## 1 Introduction  
Under the drive of device miniaturization, high-density packaging technologies, such as ball grid array (BGA), chip scale package (CSP), and wafer level packaging (WLP), have been applied in the advanced electronical equipment, which decreases the joint size and calls for excellent property of packaging materials [1]. To meet the actual requirement of electronic industries, packaging materials for electronic devices are required to have not only perfect electrical conductivity, wettability, and mechanical properties, but also low cost and high reliability. Traditional SnPb solders were widely used in electronic packaging because of low cost and excellent performance. However, due to the toxicity of lead, most countries have implemented a series of laws such as waste electrical and electronic equipment (WEEE) and restriction of hazardous substances (RoHS), to restrict the use of lead in various electronic and electrical products [2]. Electrically conductive adhesives (ECAs) and lead-free solders were considered as promising alternatives of SnPb solders. ECAs, mainly composed of matrix polymer and conductive fillers, offer numerous advantages such as environmental friendliness, lower processing temperature, and fine pitch capability [3]. However, the application of ECAs is limited due to its lower thermal conductivity and electrical conductivity, poor mechanical properties, and low reliability [4]. Especially, ECAs can hardly meet the requirements of three-dimension integrated circuits (3D ICs) packaging technology, which easily produces high current density and Joule heating. Therefore, compared with ECAs, the development of lead-free solder alloys with better reliability, thermal, and electrical properties to replace SnPb solders has always been one of research focuses in the field of microelectronic joining. Although the application of SnPb solder is limited, its excellent performance makes it a reference standard for the development of new lead-free solders [5]. Nowadays, several Sn-based lead-free solder alloys have been synthesized, such as SnAgCu, SnZn, and SnBi solder alloys. However, these lead-free solders still have some shortages such as high melting point, poor wettability and reliability, and high cost, which fail to meet the actual production needs. As a result, it is urgent that an effective way to improve the performance of the solder alloys is found. Currently, researchers mainly improve the performance of lead-free solders by microalloying and nanoparticles strengthening. Microalloying is a method of improving the properties of the solder by adding trace alloying elements to change the solder composition. The alloying elements can be mainly divided into rare earth (RE) elements (Ce, La, Pr, Nd, etc.) [6; 7] and other metal elements (Ga, In, Mg, Ni, Ag, etc.) [8; 9; 10; 11]. However, there are still some shortcomings in this method, which limits the widely application of the lead-free solders containing minor alloying elements. For example, adding excessive RE elements will result in the formation of Sn whisker on the surface of solder joints, causing premature failure of electronic device [12; 13]. Due to excellent physicochemical properties and size effects, nanoparticles play a significant role in refining grains, increasing mechanical properties, and improving wettability of lead-free solder [14; 15; 16; 17; 18; 19]. Therefore, more and more researchers are focused on the preparation and performance characterization of the composite solders reinforced by various nanoparticles. Up to now, types of nanoparticles studied extensively mainly includes metals (Cu, Ni, Mn, Co, etc. ), oxides (La\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), etc. ), ceramic (SiC, TiC, etc. ), and carbon materials (carbon nanotubes (CNTs), graphene nanosheets (GNSs), etc.). Although some research results have been achieved, there are still some deficiencies in the strengthening effect of nanoparticles.