Fixing a PDN topology defect after final routing can take 2 to 3 weeks of signal-net re-routing in a dense design; finding the same defect at placement can take about 6 hours. Late strap changes displace signal nets, trigger DRC cleanup, and create ECO cycles. Early edits preserve routing options.
Most teams still discover these issues at signoff because that was once the only practical point for accurate analysis. SPEF arrived after routing, so power checks waited with it. Current vectorless tools close much of that timing gap and can screen IR drop during placement.
Why Signoff Became the Default Checkpoint
Historically, signoff-only analysis was rational. Accurate resistance and capacitance data depended on the routed design, captured in SPEF. Before that file existed, teams could not model the completed interconnect with enough confidence to make detailed power decisions.
The usual workaround was extra PDN margin during floorplanning. That worked reasonably well on mature nodes with established tape-out history and familiar power distributions. It became less dependable at new nodes or in designs where compute density created local current demand far above the floorplan average.
Placement-stage vectorless analysis uses cell density, estimated activity, and the planned PDN topology. Its numbers can differ materially from final results, but it reliably exposes severe concentration and topology problems. That makes it useful for triage before routing choices become expensive.
A Three-Checkpoint Power Analysis Method
Use three checkpoints: placement, global routing, and post-route, each answering a different power question.
At placement, the goal is topology validation, not a perfect prediction. An acceptable result is no region above twice the target threshold, rather than zero provisional violations. At this stage we can move macros, alter the grid, or reserve additional straps.
Global routing adds estimated wire resistance and gives us a sharper view of current paths. Strap widths and via arrays can still be changed here, before detailed signal routing makes those edits disruptive. This checkpoint narrows uncertainty while useful physical options remain.
Post-route analysis uses the complete SPEF for signoff. We check for VDD violations and electromigration violations. When the first two checkpoints were used properly, this run should confirm the design rather than reveal a topology defect for the first time.
The Via Electromigration Check Teams Miss
Standard post-route electromigration scripts usually inspect wire segments. Via electromigration is a separate concern, and it is easy to omit even when wire checks are enabled.
PDN strap current often passes through via arrays. Excess current does not necessarily create an immediate open. Instead, the stressed vias can accumulate damage and raise resistance gradually, producing an in-field power integrity failure.
A via EM check requires the full post-route SPEF and the foundry PDK current limits. Many default EM configurations omit it, so we add the rule explicitly. The setup is usually one-time work, and the correct execution point is after routing but before GDSII generation.
28nm Example: 6 Hours, Not 3 Weeks
In one synthetic 28nm ASIC scenario for edge inference, placement analysis found a high-density compute region at an estimated 80mV IR drop against a 30mV target. The result was not a subtle modeling disagreement; it marked a topology that needed attention.
We added two horizontal M3 straps and widened the vertical M4 straps by 50%, then edited the floorplan and reran placement. That exposed a macro blockage conflict, which we resolved before routing. The complete loop took 6 hours.
Had signoff found the same condition, the fix would have required re-routing displaced signal nets, correcting DRC violations, and repeating ECO cycles. The team estimated 2 to 3 weeks for that route-stage repair.
Where Early Estimates Help Most
The common objection is accuracy. Early vectorless estimates can be wrong by 2x, and teams reasonably worry about over-engineering the PDN around uncertain numbers.
The accuracy objection is valid for fine-grained intermediate estimates. It does not apply to outliers where halving the estimate still leaves the region beyond the acceptable limit. Those cases justify an early topology change without pretending the placement result is final.
The greatest benefit goes to teams entering new process nodes or designing unusual power-density distributions. Conservative floorplan margins are least dependable in exactly those conditions, making early outlier detection more valuable than waiting for extracted signoff data.