Disclosed is a semiconductor plastic package having a structure in which a metal sheet having a size nearly equivalent to a printed wiring board is disposed nearly in the central portion in the thickness direction of the printed wiring board. The metal sheet and a signal propagation conductive circuit on a front surface of the printed wiring board are insulated from each other with a thermosetting resin composition. The conductive circuit on the printed wiring board surface is connected to a conductive circuit formed on an opposite surface of the printed wiring board, or to a conductive circuit pad formed for being connected with solder balls with a conductive through hole. A semiconductor chip, wire and bonding pad are encapsulated with a resin. The semiconductor plastic package has at least one blind via hole made in the opposite surface so as to be directly connected to the metal sheet. The blind via hole has an inner wall rendered thermally conductive. The printed wiring board is provided, on the semiconductor-chip-mounting side, with an elevated metal portion, a plurality of metal protrusions each having the form of a frustum of a pyramid or a cone, or at least one via hole having an inner wall rendered thermally conductive. A process for the production of a printed wiring board used for the above plastic package is also disclosed.