# Interface Growth and Void Formation in Sn/Cu and Sn0.7 Cu/Cu Systems
## 3 Results and Discussion
### Growth Behaviors of IMC Layer  
The thicknesses of the total IMCs, Cu\({}_{6}\)Sn\({}_{5}\), and Cu\({}_{3}\)Sn layer for the three kinds of solder joints aged at different aging periods are shown in Figure 5. It was observed that the thickness of the total IMC layer increased with the aging time. EP Cu and Cu addition (0.7%) seemed to slightly affect the total IMC thickness. However, the growth behaviors of both the Cu\({}_{6}\)Sn\({}_{5}\) and Cu\({}_{3}\)Sn layer were sensitive to the EP Cu and SnO.7 Cu solder. With the increase of the aging time, as presented in Figure 5b, the thickness of the Cu\({}_{6}\)Sn\({}_{5}\) layer in the Sn/HP Cu joints was relatively stable, while the thickness of the Cu\({}_{6}\)Sn\({}_{5}\) layer in the Sn/EP Cu joints was decreased. The thickness of the Cu\({}_{6}\)Sn\({}_{5}\) layer in the Sn0.7 Cu/EP Cu joints gradually increased. For the thickness of the Cu\({}_{3}\)Sn layer, as seen in Figure 5c, it was observed that the thickness of the Cu\({}_{3}\)Sn layer increased with the aging time. The growth rate of the Cu\({}_{3}\)Sn layer in the Sn/EP Cu joint was larger than that of Sn/HP Cu. While in the SnO.7 Cu/EP Cu joint, the growth rate of the Cu\({}_{3}\)Sn layer slowed down. This suggested that the EP Cu substrate promoted growth of the Cu\({}_{3}\)Sn layer, while Cu addition (0.7%) had the function of suppressing growth of the Cu\({}_{3}\)Sn layer.  
[PERSON] et al. [13] investigated the effects of Cu on the interfacial reaction of Sn-Ag/Cu solder joints. They also found that increasing the amount of Cu in Sn-Ag solder significantly reduced the thickness of the Cu\({}_{3}\)Sn layer. The proportion of Cu\({}_{6}\)Sn\({}_{5}\) vs Cu\({}_{3}\)Sn in the Sn-3.0 Ag-0.5 Cu/Cu solder joint after aging at 150 \({}^{\circ}\)C for 28 days was about 6:4. On other hand, a Cu\({}_{6}\)Sn\({}_{5}\)/Cu\({}_{3}\)Sn ratio of about 7:3 was observed in the Sn-3.0 Ag-1.0 Cu/Cu joint. In addition, [PERSON] et al. [22] studied IMC growth in Sn-Ag/Cu solder joints. They found that the thickness ratio of Cu\({}_{6}\)Sn\({}_{5}\) to Cu\({}_{3}\)Sn was about 2:1 as aged at 150 \({}^{\circ}\)C for 700 h. In our work, the Cu\({}_{6}\)Sn\({}_{5}\)/Cu\({}_{3}\)Sn ratios were 1:1 in the Sn/HP Cu solder joint, 1:6 in the Sn/EP Cu, and 2:1 in the Sn0.7 Cu/EP Cu as aged at 150 \({}^{\circ}\)C for 480 h. The results indicated that the EP Cu substrate promotes growth of the Cu\({}_{3}\)Sn layer, and Cu addition (0.7%) is beneficial in retarding the growth of the Cu\({}_{3}\)Sn layer.  
As the temperature increased to 180 \({}^{\circ}\)C, as seen in Figure 5d, it was observed that the thickness of the total IMC layer of Sn/EP Cu increased with the aging temperature, and the Cu\({}_{6}\)Sn\({}_{5}\)/Cu\({}_{3}\)Sn ratio was 1:6 in the Sn/ EP Cu as aged at 150 \({}^{\circ}\)C for 480 h, and 1:8 in the Sn/ EP Cu was aged at 180 \({}^{\circ}\)C  
Figure 4: Interfacial microstructure of Sn/EP Cu joints with different aging conditions, (**a**) 150 \({}^{\circ}\)C for 120 h, (**b**) 180 \({}^{\circ}\)C for 72 h.  
for 480 h. The results indicated that aging temperature could change the reaction occurring at the Cu\({}_{6}\)Sn\({}_{5}\)/Cu\({}_{3}\)Sn interface.