Interface Phonon Transmission Behaviors and Their Resultant Influence on Thermal Transport in Cu/Diamond Composites

ABSTRACT In high‐power electronic packaging, Cu/diamond composites are attractive heat‐spreader candidates owing to diamond's ultrahigh thermal conductivity. However, their practical thermal performance is often limited by the intrinsically low thermal boundary conductance (TBC) at Cu/diamond interfaces, which can be improved by introducing a manufacturable carbide interlayer. To elucidate the phonon‐mediated interfacial thermal transport mechanisms affected by interlayer structures, the present work performs a systematic analysis based on density functional theory (DFT) and the diffuse mismatch model (DMM). The results reveal that TBC is jointly determined by phonon group velocity, PDOS overlap, transmission probability, and the effective participating frequency range, providing qualitative criteria for selecting carbide interlayers. The analysis further indicates that neglecting optical–phonon contributions and restricting phonon branch conversion can substantially underestimate TBC. The interfacial phonon transport is highly sensitive to interlayer thickness and composition: When thickness control is challenging, high‐thermal–conductivity carbides (B 4 C and WC) better retain interfacial thermal conductance (ITC) at large thickness. Residual B or Cr can provide an additional phonon‐coupling pathway and further improve ITC. This work deepens the understanding of phonon‐mediated interfacial thermal transport and provides mechanistic guidance for interlayer design in Cu/diamond heat spreaders.