A method includes dispensing an underfill between a first package component and a second package component, wherein the first package component is placed on a lower jig, and the second package component is over and bonded to the first package component. A through-opening is in the lower jig and under the first package component. The underfill is cured, wherein during the step of curing the underfill, a force is applied to flatten the first package component. The force is applied by performing an action selected from the group consisting of vacuuming and air blowing through the through-opening.