In accordance with some embodiments, a package structure and a method for forming a package structure are provided. The package structure includes a semiconductor die and a molding compound partially or completely encapsulating the semiconductor die. The package structure also includes a through package via in the molding compound. The package structure further includes an interfacial layer between the through package via and the molding compound. The interfacial layer includes an insulating material and is in direct contact with the molding compound.