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It is not necessary that the power pads & the ground pads are same on a chip.....
After the power is laid down after floorplan, we run IR drop analysis on the power as well as ground and a check is made on
1) How much a VDD is possible to fall at the power pins of a hard macro/ memory
2) How much a bump is possible upto the VSS pins of memory/macro
3) there is limit on both bump in VSS and fall in VDD. The output of IR drop tool is a IR drop map and based on which side the drop/bump is more, we add VDD/VSS pads
to compensate the drop.
Thus the no. of pads can definitely vary. It all depend on the way you build the power/ground mesh...