CID Layout & Routing Techniques Flashcards

What is the '3W rule' in PCB routing and what does it prevent?

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.

What is a differential pair in PCB routing and why must the two traces be length-matched?

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.

What is the purpose of a 'guard trace' (guard ring) around a sensitive analog signal trace?

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.

Why should high-speed signal traces avoid 90-degree corners in routing?

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.

What is 'back-drilling' in PCB fabrication and why is it used for high-speed designs?

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.

Which routing layer strategy is typically recommended for differential pairs in a standard 6-layer PCB stackup?

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.