Electromagnetic compatibility (EMC) is a key indicator for measuring electronic products. The layout of the PCB and the arrangement of components within the product serve as both sources of interference and targets for interference. Reducing and mitigating these electromagnetic interferences is crucial for enhancing the EMC of products. This article discusses the design aspects of PCB layout, component placement, and wiring to improve the electromagnetic interference resistance of electronic products.
With the development of the electrical era, various sources of electromagnetic waves have proliferated in human living environments, such as radio broadcasts, television, microwave communications, household appliances, power transmission line electromagnetic fields, and high-frequency electromagnetic fields. When the intensity of these electromagnetic fields exceeds a certain limit and persists for a sufficient duration, they may pose risks to human health and interfere with other electronic devices and communications. Therefore, protective measures are necessary. During the development, production, and use of electronic products, concepts such as electromagnetic interference and shielding are often raised. The core of normal operation of electronic products is the coordinated working process between the circuit board and the components mounted on it. It is essential to enhance the performance indicators of electronic products and reduce the impact of electromagnetic interference.
1. PCB Design
Printed circuit boards (PCBs) are the supporting components for electronic circuit elements and devices, providing electrical connections between them. They are the fundamental building blocks of various electronic devices, and the performance of the PCB directly affects the quality and performance of the electronic equipment. With the advancement of integrated circuits, SMT technology, and micro-assembly technology, high-density and multifunctional electronic products are becoming increasingly common, leading to complex wiring, numerous components, and dense installations on PCBs, which inevitably intensifies interference among them. Therefore, suppressing electromagnetic interference has become a key factor in whether an electronic system can operate normally. Similarly, as technology advances, the density of PCBs continues to increase, and the quality of PCB design significantly impacts circuit interference and anti-interference capabilities. To achieve optimal performance in electronic circuits, good PCB design is a crucial factor in electromagnetic compatibility, in addition to component selection and circuit design.
1.1 Reasonable PCB Layer Design
Choosing the appropriate number of PCB layers based on the complexity of the circuit can effectively reduce electromagnetic interference, significantly decrease the PCB size, and shorten the length of current loops and branch wiring, thereby greatly reducing cross-interference between signals. Experiments have shown that, with the same material, a four-layer board has 20 dB lower noise than a two-layer board. However, as the number of layers increases, the manufacturing process becomes more complex, and the manufacturing cost rises. In multilayer board wiring, it is best to use a “zigzag” mesh wiring structure between adjacent layers, meaning that the wiring direction of adjacent layers should be perpendicular to each other. For example, horizontal wiring on the top layer and vertical wiring on the next layer, connected by vias.
1.2 Reasonable PCB Size Design
If the PCB size is too large, it will lead to increased trace lengths, higher impedance, reduced noise immunity, and increased device size and cost. If the size is too small, it will result in poor heat dissipation and increased susceptibility to interference from adjacent traces. Generally, the physical boundary of the PCB’s outer dimensions is determined on the mechanical layer, while the keepout layer defines the effective area for layout and wiring. The optimal shape and size of the PCB are usually determined based on the number of functional units in the circuit and the overall arrangement of all components. Rectangular shapes with a length-to-width ratio of 3:2 are typically chosen. For circuit board dimensions greater than 150 mm x 200 mm, mechanical strength should be considered.
2. PCB Layout
In PCB design, product designers often focus solely on increasing density, minimizing space usage, simplifying production, or pursuing aesthetics and uniform layout, neglecting the impact of circuit layout on electromagnetic compatibility, which can lead to significant signal radiation and mutual interference. Poor PCB wiring can lead to more electromagnetic compatibility issues rather than eliminating them.
The layout and wiring of components in digital circuits, analog circuits, and power circuits differ in characteristics, and the interference they generate and methods for suppressing interference vary. Due to frequency differences, high-frequency and low-frequency circuits have different interference characteristics and suppression methods. Therefore, when arranging components, digital circuits, analog circuits, and power circuits should be placed separately, and high-frequency circuits should be isolated from low-frequency circuits. If conditions allow, they should be isolated or made into separate circuit boards. Special attention should also be paid to the distribution of strong and weak signal devices and the direction of signal transmission.
2.1 Component Layout on PCB
The layout of PCB components, like other logic circuits, should place related devices as close as possible to achieve better noise immunity. The positions of components on the printed circuit board should fully consider electromagnetic interference issues. One principle is to keep the leads between components as short as possible. In layout, the analog signal section, high-speed digital circuit section, and noise source section (such as relays, high-current switches, etc.) should be reasonably separated to minimize signal coupling between them.
Clock generators, crystal oscillators, and the clock input pins of CPUs are prone to noise and should be placed close to each other. Noise-prone devices, small current circuits, and large current circuits should be kept as far away from logic circuits as possible. If feasible, separate circuit boards should be used for these components, which is very important.
General layout requirements for PCB components: The layout of circuit elements and signal paths must minimize unwanted signal coupling.
1) Low-level signal paths should not be close to high-level signal paths and unfiltered power lines, including circuits that can generate transient processes.
2) Low-level analog circuits and digital circuits should be separated to avoid common impedance coupling from shared return paths.
3) High, medium, and low-speed logic circuits should occupy different areas on the PCB.
4) When arranging circuits, minimize the length of signal lines.
5) Ensure that there are no excessively long parallel signal lines between adjacent boards, adjacent layers of the same board, or adjacent wiring on the same layer.
6) EMI filters should be placed as close as possible to the electromagnetic interference source and on the same circuit board.
7) DC/DC converters, switching elements, and rectifiers should be placed as close as possible to transformers to minimize wire lengths.
8) Voltage regulation components and filter capacitors should be placed as close as possible to rectifier diodes.
9) The printed board should be partitioned according to frequency and current switching characteristics, with greater distances between noise-generating and non-noise-generating components.
10) Sensitive wiring should not run parallel to high current or high-speed switching lines.
11) Component layout should also pay special attention to heat dissipation. For high-power circuits, heat-generating components such as power transistors and transformers should be dispersed and placed near the edges to facilitate heat dissipation, avoiding concentration in one area and keeping high capacitance components at a distance to prevent premature aging of electrolytic fluid.
2.2 PCB Wiring
A PCB consists of a multilayer structure using a series of laminations, routing, and pre-impregnation. In multilayer PCBs, signal lines are often routed on the outermost layer for ease of debugging.
In high-frequency situations, the distribution inductance and capacitance of traces, vias, resistors, capacitors, and connectors on the printed circuit board cannot be ignored. Resistors can reflect and absorb high-frequency signals. The distributed capacitance of traces also plays a role. When the trace length exceeds 1/20 of the corresponding wavelength of the noise frequency, an antenna effect occurs, causing noise to be emitted outward through the traces.
Connections on printed circuit boards are mostly completed through vias. A via can introduce approximately 0.5 pF of distributed capacitance, and reducing the number of vias can significantly improve speed.
An integrated circuit’s packaging material introduces 2 to 6 pF of capacitance. A connector on a circuit board has 520 nH of distributed inductance. A 24-pin dual in-line package IC socket introduces 4 to 18 nH of distributed inductance.
General requirements to avoid the impact of PCB wiring distributed parameters include:
1) Increase the spacing of traces to reduce capacitive coupling crosstalk.
2) When wiring double-sided boards, the traces on both sides should be perpendicular, diagonal, or curved to avoid parallelism, thereby reducing parasitic coupling; printed traces used for circuit inputs and outputs should avoid adjacent parallelism to prevent feedback, and it is best to add ground lines between these traces.
3) Sensitive high-frequency lines should be routed away from high-noise power lines to reduce mutual coupling; high-frequency digital circuit traces should be thinner and shorter.
4) Widen power and ground lines to reduce their impedance.
5) Use 45° bends instead of 90° bends in wiring to minimize high-frequency signal emissions and coupling.
6) For address or data lines, the length differences should not be too large; otherwise, shorter lines should be bent to compensate.
7) High current signals, high voltage signals, and small signals should be isolated (the isolation distance depends on the withstand voltage; typically, at 2 kV, the distance should be 2 mm, and for higher voltages, the distance should be proportionally increased. For example, for a 3 kV withstand voltage test, the distance between high and low voltage lines should be more than 3.5 mm. In many cases, grooves are also made on the printed circuit board between high and low voltage areas to avoid creeping discharge).
3. Circuit Design in PCB
When designing electronic circuits, practical performance is often prioritized over electromagnetic compatibility characteristics and suppression of electromagnetic interference. To achieve electromagnetic compatibility during PCB layout based on circuit schematics, necessary measures must be taken, such as adding additional circuits to enhance the product’s electromagnetic compatibility performance. The following circuit measures can be adopted in actual PCB design:
1) A resistor can be connected in series on the PCB traces to reduce the rise and fall rates of control signal lines.
2) Provide some form of damping (high-frequency capacitors, reverse diodes, etc.) for relays.
3) Signals entering the printed board should be filtered, and signals transitioning from high noise areas to low noise areas should also be filtered, while using series termination resistors to reduce signal reflections.
4) Unused MCU pins should be connected to power or ground through appropriate matching resistors or defined as output pins. All pins on integrated circuits that should be connected to power or ground must be connected, avoiding floating pins.
5) Unused gate inputs should not be left floating but should be connected to power or ground through appropriate matching resistors. The positive input of unused operational amplifiers should be grounded, and the negative input should be connected to the output.
6) Each integrated circuit should have a high-frequency decoupling capacitor. A small high-frequency bypass capacitor should be added next to each electrolytic capacitor.
7) Use large-capacity tantalum or polyester capacitors instead of electrolytic capacitors for energy storage capacitors on the circuit board. When using tubular capacitors, the casing should be grounded.
4. Conclusion
With the continuous advancement of technology, the miniaturization and intelligence of various electronic devices have become mainstream trends. At the same time, the operating environments of electronic products or devices are becoming increasingly complex, requiring continuous development and maturation of anti-interference technology and electromagnetic compatibility technology.