What are the wiring requirements of the circuit board?

In PCB design, wiring is an important step to complete product design. It can be said that the preparatory work in the early stage is done for it. In the whole PCB, the wiring design process has the highest clarity, the least skill and the largest workload. PCB wiring has single-sided wiring, double-sided wiring and multi-layer wiring. There are also two ways of wiring: automatic wiring and interactive wiring. Before automatic wiring, strict wires can be alternately wired in advance, and the edges of input and output terminals should be avoided to be adjacent and parallel to avoid reflection interference. If necessary, grounding wire should be added for isolation, and the wiring of two adjacent layers should be perpendicular to each other, which is easy to produce parasitic coupling in parallel.

The routing rate of automatic routing depends on good layout, and the routing rules can be set in advance, including the number of bends, through holes, steps and so on. Generally, tentative warp laying is carried out first, short wires are connected quickly, and then maze wiring is carried out. First of all, the wires to be laid are globally optimized, and the laid wires can be disconnected as needed. And try to rewire to improve the overall effect.

For the current high-density PCB design, I feel that through holes are not suitable, which wastes a lot of valuable wiring channels. In order to solve this contradiction, blind hole and buried hole technology appeared, which not only completed the function of through hole, but also saved many wiring channels, making the wiring process more convenient, smooth and perfect. The design process of PCB is a complex and simple process. To master it well, it needs the experience of the majority of electronic engineering designers.

1 Handling of power supply and ground wire

Even if the wiring in the whole PCB board is well done, the interference caused by poor consideration of power supply and ground wire will reduce the performance of the product and sometimes even affect the success rate of the product. Therefore, we should pay attention to the wiring of electricity and ground wire, minimize the noise interference caused by electricity and ground wire, and ensure the product quality.

For every engineer engaged in electronic product design, it is clear how the noise between the ground wire and the power wire is caused. Now, only noise suppression by reducing types will be described:

(1), it is well known to add decoupling capacitance between power supply and ground.

(2) Widen the width of power supply and ground wire as much as possible, preferably the ground wire is wider than the power supply wire. Their relationship is: ground wire > power wire > signal wire. Generally, the width of signal line is 0.2 ~ 0.3 mm, and the narrowest can reach 0.05~0.07mm, and the power line is1.2 ~ 2.5 mm..

For the PCB of digital circuit, a wide grounding conductor can be used to form a loop, that is, a grounding net (the ground of analog circuit cannot be used in this way).

(3) Use a large area of copper layer as the ground wire, and connect all the unused ground wires on the printed board as the ground wire. Or make it into a multilayer board, a layer of power supply and a layer of ground wire.

2 * * Processing of Digital Circuit and Analog Circuit

Nowadays, many PCBs are no longer single functional circuits (digital or analog circuits), but are composed of digital and analog circuits. Therefore, it is necessary to consider the mutual interference between them, especially the noise interference of the ground wire.

The digital circuit has high frequency and the analog circuit has strong sensitivity. For signal lines, high-frequency signal lines should be as far away from sensitive analog circuit devices as possible. For the ground wire, only one node of the whole PCB leads to the outside world, so it is necessary to deal with the digital, analog and grounding problems inside the PCB. In fact, the digital ground and analog ground inside the board are separated and not connected with each other, only at the interface between PCB and the outside world (such as plug). Digital ground is slightly short-circuited to analog ground. Please note that there is only one connection point. Some of them are not on the PCB, which is determined by the system design.

3 The signal wire is laid on the electrical (grounding) layer.

When wiring multi-layer printed boards, because there are not many wires left in the signal line layer, adding more layers will cause waste and correspondingly increase the production workload and cost. To solve this contradiction, we can consider wiring on the electrical (grounding) layer. First of all, we should consider the power level, and then the class. Because it is best to preserve the integrity of the formation.

4 Treatment of connecting legs in large area conductors

In a large area of grounding (electricity), the pins of common components are connected to it, so the treatment of pins needs to be considered comprehensively. As far as electrical performance is concerned, it is best for the bonding pads of the component pins to be completely connected with the copper surface, but there are some hidden dangers in the welding and assembly of components, such as: ① welding requires a high-power heater. ② It is easy to cause virtual solder joints. Therefore, considering the electrical performance and process requirements, a cross-shaped pad is made, which is called thermal shield, commonly known as thermal pad. This can greatly reduce the possibility of virtual solder joints caused by excessive heat dissipation in the cross section during welding. The grounding pin of multilayer board is treated the same.

5 the role of network system in wiring

In many CAD systems, wiring is determined according to the network system. The grid is too dense, the paths increase, but the step size is too small, and the data in the drawing field is too large, which will inevitably require higher storage space for equipment and have a great impact on the running speed of computer electronic products. However, some paths are invalid, such as those occupied by pads of component legs or by mounting holes and fixing holes. The grid is too sparse and the paths are too few, which has a great influence on the distribution rate. Therefore, a reasonable grid system is needed to support wiring.

The distance between the legs of a standard part is 0. 1 inch (2.54 mm), so the basis of a grid system is generally 0. 1 inch (2.54 mm) or an integer multiple of less than 0. 1 inch, such as 0.05 inch, 0.025 inch, 0.02 inch, etc.

6 Design Rule Check (DRC)

After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules formulated by the designer, and at the same time confirm whether the formulated rules conform to the requirements of PCB production technology. General inspection includes the following aspects:

(1), whether the distances between lines, lines and component pads, lines and vias, component pads and vias, and vias and vias are reasonable and meet the production requirements.

(2) Is the width of the power line and the ground line appropriate and the coupling tight (low wave impedance)? Is there any place on the PCB where the ground wire can be widened?

(3) Whether the best measures have been taken for the key signal lines, such as the shortest length, protection lines, and obvious separation of input lines and output lines.

(4) Whether the analog circuit and the digital circuit have their own independent grounding wires.

(Whether the graphics (such as icons and labels) added on PCB will cause signal short circuit.

(6) Modify some unsatisfactory linetypes.

(Is there a process line on the PCB? Whether the solder resist meets the production process requirements, whether the solder resist size is appropriate, and whether there are characters on the device pads, so as not to affect the quality of electrical appliances.

(8) Whether the edge of the outer frame of the power supply layer in the multilayer board is narrowed, such as the copper foil of the power supply layer exposed outside the board, it is easy to cause short circuit.

Second PCB layout

In design, layout is an important link. The result of layout will directly affect the effect of wiring, so it can be considered that reasonable layout is the first step of PCB design success.

There are two ways of layout, one is interactive layout and the other is automatic layout. Generally, it is adjusted through interactive layout on the basis of automatic layout. In the layout, the doors can be redistributed according to the wiring situation, and the two doors can be interchanged, making it the best layout for wiring. After the layout is completed, the design documents and related information can be returned and marked on the schematic diagram, so that the relevant information in the PCB board is consistent with the schematic diagram, so that the future filing and design modification can be synchronized, and the simulation related information can be updated, thus verifying the electrical performance and function of the circuit at the board level.

-Consider the overall aesthetic feeling

Whether a product is successful or not depends on the internal quality and the overall beauty. Only when both are perfect can the product be considered successful.

On a PCB, the layout of components should be balanced, dense and orderly, not top-heavy.

-Layout check

Is the printed board size consistent with the size of the processing drawing? Can it meet the requirements of PCB manufacturing process? Are there any positioning marks?

Are there any conflicts between two-dimensional space and three-dimensional space?

Is the component layout orderly? Are you all finished?

Can parts that need to be replaced frequently be easily replaced? Is it convenient to insert the patch panel into the equipment?

Is there an appropriate distance between the heat sensitive element and the heating element?

Is it convenient to adjust the adjustable parts?

Where heat dissipation is needed, is a radiator installed? Is the air flow smooth?

Is the signal flow smooth and the interconnection shortest?

It's plugs, sockets, etc. Contradictions with mechanical design?

Have you considered the interference of the line?

The third kind of high-speed PCB design

(A), the challenges faced by electronic system design

With the large-scale improvement of system design complexity and integration, electronic system designers are engaged in circuit design above 100MHZ, and the working frequency of the bus has reached or exceeded 50MHZ, and some even exceeded 100MHZ. At present, about 50% of the designed clock frequency exceeds 50MHz, and nearly 20% of the designed clock frequency exceeds 120MHz.

When the system works at 50MHz, there will be transmission line effect and signal integrity problems; When the system clock reaches 120MHz, the PCB based on the traditional method will not work unless the knowledge of high-speed circuit design is used. Therefore, high-speed circuit design technology has become a design means that electronic system designers must adopt. Only by using the design technology of high-speed circuit designers can the controllability of the design process be realized.

(2) What is a high-speed circuit?

It is generally believed that if the frequency of digital logic circuits reaches or exceeds 45MHZ~50MHZ, and the circuits operating above this frequency have occupied a certain proportion in the whole electronic system (for example, 1/3), it is called high-speed circuits.

In fact, the harmonic frequency of the signal edge is higher than the frequency of the signal itself, which is the unexpected result of signal transmission caused by the rising edge and falling edge (or signal jump) of the signal. Therefore, it is generally believed that if the line propagation delay is greater than the rise time of 1/2 digital signal driver, this kind of signal is considered as a high-speed signal, resulting in transmission line effect.

Signal transmission occurs when the signal state changes, such as rising or falling time. It takes a fixed time for the signal to pass from the driving end to the receiving end. If the transmission time is less than the rise or fall time of 1/2, the reflected signal from the receiving end will reach the driving end before the signal changes state. Conversely, the reflected signal will reach the driving end after the signal changes state. If the reflected signal is strong, the superimposed waveform may change the logic state.

(3), the determination of high-speed signal

We defined the precondition of transmission line effect above, but how do we know whether the line delay is greater than the signal rise time of 1/2 driver? In general, the typical value of signal rise time can be given in the device manual, and the signal propagation time is determined by the actual wiring length in PCB design. The following figure shows the correspondence between signal rise time and allowable wiring length (delay).

The delay per unit inch on PCB is 0. 167ns. However, if there are many vias, many device pins and many constraints on the grid line, the delay will increase. Usually, the signal rise time of high-speed logic devices is about 0.2ns, and if there is GaAs chip on the board, the maximum wiring length is 7.62 mm

Let Tr be the signal rise time and Tpd be the signal line propagation delay. If Tr≥4Tpd, the signal falls in a safe area. If 2Tpd≥Tr≥4Tpd, the signal falls in the uncertain region. If Tr≤2Tpd, the signal falls in the problem area. For signals falling in uncertain areas and problem areas, high-speed wiring method should be adopted.

(4) What is a transmission line?

The traces on PCB can be equivalent to series and parallel capacitor, resistor and inductor structures, as shown in the following figure. The typical value of series resistance is 0.25-0.55 ohm/ft, and the resistance value of parallel resistance is usually higher because of insulation layer. After parasitic resistance, capacitance and inductance are added to the actual PCB wiring, the final impedance on the wiring is called characteristic impedance Zo. The wider the wire diameter, the closer it is to the power supply/ground, or the higher the dielectric constant of the isolation layer, the smaller the characteristic impedance. If the impedance of the transmission line and the receiving end do not match, the output current signal and the final stable state of the signal will be different, which will cause the signal to reflect at the receiving end, and this reflected signal will be sent back to the signal transmitting end for reflection again. With the decrease of energy, the amplitude of the reflected signal will decrease until the voltage and current of the signal reach stability. This effect is called oscillation, and the oscillation of the signal can often be seen on the rising edge and falling edge of the signal.

(5) Transmission line effect

Based on the transmission line model defined above, to sum up, the transmission line will bring the following effects to the whole circuit design.

reflected signal

Delay and timing error delay &; Timing error

Multi-crossing logic level threshold error error switching

Overshoot and undershoot/undershoot

Noise caused by crosstalk (or crosstalk)

electromagnetic radiation

5. 1 reflected signal

If the wiring is terminated incorrectly (terminal matching), the signal pulse from the driving end will be reflected at the receiving end, which will cause unexpected influence and distort the signal profile. When the distortion is obvious, it will lead to all kinds of errors and lead to design failure. At the same time, the sensitivity of distorted signal to noise increases, which will also cause design failure. If the above situation is not considered enough, EMI will increase significantly, which will not only affect its own design results, but also cause the failure of the whole system.

The main reasons for the reflected signal are: the wiring is too long; Transmission line termination mismatch, excessive capacitance or inductance, and impedance mismatch.

5.2 Delay and Timing Error

The signal delay and timing error are as follows: when the signal changes between the high and low thresholds of logic level, the signal does not jump for a period of time. Excessive signal delay may lead to timing errors and equipment dysfunction.

When there are multiple receivers, problems usually occur. Circuit designers must determine the worst-case time delay to ensure the correctness of the design. The reasons of signal delay: driving overload and long wiring.

5.3 Multiple crossing logic level threshold error

In the process of jumping, the signal may cross the logic level threshold many times, which may lead to such errors. The error of crossing the logic level threshold for many times is a special form of signal oscillation, that is, the signal oscillation occurs near the logic level threshold, and crossing the logic level threshold for many times will lead to logic dysfunction. The reasons for the reflected signal are: the trace is too long, the transmission line is not terminated, the capacitance or inductance is too large, and the impedance is not matched.

5.4 Overshoot and undershoot

Overshoot and undershoot come from two reasons: the line is too long or the signal changes too fast. Although the receiving end of most components is protected by input protection diodes, sometimes these overshoot levels will far exceed the power supply voltage range of components and damage components.

5.5 crosstalk

Crosstalk means that when a signal passes through a signal line, related signals will be induced on the signal line adjacent to it on the PCB board, which is called crosstalk.

The closer the signal line is to the ground line, the larger the line spacing and the smaller the crosstalk signal. Asynchronous signals and clock signals are more prone to crosstalk. Therefore, the way to eliminate crosstalk is to remove the signal with crosstalk or shield the signal with serious interference.

5.6 electromagnetic radiation

Emi (electromagnetic interference) means electromagnetic interference, and its problems include excessive electromagnetic radiation and sensitivity to electromagnetic radiation. Electromagnetic interference means that when the digital system is powered on, it will radiate electromagnetic waves to the surrounding environment, thus interfering with the normal work of electronic equipment in the surrounding environment. The main reason is that the working frequency of the circuit is too high and the layout is unreasonable. At present, there are software tools for electromagnetic interference simulation, but the electromagnetic interference simulator is expensive, and it is difficult to set simulation parameters and boundary conditions, which will directly affect the accuracy and practicability of simulation results. The most common way is to apply the design rules of EMI control to all aspects of design to realize the rule-driven and control of all aspects of design.

(VI) Methods to avoid transmission line effect

In view of the influence caused by the above transmission line problems, we will talk about the methods to control these influences from the following aspects.

6. 1 Strictly control the routing length of key network cables.

If there are high-speed edge jumps in the design, the transmission line effect on PCB must be considered. Nowadays, fast integrated circuit chips with very high clock frequency have such problems. There are some basic principles to solve this problem: if CMOS or TTL circuits are used for design, the working frequency should be less than 10MHz and the wiring length should be no more than 7 inches. The wiring length at 50MHz shall not exceed 1.5 inch. If the operating frequency reaches or exceeds 75MHz, the wiring length should be 1 inch. The maximum wiring length of GaAs chip should be 0.3 inch. If this standard is exceeded, transmission line problems will occur.

6.2 Reasonably plan the topology of wiring.

Another way to solve the transmission line effect is to choose the correct wiring path and terminal topology. The topological structure of wiring refers to the wiring sequence and wiring structure of a network cable. When using high-speed logic devices, unless the branch length of the trace is kept short, the signal with fast edge change will be distorted by the branch trace on the signal trunk. Generally, PCB wiring adopts two basic topologies, namely daisy-chain wiring and star-shaped wiring.

For daisy-chain wiring, the wiring starts from the driving end and reaches each receiving end in turn. If a series resistor is used to change the signal characteristics, the position of the series resistor should be close to the driving end. In controlling the harmonic interference of wiring, daisy-chain wiring has the best effect. However, this routing method has the lowest routing rate, and 100% routing is not easy. In practical design, we make the branch length of daisy-chain wiring as short as possible, and the safe length value should be: stub delay.

For example, the length of the branch end in a high-speed TTL circuit should be less than 1.5 inch. This topology takes up less wiring space and can be terminated by a single resistance matching. However, this wiring structure makes the signal reception of different signal receiving ends asynchronous.

Star topology can effectively avoid the problem of asynchronous clock signals, but it is very difficult to manually complete wiring on high-density PCB. Using automatic router is the best way to complete star wiring. A termination resistor is required on each branch. The resistance of the termination resistor should match the characteristic impedance of the connecting wire. This can be calculated manually, or the characteristic impedance value and terminal matching resistance value can be calculated by CAD tools.

In the above two examples, a simple termination resistor is used, but in fact, you can choose to use a more complex matching terminal. The first option is the RC matching terminal. RC matching terminal can reduce power consumption, but it can only be used when the signal works stably. This method is most suitable for matching clock line signals. Its disadvantage is that the capacitance in the RC matching terminal may affect the shape and propagation speed of the signal.

Series resistance matching terminals will not generate additional power consumption, but will slow down signal transmission. This method is used in bus driver circuit which is little affected by time delay. The advantage of series resistance matching terminal is that it can reduce the number of devices used on the board and reduce the connection density.

The last method is to separate the matched terminals. In this way, the matching element needs to be placed near the receiving end. Its advantage is that it will not lower the signal and can avoid noise well. Usually used for TTL input signals (ACT, HCT, FAST).

In addition, the package type and installation type of terminal matching resistor must also be considered. Generally, SMD surface mount resistance is lower than the inductance of through-hole components, so SMD packaging components become the first choice. If you choose ordinary series resistors, you can choose vertical and horizontal installation methods.

In the vertical mounting mode, one mounting pin of the resistor is very short, which can reduce the thermal resistance between the resistor and the circuit board and make the heat of the resistor more easily dissipated into the air. However, a longer vertical installation will increase the inductance of the resistor. The horizontal installation mode has lower inductance because of lower installation. However, the overheated resistance will drift, and in the worst case, the resistance will become an open circuit, which will lead to the failure of PCB wiring termination matching and become a potential failure factor.

6.3 Methods to Suppress Electromagnetic Interference

Solving the problem of signal integrity will improve the electromagnetic compatibility (EMC) of PCB. Among them, it is very important to ensure good grounding of PCB. For complex design, using signal layer and ground layer is a very effective method. In addition, reducing the signal density of the outermost layer of the circuit board as much as possible is also a good way to reduce electromagnetic radiation. This method can be realized by using "surface layer" technology to design and manufacture PCB. Surface layers are realized by adding a combination of thin insulating layers and micropores, so as to penetrate these layers on a common process PCB. Resistors and capacitors can be buried under the surface layer, and the wiring density per unit area will be nearly doubled, thus reducing the volume of PCB. The reduction of PCB area has a great influence on the topology of wiring, which means that the current loop is reduced, the length of branch wiring is reduced, and the electromagnetic radiation is approximately proportional to the area of current loop; At the same time, small size means that high-density pins can be used to package devices, thus reducing the length of connecting wires, thus reducing current loops and improving electromagnetic compatibility characteristics.

6.4 Other available technologies

In order to reduce the instantaneous voltage overshoot on the power supply of the integrated circuit chip, decoupling capacitors should be added to the integrated circuit chip. This can effectively remove the influence of burr on the power supply and reduce the radiation of the power supply circuit to the printed board.

When the decoupling capacitor is directly connected to the power supply pin of the integrated circuit instead of the power supply layer, its burr smoothing effect is the best. This is why some devices have decoupling capacitors on their sockets, while others require that the distance between the decoupling capacitors and the devices be small enough.

Any high-speed and high-power devices should be put together as far as possible to reduce the instantaneous overshoot of power supply voltage.

If there is no power plane, a long power connection will form a loop between the signal and the loop, becoming a radiation source and a susceptible circuit.

The situation that wires form a loop that does not pass through the same network cable or other wires is called open loop. If the loop passes through other wires of the same network cable, a closed loop is formed. In both cases, antenna effect (wire antenna and loop antenna) will be formed. Antenna will produce EMI radiation, and it is also a sensitive circuit. Closed loop is a problem that must be considered, because the radiation it produces is approximately proportional to the closed loop area.

Concluding remarks

High-speed circuit design is a very complicated design process. Use ZUKEN's routing editor and EMC/EMI analysis software (INCASES, Hot-Stage) to analyze and find the problems. The method introduced in this paper is specifically aimed at solving these high-speed circuit design problems. In addition, many factors need to be considered when designing high-speed circuits, and these factors are sometimes opposite to each other. If high-speed devices are placed close to each other, the delay may be reduced, but crosstalk and significant thermal effects may occur. Therefore, in the design, it is necessary to weigh various factors and make a comprehensive compromise consideration; It not only meets the design requirements, but also reduces the design complexity. The adoption of high-speed PCB design means constitutes the controllability of the design process. Only controllable, reliable and successful!