After a long routing, the voltage transmitted along the wire will have IR drop. In order to reduce IR drop effect, the designer often employ a gap block to pick up power via well potential.
But how long the wire routing ,we should insert a gap block?
the guildline is that when the power wire rout about 20~30 um, we should insert a gap block.
For example, UMC 90nm Process 8TSRAM Cell area is 2.3x0.905=2.08 um^2, we insert one gap block per 32 SRAM Cell in vertical direction .
UMC 0.18um Proces 8TSRAM Cell area is 3.04*3.36=10.2 um^2 , we insert one gap block per 8 SRAM Cell in vertical direction.