Design reliability principle of circuit design and PCB wiring
At present, printed circuit board is still the main assembly mode of electronic equipment and systems. Practice has proved that even if the circuit schematic design is correct and the printed circuit board design is improper, the reliability of electronic equipment will be adversely affected. For example, if two thin parallel lines of PCB are close together, the delay of signal waveform will be formed, and the reflected noise will be formed at the terminal of transmission line. Therefore, when designing PCB, we should pay attention to the correct method.
Grounding is an important method to control interference in electronic equipment. If the grounding and shielding can be used correctly, most interference problems can be solved. The structure of ground wire in electronic equipment includes system ground, shell 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) select single point grounding and multi-point grounding correctly in the low-frequency circuit, the signal working frequency is less than 1MHz, its wiring and inductance between devices have little influence, while the circulation formed by the grounding circuit has great influence on the interference, so one point grounding should be adopted. When the working frequency of the signal is greater than 10MHz, the impedance of the ground wire becomes large. At this time, the impedance of the ground wire 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 shall not exceed 1 / 20 of the wavelength, otherwise multi-point grounding method shall be adopted.
2) separate the digital circuit and 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 wires of the power supply end respectively. Increase the grounding area of linear circuit as much as possible.
3) if the ground wire is as thin as possible, the ground potential will change with the change of the current, resulting in the instability of the timing signal level of the electronic equipment and the deterioration of the anti noise performance. Therefore, the ground wire should be thickened as much as possible so that it can pass through the allowable current of three positions on the printed circuit board. If possible, the width of the earth wire should be greater than 3mm
4) when the ground wire is used to form a closed loop to design the ground wire system of printed circuit board which is only composed of digital circuit, making the ground wire into a closed loop can obviously improve the anti noise ability. The reason lies in: there are many integrated circuit components on the printed circuit board, especially when there are components with more power consumption, because of the limitation of the thickness of the ground wire, a large potential difference will be generated on the ground junction, resulting in the reduction of the anti noise ability. If the grounding 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 EMC design
EMC design EMC 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, but also reduce the electromagnetic interference of electronic equipment to other electronic equipment.
1) select a reasonable conductor width. The impact interference caused by transient current on printed wire is mainly caused by the inductance component of printed wire. Therefore, the inductance of printed wire should be minimized. The inductance of printed wire is directly proportional to its length and inversely proportional to its width, so a short and precise wire is advantageous to restrain interference. The signal line of clock lead, line driver or bus driver often contains 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 wire can be selected between 0.2-1.0mm.
2) adopting the correct wiring strategy and adopting the equal wiring can reduce the conductor inductance, but the mutual inductance and distributed capacitance between the conductors increase. If the layout allows, it is better to adopt the well shaped network wiring structure, the specific method is that one side of the printed board is horizontally wired, the other side is longitudinally wired, and then the crossing holes are connected with metallized holes.
In order to suppress crosstalk between printed circuit board wires, long-distance equal routing shall be avoided as much as possible in the design of wiring, the distance between wires shall be opened as much as possible, and the signal wire, ground wire and power wire shall not be crossed as much as possible. A grounded printed wire is set between some signal wires which are very sensitive to interference, which can effectively suppress crosstalk.
In order to avoid the electromagnetic radiation caused by the high frequency signal passing through the printed wire, the following points shall be paid attention to when wiring the printed circuit board:
● try to reduce the discontinuity of printed wire, for example, the width of the wire shall not be changed suddenly, and the corner of the wire shall be greater than 90 degrees. It is forbidden to loop the wire, etc.
● the clock signal lead wire is most likely to produce electromagnetic radiation interference. When wiring, it shall be close to the ground circuit, and the driver shall be close to the connector.
● the bus driver shall be close to the bus to be driven. For those leads leaving the PCB, the driver should be next to the connector.
● one signal ground wire shall be clamped between each two signal wires for data bus wiring. It is best to place the ground loop next to the least important address lead, which often carries high frequency current.
● when arranging high-speed, medium speed and low-speed logic circuits on printed boards, arrange devices. 3. Suppress reflection interference. In order to suppress reflection interference at the terminals of printed lines, in addition to special needs, the printed circuit should be shortened as much as possible
3 configuration of decoupling capacitor
In the DC power circuit, the change of load will cause power 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 the load change, which is a routine method for the reliability design of PCB. The configuration principle is as follows:
● the input end of the power supply is connected with a 10-100uf electrolytic capacitor. If the position of the printed circuit board allows, the anti-interference effect of the electrolytic capacitor above 100uF will be better.
● a 0.01uF ceramic capacitor is configured for each IC chip. If the space of printed circuit board is too small to be installed, a tantalum electrolytic capacitor of 1-10uf can be configured for every 4-10 chips. The high frequency impedance of this device is very small, the impedance is less than 1 Ω in the range of 500khz-20mhz, and the leakage current is very small (less than 0.5ua).
● for devices with weak noise capability and large current change during shutdown and memory devices such as ROM and ram, decoupling capacitance 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 the high-frequency bypass capacitor shall not have lead
4 dimensions of printed circuit board and arrangement 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 will not be good, and it is easy to be interfered by adjacent lines.
In terms of device arrangement, as in other logic circuits, the devices related to each other should be placed as close as possible, so as to obtain better anti noise effect. As shown in Figure 2. The clock inputs of time generator, crystal oscillator and CPU are easy to generate noise, so they should be closer to each other. Devices, small current circuits and large current circuits that are easy to generate noise shall be far away from logic circuits as far as possible. If possible, circuit boards shall be made separately, which is very important.
5 thermal design from the perspective of heat dissipation
The printed board shall be installed vertically, and the distance between the board and the Board shall not be less than 2cm in general, and the arrangement of devices on the printed board shall follow certain rules: for the equipment adopting free convection air cooling, it is better to arrange the integrated circuit (or other devices) in longitudinal and rectangular form; for the equipment adopting forced air cooling, it is better to arrange the integrated circuit (or its The devices are arranged in a horizontal long way. The devices on the same PCB shall be arranged in different areas according to their heat value and heat dissipation degree as far as possible. The devices with small heat value or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) shall be placed on the top stream (entrance) of the cooling air flow. The devices with large heat value or good heat resistance (such as power transistors, large-scale capacitors, etc.) Mode integrated circuit, etc.) is placed at the downstream of the cooling air flow.
In the horizontal direction, the high-power devices shall be arranged as close as possible to the edge of the printed circuit board to shorten the heat transfer path; in the vertical direction, the high-power devices shall be arranged as close as possible to the top of the printed circuit board to reduce the impact of these devices on the temperature of other devices during operation. The devices sensitive to the temperature shall be placed in the lowest temperature area (such as the bottom of the device), never It should be placed directly above the heating device, and multiple devices should be staggered on the horizontal plane. The heat dissipation of PCB in the equipment mainly depends on the air flow, so the air flow path should be studied in the design, and the devices or PCB should be configured reasonably. When air flows, it always tends to flow where the resistance is small, so when the devices are arranged on the printed circuit board, it is necessary to avoid leaving a large airspace in a certain area. The same problem should be paid attention to in the configuration of multiple printed circuit boards in the whole machine.
A large number of practical experience shows that the reasonable arrangement of devices can effectively reduce the temperature rise of printed circuit, so that the failure rate of devices and equipment can be significantly reduced. The above mentioned are just some general principles for reliability design of printed circuit board. The reliability of printed circuit board is closely related to the specific circuit, and it is not necessary to carry out corresponding design according to the specific circuit The reliability of printed circuit board can be guaranteed to the greatest extent.