**8. Corrosion behavior of Pb-free nano-composite solder joints**

This author has recently worked on the development of nano-composite solders in microelectronic packaging by applying two methods of fabrication: mechanical mixing of inert nano-particles (Fig. 10) and precipitation of nano-IMC in the solder matrix (Fig. 11) [57]. The average size of the nominally spherical nano-Al2O3 particles was 100 nm in diameter.

Notably, the addition of nano-particles decreased the size of dendrite β-Sn grains, the needle-like Ag3Sn grains, and Ag3Sn phase located between the average spacing. When 1 wt% was added, the superfine spherical nano-Ag3Sn grains were about 0.16 ± 0.06μm in length and 0.15 ± 0.05 μm in diameter, and the average spacing between them was a significant improvement (0.14 ± 0.05μm), significantly smaller than the sizes found in the SAC composite solder. However, large Ag3Sn IMCs were not observed in the Pb-free SAC solder. Another, author reported that the effects of nano-TiO2 particles on the interfacial microstructures and bonding strength of Sn3.5Ag0.5Cu nano-composite solder joints in ball grid array (BGA) packages with immersion Sn surface finishes [58]. It is clearly shown in Fig. 12a, b that the discontinuous Cu6Sn5 IMC layer grows with a rough scallop shape (Mark A), and wicker-Cu6Sn5 IMC forms on the rough scallop-shaped Cu6Sn5 IMC layer (Mark B) and grows into the SAC solder matrix. However, the addition of a small percentage of nano-TiO2 particles alters the Pb-free Sn3.5Ag0.5Cu composite solder/pad interface morphology after reflowing, as shown in the SEM micrographs in Fig. 12c, d. Only the continuous scallop-shaped Cu6Sn5 IMC layer was detected at the interface. However, the wicker-Cu6Sn5 IMC disappeared at the interface with the Cu pads. In addition, the number of Ag3Sn IMC forms increased in the eutectic area when the content of nano- TiO2 particles was increased to 0.25–1 wt%. It is interesting that the smallest

Corrosion Resistance of Pb-Free and

composite solder[57].

Novel Nano-Composite Solders in Electronic Packaging 121

Fig. 11. SEM image of the (a) Sn3.5Ag0.5Cu solder and (b) Sn3.5Ag0.5Cu -1TiO2 nano-

thickness of the IMC layer was achieved with the addition of 1 wt% of nano-TiO2 particles. The thickness of the Cu6Sn5 IMC layer was reduced by 51%. The results indicate that the growth of the Cu6Sn5 IMC layer at the solder/pad interfaces of Sn3.5Ag0.5Cu is depressed through the small addition of nano-TiO2 particles[58]. With the addition of 0.5– 1 wt% nano-TiO2 particles, fracture occurred in all of the solder joints as cracks propagated through the Sn3.5Ag0.5Cu composite solder balls, which ruptured mostly along the submicro Ag3Sn IMC and solder matrix, as shown in Fig. 13a, b. This phenomenon is similar to that occurring in Pb-free Sn0.7Cu composite solder BGA packages[59].

Fig. 10. The nano-Al2O3 particles used in this study: (a) FE-SEM micrograph, and (b) X-ray diffraction spectrum[57].

thickness of the IMC layer was achieved with the addition of 1 wt% of nano-TiO2 particles. The thickness of the Cu6Sn5 IMC layer was reduced by 51%. The results indicate that the growth of the Cu6Sn5 IMC layer at the solder/pad interfaces of Sn3.5Ag0.5Cu is depressed through the small addition of nano-TiO2 particles[58]. With the addition of 0.5– 1 wt% nano-TiO2 particles, fracture occurred in all of the solder joints as cracks propagated through the Sn3.5Ag0.5Cu composite solder balls, which ruptured mostly along the submicro Ag3Sn IMC and solder matrix, as shown in Fig. 13a, b. This phenomenon is similar to that occurring in Pb-free Sn0.7Cu composite solder BGA

Fig. 10. The nano-Al2O3 particles used in this study: (a) FE-SEM micrograph, and (b) X-ray

packages[59].

diffraction spectrum[57].

Fig. 11. SEM image of the (a) Sn3.5Ag0.5Cu solder and (b) Sn3.5Ag0.5Cu -1TiO2 nanocomposite solder[57].

Corrosion Resistance of Pb-Free and

packages after ball shear tests [58].

Novel Nano-Composite Solders in Electronic Packaging 123

Fig. 13. Fractography of the Sn3.5Ag0.5Cu-1TiO2 nano-composite solder joints in BGA

To achieve high reliability, solder materials must have high resistance to corrosive conditions such as moisture, air pollutants from industry, and oceanic environments[54]. Although corrosion of solder alloys is not currently a major problem for electronic devices used in normal environments, it may be a problem when they are used in harsh environments, such as oceanic environments. However, there is a lack of information regarding the corrosion resistance of nano-composite solders in corrosive environments.

Figure 14 shows the polarization curves of the Sn3.5Ag0.5Cu solder and the Sn3.5Ag0.5Cu nano-composite solder in 3.5 wt.% NaCl solution[60]. From the polarization curves, the corrosion potential (*Φ*corr), the breakdown potential (*Φ*b), and the dynamic corrosion current density (*I*corr) have been determined (Table 6). The width of the passive region on the anodic polarization curves (Δ*Φ* = *Φ*b - *Φ*corr) in Table 6 indicates the pitting

Fig. 12. Morphology of intermetallic compounds formed at the interfaces of the as-reflowed solder joints: (a) Sn3.5Ag0.5Cu, ( b) (a) magnifications ; (c) d Sn3.5Ag0.5Cu-0.75TiO2; (d) (c) magnifications [58].

Fig. 12. Morphology of intermetallic compounds formed at the interfaces of the as-reflowed solder joints: (a) Sn3.5Ag0.5Cu, ( b) (a) magnifications ; (c) d Sn3.5Ag0.5Cu-0.75TiO2; (d) (c)

magnifications [58].

Fig. 13. Fractography of the Sn3.5Ag0.5Cu-1TiO2 nano-composite solder joints in BGA packages after ball shear tests [58].

To achieve high reliability, solder materials must have high resistance to corrosive conditions such as moisture, air pollutants from industry, and oceanic environments[54]. Although corrosion of solder alloys is not currently a major problem for electronic devices used in normal environments, it may be a problem when they are used in harsh environments, such as oceanic environments. However, there is a lack of information regarding the corrosion resistance of nano-composite solders in corrosive environments.

Figure 14 shows the polarization curves of the Sn3.5Ag0.5Cu solder and the Sn3.5Ag0.5Cu nano-composite solder in 3.5 wt.% NaCl solution[60]. From the polarization curves, the corrosion potential (*Φ*corr), the breakdown potential (*Φ*b), and the dynamic corrosion current density (*I*corr) have been determined (Table 6). The width of the passive region on the anodic polarization curves (Δ*Φ* = *Φ*b - *Φ*corr) in Table 6 indicates the pitting

Corrosion Resistance of Pb-Free and

Solder

Sn3.5Ag0.5Cu-0.5TiO2

polarization tests[60].

0.5Al2O3 [60].

Novel Nano-Composite Solders in Electronic Packaging 125

Sn3.5Ag0.5Cu All area 69.26 4.05 0.37 13.40 12.60

flake 72.97 0.43 - 15.71 9.58

mushroom 74.87 1.29 - 17.97 5.76

flake 64.22 0.50 15.03 20.25

Table 7. Surface element concentration of different solders after potentiodynamic

Fig. 14. The potentiodynamic polarization curves of the nano-composite solder in a 3.5wt.% NaCl solution: (a) Sn3.5Ag0.5Cu solder; (b) Sn3.5Ag0.5Cu-0.5TiO2; and (c) Sn3.5Ag0.5Cu-

All area 68.7 3.82 0.68 13.39 13.41

Surface element concentration (wt.%)

Sn Ag Cu Cl O

resistibility or the stability of the passive film on the Sn3.5Ag0.5Cu composite alloy surface. The corrosion potential (*Φ*corr) of the Sn3.5Ag0.5Cu nano-composite solder is slightly more passive than that of the Sn3.5Ag0.5Cu solder. This implies that a finer grain size produces more grain *b*oundaries, which act as corrosion barriers. On the other hand, the breakdown potential (*Φ*b) of the Sn3.5Ag0.5Cu nano-composite solders becomes much more passive with the addition of oxide nanoparticles. As Table 6 also indicates, the Sn3.5Ag0.5Cu solders possess a higher pitting tendency (smaller Δ*Φ* value) than the Sn3.5Ag0.5Cu nano-composite solders. Rosalbino et al. reported that the pit formation at the surface of Sn–Ag–M alloys is due to the dissolution of the tin-rich phase [56]. In addition, the corrosion current densities were obtained by using the TAFEL extrapolation method. The corrosion current densities of the Sn3.5Ag0.5Cu solders and Sn3.5Ag0.5Cu nano-composite solders were very similar.


Φcorr : corrosion potential; Icorr: corrosion current density; Φb : breakdown potential; ΔΦ = Φcorr - Φb.

Table 6. Corrosion properties in a 3.5 wt.% NaCl solution for the nano-composite solder [60].

Many studies have reported that the corrosion behavior of alloys depends on the second phase distribution, shown to be Mg alloy[61, 62] and Al alloy[63]. In the Sn3.5Ag0.Cu nanocomposite solder alloys, the microstructure had finer β-Sn grains, a large amount of Ag3Sn particles, and a small amount of oxidize nanoparticles. This leads to improvement of the corrosion behavior of the Sn3.5Ag0.5Cu nano-composite solder, such as greater corrosion resistance, the lower pitting tendency, and the smaller corrosion current density, respectively.

The corrosion products of Sn3.5Ag0.5Cu and Sn3.5Ag0.5Cu nano-composite solder have similar microstructures (Fig. 15). The corrosion products of Sn3.5Ag0.5Cu solder after polarization have a larger flake-like shape (Mark a) and small mushroom-like shape, and are loosely distributed on the surface, with different orientations (Fig.15a). On the other hand, the corrosion products of Sn3.5Ag0.5Cu nano-composite solder after polarization tests have only a flake-like shape, as shown in Fig. 15b (Mark a). Table 7 shows the surface element concentrations of solder corrosion products from EDS. According to the EDS analysis, the corrosion products of Sn3.5Ag0.5Cu and Sn3.5Ag0.5Cu nano-composite solder contain mainly Sn, O, and Cl (Fig.16). It can be seen that the corrosion products of the Sn3.5Ag0.5Cu solders and Sn3.5Ag0.5Cu nano-composite solders have slightly different compositions.

resistibility or the stability of the passive film on the Sn3.5Ag0.5Cu composite alloy surface. The corrosion potential (*Φ*corr) of the Sn3.5Ag0.5Cu nano-composite solder is slightly more passive than that of the Sn3.5Ag0.5Cu solder. This implies that a finer grain size produces more grain *b*oundaries, which act as corrosion barriers. On the other hand, the breakdown potential (*Φ*b) of the Sn3.5Ag0.5Cu nano-composite solders becomes much more passive with the addition of oxide nanoparticles. As Table 6 also indicates, the Sn3.5Ag0.5Cu solders possess a higher pitting tendency (smaller Δ*Φ* value) than the Sn3.5Ag0.5Cu nano-composite solders. Rosalbino et al. reported that the pit formation at the surface of Sn–Ag–M alloys is due to the dissolution of the tin-rich phase [56]. In addition, the corrosion current densities were obtained by using the TAFEL extrapolation method. The corrosion current densities of the Sn3.5Ag0.5Cu solders and Sn3.5Ag0.5Cu

nano-composite solders were very similar.

ΔΦ = Φcorr - Φb.

respectively.

different compositions.

[60].

Solder *Φ*corr

(mVSCE)

Φcorr : corrosion potential; Icorr: corrosion current density; Φb : breakdown potential;

*Φ*b (mVSCE)

Sn3.5Ag0.5Cu -662.1 -284.1 378 0.36

Sn3.5Ag0.5Cu-0.5TiO2 -651.4 -95.1 556 0.27

Sn3.5Ag0.5Cu-0.5Al2O3 -642.1 -146.2 496 0.40

Table 6. Corrosion properties in a 3.5 wt.% NaCl solution for the nano-composite solder

Many studies have reported that the corrosion behavior of alloys depends on the second phase distribution, shown to be Mg alloy[61, 62] and Al alloy[63]. In the Sn3.5Ag0.Cu nanocomposite solder alloys, the microstructure had finer β-Sn grains, a large amount of Ag3Sn particles, and a small amount of oxidize nanoparticles. This leads to improvement of the corrosion behavior of the Sn3.5Ag0.5Cu nano-composite solder, such as greater corrosion resistance, the lower pitting tendency, and the smaller corrosion current density,

The corrosion products of Sn3.5Ag0.5Cu and Sn3.5Ag0.5Cu nano-composite solder have similar microstructures (Fig. 15). The corrosion products of Sn3.5Ag0.5Cu solder after polarization have a larger flake-like shape (Mark a) and small mushroom-like shape, and are loosely distributed on the surface, with different orientations (Fig.15a). On the other hand, the corrosion products of Sn3.5Ag0.5Cu nano-composite solder after polarization tests have only a flake-like shape, as shown in Fig. 15b (Mark a). Table 7 shows the surface element concentrations of solder corrosion products from EDS. According to the EDS analysis, the corrosion products of Sn3.5Ag0.5Cu and Sn3.5Ag0.5Cu nano-composite solder contain mainly Sn, O, and Cl (Fig.16). It can be seen that the corrosion products of the Sn3.5Ag0.5Cu solders and Sn3.5Ag0.5Cu nano-composite solders have slightly

Δ*Φ* (mV)

*I*corr (μA/cm2)


Table 7. Surface element concentration of different solders after potentiodynamic polarization tests[60].

Fig. 14. The potentiodynamic polarization curves of the nano-composite solder in a 3.5wt.% NaCl solution: (a) Sn3.5Ag0.5Cu solder; (b) Sn3.5Ag0.5Cu-0.5TiO2; and (c) Sn3.5Ag0.5Cu-0.5Al2O3 [60].

Corrosion Resistance of Pb-Free and

reported [46, 64,65]:

the following reaction[66]:

Sn3O(OH)2Cl2.

polarization tests[60].

Novel Nano-Composite Solders in Electronic Packaging 127

The dehydration of Sn(OH)2 and Sn(OH)4 into SnO and SnO2, respectively, has also been

However, Yu et al., after investigating the corrosion properties of Sn9Zn and Sn8Zn3Bi solder in NaCl solution, postulated the formation of a tin oxyhydroxychloride according to

In addition, Li et al.[46] studied the corrosion properties of Sn-Ag, Sn–Ag–Cu, Sn–Cu, and SnPb solder in 3.5wt.% NaCl solution with different scanning rates, and their results showed that the corrosion product on the surface was tin oxide chloride hydroxide (Sn3O(OH)2Cl2). In our case, the presence of such a surface layer, instead of a tin oxychloride layer, cannot be ruled out due to the detection limits of energy-dispersive spectroscopy. In order to understand the reaction during the corrosion products, XRD has been used to analyse the corrosion products on the surface after the polarization tests (Fig. 17). The results show that all the Sn3.5Ag0.5Cu and Sn3.5Ag0.5Cu solder materials have the same corrosion product, Sn3O(OH)2Cl2, which is a complex oxide chloride hydroxide of tin[67]. This further confirms that the corrosion product on the Sn3.5Ag0.5Cu composite solders is

Fig. 16. EDS analysis of corrosion product of the Sn3.5Ag0.5Cu nano-composite solder after

Sn + 2OH� − 2e� = SnO + H�O (4)

Sn + 4H�O = Sn(OH)� + 4e� + 4H� (5)

Sn(OH)� + 2OH� + 2e� = Sn(OH)� (6)

Sn(OH)� + 2OH� = SnO + H�O (7)

SnO + H�O + 2OH� + 2e� = Sn(OH)� (8)

3Sn + 4OH� + 2Cl� = Sn�O(OH)�Cl� + H�O (9)

Fig. 15. Microstructure of the corrosion products on different solders after polarization tests (a) Sn3.5Ag0.5Cu solder, (b) Sn3.5Ag0.5Cu nano-composite solder[60].

During polarization testing in NaCl solution, the only possible cathodic reaction is oxygen reduction [49, 64]:

$$\text{H}\_2\text{O}\_2 + 4\text{e}^- + 2\text{H}\_2 \rightarrow 4\text{OH}^- \tag{1}$$

When the current density reaches about 10 mA/cm2, many hydrogen bubbles evolve from the cathode due to the hydrogen evolution on the cathode:

$$2\text{H}\_2\text{O} + 2\text{e}^- \rightarrow \text{H}\_2 + 2\text{OH}^- \tag{2}$$

The reactions on the anode are quite complicated. Some possible anodic reactions have been reported in the literature [46, 64-66], as displayed below:

$$\text{Sn} + 2\text{OH}^- - 2\text{e}^- = \text{Sn(OH)}\_2 \tag{3}$$

Fig. 15. Microstructure of the corrosion products on different solders after polarization tests

During polarization testing in NaCl solution, the only possible cathodic reaction is oxygen

When the current density reaches about 10 mA/cm2, many hydrogen bubbles evolve from

The reactions on the anode are quite complicated. Some possible anodic reactions have been

O� + 4e� + 2H� → 4OH� (1)

2H�O + 2e� → H� + 2OH� (2)

Sn + 2OH� − 2e� = Sn(OH)� (3)

(a) Sn3.5Ag0.5Cu solder, (b) Sn3.5Ag0.5Cu nano-composite solder[60].

the cathode due to the hydrogen evolution on the cathode:

reported in the literature [46, 64-66], as displayed below:

reduction [49, 64]:

$$\text{Sn} + 2\text{OH}^- - 2\text{e}^- = \text{SnO} + \text{H}\_2\text{O} \tag{4}$$

$$\text{Sn} + 4\text{H}\_2\text{O} = \text{Sn(OH)}\_4 + 4\text{e}^- + 4\text{H}^+ \tag{5}$$

The dehydration of Sn(OH)2 and Sn(OH)4 into SnO and SnO2, respectively, has also been reported [46, 64,65]:

$$\text{Sn(OH)}\_{2} + 2\text{OH}^{-} + 2\text{e}^{-} = \text{Sn(OH)}\_{4} \tag{6}$$

$$\text{Sn(OH)}\_{2} + 2\text{OH}^{-} = \text{SnO} + \text{H}\_{2}\text{O} \tag{7}$$

$$\text{SnO} + \text{H}\_2\text{O} + 2\text{OH}^- + 2\text{e}^- = \text{Sn(OH)}\_4 \tag{8}$$

However, Yu et al., after investigating the corrosion properties of Sn9Zn and Sn8Zn3Bi solder in NaCl solution, postulated the formation of a tin oxyhydroxychloride according to the following reaction[66]:

$$\rm{^2SSn} + 4\rm{OH}^- + 2\rm{Cl}^- = \rm{Sn}\_3\rm{O}(\rm{OH})\_2\rm{Cl}\_2 + \rm{H}\_2\rm{O} \tag{9}$$

In addition, Li et al.[46] studied the corrosion properties of Sn-Ag, Sn–Ag–Cu, Sn–Cu, and SnPb solder in 3.5wt.% NaCl solution with different scanning rates, and their results showed that the corrosion product on the surface was tin oxide chloride hydroxide (Sn3O(OH)2Cl2). In our case, the presence of such a surface layer, instead of a tin oxychloride layer, cannot be ruled out due to the detection limits of energy-dispersive spectroscopy. In order to understand the reaction during the corrosion products, XRD has been used to analyse the corrosion products on the surface after the polarization tests (Fig. 17). The results show that all the Sn3.5Ag0.5Cu and Sn3.5Ag0.5Cu solder materials have the same corrosion product, Sn3O(OH)2Cl2, which is a complex oxide chloride hydroxide of tin[67]. This further confirms that the corrosion product on the Sn3.5Ag0.5Cu composite solders is Sn3O(OH)2Cl2.

Fig. 16. EDS analysis of corrosion product of the Sn3.5Ag0.5Cu nano-composite solder after polarization tests[60].

Corrosion Resistance of Pb-Free and

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**6** 

Swe-Kai Chen

*Taiwan* 

*National Tsing Hua University, Hsinchu,* 

**Electrochemical Passive Properties** 

**of AlxCoCrFeNi (x = 0, 0.25, 0.50, 1.00) High-Entropy Alloys in Sulfuric Acids** 

*Center for Nanotechnology, Materials Science, and Microsystems (CNMM),* 

**1.1 Pseudo-unitary lattice with a characteristic parameter as a description of** 

In the summer of 1995, J.W. Yeh and the author (SKC) started the study of multi-principalelement alloys which was called, then, alloys with high randomness and now the high-entropy alloys (HEAs). SKC checked the first 10 equal-molar alloys, which was designed by Yeh that contained from 6 to 9 elements in the alloys out of one of Al, Cu, and Mo, together with Ti, V, Fe, Ni, Zr, Co, Cr, Pd, and B, with a home-made vacuum-arc remelter, and the author observed that the alloy series containing Mo can be made most easily, while the ones containing 3 at% B are the ones most difficult in melting, and 6 out of 10 can be formed in the water-cooled copper mold of the remelter, i.e., the existence of the HEAs was demonstrated by experiments. The alloys were aimed at that time to design as another kind of bulk glass alloys, and based on the high configurational entropy of R ln(n), n between 5 and 13, similar to the mixing of different gases [1]. No conclusions were drawn with XRD patterns of these alloys that were found two years later to be composed with peaks from a single simple lattice cell like FCC A1 or BCC A2, although some evidence of existence of amorphous phase was observed from TEM diffraction patterns and high resolution images [2,3]. The simple crystalline phases instead of amorphous ones were continuously found in alloys like in AlCoCrCuFeNi during research of HEAs in these 10 to 20 years, and identified with a so-called extended FCC or BCC

As multiple principal element alloys, high-entropy alloys (HEAs) comprise at least five elements whose concentration for each one ranges between 5 at % and 35 at % [5]. Attributes of forming a simple solid solution and nano-particle precipitation, as well as achieving a high hardness and strength, and excellent high-temperature oxidation resistance make HEAs highly promising for application and research and development of these alloys [6-9]. Properties of AlxCoCrFeNi (0 ≤ x ≤ 1) HEAs vary significantly with x [10]. For instance, the alloy structure changes from FCC to BCC for increased Al content x. Besides, the coefficient of thermal expansion decreases with x. Both properties are closely related to the bond strength of alloys. Moreover, electrical resistivity of AlxCoCrFeNi alloys is large, i.e.,

**multi-principal alloys – The high-entropy alloys (HEAs)** 

unit cell that SKC called it a pseudo-unitary lattice in 2010 [4].

approximately up to 200 cm [11].

**1. Introduction** 

