Five methods for reliability design of the hottest

Five methods for reliability design of printed circuit boards

at present, printed circuit boards are still the main assembly method for electronic equipment used in various 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 end of the transmission line. Therefore, when designing PCB, we should pay attention to the correct method

I. ground wire design in electronic equipment, grounding is an important method to control interference

If grounding and shielding can be correctly combined, most interference problems can be solved. The ground wire structures in electronic equipment generally include system ground, chassis ground (shielding ground), digital ground (logic ground) and analog ground. The following points should be paid attention to in the design of ground wire:

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

2. The demand of domestic automobile industry basically depends on imported digital circuits and analog circuits. 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 wires of the two should not be mixed, and they should be connected with the ground wires of the power end respectively. Try to increase the grounding area of linear circuit

3. Try to thicken the grounding wire. 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 should 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 should be greater than 3mm 4. When designing the ground wire system of the 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 when there are components that consume a lot of power, 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

II. 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 electronic equipment not only suppress all kinds of external interference, make electronic equipment work normally in a specific electromagnetic environment, but also reduce the electromagnetic interference of electronic equipment itself to other electronic equipment

1. Choose a reasonable wire width. Since the impact interference caused by transient current on the printed wire is mainly caused by the inductance of the printed wire, the inductance of the printed wire should be minimized. The inductance of the printed wire is directly proportional to its length and inversely proportional to its width, so a short and precise wire is beneficial to suppress interference. The signal wire of clock lead, row driver or bus driver often carries large transient current, and the printed wire should be as short as possible. For discrete component circuits, when the width of printed wire 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. Adopt the correct cabling strategy. Using equal wiring can reduce the conductor inductance, but the mutual inductance and distributed capacitance between conductors increase. If the layout allows, it is best to use the well shaped wiring structure. The specific method is to wiring horizontally on one side of the printed board and longitudinally on the other side, and then connect it with metallized holes at the cross holes

Mr. you said that in order to suppress the crosstalk between the conductors of the printed board, long-distance equal routing should be avoided as far as possible in the design of wiring, the distance between wires should be opened as far as possible, and the signal wire should not cross the ground wire and the power wire as far as possible. Setting a grounded printed line between some signal lines that are very sensitive to interference can effectively suppress crosstalk

in order to avoid the electromagnetic radiation generated when high-frequency signals pass through the printed wiring, the following points should also be paid attention to when wiring the printed circuit board:

● try to reduce the discontinuity of the printed wiring, for example, do not change the width of the wiring, the corner of the wiring should be greater than 90 degrees, and circular wiring is prohibited

● the clock signal lead is most likely to produce electromagnetic radiation interference. When wiring, it should be close to the ground circuit, and the driver should be close to the connector

● the bus driver shall be close to the bus to be driven. For those leads that leave the printed circuit board, the driver should 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 should be arranged

3 Suppress reflection interference. In order to suppress the reflection interference at the terminal of the printed line, in addition to the great difference between the data automatically collected during ultra fast compression and the data collected during slow compression or static compression, the length of the printed line should be shortened as much as possible and the slow circuit should be used. 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 at the end of the transmission line. According to experience, terminal matching measures should be taken when the printed line of TTL circuit with general high speed is longer than 10cm. The resistance value of the matching resistance should be determined according to the maximum value of the output driving current and absorption current of the integrated circuit

III. decoupling capacitor configuration

in the DC power 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. Configuring decoupling capacitors can suppress the noise caused by load changes, which is a conventional method of reliability design of printed circuit boards. 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 (2) is allowed by the organization of new material folding project, the anti-interference effect of electrolytic capacitors above 100uF will be better

● each IC chip is equipped with a 0.01uF ceramic capacitor. If the space of 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 ability and large current change during shutdown, and memory 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 wire of decoupling capacitor should not be too long, especially the high-frequency bypass capacitor should not have lead wire

IV. the size of printed circuit board and the layout of devices

the size of 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 poor, 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 prone to generate noise, so they should be closer to each other. Devices, low current circuits, high current circuits, etc. that are prone to noise should be kept away from logic circuits as far as possible. If possible, another circuit board should be made, which is very important

v. thermal design from the perspective of conducive to heat dissipation

the printed board is best 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 best to arrange integrated circuits (or other devices) in a longitudinal manner, and for equipment cooled by forced air, It is better to arrange the integrated circuits (or other devices) in a horizontal and long manner. The devices on the same printed board should be arranged in zones according to their calorific value and degree of heat dissipation as far as possible. Devices with low calorific value or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed at the top stream (inlet) of the cooling air flow, and devices with high calorific value or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) should be placed at the bottom stream 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 should be arranged as close as possible to the top of the printed board, so as to reduce the impact of these devices on the temperature of other devices when they work

devices that are sensitive to temperature are best placed in the area with the lowest temperature (such as the bottom of the equipment). Never put it directly above the heating device. 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 in places with low resistance, so when configuring devices on printed circuit boards, 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

a lot of practical experience shows that using a reasonable device arrangement can effectively reduce the temperature rise of the printed circuit, so that the failure rate of devices and equipment can be significantly reduced. The above are only some general principles of 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