A cooling block assembly for cooling a heat-generating electronic component , comprising: an upper block portion defining at least one internal fluid conduit for circulating a cooling fluid therethrough, each of the at least one internal fluid conduit having an inlet and an outlet for receiving and discharging the cooling fluid respectively, the upper block portion comprising a heat spreading base having a base body , a lower surface of the base body being configured to face toward the heat-generating electronic component ; a boss connected to a main portion of the base body , the boss being disposed on a lower side of the base body and being spaced from the lower surface of the base body in a height direction of the cooling block assembly , wherein notches are defined between the lower surface of the base body and the boss , the boss having a thermal transfer surface configured to be in thermal contact with the heat-generating electronic component , the thermal transfer surface being offset from the lower surface of the base body in the height direction of the cooling block assembly , the boss being dimensioned such that a periphery of the thermal transfer surface is smaller than a periphery of the base body , the periphery of the thermal transfer surface being contained within the periphery of the base body in a projection thereof on a plane parallel to the thermal transfer surface ; and a plurality of heat distributing devices configured for distributing heat through a phase change of a working substance contained within each heat distributing device , each heat distributing device being disposed partially in a corresponding notch between the boss and the lower surface of the base body to distribute heat from the boss to the lower surface of the base body . an upper block portion defining at least one internal fluid conduit for circulating a cooling fluid therethrough, each of the at least one internal fluid conduit having an inlet and an outlet for receiving and discharging the cooling fluid respectively, the upper block portion comprising a heat spreading base having a base body , a lower surface of the base body being configured to face toward the heat-generating electronic component ; a boss connected to a main portion of the base body , the boss being disposed on a lower side of the base body and being spaced from the lower surface of the base body in a height direction of the cooling block assembly , wherein notches are defined between the lower surface of the base body and the boss , the boss having a thermal transfer surface configured to be in thermal contact with the heat-generating electronic component , the thermal transfer surface being offset from the lower surface of the base body in the height direction of the cooling block assembly , the boss being dimensioned such that a periphery of the thermal transfer surface is smaller than a periphery of the base body , the periphery of the thermal transfer surface being contained within the periphery of the base body in a projection thereof on a plane parallel to the thermal transfer surface ; and a plurality of heat distributing devices configured for distributing heat through a phase change of a working substance contained within each heat distributing device , each heat distributing device being disposed partially in a corresponding notch between the boss and the lower surface of the base body to distribute heat from the boss to the lower surface of the base body . <EOS>