A composite substrate made of a circuit board mounted on a lead frame is used for an electronic system package. High heat generated electronic components are adapted to mount on the lead frame and relatively low heat generated electronic components are adapted to mount on the circuit board. Metal lines are used for electrical coupling between the circuitry of the IC chip and the circuit board. An electronic system with the composite substrate gains both advantages—good circuitry arrangement capability from the circuit board and good heat distribution from the lead frame.