## 5 Conclusions  
Although lots of research results have been achieved in nanoparticle-reinforced lead-free solder, there are still several shortages existing in the application of the composite solder. Firstly, nanoparticle-reinforced lead-free solders remain in the experimental research stage and hardly replace the SnPb solder to meet the requirement of electronic package industries. Secondly, compared with microalloying, the amount of research on nanoparticle-reinforced lead-free solders is still relatively less and the influence mechanism of nanoparticles on the reliability of the composite solders in various service environments is insufficient. What is more, the preparation process of nanoparticle-reinforced composite solder is complex and the cost is high, which limits its industrial application. To further improve the properties of nanoparticle-reinforced lead-free solders, future development trends can be carried out in the following aspects:  
1. With the development of electronic devices towards miniaturization and high performance, stricter requirements have been put forward for the reliability of lead-free solders. The current research mainly focuses on the effect of thermal stress and current stress on the performance of the solders. However, more and more electronic devices are applied to harsh environments such as cryogenic temperatures [93; 94] and irradiation [95]. Therefore, research on the reliability of the solder bearing nanoparticles in these harsh environments will become a hot spot in this subject.
2. It has been proven that the coupling effects of RE and nanoparticles can further refine the microstructure and improve the mechanical properties of lead-free solders [96]. Thus, the combined addition of RE and nanoparticles may become a new reinforcement method to improve the performance of the solders.
3. Due to small tendency of agglomeration and fine physical and chemical properties, carbon-based nanomaterials can effectively improve the microstructure of lead-free solders and improve the  
Figure 16: Microstructure of (**a**) SnZnBi, (**b**) SnZnBi-0.5 Ag, and (**c**) SnZnBi-0.5 Ni solder alloys at 85 \({}^{\circ}\)C/85% relative humidity (RH) after exposure for 250 h and (**d**) energy dispersive spectrometer (EDS) analysis of region P in (a) [76].  
properties [97; 98]. Consequently, the development of new carbon-based nanomaterial-reinforced solders with better performance may become a new direction for the future research.
* At present, lead-free solders bearing nanoparticles are mainly fabricated by mechanical mixing of the solder particles and nanoparticles. It is claimed that the nanoparticle-reinforced lead-free solders can also be directly prepared by adding nanoparticles though flux doping [37; 99] and in-situ formation [100]. Therefore, exploring new preparation methods with simpler processes and low cost attracts wide attention from researchers.  
[PERSON] conceived the idea; [PERSON] and [PERSON] wrote the paper; [PERSON], [PERSON], and [PERSON] advised the paper. All authors reviewed the paper.  
This work was funded by the National Natural Science Foundation of China, grant No.51675269 and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).  
The authors declare no conflict of interest.