The Printed circuit board (PCB) is the support of circuit components and devices in electronic products. It provides electrical connections between circuit components and devices. With the rapid development of electrical technology, the density of PGB is getting higher and higher. The ability to resist interference is very important. Therefore, in the design of PCB. Must comply with the general principles of PCB design, and should meet the requirements of the anti-interference design.
To obtain the best performance of electronic circuits, the layout of components and wires is very important. In order to design the PCB with good quality and low cost. The following general principles should be followed:
First, consider the PCB size. 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 the adjacent lines are susceptible to interference. After determining the PCB size. Then determine the location of the special components. Finally, according to the functional unit of the circuit, all components of the circuit layout.
The following principles should be observed when determining the location of special components:
- (1) Shorten the connection between high-frequency components as much as possible, and try to reduce their distribution parameters and electromagnetic interference between each other. Vulnerable 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, and the distance between them should be increased to avoid accidental short circuits caused by the discharge. The components with high voltage should be placed as far as possible in a place where the hands are not easy to reach during debugging.
- (3) Components weighing more than 15g shall be fixed with brackets and then welded. The large, heavy, and caloric components should not be installed on the printed board. Instead, they should be installed on the chassis floor of the whole machine, and the heat dissipation problem should be considered. Heat-sensitive elements should be kept away from heating elements.
- (4) for potentiometer, adjustable inductor coil, variable capacitor, micro switch, and other adjustable components of the layout should consider the structure of the machine requirements. If the machine adjustment, should be placed on the printed board above the easy-to-adjust place; If the machine is adjusted, its position should be adapted to the position of the adjusting knob on the chassis panel.
- (5) The position occupied by printing and pulling positioning holes and fixing brackets should be set aside.
According to the functional unit of the circuit. The layout of all components of the circuit shall comply with the following principles:
- (1) Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation, and make the signal keep the same direction as far as possible.
- (2) Take the core component of each functional circuit as the center and layout around it. Components should be evenly, neatly, and compactly arranged on the PCB. Minimize and shorten the leads and connections between components.
- (3) For circuits operating at high frequency, the distribution parameters between components should be considered. General circuits should be arranged in parallel as far as possible. In this way, not only beautiful. And easy to install and weld. 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 best shape of a circuit board is a rectangle. The aspect ratio is 3:2 to 4:3. When the circuit board size is larger than 200x150mm. The mechanical strength to which the circuit board is subjected should be considered.
The wiring principles are as follows:
- (1) The wires used in the input and output terminals should be avoided from being adjacent and parallel as far as possible. It is better to add the ground wire between the lines to avoid the occurrence of feedback coupling.
- (2) The minimum width of the printed photographic wire is mainly determined by the adhesion strength between the wire and the insulating base lever and the value of the current flowing through them. When the thickness of the copper foil is 0.05mm, the width is 1 ~ 15mm. The current through 2A, the temperature is not higher than 3 degrees Celsius, so. The wire width of 1.5mm can meet the requirements. For, in particular, 0.02~0.3mm wire widths are usually chosen. Of course, use wide lines whenever possible. Especially power cables and ground cables. The minimum spacing between wires is mainly determined by the worst-case interwire insulation resistance and breakdown voltage. For integrated circuits, especially digital circuits, as long as the process allows, the spacing can be made as small as 5~8mm.
- (3) Printed wire turns are generally circular arcs, and right angles or included angles will affect the electrical performance in high-frequency circuits. In addition, try to avoid the use of large areas of copper foil, otherwise. When heated for a long time, copper foil is prone to expand and fall off. When large areas of copper foil must be used, it is best to use a grid shape. This is conducive to the elimination of copper foil and substrate binder heat-produced volatile gas.
The center hole of the pad should be slightly larger than the diameter of the device lead. The too-large pad is easy to form virtual welding. Generally, the outer diameter of pad D is not less than (D +1.2) mm, where D is the lead aperture. For high-density digital circuits, the minimum diameter of the pad should be (D +1.0) mm.
PCB and circuit anti-interference measures
Printed circuit board anti-interference design and the specific circuit have a close relationship, here only on PCB anti-interference design of several common measures to do some explanation.
➡️Power line Design
According to the size of the printed circuit board current, rent the power cable width as much as possible to reduce the loop resistance. At the same time, the direction of power cables and ground cables is consistent with the direction of data transmission, which helps to enhance the anti-noise ability.
➡️Ground wire design
The principles of ground wire design are:
- (1) Digital and analog separation. If there are both logic and linear circuits on the circuit board, keep them as separate as possible. The ground of the low-frequency circuit should be grounded in parallel by a single point as far as possible. If the actual wiring is difficult, the ground can be partially connected in series and then grounded in parallel. High-frequency circuits should use multi-point series grounding, the ground wire should be short and rent, and high-frequency components around as far as possible with a large area of grid foil.
- (2) Ground cables should be as bold as possible. If the grounding line is very flexible, the grounding potential will change with the change of the current, so that the anti-noise performance is reduced. Therefore, the ground wire should be made thicker so that it can pass through three times the allowable current on the printed board. If possible, the ground cable should be at least 2 to 3mm.
- (3) The grounding wire forms a closed loop. Only composed of printed boards, the ground circuit of which is spread into a cluster loop mostly can improve the noise resistance.
➡️Capacitor configuration of the lotus root
One of the common practices is to configure appropriate decoupling capacitors in the key parts of the printed board.
The general configuration principle of the capacitor is as follows:
- (1) The input end of the power supply is bonded to an electrolytic capacitor of 10 ~ 100uF. If possible, a connection above 100uF would be better.
- (2) in principle, each chip should be equipped with a 0.01pF chip capacitor. In case of insufficient space on the printed board, a 1 ~ 10pF capacitor can be arranged every 4~8 chips.
- (3) For devices with weak anti-noise ability and large power supply changes when switched off, such as RAM and ROM storage devices, the decoupling capacitor should be directly connected between the power line and the ground line of the chip.
- (4) The capacitor lead should not be too long, especially the high-frequency bypass capacitor should not have a lead.
In addition, the following two points should be noted:
- (1) There are contactors, relays, buttons, and other components on the printed board. When operating them, they will produce a large spark discharge, and the RC circuit as shown in the attached figure must be used to absorb the discharge current. Generally, R ranges from 1 to 2K and C ranges from 2.2 to 47UF.
- (2) The input impedance of CMOS is very high, and it is easy to be induced, so it is necessary to ground or connect the positive power supply to the non-terminal when using.