Abstract: A method for manufacturing a plastic encapsulated integrated circuit (IC) package has steps for placing a diamond substrate in a lower cavity of an encapsulation mold such that the diamond substrate in the finished package underlies the die attach pad and a portion of the leads in close proximity to each. Pins are provided in lower cavities of molds to support and/or position diamond substrates to lie close to both die attach pads and leads to facilitate efficient heat transfer from an operating IC, through the die attach pad, into and through the diamond substrate, and finally to the leads leading from the encapsulated package. Apparatus is disclosed for positioning and supporting diamond substrates, and combination heat slugs for the purpose are disclosed, having diamond substrates bonded to metal slugs.