Технология PCB - Руководство по проектированию высокоскоростных PCB: проектирование надёжности PCB

At present, электронное оборудование по - прежнему используется в различных электронных оборудованиях и системах печатныйcircuit boards as the main assembly method. Practice has proved that even if the circuit schematic проектировать is correct and the печатныйплата цепи is not properly проектироватьed, it will adversely affect the reliability of electronic equipment. For example, if two thin parallel lines of the печатныйboard are close together, it will cause a delay in the signal waveform, and reflection noise will be formed at the end of the transmission line. Therefore, when проектироватьing a печатныйплата цепи, care should be taken to adopt the correct method.

1. Ground wire проектировать

в электронном оборудовании, grounding is an important method to control interference. If the grounding and shielding can be properly combined and used, Большинство проблем с помехами можно решить. The ground structure of electronic equipment roughly includes system ground, chassis ground (shield ground), digital ground (logical ground), and analog ground. The following points should be paid attention to in the ground wire проектировать:

1. Correctly choose single-point grounding and multi-point grounding;

In the low-frequency circuit, the operating frequency of the signal is less than 1MHz, and the inductance between its wiring and the device has less influence, and the circulating current formed by the grounding circuit has a greater influence on the interference, so one point grounding should be adopted. When the signal operating frequency is greater than 10MHz, the ground wire impedance becomes very large. At this time, сопротивление заземления должно быть сведено к минимуму, and the nearest multiple points should be used for grounding. When the working frequency is 1～10MHz, Если принять точку приземления, the length of the ground wire should not exceed 1/20 of the wavelength, В противном случае следует применять многоточечный способ приземления.

2. Separate the digital circuit from the analog circuit;

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

3. Make the ground wire as thick as possible;

If the ground wire is very thin, the ground potential will change with the current change, causing the timing signal level of the electronic device to be unstable and the anti-noise performance to deteriorate. Therefore, the grounding wire should be as thick as possible so that it can pass the allowable current on the печатныйcircuit board. If possible, the width of the ground wire should be greater than 3mm.

4. Form the grounding wire into a closed loop;

When проектироватьing the ground wire system of the печатныйcircuit board composed of only digital circuits, making the ground wire into a closed loop can significantly improve the anti-noise ability. причина в том, что на основе многих компонентов интегральных схем печатныйcircuit board, especially when there are components with high power consumption, due to the limitation of the thickness of the ground wire, a large potential difference will be generated on the ground junction, which will cause the anti-noise ability to decrease , 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.

2. Electromagnetic compatibility проектировать

Electromagnetic compatibility refers to the ability of electronic equipment to work in a coordinated and effective manner in various electromagnetic environments. The purpose of электромагнитная совместимость проектировать is to enable electronic equipment to suppress all kinds of external interference, so that the electronic equipment can work normally in a specific electromagnetic environment, and at the same time to reduce the electromagnetic interference of the electronic equipment itself to other electronic equipment.

1. Choose a reasonable wire width. Since the impact interference generated by the transient current on the печатныйлиния в основном вызвана индуктивностью трансформатора печатныйwires, the inductance of the печатныйwires should be minimized. The inductance of the печатныйwire is proportional to its length and inversely proportional to its width, so short and precise wires are beneficial to suppress interference. The signal lines of clock leads, Row драйвер или шинный драйвер обычно имеет большой переходный ток, and the печатныйwires should be as short as possible. For discrete component circuits, the печатныйwire width is about 1.5mm, which can fully meet the requirements; for integrated circuits, the печатныйwire width can be selected between 0.2mm and 1.0mm.

2. Adopting the correct wiring strategy and using equal wiring can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is best to use a grid-shaped wiring structure in the shape of a cross. в частности печатныйboard is horizontal. Wiring, вертикальная сторона провода, and then use metallized holes at the cross hole to connect. In order to suppress the crosstalk between the conductors of the печатныйcircuit board, when проектироватьing the wiring, you should try to avoid long-distance equal wiring, extend the distance between the wires as much as possible, Постарайтесь не пересекать сигнальные линии с Заземляющими и силовыми линиями. Setting a grounded печатныйline 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 печатныйwires, the following points should be paid attention to when the печатныйcircuit board is wired:

следует свести к минимуму разрывы в печатных линиях, например, не следует резко менять ширину линии и угол поворота линии должен быть выше 90 градусов, чтобы запретить циркуляцию.

—The clock signal lead is most likely to produce electromagnetic radiation interference. When routing the wire, it should be close to the ground loop, and the driver should be close to the connector.

—The bus driver should be close to the bus to be driven. For those leads that leave the печатныйcircuit board, the driver should be next to the connector.

проводка шины данных должна быть снабжена сигнальной линией между двумя линиями. желательно, чтобы заземленный контур был расположен рядом с линией отвода наиболее важных адресов, поскольку последний обычно имеет высокочастотный ток.

—When arranging high-speed, логическая схема с верхними и средними скоростями печатныйcircuit board, the devices should be arranged in the manner shown in Figure 1.

3. подавление помех от отражения, чтобы подавить помехи от отражения в конце датчика печатныйline, in addition to special needs, the length of the печатныйline should be shortened as much as possible and a slow circuit should be used. Terminal matching can be added when necessary, that is, a matching resistor of the same resistance is added to the end of the transmission line to the ground and the power terminal. According to experience, для схем TTL, terminal matching measures should be adopted when the печатныйlines are longer than 10cm. The resistance value of the matching resistor should be determined according to the maximum value of the output drive current and the absorption current of the integrated circuit.

Three, decoupling capacitor configuration

In the DC power supply loop, the change of the load will cause the power supply noise. For example, in digital circuits, когда схема из одного состояния в другой, на линии электропитания создаётся большой пиковый ток, forming a transient noise voltage. The configuration of decoupling capacitors can suppress the noise generated by load changes, which is a common practice in the reliability проектировать of печатныйcircuit boards. The configuration principles are as follows:

—Connect a 10-100uF electrolytic capacitor across the power input. If the location of the печатныйcircuit board allows, the anti-interference effect of using an electrolytic capacitor above 100uF will be better.

— Configure a 0.01uF ceramic capacitor for each integrated circuit chip. If the печатныйcircuit board space is small and cannot be installed, каждый 4 - 10 чипов может настраиваться на 1 - 10уф танталовый электролитический конденсатор. The high-frequency impedance of this device is particularly small, and the impedance is less than 1Ω 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 changes during turn-off and storage devices such as ROM and RAM, a decoupling capacitor should be directly connected between the power line (Vcc) and ground (GND) of the chip.

—Leads of decoupling capacitors cannot be too long, especially high-frequency bypass capacitors.

четвёртый, the size of the печатныйcircuit board and the layout of the device

размер печатной платы должен быть умеренным. когда это слишком большое, печатные линии будут длинными, сопротивление увеличится, что не только снижает шумоустойчивость, но и увеличивает расходы.

In terms of device layout, like other logic circuits, взаимосвязанное оборудование должно быть как можно ближе, с тем чтобы повысить шумоустойчивость. Как показано на диаграмме 2. The time generator, crystal oscillator, часовой ввод процессора легко генерирует шум, so they should be closer to each other. It is very important that noise-prone devices, low-current circuits, and high-current circuits should be kept away from logic circuits as much as possible. If possible, separate circuit boards should be made. This is very important.

Five, thermal проектировать

с точки зрения теплоотдачи, the печатныйplate is best installed upright, расстояние между плитами не должно быть меньше 2cm, and the arrangement of the devices on the печатныйplate should follow certain rules:

¢ For equipment that uses free convection air cooling, it is best to arrange integrated circuits (or other devices) in a longitudinal manner, Как показано на рисунке 3; для оборудования с принудительным воздушным охлаждением, it is best to arrange integrated circuits (or other devices) ) Arranged horizontally, as shown in Figure 4.

¢ The devices on the same печатныйboard should be arranged as far as possible according to their calorific value and degree of heat dissipation. Devices with low calorific value or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be cooled At the top of the airflow (at the entrance), devices with large heat or heat resistance (such as power transistors, большая интегральная схема, etc.) are placed at the most downstream of the cooling airflow.

¢ In the horizontal direction, high-power devices should be placed as close as possible to the edge of the печатныйboard to shorten the heat transfer path; in the vertical direction, high-power devices should be placed as close to the top of the печатныйboard as possible to reduce the temperature of other devices when these devices are working. Influence.

- оборудование, чувствительное к температуре, лучше всего размещать в зоне с наименьшей температурой (например, внизу устройства). не поместите его прямо над нагревателем. лучше разойтись по горизонтали с несколькими устройствами.

¢The heat dissipation of the печатныйboard in the equipment mainly relies on air flow, so the air flow path should be studied during the проектировать, and the device or печатныйcircuit board should be reasonably configured. When air flows, it always tends to flow in places with low resistance, so when configuring devices on a печатныйcircuit board, не покидать район. The configuration of multiple печатныйcircuit boards in the whole machine should also pay attention to the same problem.

A lot of practical experience has shown that the use of a reasonable device arrangement can effectively reduce the temperature rise of the печатныйcircuit, so that the failure rate of devices and equipment is significantly reduced.

The above are only some general principles for the reliability проектировать of печатныйcircuit boards. The reliability of печатныйcircuit boards is closely related to specific circuits. In the проектировать, it is not necessary to perform corresponding processing according to specific circuits in order to ensure the printing to the greatest extent. The reliability of the circuit board.

Sixth, product interference suppression program

1 Ground

1.1 The signal ground of the device

Purpose: To provide a common reference potential for any signal in the device.

Method: The signal grounding system of the equipment can be a metal plate.

1.2 Basic signal grounding method

There are three basic signal grounding methods: floating ground, single-point grounding, and multi-point grounding.

1.2.1 Floating Ground Purpose: Isolate the circuit or equipment from the common ground wire that may cause circulating currents. Floating ground also makes it easy to coordinate between circuits of different potentials. Disadvantages: It is easy to accumulate static electricity and cause strong electrostatic discharge. A compromise solution: connect a bleeder resistor.

1.2.2 Single-point grounding: Only one physical point in the line is defined as the grounding reference point, and all grounding needs to be connected here. недостаток: не подходит для высокочастотных ситуаций.

1.2.3 Multi-point grounding method: All points that need to be grounded are directly connected to the ground plane closest to it, so that the length of the grounding wire is the shortest. Disadvantages: maintenance is troublesome.

1.2.4 Mixed grounding Choose single-point and multi-point grounding as required.

1.3 Treatment of signal grounding wire (lap joint)

Bonding is the establishment of a low-impedance path between two metal points.

There are direct and indirect overlapping methods.

Regardless of the lap method, the most important thing is to emphasize a good lap.

1.4 Grounding of the equipment (connect to the earth)

The equipment is connected with the earth, with the earth as a reference point, the purpose is:

1) Realize the safety grounding of the equipment

2) заряд, накапливаемый в воздушном корпусе во избежание внутренних разрядов в устройстве.

3) The working stability of the connected high equipment, избежать изменения электрического потенциала устройства на землю под действием внешней электромагнитной среды.

1.5 Method of pulling the ground and grounding resistance Ground rod.

1.6 Grounding of electrical equipment

2 shield

2.1 Electric field shielding

2.1.1 The mechanism of electric field shielding Coupling between distributed capacitances Processing method:

1) Increase the distance between A and B.

2) B is as close to the grounding plate as possible.

3) Insert a metal shield between A and B.

2.1.ключевые точки защиты электрического поля проектировать:

1) The shielding plate is programmed to control the protected object; the shielding plate must be well grounded.

2) Pay attention to the shape of the shielding plate.

3) The shielding board should be a good conductor, толщина не требуется, and the strength should be sufficient.

2.2 Magnetic field shielding

2.2.механизм защиты магнитного поля

низкое магнитное сопротивление материала с высокой магнитной проницаемостью играет роль магнитного шунта, что значительно снижает магнитное поле в экране.

2.2.2 Key points of magnetic field shielding проектировать

1) Use high permeability materials.

2) увеличить толщину стенки кожуха.

3) защита не должна быть ближе к экрану.

4) обратите внимание на конструкцию.

5) For strong use of double-layer magnetic shields.

2.3 Mechanism of electromagnetic field shielding

1) The reflection of the surface.

2) Absorption inside the shield.

2.3.2 The effect of materials on electromagnetic shielding

2.4 The actual electromagnetic shielding body

Seven, electromagnetic compatibility проектировать inside the product

1 Electromagnetic compatibility in печатныйcircuit board проектировать

1.1 Common impedance coupling problems in печатныйcircuit boards The digital ground is separated from the analog ground, and the ground wire is widened.

1.2 The layout of the печатныйcircuit board

»When mixing high speed, medium speed and low speed, pay attention to different layout areas.

»It is necessary to separate low analog circuit and digital logic.

1.3 прокладка печатных плат (односторонняя или двусторонняя)

»Dedicated zero-volt line, the wiring width of the power line is ‰¥1mm.

как можно ближе к линиям электропитания и заземления, and the power and ground on the entire печатныйраспределительный щит должен быть расположен в форме "колодца", чтобы уравновесить ток распределительных линий.

необходимо выделить линии нулевого Вольта для аналоговых схем.

»In order to reduce the crosstalk between lines, the distance between печатныйlines can be increased if necessary, and some zero-volt lines should be inserted as isolation between lines.

вилка питания Мата печатныйцепь должна также установить больше проводов нулевого напряжения, как изоляция между проводами.

заострить внимание на размерах контура проводов в токе.

если это возможно, на входе контрольной линии (на печатных платах) добавлена развязка R - C для устранения возможных помех при передаче.

»The line width on the printing arc should not be changed suddenly, and the wire should not be suddenly cornered (‰¥90 degrees).

1.4 Helpful suggestions for using logic circuits on печатныйcircuit boards

»No need for those that can use high-speed logic circuit.

»Add a decoupling capacitor between the power supply and the ground.

»Pay attention to the waveform distortion in long-line transmission.

»Use the R-S trigger as a buffer for the coordination between the button and the electronic circuit.

1.4.1 когда работает логическая схема, the power line interference introduced and the suppression method

1.4.2 Distortion in the transmission of the output waveform of the logic circuit

1.4.3 Coordination between button operation and electronic circuit work

1.5 The interconnection of the печатныйcircuit board is mainly the crosstalk between the lines, and the influencing factors:

»Right angle wiring

»Shielded wire

»Impedance matching

»Long-term drive

2 Electromagnetic compatibility in switching power supply проектировать

2.1 Disturbance and suppression of switching power supply to grid conduction

источник домогательств:

‘  Non-linear flow.

‘¡The conduction common mode noise generated by the radiant coupling between the power transistor case and the heat sink in the primary circuit at the input end of the power supply.

Suppression method:

‘  "Trimming" the switching voltage waveform.

‘¡ Install an insulating gasket with a shielding layer between the transistor and the heat sink.

Добавить сетевой фильтр в входную схему питания.

2.2 Radiation disturbance and suppression of switching power supply

Pay attention to radiation disturbance and suppression

Suppression method:

‘  Reduce the loop area as much as possible.

‘¡The layout of the positive load current conductor on the печатныйcircuit board.

‘¢Use soft recovery diodes in the secondary line rectification circuit or connect polyester film capacitors in parallel with the diodes.

форма переключателя транзистора « подстраивания».

2.3 Reduction of output noiseThe reason is the diode reverse current

Steep changes and loop distribution inductance. The diode junction capacitance forms high-frequency attenuated oscillation, and the equivalent series inductance of the filter capacitor weakens the filtering effect. Therefore, Решение проблемы пиковых помех выходной волны путем добавления небольшой индуктивности и высокочастотной ёмкости.

соединение внутри устройства

3.1 Методы электромагнитной связи и подавления между линиями

Coupling to the magnetic field:

‘ The best way to reduce the loop area of interference and sensitive circuits is to use twisted pair and shielded wires.

‘¡ Increase the distance between the lines (to reduce the mutual inductance).

‘¢Try to make the interference source line and the induced line be wired at right angles.

For capacitive coupling:

‘  Increase the distance between lines.

‘¡ The shielding layer is grounded.

уменьшение входного сопротивления чувствительных схем.

‘£ If it is possible to use balanced circuits as input in sensitive circuits, преодоление помех от источника помех к чувствительным схемам.

3.2 General wiring method:

According to the power classification, the wires of different classifications should be bundled separately, расстояние между разделёнными пучками должно составлять от 50 до 75 мм.

4 Grounding of shielded cables

4.1 Commonly used cables

»Twisted pair is very effective when used below 100KHz, and it is limited due to uneven characteristic impedance and the resulting waveform reflection at high frequencies.

»With shielded twisted pair, the signal current flows on the two inner wires, and the noise current flows in the shielding layer, so the coupling of the common impedance is eliminated, Любые помехи одновременно индуктируются двумя проводами, чтобы устранить шум .

»The ability of unshielded twisted pair to resist electrostatic coupling is worse. But it still has a good effect on preventing magnetic field induction. экранированный эффект от незащищённой двухжильной проводки прямо пропорциональн крутке на единицу длины проволоки.

»The coaxial cable has more uniform characteristic impedance and lower loss, so that it has better characteristics from true current to very high frequency.

»Unshielded ribbon cable.

The best wiring method is to alternate between the signal and the ground. The second method is one ground, два сигнала и один заземление, and so on, или специальный наземный самолет.

4.2 Grounding of the cable shielding layer

Короче говоря, метод прямого заземления нагрузки неуместен, так как экран с двумя концами заземления обеспечивает шунтирование тока в магнитно - индукционном контуре, что снижает производительность магнитной защиты.

4.3 Termination method of cable

In high-demand occasions, a complete 360° package should be provided for the inner conductor, and a coaxial connector should be used to ensure the integrity of the electric field shielding.

защита от статического электричества

электростатический разряд может войти в электронную цепь тремя способами: прямая проводимость, capacitive coupling and inductive coupling.

Electrostatic discharge directly to the circuit often causes damage to the circuit. Discharge to adjacent objects through capacitive or inductive coupling will affect the stability of the circuit.

Protection method:

‘  Establish a complete shielding structure, металлическая оболочка с заземленным экраном может высвобождать ток разряда на землю.

- заземление металлических корпусов может ограничить повышение потенциала оболочки и привести к разряду внутренней цепи и оболочки.

‘¢ If the internal circuit is to be connected to the metal shell, a single point grounding should be used to prevent the discharge current from flowing through the internal circuit.

- добавлена защита к входу кабеля.

‘¤Add a protective ring at the entrance of the печатныйboard (the ring is connected to the grounding terminal).