At present, the printed circuit board is still the main assembly method for all kinds of electronic equipment and systems. Practice has proved that even if the circuit schematic design is correct and the printed circuit board is improperly designed, the reliability of electronic equipment will be adversely affected. For example, if the two thin parallel lines of the printed board are close together, a delay in the signal waveform will be formed, forming reflected noise at the end of the transmission line. Therefore, when designing printed circuit boards, attention should be paid to the correct method.
A, ground design
    In electronic equipment, grounding is an important method to control interference. If the grounding and shielding can be correctly combined, most interference problems can be solved. The ground wire structure in electronic equipment is roughly systematic, shell ground (shield ground), digital ground (logic) and analog ground. The following points should be noted in the design of ground wire:
    1. The correct choice of single point grounding and multi-point grounding in the low-frequency circuit, the signal operating frequency is less than 1MHz, its wiring and the inductance between the device has less influence, and the grounding circuit formed by the circulation has a greater impact on the interference, so should use a point grounding. When the signal operating frequency is greater than 10MHz, the ground impedance becomes large. In this case, the ground impedance should be reduced as much as possible, and the nearest multipoint grounding should be used. When the operating frequency is 1 ~ 10MHz, if one point grounding is used, the ground length should not exceed 1/20 of the wavelength, otherwise the multi-point grounding method should be used.
    2. Separate the digital circuit from the analog circuit. Both high-speed logic circuits and linear circuits on the circuit board should be separated as far as possible, and the ground wires of the two should not be mixed and connected to the ground wires of the power supply. The ground area of the linear circuit should be increased as much as possible.
    3. Make the ground wire thicker as far as possible If the ground wire is very thin, the ground potential changes with the change of current, resulting in the timing signal level of the electronic device is unstable, and the anti-noise performance deteriorates. Therefore, the ground wire should be as thick as possible, so that it can pass three allowable currents located on the printed circuit board. If possible, the ground cable should be wider than 3mm.
    4. When designing the ground wire as a closed-loop circuit system composed only of digital circuit boards, making the ground wire as a closed-loop circuit can significantly improve the anti-noise ability. The reason is that there are many integrated circuit components on the printed circuit board, especially in the case of power-consumption components, due to the limitation of the thickness of the grounding wire, it will produce a large potential difference on the ground junction, resulting in a decrease in anti-noise ability, if the grounding structure into a loop, it will reduce the potential difference value and improve the anti-noise ability of electronic equipment.
B, electromagnetic compatibility design
    Electromagnetic compatibility refers to the ability of electronic devices to work harmoniously and effectively in various electromagnetic environments. The purpose of electromagnetic compatibility design is to enable electronic equipment to suppress various external interference, so that electronic equipment can work normally in a specific electromagnetic environment, and at the same time reduce the electromagnetic interference of electronic equipment itself on other electronic equipment.
    1. Choose a reasonable wire width Because the impact interference caused by the transient current on the printed line is mainly caused by the inductive component of the printed wire, so the inductance of the printed wire should be minimized. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so a short and fine wire is advantageous for suppressing interference. Signal lines for clock leads, line drivers, or bus drivers often carry large transient currents, and printed wires should be as short as possible. For discrete component circuits, when the width of the printed wire is about 1.5mm, the requirements can be fully met; For integrated circuits, the printed wire width can be selected between 0.2 and 1.0mm. 2. Using the correct wiring strategy The use of equal wiring can reduce the inductance of the wire, but the mutual inductance and distributed capacitance between the wires increase, if the layout allows, it is best to use the well pattern network wiring structure, the specific approach is one side of the printed board transverse wiring, the other side longitudinal wiring, and then connected with metalized holes at the cross hole. In order to suppress the crosstalk between the printed board wires, long-distance equal wiring should be avoided as far as possible when designing the wiring.
C, decoupling capacitor configuration
    In the DC power supply circuit, the change of load will cause the power supply noise. For example, in a digital circuit, when the circuit is converted from one state to another, a large spike current is generated on the power line, forming a transient noise voltage. The configuration of decoupling capacitors can suppress noise caused by load changes and is a common practice in the reliability design of printed circuit boards. The configuration principles are as follows:
    ● The power input end is connected to an electrolytic capacitor of 10 ~ 100uF. If the position of the printed circuit board allows, the anti-interference effect of the electrolytic capacitor above 100uF will be better.
    ● Configure a 0.01uF ceramic capacitor for each IC chip. If the printed circuit board space is small and cannot be installed, every 4 to 10 chips can be configured with a 1 to 10uF tantalum electrolytic capacitor, the high-frequency impedance of this device is particularly small, the impedance is less than 1Ω in the range of 500kHz to 20MHz, and the leakage current is very small (below 0.5uA).
    ● For devices with weak noise capacity and large current change when turned off, and memory devices such as ROM and RAM, the decoupling capacitor should be directly connected between the power line (Vcc) and the ground (GND) of the chip.
    ● The lead of the decoupling capacitor cannot be too long, especially the high-frequency bypass capacitor cannot have a lead.
D, the size of the printed circuit board and the layout of the device
    The size of the printed circuit board should be moderate, too large when the printed line is long, the impedance increases, not only the anti-noise ability decreases, the cost is high; Too small, the heat dissipation is not good, and it is susceptible to interference from nearby lines. In terms of device layout, as with other logic circuits, the related devices should be placed as close as possible, so that a better anti-noise effect can be obtained. The clock generator, crystal oscillator and CPU clock input are prone to noise, to be closer to each other. Noise prone devices, small current circuits, large current circuits, etc. should be as far away from the logic circuit as possible, if possible, should be another circuit board, this is very important.
E, heat dissipation design
    From the point of view that is conducive to heat dissipation, the printed plate is best installed upright, the distance between the board and the board should generally not be less than 2cm, and the arrangement of the device on the printed plate should follow certain rules:
    For devices that use free convection air cooling, it is best to arrange integrated circuits (or other devices) in a longitudinal manner; For devices that use forced air cooling, it is best to arrange the integrated circuit (or other device) in a horizontal way.
    The devices on the same printed board should be arranged as far as possible according to the size of their heat and the degree of heat dissipation, and the devices with small heat or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed at the upper stream (entrance) of the cooling air flow. Devices with large heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed at the downstream of the cooling stream.
    In the horizontal direction, the high-power devices are arranged as close as possible to the edge of the printed board in order to shorten the heat transfer path; In the vertical direction, the high-power devices are arranged as close as possible to the printed board, in order to reduce the impact of these devices on the temperature of other devices when they work.
    The device that is more sensitive to temperature is best placed in the lowest temperature area (such as the bottom of the device), do not put it right above the heating device, and multiple devices are best staggered on the horizontal plane.
    The heat dissipation of the printed board in the equipment mainly depends on the air flow, so the air flow path should be studied in the design, and the device or printed circuit board should be reasonably configured. When the air flows, it always tends to flow where the resistance is low, so when configuring the device on the printed circuit board, it is necessary to avoid leaving a large airspace in a certain area.