## 3 Results and Discussion  
### Microstructure  
The continuous intermetallic compound (IMC) layer, forming at the interface of the solder and Cu during the soldering process, is fundamental to the efficient metallurgical combination. However, the brittle IMC layer is sensitive to the service conditions and the performance of the solder joint is greatly determined by the thickness and morphology of the IMC layer [13]. Figure 2 shows the interfacial microstructure of the Sn50 Pb49 Sb1/Cu solder joint at different irradiation doses. According to Figure 2a and the results of the EDS analysis, a continuous Cu\({}_{6}\)Sn\({}_{5}\) layer with a \"saw tooth-like\" morphology was observed at the interface of the as-soldered joint, which could lead to the conclusion that successful solder joints are prepared due to non-existent defects. As can be seen in Figure 2b, the thickness and morphology of the Sn50 Pb49 Sb1/Cu solder joint barely changed after being irradiated for 960 h, and the EDS results in Figure 2d show that the composition of the IMC layer was still Cu\({}_{6}\)Sn\({}_{5}\). Therefore, the effect of \(\gamma\)-ray irradiation on the interfacial microstructure of the Sn50 Pb49 Sb1/Cu solder joint could be considered unapparent. Figure 1: Diagram of 45\({}^{\circ}\) pull test of solder joints on QFP lead. Figure 2: The interfacial microstructure of the Sn50 Pb49 Sb1/Cu solder joint at different irradiation times: (**a**) 0 h; (**b**) 960 h; (**c**) EDS results of point A; (**d**) EDS results of point B. In the SnPb solder, tin and lead dissolved mutually, forming a solid solution. No IMC was generated based on the Sn-Pb binary phase diagram, as shown in Figure 3. Moreover, Sb could not react with neither Sn nor Pb, and only dissolved in the Sn-based solid solution [14]. Figure 4 shows the solder-side microstructure of the Sn50 Pb49 Sb1/Cu solder joint at various irradiation doses. As can be seen from Figure 4,e, the element mapping results of Figure 4 show that the microstructure of the solder joint consists of two phases: an Sn-based solid solution that is red in Figure 4, and a Pb-based solid solution that is green in Figure 4. Moreover, the trace amount of Sb only dissolved in the Sn-based solid solution. In the as-soldered joint, the Pb-based solid solution with a flat and intact morphology was observed, as shown in Figure 4. However, micro-  
There are two types of \(\gamma\) photons that \({}^{60}\)Co emits: 1.17 MeV and 1.33 MeV. Based on this energy level, the Compton effect mainly occurred in the Sn50 Pb49 Sb1/Cu solder joint subjected to \(\gamma\)-ray irradiation, as shown in Figure 5. Displaced atoms induced directly by the collision of the \(\gamma\) photon and nucleus could be ignored due to their low cross section, but energetic electrons generated by \(\gamma\)-ray irradiation through the Compton effect could bring about many point defects [15]. The energy relationship of the incident \(\gamma\) photon and energetic electron (\(E_{\text{e}}\)) is given by:  
\[E_{e}=\frac{2E_{\gamma}^{2}}{m_{e}c^{2}+2E_{\gamma}^{2}} \tag{1}\]  
where \(E_{\gamma}\) is the energy of the incident \(\gamma\) photon, and m\({}_{\text{e}}\)c\({}^{2}\) is the rest energy of electron. Figure 6 shows the schematic diagram of micro-defect generation in the Pb-based solid solution during the irradiation process. Based on Equation (1), the energetic electron induced by the \(\gamma\) photon could knock the Pb atom out and form both a displaced atom and a vacant lattice site, which is known as Frenkel disorder, as shown in Figure 6a. During the irradiation process, a part of these defects were annihilated by migration, whereas other parts retained and expanded with the accumulation of irradiation dose, forming micro-voids and micro-cracks, as illustrated in Figure 6b,c. In addition, it is well known that the interaction of the Compton process is proportional to the atomic number (Z) of the target material.