A device includes a package component having conductive features on a top surface, and a polymer region molded over the top surface of the first package component. A plurality of openings extends from a top surface of the polymer region into the polymer region, wherein each of the conductive features is exposed through one of the plurality of openings. The plurality of openings includes a first opening having a first horizontal size, and a second opening having a second horizontal size different from the first horizontal size.