This study evaluates the reliability of die interconnect layers in a stacked die system fabricated using two different sintering agents: a microparticle-based silver paste and preform-based copper nanowires, intended for high-power packaging applications. Single-step sintering is performed to attach both dies at the same time, offering a faster and efficient assembly for improving scalability in manufacturing. The copper nanowires are sintered using KlettSintering method at 230° C for 5 minutes under a pressure of 20 MPa, while the silver paste was sintered pressure-free at 280 °C for 45 minutes in nitrogen. The assembled units were characterized using shear strength and microstructural analysis. Both sintering methods showed high porosity in the top die-attach layer compared to the bottom die-attach layer, which is reflected in lower shear values for top die (35 MPa) compared to bottom die (45 MPa) in the silver sintered unit. The long-term reliability of the die-stack systems was assessed through a 500 -hour high-temperature storage test and a 500-cycle temperature cycling test, revealing significant impacts of thermo-mechanical stresses on die attach layers of both sintered units. The KlettSintered system maintained consistent performance throughout the reliability tests but exhibited coarsening and oxidation during temperature cycling. Furthermore, the study identifies areas for potential improvement, particularly in improving multi-die sintering performance in a single-step process, which is crucial for ensuring durability in high-power applications.