CID CID Layout & Routing Techniques

Question 1

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

Accepted Answer

The edge-to-edge spacing between traces should be at least 3 times the trace width to minimize inductive crosstalk

Answer

Traces must be routed at 3-inch intervals to prevent shorts

Answer

All traces must have a 3-ohm termination resistor

Answer

Traces must cross at 3-degree angles only

Question 2

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

Accepted Answer

Two traces carrying equal and opposite signals; length matching ensures signals arrive simultaneously to cancel common-mode noise

Answer

Two traces carrying the same signal for redundancy; length matching is optional

Answer

A pair of power and ground traces; matched to balance current

Answer

Two traces on opposite layers; matched for thermal symmetry

Question 3

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

Accepted Answer

To carry the analog signal's return current and reduce EMI coupling from adjacent traces

Answer

To increase trace width for lower resistance

Answer

To act as a fuse if the trace carries too much current

Answer

To define the board's keep-out region

Question 4

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

Accepted Answer

They create impedance discontinuities and can cause etching problems during manufacturing, acting as antennas

Answer

90-degree corners increase trace length unnecessarily

Answer

Manufacturing equipment cannot produce 90-degree corners

Answer

90-degree corners increase via count

Question 5

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

Accepted Answer

Removing unused via stubs below the signal layer connection to eliminate stub resonance effects

Answer

Drilling holes from the back to add more connectors

Answer

Creating blind vias from the bottom layer

Answer

Adding thermal relief to bottom-side components

Question 6

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

Accepted Answer

Route on inner signal layers with adjacent reference planes to maintain controlled impedance

Answer

Route on the top and bottom copper layers for easy access

Answer

Route on power planes to share copper

Answer

Use any layer without regard to adjacent planes