Corrosion Resistance of Pb-Free and Novel Nano-Composite Solders in Electronic Packaging

Tin-lead (Sn-Pb) alloys for metal interconnections were first used about 2000 years ago. Recently, the use of alloys has become essential for the interconnection and packaging of virtually all electronic products and circuits. Sn-Pb solder alloys have been widely used in the modern electronics industry because of their low melting points, good wettability, good corrosion resistance, low cost, reasonable electrical conductivity, and satisfactory mechanical properties. However, due to health concerns, recent legislation, and market pressures [1], the electronic industry is moving toward green manufacturing as a global trend. In the area of packaging, mainly driven by European RoHS (Reduction of Hazardous Substances), lead was banned effective July 1, 2006, except in some exempt items. In addition, Pb and Pb-containing compounds, as cited by the Environmental Protection Agency (EPA) of the US, are listed among the top 17 chemicals posing the greatest threat to human life and the environment [2] because of lead's toxicity [3]. In the electronics industry, the lead generated by the disposal of electronic assemblies is considered hazardous to the environment. Therefore, developing viable alternative Pb-free solders for electronic assemblies is of principal importance.


Nano-composite solders
As electronic devices continue to become lighter and thinner, they require much smaller solder joints and fine-pitch interconnections for microelectronic packaging. For example, portable electronic devices, such as portable computers and mobile phones, have become thinner and smaller while adding more complicated functions. The miniaturization of these electronic devices demands better solder-joint reliability. Hence, in all chip connection and ball grid array (BGA) technologies, solder interconnection through flip-chip assembly has been proven to offer the highest density of input/output (I/O) connections in a limited space. To meet the insatiable appetite for ever-finer I/O pitches and ever-higher I/O densities, C4 (controlled collapse chip connection) technology was developed by IBM in the mid 1960s, and this technology was applied to future microelectronic packaging. According to the International Technology Roadmap for Semiconductors (ITRS), the pad pitch may fall below 20 μm by the year 2016 [11]. In some flip chip packages, solder balls of 20μm in size are used to connect the pads on the chip and the print circuit board (Fig. 1). Furthermore, Thru-Silicon-Via (TSV) technologies are also lurking on the horizon as the next-generation higher-density chip connection technology, and they also require fine-pitch Pb-free solder interconnections.
The conventional solder technology may not guarantee the required performance at such pitches due to characteristics such as higher diffusivity and softening [12]. In order to solve these problems, efforts have been made to develop new Pb-free solders with a low melting point, good mechanical properties, better microstructure properties, and high creep resistance. Recently, Pb-free solders doped with nano-sized, nonreacting, noncoarsening oxide dispersoids have been identified as potential materials that could provide higher microstructure stability and better mechanical properties than the conventional solders [13][14][15][16][17][18][19][20][21][22][23][24]. Tsao et al. [14][15][16] [19] or multi-walled carbon nanotubes [20,21] on the mechanical properties of nano-composite solders. The data on the enhancement of the mechanical properties of nano-composite solders collected from some of the literature are listed in Table  2 [13,14,16,22,23]. Here, it should be stressed that although the addition of nanoparticles into solder matrices can improve the creep behavior [24], the effects on the corrosion resistance and mechanical properties of the nano-composite solders cannot be ignored.

The interfacial intermetallic compound (IMC) layers
In connected metals, all the common base materials, coatings, and metallizations, such as Cu, Ni, Ag, and Au, form intermetallic compounds (IMC) with Sn, which is the major element in Sn solders. Cu is the material most frequently used for leads and pads on flip chip substrates and printed wiring boards. It is now known that in the solder/Cu interfacial reaction, Sn reacts rapidly with Cu to form Cu 3 Sn (ε-phase) and Cu 6 Sn 5 (η-phase) [25]. Other metal substrate/solder interfacial reactions form IMCs, such as Ag 3 Sn [26] (Sn solder/Ag ), Sn-Ni [27] (Sn solder/Ni), Ag-In [28] (In solder/Ag) and Cu-In IMC [29] (In solder/Cu). These intermetallic compounds are generally more brittle than the base metal, which can have an adverse impact on the solder joint reliability. Excessive thickness may also decrease solder joint ductility and strength [30][31][32][33][34]. Recently, we found that a great number of nano-Ag 3 Sn particles form on the Cu 6 Sn 5 IMC when the solders contain Ag 3 Sn precipitate phase after a Pb-free Sn3.5Ag0.5Cu (SAC) nano-composite solder/Cu substrate interface reaction [30,31]. These nanoparticles apparently decrease the surface energy and hinder the growth of the Cu 6 Sn 5 IMC layer during soldering and aging. All these results indicate that Gibbs absorption theory can be used to explain the formation of these nanoparticles and their effects on the surface energy of the IMC. Many studies have reported that nano-sized, nonreacting, noncoarsening oxide dispersoid particles, such as TiO 2 [30][31][32], Al 2 O 3 [33], Y 2 O 3 [34], CNTs [35], and ZrO 2 [36] can affect the growth rate of interfacial IMC.

Corrosion behavior of Pb-free solder joints
The diversity of materials, drive toward miniaturization, and globalization have significantly contributed to the corrosion of microelectronic devices [37]. However, the key point is that solder joints are often exposed to corrosive environments that can accelerate the corrosion process. Although corrosion resistance is an important parameter in choosing solder alloys, the corrosion behavior of Sn-Pb solder joints was rarely of interest because the oxide that forms on the tin-lead alloy is relatively stable. Mori et al. showed that both Pbrich and Sn-rich phases dissolve when the Sn-Pb solder alloy is immersed in corrosive solution, and the corrosion rate is slower than that of the Sn-Ag solder [38,39]. Compared to traditional Sn-Pb solders, Sn-Ag-Cu solders are easily corroded in corrosive environments due to their special structures (as shown in Fig. 3). The presence of Ag 3 Sn in Sn-Ag-Cu solders accelerates the dissolution of tin from the solder matrix into a corrosive medium  [39].
because of the galvanic corrosion mechanism [39]. When corrosion occurs in the solder joints, it may change the microstructure of corroded regions and provide crack initiation sites, thereby decreasing the mechanical properties of the joints. Lin and Lee have investigated both Sn-Pb and Sn-Ag-Cu solder alloy wafer-level packages, with and without pretreatment by 5% NaCl salt spray, with thermal cycling to failure. The salt spray test did not reduce the characteristic lifetime of the Sn-Pb solder joints, but it did reduce the lifetime of the Sn-Ag-Cu solder joints by over 43% (Fig.4). The characteristic lifetime cycle number was 1384 for the as-assembled and non-salt spray treated components, but it was only 786 for the components which were treated in 5 wt.% NaCl salt spray for 96 h. In addition, the presence of multiple corrosion sites per solder joint poses an additional risk factor to the structural stability of the joint, for corrosion sites are all potential crack initiation sites.

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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 Ag 3 Sn and Cu 6 Sn 5 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].

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 [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]. 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 Cu 6 Sn 5 and Cu 3 Sn, 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 Cu 3 Sn layers than with Cu 6 Sn 5 layers. The galvanic current densities of the Sn37Pb solders of Cu 3 Sn, Cu, and Cu 6 Sn 5 are about 38, 16, and 5 (μA/cm 2 ), respectively. Fig. 7. The galvanic current densities of the solder with respect to intermetallic compounds Cu 6 Sn 5 and Cu 3 Sn, and Cu substrate, in a 3.5 wt.% solution [45].

Metals used in solder Metals used in microelectronics
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 mV SCE , the passivation current densities of all specimens are around 10 -1 A/cm 2 , with the declining sequence of Sn37Pb ≥ Cu 6 Sn 5 > Cu 3 Sn > Cu.   [45].
It can be seen that the galvanic corrosion behavior of Cu 3 Sn is generally greater than that of Cu 6 Sn 5 for the flip chip package in a 3.5 wt. % NaCl solution environment. This indicates that the formation of IMC Cu 3 Sn and Cu 6 Sn 5 layers causes many problems with corrosion behavior and reliability.  www.intechopen.com

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][53][54][55]. Lin et al. [47][48][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 Ag 3 Sn IMC is retained after the polarization test. Hence, the Ag 3 Sn 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][47][48][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, SnO 2 , SnCl 2 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 (Sn 3 O(OH) 2 Cl 2 ).

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-Al 2 O 3 particles was 100 nm in diameter.
Notably, the addition of nano-particles decreased the size of dendrite β-Sn grains, the needle-like Ag 3 Sn grains, and Ag 3 Sn phase located between the average spacing. When 1 wt% was added, the superfine spherical nano-Ag 3 Sn 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 Ag 3 Sn IMCs were not observed in the Pb-free SAC solder. Another, author reported that the effects of nano-TiO 2 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 Cu 6 Sn 5 IMC layer grows with a rough scallop shape (Mark A), and wicker-Cu 6 Sn 5 IMC forms on the rough scallop-shaped Cu 6 Sn 5 IMC layer (Mark B) and grows into the SAC solder matrix. However, the addition of a small percentage of nano-TiO 2 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 Cu 6 Sn 5 IMC layer was detected at the interface. However, the wicker-Cu 6 Sn 5 IMC disappeared at the interface with the Cu pads. In addition, the number of Ag 3 Sn IMC forms increased in the eutectic area when the content of nano-TiO 2 particles was increased to 0.25-1 wt%. It is interesting that the smallest thickness of the IMC layer was achieved with the addition of 1 wt% of nano-TiO 2 particles. The thickness of the Cu 6 Sn 5 IMC layer was reduced by 51%. The results indicate that the growth of the Cu 6 Sn 5 IMC layer at the solder/pad interfaces of Sn3.5Ag0.5Cu is depressed through the small addition of nano-TiO 2 particles [58]. With the addition of 0.5-1 wt% nano-TiO 2 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 Ag 3 Sn 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].     13. Fractography of the Sn3.5Ag0.5Cu-1TiO 2 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 (Φcorr), the breakdown potential (Φ b ), and the dynamic corrosion current density (Icorr) have been determined ( Table 6). The width of the passive region on the anodic polarization curves (ΔΦ = Φ b -Φcorr) in Table 6 indicates the pitting resistibility or the stability of the passive film on the Sn3.5Ag0.5Cu composite alloy surface. The corrosion potential (Φcorr) 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 boundaries, 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 ΔΦ 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.  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 Ag 3 Sn 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.  www.intechopen.com 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]: When the current density reaches about 10 mA/cm 2 , many hydrogen bubbles evolve from the cathode due to the hydrogen evolution on the cathode: H O+ e →H + OH The reactions on the anode are quite complicated. Some possible anodic reactions have been reported in the literature [46,[64][65][66], as displayed below: Sn + OH − e = Sn OH

Sn + OH − e =SnO+H O (4)
Sn + H O=Sn OH + e + H The dehydration of Sn(OH) 2 and Sn(OH) 4 into SnO and SnO 2 , respectively, has also been reported [46,64,65]: Sn OH + OH + e = Sn OH Sn OH + OH =SnO+H O SnO + H O+ OH + e = Sn OH 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]: 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 (Sn 3 O(OH) 2 Cl 2 ). 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, Sn 3 O(OH) 2 Cl 2 , 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 Sn 3 O(OH) 2 Cl 2 . Fig. 16. EDS analysis of corrosion product of the Sn3.5Ag0.5Cu nano-composite solder after polarization tests [60]. www.intechopen.com