PCB Design 5

Question 1

What is the 'critical length' of a PCB trace in high-speed design?

Accepted Answer

The trace length at which the propagation delay becomes a significant fraction of the signal rise time, requiring controlled impedance routing

Answer

Critical length is the trace length (typically 1/6 of the wavelength or based on rise time) beyond which the trace behaves as a transmission line and requires controlled impedance to prevent reflections.

Question 2

What is the function of a 'fiducial mark' on a PCB?

Accepted Answer

To provide visual reference points for automated pick-and-place and optical inspection machines to locate the board and components accurately

Answer

Fiducial marks are small copper circles (usually with a clear solder mask opening) used by pick-and-place machines and AOI systems as reference points to align the board and accurately place components.

Question 3

What is 'return current path' and why does it matter in PCB design?

Accepted Answer

The physical path current takes from the load back to the power supply through the ground plane

Answer

The return current path is how current flows back to its source through the ground plane; at high frequencies, return currents tend to flow directly beneath the signal trace, so uninterrupted ground planes are essential for low EMI.

Question 4

What is the purpose of a 'net class' in PCB design software?

Accepted Answer

To group nets with similar routing requirements (e.g., trace width, clearance, impedance) so design rules can be applied consistently

Answer

Net classes allow designers to group nets (such as high-speed differential pairs or power rails) and apply specific design rules like minimum trace width, clearance, and via size to all nets in the class at once.

Question 5

What is 'via stitching' used for in PCB design?

Accepted Answer

Placing multiple vias around the perimeter of an RF area or along the edge of a ground plane to create a low-impedance connection and reduce EMI leakage

Answer

Via stitching connects multiple ground plane layers with many closely spaced vias to create a solid low-impedance ground structure, reduce EMI, and prevent RF signal leakage between board regions.

PCB - Design Practice Test

PCB - Design Practice Test

PCB Design 5