PCB design principles and circuit anti-interference measures, rely on these few tricks

Printed circuit boards (PCBs) are the support for circuit components and devices in electronic products. They provide electrical connections between circuit components and devices. With the rapid development of electronic technology, the density of PCBs is getting higher and higher.

The quality of PCB design has a great impact on the anti-interference ability.

Practice has proved that even if the circuit schematic is designed correctly, improper printed circuit board design will have an adverse effect on the reliability of electronic products.

For example, if two thin parallel lines on the printed circuit board are close to each other, a delay in the signal waveform will be formed, and reflected noise will be formed at the terminal of the transmission line. Therefore, when designing printed circuit boards, attention should be paid to using the correct method, complying with the general principles of PCB design, and meeting the requirements of anti-interference design.

1.General principles of PCB design

In order to achieve the performance of electronic circuits, the layout of components and the layout of wires are very important. In order to design a PCB with good quality and low cost, the following general principles should be followed:

2.Layout

First, the size of the PCB should be considered. When the PCB size is too large, the printed lines are long, the impedance increases, the anti-noise ability decreases, and the cost increases; when it is too small, the heat dissipation is not good, and the adjacent lines are susceptible to interference. After determining the size of the PCB, determine the location of special components. , According to the functional units of the circuit, lay out all the components of the circuit.

2.When determining the location of special components, the following principles should 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 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 leading to accidental short circuits. Components with high voltage should be placed in places that are difficult to reach during debugging.

(3) Components weighing more than 15g should be fixed with a bracket and then soldered.

Those large, heavy and heat-generating components should not be installed on the printed circuit board, but should be installed on the chassis bottom plate of the whole machine, and heat dissipation should be considered. Thermistors should be kept away from heating components.

(4) The layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, and micro switches should take into account the structural requirements of the whole machine.

If it is adjusted inside the machine, it should be placed in a convenient place on the printed circuit board for adjustment; if it is adjusted outside the machine, its position should be consistent with the position of the adjustment knob on the chassis panel.

(5) The space occupied by the printed circuit board positioning holes and the fixing bracket should be reserved.
According to the functional unit of the circuit. When laying out all the components of the circuit, the following principles should 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 components of each functional circuit as the center and lay out around it. Components should be arranged evenly, neatly and compactly on the PCB. Minimize and shorten the leads and connections between components.

(3) For circuits working at high frequencies, the distribution parameters between components should be considered. In general, components should be arranged in parallel as much as possible in the circuit. This is not only beautiful, but also easy to assemble and solder, and 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 length and width are 3:2 or 4:3. When the size of the circuit board surface is greater than 200×150mm, the mechanical strength of the circuit board should be considered. 2.

WirinThe wiring principles are as follows:

(1) The wires used for input and output terminals should be avoided to be adjacent and parallel. Add ground wires between wires to avoid feedback coupling.
(2) The width of the printed circuit board wire is mainly determined by the adhesion strength between the wire and the insulating substrate and the current value flowing through them. When the copper foil thickness is 0.5mm and the width is 1~15mm, the temperature will not be higher than 3℃ when a current of 2A passes through. Therefore, a wire width of 1.5mm can meet the requirements. For integrated circuits, especially digital circuits, a wire width of 0.02~0.3mm is usually selected. Of course, as long as it is allowed, wide wires should be used as much as possible, especially for power lines and ground wires. The spacing of the wires is mainly determined by the insulation resistance and breakdown voltage between the wires in the worst case. For integrated circuits, especially digital circuits, as long as the process allows, the spacing can be less than 5~8mil.
(3) The bends of printed wires are generally arc-shaped, while right angles or included angles will affect the electrical performance in high-frequency circuits. In addition, try to avoid using large-area copper foil, otherwise, when heated for a long time, the copper foil is prone to expansion and falling off. When a large area of ​​copper foil must be used, use a grid shape. This is conducive to eliminating the volatile gas generated by the heat of the adhesive between the copper foil and the substrate.

1.Pad
The center hole of the pad 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 pad diameter can be (d+1.0) mm.

2.PCB and circuit anti-interference measures
The anti-interference design of the printed circuit board is closely related to the specific circuit. Here are some common measures for PCB anti-interference design.

3.Power line design
According to the size of the printed circuit board current, try to thicken the power line width and reduce the loop resistance. At the same time, make the direction of the power line and ground line consistent with the direction of data transmission, which helps to enhance the anti-noise ability.

4.Ground line design In the design of electronic products, grounding is an important method to control interference. If grounding and shielding can be used correctly together, most interference problems can be solved. The ground wire structure in electronic products generally includes system ground, chassis ground (shielded ground), digital ground (logic ground) and analog ground.

5.The following points should be noted in ground wire design:

(1) Correctly choose single-point grounding and multi-point grounding
In low-frequency circuits, the operating frequency of the signal is less than 1MHz, and the inductance between its wiring and devices has little effect, while the loop formed by the grounding circuit has a greater impact on interference, so a single-point grounding method should be used. When the signal operating frequency is greater than 10MHz, the ground wire impedance becomes very large. At this time, the ground wire impedance should be reduced as much as possible, and multi-point grounding should be used nearby. When the operating frequency is between 1 and 10MHz, if a single-point grounding is used, the ground wire length should not exceed 1/20 of the wavelength, otherwise a multi-point grounding method should be used.

(2) Separate digital ground from analog ground.

There are both high-speed logic circuits and linear circuits on the circuit board. They should be separated as much as possible, and the ground wires of the two should not be mixed. They should be connected to the ground wire of the power supply end respectively. The ground of low-frequency circuits should be connected in parallel at a single point 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 wire should be short and thick. A large grid-shaped ground foil should be used around high-frequency components. The grounding area of ​​linear circuits should be increased as much as possible.

(3) 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, causing the timing signal level of electronic products to be unstable and the anti-noise performance to decrease. Therefore, the ground wire should be as thick as possible so that it can pass three times the allowable current of the printed circuit board. If possible, the width of the ground wire should be greater than 3mm.

(4) The ground wire forms a closed loop.

When designing a ground wire system for a printed circuit board composed only of digital circuits, making the ground wire a closed circuit can significantly improve the anti-noise ability. The reason is that there are many integrated circuit components on the printed circuit board. Especially when there are components that consume a lot of power, due to the limitation of the thickness of the ground wire, a large potential difference will be generated on the ground wire, causing the anti-noise ability to decrease. If the ground wire is formed into a loop, the potential difference will be reduced, and the anti-noise ability of the electronic equipment will be improved.

1.Decoupling capacitor configuration

One of the common practices in PCB design is to configure appropriate decoupling capacitors at various key parts of the printed circuit board. The general configuration principle of decoupling capacitors is:

(1) A 10~100uf electrolytic capacitor is connected across the power input terminal. If possible, it is better to connect 100uF or more.

(2) In principle, each integrated circuit chip should be arranged with a 0.01pF ceramic capacitor. If the printed circuit board space is not enough, a 1~10pF tantalum capacitor can be arranged for every 4~8 chips.

(3) For devices with weak anti-noise ability 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.

(4) The capacitor lead cannot be too long, especially the high-frequency bypass capacitor cannot have leads.

In addition, the following two points should be noted:

(1) When there are contactors, relays, buttons and other components on the printed circuit board, they will generate large spark discharges when they are operated, and an RC circuit must be used to absorb the discharge current. Generally, R is 1~2K and C is 2.2~47uF.
(2) The input impedance of CMOS is very high and is easily inductive, so the unused end should be grounded or connected to the positive power supply when in use.