## 1 Introduction  
In microelectronic packaging, solder joints play an electrical, mechanical, and thermal role between electronic devices and conductive substrates [1; 2]. With miniaturized trends of integrated circuits in electronic devices, the reliability of solder joints becomes particularly important, especially in severe environments, such as high-temperature aging, thermal cycling, drop reliability, and so on [3; 4; 5]. In consideration of close relationship between microstructures and properties, interfacial microstructure at the solder-substrate interface should be studied to enhance the mechanical reliability of the joint. Numerous studies [6; 7; 8] have already demonstrated that one solder with more refined microstructures and thinner interfacial intermetallic compound (IMC) layers usually has a better joint reliability. To achieve this, several research groups are focused on developing composite materials doped with different types of micro- and nano-scale metallic e.g., Ag [9], Ni [10], In [11], Al [12], Co [13]; ceramic e.g., TiO\({}_{2}\), ZrO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and TiO\({}_{2}\) etc. [9; 14; 15; 16] and rare earth (RE) elements [17; 18; 19]. This is because most of these reinforcements are surface-active and prone to have adsorptions on the surfaces of the grain. Therefore, a pinning effect was exerted by them on grain growth. [PERSON] et al. [20] studied the effect of RE La on the microstructures and mechanical properties of SAC alloy at a temperature of 150 \({}^{\circ}\)C. Corresponding results showed that the average grain size of IMCs distributed in La-doped solder was refined by up to 40% and the coarsening rate of the IMCs is slowed by 70% when compared with those in the non-doped solder. Corresponding yield stress and tensile strength are enhanced by 20% by minute lanthanum doping. In addition, the aging of nanoparticle-dopedsolder was also investigated, such as SAC387 solder doped with Co nanoparticle aging at 150 \({}^{\circ}\)C for up to 1200 h. It was found that Co nanoparticles effectively suppressed the growth of interfacial Cu\({}_{3}\)Sn IMCs but contribute to the growth of Cu\({}_{6}\)Sn\({}_{5}\). Co nanoparticles will also experience surface dissolution during reflow processes [21]. Although nanoparticles are the research hotspot presently, doping technology and agglomeration are still key issues that remain to be resolved, especially for the economically and easily available ceramic nanoparticles [22; 23; 24]. Hence, in this study, RE Pr was taken to modify microstructure and properties of solders based on previous research on solders modified by RE alloys [17; 18]. For solder selection, lead-free solder has been adopted in this study according to the directives of Waste Electrical and Electronic Equipment (WEEE) and Restriction of Hazardous Substances (RoHS) [25]. Among the alternative lead-free Sn-based solder alloys such as Sn-9 Zn (198 \({}^{\circ}\)C), Sn-58 Bi (138 \({}^{\circ}\)C), Sn-3.5 Ag (221 \({}^{\circ}\)C), Sn-0.7 Cu (221 \({}^{\circ}\)C), and Sn-3.0 Ag-0.5 Cu (217 \({}^{\circ}\)C) [26; 27; 28; 29], Sn-Ag-Cu has received wide acceptance due to its superior comprehensive properties. However, for fear of plate-like Ag\({}_{3}\)Sn IMCs in Sn-Ag-Cu high-Ag solder ruining its drop reliability [7], the content of Ag was decreased to 0.3 wt.% in this study. Lots of previous research has only focused on studying RE alloys on the microstructure of solder matrixes and solid-liquid reactions during soldering processes, whereas the reaction mechanism during solid-solid processes should also be given attention. This could help researchers to better understand the detailed failure mechanism of electronic device in service. Hence, this work is devoted to investigating the evolution of interfacial microstructures and the shear strength of SAC0307/Cu and SAC0307-0.05 Pr/Cu solder under isothermal aging treatment (150 \({}^{\circ}\)C; up to 840 h).