Designing a circuit is a complex and meticulous process. Every detail matters, as even the slightest error, component deviation, or trace thickness can lead to circuit failure. To ensure a successful design, careful review of the layout is necessary, along with a thorough understanding of basic design rules and techniques outlined in the PCB design guide. In this article, TechSparks will provide essential pointers to enhance your PCB design process. Let’s get started.
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In circuit design, the 3W rule plays a crucial role as a key design criterion, encompassing the width of traces, the spacing between them, and the clearance between traces and surrounding areas on the board. Careful consideration of these parameters at an early stage is PCB design basics, and additionally requires strict adherence throughout the layout and trace process.
- Width: Proper line width design is essential to ensure smooth current flow and control resistance. Insufficient line width may result in excessive resistance, impacting signal transmission and power consumption. It is imperative to calculate the line width based on current load and heat dissipation requirements to achieve circuit board stability and reliability.
- Spacing: Maintaining an appropriate line-to-line distance effectively reduces crosstalk between traces, minimizing signal interference and enhancing signal integrity. Adhering to the 3W rule, a line-to-line distance not less than 3 times the line width ensures that 70% of electric fields do not interfere with each other, ensuring signal stability and accuracy. Increasing the spacing to 10 times the line width can achieve 98% electric field isolation, further enhancing signal integrity.
- Clearance: Blank areas are vital for preventing electric fields between wires and other components from causing interference, thus reducing signal interference and noise. During layout, designers must allocate sufficient blank space, especially for high-frequency or high-speed signal lines.
In the internal structure of a circuit board, changes in the electric field between the power plane and the ground plane, especially near the board’s edge, can lead to electromagnetic interference radiating from the edge, known as the edge effect.
To address the edge effect issue, a method called “shrinking the power layer” can be employed. This involves reducing the distance between the power plane and the ground plane, effectively reducing the size of the power plane. As a result, the electric field is confined within the ground plane’s boundary. For instance, using a unit “H,” if the power layer is retracted by 20H, approximately 70% of the electric field will be confined to the edge of the ground plane. Similarly, a retraction of 100H will confine nearly 98% of the electric field within the ground plane’s edge.
Since the rise and fall of signals in high-frequency circuits are often done in a short period of time, using this PCB layout guidelines for this will improve the control of electromagnetic interference radiation and reduce potential problems caused by edge effects.
The selection of the number of printed board layers is an essential consideration in circuit design, especially when dealing with high clock frequencies of around 5MHz or pulse rise times less than 5ns. In such cases, it is generally recommended to opt for a multilayer structure. This circuit structure provides better signal integrity and control of electromagnetic interference, thereby ensuring the quality of signal transmission.
However, cost constraints or other factors may lead to the choice of a double-layer panel structure, consisting of just two layers: the top and the bottom. In such situations, to optimize signal routing and minimize the length of signal traces, TechSparks suggests designing one side of the printed board, typically the bottom layer, as a complete ground plane. This approach provides a robust ground plane reference, effectively reducing signal return paths and mitigating electromagnetic interference (EMI) issues.
Ground Loop Rules
To minimize radiation and external interference, it is crucial to keep the ring area formed by the signal line and its loop as small as possible. Here are some PCB layout tips to achieve this:
- When dividing the ground plane, carefully consider the distribution of the ground plane and important signal traces to avoid potential problems caused by slotting in the ground plane. Proper ground plane management helps reduce unwanted coupling and interference.
- In double-layer PCB designs, leave adequate space for the power supply and fill this area with the reference ground. Ensure effective connections between the double-sided ground signals by adding necessary holes.
- For applications requiring high-speed signals, focus on addressing the signal return path problem. TechSparks recommends using multi-layer PCBs to enhance performance and increase circuit density.
Crosstalk Control Tips
Crosstalk is a common issue in circuit design, arising from mutual interference between long parallel wires carrying different signals on the PCB. This interference results from the distributed capacitance and distributed inductance between these parallel lines, leading to signal distortion or loss when transmitted on one wire due to the electric or magnetic field of another wire.
To effectively overcome crosstalk, mastering the following PCB design skills is essential:
- Follow the 3W rule and increase the spacing between parallel wiring between different nets to reduce the possibility of interference.
- Implement ground isolation lines, also known as ground spacers, in the design. By inserting grounded ground spacers between parallel lines, a shielding effect is achieved, effectively isolating signal lines.
- During the design process, reducing the distance between the wiring layer and the ground plane can further improve the PCB’s crosstalk immunity.
Circuit Shielding Protection Tips
To minimize signal loop area and address the integrity of crucial high-frequency signals such as clock and synchronization signals, adhering to the ground loop rule is crucial in PCB design.
For vital and high-frequency signals, a copper-axis cable shielding structure is recommended. This design involves isolating the signal line’s upper, lower, left, and right sides with ground wires, forming a closed shielding structure. This shielding effectively prevents signal radiation and interference, suppressing the signal propagation path.
The connection between the shielding ground and the actual ground plane is of utmost importance. A robust and effective connection ensures that the shielding ground provides optimal shielding effectiveness. This limits signal radiation and interference to a minimal range.
In addition, separating sensitive signals, equal-length signal lines, and using filters and suppressors are also common methods used by designers.
Trace Layout Tips
In the wiring layout, careful consideration of the direction and orientation of signal traces. Ideally, adjacent layers should have an orthogonal structure, minimizing the possibility of signal lines running in the same direction on adjacent layers. While strict orthogonality may not always be feasible due to space limitations, it is sufficient to ensure that adjacent layer signals are not parallel.
In circuits, reflections can occur when transmission speeds are high, which can negatively impact signal quality. To avoid such situations, it is essential to keep the wiring width of the same network consistent. In cases where complete consistency is not possible, such as in connector pinouts or BGA package pinouts, minimizing the effective length of the inconsistent part in the middle is recommended.
Impedance matching becomes critical when the delay time of PCB wiring exceeds 1/4 of the signal rise time or fall time, effectively making the wiring act as a transmission line. To ensure accurate signal transmission, various impedance matching methods can be employed based on the network connection mode and wiring topology.
Component Layout Partitioning Guidelines
For high-speed components, it is essential to place them at the interface with the shortest path to other high-speed components or interface pins. To reduce interference between high-frequency and low-frequency signals, consider segregating high-speed and low-speed components into different areas and minimizing the crossing of high-frequency signals with low-frequency signals.
In the ground plane layout, dividing the ground plane into digital and analog ground segments can effectively reduce interference between digital and analog signals. High-speed interfaces can benefit from a single-point connection that connects digital and analog grounds, eliminating potential ground loops between them.
For hybrid circuits containing both digital and analog components, placing them on opposite sides of the PCB helps reduce interference between the two parts by isolating their respective ground planes. Designate a ground isolation area between the digital and analog sections to further avoid potential ground loops.
Effective heat distribution and cooling layout are crucial for high-power devices. Plan a reasonable heat dissipation layout to ensure efficient heat dissipation, preventing performance degradation or damage due to excessive temperatures. Distribute multiple high-power components strategically across the PCB to balance heat distribution throughout the board.
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