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PCB Design Essentials

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PCB wiring work for many engineers is a very boring work. This is actually just a primary understanding, a good PCB design engineer can still do a lot of work and can solve a lot of product design problems. Taili PCB will share some points of PCB design wiring, for reference only.

1. General Practice

In the PCB design, in order to make the high frequency PCB , high-speed, analog circuit board design more reasonable, better anti-interference performance, should be considered from the following aspects . Reasonable selection of layers. When routing high-frequency and high-speed circuit boards in PCB design, using the middle inner layer as the power supply and ground layer can play a shielding role and effectively reduce parasitic inductance. It also reduces cross interference between signals.
  • Cabling mode: The cabling is turned at a 45° Angle or an arc, which can reduce the reflection and coupling of high-frequency and high-speed signals.
  • The length of the cable. If there is no special requirement, the shorter the length of the cable, the better (the loss requirement should be determined according to the actual situation). When adjacent wiring, the shorter the parallel distance between lines, the better.
  • Orifice design and quantity. When designing through the hole, it is necessary to pay attention to make the impedance of the hole and the impedance of the transmission line consistent with each other or as much as possible. In the meantime. The fewer the number of pass holes, the better, because pass holes can easily cause impedance discontinuity.
  • Cabling direction between adjacent layers. The wiring direction between layers should be vertical, that is, the upper layer is horizontal, and the adjacent layer is vertical, which can reduce the interference between signals.
  • In many cases, engineers believe that wrapping an important signal line can significantly improve the anti-interference ability of the signal, but care must be taken to avoid introducing new problems, such as whether the space is reduced or the impedance is changed. Of course, you can also package the interference source so that it cannot interfere with other signals. High-speed PCB design, the protection ground wire to or not, this is a problem?
  • Signal line. The signal line cannot be looped to reduce the noise introduced by the loop.

2 .Wiring Priority

  • Key signal line priority: analog small signal, high-speed signal, clock signal and synchronization signal and other key signals priority wiring
  • Density first principle: Start wiring from the most complex connected devices on the board. Start with the most densely wired area of the board
Be careful
Try to provide a special wiring layer for key signals such as clock signals, high-frequency signals, and sensitive signals, and ensure the minimum loop area. If necessary, manual priority wiring, shielding and increasing safety spacing should be adopted. Guarantee signal quality.
The EMC environment between the power layer and the ground is poor, and interference sensitive signals should be avoided.
The network with impedance control requirements should be routed as far as possible according to the line length and line width requirements.

3 . Clock Wiring

The clock line is one of the biggest factors affecting EMC. Drill fewer holes on the clock line, avoid running parallel to other signal lines, and keep away from noise or heat sources to avoid interference with the signal line. At the same time, the power supply part of the board should be avoided to prevent the power supply and the clock from interfering with each other.
If there is a special clock chip on the board, it can not go under the line, should be laid under the copper, if necessary, it can also be special. For many chip reference crystal oscillators, these crystal oscillators should not be traced below, to lay copper isolation.
  • The clock driver is arranged in the center of the PCB rather than the periphery of the board, the layout is as close as possible, the line is smooth, short, non-right Angle, non-T-shaped.
  • Avoid interference between clocks and signals, avoid parallel wiring of several signals, use GND shielding layer to wrap isolation when necessary, and distance between different clocks or signals is as far as possible.
  • The clock signal should, as far as possible, not use a cross-boundary split plane.
  • If it is a differential clock line, be sure to pay attention to the equal length.
  • Clock crystal oscillator: the clock line first passes through the load capacitor, and then reaches the crystal oscillator, and the surrounding holes are punched and GND shielding.
  • Homologous clock: the parallel matching resistance of the clock line is close to the load chip, and the series resistance is close to the clock chip or CPU.

4. Right Angle Wiring

Right Angle wiring is generally required to avoid the situation in PCB wiring, and it has almost become one of the standards to measure the quality of wiring, so how much impact will right Angle wiring have on signal transmission? In principle, right-angle wiring will cause the line width of the transmission line to change, resulting in impedance discontinuity.
In fact, not only right Angle wiring, ton Angle, acute Angle wiring may cause impedance changes.
The impact of right-angle wiring on the signal is mainly reflected in three aspects:
  • One is that the corner can be equivalent to the capacitive load on the transmission line, slowing down the rise time.
  • Second, impedance discontinuity will cause signal reflection.
  • The third is the EMI generated by the right Angle tip.

5. Differential Wiring

Differential signal is used more and more widely in high-speed circuit design, and most of the signals in the circuit adopt differential line structure.
The difference line is used for anti-interference, and its advantages can be explained from two angles.
First, in a signal of the same level amplitude, the peak-to-peak value of the differential line is twice that of the single-terminal line.
Second, in the same circuit environment, because the single-ended routing refers to the ground plane, the external interference is greatly different from the same interference on the ground plane, resulting in the interference in the routing line and the interference in the return path cannot cancel each other (the single-ended routing voltage reference is the ground plane). Because the difference line is parallel isometric lines, in the same circuit environment, the coupling degree of the two lines is very high, when the same interference source, the interference degree on the two antennas is close, and the voltage reference point of the difference line is the corresponding another line, rather than the ground plane, for common mode interference has a better suppression ability.
Differential lines want higher anti-interference capabilities to obtain low bit error rates and improve transmission rates, but they require an additional line than a single terminal line to act as the signal's return line. Therefore, only in the pursuit of higher transmission rate or stronger anti-interference ability of the design will be willing to increase the number of transmission lines to ensure the transmission rate and stronger anti-interference ability.
For PCB engineers, when designing differential transmission lines, it is necessary to do a good job of the internal equal length of the difference and the consistency of the impedance (equal spacing).
The equilength is to ensure that the two differential signals maintain opposite polarity at all times and reduce the common-mode component. The equidistance is mainly to ensure that the differential impedance of the two is consistent and reduce reflection. As for what we usually call tight coupling or loose coupling, it depends on the situation. Is differential coupling really better than loose coupling?

6. Serpentine Line

Serpentine line is a kind of routing method often used in Layout. Its main purpose is to adjust the transmission line delay, especially to meet the internal or inter-pair isometric length of the transmission line, or to meet the requirements of the system timing and targeted design.

7. Power Supply, Ground Processing

Even if the wiring in the entire PCB board is completed well, the interference caused by the inconsiderate power supply and ground wire will reduce the performance of the product, and sometimes even affect the success rate of the product. Therefore, the wiring of the power supply and ground wire should be taken seriously, and the noise interference caused by the power supply and ground wire should be reduced to a minimum to ensure the quality of the product.
The PCB routing rules for ground and power cables are as follows:
Add a decoupling capacitor between the power supply and the ground wire.
Try to widen the width of the power cable and ground cable. It is best to make the ground cable wider than the power cable.
In the PCB of a high-speed digital circuit, a wide ground wire is used to form a loop, and it is best to have a complete ground plane for reference. Analog circuits cannot be used in this way.
Use a large area of copper layer as a ground wire, and connect the places that are not used on the printed board with the ground as a ground wire, or make a multi-layer board, and the power supply and ground wire occupy one layer each.
For areas with dense through holes, pay attention to avoid holes connecting with each other in the holloway area of the power supply and the formation, forming a division of the plane layer, thereby damaging the integrity of the plane layer and increasing the loop area of the signal line in the formation.
Ground circuit rules:
The ground loop is as small as possible, that is, the ring area of the signal line and its loop should be as small as possible, the smaller the ring area, the less external radiation, and the smaller the noise of receiving external interference.
Decoupling capacitance rule:
  • Add the necessary decoupling capacitance on the PCB to filter out the interference signal on the power supply and make the power signal stable. The layout of the decoupling capacitor and the wiring mode of the power supply will directly affect the stability of the entire system, and sometimes even the success or failure of the design.
  • In PCB design, the current should generally be filtered through the filter capacitor filter before being used by the device.
  • In high-speed circuit design, whether the decoupling capacitor can be used correctly is related to the stability of the entire board.

8. Common Ground Processing of Digital Circuit and Analog Circuit

There are now many PCBS that are no longer single-function 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 interference between them, especially the noise interference on the ground wire.
The frequency of the digital circuit is high, the sensitivity of the analog circuit is strong, for the signal line, the high-frequency signal line is as far away from the sensitive analog circuit device as possible, for the ground line, the entire PCB has only one node to the outside world, so it must be processed inside the PCB, the problem of the number and mode common ground, and the digital and analog inside the board are actually separated, and they are not connected to each other. Only at the interface between the PCB and the outside world (such as plugs, etc.). There is a bit of a short connection between digital and analog, but note that there is only one connection point. There are also common on the PCB, which is determined by the system design.

9. The Signal Cable is Distributed on the Power Supply or Ground Plane

In the multi-layer PCB wiring, because there is not much left in the signal line layer, more layers will cause waste and will increase the production of a certain amount of work, and the cost will increase accordingly, in order to solve this contradiction, you can consider wiring on the power supply and ground plane layer. The power layer should be considered first, followed by the formation. Because it's best to preserve the integrity of the plane.

10. Design Rule Check (DRC)

After the wiring design is completed, it is necessary to carefully check whether the wiring design meets the rules formulated by the designer, and it is also necessary to confirm whether the rules formulated meet the needs of the printed board production process, the general inspection has the following aspects:
  • Whether the distance between wire and wire, wire and component pad, wire and through hole, component pad and through hole, through hole and through hole is reasonable, and whether it meets the production requirements.
  • Is the width of the power cord and ground cable appropriate? Is the power supply tightly coupled to the ground plane?
  • Whether the best measures are taken for the key signal lines, such as the length, the protection line, the distance between the transmit (TX) line and the receive (RX) line (some require layered wiring) and so on.
  • Whether the analog circuit and the digital circuit part have their own independent ground lines or how to connect them.
  • Is there a process line on the PCB? Whether the solder resistance meets the requirements of the production process, whether the solder resistance size is appropriate, whether the character mark is pressed on the device pad, so as not to affect the quality of Denso.
  • Whether the outer frame edge of the power supply formation in the multi-layer board is reduced, such as the copper foil of the power supply formation exposed outside the board is easy to cause short circuit.

11.  Check the 3W, 3H Principle

The 3W principle means that the center distance between the signal line and the signal line is 3 times the line width.
The 3H principle means that the center distance between the signal line and the signal line is 3 times the distance from the signal line to the reference layer.
Whether it is 3W or 3H principle, it is to reduce crosstalk between signal lines. Especially between high-speed signal lines or high-frequency signal lines. As long as the 3H or 3W principle can be satisfied, then the crosstalk will be very small. However, for miniaturized product design, it is difficult to meet the 3W or 3H principle. In addition, the main source of crosstalk is no longer just the effect between transmission lines.
For PCB design, or different people have different opinions, every engineer has his own understanding.

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