A two-phase liquid cooling system for an electronic component comprised of flexible sealed bag which is partially filled with a liquid coolant. Sufficient residual non-condensing gas is maintained in the bag so that some of the gas dissolves in the liquid coolant when the device is not operating and at ambient temperature. During warmup, the residual gas comes out of solution and creates nucleation sites that assist in initiating boiling. The bag is air and fluid-impermeable, and has sufficient flexibility such that as coolant vaporizes, the bag expands to maintain the internal bag pressure substantially the same as the ambient environmental pressure. The bag may also be provided with a metal heat spreader plate which passes through a wall of the bag an assists with transferring heat from the component to the coolant. The heat spreader plate may be specially treated to allow the flexible bag material to by directly heat sealed to the plate and to provide nucleation sites to enhance coolant boiling.