PCB Design 5 — Questions and Answers
Question 1: What is the 'critical length' of a PCB trace in high-speed design?
- The maximum allowed trace length before a signal degrades
- The trace length at which the propagation delay becomes a significant fraction of the signal rise time, requiring controlled impedance routing (Correct answer)
- The minimum trace length needed for a valid connection
- The length at which a trace must be increased in width
Correct answer: The trace length at which the propagation delay becomes a significant fraction of the signal rise time, requiring controlled impedance routing
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?
- To indicate the board revision number
- To provide visual reference points for automated pick-and-place and optical inspection machines to locate the board and components accurately (Correct answer)
- To mark the location of through-hole components
- To indicate regions where no copper should be placed
Correct answer: To provide visual reference points for automated pick-and-place and optical inspection machines to locate the board and components accurately
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?
- The physical path current takes from the load back to the power supply through the ground plane (Correct answer)
- The path a signal takes from output to input through feedback
- The current path through the solder mask
- The reverse current flow through a protection diode
Correct answer: The physical path current takes from the load back to the power supply through the ground plane
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?
- To categorize components by manufacturer
- To group nets with similar routing requirements (e.g., trace width, clearance, impedance) so design rules can be applied consistently (Correct answer)
- To define the power consumption of a net
- To assign signal names to schematic symbols
Correct answer: To group nets with similar routing requirements (e.g., trace width, clearance, impedance) so design rules can be applied consistently
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?
- Connecting signal traces across multiple layers
- 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 (Correct answer)
- Filling unused board area with vias to reduce weight
- Creating a thermal path for heat dissipation from components
Correct 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
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.
Question 6: What is 'starved thermals' (or insufficient thermal relief) on a pad?
- A pad with too many connections to the copper pour, creating a strong thermal bond that hinders soldering (Correct answer)
- A pad with too few or too-narrow spokes connecting to the copper pour, leaving the pad thermally isolated
- A defect where the pad is not connected to any net
- A pad that has no solder mask opening
Correct answer: A pad with too many connections to the copper pour, creating a strong thermal bond that hinders soldering
Starved thermals (overconnected pads) occur when a pad has too many or overly wide spokes to a copper pour, making it difficult to heat the pad to soldering temperature because the pour sinks heat away too effectively.
Question 7: In multilayer PCB design, what is the primary advantage of using a 'power plane' layer over routed power traces?
- Power planes reduce the number of layers needed
- Power planes provide a low-impedance, low-inductance power distribution with inherent decoupling capacitance against the adjacent ground plane (Correct answer)
- Power planes are easier to manufacture than traced power routes
- Power planes eliminate the need for decoupling capacitors
Correct answer: Power planes provide a low-impedance, low-inductance power distribution with inherent decoupling capacitance against the adjacent ground plane
A dedicated power plane provides extremely low impedance power distribution across the entire board and forms a distributed capacitor with the adjacent ground plane, inherently improving decoupling at high frequencies.
PCB Design 5
What is the 'critical length' of a PCB trace in high-speed design?