Design Reliability Principle in Circuit Design and PCB Wiring

At present, printed circuit board is still the main assembly method for electronic equipment and systems. Practice has proved that even if the circuit schematic design is correct and the printed circuit board design is improper, it will have an adverse impact on the reliability of electronic equipment. For example, if two thin parallel lines of the printed board are close together, the delay of the signal waveform will be formed and the reflected noise will be formed at the terminal of the transmission line. Therefore, when designing printed circuit board, we should pay attention to the correct method.

1、 Grounding

Ground wire design in electronic equipment, grounding is an important method to control interference. Most interference problems can be solved if grounding and shielding can be correctly combined. The ground wire structures in electronic equipment generally include system ground, shell ground (shielding ground), digital ground (logic ground) and analog ground. The following points shall be noted in the design of ground wire:

1) Correctly select single point grounding and multi-point grounding. In low-frequency circuit, the working frequency of signal is less than 1MHz, its wiring and inductance between devices have little influence, while the circulating current formed by grounding circuit has great influence on interference, so one point grounding should be adopted. When the signal operating frequency is greater than 10MHz, the ground wire impedance becomes large. At this time, the ground wire impedance shall be reduced as much as possible, and the nearest multi-point grounding shall be adopted. When the working frequency is 1 10MHz, if one point grounding is adopted, the length of ground wire shall not exceed 1 / 20 of the wavelength, otherwise multi-point grounding method shall be adopted.

2) Separate the digital circuit from the analog circuit. There are both high-speed logic circuit and linear circuit on the circuit board. They should be separated as far as possible. The ground wires of the two should not be mixed, and they should be connected with the ground wire of the power terminal respectively. The grounding area of linear circuit shall be increased as much as possible.

3) The grounding wire shall be thickened as much as possible. If the grounding wire is very thin, the grounding potential will change with the change of current, resulting in unstable timing signal level of electronic equipment and deterioration of anti noise performance. Therefore, the grounding wire shall be thickened as much as possible so that it can pass the allowable current of three positions on the printed circuit board. If possible, the width of the grounding wire shall be greater than 3mm

4) When designing the ground wire system of printed circuit board composed of only digital circuits, making the ground wire into a closed loop can significantly improve the anti noise ability. The reason is that there are many integrated circuit components on the printed circuit board, especially in case of components with high power consumption, due to the limitation of the thickness of the grounding wire, a large potential difference will be generated on the ground junction, resulting in the decline of anti noise ability. If the grounding structure is formed into a loop, the potential difference will be reduced and the anti noise ability of electronic equipment will be improved.

2、 EMC design

Electromagnetic compatibility design electromagnetic compatibility refers to the ability of electronic equipment to work coordinately and effectively in various electromagnetic environments. The purpose of EMC design is to make the electronic equipment not only suppress all kinds of external interference, make the electronic equipment work normally in the specific electromagnetic environment, but also reduce the electromagnetic interference of the electronic equipment itself to other electronic equipment.

1) Choose a reasonable conductor width. The impact interference caused by transient current on the printed wire is mainly caused by the inductance of the printed wire. Therefore, the inductance of the printed wire should be reduced as much as possible. The inductance of printed wire is directly proportional to its length and inversely proportional to its width. Therefore, short and precise wire is beneficial to suppress interference. The signal lines of clock leads, row drivers or bus drivers often carry large transient currents, and the printed wires should be as short as possible. For discrete component circuits, when the printed wire width is about 1.5mm, it can fully meet the requirements; For integrated circuits, the width of printed wires can be selected between 0.2 1.0mm.

2) Using the correct wiring strategy and equal routing can reduce the conductor inductance, but the mutual inductance and distributed capacitance between conductors increase. If the layout allows, it is best to adopt the well shaped mesh wiring structure. The specific method is to route horizontally on one side of the printed board and longitudinally on the other side, and then connect with metallized holes at the cross holes.

In order to suppress the crosstalk between printed board conductors, long-distance equal routing shall be avoided as far as possible in the design of wiring, the distance between lines shall be opened as far as possible, and the signal line, ground wire and power line shall not cross as far as possible. A grounded printed line is set between some signal lines that are very sensitive to interference, which can effectively suppress crosstalk.

In order to avoid the electromagnetic radiation generated when the high-frequency signal passes through the printed wire, the following points shall also be paid attention to when wiring the printed circuit board:

The discontinuity of printed wires shall be minimized. For example, the width of wires shall not change suddenly, the corners of wires shall be greater than 90 degrees, and circular routing is prohibited.

The clock signal lead is most likely to generate electromagnetic radiation interference. When routing, it shall be close to the ground circuit, and the driver shall be close to the connector.

The bus driver shall be next to the bus to be driven. For those leads leaving the printed circuit board, the driver shall be close to the connector.

The wiring of data bus shall be clamped with a signal ground wire between each two signal wires. It is best to place the ground loop next to the least important address lead, because the latter often carries high-frequency current.

When high-speed, medium speed and low-speed logic circuits are arranged on the printed board, the devices shall be arranged. 3. Suppression of reflection interference in order to suppress the reflection interference at the terminal of the printed line, in addition to special needs, the length of the printed line shall be shortened as much as possible and the slow circuit shall be adopted. If necessary, terminal matching can be added, that is, a matching resistance with the same resistance value can be added to the ground and the power supply terminal at the end of the transmission line. According to experience, terminal matching measures should be adopted for TTL circuits with general high speed when the printed line is longer than 10cm. The resistance value of matching resistance shall be determined according to the maximum value of output driving current and absorption current of integrated circuit.

3、 Decoupling capacitor configuration

In the DC power supply circuit, the change of load will cause power supply noise. For example, in a digital circuit, when the circuit changes from one state to another, a large peak current will be generated on the power line, forming a transient noise voltage. The configuration of decoupling capacitor can suppress the noise caused by load change, which is a conventional method of reliability design of printed circuit board. The configuration principles are as follows:

A 10 100uF electrolytic capacitor is connected across the power input end. If the position of the printed circuit board allows, the anti-interference effect of electrolytic capacitor above 100uF will be better.

Each IC chip is equipped with a 0.01uF ceramic capacitor. If the printed circuit board is too small to fit, a 1 10uF tantalum electrolytic capacitor can be configured for every 4 10 chips. The high-frequency impedance of this device is particularly small, the impedance is less than 1 Ω in the range of 500KHz 20MHz, and the leakage current is very small (below 0.5ua).

For devices with weak noise capability and large current change during shutdown and memory devices such as ROM and ram, decoupling capacitors shall be directly connected between the power line (VCC) and ground line (GND) of the chip.

The lead of decoupling capacitor shall not be too long, especially for high-frequency bypass capacitor

4、 Dimensions of printed circuit boards and arrangement of devices

The size of the printed circuit board should be moderate. If it is too large, the printed line will be long and the impedance will increase, which will not only reduce the anti noise ability, but also increase the cost; If it is too small, the heat dissipation is not good, and it is easy to be disturbed by adjacent lines.

In terms of device layout, like other logic circuits, relevant devices should be placed as close as possible, so as to obtain better anti noise effect. As shown in Figure 2. The clock input terminals of time generator, crystal oscillator and CPU are easy to generate noise, so they should be closer to each other. Devices, low current circuits and high current circuits that are prone to noise should be far away from logic circuits as far as possible. If possible, another circuit board should be made, which is very important.

5、 Thermal design starts from the point of view conducive to heat dissipation

The printed board should be installed vertically, and the distance between boards should generally not be less than 2cm, and the arrangement of devices on the printed board should follow certain rules: for equipment cooled by free convection air, it is better to arrange the integrated circuits (or other devices) in a vertical and rectangular manner; For the equipment using forced air cooling, it is better to arrange the integrated circuits (or other devices) in a horizontal and long manner. The devices on the same printed board shall be arranged in zones according to their calorific value and heat dissipation degree as far as possible. The devices with low calorific value or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) shall be placed at the uppermost flow (inlet) of the cooling air flow, Devices with high calorific value or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed at the downstream of the cooling air flow.

In the horizontal direction, high-power devices are arranged as close to the edge of the printed board as possible to shorten the heat transfer path; In the vertical direction, high-power devices shall be arranged as close as possible to the upper part of the printed board, so as to reduce the impact of these devices on the temperature of other devices during operation. Devices sensitive to temperature are best placed in the area with the lowest temperature (such as the bottom of the equipment). Never put them directly above the heating devices. Multiple devices are best staggered on the horizontal plane. The heat dissipation of printed boards in the equipment mainly depends on air flow, so the air flow path should be studied in the design, and the devices or printed circuit boards should be configured reasonably. When air flows, it always tends to flow where the resistance is small, so when configuring devices on the printed circuit board, it is necessary to avoid leaving a large airspace in a certain area. The configuration of multiple printed circuit boards in the whole machine should also pay attention to the same problem.

6、 Summary

A large number of practical experience shows that adopting a reasonable device arrangement can effectively reduce the temperature rise of the printed circuit, so as to significantly reduce the failure rate of devices and equipment. The above are only some general principles for the reliability design of the printed circuit board. The reliability of the printed circuit board is closely related to the specific circuit, and there is no need to deal with it according to the specific circuit in the design, In order to ensure the reliability of printed circuit board to the greatest extent.

Design Reliability Principle in Circuit Design and PCB Wiring 1

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