Stacked semiconductor dies are provided with selective capillary under fill to avoid wafer warpage during curing. In one embodiment, a method of manufacturing a semiconductor device includes forming at least three stacks of semiconductor dies over a substrate, the stacks spaced apart from one another by gaps. A first sealing material such as a capillary under fill material is deposited into a first subset of the gaps. A second sealing material such as a mold resin is deposited into a second subset of the gaps. The first and second sealing materials are cured, and the die stacks are then singulated.