How do you calculate and round a core and die consistently?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Rounding core/die dimensions isn't a cosmetic formatting step — every snap to a legal grid or row/site multiple is a real physical decision that shifts area and utilisation, so it has to be done deliberately, not left to whatever the tool defaults to. Start with the ideal core dimensions from your area math, then snap width and height to legal manufacturing grid values and to whole row/site multiples — you can't have a half-row hanging off the edge.
Technical Explanation
- After snapping, always recompute actual core area and actual utilisation from the rounded numbers — the pre-rounding number is not what you'll actually get.
- Worked example: an ideal target of W = 1200 um and H = 800 um snaps under this discipline; separately, applying asymmetric die-to-core offsets of Left/Right/Bottom/Top = 60/90/50/70 um against that core gives a die of 1350 um x 920 um before any additional grid snapping is applied to the die itself.
- Asymmetric offsets exist for real reasons (I/O ring width differences, keepout zones, corner cell requirements) — don't default to symmetric margins just because it's simpler to compute.
- Always re-verify the final rounded/offset numbers against the site grid and manufacturing grid one more time — a die dimension that looks fine in isolation can still land off-grid once offsets are added.
Formula Or Decision Rule
W = sqrt(A x AR); H = sqrt(A / AR); die W = W+L+R; die H = H+B+T.
What To Check
- Warning sign: Rounded values are reported without recomputing actual area or occupancy.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Snap, recalculate, and place both ideal and final values in the review record.
Command Checks & Actions
report_design-floorplan: reports the floorplan state understood by the tool
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.
Healthy, Suspicious & Hard-stop Results
- Healthy: Calculate ideal core dimensions, snap them to legal grids or row/site multiples, recompute actual area and utilisation, then add asymmetric offsets.
- 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
” 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 rounded values are reported without recomputing actual area or occupancy. ”
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