# Interface Growth and Void Formation in Sn/Cu and Sn0.7 Cu/Cu Systems
## 3 Results and Discussion
### Discussion  
According to the results obtained, the voids did not appear at the Sn/HP Cu interface, while they formed at the Sn/EP Cu interfaces. This implied that EP Cu may be the main cause for the formation of voids. In the process of electroplating, a number of impurities and defects were introduced into the deposited Cu film [23]. Impurity elements, such as C, O, Cl, and S, existed in the EP Cu based on XPS analysis, as seen in Figure 6. [PERSON] and [PERSON] [23] reported that the organic impurity, poly ethylene glycol (PEG-), was absorbed at the grain boundary of EP Cu film. [PERSON] et al. [18] showed that the segregation of impurities (like \(S\)) to the Cu/Cu\({}_{3}\)Sn interface might lower the nucleation energy of KVs, facilitating the formation and growth of Kirkendall voids. As seen in Figure 2, during the thermal aging, the Cu\({}_{3}\)Sn/EP Cu interface shifted towards the substrate gradually through the consumption of Cu film. The impurities contained in the film would involve the reaction interface. The Cu\({}_{3}\)Sn/EP Cu interface was the most active area for the voiding, which also served as the heterogeneous sites. Therefore, all evidence indicates that the impurities are obviously a dominant reason for voiding. Moreover, according to the experimental results, it was found that the voiding level increases with growth of the Cu\({}_{3}\)Sn layer. Figure 5: Relationship between aging time and average thickness of total IMCs (**a**), Cu\({}_{6}\)Sn\({}_{5}\) (**b**), and Cu\({}_{3}\)Sn (**c**) layer for the three kinds of solder joints after aging at 150 \({}^{\circ}\)C and 180 \({}^{\circ}\)C (**d**), respectively. For the Sn-Cu system, the chemical reactions at the Cu/Cu\({}_{3}\)Sn and Cu\({}_{3}\)Sn/Cu\({}_{6}\)Sn\({}_{5}\) interfaces were represented by chemical equations [24]. At the Cu/Cu\({}_{3}\)Sn interface on the Cu\({}_{3}\)Sn side (the elements in square brackets denote the diffusing species):  
\[3 Cu+[Sn]\to Cu_{3}Sn \tag{1}\]  
At the Cu\({}_{3}\)Sn/Cu\({}_{6}\)Sn\({}_{5}\) interface on the Cu\({}_{3}\)Sn side:  
\[Cu_{6}Sn_{5}+9[Cu]\to 5 Cu_{3}Sn \tag{2}\]  
\[Cu_{6}Sn_{5}\to 3[Sn]+2 Cu_{3}Sn \tag{3}\]  
At the Cu\({}_{3}\)Sn/Cu\({}_{6}\)Sn\({}_{5}\) interface on the Cu\({}_{6}\)Sn\({}_{5}\) side:  
\[5 Cu_{3}Sn\to 9[Cu]+Cu_{6}Sn_{5} \tag{4}\]  
\[2 Cu_{3}Sn+3[Sn]\to Cu_{6}Sn_{5} \tag{5}\]  
According to the equations above, the interfacial reaction was a dynamic process. At the Cu\({}_{3}\)Sn/Cu\({}_{6}\)Sn\({}_{5}\) interface, the reaction direction in Equations (2) and (3) was opposite to that in Equations (4) and (5). The final reaction direction, i.e., the migration direction of the Cu\({}_{3}\)Sn/Cu\({}_{6}\)Sn\({}_{5}\) interface, was determined, to a large extent, by the diffusion fluxes of Cu and Sn. It is well known that copper was the dominant diffusing species in the Sn/Cu reaction couple [25]. The grain boundary diffusion was a dominant diffusion mechanism for the Cu atom [26]. We plotted some sketch maps reflecting the relation between interface behavior and high-purity Cu (HP Cu), electroplated Cu (EP Cu) and Cu addition (0.7%), respectively, based on the experimental results, as shown in Figure 7. I was observed that EP Cu was smaller in grain size than that of HP Cu. It is known to all that finer grained Cu film could supply much more diffusion paths for Cu atom diffusion, resulting in a bigger atom flux shifting out of the interface. Hence, a much bigger Cu flux would be generated in the EP Cu. This was favorable for the reactions on the Cu\({}_{3}\)Sn side (Equation (2) and Equation (3)). In addition, according to Equation \(Jv=J_{Cu}-J_{Su}\), the finer EP Cu was able to supply a bigger \(J_{Cu}\) due to more diffusion paths, which was expected to generate more considerable vacancies at the EP Cu/IMC interface, relative to the HP Cu/IMC. At last, the vacancies gathered together to form the micro voids, or KVs.