Mounting substrate

A mounting substrate for LSIs having a multi-layered thin film wiring portion formed on a silicon or ceramic substrate, wherein said thin film wiring portion includes a vertically stacked arrangement of plural, preformed wiring units, each wiring unit having first and second, opposing main surfaces and including wirings composed of a plurality of wiring layers provided as a multilevel arrangement of conductive layers, each wiring layer within a wiring unit being isolated by an insulator from an adjacent, higher level wiring layer except for portions thereof extending vertically via through holes of said insulator and provided for interconnecting said wiring layer and said adjacent, higher level wiring layer thereof, said wirings between respective wiring units being electrically connected through connecting pads formed at the main surfaces thereof, the pads at a main surface of said wiring unit being provided from a same level layer of said multilevel arrangement as that of a corresponding surface conductive layer of said wiring unit, each said wiring unit except for an uppermost wiring unit of said vertically stacked arrangement including two, internally provided signal line layers, a power supply layer and a ground layer, said power supply layer and said ground layer are formed at said opposing, main surfaces thereof, respectively, and which surfaces correspond to substantially flat, upper and lower surfaces of the insulator associated with said wiring unit, and interconnections of the respective layers of said wiring unit are provided via through holes in said insulator.