A device comprises a bottom package mounted on a printed circuit board, wherein the bottom package comprises a plurality of first bumps formed between the bottom package and the printed circuit board, a first underfill layer formed between the printed circuit board and the bottom package, a semiconductor die mounted on the bottom package and a top package bonded on the bottom package, wherein the top package comprises a plurality of second bumps and the top package and the bottom package form a ladder shaped structure. The device further comprises a second underfill layer formed between the bottom package and the top package, wherein the second underfill layer is formed of a same material as the first underfill layer.