32 Common PCB Design Questions and Answers

Circuit design is the foundation of electronics, and good layout directly affects the performance of PCB. With continuous updates to PCB design software, more advanced features are unlocked, with automatic layout and routing being the most noteworthy. However, when dealing with complex designs, designer intervention and optimization remain crucial, and complete reliance on tools is not advisable.

Therefore, mastering design layout techniques and understanding basic rules are essential for enhancing a designer’s professional competence. TechSparks provides the 100 frequently PCB design Q&A here to facilitate your learning process! Let’s get started!

Question 1: What are the key considerations for routing high-speed signals?

  • Ensure impedance matching for high-speed PCB;
  • Pay attention to spacing between high-speed signal lines and other signal lines;
  • Differential traces perform better, but ensure matching and phase consistency between differential pairs;
  • Consider shielding to prevent electromagnetic interference;
  • Question 2: How to avoid the impact of vias on PCB performance?

  • Reduce the number and density of vias while meeting requirements;
  • Avoid multiple crossings of high-speed signal lines through vias;
  • Select a more suitable type of PCB via based on project requirements;
  • Check if the manufacturer has advanced equipment like laser drilling;
  • Question 3: Is it better to have more decoupling capacitors?

    The significance of decoupling capacitors is to provide power stability and reduce power noise. However, it's clear that more isn't necessarily better. The quantity should be determined based on specific circuit requirements and design principles. For example:

    • Each integrated circuit should have at least one decoupling capacitor;
    • Large ICs or high-frequency circuits may require more decoupling capacitors;
    • If multiple decoupling capacitors are needed, they should be distributed logically in different areas rather than just near the power pins;

    Question 4: What are the standards for a high-quality PCB board?

    Well-thought-out circuit layout, Sound physical structure, High-quality substrate materials, Adequate power plane redundancy, High-frequency impedance matching, Concise low-frequency routing.

    Question 5: Should ASIC be ground-isolated in high-speed signal chain applications?

    The manual for ADI mixed-signal chips suggests that the choice should be made based on specific application requirements and design goals. During the implementation process, the optimal solution can be determined through simulation and testing.

    Question 6: What is the purpose of serpentining traces?

    Serpentine traces are typically used in high-speed digital signal transmission lines. Their primary objectives include reducing signal transmission loss, minimizing signal delay skew, enhancing signal immunity to interference, and preventing signal crosstalk.

    Question 7: How to mitigate EMC/EMI issues?

  • Place the clock generator away from external connectors;
  • Use inner-layer routing for high-speed signals;
  • Implement proper return path designs to minimize signal return path length and loop area;
  • Choose components that are low-noise, low-radiation, and have undergone EMI testing and certification;
  • Ensure a solid connection between the PCB and the enclosure ground points.
  • Question 8: What are the considerations for transmission line design in RF broadband circuits?

    • Choose appropriate PCB substrate to avoid adverse effects on signal transmission characteristics, such as dielectric constant, loss tangent, and conductivity.
    • Develop accurate transmission line models to predict signal behavior.
    • Consider the impact of component parameters on circuit performance.
    • Account for factors like bandwidth, cutoff frequency, and filtering characteristics.
    • Ensure that the matching network between the transmission lines and components is effective.

    Question 9: How to choose the package type for components like resistors and capacitors?

  • Consider circuit performance requirements, including frequency response, capacitance, inductance, etc.
  • Smaller packages can save space but may increase soldering and maintenance difficulty.
  • Smaller packages might introduce more parasitic parameters.
  • Some high-power components may require larger packages for better heat dissipation.
  • Question 10: What is the priority for signal lines and ground lines in double-sided PCB design?

    It depends on the specific circuit and application requirements. For example, in high-speed digital signals, analog circuits, and high-frequency RF circuits, the priority is given to signal lines. In mixed-signal circuits, power distribution, and physical constraints, the priority is given to ground lines.

    Question 11: What should be considered when designing a multi-layer PCB?

  • Proper layer planning, such as separating analog and digital signals.
  • Selecting suitable vias to facilitate signal transmission and connections between layers.
  • Avoiding interruptions or splits in ground planes.
  • Segregating different power voltages or power domains into separate power planes.
  • Adding heat dissipation layers and heat sinks to control temperature if there are high-power components.
  • Question 12: Should the design be based on the CPU frequency or the data interaction frequency of external components?

    For data interaction between the CPU and external components, the design is typically based on the data interaction frequency of the external components, rather than solely on the CPU's frequency. This is because external components may have their own clocks and operating frequencies, which may differ from the CPU's frequency.

    Question 13: How to determine whether noise is caused by an operational amplifier device or wiring defects?

    In most cases, distinguishing whether noise is caused by the operational amplifier device itself or wiring defects can be a challenging task. Here are some common methods and considerations:

    • Use an oscilloscope or spectrum analyzer to observe the spectral characteristics of the signal and noise.
    • Check for voltage fluctuations and ripple on the power supply lines.
    • Ensure a good ground plane and minimize ground loop paths.
    • If there is suspicion of operational amplifier device failure, consider replacing the device with the same model or conducting comparative tests with different device models.

    Question 14: Where does the heat come from in a PCB?

    The sources of heat can be categorized into three main parts: component heating is the primary source; PCB board heating is secondary; and heat conduction from external systems can be negligible.

    Question 15: Is it necessary to have a ground plane on the outer layer in a PCB with MCU control?

    In most cases, at least one complete ground plane is required to meet circuit connectivity, signal integrity, and EMC/EMI requirements.

    Question 16: How to Handle Multiple ADC Chips on the Board?

  • Try to place the ADC chips close together.
  • Establish a single-point connection for the analog and digital grounds beneath the chips.
  • Ensure each chip has its power supply.
  • Maintain clock synchronization among the chips.
  • Question 17: What are the requirements for ground design in analog and digital circuits?

    Analog Circuits: Grounding should be comprehensive, with minimal loop area and impedance matching.

    Digital Circuits: Low-frequency circuits have no specific requirements, while high-frequency circuits need to consider impedance matching and ground integrity.

    Question 18: How to avoid vias in high-speed PCB?

  • Employ interlayer connections or other methods to reduce the number of vias.
  • Use blind vias and buried vias to minimize the intersections between signal lines and vias.
  • Ensure an adequate distance between routing paths and vias.
  • Question 19: How to choose the thickness of power traces?

    It is important to ensure that the thickness of power traces can handle the required current. Thin power traces may lead to voltage drop and overheating issues. It is recommended to use circuit simulation tools to make an appropriate selection or rely on the empirical rule of 0.15× trace width = A (not applicable in all cases).

    Question 20: Do you need to distribute digital and analog circuits to different layers of a multi-layer PCB?

    It depends on the specific project requirements. High-precision analog circuits may require the separation of digital and analog circuits into different layers, using ground or partition layers to isolate them for higher levels of isolation and noise suppression. In conventional circuits, just place them separately.

    Question 21: How to avoid crosstalk issues in high-speed signals?

  • Maintain a safe distance between high-speed signal lines.
  • Avoid parallel routing of signal lines whenever possible.
  • Utilize shielding techniques such as shielding enclosures, shielding boxes, and shielded cables.
  • Implement differential signal transmission.
  • Place different types of signal lines on separate layers.
  • Consider the paths, intersections, and relationships of signal lines.
  • Question 22: How to decide on the ground plane?

    The ground plane can act as a signal return path but can also introduce parasitic capacitance with signal lines. The decision depends on whether the parasitic capacitance significantly impacts the signal. If it does, a reevaluation is necessary.

    Question 23: Is copper cladding required in multi-layer PCB design?

    Copper cladding can provide EMI/RFI protection, improved thermal management, and signal integrity. If there is a complete ground plane and power plane within the circuit, the top and bottom layers may not require copper cladding.

    Question 24: Does pairing fine-pitch components with wide traces lead to impedance mismatch issues?

    It does not lead to impedance mismatch issues because the impedance of a device is primarily determined by its internal circuitry and design rather than the width of its pins. The impedance of devices is typically detailed in datasheets or specifications, and designers can ensure impedance matching for signals based on these specifications.

    Question 25: How can you remedy the situation if differential lines cannot be achieved?

  • Implement a serpentine routing pattern to increase the length and achieve equal lengths for the lines. Some design software offers automatic serpentine routing features to simplify this process.
  • Rearrange components to shorten the length of one of the differential lines.
  • Use specialized delay lines to adjust the arrival times of signals.
  • Question 26: How can you prevent interference from PWM and other abrupt signals on analog signals (e.g., op-amps)?

  • Physically separate the PWM signal lines from the analog signal lines.
  • Shield the PWM signal lines using shielding enclosures or layers.
  • Add low-pass filters at the output or input to attenuate high-frequency components.
  • Ensure that decoupling capacitors are adequately used on the power lines of op-amps and other analog circuits.
  • Introduce ferrite beads or inductors between the PWM signal lines and the op-amp input.
  • Question 27: What should be considered when routing LVDS signals?

    LVDS signals consist of a pair of differential signals, typically composed of a positive and a negative signal. To ensure proper LVDS signal routing:

    • Ensure that the lengths of the signal traces are equal.
    • Maintain consistent spacing between the differential pairs.
    • Add appropriate termination resistors at both the LVDS signal's transmitting and receiving ends.

    Question 28: What protections are needed at the input interfaces for signals and power?

  • Overvoltage Protection: voltage regulators, voltage limiters, and discharge diodes.
  • Overcurrent Protection: fuses, current sensors, and current limiters.
  • ESD Protection: ESD suppressors, ESD diodes.
  • Question 29: Is it advisable to use serpentine routing when size constraints are present?

    If you determine that serpentine routing is the only viable option due to size constraints, it is essential to pay attention to the length and curvature during the design phase to avoid introducing additional capacitance and inductance.

    Question 30: How can you layout an amplifier to suppress the introduction of ripple?

  • If the circuit includes a switching power supply, you can use voltage regulators, capacitor filters, and magnetic components to reduce ripple in the output.
  • Place an adequate amount of decoupling capacitors between the signal source and the amplifier, as well as between the power pins of the amplifier.
  • Question 31: When designing high-frequency circuits, which is preferable, lumped parameters or distributed parameters?

    Lumped parameter models are simpler but may have some inaccuracies. Distributed parameter models offer higher accuracy and complete waveforms but require designers to have a detailed understanding of transmission line characteristics.

    Question 32: How long can the wiring be for high-speed logic devices?

    Wiring length isn't the primary concern; what's crucial is maintaining symmetry and minimizing disparities in the wiring. This is important to prevent logic errors caused by delays.

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