The embodiments of the present disclosure have some advantageous features. By designing centripetal elongated openings, the solder regions in the centripetal elongated openings can endure higher stresses without failure than the solder regions in non-elongated openings. The locations of the centripetal elongated openings are selected according to the stresses suffered by the solder regions. Simulation results indicated that when the corner openings of the bottom package 100 are centripetal elongated openings, the respective package fails after 769 thermal cycles in the reliability test. In comparison, when the corner openings of the bottom package 100 are non-elongated openings, the respective package fails after 604 thermal cycles. When the corner openings of the bottom package 100 are elongated openings with the widthwise directions extending toward the neutral-stress point, the respective bottom package fails after 574 thermal cycles. These results indicate that a package with centripetal elongated openings have improved reliability and can endure more thermal circles before they fail.

In accordance with some embodiments of the present disclosure, a package includes a corner, a device die having a front side and a backside, and a molding material molding the device die therein. A plurality of redistribution lines is on the backside of the device die. The plurality of redistribution lines includes a plurality of metal pads. A polymer layer contacts the plurality of metal pads. A plurality of openings is disposed in the polymer layer, with the plurality of metal pads aligned to and exposed to the plurality of openings. The plurality of openings includes a corner opening that is elongated and an additional opening farther away from the corner than the corner opening. The additional opening is non-elongated.