What is the difference between wire width, spacing, and pitch?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Width is how thick the wire itself is, measured straight across it. Spacing is the empty gap between the edge of one shape and the edge of its neighbor.
Technical Explanation
- Width is how thick the wire itself is, measured straight across it.
- Spacing is the empty gap between the edge of one shape and the edge of its neighbor.
- Pitch is the center-to-center distance between repeating features — like the spacing between parallel routing tracks.
- For two equal-width wires sitting on adjacent, evenly spaced tracks: pitch = width + spacing (the worked example here lands at 0.028 um).
- For unequal widths, it's pitch minus half of each wire's width to get the actual gap.
- This is pure geometry, not a DRC checker — real spacing rules also depend on parallel-run length, masks, line ends, neighboring shapes, and via enclosures, so a correct arithmetic answer can still fail signoff DRC.
- Don't confuse routing-track pitch with the pitch of what actually gets routed — some tracks are reserved, a wide wire can eat more than one track, and a single default pitch tells you nothing about real capacity near a macro or power strap.
Visual Verification
Center-to-center track pitch equals the edge-to-edge spacing gap plus half the width of each adjacent wire (Pitch = Spacing + W1/2 + W2/2). Width and spacing are independent physical parameters.
Command Checks & Actions
set_technology -node 7Applies the technology file's routing pitch, width, and spacing rules for the node before placement or routing runs -- these three numbers live in the technology file, not in a runtime report.
set_technology -report_onlyReports which technology settings, including these routing rules, are actually active for the current session without changing anything.
Common Mistake
The Trap: Candidates often mistakenly calling the empty gap pitch, or treating pitch = width + spacing as universally valid for unequal-width wires.
Follow-up Question & Model Response
"Does doubling wire width automatically double routing capacity?"
Candidate Model Response: No. It consumes more space and may trigger larger spacing rules, even if resistance is reduced.
Practical Example
Tapeout Scenario: At 0.060 um center spacing, a 0.020 um wire beside a 0.040 um wire leaves 0.030 um between edges: 0.060 - 0.010 - 0.020. It would be incorrect to subtract only one of the widths.
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