A package board module wherein a semiconductor chip such as an LSI is mounted on the topside surface of a package board, and a package mounted module wherein the package board is mounted on the motherboard of a large-sized computer or the like. A stiffener for supporting the package board and/or a stiffener for supporting the motherboard each has a bimetal structure wherein a first member and a second member having mutually different thermal expansion coefficients are respectively adhered to each other, so as to cause the stiffeners to warp in harmony with the warpage of the package board and the motherboard caused by a temperature change, thereby preventing stress from arising in the solder-bonded portions.