A wiring board assembly includes a rectangular plate-shaped wiring board having a plurality of resin insulation layers and conduction layers alternately laminated together to define opposite first and second main surfaces and a plurality of connection terminals arranged on the first main surface for surface contact with terminals of a chip and a rectangular frame-shaped reinforcing member fixed to the first main surface of the wiring board with the connection terminals exposed through an opening of the reinforcing member. The reinforcing member has a plurality of structural pieces separated by slits extending from an inner circumferential surface to an outer circumferential surface of the reinforcing member.