A heat dissipating system utilizes a thin layer of a high conductivity liquid metal to provide cooling for electronic sub-assemblies, such as semiconductor chips. The liquid metal preferably is gallium or its alloy which transfers heat from the chip to a heat sink or heat spreader. The system includes one or more vents that allow for filling and venting of the space occupied by the liquid. It also utilizes a flexible seal, such as an O-ring or a membrane held in place with a retainer ring, or a plug that seals the filling vent. The seal flexes to accommodate expansion and contraction of the liquid, as well as phase changes from the liquid phase to the solid phase.