Congestion Analysis
Predict routing risk before it becomes a blocker.
Overview
Congestion analysis estimates where routing demand exceeds available tracks. It is most valuable early, during floorplanning and placement.
Hotspots often appear near macros, narrow channels, or high pin-density blocks.
What to Look For
High demand-to-supply ratios on horizontal and vertical layers.
Pins clustered in a small area with limited routing tracks.
Long detours that inflate RC delay and timing risk.
Mitigation
Add whitespace or widen channels between macros.
Spread high-fanout logic or insert buffers to reduce density.
Reserve routing resources for clocks and critical nets.
Related Topics
Congestion Analysis in the Physical Design Flow
Congestion Analysis is not an isolated step in backend implementation. It is one of the earliest indicators that a placement or macro arrangement may fail later during detailed routing, timing closure, or signal integrity cleanup. 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 congestion analysis. It helps turn theory into repeatable engineering practice and also improves project documentation quality.
- Review congestion maps by layer and direction, not only a single global score
- Correlate hotspots with macro channels, pin density, and high-fanout net regions
- Check whether timing fixes are increasing local density or routing demand
- Use spacing, blockages, or placement spreading only after identifying the root cause
- Re-run analysis after each floorplan or placement change and compare reports
Track the result of each change with measurable data instead of intuition alone. Measure demand-to-supply ratios, overflow hotspots, detour length, and post-route timing impact before and after mitigation. 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.