Chapter 4 Power Planning, IR Drop & EM

Metal Layers for Rings and Stripes Rings and stripes are built on the top (thick) metal layers because those layers have low resistivity, which minimises IR drop in the power network. KEY Top metals carry rings/stripes - low resistivity means less IR drop.

Electromigration Electromigration (EM) is the gradual movement of metal atoms caused by momentum transfer from conducting electrons. Sustained current in a wire eventually produces opens or shorts. EM cannot be eliminated, only mitigated - the main method is to use wider wires so the conductor stays wide enough to carry current reliably over the IC's lifetime. KEY EM = metal atoms drift under high current, mitigate it with wider wires.

Importance of IR-Drop Analysis IR drop sets the actual voltage seen at the pins of standard cells. The acceptable IR-drop budget is fixed at project start and feeds into the derate value. If IR drop exceeds the budget, the derate must be updated; without that update, timing analysis becomes optimistic - for example, the reported setup slack is better than reality. KEY IR drop sets real cell voltage; exceeding the budget makes timing optimistic unless derate is updated.

Static vs Dynamic IR Drop Static IR drop is the voltage drop when a constant current flows through the power network's resistance, with the circuit in steady state. Dynamic IR drop is the drop when a high transient current is drawn