Patent ID: 8253033

Claim:
A circuit board comprising: a lower sub-board including a first base layer having an upper surface, and a first wiring pattern provided on the upper surface of the first base layer; an upper sub-board including a second base layer having a lower surface, and a second wiring pattern provided on the lower surface of the second base layer; a connection layer including an insulating layer having a lower surface and an upper surface, the lower surface of connection layer being joined to the upper surface of the first base layer, the upper surface of connection layer being situated on the lower surface of the second base layer, and a via-conductor passing through the insulating layer, the via-conductor being connected to the first wiring pattern and the second wiring pattern, wherein the upper sub-board and the connection layer expose a portion of the upper surface of the lower sub-board, the insulating layer includes thermosetting resin, inorganic filler dispersed in the thermosetting resin, and elastomer dispersed in the thermosetting resin, and a fillet layer covering an outer periphery of an edge surface of the connection layer at the exposed portion of the upper surface of the lower sub-board, said fillet layer continuously extending from a top surface of said insulating layer to a bottom surface of said insulating layer, wherein the fillet layer contains the thermosetting resin of the insulating layer and does not contain the inorganic filler.