A thermosiphon cooling assembly includes a housing having a lower portion and an upper portion. A refrigerant is disposed in the lower portion for liquid-to-vapor transformation. A boiler plate defines an outer bottom wall of the housing for transferring heat from the electronic device to the refrigerant. This transfer of heat to the refrigerant leads to a liquid-to-vapor transformation and a pressure build up in the housing. The boiler plate is flexible for reacting with the electronic device in response to the pressure build up in the housing. The boiler plate includes an inflexible first area for overlying the electronic device and a flexible second area which surrounds the first area of the boiler plate. The second area has a thickness t2 less than the thickness t, of the first area thus allowing the boiler plate to flex in the second area.