The present invention relates to a method of manufacturing a circuit board, more precisely relates to a thin circuit board on which cable patterns can be highly concentrated. 
A conventional method of manufacturing a printed circuit board, in which multilayered cable patterns are formed on both sides of a core substrate by a buildup process, will be explained with reference to FIGS.  8A–9D.
FIGS.  8A–8F show the steps of forming a core section, in which cable patterns are piled on the both surfaces.  In FIG.  8A, copper films 11 are adhered on a core substrate 10.  The core substrate 10 comprises a core member 10a, which is made of epoxy resin including glass cloth, and the copper films 11, which respectively cover an upper surface and a lower surface of the core member 10a.  
In FIG.  8B, through-holes 12 are bored, by a drill, in the core substrate 10.  An inner diameter of each through-hole 12 is about 250 μm.  In FIG.  8C, inner faces of the through-holes 12 are coated with copper layers 14 by plating, so that cable patterns on the upper surface and the lower surface of the core substrate 10 can be electrically connected. 
In FIG.  8D, the through-holes 12 are filled with resin 16 so as to form cable patterns on the upper surface and the lower surface of the core substrate 10.  In FIG.  8E, copper layers 18 are formed on the both surfaces of the core substrate 10 as lid layers.  By forming the lid layers, the whole surfaces of the core substrate 16 including end faces of the resin 16 can be covered with the copper lid layers 18. 
In FIG.  8F, cable patterns 20 are formed on the both surfaces of the core substrate 10, by etching the copper layers 14 and 18 and the copper films 11, so as to form the core section 22.  Note that, in this example, the cable patterns 20 are formed by a subtract process, so concentration of the cable patterns 20 is limited. 
FIGS.  9A–9D show the steps of forming a printed circuit board, in which cable patterns are formed on the both surfaces of the core section 22. 
In FIG.  9A, cable patterns 24 are formed on the both surfaces of the core section 22 by a buildup process.  Symbols 26 stand for insulating layers.  The cable patterns 24 in different layers are electrically connected by vias 28.  In FIG.  9B, the surfaces of the substrate, on which the cable patterns 24 are formed, are coated with photosensitive solder resist 30, then they are exposed and developed, so that prescribed parts of the surfaces of the substrate are coated with the solder resist 30.  In FIG.  9C, surfaces of the cable patterns 24 are coated by electroless nickel plating and electroless gold plating.  Further, the exposed surfaces of the cable patterns are coated with protection layers 32 by plating.  In FIG.  9D, solder bumps 34 are formed at electrodes of the cable patterns 24.  By the above described steps, the printed circuit board 36 is completed. 
These days, thin and compact semiconductor devices are required, so thin circuit boards, on which semiconductor devices will be mounted, having highly concentrated cable patterns are required.  However, the through-holes are bored in the substrate by a drill, so the inner diameter of each through-hole 12 must be about 250 μm.  Namely, it is impossible to bore the through-holes 12 with narrower separations, so that concentration of cable patterns must be limited.  In the conventional printed circuit board having the core substrate, separations between electrodes of a semiconductor chip to be mounted are, for example, 200 μm, but separations between electrodes for connecting with external devices are, for example, 200 μm, therefore separations between cable patterns must be made wider toward the electrodes for connecting with external devices.  Concentration of cable patterns in the printed circuit board is further limited. 
The core substrate 10 of the thin circuit board must be thin.  However, a special manufacturing line, in which thin core substrates can be conveyed and treated, is required.  Thin substrates are apt to be deformed by stresses, which are generated in the steps of forming the insulating layers and the plated layers.  Therefore, it is difficult to control size of the thin circuit board, so that accuracy of the thin circuit board, in which highly concentrated cable patterns will be formed, must be lower.