10_Routing.md

Routing — This Is Not a Solver

Purpose of This Chapter

Routing is where many people expect miracles.

They should not.

Routing does not:

• fix timing
• fix placement
• fix CTS
• fix bad design decisions

Routing only answers one question:

Can this design be physically connected without violating rules?

If the answer is “no”, the design was already broken earlier.

What Routing Actually Does

Routing takes:

• placed cells
• a clock tree
• legal metal layers
• spacing and width rules

and attempts to:

• connect all nets
• obey DRC rules
• minimize congestion

That is all.

Routing Phases in OpenROAD

1. Global Routing

• estimates routing paths
• builds congestion maps
• does not draw exact wires

This phase answers:

“Is routing even possible?”

2. Detailed Routing

• creates real wires
• inserts vias
• enforces all DRC rules

This phase answers:

“Can it be routed legally?”

The Single Most Important Rule

If global routing looks bad, stop.

Do not proceed to detailed routing.

Detailed routing will not fix congestion. It will only fail later and harder.

Reading Global Routing Results

You must look at:

• congestion heat maps
• overflow reports
• hot spots

Red flags:

• severe congestion near clock spines
• long narrow channels
• dense standard-cell clusters

These indicate placement or floorplan failure.

Common Routing Failure Patterns

Pattern 1: Overflow everywhere

Cause:

• too small core area
• too aggressive utilization
• poor pin placement

Correct action:

• rollback
• fix floorplan

Pattern 2: Routing completes but DRC explodes

Cause:

• metal spacing violations
• via density problems
• antenna issues

Correct action:

• analyze rule type
• fix root cause (not router settings)

Pattern 3: Routing completes but timing collapses

Cause:

• routing added delay to already-bad paths

Correct action:

• timing should have been fixed before routing

Antenna Violations (Do Not Ignore)

Antenna violations are:

• not cosmetic
• not optional
• not safe to waive blindly

They indicate:

• real manufacturing risks

Fix them structurally or with proper diode insertion.

Routing vs Timing Reality

Routing always increases delay.

If timing was barely passing before routing: it will fail after routing.

This is normal and expected.

What NOT to Do During Routing

• Do not increase router effort blindly
• Do not ignore congestion warnings
• Do not treat DRC as optional
• Do not keep routing “until it passes”

Routing failure means earlier failure.

Healthy Routing Indicators

A healthy routing stage shows:

• low congestion
• limited DRC violations
• predictable timing degradation
• stable memory usage

If routing feels “fragile”: it is.

Output Artifacts After Routing

You should get:

• routed DEF
• clean or manageable DRC
• SPEF for STA
• GDS-ready layout

If any of these are missing: rollback.

Why This Chapter Exists

Routing does not reward optimism.

It punishes earlier mistakes.

Treat routing as a validation step, not a repair tool.

Next Chapter

If routing is clean:

• 11_STA.md — Timing Is a Contract, Not a Guess