In PCB wiring guidelines, there is a principle known as “key signal line priority.” This means that key signals, such as power supply, analog signals, high-speed signals, clock signals, differential signals, synchronization signals, and others, should be prioritized. Let’s now explore the wiring requirements for these critical signals in greater detail.
### How to Wire PCB Key Signals
#### Analog Signal Wiring Requirements
Analog signals are characterized by their poor resistance to interference, so protecting these signals is a key concern during routing. The handling of analog signals generally follows these guidelines:
1. To improve their resistance to interference, keep the signal traces as short as possible.
2. For some analog signals, the requirement for impedance control can be relaxed, allowing for thicker traces if needed.
3. Limit the routing area for analog signals, ideally keeping them within a designated analog region and away from digital signals.
#### High-Speed PCB Signal Wiring Requirements
1. **Use of Multilayer PCBs**
High-speed signal circuits typically feature high integration and dense routing. The use of multilayer boards is not only essential for routing but also an effective way to reduce interference. Proper selection of layer count can significantly reduce the board size and enable the use of intermediate layers for shielding. This improves grounding, minimizes parasitic inductance, shortens signal transmission lengths, and greatly reduces cross-interference between signals.
2. **Minimize Trace Bends**
It is advisable to minimize the number of bends in high-speed signal traces.
1. **The less the lead bends between the pins of high-speed circuit devices, the better**
For high-speed signal routing, it is preferable to use as few bends as possible between the pins of the device. Ideally, the lead should follow a straight line. When a bend is necessary, a 45° angle or a curved arc can be used. This approach is typically employed to enhance the mechanical strength of the steel foil in low-frequency circuits. However, in high-speed circuits, it serves an additional purpose: it helps to minimize external emissions and mutual coupling of high-speed signals, reducing signal radiation and reflection.
2. **The shorter the lead, the better**
For high-speed signal wiring, shorter leads between the pins of the circuit device are preferable. Longer leads increase distributed inductance and capacitance, which negatively impacts high-frequency signal transmission. Additionally, long leads alter the characteristic impedance of the circuit, leading to signal reflection and potential oscillations.
3. **The fewer the lead layer alternations, the better**
The number of layer transitions (vias) between the pins of high-speed circuit devices should be minimized. Fewer vias result in lower distributed capacitance—each via can add approximately 0.5 pF, which increases circuit delay. Reducing the number of vias can significantly improve the circuit’s speed.
4. **Minimize parallel cross-interference**
High-speed signal routing should avoid “cross-interference” caused by closely parallel signal traces. If parallel routing cannot be avoided, place a large ground area opposite the signal traces to significantly reduce interference.
5. **Avoid branches and stubs**
High-speed signal routing should avoid branching or creating stubs. Stubs can significantly affect impedance, leading to signal reflection and overshoot. To minimize these effects, it’s recommended to use a daisy-chain layout, which reduces the impact on the signal.
### How to route key signals on a PCB
6. **Route high-frequency signal lines on inner layers**
Surface-mounted high-frequency signal traces are prone to generating significant electromagnetic radiation and are also more susceptible to external electromagnetic interference. It is better to route high-frequency signals between the power and ground layers, as the radiation generated by these layers will absorb the electromagnetic waves.
### Clock signal routing requirements
In digital circuit design, the clock signal oscillates between high and low states, which is crucial for circuit operation. The clock network plays a central role in the performance of the circuit and is also a major source of electromagnetic radiation. Special attention must be paid to clock signal routing during PCB layout. The clock tree should be planned early on, and the relationships between various clocks must be clearly defined to ensure proper signal routing. In addition, the clock signal is often a challenging aspect of EMC (Electromagnetic Compatibility) design, so special care should be taken for circuits requiring EMC compliance.
Along with the usual impedance control and equal length requirements for clock lines, the following considerations should also be addressed:
1. Select the optimal layer for clock signal routing.
2. Avoid crossing or routing clock signals through split planes.
3. Maintain a minimum distance of 3W between clock signals and other traces.
4. For designs requiring EMC compliance, use inner layers for longer clock traces.
5. Ensure proper termination matching for clock signals.
6. Use a star structure for clock signal distribution rather than a daisy chain. In a star configuration, all clock loads should be connected directly to the clock driver.
7. Keep traces connected to the crystal oscillator’s input/output terminals as short as possible to minimize noise interference and the impact of distributed capacitance.
8. Connect the crystal capacitor ground pin with the shortest and widest possible trace, and minimize vias near the digital ground pin closest to the crystal oscillator.
9. Since clock signals typically have fast edge transitions and high external crosstalk, surround clock traces with ground planes to reduce distributed capacitance and minimize crosstalk. For high-frequency clock signals, use low-voltage clocks and ground-wrapped designs, paying attention to the integrity of the ground connection.
### Differential signal routing requirements
Differential signals, also known as differential pairs, consist of two signals with opposite polarities that transmit data based on the voltage difference between them. To ensure the signals remain perfectly matched, the differential traces should be routed in parallel, with consistent line width and spacing.
**Key points for routing differential signals on a PCB:**
1. **Equal length:** The two differential traces should be as close in length as possible to maintain opposite polarities, minimizing common-mode components.
2. **Equal width and constant spacing:** The width of the traces and the distance between them should be kept consistent to maintain signal integrity.
### General PCB routing tips
For critical signals such as clock, high-frequency, and sensitive signals, always try to dedicate specific layers for routing these signals. Minimize the loop area and use techniques such as shielding and increasing the clearance to ensure optimal signal integrity.
If your have any questions about PCB ,please contact me info@wellcircuits.com
### How to Wire PCB Key Signals
#### Analog Signal Wiring Requirements
Analog signals are characterized by their poor resistance to interference, so protecting these signals is a key concern during routing. The handling of analog signals generally follows these guidelines:
1. To improve their resistance to interference, keep the signal traces as short as possible.
2. For some analog signals, the requirement for impedance control can be relaxed, allowing for thicker traces if needed.
3. Limit the routing area for analog signals, ideally keeping them within a designated analog region and away from digital signals.
#### High-Speed PCB Signal Wiring Requirements
1. **Use of Multilayer PCBs**
High-speed signal circuits typically feature high integration and dense routing. The use of multilayer boards is not only essential for routing but also an effective way to reduce interference. Proper selection of layer count can significantly reduce the board size and enable the use of intermediate layers for shielding. This improves grounding, minimizes parasitic inductance, shortens signal transmission lengths, and greatly reduces cross-interference between signals.
2. **Minimize Trace Bends**
It is advisable to minimize the number of bends in high-speed signal traces.
1. **The less the lead bends between the pins of high-speed circuit devices, the better**
For high-speed signal routing, it is preferable to use as few bends as possible between the pins of the device. Ideally, the lead should follow a straight line. When a bend is necessary, a 45° angle or a curved arc can be used. This approach is typically employed to enhance the mechanical strength of the steel foil in low-frequency circuits. However, in high-speed circuits, it serves an additional purpose: it helps to minimize external emissions and mutual coupling of high-speed signals, reducing signal radiation and reflection.
2. **The shorter the lead, the better**
For high-speed signal wiring, shorter leads between the pins of the circuit device are preferable. Longer leads increase distributed inductance and capacitance, which negatively impacts high-frequency signal transmission. Additionally, long leads alter the characteristic impedance of the circuit, leading to signal reflection and potential oscillations.
3. **The fewer the lead layer alternations, the better**
The number of layer transitions (vias) between the pins of high-speed circuit devices should be minimized. Fewer vias result in lower distributed capacitance—each via can add approximately 0.5 pF, which increases circuit delay. Reducing the number of vias can significantly improve the circuit’s speed.
4. **Minimize parallel cross-interference**
High-speed signal routing should avoid “cross-interference” caused by closely parallel signal traces. If parallel routing cannot be avoided, place a large ground area opposite the signal traces to significantly reduce interference.
5. **Avoid branches and stubs**
High-speed signal routing should avoid branching or creating stubs. Stubs can significantly affect impedance, leading to signal reflection and overshoot. To minimize these effects, it’s recommended to use a daisy-chain layout, which reduces the impact on the signal.
### How to route key signals on a PCB
6. **Route high-frequency signal lines on inner layers**
Surface-mounted high-frequency signal traces are prone to generating significant electromagnetic radiation and are also more susceptible to external electromagnetic interference. It is better to route high-frequency signals between the power and ground layers, as the radiation generated by these layers will absorb the electromagnetic waves.
### Clock signal routing requirements
In digital circuit design, the clock signal oscillates between high and low states, which is crucial for circuit operation. The clock network plays a central role in the performance of the circuit and is also a major source of electromagnetic radiation. Special attention must be paid to clock signal routing during PCB layout. The clock tree should be planned early on, and the relationships between various clocks must be clearly defined to ensure proper signal routing. In addition, the clock signal is often a challenging aspect of EMC (Electromagnetic Compatibility) design, so special care should be taken for circuits requiring EMC compliance.
Along with the usual impedance control and equal length requirements for clock lines, the following considerations should also be addressed:
1. Select the optimal layer for clock signal routing.
2. Avoid crossing or routing clock signals through split planes.
3. Maintain a minimum distance of 3W between clock signals and other traces.
4. For designs requiring EMC compliance, use inner layers for longer clock traces.
5. Ensure proper termination matching for clock signals.
6. Use a star structure for clock signal distribution rather than a daisy chain. In a star configuration, all clock loads should be connected directly to the clock driver.
7. Keep traces connected to the crystal oscillator’s input/output terminals as short as possible to minimize noise interference and the impact of distributed capacitance.
8. Connect the crystal capacitor ground pin with the shortest and widest possible trace, and minimize vias near the digital ground pin closest to the crystal oscillator.
9. Since clock signals typically have fast edge transitions and high external crosstalk, surround clock traces with ground planes to reduce distributed capacitance and minimize crosstalk. For high-frequency clock signals, use low-voltage clocks and ground-wrapped designs, paying attention to the integrity of the ground connection.
### Differential signal routing requirements
Differential signals, also known as differential pairs, consist of two signals with opposite polarities that transmit data based on the voltage difference between them. To ensure the signals remain perfectly matched, the differential traces should be routed in parallel, with consistent line width and spacing.
**Key points for routing differential signals on a PCB:**
1. **Equal length:** The two differential traces should be as close in length as possible to maintain opposite polarities, minimizing common-mode components.
2. **Equal width and constant spacing:** The width of the traces and the distance between them should be kept consistent to maintain signal integrity.
### General PCB routing tips
For critical signals such as clock, high-frequency, and sensitive signals, always try to dedicate specific layers for routing these signals. Minimize the loop area and use techniques such as shielding and increasing the clearance to ensure optimal signal integrity.
If your have any questions about PCB ,please contact me info@wellcircuits.com
