PCB Design 2 — Questions and Answers
Question 1: What is the purpose of a copper pour (flood) on a PCB layer?
- To reduce board weight
- To fill unused areas with copper, improving ground planes and reducing impedance (Correct answer)
- To add decorative patterns to the board
- To isolate signal traces from each other
Correct answer: To fill unused areas with copper, improving ground planes and reducing impedance
Copper pours fill unused PCB areas with copper connected to a net (usually GND), improving ground planes, reducing EMI, and helping with thermal dissipation.
Question 2: Which IPC standard defines acceptability criteria for printed circuit board assemblies?
- IPC-2221
- IPC-A-610 (Correct answer)
- IPC-7711
- IPC-6012
Correct answer: IPC-A-610
IPC-A-610 is the globally recognized standard for acceptability of electronic assemblies, defining workmanship criteria for PCB assembly.
Question 3: What does 'controlled impedance' mean in PCB design?
- Limiting the number of vias on a trace
- Designing trace geometry so that the characteristic impedance matches a target value (Correct answer)
- Using only right-angle bends in traces
- Controlling the resistance of power traces
Correct answer: Designing trace geometry so that the characteristic impedance matches a target value
Controlled impedance means designing trace width, dielectric thickness, and materials so the trace's characteristic impedance matches the system requirement (e.g., 50Ω for RF).
Question 4: What is a 'design rule check' (DRC) in PCB CAD tools?
- A schematic verification step
- An automated check that verifies the layout meets specified spacing, clearance, and width constraints (Correct answer)
- A process for checking BOM accuracy
- A thermal simulation of the board
Correct answer: An automated check that verifies the layout meets specified spacing, clearance, and width constraints
DRC is an automated CAD function that checks the PCB layout against predefined rules for trace width, clearance, drill sizes, and other manufacturing constraints.
Question 5: What is the function of a silkscreen layer on a PCB?
- To provide electrical connectivity between layers
- To print component reference designators, logos, and assembly markings on the board surface (Correct answer)
- To protect the copper from oxidation
- To define the board outline
Correct answer: To print component reference designators, logos, and assembly markings on the board surface
The silkscreen (or legend) layer is a non-conductive ink layer printed on the PCB surface to show component designators, polarity markings, logos, and other assembly aids.
Question 6: In a differential pair routing, why must both traces be kept the same length?
- To minimize board area usage
- To ensure signals arrive simultaneously, preserving signal integrity and common-mode rejection (Correct answer)
- To reduce manufacturing cost
- To avoid crossing power planes
Correct answer: To ensure signals arrive simultaneously, preserving signal integrity and common-mode rejection
Length matching in differential pairs ensures both signals arrive at the receiver at the same time, maintaining the phase relationship and maximizing common-mode noise rejection.
Question 7: What is a 'keepout zone' in PCB design?
- A region reserved for high-voltage components only
- A defined area where specific elements (copper, components, vias) are prohibited from being placed (Correct answer)
- A zone where trace routing is mandatory
- An area designated for heat sinks only
Correct answer: A defined area where specific elements (copper, components, vias) are prohibited from being placed
A keepout zone is a defined PCB region where placement rules prohibit certain items such as copper pours, components, or vias, often used near connectors, board edges, or RF antennas.
PCB Design 2
What is the purpose of a copper pour (flood) on a PCB layer?