5 methods for reliability design of printed circuit boards

At present, electronic equipment is used in various electronic devices and systems, and printed circuit boards are still the main assembly method. Practice has proved that even if the schematic design of the circuit is correct and the printed circuit board is not properly designed, it will adversely affect the reliability of the electronic device. For example, if the two thin parallel lines of the printed board are in close proximity, a delay in the signal waveform is formed, and reflected noise is formed at the end of the transmission line. Therefore, when designing a printed circuit board, care should be taken to use the correct method.

First, the ground line design in electronic equipment, grounding is an important method to control 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 it should be adopted. Ground a little. When the signal operating frequency is greater than 10MHz, the ground line impedance becomes very large. At this time, the ground line 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 power ground. Try to increase the grounding area of ​​the linear circuit.

3. If the grounding wire is as thick as possible, the grounding potential changes with the change of the current, which causes 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. Make the grounding wire form a closed-loop circuit. When designing the grounding system of a printed circuit board consisting only of digital circuits, making the grounding wire into a closed-loop circuit can significantly improve noise immunity. ability. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are many power-consuming components, due to the limitation of the grounding wire thickness, 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.

Second, 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 that short and precise 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. Adopt the correct wiring strategy. The use of equal traces can reduce the wire inductance, 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 the crosstalk between the printed circuit board wires, the long distance equalization should be avoided when designing the wiring, and the distance between the wires and the wires should be as far as possible. The signal wires should not cross the ground wire and the power cable as much as possible. A grounded trace is placed between some signal lines that are very sensitive to interference, which effectively suppresses crosstalk.

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

● Minimize the discontinuity of the printed conductor. For example, the width of the wire should not be abrupt. The corner of the wire should be greater than 90 degrees to prohibit the loop.

● The clock signal lead is most likely to cause electromagnetic radiation interference. When routing, it should be close to the ground loop. The driver should be next to the connector.

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

● The wiring of the data bus should be a signal ground between every two signal lines. It is best to place the ground loop next to the least important address leads, as the latter often carry high frequency currents.

● When placing high speed, medium speed and low speed logic circuits on the printed board, the devices should be arranged

3. Suppress reflection interference. In order to suppress the reflection interference that occurs at the end of the printed line, in addition to special needs, the length of the printed line should be shortened as much as possible and a slow circuit should be used. Terminal matching can be added if necessary, that is, a matching resistor of 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 the TTL circuit with faster speed, the terminal matching method should be adopted when the printed line is longer than 10cm. The resistance of the matching resistor should be determined according to the output drive current of the integrated circuit and the maximum value of the sink current.

Third, 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. The configuration of decoupling capacitors can suppress the noise generated by load changes. It is a common practice for the reliability 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 better.

• Configure a 0.01uF ceramic capacitor 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 lead of the decoupling capacitor should not be too long, especially the high frequency bypass capacitor can not be leaded.

Fourth, the size of printed circuit boards 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. as shown in picture 2. Clock generators, crystal oscillators, and CPU clock inputs are prone to noise and should be close to each other. Devices that are prone to noise, small current circuits, high-current circuits, etc. should be kept away from logic circuits as much as possible. If possible, it is important to make a separate board.

Fifth, the thermal design is from the perspective of facilitating heat dissipation

The printed version is preferably mounted upright, the distance between the board and the board is generally not less than 2 cm, and the arrangement of the device on the printed board should follow certain rules: for equipment using free convection air cooling, it is best The integrated circuits (or other devices) are arranged in a vertically long manner. For devices employing forced air cooling, it is preferred to arrange the integrated circuits (or other devices) in a horizontally long manner. 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 in the cooling airflow. The uppermost (inlet), heat-generating or heat-resistant devices (such as power transistors, large-scale integrated circuits, etc.) are placed at the most downstream of the cooling airflow. In the horizontal direction, high-power devices are placed as close as possible to the edges of the printed board to shorten the heat transfer path; in the vertical direction, high-power devices are placed as close as possible to the top of the printed board to reduce the effects of these devices on the temperature of other devices. .

For temperature-sensitive devices, it is best to place them in the lowest temperature area (such as the bottom of the device). Do not place them directly above the heat-generating device. Multiple devices are preferably staggered in a horizontal plane. The heat dissipation of the printed circuit board in the device mainly depends on the air flow, so the air flow path should be studied during the design, and the device or the 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. The same problem should be noted in the configuration of multiple printed circuit boards in the whole machine.

A large number of practical experience shows that the reasonable device arrangement can effectively reduce the temperature rise of the printed circuit, thus significantly reducing the failure rate of devices and devices. The above is only some general principles for the reliability design of printed circuit boards. The reliability of the circuit board is closely related to the specific circuit. In the design, the corresponding circuit is not required to be processed accordingly, so as to ensure the reliability of the printed circuit board to the utmost extent.

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