A method includes placing metal-core solder balls on conductive pads of a first semiconductor device, wherein the metal-core solder balls include a metal core surrounded by a solder material, and forming a device structure, forming the device structure including placing the first semiconductor device on a carrier substrate, encapsulating the first semiconductor device with an encapsulant, wherein the encapsulant covers the metal-core solder balls, performing a planarization process on the encapsulant, wherein the planarization process exposes the metal-core solder balls, and forming a redistribution structure over the encapsulant and the first semiconductor device, wherein the redistribution structure is electrically connected to the metal-core solder balls.