PCB Design 4 — Questions and Answers
Question 1: What is the recommended placement strategy for bypass (decoupling) capacitors?
- Place them at the board edge for easy access
- Place them as close as possible to the power pins of the IC they are decoupling (Correct answer)
- Place them near the power connector
- Group all decoupling caps together in one area of the board
Correct answer: Place them as close as possible to the power pins of the IC they are decoupling
Decoupling capacitors must be placed as close as possible to the IC power pins to minimize the inductance of the loop between the capacitor and the IC, providing effective high-frequency filtering.
Question 2: What is 'impedance matching' and why is it important for high-speed PCB traces?
- Making all traces the same physical length
- Designing the trace characteristic impedance to match the source and load impedance to prevent signal reflections (Correct answer)
- Matching the resistance of power and ground traces
- Ensuring equal copper weight on all layers
Correct answer: Designing the trace characteristic impedance to match the source and load impedance to prevent signal reflections
Impedance matching ensures the trace, source, and load all have the same characteristic impedance (e.g., 50Ω), preventing signal reflections that cause distortion and data errors.
Question 3: What is the purpose of 'thermal relief' connections to SMD pads on a copper pour?
- To improve high-frequency performance of the pad
- To reduce the thermal mass at the pad, making soldering easier by preventing the pour from acting as a heat sink (Correct answer)
- To increase the current carrying capacity of the pad
- To prevent solder bridges between pads
Correct answer: To reduce the thermal mass at the pad, making soldering easier by preventing the pour from acting as a heat sink
Thermal relief connects a pad to the copper pour through narrow spokes, reducing the thermal conductivity to the pour so the soldering iron can heat the pad to soldering temperature without the pour draining heat away.
Question 4: What is a 'ground plane split' and when should it be used with caution?
- Splitting the ground pour between multiple layers to save copper
- Dividing the ground plane into separate sections (e.g., analog and digital GND) to isolate noise domains (Correct answer)
- Using multiple GND vias in one area
- Removing the ground layer in a specific board region
Correct answer: Dividing the ground plane into separate sections (e.g., analog and digital GND) to isolate noise domains
A split ground plane separates analog and digital ground sections to prevent digital switching noise from coupling into sensitive analog circuitry, but must be implemented carefully to avoid return current discontinuities.
Question 5: What does 'BGA fanout' refer to in PCB design?
- The process of routing power to a BGA component
- The routing strategy of connecting BGA ball pads to the rest of the circuit using escape vias and traces (Correct answer)
- Removing a BGA from the board for rework
- The thermal analysis of BGA components
Correct answer: The routing strategy of connecting BGA ball pads to the rest of the circuit using escape vias and traces
BGA fanout is the process of routing escape traces from the dense BGA ball grid pads outward using vias (often microvias) to access inner rows of pads and connect to the rest of the PCB.
Question 6: What is the difference between 'ENIG' and 'HASL' PCB surface finishes?
- ENIG uses lead-free solder while HASL uses leaded solder
- ENIG (Electroless Nickel Immersion Gold) provides a flat, solderable gold surface, while HASL (Hot Air Solder Leveling) coats pads with solder using a hot air leveling process (Correct answer)
- HASL is used only for flexible PCBs
- ENIG is applied before drilling while HASL is applied after
Correct answer: ENIG (Electroless Nickel Immersion Gold) provides a flat, solderable gold surface, while HASL (Hot Air Solder Leveling) coats pads with solder using a hot air leveling process
ENIG deposits a flat nickel/gold surface ideal for fine-pitch SMD components, while HASL applies a layer of solder to pads and then levels it with hot air, leaving a slightly uneven surface.
Question 7: What is a 'design for manufacturability' (DFM) review in PCB design?
- A review to minimize the number of components on the board
- An analysis of the PCB design to identify and correct features that could cause manufacturing defects or increase production cost (Correct answer)
- A simulation of the board's electrical performance
- A check for compliance with regulatory standards
Correct answer: An analysis of the PCB design to identify and correct features that could cause manufacturing defects or increase production cost
DFM review examines the PCB layout for issues like insufficient clearances, acid traps, unsupported hole sizes, or problematic component placements that could cause manufacturing defects or increase cost.
PCB Design 4
What is the recommended placement strategy for bypass (decoupling) capacitors?