**3. Classification of low melting temperature solders and terminologies for low melting temperature solder design**

Conventional, low melting temperature solders are fabricated using either Sn–Bi or Sn–In [12]. In particular, Sn–Bi solders have received considerable attention because of their outstanding merits, including high wetting behavior, large creep resistance, and low coefficient of thermal expansion [12–14]. However, the relatively low mechanical strength and melting temperature (138°C) of these materials require improvement for their more effective use in flexible interconnection applications [3, 15]. Comparatively, Sn–In solder, which has a low melting temperature (118 °C), has excellent electrical and thermal properties [12, 16]. However, the price of In is very high, and this material includes high amounts of IMCs, which degrade the mechanical properties of the solder [16]. Thus, the incorporation of additives, including alloying, doping, or the use of reinforcement materials, into Sn–Bi or Sn–In alloy systems has been considered a useful strategy for improving the mechanical properties.

### **3.1. Microstructure with regard to grain size and solid solution**

The microstructure reflects the mechanical properties of a solder [15–21]. Based on the microstructural analysis of low melting point solders, specific phases and their distribution in the microstructure can be observed, and these characteristics can be used to describe intended properties [15–21]. In this section, we show how the microstructure of low melting point solders is altered by the incorporation of an additive, especially with regard to grain size and solid solution.

The representative microstructure of eutectic Sn–Bi alloy is shown in **Figure 1**; in this structure, granular Sn-rich grains and similarly granular Bi-rich grains can be seen. The dark and bright gray regions are Sn and Bi, respectively; these regions appear as interlocked lamellar structures. Upon incorporation of an additive into the Sn–Bi solder, the solder microstructure was found to be remarkably transformed due to the formation of finer grains or the presence of new solid solutions or precipitates. Usually, a plausible explanation for this is that Sn- and Bi-rich grains are heterogeneously nucleated in the formation of certain IMCs. For example, Mokhtari et al. demonstrated that the addition of In or Ni can modify the microstructure of Sn–Bi solder; in particular, the addition of 0.5 wt.% In was able to suppress the coarsening of the Bi-rich phase, which means that the Sn–Bi–0.5In solder comprised primary Sn dendrites and eutectic phases [13, 15]. Comparatively, Ni appears to have been included in the Sn phase since Ni-containing Sn–Bi solder exhibited eutectic phases and the Ni<sup>3</sup> Sn4 IMC but did not show any sign of coarsening due to the Bi-rich phase.

**Figure 2** exhibits the cross-sectional microstructure of a Sn–40Bi–2Zn–0.1Cu solder alloy composed of Sn-, Bi-, Zn-, and Cu-rich phases. For the cooling (solidification) process of the Sn–40Bi–2Zn–0.1Cu solder alloy from the liquid state, the following procedure took place: L (liquid) → L + primary Sn → primary Sn + eutectic (β-Sn + Bi-rich) + eutectic (β-Sn + Zn-rich) → primary Sn + secondary precipitated Bi + eutectic (β-Sn + secondary precipitated Bi + Bi-rich) + eutectic

**Figure 1.** SEM micrographs of (a) eutectic Sn–Bi, (b) Sn–Bi–0.5In, and (c) Sn–Bi–0.5Ni [15].

Low Melting Temperature Solder Materials for Use in Flexible Microelectronic Packaging... http://dx.doi.org/10.5772/intechopen.70272 11

merits, including high wetting behavior, large creep resistance, and low coefficient of thermal expansion [12–14]. However, the relatively low mechanical strength and melting temperature (138°C) of these materials require improvement for their more effective use in flexible interconnection applications [3, 15]. Comparatively, Sn–In solder, which has a low melting temperature (118 °C), has excellent electrical and thermal properties [12, 16]. However, the price of In is very high, and this material includes high amounts of IMCs, which degrade the mechanical properties of the solder [16]. Thus, the incorporation of additives, including alloying, doping, or the use of reinforcement materials, into Sn–Bi or Sn–In alloy systems has been

The microstructure reflects the mechanical properties of a solder [15–21]. Based on the microstructural analysis of low melting point solders, specific phases and their distribution in the microstructure can be observed, and these characteristics can be used to describe intended properties [15–21]. In this section, we show how the microstructure of low melting point solders is altered by the incorporation of an additive, especially with regard to grain size and solid solution. The representative microstructure of eutectic Sn–Bi alloy is shown in **Figure 1**; in this structure, granular Sn-rich grains and similarly granular Bi-rich grains can be seen. The dark and bright gray regions are Sn and Bi, respectively; these regions appear as interlocked lamellar structures. Upon incorporation of an additive into the Sn–Bi solder, the solder microstructure was found to be remarkably transformed due to the formation of finer grains or the presence of new solid solutions or precipitates. Usually, a plausible explanation for this is that Sn- and Bi-rich grains are heterogeneously nucleated in the formation of certain IMCs. For example, Mokhtari et al. demonstrated that the addition of In or Ni can modify the microstructure of Sn–Bi solder; in particular, the addition of 0.5 wt.% In was able to suppress the coarsening of the Bi-rich phase, which means that the Sn–Bi–0.5In solder comprised primary Sn dendrites and eutectic phases [13, 15]. Comparatively, Ni appears to have been included in the Sn phase

**Figure 2** exhibits the cross-sectional microstructure of a Sn–40Bi–2Zn–0.1Cu solder alloy composed of Sn-, Bi-, Zn-, and Cu-rich phases. For the cooling (solidification) process of the Sn–40Bi–2Zn–0.1Cu solder alloy from the liquid state, the following procedure took place: L (liquid) → L + primary Sn → primary Sn + eutectic (β-Sn + Bi-rich) + eutectic (β-Sn + Zn-rich) → primary Sn + secondary precipitated Bi + eutectic (β-Sn + secondary precipitated Bi + Bi-rich) + eutectic

Sn4

IMC but did not

considered a useful strategy for improving the mechanical properties.

since Ni-containing Sn–Bi solder exhibited eutectic phases and the Ni<sup>3</sup>

**Figure 1.** SEM micrographs of (a) eutectic Sn–Bi, (b) Sn–Bi–0.5In, and (c) Sn–Bi–0.5Ni [15].

show any sign of coarsening due to the Bi-rich phase.

**3.1. Microstructure with regard to grain size and solid solution**

10 Recent Progress in Soldering Materials

**Figure 2.** SEM micrograph and EDS analysis results of (a) Cu<sup>6</sup> Sn5 phase in Sn–40Bi–0.1Cu, (b) CuZn<sup>2</sup> phase in Sn–40Bi– 2Zn–0.1Cu, and (c) Cu<sup>5</sup> Zn<sup>8</sup> phase in Sn–40Bi–2Zn–0.1Cu [17].

(β-Sn + Zn-rich). Different from the reference Sn–40Bi–0.1Cu solder alloy, Cu<sup>6</sup> Sn5 precipitates were not found in the Sn–40Bi–2Zn–0.1Cu solder because Cu reacts more strongly with Zn than Sn does; thus, all of the Cu was consumed in the formation of Cu–Zn IMCs [17]. These results were also very similar to the studies of Islam and Li [22, 23]. According to the X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) analysis, globular CuZn<sup>2</sup> and blocky Cu5 Zn<sup>8</sup> precipitates were formed in Sn–40Bi–2Zn–0.1Cu solder; in particular, Zn atoms segregated between the Sn- and Bi-rich matrices and reacted with Cu atoms to form Cu–Zn IMC particles [17].

**Figure 3** shows the microstructures of the Sn–58Bi and Sn–Bi–Sb alloys. The Sn–58Bi alloy shows a typical eutectic structure. The dark region is the Sn phase; the white region represents the Bi phase. For the Sn–Bi–Sb alloys, each part contains two phases, the dark and the light phases shown, respectively, in the images. When the Bi content changes, the proportion of each structure does not greatly change. However, the proportion of quasi-peritectic structure increases, as the Sb content increases [18]. For the Sn–Bi–Sb alloy, the intensity of the Sn-rich phases increased when the Sb content increased [18].

The near eutectic solder (In–50Sn) is presented in **Figure 4a**, showing a cross-sectional microstructure consisting of a combination of lamellar and irregular phases. In particular, the presence of a mixture of Sn-rich and In-rich phases and the In<sup>3</sup> Sn IMC was determined by XRD and EDS analyses. Comparatively, **Figure 4b** shows the cross-sectional microstructure of the In–30Sn solder, which consisted of only two specific phases: an In-rich phase and the In<sup>3</sup> Sn IMC but the absence of Sn-rich phases and/or other Sn IMCs [16].

As can be seen in **Figure 5**, the interfacial bonding of low melting temperature Sn–In solder on the Cu substrate is presented according to the reflow temperature and duration. The reflow at 180°C for 20 min selected was the optimal condition due to the high bonding strength of 6.5 MPa and the interfacial layer with less microvoids or mechanical cracks [20]. Especially, the reflow temperature of 180°C was high enough for the active diffusion of the low melting

**Figure 3.** SEM micrographs of Sn–58Bi and Sn–Bi–Sb alloys: (a and d) Sn–58Bi, (b and e) Sn–52Bi–1.8Sb, (c and f) Sn–48Bi–1.8Sb, (g) Sn–48Bi–1.4Sb, (h) Sn–48Bi–1.8Sb, and (i) Sn–48Bi–2.4Sb [18].

Low Melting Temperature Solder Materials for Use in Flexible Microelectronic Packaging... http://dx.doi.org/10.5772/intechopen.70272 13

**Figure 4.** SEM micrographs of (a) In–50Sn and (b) In–30Sn solders [16].

the Bi phase. For the Sn–Bi–Sb alloys, each part contains two phases, the dark and the light phases shown, respectively, in the images. When the Bi content changes, the proportion of each structure does not greatly change. However, the proportion of quasi-peritectic structure increases, as the Sb content increases [18]. For the Sn–Bi–Sb alloy, the intensity of the Sn-rich

The near eutectic solder (In–50Sn) is presented in **Figure 4a**, showing a cross-sectional microstructure consisting of a combination of lamellar and irregular phases. In particular, the pres-

and EDS analyses. Comparatively, **Figure 4b** shows the cross-sectional microstructure of the In–30Sn solder, which consisted of only two specific phases: an In-rich phase and the In<sup>3</sup>

As can be seen in **Figure 5**, the interfacial bonding of low melting temperature Sn–In solder on the Cu substrate is presented according to the reflow temperature and duration. The reflow at 180°C for 20 min selected was the optimal condition due to the high bonding strength of 6.5 MPa and the interfacial layer with less microvoids or mechanical cracks [20]. Especially, the reflow temperature of 180°C was high enough for the active diffusion of the low melting

**Figure 3.** SEM micrographs of Sn–58Bi and Sn–Bi–Sb alloys: (a and d) Sn–58Bi, (b and e) Sn–52Bi–1.8Sb, (c and f)

Sn–48Bi–1.8Sb, (g) Sn–48Bi–1.4Sb, (h) Sn–48Bi–1.8Sb, and (i) Sn–48Bi–2.4Sb [18].

Sn IMC was determined by XRD

Sn

phases increased when the Sb content increased [18].

12 Recent Progress in Soldering Materials

ence of a mixture of Sn-rich and In-rich phases and the In<sup>3</sup>

IMC but the absence of Sn-rich phases and/or other Sn IMCs [16].

temperature solders, although they were dissolved in the Cu substrates and formed IMCs such as Cu6 (Sn, In)5 and Cu11(In, Sn)9 [20]. Therefore, the joint can sustain high service temperatures because it is formed completely from these IMCs.

Chen et al. determined the interfacial reactions in Sn–51In, Sn–20In, and Sn–20In–2.8Ag on Ag substrates reacted at various temperatures, with results shown in **Figure 6** [21]. Particularly for the Sn–51In/Ag couples, the reaction products are AgIn2 and Ag2 In phases at 150°C and 100°C; only Ag2 In is formed at lower temperatures [21]. Due to the formation of different reaction phases, the reaction layer in the Sn–51In/Ag couples grows more slowly at 100°C than is the case for samples reacted at lower temperatures [21]. The interfacial reaction rates in the Sn–20In/Ag couples are much slower than those in the Sn–51In/Ag couples [21]. In the Sn–20In/Ag couples, the ζ-phase is formed at 250°C, and both ζ-Ag/AgIn<sup>2</sup> phases are formed at 125°C; however, no noticeable interfacial reactions are observed to have reacted at 75 and 100°C over a period of 1440 h [21].

The species of IMCs at the interface between the eutectic Sn–In solder and the single crystalline Cu substrate were systematically investigated using scanning electron microscopy (SEM) (**Figure 7**). After reflowing at 160°C for 5 s, two kinds of IMC were formed in three sublayers from the solder to the substrate side; the formed materials were a Cu(In, Sn)2 layer with tetragonal crystal structure, a coarse-grain Cu2 (In, Sn) sublayer, and a fine-grain Cu2 (In, Sn) sublayer with hexagonal crystal structure [24]. The morphology of the Cu(In, Sn)2 grains is chunk type, the largest grain size. In the process of increased liquid soldering, this Cu(In, Sn)2 layer is prone to spalling into the solder, leaving a duplex structure of Cu2 (In, Sn) as the dominating IMC, which should be paid attention during phase identification [24]. The fine-grain Cu<sup>2</sup> (In, Sn) shows a granule-type morphology with the smallest grain size; this material distributes homogeneously on the entire Cu substrates [24]. However, coarse-grain Cu2 (In, Sn) is substrate dependent and has an elongated morphology on single crystalline Cu surfaces [24].

The SEM micrographs (**Figure 8**) show Sn–58Bi solders doped with different weight fractions of graphene nanosheets (GNSs). In particular, **Figure 8a** shows the typical lamellar structure of the eutectic Sn–58Bi solder, in which the dark regions represent the Sn-rich phase, while the white regions represent the Bi-rich phase. Compared to the

**Figure 5.** SEM micrographs (top) of as-received coating surface: (a) top view and (b) cross section. Cross-sectional SEM images (bottom) show that at the interfacial layers between the Sn-based solder and the cu substrate, the two components reacted and diffused under the following conditions (reflow temperature (°C) and duration (min)): (a) 140 and 5; (b) 140 and 20; (c) 160 and 5; (d) 160 and 20; (e) 180 and 5; and (f) 180 and 20 [20].

standard Sn–58Bi solder alloy, the microstructure of the composite solder is refined due to the increased content of graphene nanosheets, as shown in **Figure 8b–e**. The average grain size of the pure sample was about 1.6 μm, which is higher than that of the graphene Low Melting Temperature Solder Materials for Use in Flexible Microelectronic Packaging... http://dx.doi.org/10.5772/intechopen.70272 15

**Figure 6.** Backscattered electron imaging (BEI) micrographs (top) of (a) Sn–1.98 at.% In–8.03 at.% Ag, (b) Sn–10.02 at.% In–10.01 at.% Ag, (c) Sn–16.99 at.% In–80.01 at.% Ag, and (d) Sn–29.99 at.% In–68.99 at.% Ag annealed at 250°C for 12 weeks. BEI micrographs (bottom) of Sn–51In/Ag couple reacted at (a) 75°C, (b) 50°C, and (c) 25°C for 120 h [21].

standard Sn–58Bi solder alloy, the microstructure of the composite solder is refined due to the increased content of graphene nanosheets, as shown in **Figure 8b–e**. The average grain size of the pure sample was about 1.6 μm, which is higher than that of the graphene

**Figure 5.** SEM micrographs (top) of as-received coating surface: (a) top view and (b) cross section. Cross-sectional SEM images (bottom) show that at the interfacial layers between the Sn-based solder and the cu substrate, the two components reacted and diffused under the following conditions (reflow temperature (°C) and duration (min)): (a) 140 and 5; (b) 140

and 20; (c) 160 and 5; (d) 160 and 20; (e) 180 and 5; and (f) 180 and 20 [20].

14 Recent Progress in Soldering Materials

**Figure 7.** Cross-sectional SEM micrograph at the interface between eutectic Sn–In solder and single crystalline (1 1 1) Cu after reflowing at 160°C for 5 s [24].

nanosheet-doped solder samples [19]. The size reductions of the Sn–58Bi–graphene nanosheet (0.01, 0.03, 0.05, 0.1 wt.%) solders were about 28, 55, 30, and 32%, respectively [19]. These results indicate that the growth of grains was suppressed, and the microstructure of the Sn–58Bi solder alloys was refined by the addition of graphene nanosheets; in addition, the solder alloy with 0.03 wt.% graphene nanosheet addition showed the smallest average grain size [19]. The microstructure of local refinement can be found in **Figure 8b** for the 0.01 wt.% graphene nanosheet addition; meanwhile, uniform refinement of the structure and small Bi grains appear in the Sn–58Bi reinforced with 0.03 wt.% graphene nanosheets, shown in **Figure 8c**. These images indicate that the growth of grains can be suppressed by the presence of graphene nanosheets, which is caused by the high barrier of graphene nanosheets against the diffusion of metal atoms [19]. However, with more graphene nanosheets introduced, the growth of metal grains along the surface of the graphene nanosheets can be promoted [19]. As a result, the distinct feature of the uniformly distributed dendritic structure in Sn–58Bi reinforced with 0.05 wt.% graphene nanosheets can be seen in **Figure 8d**; this structure shows that graphene nanosheets can promote the growth of Bi dendrites. Moreover, local grain aggregation recurs and partial grains growth follows an inverse pattern when the content of graphene nanosheets reaches 0.1 wt.%, as shown in **Figure 8e**. This result is attributed to the increasing addition of graphene nanosheets, which undergo effective bonding with solders [19].

Low Melting Temperature Solder Materials for Use in Flexible Microelectronic Packaging... http://dx.doi.org/10.5772/intechopen.70272 17

**Figure 8.** SEM micrographs of (a) Sn–58Bi, (b) Sn–58Bi–0.01GNSs, (c) Sn–58Bi–0.03GNSs, (d) Sn–58Bi–0.05GNSs, and (e) Sn–58Bi–0.1GNSs [19].

nanosheet-doped solder samples [19]. The size reductions of the Sn–58Bi–graphene nanosheet (0.01, 0.03, 0.05, 0.1 wt.%) solders were about 28, 55, 30, and 32%, respectively [19]. These results indicate that the growth of grains was suppressed, and the microstructure of the Sn–58Bi solder alloys was refined by the addition of graphene nanosheets; in addition, the solder alloy with 0.03 wt.% graphene nanosheet addition showed the smallest average grain size [19]. The microstructure of local refinement can be found in **Figure 8b** for the 0.01 wt.% graphene nanosheet addition; meanwhile, uniform refinement of the structure and small Bi grains appear in the Sn–58Bi reinforced with 0.03 wt.% graphene nanosheets, shown in **Figure 8c**. These images indicate that the growth of grains can be suppressed by the presence of graphene nanosheets, which is caused by the high barrier of graphene nanosheets against the diffusion of metal atoms [19]. However, with more graphene nanosheets introduced, the growth of metal grains along the surface of the graphene nanosheets can be promoted [19]. As a result, the distinct feature of the uniformly distributed dendritic structure in Sn–58Bi reinforced with 0.05 wt.% graphene nanosheets can be seen in **Figure 8d**; this structure shows that graphene nanosheets can promote the growth of Bi dendrites. Moreover, local grain aggregation recurs and partial grains growth follows an inverse pattern when the content of graphene nanosheets reaches 0.1 wt.%, as shown in **Figure 8e**. This result is attributed to the increasing addi-

**Figure 7.** Cross-sectional SEM micrograph at the interface between eutectic Sn–In solder and single crystalline (1 1 1) Cu

after reflowing at 160°C for 5 s [24].

16 Recent Progress in Soldering Materials

tion of graphene nanosheets, which undergo effective bonding with solders [19].

#### **3.2. Electrical conductivity modification by supplementation with conductive additives cd, Sb, cu, Ag, Zn, in, Ni, and carbon nanomaterials**

The solder bump size on a packaging substrate decreases as a result of the electronic components being miniaturized [25–28]. Simultaneously, the pitch distance drops to the submicron level [27, 28]. The pin count increases to meet the demands of rapid signal transmission and high current load [28]. Thus, the electrical property of a solder becomes one of the most important factors [29–31]. In the literature, however, there is not much research on the electrical properties of low melting point solders. Thus, determining the electrical property for a low melting point solder can be of great use to researchers and engineers, especially those who design solder alloys. Meanwhile, the eutectic Sn–Bi solder has a relatively high electrical resistivity of 30–35 μΩ·cm due to the electrical resistivity of Bi (115 μΩ·cm), while the eutectic Sn–In solder has a very low electrical resistivity of 10–15 μΩ·cm due to the electrical resistivity of In (8 μΩ·cm) [30].

Altıntas et al. determined that the electrical conductivity varies with temperature for low melting point solders: Sn–41.39 at.% Cd–6.69 at.% Sb, Sn–49 at.% In–1 at.% Cu, Sn–50 at.% Ag–10 at.% Bi, and Sn–32 at.% Bi–3 at.% Zn alloys; these values were determined by the fourpoint probe method, as shown in **Figure 9** [31]. The electrical conductivities of all solder alloys in the present work were found to decrease linearly with increasing temperature. The electrical conductivity values as a function of temperature were found to be in ranges of 4.35–2.76, 5.00– 3.43, 5.30–4.58, and 1.52–1.39 (× 106 )/Ω·m for Sn–Cd–Sb, Sn–In–Cu, Sn–Ag–Bi, and Sn–Bi–Zn solder alloys, respectively [31]. By extrapolating the electrical conductivity lines to their melting temperature, values of electrical conductivity for Sn–Cd–Sb, Sn–In–Cu, Sn–Ag–Bi, and Sn– Bi–Zn at their melting temperatures were determined to be 2.61, 3.26, 4.57, and 1.35 (× 10<sup>6</sup> )/Ω·m, respectively, as shown in **Table 1**.

**Figure 10** depicts the resistivity of all Sn–Bi solder alloys according to the increase of the Ni amount. No significant change in the electrical resistivity was detected following the addition of Ni. Although the existence of IMCs in the solders could induce a higher electrical resistivity, in this research the effect on the solders' electrical resistivity in the presence of Ni<sup>3</sup> Sn4 is not apparent [29].

**Figure 9.** Electrical conductivity measurements according to temperature for (a) Sn–41.39Cd–6.69Sb, (b) Sn–49In–1Cu, (c) Sn–50Ag–10Bi, and (d) Sn–32Bi–3Zn (at.%) solder alloys [31].

Low Melting Temperature Solder Materials for Use in Flexible Microelectronic Packaging... http://dx.doi.org/10.5772/intechopen.70272 19


**Table 1.** Some electrical properties of solid phase for Sn–41.39 at.% Cd–6.69 at.% Sb, Sn–49 at.% In–1 at.% Cu, Sn–50 at.% Ag–10 at.% Bi, Sn–32 at.% Bi–3 at.% Zn [31].

The incorporation of carbon nanomaterials with graphene structures can also impart much more rapid electron transfer than that can be obtained using conventional Sn–Bi solder [32]. Subsequently, reinforcement with carbon nanomaterials having high thermal conductivity can be used to tailor a network structure to effectively transfer the outer thermal energy to the solder matrix [32]. **Figure 11** shows the calculated current efficiency versus different MWCNT additions in the solder alloy. This implies that the current efficiency is dependent on the concentration of

**Figure 10.** Electrical resistivity of Sn–58Bi–xNi [29].

Altıntas et al. determined that the electrical conductivity varies with temperature for low melting point solders: Sn–41.39 at.% Cd–6.69 at.% Sb, Sn–49 at.% In–1 at.% Cu, Sn–50 at.% Ag–10 at.% Bi, and Sn–32 at.% Bi–3 at.% Zn alloys; these values were determined by the fourpoint probe method, as shown in **Figure 9** [31]. The electrical conductivities of all solder alloys in the present work were found to decrease linearly with increasing temperature. The electrical conductivity values as a function of temperature were found to be in ranges of 4.35–2.76, 5.00–

solder alloys, respectively [31]. By extrapolating the electrical conductivity lines to their melting temperature, values of electrical conductivity for Sn–Cd–Sb, Sn–In–Cu, Sn–Ag–Bi, and Sn– Bi–Zn at their melting temperatures were determined to be 2.61, 3.26, 4.57, and 1.35 (× 10<sup>6</sup>

**Figure 10** depicts the resistivity of all Sn–Bi solder alloys according to the increase of the Ni amount. No significant change in the electrical resistivity was detected following the addition of Ni. Although the existence of IMCs in the solders could induce a higher electrical resistivity, in this research the effect on the solders' electrical resistivity in the presence of Ni<sup>3</sup>

**Figure 9.** Electrical conductivity measurements according to temperature for (a) Sn–41.39Cd–6.69Sb, (b) Sn–49In–1Cu,

(c) Sn–50Ag–10Bi, and (d) Sn–32Bi–3Zn (at.%) solder alloys [31].

)/Ω·m for Sn–Cd–Sb, Sn–In–Cu, Sn–Ag–Bi, and Sn–Bi–Zn

)/Ω·m,

Sn4 is

3.43, 5.30–4.58, and 1.52–1.39 (× 106

18 Recent Progress in Soldering Materials

respectively, as shown in **Table 1**.

not apparent [29].

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

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].
