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Guide de conception des PCB à grande vitesse VIII: conception de la fiabilité des PCB
Technologie PCB
Guide de conception des PCB à grande vitesse VIII: conception de la fiabilité des PCB

Guide de conception des PCB à grande vitesse VIII: conception de la fiabilité des PCB

2021-08-19
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Author:IPCB

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. Alors..., 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, La plupart des problèmes d'interférence peuvent être résolus. The ground structure of electronic equipment roughly includes system ground, chassis ground (shield ground), digital ground (logical ground), Et analogique. 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, Les points les plus proches doivent être utilisés pour la mise à la terre.. When the working frequency is 1~10MHz, if one-point grounding is adopted, the length of the ground wire should not exceed 1/(((((2)))))0 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. Ils devraient être aussi séparés que 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, Le potentiel de mise à la terre variera en fonction du courant, causing the timing signal level of the electronic device to be unstable and the anti-noise performance to deteriorate. Alors..., the grounding wire should be as thick as possible so that it can pass the allowable current on the printed circuit board. Si 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, La capacité de lutte contre le bruit peut être grandement améliorée en faisant du fil de terre une boucle fermée.. 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. La compatibilité électromagnétique est conçue pour permettre aux appareils électroniques de supprimer diverses interférences externes, so that the electronic equipment can work normally in a specific electromagnetic environment, En même temps, réduire l'interférence électromagnétique de l'équipement électronique lui - même sur d'autres équipements électroniques.


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. Pour les circuits à éléments discrets, the printed wire width is about 1.5.mm, Entièrement conforme aux exigences; Circuits intégrés, 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, Mais l'inductance mutuelle et la capacité distribuée entre les conducteurs augmentent. 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, De l'autre côté du câblage vertical, 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, Lors de la conception du câblage, you should try to avoid long-distance equal wiring, extend the distance between the wires as much as possible, Essayez de ne pas croiser la ligne de signal avec la ligne au sol et la ligne électrique. 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, Les points suivants doivent être notés lors du câblage des circuits imprimés:


. 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, Le conducteur doit être près du connecteur.

.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, L'équipement doit être disposé comme indiqué à la figure 1..


3. Inhibition of reflection interference In order to suppress the reflection interference that appears at the terminal of the printed line, En plus des besoins spéciaux, 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, Ajouter une résistance correspondante de la même résistance à l'extrémité de la ligne de transmission pour se connecter au sol et aux bornes d'alimentation. D'après l'expérience, for general faster TTL circuits, Lorsque la longueur de la ligne d'impression dépasse 10 cm, des mesures d'appariement terminal doivent être prises.. 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.


Trois., decoupling capacitor configuration

In the DC power supply loop, the change of the load will cause the power supply noise. For example, Dans les circuits numériques, 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. La configuration du condensateur de découplage supprime le bruit causé par les changements de charge, 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.

. Configurer un 0.01uF ceramic capacitor for each integrated circuit chip. Si la carte de circuit imprimé est trop petite pour être installée, one 1-10uF tantalum electrolytic capacitor can be configured for every 4-10 chips. L'impédance à haute fréquence de cet équipement est particulièrement faible, and the impedance is less than 1Ω in the range of 500kHz-20MHz. And the leakage current is very small (less than 0.5uA).

.Convient aux équipements à faible bruit et à forte variation de courant pendant l'arrêt, ainsi qu'aux dispositifs de stockage tels que rom et 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. Quand il est trop grand, the printed lines will be long and the impedance will increase, Cela réduit non seulement la résistance au bruit, but also increase the cost.

En termes de disposition de l'équipement, 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. Générateur de temps, crystal oscillator, Les entrées d'horloge du processeur sont sujettes au bruit, so they should be closer to each other. It is very important that noise-prone devices, low-current circuits, Les circuits à courant élevé doivent être aussi éloignés que possible des circuits logiques.. If possible, separate circuit boards should be made. This is very important.


V, thermal design

Du point de vue de la dissipation de chaleur, il est préférable d'installer la plaque d'impression verticalement, la distance entre la plaque et la plaque ne doit pas être inférieure à 2 cm, et la disposition de l'équipement sur la plaque d'impression doit suivre certaines règles:


Pour les équipements refroidis à l'air libre, il est préférable de placer les circuits intégrés (ou d'autres équipements) longitudinalement, comme le montre la figure 3; Pour les équipements utilisant un refroidissement par air forcé, il est préférable de placer les circuits intégrés (ou d'autres équipements) horizontalement, comme le montre la figure 4.

Dans la mesure du possible, l'équipement d'une même carte imprimée doit être disposé en fonction de son pouvoir calorifique et de son degré de dissipation de chaleur. Les équipements à faible pouvoir calorifique ou à faible résistance à la chaleur (tels que les petits Transistors de signalisation, les petits circuits intégrés, les condensateurs électrolytiques, etc.) doivent être refroidis au Sommet du flux d'air (entrée), tandis que les équipements à haute résistance à la chaleur ou à la chaleur (tels que les Transistors de puissance, les grands circuits intégrés, etc.) doivent être placés à l'extrémité inférieure du flux d'air de refroidissement.

Dans le sens horizontal, les équipements de grande puissance doivent être situés le plus près possible du bord de la carte imprimée afin de réduire la trajectoire de transfert de chaleur; Dans le sens vertical, les équipements de grande puissance doivent être situés le plus près possible du Haut de la carte imprimée afin de réduire la température pendant le fonctionnement des autres équipements. 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. Il est préférable de décaler plusieurs équipements sur une surface plane.

La dissipation de chaleur des circuits imprimés dans l'équipement dépend principalement du débit d'air, de sorte que la trajectoire du débit d'air doit être étudiée lors de la conception et que l'équipement ou les circuits imprimés doivent être configurés rationnellement. Lorsque l'air circule, il tend toujours à s'écouler à faible résistance, de sorte qu'il faut éviter de laisser plus d'espace dans une zone particulière lors de la configuration de l'équipement sur une carte de circuit imprimé. La configuration de plusieurs circuits imprimés dans l'ensemble de la machine doit également tenir compte du même problème.

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. La fiabilité des circuits imprimés est étroitement liée à des circuits spécifiques.. 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. Le sol flottant facilite également la coordination entre les circuits à différents potentiels. 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, La terre comme point de référence, 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 points clés de la conception du blindage du champ électrique:

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, Épaisseur non requise, 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.

Augmenter l'épaisseur de la paroi du bouclier.

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

ATL

Seven, electromagnetic compatibilitydesign inside the product

1 Electromagnetic compatibility in printed circuit board design

1.1. Problèmes courants de couplage d'impédance dans les circuits imprimés, and the ground wire is widened.

1.2 The layout of the printed circuit board

※When mixing high speed, medium speed and low speed, Notez les différentes zones de mise en page.

※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.

Il est nécessaire de fournir des lignes de zéro volt spécifiquement pour les circuits analogiques.

Pour réduire les échanges entre les lignes, Si nécessaire, augmenter la distance entre les lignes tracées, Certaines lignes à tension nulle doivent être insérées entre les lignes comme isolant..

※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.

Le poids de ligne sur l'arc d'impression ne doit pas changer brusquement, 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.

Ajouter un condensateur de découplage entre l'alimentation électrique et le sol.

※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. Bruit en mode commun induit par le couplage radiatif entre le boîtier du Transistor de puissance et le radiateur dans le circuit primaire de l'entrée de puissance.

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. Disposition des conducteurs de courant de charge positive sur les circuits imprimés.

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

4. Forme d'onde de commutation du Transistor de compensation.


2.3 réduction du bruit de sortie due au courant inverse de la diode

Changement brusque et inductance distribuée en boucle. The diode junction capacitance forms high-frequency attenuated oscillation, L'inductance en série équivalente de la capacité de filtrage affaiblit l'effet de filtrage. Therefore, La solution à l'interférence de pointe de l'onde de sortie est d'ajouter une petite inductance et une capacité à haute fréquence.


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 méthode générale de câblage:

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.

Dans le cas des paires Torsadées blindées, le courant de signal s'écoule sur les deux fils internes et le courant sonore s'écoule dans le bouclier, de sorte que le couplage de l'impédance commune est éliminé et toute interférence sera détectée simultanément sur les deux fils pour éliminer le bruit.

※The ability of unshielded twisted pair to resist electrostatic coupling is worse. Mais il est encore très efficace pour empêcher la détection de champ magnétique. 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.

Câble ruban non blindé.

The best wiring method is to alternate between the signal and the ground. La deuxième approche est une raison, two signals and one ground, Attendez un peu!, 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 méthode de terminaison des câbles

In high-demand occasions, Un emballage complet à 360° doit être prévu pour les conducteurs internes., Des connecteurs coaxiaux doivent être utilisés pour assurer l'intégrité du blindage du champ électrique..


5 Protection against static electricity

Electrostatic discharge can enter electronic circuits in three ways: direct conduction, Couplage capacitif et inductif.

Une décharge directe dans un circuit endommage généralement le circuit. La stabilité du circuit peut être affectée par la décharge d'objets adjacents par couplage capacitif ou inductif.


Protection method:

1. Construire une structure de blindage complète, and a metal shielded shell with a ground can release the discharge current to the ground.

2. La mise à la terre de l'enceinte métallique peut limiter l'augmentation du potentiel de l'enceinte et provoquer une décharge entre le circuit interne et l'enceinte.

3. Si le circuit interne doit être raccordé à un boîtier métallique, un seul point de mise à la terre doit être utilisé pour empêcher le courant de décharge de passer à travers le circuit interne.

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).