At present, electronic devices continue to utilize printed circuit boards (PCBs) as the primary assembly method in various systems. Experience has demonstrated that even if the circuit schematic design is correct, improper PCB design can negatively impact the reliability of electronic equipment. For instance, if two narrow parallel traces on a PCB are positioned too close together, it can lead to signal waveform delays and generate reflection noise at the end of the transmission line. Therefore, it is crucial to employ the correct methods during PCB design.
1. PCB Design Ground Wire Design
In electronic equipment, grounding is a vital technique for managing interference. Proper integration of grounding and shielding can effectively address most interference issues. The grounding structure of electronic equipment typically comprises system ground, chassis ground (shield ground), digital ground (logical ground), and analog ground. Attention should be paid to the following points in ground wire design:
1. Proper Selection of Single-Point and Multi-Point Grounding
In low-frequency circuits, where the signal frequency is below 1 MHz, wiring and device inductance have minimal impact, while the circulating currents in the grounding circuit significantly influence interference; hence, single-point grounding is recommended. When the signal frequency exceeds 10 MHz, the ground wire impedance increases substantially. In such cases, it’s essential to minimize ground wire impedance by using multiple nearby grounding points. For working frequencies between 1 and 10 MHz, if single-point grounding is utilized, the ground wire length should not exceed 1/20 of the wavelength; otherwise, multi-point grounding should be employed.
2. Separate Digital Circuits from Analog Circuits
The circuit board comprises both high-speed logic circuits and linear circuits. It is essential to keep these circuits as far apart as possible, ensuring their ground wires are not interconnected. Each type should connect to its respective ground wire at the power supply terminal. Additionally, aim to maximize the grounding area for the linear circuit.
3. Use Thick Ground Wires
Thin ground wires can cause fluctuations in ground potential with changes in current, leading to instability in timing signals and reduced noise immunity. Therefore, the ground wire should be as thick as feasible to handle the permissible current on the printed circuit board. Ideally, the width of the ground wire should exceed 3mm.
4. Create a Closed Loop for Ground Wires
In designing the ground wire system for printed circuit boards containing only digital circuits, forming the ground wire into a closed loop can greatly enhance noise immunity. This is particularly important due to the presence of numerous integrated circuit components, especially those with high power consumption. The limitations imposed by thin ground wires can generate significant potential differences at ground junctions, diminishing noise resistance. A looped grounding structure reduces this potential difference, thereby improving the noise immunity of electronic equipment.
Two, PCB Design for Electromagnetic Compatibility
Electromagnetic compatibility (EMC) refers to the ability of electronic equipment to function harmoniously within diverse electromagnetic environments. The objective of EMC design is to enable electronic devices to suppress various external interferences, ensuring normal operation in specified electromagnetic environments while minimizing the electromagnetic interference emitted by the devices themselves.
1. Select Appropriate Wire Width
The impact interference from transient currents on printed lines is largely influenced by the inductance of these traces, which should be minimized. The inductance is directly proportional to the trace length and inversely proportional to its width, making shorter, wider traces advantageous for reducing interference. Signal lines, such as clock leads and row or bus drivers, often carry significant transient currents, necessitating short traces. For discrete component circuits, a trace width of approximately 1.5mm suffices, while for integrated circuits, widths can range from 0.2mm to 1.0mm.
2. Implement an Effective Routing Strategy
Employing uniform routing can lower wire inductance; however, it also increases mutual inductance and distributed capacitance among traces. If layout permits, a grid-based routing structure is ideal. This involves routing one side of the PCB horizontally and the opposite side vertically, connecting them through vias at intersecting points. To minimize crosstalk between traces, avoid long-distance uniform routing during the design phase.
Three, Configuration of Decoupling Capacitors in Circuit Board Design
In the DC power supply loop, load variations can induce power supply noise. For instance, in digital circuits, transitions between states can generate large spike currents on the power line, creating transient noise voltages. The strategic placement of decoupling capacitors effectively mitigates noise from load changes, a common practice for enhancing the reliability of printed circuit boards. The configuration principles include:
– A 10-100µF electrolytic capacitor should be connected across the power input terminal. If space allows, using an electrolytic capacitor above 100µF can yield even better anti-interference results.
– A 0.01µF ceramic capacitor should be placed for each integrated circuit chip. If space is limited, one 1-10µF tantalum electrolytic capacitor may be used for every 4-10 chips. This capacitor type offers low high-frequency impedance, under 1Ω from 500kHz to 20MHz, and has minimal leakage current (below 0.5µA).
– For devices sensitive to noise and experiencing significant current changes during shutoff, such as ROM and RAM, a decoupling capacitor should be directly connected between the power line (Vcc) and ground (GND) of the chip.
– The leads of decoupling capacitors should be kept short, particularly for high-frequency bypass capacitors.
Four, PCB Design: Size and Device Layout
The size of the PCB should be appropriate; overly large boards result in longer printed traces, increasing impedance, which diminishes noise immunity and elevates costs. In terms of device layout, devices that are interconnected should be positioned closely together to enhance noise resistance. Components like clock generators, crystal oscillators, and CPU clock inputs, which are prone to noise, should be placed near each other. It is crucial to maintain distance between noise-sensitive devices, low-current circuits, and high-current circuits from logic circuits, and, if feasible, utilize separate circuit boards.
5. Circuit Board Design and Heat Dissipation
From a thermal management perspective, it is optimal to install the PCB upright, ensuring a minimum spacing of 2cm between boards. The arrangement of components on the PCB should follow specific guidelines:
– For equipment relying on free convection air cooling, integrated circuits (or other devices) should be oriented vertically; for forced air cooling, a horizontal arrangement is preferable.
– Components on the same PCB should be organized based on their heat generation and dissipation characteristics. Devices with low heat output or poor heat resistance (e.g., small signal transistors, small-scale integrated circuits, electrolytic capacitors) should be positioned within the cooling airflow, preferably at the entrance, while high-heat or heat-resistant devices (e.g., power transistors, large-scale integrated circuits) should be placed downstream in the airflow.
– In the horizontal layout, position high-power devices close to the edge of the PCB to minimize the heat transfer path; vertically, high-power devices should be near the top of the PCB to mitigate their thermal impact on other components.
– Temperature-sensitive devices should be located in the coolest areas (e.g., at the bottom) and should not be directly above heating devices. Staggering multiple devices on the horizontal plane is also advisable.
– PCB heat dissipation primarily relies on airflow, so the design should account for airflow paths and strategically configure devices or printed circuit boards. Since air tends to flow through low-resistance areas, avoid creating large air pockets in specific sections of the layout.
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Feel free to ask for further refinements or any specific adjustments!
1. PCB Design Ground Wire Design
In electronic equipment, grounding is a vital technique for managing interference. Proper integration of grounding and shielding can effectively address most interference issues. The grounding structure of electronic equipment typically comprises system ground, chassis ground (shield ground), digital ground (logical ground), and analog ground. Attention should be paid to the following points in ground wire design:
1. Proper Selection of Single-Point and Multi-Point Grounding
In low-frequency circuits, where the signal frequency is below 1 MHz, wiring and device inductance have minimal impact, while the circulating currents in the grounding circuit significantly influence interference; hence, single-point grounding is recommended. When the signal frequency exceeds 10 MHz, the ground wire impedance increases substantially. In such cases, it’s essential to minimize ground wire impedance by using multiple nearby grounding points. For working frequencies between 1 and 10 MHz, if single-point grounding is utilized, the ground wire length should not exceed 1/20 of the wavelength; otherwise, multi-point grounding should be employed.
2. Separate Digital Circuits from Analog Circuits
The circuit board comprises both high-speed logic circuits and linear circuits. It is essential to keep these circuits as far apart as possible, ensuring their ground wires are not interconnected. Each type should connect to its respective ground wire at the power supply terminal. Additionally, aim to maximize the grounding area for the linear circuit.
3. Use Thick Ground Wires
Thin ground wires can cause fluctuations in ground potential with changes in current, leading to instability in timing signals and reduced noise immunity. Therefore, the ground wire should be as thick as feasible to handle the permissible current on the printed circuit board. Ideally, the width of the ground wire should exceed 3mm.
4. Create a Closed Loop for Ground Wires
In designing the ground wire system for printed circuit boards containing only digital circuits, forming the ground wire into a closed loop can greatly enhance noise immunity. This is particularly important due to the presence of numerous integrated circuit components, especially those with high power consumption. The limitations imposed by thin ground wires can generate significant potential differences at ground junctions, diminishing noise resistance. A looped grounding structure reduces this potential difference, thereby improving the noise immunity of electronic equipment.
Two, PCB Design for Electromagnetic Compatibility
Electromagnetic compatibility (EMC) refers to the ability of electronic equipment to function harmoniously within diverse electromagnetic environments. The objective of EMC design is to enable electronic devices to suppress various external interferences, ensuring normal operation in specified electromagnetic environments while minimizing the electromagnetic interference emitted by the devices themselves.
1. Select Appropriate Wire Width
The impact interference from transient currents on printed lines is largely influenced by the inductance of these traces, which should be minimized. The inductance is directly proportional to the trace length and inversely proportional to its width, making shorter, wider traces advantageous for reducing interference. Signal lines, such as clock leads and row or bus drivers, often carry significant transient currents, necessitating short traces. For discrete component circuits, a trace width of approximately 1.5mm suffices, while for integrated circuits, widths can range from 0.2mm to 1.0mm.
2. Implement an Effective Routing Strategy
Employing uniform routing can lower wire inductance; however, it also increases mutual inductance and distributed capacitance among traces. If layout permits, a grid-based routing structure is ideal. This involves routing one side of the PCB horizontally and the opposite side vertically, connecting them through vias at intersecting points. To minimize crosstalk between traces, avoid long-distance uniform routing during the design phase.
Three, Configuration of Decoupling Capacitors in Circuit Board Design
In the DC power supply loop, load variations can induce power supply noise. For instance, in digital circuits, transitions between states can generate large spike currents on the power line, creating transient noise voltages. The strategic placement of decoupling capacitors effectively mitigates noise from load changes, a common practice for enhancing the reliability of printed circuit boards. The configuration principles include:
– A 10-100µF electrolytic capacitor should be connected across the power input terminal. If space allows, using an electrolytic capacitor above 100µF can yield even better anti-interference results.
– A 0.01µF ceramic capacitor should be placed for each integrated circuit chip. If space is limited, one 1-10µF tantalum electrolytic capacitor may be used for every 4-10 chips. This capacitor type offers low high-frequency impedance, under 1Ω from 500kHz to 20MHz, and has minimal leakage current (below 0.5µA).
– For devices sensitive to noise and experiencing significant current changes during shutoff, such as ROM and RAM, a decoupling capacitor should be directly connected between the power line (Vcc) and ground (GND) of the chip.
– The leads of decoupling capacitors should be kept short, particularly for high-frequency bypass capacitors.
Four, PCB Design: Size and Device Layout
The size of the PCB should be appropriate; overly large boards result in longer printed traces, increasing impedance, which diminishes noise immunity and elevates costs. In terms of device layout, devices that are interconnected should be positioned closely together to enhance noise resistance. Components like clock generators, crystal oscillators, and CPU clock inputs, which are prone to noise, should be placed near each other. It is crucial to maintain distance between noise-sensitive devices, low-current circuits, and high-current circuits from logic circuits, and, if feasible, utilize separate circuit boards.
5. Circuit Board Design and Heat Dissipation
From a thermal management perspective, it is optimal to install the PCB upright, ensuring a minimum spacing of 2cm between boards. The arrangement of components on the PCB should follow specific guidelines:
– For equipment relying on free convection air cooling, integrated circuits (or other devices) should be oriented vertically; for forced air cooling, a horizontal arrangement is preferable.
– Components on the same PCB should be organized based on their heat generation and dissipation characteristics. Devices with low heat output or poor heat resistance (e.g., small signal transistors, small-scale integrated circuits, electrolytic capacitors) should be positioned within the cooling airflow, preferably at the entrance, while high-heat or heat-resistant devices (e.g., power transistors, large-scale integrated circuits) should be placed downstream in the airflow.
– In the horizontal layout, position high-power devices close to the edge of the PCB to minimize the heat transfer path; vertically, high-power devices should be near the top of the PCB to mitigate their thermal impact on other components.
– Temperature-sensitive devices should be located in the coolest areas (e.g., at the bottom) and should not be directly above heating devices. Staggering multiple devices on the horizontal plane is also advisable.
– PCB heat dissipation primarily relies on airflow, so the design should account for airflow paths and strategically configure devices or printed circuit boards. Since air tends to flow through low-resistance areas, avoid creating large air pockets in specific sections of the layout.
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Feel free to ask for further refinements or any specific adjustments!
