## 4 Results and Discussion
### Thermal Annealing with Constant Electric Currentand Current Density (Uniform Cross-Sectional Areas)  
In this experiment, two 5:5 samples (constant cross-section samples) were heated and subjected to a current fixed at 0.8 A. One sample was heated at 130 \({}^{\circ}\)C and was observed after 3 days and 4 days; the other was heated at 180 \({}^{\circ}\)C and was also observed after 3 days and 4 days. The IMC formation in the samples' anode and cathode ends were monitored and measured. As shown in Figure 9, the holes and cracks in the IMC layer of the sample heated at 130 \({}^{\circ}\)C were few. The resulting IMC formations and their corresponding thickness for the sample heated at 130 \({}^{\circ}\)C are summarized in Table 3.  
Figure 8: Intermetallic compound (IMC) formation after thermal annealing: (**a**) sample heated at 130 \({}^{\circ}\)C for 3 days; (**b**) sample heated at 180 \({}^{\circ}\)C for 4 days. Figure 9: SEM images of 5:5 sample heated at 130 \({}^{\circ}\)C and subjected to a current of 0.8 A. (**a**) Anode side (indicated on red circle) for 3 days; (**b**) Cathode side for 3 days; (**c**) Anode side for 4 days; (**d**) Cathode side for 4 days. For the 5:5 samples with constant current and heated at 180 \({}^{\circ}\)C, it was observed that the IMC formation was considerably thicker after 3 days and 4 days of observation. The high temperature provided Cu\({}_{6}\)Sn\({}_{5}\) with great growth energy; and because Cu\({}_{3}\)Sn growth was controlled by Cu\({}_{6}\)Sn\({}_{5}\), the latter was thicker than the former. Table 4 and Figure 10 show the IMC formations and their corresponding thickness in the sample heated at 180 \({}^{\circ}\)C. It was also observed that the IMC in the anode was thicker than that in the cathode end. This is because the atoms in the cathode migrated out, while those in the anode accumulated on the site. \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**IMC**} & \multicolumn{2}{c}{**Anode**} & \multicolumn{2}{c}{**Cathode**} \\ \cline{2-5}  & **3 Days** & **4 Days** & **3 Days** & **4 Days** \\ \hline Cu\({}_{6}\)Sn\({}_{5}\) & 7.65 & 7.68 & 5.3 & 6.56 \\ \hline Cu\({}_{3}\)Sn & 2.69 & 4.67 & 2.69 & 3.29 \\ \hline Total IMC & 10.34 & 12.35 & 7.99 & 9.86 \\ \hline \end{tabular}
\end{table} 
Table 4: The IMC formation and their corresponding thickness in the sample heated at 180 \({}^{\circ}\)C and subjected to a current of 0.8 A for the 55 sample. Figure 10: SEM images of 5:5 sample heated at 180 \({}^{\circ}\)C and subjected to a current of 0.8 A. (**a**) Anode side (indicated on red circle) for 3 days; (**b**) Cathode side for 3 days; (**c**) Anode side for 4 days; (**d**) Cathode side for 4 days. \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**IMC**} & \multicolumn{2}{c}{**Anode**} & \multicolumn{2}{c}{**Cathode**} \\ \cline{2-5}  & **3 Days** & **4 Days** & **3 Days** & **4 Days** \\ \hline Cu\({}_{6}\)Sn\({}_{5}\) & 1.71 & 2.02 & 1.6 & 1.67 \\ \hline Cu\({}_{3}\)Sn & 1.56 & 1.85 & 1.39 & 1.51 \\ \hline Total IMC & 3.27 & 3.87 & 2.99 & 3.18 \\ \hline \end{tabular}
\end{table} 
Table 3: The IMC formation and their corresponding thickness in the sample heated at 130 \({}^{\circ}\)C and subjected to a current of 0.8 A. Thermal Annealing with Constant Current and Varied Current Density (Different Cross-Sectional Areas)  
In the last experiment, the samples used had different cross-sectional areas (3:7 and 1:9) and were subjected to a current of 0.8 A. Both were heated at 130 \({}^{\circ}\)C and 180 \({}^{\circ}\)C separately for 3 and 4 days. The current density was analyzed using the finite element method software ANSYS.