IR Drop Analysis

Overview

IR drop is the voltage loss across the power grid due to resistance. Static IR drop is the steady-state loss, while dynamic IR drop occurs during switching bursts.

Severe IR drop can slow cells or cause functional failure.

Causes

Insufficient strap width or via density in the PDN.

High local switching activity without nearby decap support.

Long power paths from pads to active regions.

Mitigation

Widen straps, add straps, or use higher metal layers.

Insert decap cells near high-activity blocks.

Balance power distribution across the die.

IR Drop Analysis in the Physical Design Flow

IR Drop Analysis is not an isolated step in backend implementation. IR drop directly impacts timing, noise margin, and silicon reliability, so it must be analyzed as both a power-grid design problem and a workload-dependent verification problem. In a practical ASIC flow, engineers revisit this topic at least twice: once to prevent problems early, and again after optimization when the design context changes because of timing fixes, buffering, or routing decisions.

When using this page for learning or interview preparation, separate the topic into inputs, tool actions, and outputs. Inputs define what data must be clean before you start. Tool actions describe what the engine is optimizing. Outputs show whether the run is actually improving design quality. The most useful reviews combine those three views instead of memorizing a short definition.

Practical Checklist

Use this quick checklist while studying or debugging ir drop analysis. It helps turn theory into repeatable engineering practice and also improves project documentation quality.

Separate static IR checks from dynamic IR scenarios driven by switching activity
Correlate hotspot locations with power straps, vias, macros, and current demand
Review voltage drop on critical timing regions, not only the global maximum
Strengthen the grid using straps, vias, or decaps based on observed bottlenecks
Re-run extraction and analysis after major routing or ECO changes

Track the result of each change with measurable data instead of intuition alone. Measure worst voltage drop, spatial hotspot distribution, current density concentration, and timing sensitivity of affected regions. Keeping a small log of assumptions, changes, and outcomes will make this topic easier to revise later and easier to explain in interviews or design reviews.