## 3 Results and Discussions
### Characterization of Sn-Cu Coated MWCNT
The formation of Cu-Sn IMC led to a total volume shrinkage of 8.5% [18]. The volume shrinkage was compensated by a decrease in thickness of the Cu/Sn/Cu sandwich, however, at the impingement interface (middle region), volume shrinkage was accompanied by the formation of voids [19]. It was reported that these voids could be controlled by optimizing the experimental parameters, such as applied pressure, reflow time, and interlayer thickness [17; 20]. In the case of the Cu-Sn powder TLP system, voids were more prominently subjected to the large difference in diffusivity of Cu and Sn [21]. The inhomogeneity in mixing the Cu and Sn powder further enhanced the void formation due to the presence of residual Sn (excess unreacted Sn) and IMC formation, resulting in volume shrinkage [22]. Sn coated Cu particles proved to be efficient in reducing the diffusion distance of Cu and Sn. However, voids still occurred due to the accommodation of spherical particles (shape factor) [10]. These voids could significantly reduce the shear strength of the joints and weaken the interconnection. With the use of Sn coated Cu-MWCNT composite powders in the present work, the voids at the joints were reduced when compared to the conventional Cu/Sn foil/Cu TLP [23] and Cu/Sn coated Cu/Cu TLP [10]. This could be due to the flake-like morphology of the Sn-Cu-MWCNT composite. Under the applied pressure, Sn-Cu-MWCNT flakes could rearrange themselves, forming a highly dense compact. However, low diffusion among the particles resulted in the irregular shape pores due to incomplete coalescence, as seen in Figure 3b. With increasing reflow time, the pores shape changed from irregular to spherical, as shown in Figure 3d. This showed the driving force for the pores to attain the shape with minimum interface energy, which was only possible through sufficient diffusion time [10]. The high magnification microstructures in Figure 3b showed the presence of a high fraction of Cu\({}_{3}\)Sn phases (dark grey) along with a few Cu\({}_{6}\)Sn\({}_{5}\) phases (light grey). After eight minutes of bonding time, the microstructure in Figure 3c showed the complete transformation of Cu\({}_{6}\)Sn\({}_{5}\) to Cu\({}_{3}\)Sn. Composition analysis corresponding to the region A of Figure 3d showed a 3:1 atomic ratio of the elements Cu and Sn, thus confirming the formation of Cu\({}_{3}\)Sn. Cu\({}_{3}\)Sn are highly desirable phases in interconnect applications, as they can withstand an operating temperature up to 676 \({}^{\circ}\)C [2]. During the joining process, as the temperature reaches 232 \({}^{\circ}\)C, Sn coated on the Cu-MWCNT melts and reacts with copper to form Cu-Sn IMC. At the interface of Sn and Cu-MWCNT, Cu\({}_{3}\)Sn forms according to Equation (2), whereas in Sn rich regions (surface of Sn coated Cu-MWCNT), Cu diffusion leads to the formation of Cu\({}_{6}\)Sn, according to Equation (1). This results in Cu\({}_{3}\)Sn and Cu\({}_{6}\)Sn\({}_{5}\) mixed phases, as observed in Figure 3a,b. With the increase in reflow time, Cu diffusion proceeds and leads to the transformation of Cu\({}_{6}\)Sn\({}_{5}\) to Cu\({}_{3}\)Sn, as per Equation (3), resulting in the single Cu\({}_{3}\)Sn phase, as seen in Figure 3c,d. The explained mechanism is schematically illustrated in Figure 4. Moreover, it is reported that that diffusion of Cu is faster in Cu\({}_{6}\)Sn\({}_{5}\) than in Sn [24]. In the present work, the Cu\({}_{6}\)Sn\({}_{5}\) IMC formed during Sn electroless coating on Cu-MWCNT [as detected through XRD in Figure 1b] is considered an added advantage for enhancing Cu diffusion and thereby enhancing the complete transformation of Cu\({}_{6}\)Sn\({}_{5}\) to Cu\({}_{3}\)Sn. Figure 4: Schematic illustrating the mechanism of Cu\({}_{3}\)Sn-MWCNT formation during TLP bonding. Figure 5a shows the shear strength of copper to copper joints bonded with Sn coated Cu-MWCNT powders reflowed at 260 \({}^{\circ}\)C as a function of bonding time. Initially, for 3 min of bonding time, the shear strength was 29.4 MPa.