Thermal dissipation is a major concern in microelectronics, especially for compact packages and 3D circuits where the dense stacking of thin silicon layers leads to a significant increase of heat densities. Direct hybrid bonding is considered as one of the most promising technologies for future 3D-ICs. Its face-to-face structure allows significant inter-connexion capabilities but it also implies increased thermal densities that will be reflected in both tiers due to the lack of insulating barriers. A specific test vehicle for 3D hybrid bonding including heaters and temperature sensors on each tiers has been fabricated and characterized. Several packaging configurations including different silicon thicknesses, substrate thermal design or the integration of a patterned graphite heat spreader have been tested. The best results were obtained with the integration of the graphite heat spreader which led to a reduction in thermal resistance by 11%. These experimental results have been retro-simulated to establish a thermal model. This model was then used to analyse the heat path and explore the thermal impact of the different packaging parameters.