Alumina substrates have been developed for a flip-chip mount ECU ( Electronic Control Unit ). On this substrate, large sized IC chips as a flip-chip and other electronic components have been mounted. Cu thick film circuit has also been mounted on the other surface. This circuit consists of printed resistors by connecting tungsten metallized layer and Cu thick film through Cu plating. In order to mount the large sized flip-chip IC, the substrate needs strictly precise dimensions. The tolerance of the dimensions are within ± 0.1 % and the warp of the flip-chip area is less than 10 μm per 10 mm.