Printed circuit board (PCB) is the support of circuit components and components in electronic products. It provides the electrical connection between circuit components and components. It is the most basic component of various electronic equipment, and its performance is directly related to the quality of electronic equipment. With the development of information society, all kinds of electronic products often work together, and the interference between them is more and more serious. Therefore, electromagnetic compatibility becomes the key to the normal operation of an electronic system. Similarly, with the development of electrical technology, the density of PCB is getting higher and higher. The quality of PCB design has a great influence on the interference and anti-interference ability of circuit. In addition to the choice of components and circuit design, good PCB wiring is also a very important factor in electromagnetic compatibility for optimal performance of electronic circuits.

Since the PCB is an inherent component of the system, enhancing electromagnetic compatibility in the PCB wiring does not incur additional costs to the final product completion. However, in printed circuit board design, product designers often only pay attention to improve density, reduce the occupation of space, simple production, or the pursuit of beautiful, uniform layout, ignoring the impact of circuit layout on electromagnetic compatibility, so that a large number of signals radiation into the space to form harassment. A poor PCB wiring can cause more emc problems than it can eliminate. In many cases, even adding filters and components does not solve these problems. Eventually, the entire board had to be rewired. Therefore, developing good PCB wiring habits is the most cost-effective way to start.

One thing to note is that there are no strict rules for PCB wiring and no specific rules that cover all PCB wiring. Most PCB wiring is limited by the size of the circuit board and the number of copper-clad layers. Some wiring techniques that can be applied to one circuit but not to another depend on the experience of the wiring engineer. There are, however, some general rules, which will be discussed below.

For good design quality. PCB with low cost should follow the following general principles:

2. Layout of components on PCB

First of all, it is necessary to consider the PCB size is too large. When the PCB size is too large, the printed line is long, the impedance increases, the anti-noise ability decreases, and the cost increases. Too small, the heat dissipation is not good, and adjacent lines are susceptible to interference. After determining the PCB size. Then locate the special components. Finally, according to the functional unit of the circuit, all the components of the circuit are laid out.

A digital circuit in an electronic device. Analog circuit and power circuit components layout and wiring characteristics are different, they produce interference and interference suppression methods are different. Also high frequency. Because of the different frequency, the interference of low frequency circuit and the method of suppressing interference are different. So in component layout, the digital circuit should be. The analog circuit and the power supply circuit are placed separately to separate the high frequency circuit from the low frequency circuit. If there are conditions, they should be isolated or made into a circuit board separately. In addition, the layout should also pay special attention to strong. Weak signal device distribution and signal transmission direction.

In printed board layout high speed. For medium-speed and low-speed logic circuits, the components should be arranged in the manner shown in Figure 1-1.

As with other logic circuits, the components should be placed as close as possible to each other so as to achieve better anti-noise effect. The position of components on the PRINTED circuit board should take full account of emi. One of the principles is to keep the leads between components as short as possible. In terms of layout, the analog signal part, high-speed digital circuit part, and noise source part (such as relay, high current switch, etc.) should be properly separated to minimize the signal coupling between them, as shown in Figure 1-②.

Clock generator. Crystal oscillator and CPU clock input are prone to noise, to be closer to each other. Noisy devices. Low current circuit. Large current circuits should be kept away from logic circuits as far as possible. It is important to make a separate circuit board if possible.

2.1 The following principles shall be observed when determining the location of special components: (1) Shorten the connection between high-frequency components as far as possible, and try to reduce their distribution parameters and electromagnetic interference between each other. Easily disturbed components should not be too close to each other, and input and output components should be as far away as possible.

(2) There may be a high potential difference between some components or wires, so the distance between them should be increased to avoid accidental short circuit caused by discharge. Components with high voltage should be as far as possible placed in places not easily accessible by hand during debugging.

(3) Components whose weight exceeds 15g. It should be braced and then welded. Those are big and heavy. The components with high calorific value should not be installed on the printed board, but on the chassis of the whole machine, and the problem of heat dissipation should be considered. Thermal elements should be kept away from heating elements.

(4) for potentiometer. Adjustable inductor coil. Variable capacitor. The layout of adjustable components such as microswitch should consider the structural requirements of the whole machine. If the machine adjustment, should be placed on the printed board above easy to adjust the place; If the machine is adjusted outside, its position should be adapted to the position of the adjusting knob on the chassis panel.

(5) The position occupied by the positioning hole and fixing bracket of the printed board should be set aside.

2.2 Layout of all components of the circuit according to the functional units of the circuit shall comply with the following principles:

(1) Arrange the position of each functional circuit unit according to the circuit process, so that the layout is convenient for signal flow and the signal keeps the same direction as far as possible.

(2) To the core components of each functional circuit as the center, around it to carry out the layout. Components should be uniform. And tidy. Compact arrangement on PCB to minimize and shorten leads and connections between components. (3) For circuits working at high frequencies, the distributed parameters between components should be considered. In general circuits, components should be arranged in parallel as much as possible. In this way, not only beautiful, and easy to install welding, easy to mass production.

(4) Components located at the edge of the circuit board, generally not less than 2mm from the edge of the circuit board. The best shape of a circuit board is a rectangle. Length to width ratio 3:2 or 4:3. The size of the circuit board is greater than 200x150mm. Consideration should be given to the mechanical strength of the circuit board.

2.3 General layout requirements for PCB components:

Circuit elements and signal paths must be laid out to minimize the coupling of unwanted signals:

(1) The low electronic signal channel should not be close to the high level signal channel and the power line without filtering, including the circuit that can produce transient process.

(2) Separate the low level analog circuit from the digital circuit to avoid the analog circuit. The digital circuit and the common loop of the power supply produce common impedance coupling.

(3) high. In the. Low speed logic circuits use different areas on the PCB.

(4) The length of signal line should be minimized when the circuit is arranged

(5) Ensure between adjacent plates. Between adjacent layers of the same board. Do not have excessively long parallel signal cables between adjacent cables at the same layer.

(6) Electromagnetic interference (EMI) filter should be as close to EMI source as possible and placed on the same circuit board.

(7) DC/DC converter. Switching elements and rectifiers should be placed as close to the transformer as possible to minimize the length of their wires

(8) Place the voltage regulating element and filter capacitor as close as possible to the rectifier diode.

(9) The printed board is divided according to frequency and current switching characteristics, and the noise element and the non-noise element should be farther away.

(10) The noise-sensitive wiring should not be parallel to the high-current, high-speed switching line.