Experience in PCB design

- Jul 27, 2018-

I. size of printed circuit board and arrangement of devices

The size of printed circuit board should be moderate, the length of printed line should be long when it is too large, and the impedance increases.

Too small, the heat dissipation is bad, at the same time easy to be adjacent line interference.

As with other logic circuits, the interrelated devices should be placed as close as possible so as to obtain better anti-noise effect.

Clock generators, crystal oscillators, and CPU clock input terminals are prone to noise, to be closer to each other.

It is important that devices, small current circuits and large current circuits that are prone to noise should be kept as far away from the logical circuit as possible.

二、去耦电容配置

Ii. Decoupling capacitor configuration

In dc power circuit, the change of load will cause power noise.

For example, in a digital circuit, when the circuit changes from one state to another, a large spike current will be generated on the power line, forming a transient noise voltage.

It is a common practice for reliability design of printed circuit board to configure decoupling capacitors to suppress noise caused by load change.

配置原则如下

The power input end spans an electrolytic capacitor of 10 ~ 100uF. If the position of the printed circuit board is allowed, the anti-interference effect of the electrolytic capacitor with more than 100uF will be good.


A 0.01uF ceramic capacitor is configured for each IC chip.

If printed circuit board and a small space, can every 4 ~ 10 chip configuration a 1 ~ 10 uf tantalum electrolytic capacitor, this particularly small high frequency impedance of the device in 500 KHZ ~ 20 MHZ range impedance is less than 1 Ω, and leakage current is very small (below 0.5 uA).


For devices with weak noise capacity and large changes in current when switching off, and storage devices such as ROM and RAM, decoupling capacitors should be directly connected between the power line (Vcc) and ground line (GND) of the chip.


The lead of decoupling capacitors should not be too long, especially high frequency bypass capacitors should not be led.


3. Heat dissipation design


From the perspective of heat dissipation, the printing plate should be installed upright. The distance between the plate and the plate should generally not be less than 2cm, and the arrangement of devices on the printing plate should follow certain rules:


1. For devices using free-convection air cooling, it is best to arrange integrated circuits (or other devices) in vertical length;

For devices with forced air cooling, it is best to align the integrated circuits (or other devices) horizontally.


2, on the same piece of PCB device should as far as possible according to the calorific value of the size and degree of heat partition, calorific value is small or poor heat resistance devices (such as small signal transistor, small-scale integrated circuit, electrolytic capacitors, etc.) on the cooling airflow of the best () at the entrance, calorific value big or good heat resistance devices (such as power transistor, large scale integrated circuit, etc.) in the most downstream cooling airflow.


3. In the horizontal direction, high-power devices should be arranged as close as possible to the edge of the printing plate to shorten the heat transfer path;

In the vertical direction, high-power devices are arranged as close as possible to the upper part of the printing plate to reduce the influence of these devices on the temperature of other devices.


4. Devices that are sensitive to temperature should be placed in the lowest temperature area (such as the bottom of the equipment), and should not be placed directly above the heating device. Multiple devices should be staggered on the horizontal surface.


5. The heat dissipation of the printing plate in the equipment mainly depends on air flow, so the air flow path should be studied in the design, and the device or printed circuit board should be reasonably configured.

Air flow tends to flow where there is little resistance, so when configuring devices on a printed circuit board, avoid leaving a large area of space.


4. Electromagnetic compatibility design


Electromagnetic compatibility refers to the ability of electronic equipment to work harmoniously and effectively in various electromagnetic environments.

The purpose of electromagnetic compatibility design is to make electronic equipment can not only suppress various external interference, so that electronic equipment can work normally in a specific electromagnetic environment, but also reduce the electromagnetic interference of electronic equipment itself to other electronic equipment.


1. Select a reasonable wire width


Since the shock interference caused by transient current on the printed line is mainly caused by the inductance of the printed wire, the inductance of the printed wire should be minimized.

The inductance of the printed conductor is proportional to its length and inversely proportional to its width, so the short and fine conductor is beneficial to suppress the interference.

Signal lines for clock leads, line drives, or bus drivers often carry large transient currents, and printed wires should be as short as possible.

For the discrete component circuit, when the width of the printed conductor is about 1.5mm, it can fully meet the requirements.

For integrated circuits, the width of printed wires can be selected between 0.2 and 1.0mm.


2. Adopt the correct wiring strategy


The use of equal wire can reduce the inductance of wires, but the mutual inductance and distribution capacitance between wires are increased. If the layout allows, it is better to adopt the well-shaped network wiring structure.

In order to restrain the crosstalk between the wires of the printing plate, the long distance parallel wire should be avoided as far as possible when designing the wiring.


5. Ground wire design


In electronic equipment, grounding is an important method to control interference.

The correct combination of grounding and shielding can solve most of the interference problems.

The ground wire structure of electronic equipment mainly includes system ground, casing ground (shielding ground), digital ground (logical ground) and simulation ground.

The following points should be noted in the design of ground wire:


1. Correct selection of single point and multi-point grounding


In a low-frequency circuit, the working frequency of the signal is less than 1MHz, and its wiring and inductance between devices have less influence on it, while the loop current formed by the grounding circuit has a greater influence on the interference, so a bit of grounding should be adopted.

When the working frequency of the signal is greater than 10MHz, the ground impedance becomes very large. At this point, the ground impedance should be reduced as much as possible, and the nearest multi-point grounding should be adopted.

When the working frequency is 1 ~ 10MHz, if a bit of grounding is used, the length of the ground wire should not exceed 1/20 of the wavelength, otherwise, the multi-point grounding method should be adopted.


2. Separate the digital circuit from the analog circuit


There are both high speed logic circuits and linear circuits on the circuit board, so they should be separated as far as possible, and the ground lines of the two should not be mixed, respectively connected with the ground lines of the power end.

Try to increase the ground area of the linear circuit.


3. Thicken the ground wire as much as possible


If the ground wire is very thin, the ground potential will change with the change of current, causing the timing signal level of the electronic equipment to be unstable and the anti-noise performance to become bad.

Therefore, the ground wire should be thickened as much as possible so that it can pass the allowable current of three positions on the printed circuit board.

Where possible, the width of the ground wire should be greater than 3mm.


4. Make the ground wire a closed loop


When designing the ground wire system of printed circuit board consisting only of digital circuit, the ground wire can be made into a closed loop to improve the anti-noise ability.

The reason is that there are a lot of integrated circuit components on the printed circuit board, especially when there are components that consume a lot of electricity, due to the limitation of the thickness of grounding wires, a large potential difference will be generated on the ground junction, resulting in the reduction of anti-noise ability