Reliability principles of PCB design and wiring

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Reliability principles of PCB design and wiring [Copy link]

At present, electronic equipment used in various electronic equipment and systems still uses printed circuit boards as the main assembly method. 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 equipment.

Therefore, when designing printed circuit boards, attention should be paid to using the correct method.

1. Grounding

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

The following points should be noted in the ground design

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 one-point grounding should be used. When the signal operating frequency is greater than 10MHz, the ground impedance becomes very large. At this time, the ground 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 one-point grounding is used, the ground length should not exceed 1/20 of the wavelength, otherwise multi-point grounding should be used.

Separate digital circuits from analog circuits. 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 wires of the power supply end respectively. The ground area of the linear circuit should be increased as much as possible.

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 deteriorate. Therefore, the ground wire should be as thick as possible so that it can pass the allowable current of the three-position printed circuit board. If possible, the width of the ground wire should be greater than 3mm.

When designing a ground wire system for a printed circuit board consisting only of 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 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 junction, causing the anti-noise ability to decrease. If the ground structure is formed into a loop, the potential difference will be reduced, and the anti-noise ability of the electronic equipment will be improved.

2. Electromagnetic compatibility design

Electromagnetic compatibility refers to the ability of electronic equipment to work in a coordinated and effective manner in various electromagnetic environments. The purpose of electromagnetic compatibility design is to enable electronic equipment to suppress various external interferences, enable electronic equipment to work normally in a specific electromagnetic environment, and reduce the electromagnetic interference of electronic equipment itself to other electronic equipment.

Choose a reasonable wire width. Since the impact interference generated by transient current on the printed line is mainly caused by the inductance component of the printed wire, the inductance of the printed wire should be minimized. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so short and fine wires are beneficial for suppressing 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, the requirements can be fully met when the printed wire width is about 1.5mm; for integrated circuits, the printed wire width can be selected between 0.2 and 1.0mm.

Using the correct wiring strategy Using equal routing can reduce the inductance of the wires, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is best to use a tic-tac-toe mesh wiring structure. The specific method is to wire horizontally on one side of the printed board and vertically on the other side, and then connect them with metalized holes at the cross holes.

In order to suppress the crosstalk between the wires of the printed board, long-distance equal routing should be avoided as much as possible when designing the wiring, and the distance between the wires should be kept as far as possible. The signal wire should not cross the ground wire and the power wire as much as possible. Setting a grounded printed wire between some signal wires that are very sensitive to interference can effectively suppress crosstalk. To avoid electromagnetic radiation when high-frequency signals pass through printed conductors, please note:

Try to reduce the discontinuity of printed conductors, for example, the conductor width should not change suddenly, the conductor corner should be greater than 90 degrees, and loop routing is prohibited.

Clock signal leads are most likely to generate electromagnetic radiation interference. When routing, they should be close to the ground loop, and the driver should be close to the connector.

The bus driver should be close to the bus it is intended to drive. For those leads that leave the printed circuit board, the driver should be close to the connector.

The data bus should be laid out with a signal ground line between every two signal lines. It is best to place the ground loop close to the least important address pin, because the latter often carries high-frequency current.

When arranging high-speed, medium-speed and low-speed logic circuits on a printed circuit board, arrange the devices 3.Suppressing reflection interference In order to suppress the reflection interference appearing at the terminal of the printed line, except for special needs, the length of the printed line should be shortened as much as possible and a slow circuit should be used. If necessary, terminal matching can be added, that is, a matching resistor with the same resistance value is added to the ground and the power supply end at the end of the transmission line. According to experience, for TTL circuits with generally faster speeds, terminal matching measures should be adopted when the printed line is longer than 10cm. The resistance value of the matching resistor should be determined according to the maximum value of the output drive current and absorption current of the 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 switches from one state to another, a large peak current will be generated on the power line, forming a transient noise voltage. Configuring decoupling capacitors can suppress the noise caused by load changes, which is a common practice for the reliability design of printed circuit boards.

The configuration principles are as follows:

A 10~100uF electrolytic capacitor is connected across the power input terminal. If the position of the printed circuit board allows, the anti-interference effect of using an electrolytic capacitor of more than 100uF will be better.

A 0.01uF ceramic capacitor is configured for each integrated circuit chip. If the printed circuit board space 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, and the impedance is less than 1Ω in the range of 500kHz~20MHz, and the leakage current is very small (below 0.5uA).

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

The lead of the decoupling capacitor cannot be too long, especially the high-frequency bypass capacitor cannot have a lead

4. The size of the printed circuit board and the arrangement of the components

The size of the printed circuit board should be moderate. If it is too large, the printed lines 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 will be poor and it will be easily interfered by the adjacent lines.

In terms of component arrangement, just like other logic circuits, the related components should be placed as close as possible to obtain a better anti-noise effect. The clock generator, crystal oscillator and CPU clock input are all prone to noise and should be placed closer to each other. Devices prone to noise, small current circuits, large current circuits, etc. should be kept as far away from logic circuits as possible. If possible, a separate circuit board should be made. This is very important.

5. Thermal design starts from the perspective of heat dissipation

The printed board is best installed upright. The distance between the boards should generally not be less than 2cm, and the arrangement of the devices on the printed board should follow certain rules:

For equipment using free convection air cooling, it is best to arrange the integrated circuits (or other devices) in a longitudinal manner;

For equipment that uses forced air cooling, it is best to arrange the integrated circuits (or other devices) in a horizontally long manner;

The devices on the same printed circuit board should be arranged according to their heat generation and heat dissipation as much as possible. Devices with low heat generation or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed at the uppermost (entrance) of the cooling airflow, and devices with high heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) should be placed at the downstream of the cooling airflow;

In the horizontal direction, high-power devices should be arranged as close to the edge of the printed circuit board as possible to shorten the heat transfer path;

In the vertical direction, high-power devices should be placed as close to the top of the printed circuit board as possible to reduce the impact of these devices on the temperature of other devices when they are working; Temperature-sensitive devices should be placed in the lowest temperature area (such as the bottom of the device), and should never be placed directly above the heat-generating device. Multiple devices should be staggered on the horizontal plane;For devices with weak noise tolerance, large current change when turned off, and storage devices such as ROM and RAM, a decoupling capacitor should be directly connected between the power line (Vcc) and the ground line (GND) of the chip.