CID Risk Assessment & Mitigation 2 — Questions and Answers
Question 1: A CID is reviewing a high-speed design where two parallel signal traces run adjacent for 3 inches at 5 Gbps. What is the PRIMARY risk introduced by this routing?
- Increased propagation delay due to dielectric loading
- Inductive and capacitive crosstalk causing noise on the victim trace (Correct answer)
- Impedance reduction due to mutual inductance cancellation
- Increased DC resistance from skin-effect coupling
Correct answer: Inductive and capacitive crosstalk causing noise on the victim trace
Long parallel high-speed traces create both inductive and capacitive coupling, injecting crosstalk noise onto the victim net.
Question 2: Which mitigation strategy most effectively reduces common-mode EMI noise generated by a high-speed differential pair on a PCB?
- Increasing trace width to lower impedance
- Maintaining tight differential pair coupling with matched lengths (Correct answer)
- Adding series termination on each trace individually
- Routing the pair on an outer layer for easy probing
Correct answer: Maintaining tight differential pair coupling with matched lengths
Tight differential pair coupling ensures both traces carry equal and opposite currents, causing their radiated fields to cancel and suppressing common-mode EMI.
Question 3: During risk review, a designer finds that a critical clock net has no reference plane underneath it for 200 mils. What failure mode does this create?
- Reduced propagation velocity increasing setup time margin
- Return current forced to take a long detour, causing ground bounce and EMI (Correct answer)
- Increased capacitance causing false triggering at the receiver
- Lower characteristic impedance improving signal rise time
Correct answer: Return current forced to take a long detour, causing ground bounce and EMI
A gap in the reference plane forces return current to detour around it, increasing loop area, ground bounce, and radiated emissions.
Question 4: A PCB design team identifies that a DDR4 bus has unequal trace lengths within a byte lane. What risk does this create and what is the standard mitigation?
- Impedance mismatch risk; mitigated by adding series resistors to longer traces
- Skew risk causing data setup/hold violations; mitigated by serpentine length matching (Correct answer)
- Crosstalk risk; mitigated by widening the spacing between traces
- Voltage drop risk; mitigated by increasing trace width on shorter traces
Correct answer: Skew risk causing data setup/hold violations; mitigated by serpentine length matching
Unequal trace lengths in a DDR4 byte lane introduce timing skew that can violate setup and hold requirements, corrected by serpentine tuning to match lengths.
Question 5: What risk is associated with placing bypass capacitors far from the IC power pins on a high-frequency design?
- Capacitors become over-stressed due to proximity effects
- Parasitic inductance of the long path reduces the capacitor's high-frequency effectiveness (Correct answer)
- The capacitors pick up more noise from adjacent traces
- Capacitor self-resonant frequency increases beyond useful range
Correct answer: Parasitic inductance of the long path reduces the capacitor's high-frequency effectiveness
Distance from the power pin increases the series inductance in the supply path, reducing the bypass capacitor's ability to suppress high-frequency noise.
Question 6: A CID is assessing risk for a controlled-impedance microstrip trace. Which factor would MOST likely cause the fabricated impedance to deviate from the designed value?
- Choice of solder mask color
- Variation in dielectric thickness or copper width during PCB fabrication (Correct answer)
- Trace routing direction relative to fiber weave
- Via drill size used for the signal trace
Correct answer: Variation in dielectric thickness or copper width during PCB fabrication
Fabrication tolerances in dielectric thickness and copper trace width are the dominant contributors to impedance deviation in microstrip lines.
Question 7: When performing risk assessment on a high-speed PCB design, which via type poses the HIGHEST risk for signal integrity at frequencies above 10 GHz?
- Blind vias connecting layer 1 to layer 2
- Through-hole vias with a large unused stub extending beyond the signal layer (Correct answer)
- Buried vias entirely within the inner layers
- Back-drilled vias with stub length under 5 mils
Correct answer: Through-hole vias with a large unused stub extending beyond the signal layer
A through-hole via stub acts as a shunt resonator that can cause significant signal attenuation at frequencies where the stub length is a quarter wavelength.
CID Risk Assessment & Mitigation 2
A CID is reviewing a high-speed design where two parallel signal traces run adjacent for 3 inches at 5 Gbps.
What is the PRIMARY risk introduced by this routing?