The invention relates to leadframe semiconductor packages mounted on a heat-sink and fabrication thereof. A system in package (SiP) comprises a leadframe having extension leads, configured with divisional heat sinks serving as power and ground nets. A set of semiconductor dies is attached by adhesive on the central region of the lead frame. Pluralities of wire bonds electrically connect the set of semiconductor dies to the leadframe and to the divisional heat sinks respectively. An encapsulation encloses the leadframe, but leaves the extension leads and the divisional heat sink uncovered, exposing a heat dissipating surface.