# Mechanical Property of Sn-58 Bi Solder Paste Strengthened by Resin
## 1 Introduction  
The regulation of certain hazardous substances (RoHS) and waste electrical and electronic equipment (WEEE), issued in 2003 and formally implemented on 2006, has lead research on lead-free solder to become a global research hotspot [1; 2]. Finding lead-free solders to replace Sn-Pb solders has become a new challenge in materials science. Recently, environmental-friendly lead-free Sn-based solder alloys contain a variety of elements, such as Ag, Cu, Sb, Zn, Bi, and In, have been widely used in electronic packaging systems. References [3; 4; 5; 6; 7] have shown that compared with the toxicity of cadmium and antimony, Ag, Cu, Zn, Bi, and In additives are generally recognized as \"green elements\" in the electronic field. However, the most common solder alloys, such as Sn-Ag and Sn-Cu solder alloys, have melting point temperatures in excess of 200 \({}^{\circ}\)C. In the case of low-temperature soldering conditions, only Sn-Bi and Sn-In systems are common in soldering [8; 9]. Low-melting-point, lead-free solder alloys are mainly used in applications where the components can only withstand low temperatures, such as in notebook cooling modules, Surface Mounted Devices (SMDs), and thermistors [10]. At present, the solder alloys satisfying the conditions are mainly Sn-In and Sn-Bi, whose melting points are below 180 \({}^{\circ}\)C. However, the application of Sn-In alloy is limited due to the high cost and shortage of resources. Sn-Bi solder alloy is particularly suitable for products requiring low-temperature soldering conditions. Bi is a surface-active element that can improve solder wettability. In addition to Pb, Bi is the most effective element for reducing the occurrence of Sn whiskers [11; 12]. In terms of health hazards, bismuth is safer than other commonly used metals such as Sn, Ag, In, Cu, and Ni. Sn-\(\chi\)Bi-based solder alloys have an extremely broad melting point range, from 138 \({}^{\circ}\)C to 232 \({}^{\circ}\)C in the eutectic point. It does not form inter-metallic compounds (IMCs), and Bi exists mainly in Sn as a solid solution [13; 14]. Therefore, the strengthening effect of the Sn-Bi system is solid solution strengthening. Some researchers believe that Sn-B-based solder alloys are ideal substitutes for traditional Sn-Pb solderalloys; however, their inherent brittleness limits their applications [15]. Excessive concentrations of Bi atoms can adversely affect the mechanical properties of Sn-Bi solder joints and reduce tensile strength and plasticity [16]. To improve the strength of Sn-Bi solder joints, researchers have proposed many improvements. [PERSON] et al. [17] researched the influences of Ag addition to Sn-58 Bi solder, and the results revealed that the Ag addition could form a Cu-Ag layer and Bi segregation was restrained without embrittlement, which could enhance the solder joint strength. [PERSON] et al. [18] conducted a mechanical mixing method to add low-Ag Sn-0.7 Ag-0.5 Cu-2.5 Bi-0.05 Ni (SAC-BN) to Sn-Bi eutectic solder pastes, which suppressed the possibility of brittle failure of the solder joints by decreasing the relative concentration of Bi. The distribution of Sn-Ag and Sn-Cu intermetallic compounds was considered to improve the mechanical properties of solder joints. [PERSON] et al. [19] studied the effect of Sb content on the properties of Sn-Bi solders. It was found that the shear strength of the Sn-Bi-Sb solders increased as the Sb content increased. However, rather than adding a third element to the solder alloy to improve the strength, another method acts by changing the flux component in the solder paste. In addition to solvents, thixotropic agents, and activators, the most important component of flux in solder paste is rosin or synthetic resin, which can improve wettability, increase surface tension, and has a low viscosity to remove the product. A stable insulating layer can be formed to stabilize the product after soldering.