**6. Galvanic corrosion of soldering**

Corrosion of solder alloys, in the presence of a suitable electrolyte can occur either due to the potential difference between the major phases in the alloy or galvanic coupling between one or more phases of the alloy and other parts of the microelectronics device. Some metals that are frequently used in microelectronics are Cu, Au, Ag, Ni and Pd. The standard emf for these metals and metals used in solder alloys are listed in Table 3[4]. Especially, advanced packaging technologies make the solder alloy susceptible to corrosion problems

Unlike Sn-Pb joints, which have a dual phase structure and block the path of corrosion due to the existence of phase boundaries, the SAC305 joint is basically pure Sn with coarse islands of Ag3Sn and Cu6Sn5 intermetallic precipitate (Fig. 5). A corrosion crack can propagate and lead to additional corrosion along the way, without interruption from the Sn phase structure. Although both materials show strong resistance to corrosion, the localized nature of the corroded area at critical locations causes significant degradation in Sn-Ag-Cu solder joints[40].

Fig. 5. Cross-section SEM microstructure after salt spray treatment and then thermal cycling:

Corrosion of solder alloys, in the presence of a suitable electrolyte can occur either due to the potential difference between the major phases in the alloy or galvanic coupling between one or more phases of the alloy and other parts of the microelectronics device. Some metals that are frequently used in microelectronics are Cu, Au, Ag, Ni and Pd. The standard emf for these metals and metals used in solder alloys are listed in Table 3[4]. Especially, advanced packaging technologies make the solder alloy susceptible to corrosion problems

(a) Sn-Pb, and (b) SAC305 solder joint [40].

**6. Galvanic corrosion of soldering** 

[41]. Thus, in the electronics industry, corrosion has become a significant factor in recent years because of the extremely complex systems that have been developed and the increasing demand on their reliability [42, 43]. For example, using Cu and Sn metals allows fine-pitch interconnections to be fabricated at relatively low cost. These features make Cu-Sn based SLID bonding very appealing for 3D stacked applications (Fig. 6) [44].


Table 3. Δemf values for metals commonly used in microelectronics[4].

Fig. 6. Electroplated pads of 5 μm Cu and 200 nm Sn: (a) and (b) SEM image with different magnification; (c) Cross-section view under optical microscope; and (d) Cross-section view of a fluxless bonded Cu/Sn Interconnect [44].

Corrosion Resistance of Pb-Free and

behavior and reliability.

Novel Nano-Composite Solders in Electronic Packaging 117

It can be seen that the galvanic corrosion behavior of Cu3Sn is generally greater than that of Cu6Sn5 for the flip chip package in a 3.5 wt. % NaCl solution environment. This indicates that the formation of IMC Cu3Sn and Cu6Sn5 layers causes many problems with corrosion

Fig. 8. The potentiodynamic polarization curves of Sn37Pb solder, Cu6Sn5 IMC, Cu3Sn IMC,

Fig. 9. Effect of Cu content on both *Φ*corr and *I*corr during polarization of the Sn37Pb solder,

Cu6Sn5, Cu3Sn, and Cu substrate in 3.5 wt.% NaCl solution[45].

and pure Cu samples in a 3.5 wt.% NaCl solution [45].

The joining of materials with solders generally results in a multi-layer structure in which IMC are formed between substrate and solders. Such a structure in a flip chip package is a galvanic couple. The galvanic corrosion behavior of the solder bump structures have a great effect upon reliability[45]. For instance, the galvanic current densities of the Sn solder with respect to the IMC Cu6Sn5 and Cu3Sn, and base Cu have been investigated (Fig. 7). It appears that Sn solder has a greater galvanic current density and thus is very subject to corrosion, and it is especially so in coupling with the formation of Cu3Sn layers than with Cu6Sn5 layers. The galvanic current densities of the Sn37Pb solders of Cu3Sn, Cu, and Cu6Sn5 are about 38, 16, and 5 (μA/cm2), respectively.

Fig. 7. The galvanic current densities of the solder with respect to intermetallic compounds Cu6Sn5 and Cu3Sn, and Cu substrate, in a 3.5 wt.% solution [45].

Increasing the copper content, which reacts with Sn to form IMC, significantly improves the corrosion resistance of solders and increases the corrosion current density (*I*corr), as shown in Fig. 8, 9 and Table 4. At above 460 mVSCE, the passivation current densities of all specimens are around 10-1A/cm2, with the declining sequence of Sn37Pb ≥ Cu6Sn5 > Cu3Sn > Cu.


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

ΔΦ = Φcorr. - Φb, Φp: passivation range of solder alloy; Ip: passivation current density at above 460 mVSCE.

Table 4. Corrosion properties in a 3.5 wt.% NaCl solution for the Sn37Pb solder, Cu6Sn5 IMC, Cu3Sn IMC and pure Cu samples [45].

The joining of materials with solders generally results in a multi-layer structure in which IMC are formed between substrate and solders. Such a structure in a flip chip package is a galvanic couple. The galvanic corrosion behavior of the solder bump structures have a great effect upon reliability[45]. For instance, the galvanic current densities of the Sn solder with respect to the IMC Cu6Sn5 and Cu3Sn, and base Cu have been investigated (Fig. 7). It appears that Sn solder has a greater galvanic current density and thus is very subject to corrosion, and it is especially so in coupling with the formation of Cu3Sn layers than with Cu6Sn5 layers. The galvanic current densities of the Sn37Pb solders of Cu3Sn, Cu, and Cu6Sn5

Fig. 7. The galvanic current densities of the solder with respect to intermetallic compounds

Increasing the copper content, which reacts with Sn to form IMC, significantly improves the corrosion resistance of solders and increases the corrosion current density (*I*corr), as shown in Fig. 8, 9 and Table 4. At above 460 mVSCE, the passivation current densities of all specimens are around 10-1A/cm2, with the declining sequence of Sn37Pb ≥ Cu6Sn5 > Cu3Sn > Cu.

Sn37Pb -584.4 -303.0 281 6.48 67.7 Cu6Sn5 -457.7 -45.0 412 2.61 56.9 Cu3Sn -309.0 -8.9 300 48.17 18.3 Cu -192.1 236 428 391.6 6.5

Table 4. Corrosion properties in a 3.5 wt.% NaCl solution for the Sn37Pb solder, Cu6Sn5

ΔΦ (mV)

Icorr (μA/cm2)

Ip (mA/cm2)

Cu6Sn5 and Cu3Sn, and Cu substrate, in a 3.5 wt.% solution [45].

Φ<sup>b</sup> (mVSCE)

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

Specimens Φcorr

(mVSCE)

ΔΦ = Φcorr. - Φb, Φp: passivation range of solder alloy; Ip: passivation current density at above 460 mVSCE.

IMC, Cu3Sn IMC and pure Cu samples [45].

are about 38, 16, and 5 (μA/cm2), respectively.

It can be seen that the galvanic corrosion behavior of Cu3Sn is generally greater than that of Cu6Sn5 for the flip chip package in a 3.5 wt. % NaCl solution environment. This indicates that the formation of IMC Cu3Sn and Cu6Sn5 layers causes many problems with corrosion behavior and reliability.

Fig. 8. The potentiodynamic polarization curves of Sn37Pb solder, Cu6Sn5 IMC, Cu3Sn IMC, and pure Cu samples in a 3.5 wt.% NaCl solution [45].

Fig. 9. Effect of Cu content on both *Φ*corr and *I*corr during polarization of the Sn37Pb solder, Cu6Sn5, Cu3Sn, and Cu substrate in 3.5 wt.% NaCl solution[45].

Corrosion Resistance of Pb-Free and

Ep – passive potential.

solution.

diameter.

Solder Scanning rate *E*corr

Novel Nano-Composite Solders in Electronic Packaging 119

Sn37Pb 1 mV/s -588 1.905 × 10-6 -201 4.989 [51]

Sn0.7Cu 30 mV/s -688 1.78× 10-7 - 0.74 [46]

*I*corr (A/cm2)

*Ep* (mV)

*Ip* (mA/cm2) References

(mV)

Sn9Zn -940 2.691 × 10-5 -326 2.938 Sn8Zn3Bi -1291 1.380 × 10-5 9 8.035 Sn3.5Ag0.5Cu -605 5.370× 10-7 -236 4.083 Sn3.5Ag0.5Cu9In -578 7.413 × 10-6 -158 1.524

Sn3.5Ag -705 4.9× 10-7 - 0.49 Sn3.8Ag0.7Cu -727 0.89× 10-7 - 1.07

Ecorr – corrosion potential, Ip – passivation current density, Icorr – corrosion current density,

Table 5. Experimental data of the testing solders under polarization in 3.5 wt.% NaCl

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

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

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

#### **7. Corrosion behavior of Pb-free solder**

Both the particular design of the electronic system, and the manner in which it is mounted in a substrate or printed wiring board, the solder connection can be exposed to the atmosphere. The solder is thus not only exposed to air, but also moisture and other corrosives such as chlorine and sulfur compounds. The ability of the solder to be able to withstand corrosion property is therefore relevant to the long-term reliability of solder joints [4]. In addition, solder alloys are electrically connected with other metallic components in the electronic device. Some metals that are frequently used in microelectronics are Cu, Au, Ag, Ni and Pd. Therefore, there is also the potential for galvanically induced corrosion of the solder, which could exacerbate any atmospheric corrosion that might be occurring. However, the properties of these lead free alloys in corrosive environments has not been widely reported, though it is of importance in many automotive, aerospace, maritime and defence applications [46]. Some researchers have studied the corrosion behaviour of Sn–Zn–X solders [47, 48] and Sn–Zn–Ag–Al–XGa [49], but few [50, 51] have studied the corrosion properties of Sn–Ag, Sn–Cu and Sn–Ag–Cu solders. Zinc is both metallurgically and chemically active. The presence of Zn in the solder alloy results in poor corrosion resistance, which is an important problem to address before practical application of this material [49]. Hence, the electrochemical corrosion behaviour of Pb-free Sn-Zn binary solder and Sn-Zn-X (X=Bi, Ag and Al) solder alloys have been investigated in NaCl solution by potentiodynamic polarization techniques[ 52- 55]. Lin et al. [47-49] have investigated the corrosion behaviour of Sn–Zn–Al, Sn–Zn–Al–In and Sn–Zn–Ag–Al–XGa solders in 3.5% NaCl solution. They found that Sn–Zn–Al alloy [47] undergoes more active corrosion than Sn–37Pb alloy. Furthermore, they found that 5In–9(5Al–Zn)–YSn and 10In–9(5Al–Zn)–Sn alloys exhibit electrochemical passivation behaviour, and the polarization behaviours of these two alloys are similar to that of 9(5Al–Zn)–Sn alloy. Sn-Ag-M (M=In, Bi) solders exhibit poor corrosion behaviour as compared to that of Sn-Pb eutectic solder (0.1M NaCl solution)[56]. In contrast, increasing the copper content (from 0.8 to 6.7 at.%) enhances the corrosion resistance of Sn-Ag solder alloys, which exhibit improved passivity behaviour as compared to Sn-Pb eutectic solder. EPMA results indicate that the Ag3Sn IMC is retained after the polarization test. Hence, the Ag3Sn is more noble than the β-Sn phase. The pit formation on the surface of Sn–Ag– M alloys is due to the dissolution of the tin-rich phase. Wu et al. [51] has studied the corrosion behaviors of five solders in salt and acid solutions by means of polarization and EIS measurements. The Sn3.5Ag0.5Cu solder has the best corrosion resistivity due to the high content of noble or immune elements (Ag and Cu) and theorized stable structure, whereas the Sn9Zn and Sn8Zn3Bi solder have the worst corrosion behavior. Nevertheless, the four Pb-free solders exhibit acceptable corrosion properties, since there is not much difference in key corrosion parameters between them and the Sn37Pb solder. The corrosion data of the solders in 3.5 wt.% NaCl solutions are listed in Tables 5 [46, 51]. Lin and Mohanty et al. [46-49] studied the corrosion properties of Sn–Zn–X and Sn–Zn–Ag– Al–XGa in NaCl solution, and their results showed that the corrosion product on the surface could be SnO, SnO2, SnCl2 and ZnO, etc., depending on the applied potential. Li et al. [46] confirms that the corrosion product on the Sn–Pb and lead free solders is tin oxide chloride hydroxide (Sn3O(OH)2Cl2).

Both the particular design of the electronic system, and the manner in which it is mounted in a substrate or printed wiring board, the solder connection can be exposed to the atmosphere. The solder is thus not only exposed to air, but also moisture and other corrosives such as chlorine and sulfur compounds. The ability of the solder to be able to withstand corrosion property is therefore relevant to the long-term reliability of solder joints [4]. In addition, solder alloys are electrically connected with other metallic components in the electronic device. Some metals that are frequently used in microelectronics are Cu, Au, Ag, Ni and Pd. Therefore, there is also the potential for galvanically induced corrosion of the solder, which could exacerbate any atmospheric corrosion that might be occurring. However, the properties of these lead free alloys in corrosive environments has not been widely reported, though it is of importance in many automotive, aerospace, maritime and defence applications [46]. Some researchers have studied the corrosion behaviour of Sn–Zn–X solders [47, 48] and Sn–Zn–Ag–Al–XGa [49], but few [50, 51] have studied the corrosion properties of Sn–Ag, Sn–Cu and Sn–Ag–Cu solders. Zinc is both metallurgically and chemically active. The presence of Zn in the solder alloy results in poor corrosion resistance, which is an important problem to address before practical application of this material [49]. Hence, the electrochemical corrosion behaviour of Pb-free Sn-Zn binary solder and Sn-Zn-X (X=Bi, Ag and Al) solder alloys have been investigated in NaCl solution by potentiodynamic polarization techniques[ 52- 55]. Lin et al. [47-49] have investigated the corrosion behaviour of Sn–Zn–Al, Sn–Zn–Al–In and Sn–Zn–Ag–Al–XGa solders in 3.5% NaCl solution. They found that Sn–Zn–Al alloy [47] undergoes more active corrosion than Sn–37Pb alloy. Furthermore, they found that 5In–9(5Al–Zn)–YSn and 10In–9(5Al–Zn)–Sn alloys exhibit electrochemical passivation behaviour, and the polarization behaviours of these two alloys are similar to that of 9(5Al–Zn)–Sn alloy. Sn-Ag-M (M=In, Bi) solders exhibit poor corrosion behaviour as compared to that of Sn-Pb eutectic solder (0.1M NaCl solution)[56]. In contrast, increasing the copper content (from 0.8 to 6.7 at.%) enhances the corrosion resistance of Sn-Ag solder alloys, which exhibit improved passivity behaviour as compared to Sn-Pb eutectic solder. EPMA results indicate that the Ag3Sn IMC is retained after the polarization test. Hence, the Ag3Sn is more noble than the β-Sn phase. The pit formation on the surface of Sn–Ag– M alloys is due to the dissolution of the tin-rich phase. Wu et al. [51] has studied the corrosion behaviors of five solders in salt and acid solutions by means of polarization and EIS measurements. The Sn3.5Ag0.5Cu solder has the best corrosion resistivity due to the high content of noble or immune elements (Ag and Cu) and theorized stable structure, whereas the Sn9Zn and Sn8Zn3Bi solder have the worst corrosion behavior. Nevertheless, the four Pb-free solders exhibit acceptable corrosion properties, since there is not much difference in key corrosion parameters between them and the Sn37Pb solder. The corrosion data of the solders in 3.5 wt.% NaCl solutions are listed in Tables 5 [46, 51]. Lin and Mohanty et al. [46-49] studied the corrosion properties of Sn–Zn–X and Sn–Zn–Ag– Al–XGa in NaCl solution, and their results showed that the corrosion product on the surface could be SnO, SnO2, SnCl2 and ZnO, etc., depending on the applied potential. Li et al. [46] confirms that the corrosion product on the Sn–Pb and lead free solders is tin oxide

**7. Corrosion behavior of Pb-free solder** 

chloride hydroxide (Sn3O(OH)2Cl2).


Ecorr – corrosion potential, Ip – passivation current density, Icorr – corrosion current density, Ep – passive potential.

Table 5. Experimental data of the testing solders under polarization in 3.5 wt.% NaCl solution.
