## 2 SnAgCu-Based Solders
### Reliability  
After high-temperature storage, the coarsening IMC layer of solders joint could be obtained, which has been proven to be harmful to solder joints [41; 42; 43]. Competitive growth of Cu\({}_{6}\)Sn\({}_{5}\) and Cu\({}_{3}\)Sn layers reduces the mechanical properties of solder joints and increases the risk of fracture [44]. [PERSON] et al. [45] found the absorption of TiO\({}_{2}\) nanoparticles on the surface of interfacial IMCs could hinder the diffusion and reaction between Cu and Sn, so the formation and growth of IMCs was inhibited. With 0.5 wt.% TiO\({}_{2}\) nanoparticles addition, the activation energies of overall IMC layer growth increased from 42.48 KJ/mol to 60.31 KJ/mol, which meant more energy was needed for elements diffusion and IMCs growth [46]. TiC nanoparticles can also lower the growth rate of IMCs layer during aging. One reason is that the particles can be adsorbed on the surface of IMCs and the other is that the gathering of TiC particles at the interface decreases the concentration gradient of Sn. Consequently, appropriate content of TiC nanoparticles effectively inhibited the growth of Cu\({}_{6}\)Sn\({}_{5}\) and Cu\({}_{3}\)Sn layers during aging, as shown in Figure 6[47].  
The increasing current density in IC brings a deteriorative electromigration effect and the improvement of EM resistance of solder joints gradually becomes a research focus in reliability research [48; 49]. CNTs will entangle with each other after adding it in the strengthening phase in lead-free solder. Because of the excellent electrical conductivity of CNTs, the conductivity channel formed by CNTs greatly decreases the current intensity of the solder matrix, which increases the service  
Figure 5: Schematic diagram of the effect of Al\({}_{2}\)O\({}_{3}\) nanoparticles on the wetting and spreading: (a) Contribution to wetting; (b) contribution to spreading [39].  
Figure 6: Effect of TiC nanoparticles content on the thickness of IMCs layers during aging (a) Cu\({}_{6}\)Sn\({}_{5}\); (b) Cu\({}_{3}\)Sn [47].  
life of the solder joints [29; 50; 51]. After SAC-CNTs/Cu solder joints underwent aging at 100 \({}^{\circ}\)C and a current density of \(1.2\times 10^{4}\) A/cm\({}^{2}\) for 336 h, a Cu\({}_{5}\)Sn layer formed between the Cu\({}_{6}\)Sn\({}_{5}\) layer and the Cu substrate. The diffusion of Cu atoms from the solder matrix to the interface was impeded due to the pinning effect of CNTs, therefore the thickness of Cu\({}_{6}\)Sn\({}_{5}\) and Cu\({}_{3}\)Sn layers at both the anode side and cathode side decreased with the increase of CNTs content [52].  
Since lead-free solders are usually exposed to corrosion environments, it is important to improve corrosion resistance of the solder for improving reliability and mechanical properties of solder joints [53]. [PERSON] [54] found that GNSs could significantly improve corrosion resistance of SAC solder and the optimum content was 0.03 wt.%. Uniformly distributed GNSs increased the tortuosity of oxygen diffusion pathway and inhibited the diffusion of oxygen in the solder, as illustrated in Figure 7. Furthermore, the addition of GNSs refined the microstructure of the solder and the fine grains formed the initial passivation film to inhibit further corrosion.