A method for making an apparatus, a system, and apparatus, the apparatus and system each comprising a substrate with a top side, wherein the substrate has a set of cavities in the top side of the substrate, wherein the substrate has a set of conductive elements on the top side of the substrate arranged to electrically couple with a set of conductive elements of a photonic integrated circuit (PIC), wherein each cavity of the set of cavities when coupled to the PIC creates a surface tension when exposed to an underfill to cause the underfill to flow around each cavity.