## 1 Introduction  
With the rapid updating of electronic packaging and devices, micro-bumps are becoming widely used in 3D packaging [1; 2; 3]. The mechanical properties of lead-free solder alloys are dependent on the microstructural morphologies, such as the \(\beta\)-Sn phase, intermetallic precipitations, and doped nanoparticles in the solder matrix [4; 5; 6]. The decreasing size of solder joints has significant effects on the interfacial reaction, the forming of interfacial intermetallic compounds (IMCs), and the evolution of crystallization in the solder matrix, which further affect the mechanical properties [6; 7; 8]. Volume and size effects on microscale solder joints were reviewed by [PERSON] et al. [9] in 2011. Recently, more concerns were focused on the size and volume effects on solder joints due to the industrial application demands. Indeed, the size effects on solder joints of electronic devices is a complicated topic which covers the microstructure evolution (such as the formation of multiple kinds of intermetallic compounds) and the mechanical behavior (such as shear, creep, and fracture behaviors), each of which was separately discussed in the existing literature. To achieve a comprehensive up-to-date view for the electronic packaging industry, a review on recent researches is needed to build the association between microstructure and mechanical properties, by emphasizing the size effects of solder joints.  
Different types of specimens have been used to investigate the size effects of solder joints, including solder balls, unilateral solder joints, bi-lateral solder joints, line-type solder joints, lap-shear solder joints, and micro-specimens, as summarized in Figure 1. Solder balls without substrate are usually used to investigate the thermal properties such as undercooling, solidification, and phase transformation. Unilateral solder joints are suitable to study the interfacial reaction and measure the IMC growth rate, as well as shear strength. The bi-lateral solder joint type is more approximate to the solder interconnects in packaging structures, and can be used to study the interfacial reaction with different substrates. The diameters of the solder balls and unilateral and bi-lateral solder joints usually range from 0.2 to 1.0 mm. Line-type and lap-shear solder joints are usually used to test the tensile and shear properties, respectively. Furthermore, there are two classes of line-type solder joints: One has a constant solder diameter but reducing solder height, and the other has a constant solder diameter-to-height ratio. The diameters of the Cu pillars of the line-type solder joints range from 300 \(\upmu\)m to 2 mm, and the heights of solder layers range from 5 \(\upmu\)m to 2 mm. Micro-sized specimens are fabricated by focused ion beam (FIB) milling, and can be utilized to explore the mechanical properties on the microscale and have received much more attention recently. For example, the fracture toughness of the IMC Cu\({}_{6}\)Sn\({}_{5}\) can be obtained with the nanoindentation test on the microcantilever cylinder with a prefabricated notch. The microcantilever can be milled into a length of several microns, which is smaller than a single Cu\({}_{6}\)Sn\({}_{5}\) grain. The depth of the notch can be milled to hundreds of nanometers. The compression test of Cu\({}_{6}\)Sn\({}_{5}\) on the micron scale can also be conducted with FIB milling and nanoindentation. Wedge indenters are used in the microcantilever fracture test, while flat-end indenters are used in the compression test.