Abstract: A method for providing a high in-plane and through-plane thermal conductivity path between a heat producing electronic device and a heat sink is described. A vapor chamber, or other type of heat spreader, is provided that has a substantially flat top copper surface and a substantially flat bottom copper surface. A ceramic submount is also provided, where the submount has a top copper metallization layer patterned for connection to electrodes of a heat-producing die, and where the submount has a bottom copper metallization layer. Prior to a working fluid being introduced into the vapor chamber, and prior to a die being mounted on the submount, the top copper surface of the vapor chamber is diffusion bonded to the bottom copper metallization layer of the ceramic submount under heat and pressure. The working fluid is then introduced into the vapor chamber, and the chamber is sealed. Dies are then mounted on the submount. The bottom of the vapor chamber is then affixed over a heat sink.