A substrate includes a substrate body and an interconnection layer disposed on a bearing surface of the substrate body and having an annular portion and a plurality of protrusions extending outward from an outer periphery of the annular portion. A package module is formed by the substrate, a chip mounted on the bearing surface of the substrate body, and a cap enclosing the chip and having a bottom thereof adhered to the interconnection layer of the substrate by an adhesive. By means of the protrusions of the interconnection layer, the bonding area of the adhesive is increased and the spread of the adhesive is effectively concentrated.