FIG. 1 illustrates a cross-sectional view of package 400 in accordance with some exemplary embodiments of the present disclosure. Package 400 includes bottom package 100 and top package 200 over and bonded to bottom package 100. Both bottom package 100 and top package 200 are pre-formed, and are then bonded to each other to form a package on package structure. In accordance with some embodiments of the present disclosure, bottom package 100 includes device dies 102 (including 102A and 102B), with the front sides of device dies 102 facing down and bonded to Redistribution Lines (RDLs) 112. In alternative embodiments, bottom package 100 includes a single device die or more than two device dies. Device dies 102 may include semiconductor substrates 108, and integrated circuit devices (such as active devices, which include transistors, for example) 104 at the front surface (the surface facing down) of semiconductor substrates 108. Device dies 102 may include logic dies such as Central Processing Unit (CPU) dies, Graphic Processing Unit (GPU) dies, mobile application dies, or the like.