 nanoparticles reduced the thickness of the Bi IMCs along the interfacial layers [39]. More specifically, the growth rate of the IMC thickness according to the addition of Al2 O3 nanoparticles decreased by 8% compared with that of pristine solder [39]. Furthermore, the microhardness of Al2 O3 -containing solder exhibited better performance than that of pristine solder according to aging time [39]. On the other hand, the addition of Al2 O3 significantly improved the shear strength of the solder joint after aging for 48 and 288 h [39]. More specifically, after the solder was aged for these time periods at 85°C, the amplitudes of the shear strength increased by 3.5% and 2.4%, respectively, because unlike the smooth surface of the pristine solder, the surface of the Al2 O3 -containing solder showed a ductile failure (fractured) state [39].

To improve the mechanical behaviors of a Sn–58Bi/Cu joint, a minor amount of elemental Zn was alloyed into the Cu substrate [40]. The interfacial IMC growth and bending properties of Sn–58Bi/Cu and Sn–58Bi/Cu–2.29Zn were studied according to the effect of isothermal liquid and solid aging [40]. Although there was no significant change in the composition, thickness, or morphology of the interfacial IMC under liquid aging, the depressing of IMC growth at the interface between the Sn–58Bi solder and the substrate and the avoidance of the formation of Cu3 Sn IMC, Kirkendall voids, and Bi segregation at the IMC/Cu interface were realized for the Cu–Zn substrate under isothermal solid aging [40]. Joint strength and fracture behavior were also improved when using the Cu–Zn substrate [40]. There was no obvious decrease in the joint strength, and fracturing during bending was found mainly to occur in the solder matrix with ductile fracture mode or along the solder/IMC interface with partly brittle fracture mode for the Cu–Zn joint; these behaviors can be compared with the dramatically decreased joint strength and brittle fracture mode that occurred along the interface between IMC and Cu in Sn–Bi/Cu joints after aging [40].

After the addition of 0.05 wt.% Cu6

26 Recent Progress in Soldering Materials

Cu6 Sn5 Sn5

agglomeration weakening; this size generated the best real improvement [38].

dentation testing revealed that the creep resistance of the Sn–Bi–Cu6

nanoparticles exhibit a lower corrosion rate [36].

**Figure 16.** DSC of Sn–bi/RGOS nanocomposites with Sn content of 36.0 wt.% [11].

of the solder underwent brittleness caused by a change in ductility [36]. However, nanoin-

through the creep mechanism transformation [36]. In corrosion experiments, samples with

Adding different sized Ag nanoparticles to a eutectic Sn–Bi alloy system refined the grain (microstructure), suppressed the growth and expansion of the interfacial IMCs, and increased the shear strength of the solder joint [38]. To be specific, the reinforcement with 76 nm Ag nanoparticles refined the microstructure by 49.1% and enhanced the microhardness by 12.2% compared to the standard Sn–Bi solder because the extent of the formation of the Cu–Sn IMC decreased from 0.394 to 0.339, suppressing the IMC thickness by 39.7% and improving the shear strength by 18.9% after reflowing at 220°C for 180 min. However, after the addition of both larger (133 nm) and smaller (31 nm) Ag nanoparticles, such thermomechanical properties improvements were lower than those of the solder having 76 nm Ag nanoparticles [38]. These improvements might have been due to refinement and dispersion strengthening and adsorption [38]. In fact, although the solder with smaller Ag nanoparticles should have had higher property improvements than those with larger ones, the agglomeration of the smaller sized Ag nanoparticles deteriorated the overall solder's properties and reduced the practical improvements [38]. Overall, an optimal particle size was proposed to balance the theoretical improvement and the

nanoparticles in Sn–Bi solder, the tensile properties

Sn5

solder is enhanced

Sn–57.6Bi–0.4Ag solder was reinforced with tungsten (W) nanoparticles at a concentration of 0.5 wt.% [41]. Due to the dispersion of W nanoparticles and the consequently refined microstructure, the mechanical properties of the solder alloy were enhanced, as indicated by the 6.2% improvement in the microhardness [41]. During electromigration, the segregation of the Sn-rich and Bi-rich phases and the accumulation of an (Au, Ni), (Sn, Bi)4 layer at the cathode interface were also alleviated by the addition of W nanoparticles, which improved the electromigration resistance [41].

The tensile properties of eutectic Sn–Bi, Sn–Bi–0.5In, and Sn–Bi–0.5Ni solder alloys, and their shear strength as Cu/solder/Cu joints were investigated [15]. The addition of 0.5 wt.% Ni decreased the elongation property of the Sn–Bi alloy because of the formation of Ni3 Sn4 IMCs [15]. The In-bearing solder alloys exhibited the greatest elongation among all the tensile-tested solder alloys [15]. The eutectic Sn–Bi and Ni-bearing solder joints exhibited degraded shear strength owing to the formation of coarsened Bi-rich phases [15]. The thermally aged Sn–Bi solder joints on Cu substrates exhibited a harshly fractured surface structure in the IMC layers at the interfacial boundaries, whereas the thermally aged In- and Ni-containing Sn–Bi solder joints showed a smoothly fractured surface structure because of the growth suppression of Cu–Sn IMCs [15]. In particular, the as-reflowed In-containing solder joints had a dimple-like, fractured surface structure, indicating a ductile microstructure because the thermally aged In-containing solder joints retained their ductile property well, while both the coarsened, fractured surface structure and excessive IMC growth of the thermally aged Sn–Bi solder joints at the interfacial boundaries is able to explain their mechanical degradation [15].

Four different concentrations of Ni (i.e. 0.05, 0.1, 0.5, and 1.0 wt.%) were individually added to Sn–58Bi samples, and respective microstructure, tensile strength, elongation, and wettability of Sn–58Bi–xNi were subsequently measured [29]. The results indicate that Ni refined the microstructure of the solder matrix and induced the formation of the Ni<sup>3</sup> Sn4 phase; furthermore, the formation and then continuously increasing concentration of Ni3 Sn4 were proportional to the increase of Ni added to the solder [29]. Thus, the optimal concentration of Ni added to enhance the solder's tensile strength should be less than 0.1 wt.% [29]. Nevertheless, the elongation of the alloy was in fact inversely proportional to the increase of the added Ni content, although the appropriate incorporation of Ni contributed positively to the wettability of the solder alloy [29].

When the In content increased to 4% in the Sn–Bi alloy, tensile test results showed that the tensile strength increased slightly with the increase of added In, while the elongation first increased remarkably and then decreased after the addition of 2.5 wt.% In [13]. The diffused In was confirmed to participate in interfacial reactions, thereby forming Cu–Sn–In IMCs and affecting the wettability of the Sn–Bi solder on the Cu substrate [13]. Tensile strength changed slightly with increasing In addition, while the elongation increased remarkably with the addition of 2.5 wt.% In [13].

The interfacial reaction kinetics, tensile strength, and creep resistance of the Sn–58Bi–xZn (x = 0.0 or 0.7 wt.%) solder samples during liquid-state aging were investigated [33]. With the addition of 0.7 wt.% Zn, ultimate tensile strength (UTS) values of the eutectic Sn–Bi solder increased by 6.05 and 5.50% after soldering and aging, respectively; those values for the Cu/Sn–Bi/Cu solder joints also increased by 21.51 and 29.27%, respectively [33]. The increase in strengthening of the Cu/Sn–Bi–xZn/Cu solder joints can be attributed to the phase transformation at each Cu/IMC/solder interface due to the formation of finer Bi grains according to the addition of Zn [33].

The effect of Sb content on the mechanical properties of Sn–Bi solders was studied [18]. The mechanical properties of the solders/Cu joints were also evaluated [18]. The results show that the ternary alloy solders contain eutectic structures resulting from a quasi-peritetic reaction [18]. With the increase of the Sb content, the size of the eutectic structure increases [18]. A small amount of Sb has a large impact on the wettability of the Sn–Bi solders [18]. Reaction layers form during the spreading process [18]. Sb is detected in the reaction layer, while Bi is not detected [18]. The total thickness of the reaction layer between the solder and Cu increases with increased Sb [18]. The shear strength of the Sn–Bi–Sb solders also increases as the Sb content increases [18].

The mechanical properties of the melt-spun Bi–42Sn, Bi–40Sn–2In, Bi–40Sn–2Ag, and Bi–38Sn– 2In–2Ag alloys were studied using dynamic resonance and Vickers indentation techniques at room temperature and compared to the mechanical properties of the traditional Sn–Pb eutectic alloy [42]. The results show that the crystallographic structure of the Bi–42Sn alloy presents as a combination of body centered tetragonal Sn and rhombohedral Bi [42]. The two ternary alloys exhibit additional constituent phases of SnIn19 for Bi–40Sn–2In and Ag3 Sn for Bi–40Sn–2Ag alloys [42]. Attention has been paid to the role of IMCs in the mechanical and creep behavior [42]. The In- and Ag-containing solder alloys exhibited a good combination of higher creep resistance as compared with the Pb–Sn eutectic solder alloy [42]. This was attributed to the strengthening effect of Bi in the Sn matrix and the formation of InSn19 and Ag3 Sn IMCs, which act as grain refiners in the matrix material [42].

joints showed a smoothly fractured surface structure because of the growth suppression of Cu–Sn IMCs [15]. In particular, the as-reflowed In-containing solder joints had a dimple-like, fractured surface structure, indicating a ductile microstructure because the thermally aged In-containing solder joints retained their ductile property well, while both the coarsened, fractured surface structure and excessive IMC growth of the thermally aged Sn–Bi solder joints at

Four different concentrations of Ni (i.e. 0.05, 0.1, 0.5, and 1.0 wt.%) were individually added to Sn–58Bi samples, and respective microstructure, tensile strength, elongation, and wettability of Sn–58Bi–xNi were subsequently measured [29]. The results indicate that Ni refined the

tional to the increase of Ni added to the solder [29]. Thus, the optimal concentration of Ni added to enhance the solder's tensile strength should be less than 0.1 wt.% [29]. Nevertheless, the elongation of the alloy was in fact inversely proportional to the increase of the added Ni content, although the appropriate incorporation of Ni contributed positively to the wettability

When the In content increased to 4% in the Sn–Bi alloy, tensile test results showed that the tensile strength increased slightly with the increase of added In, while the elongation first increased remarkably and then decreased after the addition of 2.5 wt.% In [13]. The diffused In was confirmed to participate in interfacial reactions, thereby forming Cu–Sn–In IMCs and affecting the wettability of the Sn–Bi solder on the Cu substrate [13]. Tensile strength changed slightly with increasing In addition, while the elongation increased remarkably with the addi-

The interfacial reaction kinetics, tensile strength, and creep resistance of the Sn–58Bi–xZn (x = 0.0 or 0.7 wt.%) solder samples during liquid-state aging were investigated [33]. With the addition of 0.7 wt.% Zn, ultimate tensile strength (UTS) values of the eutectic Sn–Bi solder increased by 6.05 and 5.50% after soldering and aging, respectively; those values for the Cu/Sn–Bi/Cu solder joints also increased by 21.51 and 29.27%, respectively [33]. The increase in strengthening of the Cu/Sn–Bi–xZn/Cu solder joints can be attributed to the phase transformation at each Cu/IMC/solder interface due to the formation of finer Bi grains according

The effect of Sb content on the mechanical properties of Sn–Bi solders was studied [18]. The mechanical properties of the solders/Cu joints were also evaluated [18]. The results show that the ternary alloy solders contain eutectic structures resulting from a quasi-peritetic reaction [18]. With the increase of the Sb content, the size of the eutectic structure increases [18]. A small amount of Sb has a large impact on the wettability of the Sn–Bi solders [18]. Reaction layers form during the spreading process [18]. Sb is detected in the reaction layer, while Bi is not detected [18]. The total thickness of the reaction layer between the solder and Cu increases with increased Sb [18]. The shear strength of the Sn–Bi–Sb solders also increases as the Sb

Sn4

Sn4

phase; further-

were propor-

the interfacial boundaries is able to explain their mechanical degradation [15].

microstructure of the solder matrix and induced the formation of the Ni<sup>3</sup>

of the solder alloy [29].

28 Recent Progress in Soldering Materials

tion of 2.5 wt.% In [13].

to the addition of Zn [33].

content increases [18].

more, the formation and then continuously increasing concentration of Ni3

Sn–57.6Bi–0.4Ag solder joints with different contents of CNTs and Ni–CNTs were investigated [35]. In particular, it was possible to improve the mechanical properties of the Sn57.6Bi0.4Ag solder joints by the addition of either CNTs or Ni–CNTs, and those with the addition of 0.05 wt.% CNTs or 0.07 wt.% Ni–CNTs showed the best mechanical performance [35]. With the addition of either CNTs or Ni–CNTs, the solder joints had rougher, fractured surface structures, resulting in better bonding properties. Although reinforcement with either CNTs or Ni–CNTs improved the mechanical performance of solder joints, Ni–CNTs worked much better [35]. Ni coating was proven to significantly inhibit the aggregation of CNTs, which can induce cracks and wetting problems and even deteriorate the strength of solder joints [35].

Sn–Bi composite solders containing Ni–CNTs were successfully synthesized [43]. The mechanical properties of Sn–Bi with different weight percentages of Ni–CNT were investigated [43]. The UTS and elongation of the Sn–Bi–0.05(Ni–CNT) solder with the optimized amount (0.05 wt.%) of Ni–CNTs increased remarkably because the CNTs and Ni3 Sn4 enhanced the wettability and bondability of the composite solder [43]. However, because of the presence of CNT clusters and the intrinsic brittleness of IMCs, the UTS and elongation degraded with increased addition of Ni–CNTs [43]. That is to say, the UTS of the solder joint reached its maximum value with 0.05 wt.% Ni–CNTs addition and then degraded after increased addition of Ni–CNT [43]. Moreover, the tensile strength of the composite solder was much higher than that of the pristine solder, and subsequently, the creep resistance and hardness of the Sn–Bi–0.05(Ni–CNT) solder increased significantly compared to those of the Sn–Bi solder [43]. However, the hardness and the creep performance also decreased with 0.1 and 0.2 wt.% CNT content due to the same reasons mentioned above [43]. The CNT clusters and pore formation in the presence of the IMC with its intrinsic brittleness contributed to the decreases of hardness and creep performance [43].

The mechanical strength and ductility of the eutectic Sn–Bi solder alloy were dependent on the incorporation of MWCNTs [44]. Mechanical test results show that the bending strength of the Sn–Bi–0.03CNT composite increased by 10.5% compared to that of the reference Sn–Bi alloy, which can be attributed to the reduction in Sn-rich segregation and to grain refinement [44]. In particular, the toughness of the Sn–58Bi–0.03CNT composite increased by 48.9% compared to that of the unreinforced Sn–Bi solder alloy [44]. In addition, corresponding fracture surface comparison between the Sn–58Bi–0.03CNT composite and the monolithic Sn–58Bi alloy was performed to identify the influence of CNTs on the fracture behavior [44].

The effects of graphene nanosheets on the mechanical properties of the Sn–58Bi–0.7Zn solder joint were investigated [45]. Experimental results and finite element simulations showed that the best mechanical property improvement came from the 0.076 wt.% graphene nanosheetdoped Sn–58Bi–0.7Zn sample [45]. For the thermal aging samples, the UTS of the solder joint was also increased by 2.04% [45].

The mechanical properties (the stress expansion and strain distribution during a single lap shear test) of the Sn–Bi–graphene nanocomposite according to the weight ratio of graphene were simulated based on the theoretical calculations of the finite element method [46]. The strength of the joint was found to be mainly influenced by the shear stress; initial cracking was found to occur at the edge of the joint [46]. The shear modulus of the Sn–Bi–graphene nanocomposite was 192% greater than that of the pure Sn–Bi alloy, when the content of graphene increased to 1.0 wt.% [46]. Stress concentration was found to exist near the edge of the graphene, where initial failure may occur [46].

Graphene nanosheets were successfully incorporated at various percentages (0, 0.01, 0.03, 0.05, or 0.1 wt.%) into Sn–58Bi solder [19]. The tensile properties, wettability, corrosion resistance, microhardness, and creep behavior were subsequently improved [19]. Tensile and nanoindentation tests reveal that the composite solder with 0.1 wt.% graphene nanosheets leads to enhancements of about 14 and 38%, respectively [19]. With 0.01 wt.% graphene nanosheet addition, the elongation is 49% greater than that of the pure Sn–58Bi solder alloy [19]. The creep performance and the corrosion resistance are all enhanced by addition of graphene nanosheets [19]. The mechanism of enhancement of the graphene nanosheets of the performance of the composite solder alloy is also analyzed in this work [19]. Tensile tests reveal that the UTS of the solders rises gradually with graphene nanosheet addition; there is a 14% enhancement of tensile strength for the Sn–58Bi–0.1graphene nanosheet [19]. The huge enhancement of 49% in the elongation of Sn–58Bi–0.01graphene nanosheet and the establishment of a brittle to ductile fracture mode are induced by the strengthening effect of graphene nanosheets [19]. The wettability is improved with graphene nanosheet addition because the nanosheets lower the interfacial surface energy between the solder and the substrate [19]. Moreover, the corrosion resistance is distinctly enhanced in the Sn–58Bi–0.1graphene nanosheet, and this material retains a lower corrosion rate than that of Sn–58Bi [19]. The hardness and creep resistance leads to an obvious improvement due to the addition of graphene nanosheets [19]. The hardness is enhanced by 38%, when the addition of graphene nanosheets increases to 0.1 wt.% [19]. The enhancement of the creep behavior is further illustrated by the variation of the creep mechanism in the solder alloys [19]. Among the composite solders synthesized, the Sn–58Bi–0.1graphene nanosheet provides the best tensile strength and hardness with decreased ductility (**Table 4**) [19].

Low Melting Temperature Solder Materials for Use in Flexible Microelectronic Packaging... http://dx.doi.org/10.5772/intechopen.70272 31


by 48.9% compared to that of the unreinforced Sn–Bi solder alloy [44]. In addition, corresponding fracture surface comparison between the Sn–58Bi–0.03CNT composite and the monolithic Sn–58Bi alloy was performed to identify the influence of CNTs on the fracture

The effects of graphene nanosheets on the mechanical properties of the Sn–58Bi–0.7Zn solder joint were investigated [45]. Experimental results and finite element simulations showed that the best mechanical property improvement came from the 0.076 wt.% graphene nanosheetdoped Sn–58Bi–0.7Zn sample [45]. For the thermal aging samples, the UTS of the solder joint

The mechanical properties (the stress expansion and strain distribution during a single lap shear test) of the Sn–Bi–graphene nanocomposite according to the weight ratio of graphene were simulated based on the theoretical calculations of the finite element method [46]. The strength of the joint was found to be mainly influenced by the shear stress; initial cracking was found to occur at the edge of the joint [46]. The shear modulus of the Sn–Bi–graphene nanocomposite was 192% greater than that of the pure Sn–Bi alloy, when the content of graphene increased to 1.0 wt.% [46]. Stress concentration was found to exist near the edge of the

Graphene nanosheets were successfully incorporated at various percentages (0, 0.01, 0.03, 0.05, or 0.1 wt.%) into Sn–58Bi solder [19]. The tensile properties, wettability, corrosion resistance, microhardness, and creep behavior were subsequently improved [19]. Tensile and nanoindentation tests reveal that the composite solder with 0.1 wt.% graphene nanosheets leads to enhancements of about 14 and 38%, respectively [19]. With 0.01 wt.% graphene nanosheet addition, the elongation is 49% greater than that of the pure Sn–58Bi solder alloy [19]. The creep performance and the corrosion resistance are all enhanced by addition of graphene nanosheets [19]. The mechanism of enhancement of the graphene nanosheets of the performance of the composite solder alloy is also analyzed in this work [19]. Tensile tests reveal that the UTS of the solders rises gradually with graphene nanosheet addition; there is a 14% enhancement of tensile strength for the Sn–58Bi–0.1graphene nanosheet [19]. The huge enhancement of 49% in the elongation of Sn–58Bi–0.01graphene nanosheet and the establishment of a brittle to ductile fracture mode are induced by the strengthening effect of graphene nanosheets [19]. The wettability is improved with graphene nanosheet addition because the nanosheets lower the interfacial surface energy between the solder and the substrate [19]. Moreover, the corrosion resistance is distinctly enhanced in the Sn–58Bi–0.1graphene nanosheet, and this material retains a lower corrosion rate than that of Sn–58Bi [19]. The hardness and creep resistance leads to an obvious improvement due to the addition of graphene nanosheets [19]. The hardness is enhanced by 38%, when the addition of graphene nanosheets increases to 0.1 wt.% [19]. The enhancement of the creep behavior is further illustrated by the variation of the creep mechanism in the solder alloys [19]. Among the composite solders synthesized, the Sn–58Bi–0.1graphene nanosheet provides the best tensile strength and hardness

behavior [44].

30 Recent Progress in Soldering Materials

was also increased by 2.04% [45].

graphene, where initial failure may occur [46].

with decreased ductility (**Table 4**) [19].

**Table 4.** The improvement of various mechanical properties of the low melting temperature solders.
