PCB Technical - Eight of high-speed PCB design guidelines: PCB reliability design

At present, electronic equipment is still used in various electronic equipment and systems with printed circuit board s as the main assembly method. Practice has proved that even if the circuit schematic design is correct and the printed circuit board is not properly designed, it will adversely affect the reliability of electronic equipment. For example, if two thin parallel lines of the printed board 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 designing a printed circuit board, care should be taken to adopt the correct method.

1. Ground wire design

In electronic equipment, grounding is an important method to control interference. If the grounding and shielding can be properly combined and used, most interference problems can be solved. 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 design:

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, the ground wire impedance should be reduced as much as possible, and the nearest multiple points should be used for grounding. 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. Separate the digital circuit from the analog circuit;

There are both high-speed logic circuits and linear circuits on the circuit board. 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 printed circuit board. If possible, the width of the ground wire should be greater than 3mm.

4. Form the grounding wire into a closed loop;

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 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 design

Electromagnetic compatibility refers to the ability of electronic equipment to work in a coordinated and effective manner in various electromagnetic environments. The purpose of electromagnetic compatibility design 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 printed lines is mainly caused by the inductance of the printed wires, the inductance of the printed wires should be minimized. The inductance of the printed wire 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 drivers or bus drivers often carry large transient currents, and the printed wires should be as short as possible. For discrete component circuits, the printed wire width is about 1.5mm, which can fully meet the requirements; for integrated circuits, the printed wire 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. The specific method is that one side of the printed board is horizontal. Wiring, the other side of the vertical wiring, and then use metallized holes at the cross hole to connect. In order to suppress the crosstalk between the conductors of the printed circuit board, when designing the wiring, you should try to avoid long-distance equal wiring, extend the distance between the wires as much as possible, and try not to cross the signal wires with the ground wires and the power wires. 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 wires, the following points should be paid attention to when the printed circuit board is wired:

. Minimize the discontinuity of printed wires, for example, the width of the wires should not change suddenly, and the corners of the wires should be greater than 90 degrees to prohibit looping.

.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 printed circuit board, the driver should be next to the connector.

.The wiring of the data bus should clamp a signal ground wire between every 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 currents.

.When arranging high-speed, medium-speed and low-speed logic circuits on the printed circuit board, the devices should be arranged in the manner shown in Figure 1.

3. Inhibition of reflection interference In order to suppress the reflection interference that appears at the terminal 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 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, for general faster TTL circuits, terminal matching measures should be adopted when the printed lines 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, when the circuit changes from one state to another, a large spike current will be generated on the power line, 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 design of printed circuit boards. The configuration principles are as follows:

.Connect a 10-100uF electrolytic capacitor across the power input. If the location of the printed circuit 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 printed circuit board space is small and cannot be installed, one 1-10uF tantalum electrolytic capacitor can be configured for every 4-10 chips. 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.

Fourth, the size of the printed circuit board and the layout of the device

The size of the printed circuit board should be moderate. When it is too large, the printed lines will be long and the impedance will increase, which will not only reduce the noise resistance, but also increase the cost.

In terms of device layout, like other logic circuits, the devices related to each other should be placed as close as possible so that a better anti-noise effect can be obtained. as shown in picture 2. The time generator, crystal oscillator, and the clock input of the CPU are all prone to noise, 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 design

From the perspective of conducive to heat dissipation, the printed plate is best installed upright, the distance between the board and the board should not be less than 2cm, and the arrangement of the devices on the printed plate 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, as shown in Figure 3; for equipment that uses forced air cooling, it is best to arrange integrated circuits (or other devices) ) Arranged horizontally, as shown in Figure 4.

• The devices on the same printed board 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, large-scale integrated circuits, 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 printed board to shorten the heat transfer path; in the vertical direction, high-power devices should be placed as close to the top of the printed board as possible to reduce the temperature of other devices when these devices are working. Influence.

• The temperature-sensitive device is best placed in the lowest temperature area (such as the bottom of the device). Never place it directly above the heating device. It is best to stagger multiple devices on the horizontal plane.

•The heat dissipation of the printed board in the equipment mainly relies on air flow, so the air flow path should be studied during the design, and the device or printed circuit board should be reasonably configured. When air flows, it always tends to flow in places with low resistance, so when configuring devices on a printed circuit board, 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 has shown that the use of a reasonable device arrangement can effectively reduce the temperature rise of the printed circuit, so that the failure rate of devices and equipment is significantly reduced.

The above are only some general principles for the reliability design of printed circuit boards. The reliability of printed circuit boards is closely related to specific circuits. In the design, 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. Disadvantages: Not suitable for high frequency occasions.

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) Drain the charge accumulated on the chassis to avoid internal discharge of the device.

3) The working stability of the connected high equipment, to avoid the change of the potential of the equipment to the earth under the action of the external electromagnetic environment.

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.

2.1.2 Key points of electric field shielding design:

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, the thickness is not required, and the strength should be sufficient.

2.2 Magnetic field shielding

2.2.1 The mechanism of magnetic field shielding

The low magnetic resistance of the high magnetic permeability material acts as a magnetic shunt, which greatly reduces the magnetic field in the shield.

2.2.2 Key points of magnetic field shielding design

1) Use high permeability materials.

2) Increase the wall thickness of the shield.

3) The shielded object should not be close to the shielding body.

4) Pay attention to structural design.

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 compatibilitydesign inside the product

1 Electromagnetic compatibility in printed circuit board design

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

1.2 The layout of the printed circuit 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 Wiring of printed circuit board (single-sided or double-sided)

※Dedicated zero-volt line, the wiring width of the power line is ≥1mm.

※The power line and the ground line are as close as possible, and the power and ground on the entire printed board should be distributed in a "well" shape to balance the distribution line current.

※It is necessary to provide a zero-volt line specially for the analog circuit.

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

※The plugs of the printed circuit should also be arranged with more zero-volt wires as isolation between the wires.

※Pay special attention to the size of the wire loop in the current flow.

※If possible, add R-C decoupling at the entrance of the control line (on the printed board) in order to eliminate the interference factors that may appear in the transmission.

※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 printed circuit 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 When the logic circuit is working, 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 printed circuit 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 design

2.1 Disturbance and suppression of switching power supply to grid conduction

Source of harassment:

1. Non-linear flow.

2. 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:

1. "Trimming" the switching voltage waveform.

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

3. Add a power filter to the mains input circuit.

2.2 Radiation disturbance and suppression of switching power supply

Pay attention to radiation disturbance and suppression

Suppression method:

1. Reduce the loop area as much as possible.

2. The layout of the positive load current conductor on the printed circuit board.

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

4. "Trimming" the transistor switching waveform.

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, the solution to the spike interference in the output wave is to add a small inductance and a high-frequency capacitor.

3 Wiring inside the device

3.1 Electromagnetic coupling between lines and suppression methods

Coupling to the magnetic field:

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

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

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

For capacitive coupling:

1. Increase the distance between lines.

2. The shielding layer is grounded.

3. Reduce the input impedance of sensitive lines.

4. If it is possible to use balanced circuits as input in sensitive circuits, use the inherent common-mode suppression capability of balanced circuits to overcome the interference from interference sources to sensitive circuits.

3.2 General wiring method:

According to the power classification, the wires of different classifications should be bundled separately, and the distance between the separated wiring harnesses should be 50~75mm.

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, and any interference will be induced to the two wires at the same time to cancel the noise .

※The ability of unshielded twisted pair to resist electrostatic coupling is worse. But it still has a good effect on preventing magnetic field induction. The shielding effect of unshielded twisted pair is proportional to the number of twists per unit length of wire.

※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, two signals and one ground, and so on, or a dedicated ground plane.

4.2 Grounding of the cable shielding layer

In short, the method of directly grounding the load is inappropriate, because the shielding layer grounded at both ends provides a shunt for the magnetically induced ground loop current, which reduces the magnetic field shielding performance.

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.

5 Protection against static electricity

Electrostatic discharge can enter electronic circuits in three ways: direct conduction, 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:

1. Establish a complete shielding structure, and a metal shielded shell with a ground can release the discharge current to the ground.

2. The metal shell grounding can limit the rise of the shell potential and cause the discharge between the internal circuit and the shell.

3. 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.

4. Add protective devices at the cable entrance.

5. Add a protective ring at the entrance of the printedboard (the ring is connected to the grounding terminal).