## 3 Results and Discussions  
### Characterization of Sn-Cu Coated MWCNT  
Figure 1a,b shows the XRD analysis of Cu-MWCNT composites and Sn coated Cu-MWCNT composite powders, respectively. The CNT peaks appeared along with the copper peaks in Figure 1a, thus confirming the Cu-MWCNT composite. The XRD of the Sn coated Cu-MWCNT in Figure 1b showed the peaks corresponding to Cu, Sn, and Cu\({}_{6}\)Sn\({}_{5}\) phases. The XRD analysis suggests that few Sn reacted with Cu to form Cu\({}_{6}\)Sn\({}_{5}\) IMC during electroless coating. Figure 2a-f shows the morphology and elemental analysis of the Cu-MWCNT composite and Sn coated Cu-MWCNT composite powders investigated by SEM and EDS. As seen from Figure 2a, the copper coated morphology of MWCNT were irregular and flake-like, with average particle sizes of 8.4 \(\pm\) 5 \(\upmu\)m when compared to the initial size of 60-100 nm diameter and 1-2 \(\upmu\)m length. This could be due to the agglomeration of MWCNTs during copper coating, resulting in the Cu-MWCNT composite. Figure 1: XRD analysis of (**a**) Cu-MWCNT (multi-walled carbon nanotube) composite; (**b**) Sn coated MWCNT composite. The EDS analysis corresponding to region A [Figure 2a], as shown in Figure 2b, confirms the presence of elements C and Cu in the Cu-MWCNT composite. Figure 2c,d shows porous coating of Sn on the Cu-MWCNT composite flakes. The average particle size of Cu-MWCNT after Sn coating increased to 21.7 \(\pm\) 8 \(\upmu\)m. Figure 2e,f shows the elemental maps that confirm the coating of Cu and Sn. Due to the irregular morphology of the Cu-MWCNT composite, the Sn coating was not uniform. After Sn coating, few regions were Sn rich, while others were Cu rich, therefore we conclude that a high pressure of 10 MPa is required during the TLP process to achieve a good bonding. The cross-sectional microstructures of the Cu/Sn coated Cu-MWCNT/Cu joint after TLP bonding at 260 \({}^{\circ}\)C for 5 min and 8 min are shown in Figure 3a-e. Few pores were observed in the microstructure of the TLP joint reflowed at 260 \({}^{\circ}\)C for 8 min when compared to the 5 min sample, as shown in Figure 3a,c. Figure 2: SEM analysis of powders (**a**) Cu-MWCNT composite; (**b**) energy dispersive spectrometer (EDS) analysis of point A; (**c**) and (**d**) Sn coated Cu-MWCNT composite; (**e**) and (**f**) EDS elemental mapping of Cu and Sn, respectively. In the Cu-Sn system, Cu\({}_{6}\)Sn\({}_{5}\) and Cu\({}_{3}\)Sn were the well-established stable intermetallic phases of the Cu and Sn reaction. Firstly, the planar Cu\({}_{6}\)Sn\({}_{5}\) IMC formed at the Cu/Sn interface according to Equation (1) [16] because the thermodynamic driving force for the formation for Cu\({}_{6}\)Sn\({}_{5}\) was higher than Cu\({}_{3}\)Sn at 250 \({}^{\circ}\)C [17]. \[\textbf{6}\textbf{Cu}+\textbf{5}\textbf{Sn}\rightarrow\textbf{Cu}_{6}\textbf{ Sn}_{5} \tag{1}\]  
Cu\({}_{3}\)Sn IMC formed in the vicinity of copper rich regions either by Equation (2) or by the transformation of Cu\({}_{6}\)Sn\({}_{5}\) IMC as per Equation (3) due to the diffusion of Cu atoms [16]. \[\textbf{3}\textbf{Cu}+\textbf{Sn}\rightarrow\textbf{Cu}_{3}\textbf{Sn} \tag{2}\]  
\[\textbf{Cu}_{6}\textbf{Sn}_{5}+\textbf{9}\textbf{Cu}\rightarrow\textbf{5}\textbf{ Cu}_{3}\textbf{Sn} \tag{3}\]  
Figure 3: Cross-sectional SEM microstructure of Cu/Sn-Cu-MWCNT/Cu transient liquid phase (TLP) joint at 260 \({}^{\circ}\)C. (**a**) Low and (**b**) high magnification image of TLP bonding joint at 5 min; (**c**) and (**d**) low and high magnification image of TLP bonding joint at 8 min respectively; (**e**) EDS analysis corresponding to point A. In a layered Cu-Sn-Cu system with increasing reflow time, planar Cu\({}_{6}\)Sn\({}_{5}\) transformed to a scallop type that grew from both sides until Sn was completely consumed, causing them to ripen and impinge on each other. Meanwhile, Cu\({}_{3}\)Sn emerged in the Cu/Cu\({}_{6}\)Sn\({}_{5}\) interface and with an increase in reflow time, Cu\({}_{3}\)Sn continued to grow at the expense of Cu\({}_{6}\)Sn\({}_{5}\)[17].