The first article PCB wiring

In PCB design, wiring is an important step in completing product design. It can be said that the previous preparation work is done for it. In the entire PCB, the wiring design process has the highest limit, the most detailed skills, and the largest workload. PCB wiring includes 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, interactive pre wiring can be used to route the lines with stricter requirements. The edge lines of the input and output ends should avoid being adjacent and parallel to avoid reflection interference. If necessary, ground wire isolation should be added, and the wiring of adjacent layers should be perpendicular to each other, as parallelism can easily cause parasitic coupling.

The routing rate of automatic routing depends on a good layout, and the routing rules can be pre-set, including the number of bends in the routing, the number of through holes, the number of steps, etc. Generally, exploratory layout of warp lines is carried out first, quickly connecting the short lines, and then maze routing is carried out. The routing path of the lines to be laid is optimized globally, and it can disconnect the already laid lines as needed. And try rewiring to improve the overall effect.

I feel that through holes are not very suitable for the current high-density PCB design. It wastes many valuable wiring channels. To solve this contradiction, blind hole and buried hole technologies have emerged. It not only completes the function of through holes, but also saves many wiring channels, making the wiring process more convenient, smooth, and complete, The design process of PCB boards is a complex and simple process. To master it well, electronic engineering designers need to experience it themselves in order to obtain the true essence.

1. Treatment of power and ground wires

Even if the wiring is completed well throughout the entire PCB board, interference caused by inadequate consideration of power and ground wires can lead to a decrease in product performance and sometimes even affect the success rate of the product. So the wiring of electricity and ground wires should be taken seriously, and the noise interference generated by electricity and ground wires should be minimized to ensure the quality of the product.

For every engineering personnel engaged in electronic product design, they understand the reasons for the noise generated between the ground wire and the power line. Now, only the reduced noise suppression method is described:

(1) It is well known to add a coupling capacitor between the power supply and ground wire.

(2) Try to widen the width of the power supply and ground wire as much as possible, preferably with the ground wire wider than the power line. The relationship between them is: ground wire>power line>signal line. Typically, the signal line width is 0.2-0.3mm, with a maximum fine width of 0.05-0.07mm and a power line width of 1.2-2.5mm

A wide ground wire can be used to form a circuit for the PCB of digital circuits, which forms a ground grid for use (the ground of analog circuits cannot be used in this way)

(3) Use a large area of copper layer as the ground wire, and connect all unused areas on the printed circuit board to the ground as the ground wire. Or it can be made into a multi-layer board, with one layer for the power supply and one layer for the ground wire.

Common ground processing of digital and analog circuits

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

The frequency of digital circuits is high, and the sensitivity of analog circuits is strong. For signal lines, high-frequency signal lines should be kept as far away from sensitive analog circuit components as possible. For ground wires, the entire PCB only has one node to the outside world, so it is necessary to handle the problem of digital and analog ground inside the PCB. However, inside the PCB, digital ground and analog ground are actually separate and not connected to each other, only at the interface between the PCB and the outside world (such as plugs). There is a short circuit between the digital ground and the analog ground. Please note that there is only one connection point. There are also different ground points on the PCB, which are determined by the system design.

3 signal lines laid on the electrical (ground) layer

When wiring multi-layer printed boards, as there are not many wires left in the signal layer that have not been fully laid, adding more layers will cause waste and increase production workload, and the cost will also increase accordingly. To solve this contradiction, wiring on the electrical (ground) layer can be considered. The first consideration should be to use the power layer, followed by the geological layer. Because it is best to preserve the integrity of the strata.

Treatment of connecting legs in large-area conductors

In large-scale grounding (electricity), the legs of commonly used components are connected to them, and the treatment of the connecting legs needs to be comprehensively considered. In terms of electrical performance, it is better to fully connect the solder pads of the component legs to the copper surface. However, there are some hidden dangers in the welding and assembly of the components, such as: ① Welding requires a high-power heater. ② Easy to cause virtual solder joints. So, considering both electrical performance and process requirements, a cross shaped solder pad is made, known as a heat shield, commonly known as a thermal pad. This can greatly reduce the possibility of virtual solder joints caused by excessive heat dissipation in the cross-section during welding. The treatment of the grounding leg of the multi-layer board is the same.

The role of network systems in wiring

In many CAD systems, wiring is determined based on the network system. The grid is too dense, and although the paths have increased, the step size is too small, resulting in a large amount of data in the graph field. This inevitably requires higher storage space for devices, and also has a great impact on the computational speed of computer electronic products. And some pathways are invalid, such as those occupied by the solder pads of the component legs or occupied by mounting holes, fixing holes, etc. The sparsity of the grid and the lack of pathways have a significant impact on the deployment rate. So there needs to be a grid system with reasonable density to support the routing process.

The distance between the legs of a standard component is 0.1 inches (2.54mm), so the foundation of a grid system is generally set at 0.1 inches (2.54 mm) or multiples less than 0.1 inches, such as 0.05 inches, 0.025 inches, 0.02 inches, etc.

6 Design Rule Check (DRC)

After the wiring design is completed, it is necessary to carefully check whether the wiring design complies with the rules formulated by the designer, and also confirm whether the rules formulated meet the requirements of the printed circuit board production process. Generally, the inspection includes the following aspects:

(1) Is the distance between wire and wire, wire and component solder pads, wire and through-hole, component solder pads and through-hole, and through-hole reasonable and meets production requirements.

(2) Is the width of the power and ground wires appropriate, and is there tight coupling (low wave impedance) between the power and ground wires? Is there any place in the PCB that can widen the ground wire.

(3) Have the best measures been taken for key signal lines, such as the shortest length, adding protective lines, and clearly separating input and output lines.

(4) Are there independent ground wires for the analog circuit and digital circuit parts.

(5) Will the graphics (such as icons and labels) added to the PCB cause a signal short circuit.

(6) Modify some unsatisfactory linear shapes.

(7) Is there a process line added to the PCB? Whether the solder mask meets the requirements of the production process, whether the solder mask size is appropriate, and whether the character mark is pressed on the device solder pad to avoid affecting the quality of the electrical installation.

(8) Does the outer edge of the power layer in the multi-layer board shrink? If the copper foil of the power layer is exposed outside the board, it is easy to cause a short circuit.

Chapter 2 PCB layout

In design, layout is an important aspect. The quality of layout results will directly affect the effectiveness of wiring, so it can be considered that a reasonable layout is the first step to successful PCB design.

There are two types of layout methods, one is interactive layout, and the other is automatic layout. Generally, interactive layout is used to adjust the layout based on automatic layout. During layout, gate circuits can be reassigned according to the routing situation, and the two gate circuits can be exchanged to make it the best layout for easy wiring. After the layout is completed, the design files and related information can also be returned and annotated on the schematic, so that the relevant information in the PCB board is consistent with the schematic, so that future filing and design changes can be synchronized. At the same time, simulation related information can be updated to enable board level verification of the electrical performance and function of the circuit.

--Considering overall aesthetics

The success or failure of a product depends on both internal quality and overall aesthetics. Only when both are perfect can the product be considered successful.

On a PCB board, the layout of components should be balanced, dense and orderly, and should not be heavy or heavy on one end.

--Layout inspection

Does the size of the printed circuit board match the dimensions on the processing drawing? Can it meet the requirements of PCB manufacturing process? Is there a positioning mark?

Are there any conflicts between components in 2D and 3D space?

Is the component layout dense and orderly, arranged neatly? Have all the fabrics been laid out?

Can components that need to be replaced frequently be easily replaced? Is it convenient to insert the plugin board into the device?

Is there an appropriate distance between the thermosensitive element and the heating element?

Is it convenient to adjust adjustable components?

Have radiators been installed in areas that require heat dissipation? Is the air flow unobstructed?

Is the signal flow smooth and has the shortest interconnection?

Are plugs, sockets, etc. contradictory to mechanical design?

Has the interference issue of the line been considered?

Chapter 3 High Speed PCB Design

（1） The challenges faced by electronic system design

With the increasing complexity and integration of system design on a large scale, electronic system designers are engaged in circuit design above 100MHz, and the operating frequency of the bus has also reached or exceeded 50MHz, some even exceeding 100MHz. Currently, about 50% of the designed clock frequencies exceed 50MHz, and nearly 20% of the designed main frequencies exceed 120MHz.

When the system operates at 50MHz, transmission line effects and signal integrity issues will occur; When the system clock reaches 120MHz, PCBs designed based on traditional methods will not work unless high-speed circuit design knowledge is used. Therefore, high-speed circuit design technology has become a design tool that electronic system designers must adopt. Only by using the design techniques of high-speed circuit designers can the controllability of the design process be achieved.

（2） What is a high-speed circuit

It is generally believed that if the frequency of a digital logic circuit reaches or exceeds 45MHz~50MHz, and the circuit operating above this frequency already occupies a certain proportion of the entire electronic system (such as 1/3), it is called a high-speed circuit.

In fact, the harmonic frequency at the edge of the signal is higher than the frequency of the signal itself, which is an unexpected result of signal transmission caused by the rapid changes in the rising and falling edges (also known as signal jumps) of the signal. Therefore, it is generally agreed that if the line propagation delay is greater than 1/2 of the rise time of the digital signal driver end, such signals are considered high-speed signals and generate transmission line effects.

The transmission of signals occurs at the moment when the signal state changes, such as the rise or fall time. The signal passes through a fixed period of time from the driving end to the receiving end. If the transmission time is less than half of the rise or fall time, the reflected signal from the receiving end will reach the driving end before the signal changes state. On the contrary, the reflected signal will reach the driving end after the signal changes state. If the reflection signal is strong, the superimposed waveform may change the logical state.

（3） Determination of high-speed signals

We have defined the prerequisites for the occurrence of transmission line effects above, but how can we determine whether the line delay is greater than half of the signal rise time at the driver end? Generally, the typical value of signal rise time can be given by the device manual, while the propagation time of the signal is determined by the actual wiring length in PCB design. The following figure shows the correspondence between the signal rise time and the allowed wiring length (delay).

The delay per unit inch on the PCB board is 0.167 ns. However, if there are many via holes, device pins, and constraints set on the network cable, the delay will increase. The signal rise time of high-speed logic devices is usually about 0.2ns. If there are GaAs chips on the board, the maximum wiring length is 7.62mm.

Set Tr as the signal rise time, Tcd is the propagation delay of the signal line. If Tr ≥ 4TPD, the signal falls in a safe area. If 2Tpp ≥ Tr ≥ 4Tpp, the signal falls in an uncertain region. If Tr ≤ 2TPD, the signal falls in the problem area. For signals that fall into uncertain and problematic areas, high-speed wiring methods should be used.

（4） What is a transmission line

The wiring on the PCB board can be equivalent to the series and parallel capacitor, resistor, and inductor structures shown in the following figure. The typical value of series resistance is 0.25-0.55 ohms/foot, and due to the insulation layer, the resistance value of parallel resistance is usually very high. After adding parasitic resistance, capacitance, and inductance to the actual PCB connection, the final impedance on the connection is called the characteristic impedance Zo. The wider the wire diameter, the closer it is to the power/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 causes the signal to reflect at the receiving end. This reflected signal will be transmitted back to the signal transmitter and reflected back again. As the energy weakens, 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 and falling edges of the signal.

（5） Transmission line effect

Based on the transmission line model defined above, in summary, transmission lines will have the following effects on the entire circuit design.

·Reflected signals

·Delay&Timing errors

·Multiple crossing of logic level threshold error False Switching

·Overshot and undershot

·Induced Noise (or crosstalk)

·Electromagnetic radiation

5.1 Reflection signal

If a wire is not properly terminated (terminal matching), the signal pulse from the driver end is reflected at the receiver end, causing unexpected effects and distortion of the signal contour. When the distortion and deformation are very significant, it can lead to various errors and cause design failure. Meanwhile, the sensitivity of distorted signals to noise increases, which can also lead to design failure. If the above situation has not been sufficiently considered, EMI will significantly increase, which not only affects the design results, but also leads to the failure of the entire system.

The main reasons for the generation of reflection signals are: excessively long wiring; Transmission lines that have not been matched and terminated, excessive capacitance or inductance, and impedance mismatch.

5.2 Delay and timing errors

Signal delay and timing errors are manifested as: the signal remains unchanged for a period of time when it changes between the high and low thresholds of the logic level. Excessive signal delay may lead to timing errors and confusion in device functionality.

Usually, problems arise when there are multiple receivers. Circuit designers must determine the worst-case time delay to ensure the correctness of the design. The reason for signal delay is that the driver is overloaded and the wiring is too long.

5.3 Multiple cross logic level threshold errors

The signal may cross the logic level threshold multiple times during the jumping process, leading to this type of error. Multiple crossing of the logic level threshold error is a special form of signal oscillation, where the oscillation of the signal occurs near the logic level threshold. Multiple crossing of the logic level threshold can lead to logical dysfunction. The reasons for the generation of reflected signals include excessively long wiring, unexposed transmission lines, excessive capacitance or inductance, and impedance mismatch.

5.4 Overshoot and undershoot

Overrush and undershoot are caused by two reasons: long wiring or rapid signal changes. Although most components have input protection diodes at the receiving end, sometimes these overshoot levels can far exceed the voltage range of the component's power supply, damaging the component.

5.5 Crosstalk

Crosstalk refers to the induction of relevant signals on adjacent signal lines on a PCB board when a signal passes through a single signal line.

The closer the signal line is to the ground wire and the larger the line spacing, the smaller the crosstalk signal generated. Asynchronous signals and clock signals are more prone to crosstalk. Therefore, the method of resolving crosstalk is to move away from the signal that is experiencing crosstalk or to shield the signal that is severely interfered with.

5.6 Electromagnetic radiation

EMI (Electro Magnetic Interference) refers to electromagnetic interference, which generates problems including excessive electromagnetic radiation and sensitivity to electromagnetic radiation. EMI is manifested as the radiation of electromagnetic waves to the surrounding environment when a digital system is powered on, thereby interfering with the normal operation of electronic devices in the surrounding environment. The main reasons for its occurrence are the high operating frequency of the circuit and the unreasonable layout and wiring. At present, there are software tools available for EMI simulation, but EMI simulators are expensive and it is difficult to set simulation parameters and boundary conditions, which will directly affect the accuracy and practicality of simulation results. The most common approach is to apply the design rules that control EMI to every step of the design process, achieving rule driven and control at each stage of the design.

（6） Methods to avoid transmission line effects

We will discuss methods to control the impact of the transmission line issues mentioned above from the following aspects.

6.1 Strictly control the routing length of critical network cables

If there are high-speed jump edges in the design, it is necessary to consider the issue of transmission line effects on the PCB board. The commonly used fast integrated circuit chips with high clock frequencies now face such problems. There are some basic principles to solve this problem: if CMOS or TTL circuits are used for design, the operating frequency should be less than 10MHz, and the wiring length should not exceed 7 inches. The wiring length should not exceed 1.5 inches when operating at a frequency of 50MHz. If the operating frequency reaches or exceeds 75MHz, the wiring length should be within 1 inch. The maximum wiring length for GaAs chips should be 0.3 inches. If this standard is exceeded, there is a problem with the transmission line.

6.2 Reasonably plan the topology structure of the routing

Another method to solve the transmission line effect is to choose the correct wiring path and terminal topology. The topology of routing refers to the routing sequence and structure of a network cable. When using high-speed logic devices, signals with rapidly changing edges will be twisted by branch routing on the signal backbone unless the length of the routing branch is kept very short. Normally, PCB wiring adopts two basic topology structures, namely daisy chain wiring and star distribution.

For daisy chain wiring, the wiring starts from the driver end and reaches each receiving end in sequence. 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 terms of controlling high-order harmonic interference in wiring, daisy chain wiring has the best effect. But this wiring method has the lowest deployment rate and is not easy to achieve 100% deployment. In actual design, we aim to make the branch length of the daisy chain wiring as short as possible, and the safe length value should be: Stub Delay<=Trt * 0.1

For example, the branch end length in high-speed TTL circuits should be less than 1.5 inches. This topology takes up less wiring space and can be terminated with a single resistor matching. However, this wiring structure results in asynchronous signal reception at different signal receiving ends.

The star topology structure can effectively avoid the problem of asynchronous clock signals, but it is very difficult to manually complete wiring on high-density PCB boards. Using an automatic router is the best way to complete star routing. Terminal resistors are required on each branch. The resistance value of the terminal resistor should match the characteristic impedance of the connection. This can be calculated manually or through CAD tools to determine the characteristic impedance value and terminal matching resistance value.

In the above two examples, simple terminal resistors were used, but in practice, more complex matching terminals can be chosen. The first option is to match the RC terminal. RC matching terminals can reduce power consumption, but can only be used in situations where signal operation is relatively stable. This method is most suitable for matching and processing clock line signals. The disadvantage is that the capacitance in the RC matching terminal may affect the shape and propagation speed of the signal.

Series resistor matching terminals will not generate additional power consumption, but will slow down signal transmission. This method is used for bus drive circuits where time delay has little impact. The advantage of matching terminals with series resistors also lies in reducing the number of devices used on the board and the density of wiring.

The last method is to separate the matching terminals, which requires the matching components to be placed near the receiving end. Its advantage is that it will not lower the signal and can effectively avoid noise. Typical for TTL input signals (ACT, HCT, FAST)。

In addition, the packaging and installation types of terminal matching resistors must also be considered. SMD surface mount resistors typically have lower inductance than through hole components, making SMD packaged components the preferred choice. If you choose a regular plug-in resistor, there are two installation methods to choose from: vertical and horizontal.

In the vertical installation method, one pin of the resistor is very short, which can reduce the thermal resistance between the resistor and the circuit board, making it easier for the heat from the resistor to dissipate into the air. But longer vertical installations will increase the inductance of the resistor. The horizontal installation method has lower inductance due to lower installation. But the overheated resistance will drift, and in the worst-case scenario, the resistance will become open circuit, causing PCB wiring termination and matching failure, becoming a potential failure factor.

6.3 Methods for suppressing electromagnetic interference

Effectively addressing signal integrity issues will improve the electromagnetic compatibility (EMC) of PCB boards. It is very important to ensure that the PCB board has a good grounding. The use of a signal layer paired with a ground layer is a highly effective method for complex designs. In addition, minimizing the density of the outermost signal on the circuit board is also a good way to reduce electromagnetic radiation. This method can be achieved by using the "Build up" technology to design and manufacture PCBs. The surface area layer is achieved by adding a thin insulation layer and a combination of micropores used to penetrate these layers on a regular process PCB. Resistance and capacitance can be buried below the surface layer, and the wire density per unit area will increase by nearly twice, thus reducing the volume of the PCB. The reduction of PCB area has a huge impact on the topological structure of wiring, which means that the reduced current circuit, the reduced branch wiring length, and electromagnetic radiation are approximately proportional to the area of the current circuit; At the same time, the small volume feature means that high-density pin packaged devices can be used, which in turn reduces the length of the wiring, thereby reducing the current loop and improving electromagnetic compatibility characteristics.

6.4 Other available technologies

To reduce the instantaneous voltage overshoot on the power supply of integrated circuit chips, decoupling capacitors should be added to the integrated circuit chips. This can effectively remove the impact of burrs on the power supply and reduce the radiation of the power supply loop on the printed circuit board.

When the decoupling capacitor is directly connected to the power supply leg of the integrated circuit instead of the power supply layer, its effect of smoothing burrs is the best. That's why some device sockets have decoupling capacitors, while others require the distance between the decoupling capacitors and the device to be small enough.

Any high-speed and high power consuming devices should be placed together as much as possible to reduce instantaneous power supply voltage overshoot.

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

The situation where the wiring forms a loop that does not pass through the same network cable or other wiring is called an open loop. If the loop passes through the same network cable and other routing lines, it forms a closed loop. Both situations will result in antenna effects (line antenna and ring antenna). The antenna generates EMI radiation to the outside and is also a sensitive circuit. Closed loop is a problem that must be considered, as the radiation it generates is approximately proportional to the area of the closed loop.

Conclusion

High speed circuit design is a very complex design process, ZUKEN's high-speed circuit routing algorithm (Route Editor) and EMC/EMI analysis software (INCASES), Hot Stage is applied to analyze and discover problems. The method described in this article is specifically aimed at solving these high-speed circuit design problems. In addition, there are multiple factors that need to be considered when designing high-speed circuits, and these factors are sometimes opposed to each other. When laying out high-speed devices in close proximity, although it can reduce delay, it may generate crosstalk and significant thermal effects. Therefore, in design, it is necessary to weigh various factors and make a comprehensive compromise consideration; Meet design requirements while reducing design complexity. The adoption of high-speed PCB design methods constitutes the controllability of the design process. Only when controllable can it be reliable and successful!