A package structure and a method of forming the same are provided. A method includes forming first electrical connectors and second electrical connectors on a first side of an interposer wafer. An integrated circuit die is bonded to the first side of the interposer wafer using the first electrical connectors. A stiffener structure is attached to the first side of the interposer wafer adjacent the integrated circuit die. The stiffener structure covers the second electrical connectors in a plan view. The integrated circuit die and the stiffener structure are encapsulated with a first encapsulant. The interposer wafer and the stiffener structure are singulated to form a stacked structure.