A method includes bonding a first package component over a second package component, dispensing a first underfill between the first package component and the second package component, and bonding a third package component over the second package component. A second underfill is between the third package component and the second package component. The first underfill and the second underfill are different types of underfills.

A semiconductor package comprises: a package substrate having a first region, a second region and an opening between the first and second regions; a first semiconductor die mounted on the first region via a first set of solder bumps and having a first overhanging portion above the opening; a second semiconductor die mounted on the second region via a second set of solder bumps and having a second overhanging portion above the opening; a bridge semiconductor die mounted onto respective back surfaces of the first overhanging portion and the second overhanding portion via a third set of solder bumps, wherein the bridge semiconductor die is aligned with the opening; and an underfill layer filled between both of the first and second semiconductor dice and both of the package substrate and the bridge semiconductor die.