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Summary of printed circuit board design experience

Posted:02:05 PM January 24, 2019 writer: G

1. Printed circuit board size and device layout

The size of the printed circuit board should be moderate. When the size is too large, the printed lines are long and the impedance is increased. The noise resistance is reduced and the cost is high. If it is too small, the heat dissipation is not good, and it is susceptible to interference from adjacent lines. In terms of device layout, as with other logic circuits, the related devices should be placed as close as possible to achieve better noise immunity. Clock generators, crystal oscillators, and CPU clock inputs are prone to noise and are close to each other. Devices that are prone to noise, small current circuits, high-current circuits, etc. should be as far away as possible from the logic circuit. If possible, another board should be used. This is very important.

2. Decoupling capacitor configuration

In a DC power supply loop, changes in load can cause power supply noise. For example, in a digital circuit, when a circuit transitions from one state to another, a large spike current is generated on the power line to form a transient noise voltage. Configuring decoupling capacitors to suppress noise due to load changes is a common practice for reliable design of printed circuit boards.

The configuration principles are as follows:

The power input terminal is connected to a 10~100uF electrolytic capacitor. If the position of the printed circuit board is allowed, the anti-interference effect of the electrolytic capacitor above 100uF will be good.

A 0.01 uF ceramic capacitor is provided for each integrated circuit chip. If you encounter a small printed circuit board space and can not fit, you can configure a 1 ~ 10uF tantalum electrolytic capacitor every 4 ~ 10 chips, the high-frequency impedance of this device is particularly small, the impedance is less than 1Ω in the range of 500kHz ~ 20MHz, Moreover, the leakage current is small (0.5 uA or less).

For devices with weak noise capability, large current changes during shutdown, and memory devices such as ROM and RAM, decoupling capacitors should be directly connected between the power supply line (Vcc) and ground (GND) of the chip.

The leads of the decoupling capacitors must not be too long, especially if the high frequency bypass capacitors are not leaded.

3. The heat dissipation design

From the perspective of facilitating heat dissipation, the printed plate is preferably installed upright, and the distance between the plates and the plate is generally not less than 2 cm, and the arrangement of the devices on the printed plate should follow certain rules:

1. For devices that use free convection air cooling, it is best to arrange the integrated circuits (or other devices) in a vertically long manner; for devices that use forced air cooling, it is best to divide the integrated circuits (or other devices) horizontally. Way row.

2. The devices on the same printed board should be arranged as far as possible according to their heat generation and heat dissipation. Devices with low heat generation or poor heat resistance (such as small signal transistors, small scale integrated circuits, electrolytic capacitors, etc.) should be placed. The uppermost flow (at the inlet) of the cooling airflow, the device that generates a large amount of heat or heat (such as a power transistor, a large-scale integrated circuit, etc.) is placed at the most downstream of the cooling airflow.

3. In the horizontal direction, the high-power devices are placed as close as possible to the edge of the printed board to shorten the heat transfer path; in the vertical direction, the high-power devices are placed as close as possible to the top of the printed board, so as to reduce the temperature of other devices when these devices are operated. Impact.

4. The temperature-sensitive device should be placed in the lowest temperature area (such as the bottom of the device). Do not place it directly above the heat-generating device. Multiple devices are preferably staggered on a horizontal plane.

5. The heat dissipation of the printed circuit board in the equipment mainly depends on the air flow, so the air flow path should be studied during the design, and the device or printed circuit board should be properly configured. When the air flows, it tends to flow in a place with low resistance. Therefore, when configuring the device on the printed circuit board, avoid leaving a large air space in a certain area.

4. Electromagnetic compatibility design

Electromagnetic compatibility refers to the ability of an electronic device to work in a coordinated and efficient manner in various electromagnetic environments. The purpose of the electromagnetic compatibility design is to enable the electronic device to suppress various external interferences, enable the electronic device to work normally in a specific electromagnetic environment, and at the same time reduce the electromagnetic interference of the electronic device itself to other electronic devices.

1. Choose a reasonable wire width

Since the transient interference generated by the transient current on the printed wiring is mainly caused by the inductance component of the printed conductor, the inductance of the printed conductor should be minimized. The inductance of a printed conductor is proportional to its length and inversely proportional to its width, so short and fine conductors are advantageous for suppressing interference. Signal lines for clock leads, row drivers, or bus drivers often carry large transient currents, and the printed conductors should be as short as possible. For the discrete component circuit, when the width of the printed conductor is about 1.5mm, the requirement can be fully satisfied; for the integrated circuit, the width of the printed conductor can be selected between 0.2 and 1.0mm.

2. Using the correct wiring strategy

The use of equal traces can reduce the inductance of the wire, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is better to use a well-shaped mesh wiring structure. The specific method is that one side of the printed board is laterally routed, and the other side is longitudinally routed. Metallized holes are then connected at the intersection holes. In order to suppress crosstalk between printed circuit board conductors, it is necessary to avoid long-distance equal traces when designing wiring.

5. Ground wire design

In electronic equipment, grounding is an important method of controlling interference. If the grounding and shielding are properly combined, most of the interference problems can be solved. The ground wire structure in the electronic device is roughly systematic, chassis ground (shielded ground), digital ground (logically), and analog ground. Pay attention to the following points in the ground line design:

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

In the low-frequency circuit, the operating frequency of the signal is less than 1MHz, and the influence of the inductance between the wiring and the device is small, and the circulating current formed by the grounding circuit has a great influence on the interference, so a grounding should be adopted. 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. When the operating frequency is between 1 and 10 MHz, if a grounding is used, the grounding length should not exceed 1/20 of the wavelength. Otherwise, the multi-point grounding method should be used.

2. Separate the digital circuit from the analog circuit

The circuit board has both high-speed logic circuits and linear circuits. They should be separated as much as possible, and the ground wires of the two should not be mixed, and they are connected to the ground of the power supply. Try to increase the grounding area of the linear circuit.

3. Try to thicken the ground wire

If the grounding wire is very thin, the grounding potential changes with the change of the current, causing the timing signal level of the electronic device 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 three allowable currents on the printed circuit board. If possible, the width of the grounding wire should be greater than 3mm.

4. The grounding line constitutes a closed loop

When designing the grounding system of a printed circuit board consisting only of digital circuits, making the grounding line into a closed loop can significantly improve the noise immunity. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are 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, causing the noise resistance to decrease. If the ground structure is looped, the potential difference will be reduced to improve the noise immunity of the electronic device.

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