Experimental results of ultralightweight compact heat sinks with open-celled copper (Cu) foams under the impingement of axial fan flows are presented. The thermal resistance of the system and the pressure coefficient on heat sink base plate were measured, focusing on the influences of foam height (H-f) and impinging distance between fan exit and base plate (H). With the impinging distance fixed at H/D = 0.5 (D being the fan diameter), it is demonstrated that an optimum foam height exists at H-f/D = 0.22, providing the lowest thermal resistance. Furthermore, with the foam height fixed at H-f/D = 0.22, reducing the impinging distance to the foam height level reduces the thermal resistance further. In comparison with conventional aluminum (Al) plate-fin heat sinks under identical flow conditions, the Cu foam heat sinks require only 30% of the weight and 50% of the volume to achieve a similar level of heat dissipation performance.