## 3 Results and Discussion
### Microstructure Evolution during Aging  
Figure 3 shows the microstructure of as-soldered and -aged (up to 1440 h) solder matrix and joint. It can be seen that SAC387-0.05 Nd solder (Figure 3b) has a better refined microstructure compared with the original (Figure 3a) SAC387 solder. In addition, the amount of eutectic areas was also increased after doping 0.05 wt. % Nd. After 1440 h aging treatment, the size of IMCs in eutectic area enlarged with aging time in both solders (Figure 3c-d). Due to the refining effect of Nd, the average size of IMCs in the aged SAC387-0.05 Nd solder was obviously smaller than those in the aged SAC387 solder. Figure 3e-h shows the evolution of interfacial morphology of solder joint after aging treatment, it can be clearly observed that a continuous hill-like Cu\({}_{6}\)Sn\({}_{5}\) interfacial IMC layer (IML) [19] with sharp protruding emerged at SAC387/Cu interface (Figure 3e). Meanwhile, the growth of interfacial IMCs was inhibited in SAC387-0.05 Nd solder joint, exhibiting as flatter IMC morphology (Figure 3f). After 1440 h aging treatment, the thickness and radius of Cu\({}_{6}\)Sn\({}_{5}\) in both solder joints increased with aging time (Figure 3g,h). It can be concluded that the thickness and radius of Cu\({}_{6}\)Sn\({}_{5}\) at SAC387/Cu interface were larger than those at SAC387-0.05 Nd/Cu interface, indicating a hindering effect of Nd on the growth of interfacial IMCs. Besides, a darker interfacial IMC layer identified as Cu\({}_{3}\)Sn was  
Figure 2: Shear force of solder joint with fracture morphology after certain aging time. (**a**): 0 h, SAC387/Cu; (**b**): 720 h, SAC387/Cu; (**c**): 1440 h, SAC387/Cu; (**d**): 0 h, SAC387-0.05 Nd/Cu; (**e**): 720 h, SAC387-0.05 Nd /Cu; (**f**): 1440 h, SAC387-0.05 Nd/Cu. observed at the bottom of interfacial Cu\({}_{6}\)Sn\({}_{5}\) IMC layer in both solder joints, while the thickness of Cu\({}_{3}\)Sn layer in SAC387-0.05 Nd/Cu interface was less than that of SAC387/Cu interface. Figures 4 and 5 provided the detailed interfacial IMCs' evolution of these two types of joints aged for different hours. As can be clearly seen, interfacial Cu\({}_{6}\)Sn\({}_{5}\) and Cu\({}_{2}\)Sn IMCs grew with aging time, and those at the SAC387-0.05 Nd/Cu interface grew slower compared to those at the SAC387/Cu interface. In addition, strip-shaped Ag\({}_{3}\)Sn IMCs on the top of Cu\({}_{6}\)Sn\({}_{5}\) IMCs at SAC387/Cu interface could be clearly observed during the aging process (Figure 4). However, the size and shape of Ag\({}_{3}\)Sn IMCs was obviously reduced at SAC387-0.05 Nd/Cu interface as shown in Figure 5. Thus, these Ag\({}_{3}\)Sn IMCs play an inhibiting role of the growth of interfacial IMCs to some degree, as will be given in Section 3.4. The absorbing-refining effect of Nd during aging treatment is shown in Figure 6. Generally, the growth of IMCs in solders is dependent on the diffusion rate of Sn, Ag and Cu atoms (Figure 6a). However, element Nd is prone to be absorbed on the surface of IMCs due to its surface-active characteristic as shown in Figure 6b, which hindered the diffusion of Sn, Ag and Cu atoms [19]. As can be seen from Figure 6c, the interfacial IMCs grows depending on the diffusion of Sn and Cu atoms to the solder/Cu\({}_{6}\)Sn\({}_{5}\), Cu\({}_{6}\)Sn\({}_{5}\)/Cu\({}_{3}\)Sn and Cu\({}_{3}\)Sn/Cu interfaces at a certain rate; hence the interfacial Cu\({}_{6}\)Sn\({}_{5}\) IMCs are scallop-shaped. With trace amount of Nd added, the interfacial Cu\({}_{6}\)Sn\({}_{5}\) became flatter due to the adsorbing effect of Nd atoms on the grain surface of Cu\({}_{6}\)Sn\({}_{5}\) IMCs and hindering their growth (Figure 6d).