A block pin is legal but off-track, stacked, or on an unusable layer. What is the response?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Passing legality checks proves the pin satisfies placement/geometry rules — it says nothing about whether a router can actually get a wire into it, and those are genuinely different questions. Treat this explicitly as a pin-access defect, not a legality problem, and start by reporting the pin's actual geometry alongside the nearby track grid and available routing resources on that layer.
Technical Explanation
- Work through the standard causes one at a time: a wrong pin offset relative to the track grid, a nominal track that's actually blocked by something else, a layer mismatch (pin sits on a layer the router isn't using for that net type), corner crowding (too many pins/objects competing for the same small area), a PG shape conflict eating the access point, or a bundle/bus ordering issue that leaves this particular pin stranded.
- "Off-track" and "stacked" are two distinct sub-problems: off-track means the pin doesn't align to where the router expects to make connections, while stacked usually means multiple pins/vias are competing for the exact same access point, so only one can actually connect.
- g. widening a halo when the real problem is a layer mismatch) wastes iteration cycles.
- The fix itself is almost always one of: move the pin (if it's a movable macro/cell), spread nearby pins apart, reorder a bundle so pins aren't competing, or re-layer the connection — all done within the interface's existing constraints, not by inventing a new interface.
Formula Or Decision Rule
Decision rule: require a legal approach on an allowed, unblocked routing resource.
What To Check
- Warning sign: The pin is snapped to the nearest nominal track without checking usability.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Correct the actual cause and rerun pin placement plus local congestion.
Command Checks & Actions
- check_pin_placement -alignment true -layer_mismatch_only true -blocked_only true: checks pin-placement legality and access-related conditions
- report_pin_placement -pins <pins> -format {layer side offset}: reports pin layer, side, and offset
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.
Healthy, Suspicious & Hard-stop Results
- Healthy: Treat this as a pin-access defect. Report the pin geometry and nearby tracks/resources, then move, spread, reorder, or re-layer it within the interface constraints.
- Hard stop: required legality or physical feasibility is missing or unexplained.
Common Mistake
The Trap: Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “A block pin is legal but off-track, stacked, or on an unusable layer. ” to measurable physical evidence and an owned correction.
Follow-up Question & Model Response
"* Candidate Model Response: Model response: “I would change it if the same controlled rerun shows that the pin is snapped to the nearest nominal track without checking usability. ”
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