The solderless bonding of GaN devices to diamond as heat spreaders and graphite composites as heat sinks via Ar-irradiation-assisted thermo-compression bonding has been examined. To firmly assemble these dissimilar sub-strates, the target surfaces were sputter coated with gold (Au). The latter is an essential process to obtain activated surfaces that generate a bonded layer strong enough between the substrates, and the Au films can compensate for their original surface roughness. The material roughness – a key factor in achieving well-adhered surfaces – was measured by Scanning Probe Microscope. Thus, thanks to a short time surface activation (10 min), the substrates can be bonded at temperatures ~280 ℃ and low-load pressure (~9 MPa) conditions. The bonding results were evaluated by Scanning Acoustic Tomography (SAT).