A package on package structure providing mechanical strength and warpage control includes a first package component, a second package component, and a first set of conductive elements coupling the first package component to the second package component. A first polymer-comprising material is molded on the first package component and surrounds the first set of conductive elements. The first polymer-comprising material has an opening therein exposing a top surface of the second package component. A third package component and a second set of conductive elements couples the second package component to the third package component.