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Principles of PCB layout process design and EMC measures


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In PCB design, wiring is an important step to complete product design. PCB wiring includes single-sided wiring, double-sided wiring and multi-layer wiring. Parallel interference such as parasitic coupling will affect the stability of the line, and even cause the circuit board to change and fail to work when the interference is severe. The following aspects are generally considered in the PCB layout process design:

1. Consider the PCB size

When the PCB size is too large, the printed lines will be long, the impedance will increase, the anti-noise ability will decrease, and the cost will also increase; if the size is too small, the heat dissipation will not be good, and adjacent lines will be easily disturbed. The PCB size should be determined according to the specific circuit.

2. Determine the position of special components

Determining the position of special components is an important aspect of the PCB layout process. The layout of special components should pay attention to the following aspects:
① Shorten the connection between high-frequency components as much as possible, and try to reduce their distribution parameters and mutual electromagnetic interference. Components that are susceptible to interference cannot be too close to each other, and input and output components should be kept as far away as possible.
② There may be a high potential difference between some components or wires, and the distance between them should be increased to avoid short circuit caused by discharge. Components with high voltage should be arranged in places that are not easily accessible by hand during debugging.
③Components weighing more than 15g should be fixed with brackets and then welded. Those large, heavy and heat-generating components should not be installed on the PCB board, but should be installed on the chassis bottom plate of the whole machine, and the heat dissipation problem should be considered, and the heat-sensitive components should be kept away from the heating components.
④For the layout of adjustable components such as potentiometers, adjustable inductance coils, variable capacitors, and micro switches, the structural requirements of the whole machine should be considered. If it is adjusted in the machine, it should be placed on the printed board where it is easy to adjust; External adjustment, its position should be adapted to the position of the adjustment knob on the chassis panel. The positions used for the positioning holes of the PCB board and the fixing brackets should be reserved.

3. Layout

The layout method combining interactive layout and automatic layout is adopted. The layout method can be divided into two types, namely automatic layout and interactive layout. Before automatic routing, interactive pre-layout of lines with stricter requirements can be used to complete the layout. After the layout of special components, the layout of all components mainly follows the following principles:
①Arrange the position of each functional circuit unit according to the flow of the circuit, so that the layout is convenient for signal circulation, and the direction of the signal is kept as consistent as possible.
② Take the core components of each functional circuit as the center and make layout around it. Components should be arranged neatly and compactly on the PCB, minimizing and shortening the leads and connections between components.
③ For circuits working at high frequencies, the distribution parameters between components should be considered. In general, the components should be arranged in parallel as much as possible. In this way, it is not only beautiful but also easy to install and weld, and easy to mass produce.
④The components located on the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board. The best shape of the circuit board is a rectangle, and the aspect ratio should be 3:2 or 4:3. When the size of the circuit board is greater than 200*150mm, it should be Consider the mechanical strength to which the board is subjected.

4. Basic principles of power supply and grounding wire handling

The interference caused by insufficient consideration of the power supply and ground wire will degrade the performance of the product. Take some measures to reduce the noise interference generated by the power supply and ground wire to ensure the quality of the product. The methods are as follows:
①Add a decoupling capacitor between the power supply and the ground wire. A single power plane alone does not reduce noise because, regardless of current distribution, all systems can generate noise and cause problems, so additional filtering is required. Usually, a 1~10μF bypass capacitor is placed at the power input, and a 0.01~0.1μF capacitor is placed between the power pin and the ground pin of each component. The bypass capacitor acts as a filter, and the large capacitor (10μF) placed between the power supply and the ground on the board is to filter out low-frequency noise (such as 50/60Hz power frequency noise) generated on the board. The noise generated by the components working on the board usually resonates in the frequency range of 100MHz or higher, so the bypass capacitor placed between the power pin and the ground pin of each component is generally small (about 0.1μF ). It is best to place the capacitor on the other side of the board, directly under the component, even better if it is a surface mount capacitor.
② Try to widen the width of the power supply and ground wires. It is best that the ground wires are wider than the power wires. Their relationship is: ground wires>power wires>signal wires. Usually, the signal wire width is 0.2 ~ 0.3mm, and the thinnest width can reach 0.05~0.07mm, the power line is 1.2~2.5mm, a large area of copper layer is used as the ground wire, and the unused places on the printed board are connected to the ground as the ground wire. It is made into a multi-layer board, and the power supply and the ground wire each occupy one layer.
③ According to the principle of separating the digital ground and the analog ground. If there are both digital logic circuits and analog linear circuits on the circuit board, they should be separated as much as possible. The ground of the low-frequency circuit should be connected in parallel with a single point as much as possible. If the actual wiring is difficult, it can be partially connected in series and then connected in parallel. The high-frequency circuit should adopt multi-point series grounding, the ground wire should be short and thick, and the grid-shaped large-area ground foil should be used as much as possible around the high-frequency components to ensure that the ground wire forms a closed loop.

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