METHOD FOR FORMING VIAHOLE OF MULTILAYER CIRCUIT BOARD BY MACHINING

PURPOSE:To improve the reliability of connection of a plated layer in a viahole formed in a multilayer circuit board by machining with an inner layer circuit. CONSTITUTION:A drill bit 4 is fed from above of the surface of a multilayer circuit board 1 to a position immediately before an inner layer circuit 2 to drill a guide hole 5, and then the bottom of the hole 5 is so flatly ground by a grindstone 6 formed at its end face in a flat surface as to arrive at the layer 2 to open an unpenetrating viahole 3 from the surface to the layer 2. The layer 2 can be exposed in a wide area in the bottom of the viahole 3 formed flatly by the grinding of the grindstone 6, and the contact area of a plated layer 7 provided in the viahole 3 with the circuit 2 can be increased.