An interposer may comprise a metal layer above a substrate. A dam or a plurality of dams may be formed above the metal layer. A dam surrounds an area of a size larger than a size of a die which may be connected to a contact pad above the metal layer within the area. A dam may comprise a conductive material, or a non-conductive material, or both. An underfill may be formed under the die, above the metal layer, and contained within the area surrounded by the dam, so that no underfill may overflow outside the area surrounded by the dam. Additional package may be placed above the die connected to the interposer to form a package-on-package structure.