PCB board design principles and anti-interference measures

PCB board design principles and anti-interference measures
Printed circuit board is the support of circuit components and equipment in electronic products It provides electrical connections between circuit components and equipment With the rapid development of electronic technology, the density of PGB is getting higher and higher The anti-interference ability of quality PCB design has great influence Therefore, in the design of PCB board, the general principle of PCB board must follow the design and meet the anti-interference design requirements In order to obtain the efficiency of electronic circuits, component layout and wire layout are very important
Printed circuit board

1. Layout
First of all, it is necessary to consider PCB board When the PCB board is too large, the printed lines will be very long, the impedance will be increased, the noise resistance will be reduced, and the cost will also be increased; If it is too small, the heat dissipation is poor, and adjacent lines are vulnerable to interference In determining the PCB, determine the position of special components Layout all components of the circuit according to the functional unit of the circuit
Observe the following guidelines when locating special components:
(1) Shorten the connection between high frequency components as much as possible, and minimize their distribution parameters and mutual electromagnetic interference The easily disturbed parts shall not be too close to each other, and the input and output components shall be as far away as possible
(2) There may be a high potential difference between some components or wires High voltage components shall be arranged in places that are not easily accessible during commissioning
(3) Components weighing more than 15g.  Fix with bracket and weld Those are big and heavy The printed board shall not be installed with components that generate a lot of heat, but shall be installed on the chassis bottom plate of the whole machine. Heat dissipation shall be considered The heating element shall be kept away from the heating element
(4) For potentiometers.  Adjustable inductance coil Variable capacitors The layout of adjustable components such as microswitch shall consider the structural requirements of the whole machine If the adjustment is made inside the machine, it should be placed on the printed board where it is convenient for adjustment; If the adjustment is carried out outside the machine, its position should match the position of the adjustment knob on the chassis panel
(5) The position occupied by the positioning hole of the printed pulley and the fixing bracket should be reserved.
According to the functional unit of the circuit The following principles should be followed:
(1) Arrange the position of each functional circuit unit according to the circuit flow
(2) Centering on the element of each functional circuit Components shall be uniform Neat It is compactly arranged on the PCB board to minimize and shorten the wires and connections between components
(3) For circuits operating at high frequencies, the distribution parameters between components should be considered In general circuits, components shall be arranged in parallel as far as possible With this kind of pipe, it is not only beautiful, but also easy to install and weld It is easy to mass produce
(4) Components located at the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board.  The shape of the circuit board is rectangular The aspect ratio is 3:2 to 4:3. When the size of the circuit board is larger than 200x150mm, the mechanical strength of the circuit board should be considered
2. Wiring
The principles of wiring are as follows:
(1) The wires used at the input and output terminals should be avoided as much as possible in parallel.  Add a ground wire between the wires to avoid reverberation coupling
(2) The width of the printed wire is mainly determined by the adhesion strength between the wire and the insulating base plate and the value of the current flowing through them.  When the thickness of copper foil is 0 05mm, with a width of 1-15mm. The temperature will not be higher than 3 ℃ when passing 2A current. In addition, the wire width of 1.5mm can meet the requirements For integrated circuits, especially digital circuits, the wire width is 0.02~0. Usually 3mm is selected Of course, use as wide a wire as possible, especially power and ground wires The wire spacing is mainly determined by the insulation resistance between wires and the breakdown voltage under severe conditions For integrated circuits, especially digital circuits, as long as the process allows, the spacing can be as small as 5-8mm
(3) The bend of the printed wire is generally arc shaped In addition, avoid using large area copper foil as far as possible, otherwise, the copper foil is easy to expand and fall off when heated for a long time Use grids when large areas of copper foil must be used This is beneficial to eliminate the volatile gas generated by heating the adhesive between the copper foil and the substrate
3. Pad
The pad center hole is slightly larger than the device lead diameter.  If the mat is too large, it is easy to form virtual solder The outer diameter D of the pad is generally not less than (d+1.2) mm, where d is the diameter of the pilot hole The diameter of the pad may be (d+1.0) mm
Anti interference measures for PCB and circuit. Anti interference design of PCB is closely related to specific circuit. Here are only a few common measurement methods for PCB anti interference design
3.1 Power Cord Design
According to the size of the printed circuit board current, try to increase the width of the power line to reduce the circuit resistance At the same time Make the power cord The direction of the ground wire is consistent with that of the data transmission, which helps to enhance the anti noise capability
3.2 Lot Design
The principles of ground wire design are:
(1) The digital ground is separated from the analog ground.  If there are logic circuits and linear circuits on the circuit board, they should be separated as far as possible The grounding of low frequency circuit shall be connected in parallel at single point as far as possible When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel High frequency circuit shall be grounded at several series points, and the grounding wire shall be short and leased. Large area grid grounding foil shall be used around high-frequency components as far as possible
(2) The ground wire should be as thick as possible.  If the grounding wire is very thin, the grounding potential will change with the change of current, which will reduce the anti noise efficiency Therefore, the grounding wire should be thickened so that it can pass 3 times the allowable current on the printed board If possible, the grounding wire should be larger than 2-3 mm
(3) The ground wire forms a closed loop.  For printed boards composed of digital circuits only, most of the grounding circuits are arranged in the loop, which can improve the anti noise ability
3.3 Decoupling capacitor configuration
One of the common practices of PCB is designed to include appropriate decoupling capacitors in each key component of PCB The general configuration principles of decoupling capacitors are:
(1) The power input end is connected across an electrolytic capacitor of 10~100uf.  If possible, it is better to connect more than 100uF
(2) In principle, each integrated circuit chip shall be equipped with a 0.01pF ceramic capacitor If the space of printed board is insufficient, 1~10pF capacitor can be configured for every 4~8 pcs
(3) Weak anti-noise ability.  For devices with large power changes when turned off, such as RAM ROM storage equipment and decoupling capacitor shall be directly connected between the power line and ground wire of the chip
(4) The capacitor leads should not be too long