Electronic equipment used in all kinds of electronic equipment and systems are still in printed circuit board as the main assembly way. Practice has proved that even if the circuit principle diagram design is correct, improper printed circuit board design, also can produce adverse effect on the reliability of electronic equipment. For example, if the two parallel lines in close proximity, PCB will form the signal delay, reflection noise in transmission line terminal formation. Therefore, in the design of the printed circuit board, should pay attention to the right way.
A, ground design in electronic equipment, earthing is an important way to control interference. If you can combine grounding and shielding correct use, can solve most of the interference problem. Electronic equipment in the ground structure roughly systematically, casing (block), digital earth (logically) and simulation, etc. In ground design should pay attention to the following:
1. The correct selection of single point grounding and multipoint earthing in low frequency circuit, the signal of the working frequency is less than 1 MHZ, little impact on its inductance between wiring and devices, and grounding circuit had a greater influence on the formation of the circulation of interference, thus should use one-point earthing. When the working frequency of the signal is more than 10 MHZ, ground impedance becomes large, at this point should try to reduce the ground impedance, nearby multipoint earthing should be adopted. When the working frequency in 1 ~ 10 MHZ, if the grounding, the ground wire length should not exceed 1/20 of the wavelength, otherwise the multipoint earthing method should be adopted.
2. Separate digital circuit and analog circuit on the circuit board has both high speed logic circuit, and linear circuit, should make them apart as far as possible, and both ground not mixed, respectively connected to the power end ground. Try to increase the grounding area of linear circuit.
3. Bold grounding line as far as possible if the ground wire is very thin, grounding potential changes over electric current, the timing signal level instability of the electronic equipment, noise resistance worse. Therefore earth wire should be bold as far as possible, make it can pass three allows current in printed circuit board. If possible, the width of the ground wire should be greater than 3 mm. 4. The ground wire constitute a closed loop road design is composed of a digital circuit PCB only ground system, to make the grounding line closed loop road can obviously improve the ability to resist noise. The reason for this is that there are many integrated circuit components on printed circuit board, especially in case of more components, power consumption due to restrictions of the grounding line thickness, can produce large potential difference on the knot, cause anti-noise ability to drop, if the grounding structure into the loop, would be to reduce the potential difference value, improve the ability to resist noise electronic equipment.
B, electromagnetic compatibility design of electromagnetic compatibility is refers to the electronic equipment in various kinds of electromagnetic environment can still coordination, ability to work effectively. Electromagnetic compatibility design of the purpose is to make electronic devices can inhibit a variety of external interference, make the electronic equipment can work normally in the specific electromagnetic environment, and also can reduce the electronic device itself electromagnetic interference to other electronic equipment.
1. Choose reasonable conductor width due to transient current impact on the printed line produced by the interference is mainly caused by printed conductor inductance components, therefore should minimize the inductance of printed wiring. Printed conductor inductance is proportional to its length, its width is inversely proportional to, so the short and fine wires is favorable to suppress interference. Clock, lead line drive or bus driver signal often contains large transient current, printed wiring to short as possible. For discrete component circuit, printed wiring width at around 1.5 mm, can fully meet the requirements; For integrated circuits, printed wiring width can choose between 0.2 ~ 1.0 mm.
2. Use the correct wiring strategies use equal linear conductor inductance can be reduced, but the mutual inductance between conductor and distributed capacitance increases, the layout of permitting, had better use well glyph structure network cabling, specific side lateral approach is printed circuit board wiring, the other side vertical wiring, then at the cross hole is connected with metallized hole.
In order to prevent the PCB crosstalk between wires, when designing the wiring should try to avoid long distance equal walk the line, the distance between the lines to open and as far as possible, signal wire and ground wire and the power cord as far as possible, don't cross. In some very sensitive to interference signal lines between a grounded printing line, can effectively restrain crosstalk.
In order to avoid the high frequency signal through printed wiring of electromagnetic radiation, the printed circuit board wiring, also should pay attention to the following: to reduce the discontinuity of printed wiring as far as possible, such as wire width don't mutation, the wire should be greater than 90 degrees of corner ban ring line etc.
Clock signal leads the most susceptible to electromagnetic radiation interference, go line should close, with different ground loop drive should be close to the connector.
The bus driver should be close to its desire to drive the bus. For those who leave the printed circuit board, and lead the drive should be tightly close to the connector.
Data bus wiring should be every two signal lines between a signal ground. Best is tightly close to the least important address fuses placed ground loops, which often contains high frequency current.
In PCB layout, high speed, medium speed and low speed logic circuit should be arranged according to the picture 1 device (3) reflection interference suppression in order to prevent the reflection interference appears in the printed line terminal, in addition to the special needs, should as far as possible, shorten the length of the printed line and adopting slow circuit. Add terminal matching when necessary, that is, at the end of the transmission line to ground and added a different power supply side resistance matching resistance. According to the experience, generally faster TTL circuit, when the printed line longer than 10 cm above the terminal matching measures should be adopted. Matching resistance value should be based on the output of the integrated circuit drive current and the maximum absorption current to decide.
C, decoupling capacitors in dc power supply circuit, the change of the load will cause the power supply noise. In digital circuits, for example, when the circuit from one state into another state, will be on the power line to produce a large peak currents, form a transient noise voltage. Configuration decoupling capacitor can restrain the noise due to load changes, is the reliability of the printed circuit board design of a conventional practice, the configuration principle is as follows: low power input end of the jumper a 10 to 100 uf electrolytic capacitor, the position of the printed circuit board permitting, adoption of more than 100 uf electrolytic capacitor anti-interference effect will be better.
Low for each of the integrated circuit chip configuration a 0.01 uF ceramic capacitor. If printed circuit board and a small space, can every 4 ~ 10 chip configuration a 1 ~ 10 uf tantalum electrolytic capacitor, this particularly small high frequency impedance of the device in 500 KHZ ~ 20 MHZ range impedance is less than 1 Ω, and leakage current is very small (below 0.5 uA).
For noise Electronic Products //www.eepw.com.cn/article/150600_2.htm weak, big changes in the off current device and ROM, RAM and other storage-type devices, the chip's power cord should Direct (Vcc) and ground (GND) between the access decoupling capacitors.
decoupling capacitor lead not too long, not particularly high frequency bypass capacitor with leads of four, printed circuit board layout printed circuit board size and the size of the device should be moderate, long lines printed too large, impedance increases not only decrease noise immunity, high cost; too small, the heat is not good, and the line close to the interference vulnerability.
In other aspects of the device layout logic circuits, devices should be related to each other as much as possible to put too close to something, so you can get a better anti-noise effect. Figure 2. When the clock input of the species generator, crystal oscillator and CPU are prone to noise, to be close to each other more. The device is easy to produce noise, low current circuits, high-current circuits, etc. should be kept away from the logic circuit and, if possible, to do another circuit board, which is very important.
E, the thermal design allows for heat dissipation from the perspective of the printed version of the best upright installation, the distance between the board and the board should not generally be less than 2cm, and the arrangement of components on the printed version should follow certain rules: . For a free-convection air cooling equipment, it is best integrated circuit (or other devices) are arranged in a longitudinal manner shown in Figure 3; For devices with forced air cooling, preferably integrated circuit (or other devices) arranged by the horizontal length, as shown in Figure 4. . Devices should be aligned as far as possible in accordance with district heat and cooling degree of the same size on a piece of PCB, heat a small or poor heat resistance of the device (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) on the cooling airflow The most upstream (inlet), heat, or high heat resistance devices (such as power transistors, LSI, etc.) on the most downstream cooling flow. On the other device when the temperature in the vertical direction, the power devices are arranged as close to the top of the PCB, in order to reduce these devices work; in the horizontal direction, the power devices are arranged as close to the edge of the PCB, in order to shorten the heat transfer path . . Temperature sensitive device is preferably placed in the coldest regions (such as the bottom of the device), do not put it above the heating device, multiple devices are interleaved best layout in the horizontal plane. PCB cooling equipment mainly depends on the air flow, so the design to study the air flow path, the rational allocation of components or printed circuit boards. Always tend to place less resistance when the flow of air flow, so the printed circuit board configuration device, to avoid leaving a large airspace in an area. Printed circuit board machine in multi-block configuration should also pay attention the same problem.
A lot of practical experience has shown that the use of reasonable arrangement of devices, can effectively reduce the temperature rise of the printed circuit, so that the failure rate of devices and equipment decreased more than just publish some of the general principles of the reliability of the circuit board design, printed system reliability and specific circuit board has a close relationship, no need to be treated accordingly in the design according to the specific circuit to ensure maximum reliability printed circuit boards.
