A package structure includes an interposer, a die over and bonded to the interposer, and a Printed Circuit Board (PCB) underlying and bonded to the interposer. The interposer is free from transistors therein (add transistor), and includes a semiconductor substrate, an interconnect structure over the semiconductor substrate, through-vias in the silicon substrate, and redistribution lines on a backside of the silicon substrate. The interconnect structure and the redistribution lines are electrically coupled through the through-vias.