6 PCB design principles you must know.

菲利盟电子

6 PCB design principles you must know. [Copy link]

1. Layout First, consider the size of the PCB. When the PCB size is too large, the printed lines are long, the impedance increases, the anti-noise ability decreases, and the cost increases; if it is too small, the heat dissipation is poor, and the adjacent lines are susceptible to interference. After determining the PCB size, determine the location of special components. Finally, according to the functional units of the circuit, all the components of the circuit are laid out. When determining the location of special components, the following principles must be followed: (1) Shorten the connection between high-frequency components as much as possible, and try to reduce their distributed parameters and mutual electromagnetic interference. Components that are susceptible to interference should not 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 discharge causing accidental short circuits. Components with high voltage should be placed as far as possible in places that are not easily touched by hands during debugging. (3) Space should be reserved for the positioning holes of the printed board and the fixed bracket. When laying out all the components of the circuit according to the functional units of the circuit, the following principles must be followed: (1) Arrange the positions of each functional circuit unit according to the flow of the circuit, so that the layout is convenient for signal flow and the signal can maintain the same direction as much as possible. (2) Take the core component of each functional circuit as the center and arrange the layout around it. The components should be arranged evenly, neatly and compactly on the PCB. Try to reduce and shorten the leads and connections between the components. (3) For circuits working at high frequencies, the distribution parameters between the components should be considered. In general, the components should be arranged in parallel as much as possible in the circuit. In this way, it is not only beautiful, but also easy to install and solder, and easy to mass produce. (4) The components located at 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. The aspect ratio is 3:2 to 4:3. When the surface size of the circuit board is larger than 200x150mm, the mechanical strength of the circuit board should be considered. 2. Wiring The wiring principles are as follows: (1) The wires used for input and output terminals should be avoided to be adjacent and parallel as much as possible. It is best to add a ground wire between the wires to avoid feedback coupling. (2) The minimum width of the printed wiring is mainly determined by the adhesion strength between the wire and the insulating base and the current value flowing through them. (3) The bends of printed conductors are generally in the shape of arcs, while right angles or included angles will affect the electrical performance in high-frequency circuits. In addition, try to avoid using large areas of copper foil, otherwise, when heated for a long time, the copper foil is prone to expansion and falling off. When large areas of copper foil must be used, it is best to use a grid shape. This is conducive to removing the volatile gas generated by the heat of the adhesive between the copper foil and the substrate. 3. The center hole of the pad (through-hole device) should be slightly larger than the diameter of the device lead. A pad that is too large is prone to cold soldering. The outer diameter D of the pad is generally not less than (d+1.2) mm, where d is the lead hole diameter. For high-density digital circuits, the minimum diameter of the pad can be (d+1.0) mm. PCB and circuit anti-interference measures: The anti-interference design of the printed circuit board is closely related to the specific circuit. Here are just a few common measures for PCB anti-interference design. 1. Power line design According to the size of the printed circuit board current, try to increase the width of the power line and reduce the loop resistance. At the same time, the direction of the power line and the ground line should be consistent with the direction of data transmission, which will help enhance the anti-noise ability. 2. Ground design The principles of ground design are: (1) Digital ground and analog ground are separated. If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible. The ground of the low-frequency circuit should be connected to the ground at a single point in parallel as much as possible. If the actual wiring is difficult, it can be connected in series and then connected in parallel. High-frequency circuits should be grounded at multiple points in series. The ground line should be short and loose, and a large area of grid-shaped ground foil should be used around high-frequency components as much as possible. (2) The ground wire should be as thick as possible. If the ground wire is made of very thin wire, the ground potential will change with the change of current, which will reduce the anti-noise performance. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed circuit board. If possible, the ground wire should be more than 2~3mm. (3) The ground wire forms a closed loop. For printed circuits composed only of digital circuits, the ground circuit is mostly arranged in a closed loop to improve the anti-noise ability. 4. Decoupling capacitor configuration One of the common practices in PCB design is to configure appropriate decoupling capacitors at various key locations on the printed circuit board. The general configuration principles for decoupling capacitors are: (1) Connect a 10~100uf electrolytic capacitor across the power input terminal. If possible, it is better to connect a 100uF or higher capacitor. (2) In principle, each integrated circuit chip should be equipped with a 0.01uf~0.1uf ceramic capacitor. If the printed circuit board space is insufficient, a 1~10pF capacitor can be arranged for every 4~8 chips. (3) For devices with weak noise resistance and large power supply changes when turned off, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and the ground line of the chip. 5. Via design In high-speed PCB design, seemingly simple vias often bring great negative effects to the circuit design. In order to reduce the adverse effects of the parasitic effects of vias, the design can try to achieve (1) Considering both cost and signal quality, choose a reasonable size of via. For example, for 6-10 layer memory module PCB design, it is better to use 10/20mil (drilling/pad) vias. For some high-density small-sized boards, you can also try to use 8/18mil vias. Under current technical conditions, it is difficult to use smaller vias (when the hole depth exceeds 6 times the drill diameter, it cannot be guaranteed that the hole wall can be evenly copper plated); for power or ground vias, you can consider using larger sizes to reduce impedance. (2) Try not to change layers for signal routing on PCB boards, that is, try not to use unnecessary vias. (3) The power and ground pins should be drilled nearby, and the leads between the vias and the pins should be as short as possible. (4) Place some ground vias near the signal layer-changing vias to provide the closest loop for the signal. You can even place a large number of redundant ground vias on the PCB. 6. Some experience in reducing noise and electromagnetic interference (1) If you can use a low-speed chip, don't use a high-speed one. Use high-speed chips in key places (2) You can use a resistor in series to reduce the jump rate of the control circuit. (3) Try to provide some form of damping for relays, such as RC to set current damping (4) Use the lowest frequency clock that meets the system requirements. (5) The clock should be as close as possible to the device that uses the clock. The shell of the quartz crystal oscillator should be grounded. Use a ground wire to circle the clock area. The clock line should be as short as possible. Do not run the line under the quartz crystal or under the device that is sensitive to noise. Clock, bus, and chip select signals should be kept away from I/O lines and connectors. Clock lines perpendicular to I/O lines will cause less interference than those parallel to I/O lines. (6) Do not leave unused gate circuit inputs floating. The unused op amp positive input should be grounded, and the negative input should be connected to the output.

JLCS

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shakencity

This article is good. Save it quickly and compare it with the process of doing it to develop good habits. Very good