A package on package structure providing mechanical strength and warpage control includes a first package component coupled to a second package component by a first set of conductive elements. A first polymer-comprising material is arranged between the first package component and the second package component. The first polymer-comprising material surrounds the first set of conductive elements and the second package component. A third package component is coupled to the second package component by a second set of conductive elements. An underfill is arranged on the second package component and surrounds the second set of conductive elements. The first polymer-comprising material extends past sidewalls of the underfill.