A method of forming a package on package (PoP) structure includes forming a first die package, and bonding an external connector of a second die package to a solder paste layer of the first die package. The forming the first die package includes forming a contact pad over a substrate, attaching a metal ball with a convex surface to the contact pad, and applying a solder paste layer over a distal end of the metal ball and leaving at least a portion of the metal ball without solder paste. The forming the first die package also includes attaching a semiconductor die to the substrate, and forming a molding compound between the semiconductor die and the metal ball, where the solder paste layer has a first portion extending above an upper surface of the molding compound and a second portion extending below the upper surface of the molding compound.