CID CID Layout & Routing Techniques 1 — Questions and Answers
Question 1: What is the '3W rule' in PCB routing and what does it prevent?
- Traces must be routed at 3-inch intervals to prevent shorts
- The edge-to-edge spacing between traces should be at least 3 times the trace width to minimize inductive crosstalk (Correct answer)
- All traces must have a 3-ohm termination resistor
- Traces must cross at 3-degree angles only
Correct answer: The edge-to-edge spacing between traces should be at least 3 times the trace width to minimize inductive crosstalk
The 3W rule specifies that edge-to-edge trace spacing should be at least 3x the trace width, which reduces inductive crosstalk to approximately 70% of adjacent trace coupling.
Question 2: What is a differential pair in PCB routing and why must the two traces be length-matched?
- Two traces carrying the same signal for redundancy; length matching is optional
- Two traces carrying equal and opposite signals; length matching ensures signals arrive simultaneously to cancel common-mode noise (Correct answer)
- A pair of power and ground traces; matched to balance current
- Two traces on opposite layers; matched for thermal symmetry
Correct answer: Two traces carrying equal and opposite signals; length matching ensures signals arrive simultaneously to cancel common-mode noise
Differential pairs carry complementary signals, and length matching ensures both signals arrive at the receiver simultaneously, preserving the differential signal integrity and maximizing common-mode rejection.
Question 3: What is the purpose of a 'guard trace' (guard ring) around a sensitive analog signal trace?
- To carry the analog signal's return current and reduce EMI coupling from adjacent traces (Correct answer)
- To increase trace width for lower resistance
- To act as a fuse if the trace carries too much current
- To define the board's keep-out region
Correct answer: To carry the analog signal's return current and reduce EMI coupling from adjacent traces
A guard trace routed alongside a sensitive analog trace and connected to a low-impedance reference provides a shielded return path and reduces capacitive and inductive coupling from nearby noisy traces.
Question 4: Why should high-speed signal traces avoid 90-degree corners in routing?
- 90-degree corners increase trace length unnecessarily
- They create impedance discontinuities and can cause etching problems during manufacturing, acting as antennas (Correct answer)
- Manufacturing equipment cannot produce 90-degree corners
- 90-degree corners increase via count
Correct answer: They create impedance discontinuities and can cause etching problems during manufacturing, acting as antennas
Sharp 90-degree corners increase the effective trace width at the bend, creating an impedance discontinuity and potential EMI radiation, which is why 45-degree angles or curves are preferred.
Question 5: What is 'back-drilling' in PCB fabrication and why is it used for high-speed designs?
- Drilling holes from the back to add more connectors
- Removing unused via stubs below the signal layer connection to eliminate stub resonance effects (Correct answer)
- Creating blind vias from the bottom layer
- Adding thermal relief to bottom-side components
Correct answer: Removing unused via stubs below the signal layer connection to eliminate stub resonance effects
Back-drilling removes excess via barrel below the signal connection point, eliminating the via stub that can resonate and cause signal reflections at high frequencies.
Question 6: Which routing layer strategy is typically recommended for differential pairs in a standard 6-layer PCB stackup?
- Route on the top and bottom copper layers for easy access
- Route on inner signal layers with adjacent reference planes to maintain controlled impedance (Correct answer)
- Route on power planes to share copper
- Use any layer without regard to adjacent planes
Correct answer: Route on inner signal layers with adjacent reference planes to maintain controlled impedance
Routing differential pairs on inner signal layers sandwiched between solid reference planes provides stable, consistent impedance control and better EMI shielding.
CID CID Layout & Routing Techniques 1
What is the '3W rule' in PCB routing and what does it prevent?