What are parasitics, in plain language?
From PDVerse MMMC Interview Masterclass, part of the pdVerse Mentor Series
Direct answer
Parasitics are the unintended electrical properties inherent in physical metal wires: series resistance (R), ground capacitance (C_gnd), and lateral coupling capacitance to neighboring signal wires (C_c). They convert ideal schematic connections into distributed RC delay networks.
Mentor explanation
Parasitics transform ideal zero-delay schematic wires into distributed RC transmission networks.
Key terms
- Wire resistance (R) — governed by metal sheet resistance; lower metal layers (M1-M3) are more resistive than thick upper layers, and vias add lumped series resistance.
- Ground/area capacitance (C_gnd) — capacitance between the wire and the substrate or reference planes.
- Coupling capacitance (C_c) — capacitance to neighboring signal wires; at sub-16nm geometries, coupling capacitance can exceed 70% of total interconnect capacitance and is the primary driver of crosstalk.
- SPEF — Standard Parasitic Exchange Format, the industry-standard ASCII file that transfers extracted R and C values into STA engines.
A physical net is not a single lumped capacitor — it is a distributed RC network where neighboring wires are electrically coupled to one another.
Practical example
Distributed Net Breakdown:
Driver Pin ───[ R1 ]───┬───[ R2 ]───┬─── Receiver Pin
│ │
[C1] [C2] (Ground Caps)
│ │
GND GND
│
[Cc] ─── Neighbor Aggressor WireInterview trap
Describing wire capacitance as only "capacitance to ground." In modern sub-16nm nodes, lateral coupling to neighboring wires is the dominant component of total capacitance.
Key takeaways
- Parasitics are the unintended physical R and C created by routed metal wires and vias.
- Coupling capacitance between adjacent wires (C_c) makes up >70% of total wire capacitance.
- Parasitics must be extracted into SPEF to calculate realistic wire delays in STA.
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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