A semiconductor package comprises a first semiconductor chip, a second semiconductor chip on the first semiconductor chip, a third semiconductor chip on the second semiconductor chip and a fourth semiconductor chip on the third semiconductor chip. A first underfill layer is positioned between the second semiconductor chip and the first semiconductor chip; a second underfill layer is positioned between the third semiconductor chip and the second semiconductor chip, and a third underfill layer is positioned between the fourth semiconductor chip and the third semiconductor chip. In some embodiments, the second underfill layer comprises a material that is different than the first and third underfill layers.