A heat sink base (10) includes a rectangular body (11) made of aluminum and a circular core (12) made of copper. The body defines a circular through opening (111). A diameter of the opening is slightly less than a diameter of the core. The core is attached within the body according to the following steps: a) pressing the core into the through opening of the body; b) stamping the core to cause it to plastically deform in radial directions and thereby become firmly combined with the body; and c) removing any burring of the core flowing out from the opening such that surfaces of the core and the body are coplanarly smooth.