A method for manufacturing a semiconductor package includes: (a) providing a substrate structure, wherein the substrate structure includes a chip attach area, a bottom area opposite to the chip attach area, a lower side rail surrounding the bottom area, a first lower structure and a second lower structure, wherein the first lower structure is disposed in a first lower region of the lower side rail, and a second lower occupancy ratio is greater than a first lower occupancy ratio; (b) attaching at least one semiconductor chip to the chip attach area; and (c) forming an encapsulant to cover the at least one semiconductor chip.