## 3 Results and Discussion  
### Microstructure Evolution after Isothermal Aging  
Figure 1 provided the microstructure of SAC0307, SAC0307-0.05 Pr, and their aged samples (up to 840 h). As can be seen, solder doped with 0.05 wt.% Pr has a much more refined microstructure (Figure 1b) when compared with the original SAC0307 solder (Figure 1a). Obviously, IMCs in the eutectic area distributing on the Sn matrix in SAC0307-0.05 Pr solder were much more refined than those in SAC0307 solder. In addition, the amount of eutectic areas was also increased after doping 0.05 wt.% Pr. After an 840-h aging treatment, IMCs in the eutectic area whether in the aged SAC0307 solder or in the aged SAC0307-0.05 Pr solder grew larger (Figure 1c,d). However, the average size of  
the related IMCs in the aged SAC0307 were still larger in the aged SAC0307-0.05 Pr and the amount of eutectic areas was also smaller than those in SAC0307-0.05 Pr. It should be noted that PrSn3 phase with flower shape [30] emerged in the aged SAC0307-0.05 Pr solder. Figure 1e shows the SEM image of the microstructure of aged SAC0307-0.05 Pr. Also, PrSn\({}_{3}\) phase with the similar shape as that in Figure 1d can be clearly seen, which was demonstrated by the EDS analysis of Point 1, shown in Table 1. It was largely because RE Pr is a Sn-affinity element. With aging treatment, Sn and Pr atom diffusion were accelerated and finally they were reacted as PrSn\({}_{3}\). In addition, IMCs in the eutectic area was also analyzed by EDS technique and the results showed the darker ones are mainly Cu6 Sn5 and the bright ones are Ag3 Sn IMCs. Figure 2 gives the detailed illustration of changes in IMCs in both SAC0307 and SAC0307-0.05 Pr solders during aging process. Usually, IMCs in SAC0307 grow larger with a certain rate depending on Sn, Ag, and Cu atom diffusion rates, as can be seen from Figure 2(aI-III). For SAC0307 with 0.05 wt.% Pr doped, the growth of IMCs in the eutectic area with aging process was hindered since the surface-active Pr atoms tend to adsorb on the grain surface of IMCs. Hence, the diffusion of Sn, Ag, and Cu atoms were inhibited, as shown in Figure 2(bI-III). Figure 1: OM Microstructure of **(a)** SAC0307 solder; **(b)** SAC0307-0.05 Pr solder; **(c)** aged SAC0307 solder at 150 \({}^{\circ}\)C for 840 h; **(d)** aged SAC0307-0.05 Pr solder at 150 \({}^{\circ}\)C for 840 h, **(e)** SEM image of aged SAC0307-0.05 Pr solder at 150 \({}^{\circ}\)C for 840 h.  
Figures 3 and 4 show changes of interfacial microstructures of SAC0307/Cu and SAC0307-0.05 Pr/Cu joint subjected to different length of time of aging process. For the interfacial microstructure of SAC0307/Cu solder, a continuous hill-like Cu\({}_{6}\)Sn\({}_{5}\) interfacial IMC layer [31] with sharp protrusions emerged after initial 72-h aging (Figure 3a). Moreover, a darker interfacial IMC layer, as identified to be Cu\({}_{3}\)Sn [32] also grew near the Cu substrate. As aging time prolongs to 840 h, the radius of Cu\({}_{6}\)Sn\({}_{5}\) increased with the number decreasing (Figure 3b-d). In addition, the thickness of Cu\({}_{3}\)Sn also increased, and a detailed calculation is conducted in next section. In contrast to interfacial IMCs' growth at SAC0307/Cu interface with aging, Cu\({}_{6}\)Sn\({}_{5}\) IMCs at SAC0307-0.05 Pr/Cu interface were developed without any sharp Cu\({}_{6}\)Sn\({}_{5}\) protrusions and the interfacial IMC layer was much more flat than the original aged one, even with aging treatment (Figure 4a-d). In addition, the growth of Cu\({}_{3}\)Sn was also much slower than that at the SAC0307-0.05 Pr/Cu interface. Figure 5 provides the changes in interfacial IMCs at both SAC0307 solder/Cu interface and SAC0307-0.05 Pr solder/Cu interface during the aging process. Without Pr addition, the interfacial IMCs grows at a certain rate depending on the diffusions of Sn atoms from solder and Cu atoms from Cu substrate to the solder/Cu\({}_{6}\)Sn\({}_{5}\) or Cu\({}_{6}\)Sn\({}_{5}\)/Cu\({}_{3}\)Sn and Cu\({}_{3}\)Sn/Cu interface.