Why can a path become newly critical only after routing, when it looked fine before?
From PDVerse MMMC Interview Masterclass · pdVerse Mentor Series
Direct answer
Because pre-route timing uses estimated interconnect, and actual routing can introduce longer detours, extra vias, different layer assignments, more coupling exposure, and different real loading — all of which can push net delay and slew well past what was estimated.
Mentor explanation
Before routing, the tool is guessing at wire behavior using statistical or wireload-based estimates. Those estimates are useful for early decisions, but they're not physical truth. Once the router actually places metal, a net might take a longer path than expected to avoid congestion, switch layers (changing its resistance per unit length), pick up more neighboring aggressors, or simply end up loading a receiver differently than assumed. Any of these can degrade the net's delay and the receiver's input slew in ways the pre-route estimate never captured — which is exactly why post-route, SPEF-based signoff timing is treated as closer to the truth than pre-route numbers.
Key takeaways
- Because pre-route timing uses estimated interconnect, and actual routing can introduce longer detours, extra vias, different layer assignments, more coupling exposure, and different real loading — all of which can push net delay and slew well past what was estimated.
Self-check: can you answer this aloud?
Try a 45-second answer using this structure:
- State the direct answer.
- Explain the timing or physical reason.
- Name one caveat.
- Say how you would verify it in a real flow.
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