**5. 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 Ag3Sn in Sn-Ag-Cu solders accelerates the dissolution of tin from the solder matrix into a corrosive medium

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Novel Nano-Composite Solders in Electronic Packaging 113

Fig. 4. Weibull plot for the thermal cycling results on 5 wt.% NaCl aqueous solution (salt spray) treated WLCSP: (a) Sn-Pb solder alloy samples and (b) SAC305 solder alloy samples [40].

Fig. 3. Surface morphology changes of solder balls after the salt spray test for 96 hrs: (a) Sn-Pb solder, and (b) SAC solder[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.

Fig. 3. Surface morphology changes of solder balls after the salt spray test for 96 hrs: (a) Sn-

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.

Pb solder, and (b) SAC solder[39].

Fig. 4. Weibull plot for the thermal cycling results on 5 wt.% NaCl aqueous solution (salt spray) treated WLCSP: (a) Sn-Pb solder alloy samples and (b) SAC305 solder alloy samples [40].

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Novel Nano-Composite Solders in Electronic Packaging 115

[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

**Metals used in solder Metals used in microelectronics** 

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].

Sn 1.636 0.935 0.473 -0.114 1.123 Pb 1.626 0.925 0.463 -0.124 1.113 In 1.842 1.141 0.679 0.092 1.329 Zn 2.263 1.562 1.10 0.513 1.75

Au Ag Cu Ni Pd

based SLID bonding very appealing for 3D stacked applications (Fig. 6) [44].

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

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