# Mechanical Property of Sn-58 Bi Solder Paste Strengthened by Resin
## 3 Results and Discussion
### Fracture Analysis and Shear Strength  
Figure 5 shows the effect of different contents of epoxy resin on Sn-58 Bi solder joints, tested by a force gauge ranging from 0 wt.% to 9 wt.% according to JIS Z 3198-7. Five data points were tested for one parameter and averaged, and the standard deviations of the experimental data are shown in Figure 5. The shear strength of the Sn-58 Bi solder joint was 95 N, and as the epoxy content increased to 6 wt.%, the shear strength rose to 191 N (reaching twice that of the Sn-58 Bi solder joint alone), and then decreased rapidly.  
The fracture morphology of the solder joint is shown in Figure 6. It was observed that the flux with epoxy resin around the solder joint was deformed by shear force, as shown in Figure 6d. Epoxy resin has adhesiveness and good strength, which can affect the fracture process of the solder joint by mechanical resistance [25]. However, Figure 6b indicates that the flux with no epoxy resin does not participate in the deformation, and thus does not influence the shear force.  
In order to study the failure mechanisms of Sn-58 Bi solder joints with different epoxy contents, Sn-58 Bi solder joints with 0 wt.%, 6 wt.%, and 9 wt.% epoxy resin were selected as the characteristic parameters for analysis. The fractured joints are shown in Figure 7. It was obvious that the epoxy resin affects the morphology of the fracture surface. There was no obvious plastic deformation feature on the fracture surface of the Sn-58 Bi solder joint; the joint appeared to have experienced a brittle fracture process. Red area A displayed in Figure 7a is enlarged and shown in Figure 7(a1), and red area B displayed in Figure 7a is enlarged and shown in Figure 7(a2). A series of steps in the crack propagation direction of the area showed a river line pattern, which confirmed a typical brittle fracture. EDS analysis identified the Bi-rich phase, as shown in Figure 7(d1); this was marked as spectrum 5. The surface area of the fracture detected the bare substrate of spectrum 6 in Figure 7(d2), which indicated that the adhesive strength of the Sn-58 Bi solder paste and the substrate was poor. It was found that the fracture showed different degrees of the grain polyhedral shape of the rock-like pattern in Figure 7(a2), which was identified as intergranular fracture [26; 27; 28].  
Figure 5: Shear strength test with different epoxy resin contents.  
As the epoxy resin content increased to 6 wt.%, fracture tended to occur inside the solder. The toughness and brittleness mixing morphology of the Sn-58 Bi solder was observed on the fracture surface. The enlarged views of regions A and B marked in Figure 7b are shown in Figure 7(b1,b2), respectively. The fracture surface in Figure 7(b1) shows a sliding characteristic, and a tendency to change in toughness is observed in Figure 7(b2), indicating that the fracture property was improved through the addition of epoxy resin.  
Figure 7c shows the fracture morphology of the Sn-58 Bi solder with 9 wt.% epoxy resin. Many holes with different sizes were observed in the fracture surface. EDS analysis revealed the presence of carbon and oxygen in the holes. It is suspected that due to the excessive addition of epoxy resin to the solder paste, the flux could not be completely diffused during reflow and remained in the solder to form holes. These holes can be considered as the defect that reduced the shear strength of the Sn-58 Bi and Cu solder joint.  
However, the research in this article still has certain limitations. Future research will focus on the curing process of epoxy resin and the performance of epoxy resin. At the same time, although an increase in the shear strength was observed, the mechanical properties and electrical properties should also be considered and tested.  
Figure 6: SEM images of the fractured surface on the solder joint border. (**a,b**) Sn-58 Bi solder joint and (**c,d**) Sn-58 Bi + 6 wt.% epoxy resin (ER) solder joint; (**e**) spectrum 3 in Figure 6b, (**f**) spectrum 4 in Figure 6d.