The anti-interference design of a PCB is closely linked to the specific circuit involved. Here, we outline a few common methods for PCB anti-interference design.
1. **Power Trace Design**
Based on the current requirements of the printed circuit board, the width of the power trace should be increased to reduce loop resistance. Additionally, the power and ground traces should be aligned with the direction of data transmission, as this helps improve noise immunity.
2. **Ground Wire Design Principles**
(1) **Separation of Digital and Analog Grounds**
If both logic circuits and analog circuits exist on the same PCB, it is advisable to separate their grounds as much as possible. For low-frequency circuits, the ground should ideally be connected at a single point. In cases where this is difficult, partial series connections can be used before grounding in parallel. High-frequency circuits, on the other hand, should have multiple grounding points in series. The ground traces should be as short and direct as possible, and a large-area, grid-like ground plane should be used around high-frequency components to minimize noise.
(2) **Thickness of Ground Trace**
The ground trace should be as wide as possible. A narrow ground trace can cause voltage fluctuations due to varying currents, which reduces noise immunity. To avoid this, the ground trace should be capable of handling three times the maximum current expected on the PCB. Ideally, the trace width should be at least 2–3 mm.
(3) **Closed Loop Grounding**
For PCBs containing only digital circuits, the grounding circuit is typically arranged in a loop to enhance noise resistance. This loop configuration helps to improve the PCB’s overall anti-interference performance.
3. **Decoupling Capacitor Placement**
A common practice in PCB design is to place appropriate decoupling capacitors at key locations on the board. The general principles for decoupling capacitor placement are:
(1) Connect an electrolytic capacitor with a value between 10µF and 100µF across the power input. If possible, a 100µF or higher capacitor is preferred.
(2) Ideally, each integrated circuit chip should have a 0.01µF ceramic capacitor connected to it. If there isn’t enough space on the PCB, you can place a 1-10µF tantalum capacitor for every 4-8 chips.
(3) For components like RAM and ROM storage devices, which have low noise immunity and experience significant power fluctuations during shutdown, a decoupling capacitor should be directly connected between the power and ground pins of the chip.
(4) Keep capacitor leads as short as possible, particularly for high-frequency bypass capacitors.
(5) Components like contactors, relays, and buttons can generate large spark discharges when activated. In such cases, RC circuits should be used to absorb the discharge current. Typically, R should be between 1kΩ and 2kΩ, and C should range from 2.2µF to 47µF.
(6) CMOS devices have very high input impedance and are sensitive to induction. Therefore, unused terminals should either be grounded or connected to a positive power supply.
### 5. PCB Routing Guidelines
In PCB design, routing is a critical step in completing the overall product design. All prior work is done in preparation for this stage. Routing is the most restrictive process in PCB design, requiring both skill and considerable effort. It can be done in single-sided, double-sided, or multilayer configurations, with two primary methods: automatic routing and interactive routing. Before using automatic routing, interactive routing can be employed to manually lay out the more complex traces.
The input and output terminals should not be placed adjacent or parallel to one another to avoid reflection interference. If necessary, a ground trace should be used for isolation, and traces on adjacent layers should cross at right angles to minimize parasitic coupling.
The success of automatic routing depends largely on a well-thought-out layout. Routing rules can be predefined, including constraints on the number of bends, vias, and layer transitions. Typically, the process starts with tracing shorter paths and then routing more complex paths. The global routing should be optimized first, and connections can be adjusted or re-routed to improve overall performance.
In modern high-density PCB designs, traditional through-hole vias are becoming less viable, as they consume valuable routing space. To address this, blind and buried vias have emerged, providing the functionality of traditional vias while saving space and improving routing efficiency. This makes the routing process smoother and more comprehensive. PCB design is both a complex and straightforward process, and mastery requires hands-on experience by engineers. Only through direct involvement can one fully grasp its nuances.
### 1. Power and Ground Plane Considerations
Even if the routing of the PCB is well-executed, improper handling of the power and ground planes can introduce noise that degrades product performance and, in some cases, even reduces product yield. Therefore, careful attention must be given to the routing of power and ground traces to minimize noise interference and ensure the product meets quality standards.
Every electronic engineer working on product design understands the potential for noise between the power and ground planes. This section focuses on how to reduce noise through effective design:
### 2. Grounding in Digital and Analog Circuits
Many modern PCBs integrate both digital and analog circuits, rather than focusing on a single function. As a result, it’s important to consider the mutual interference between these circuits during layout, especially the noise generated on the ground plane.
Digital circuits operate at high frequencies and are prone to generating noise, while analog circuits are highly sensitive to interference. For signal lines, high-frequency traces should be routed as far from analog-sensitive components as possible. Since the PCB has only one external connection point for the ground, the digital and analog grounds must be isolated within the board. These two grounds should not be directly connected to each other, except at a single point (e.g., at connectors or other external interfaces). In the PCB layout, ensure that the digital ground and analog ground are only connected at this singular point, which helps mitigate interference.
### 3. Routing on Power (Ground) Layers
In multi-layer PCB designs, it is common for signal layers to become crowded, making it difficult to route additional signals efficiently. To alleviate this, signal routing on power and ground layers can be considered. Power layers should be prioritized, followed by ground layers, to preserve the integrity of the circuit and minimize interference.
### 4. Connection of Component Leads in Large Ground Areas
When connecting component leads to large ground planes, a comprehensive approach is needed. For optimal electrical performance, component pads should be connected directly to the copper plane. However, there are potential issues during soldering and assembly, such as the need for high-power soldering equipment and the risk of cold or dry solder joints.
To mitigate these issues, it’s best to use crosshatched pads (often called thermal pads) to prevent excessive heat buildup during soldering. These pads reduce the likelihood of cold solder joints by distributing heat more effectively. This approach is also applicable to power and ground connections in multi-layer PCBs, ensuring a reliable and efficient assembly process.
1. **Power Trace Design**
Based on the current requirements of the printed circuit board, the width of the power trace should be increased to reduce loop resistance. Additionally, the power and ground traces should be aligned with the direction of data transmission, as this helps improve noise immunity.
2. **Ground Wire Design Principles**
(1) **Separation of Digital and Analog Grounds**
If both logic circuits and analog circuits exist on the same PCB, it is advisable to separate their grounds as much as possible. For low-frequency circuits, the ground should ideally be connected at a single point. In cases where this is difficult, partial series connections can be used before grounding in parallel. High-frequency circuits, on the other hand, should have multiple grounding points in series. The ground traces should be as short and direct as possible, and a large-area, grid-like ground plane should be used around high-frequency components to minimize noise.
(2) **Thickness of Ground Trace**
The ground trace should be as wide as possible. A narrow ground trace can cause voltage fluctuations due to varying currents, which reduces noise immunity. To avoid this, the ground trace should be capable of handling three times the maximum current expected on the PCB. Ideally, the trace width should be at least 2–3 mm.
(3) **Closed Loop Grounding**
For PCBs containing only digital circuits, the grounding circuit is typically arranged in a loop to enhance noise resistance. This loop configuration helps to improve the PCB’s overall anti-interference performance.
3. **Decoupling Capacitor Placement**
A common practice in PCB design is to place appropriate decoupling capacitors at key locations on the board. The general principles for decoupling capacitor placement are:
(1) Connect an electrolytic capacitor with a value between 10µF and 100µF across the power input. If possible, a 100µF or higher capacitor is preferred.
(2) Ideally, each integrated circuit chip should have a 0.01µF ceramic capacitor connected to it. If there isn’t enough space on the PCB, you can place a 1-10µF tantalum capacitor for every 4-8 chips.
(3) For components like RAM and ROM storage devices, which have low noise immunity and experience significant power fluctuations during shutdown, a decoupling capacitor should be directly connected between the power and ground pins of the chip.
(4) Keep capacitor leads as short as possible, particularly for high-frequency bypass capacitors.
(5) Components like contactors, relays, and buttons can generate large spark discharges when activated. In such cases, RC circuits should be used to absorb the discharge current. Typically, R should be between 1kΩ and 2kΩ, and C should range from 2.2µF to 47µF.
(6) CMOS devices have very high input impedance and are sensitive to induction. Therefore, unused terminals should either be grounded or connected to a positive power supply.
### 5. PCB Routing Guidelines
In PCB design, routing is a critical step in completing the overall product design. All prior work is done in preparation for this stage. Routing is the most restrictive process in PCB design, requiring both skill and considerable effort. It can be done in single-sided, double-sided, or multilayer configurations, with two primary methods: automatic routing and interactive routing. Before using automatic routing, interactive routing can be employed to manually lay out the more complex traces.
The input and output terminals should not be placed adjacent or parallel to one another to avoid reflection interference. If necessary, a ground trace should be used for isolation, and traces on adjacent layers should cross at right angles to minimize parasitic coupling.
The success of automatic routing depends largely on a well-thought-out layout. Routing rules can be predefined, including constraints on the number of bends, vias, and layer transitions. Typically, the process starts with tracing shorter paths and then routing more complex paths. The global routing should be optimized first, and connections can be adjusted or re-routed to improve overall performance.
In modern high-density PCB designs, traditional through-hole vias are becoming less viable, as they consume valuable routing space. To address this, blind and buried vias have emerged, providing the functionality of traditional vias while saving space and improving routing efficiency. This makes the routing process smoother and more comprehensive. PCB design is both a complex and straightforward process, and mastery requires hands-on experience by engineers. Only through direct involvement can one fully grasp its nuances.
### 1. Power and Ground Plane Considerations
Even if the routing of the PCB is well-executed, improper handling of the power and ground planes can introduce noise that degrades product performance and, in some cases, even reduces product yield. Therefore, careful attention must be given to the routing of power and ground traces to minimize noise interference and ensure the product meets quality standards.
Every electronic engineer working on product design understands the potential for noise between the power and ground planes. This section focuses on how to reduce noise through effective design:
### 2. Grounding in Digital and Analog Circuits
Many modern PCBs integrate both digital and analog circuits, rather than focusing on a single function. As a result, it’s important to consider the mutual interference between these circuits during layout, especially the noise generated on the ground plane.
Digital circuits operate at high frequencies and are prone to generating noise, while analog circuits are highly sensitive to interference. For signal lines, high-frequency traces should be routed as far from analog-sensitive components as possible. Since the PCB has only one external connection point for the ground, the digital and analog grounds must be isolated within the board. These two grounds should not be directly connected to each other, except at a single point (e.g., at connectors or other external interfaces). In the PCB layout, ensure that the digital ground and analog ground are only connected at this singular point, which helps mitigate interference.
### 3. Routing on Power (Ground) Layers
In multi-layer PCB designs, it is common for signal layers to become crowded, making it difficult to route additional signals efficiently. To alleviate this, signal routing on power and ground layers can be considered. Power layers should be prioritized, followed by ground layers, to preserve the integrity of the circuit and minimize interference.
### 4. Connection of Component Leads in Large Ground Areas
When connecting component leads to large ground planes, a comprehensive approach is needed. For optimal electrical performance, component pads should be connected directly to the copper plane. However, there are potential issues during soldering and assembly, such as the need for high-power soldering equipment and the risk of cold or dry solder joints.
To mitigate these issues, it’s best to use crosshatched pads (often called thermal pads) to prevent excessive heat buildup during soldering. These pads reduce the likelihood of cold solder joints by distributing heat more effectively. This approach is also applicable to power and ground connections in multi-layer PCBs, ensuring a reliable and efficient assembly process.
