Now, although there are many software that can realize pcb automatic layout and routing. However, as the signal frequency continues to increase, in many cases, engineers need to understand the basic principles and skills of PCB layout and wiring to make their design perfect.
The following covers the basic principles and design skills related to PCB layout and routing, and answers the difficult problems related to PCB layout and routing in the form of questions and answers.
1. Q: What problems should we pay attention to when wiring high frequency signals?
Answer: Impedance matching of signal line; Space isolation from other signal lines; For digital high-frequency signals, the differential line will be better.
2. Q: When laying boards, if the wires are dense, there may be more vias, which will certainly affect the electrical performance of the board. How can I improve the electrical performance of the board?
Answer: For low-frequency signals, vias do not matter. For high-frequency signals, vias should be minimized. If there are many lines, multi-layer boards can be considered.
3. Q: Is the more decoupling capacitors added to the board the better?
Answer: The decoupling capacitor needs to be added with a proper value at a proper position. For example, add it to the power supply port of your analog device, and use different capacitance values to filter out stray signals of different frequencies.
4. Q: What is the standard of a good board?
Answer: The layout is reasonable, the power redundancy of the power line is sufficient, and the high frequency impedance and low frequency wiring are SIMple.
5. Q: How much do through-hole and blind hole affect the difference of signals? What are the principles of application?
Answer: The use of blind holes or buried holes is an effective way to improve the density of multilayer boards, reduce the number of layers and board size, and greatly reduce the number of plated through holes. However, by comparison, through holes are easy to realize in technology and low in cost, so they are generally used in design.
6. Q: When it comes to analog digital hybrid systems, it is suggested that the electric layer be divided, the ground plane be covered with copper, and it is also suggested that the electric layer be divided, with different grounds connected at the end of the power source, but the return path of the signal is far away. How to choose an appropriate method for specific applications?
Answer: If you have a high frequency signal line>20MHz, and the length and quantity are relatively large, you need to give at least two layers of analog high frequency signal. One layer of signal line, one layer of large area, and the signal line layer needs to make enough vias to the ground. The purpose is to:
For analog signals, this provides a complete transmission medium and impedance matching;
The ground plane isolates analog signals from other digital signals;
The ground loop is SMAll enough, because you have punched many vias, and the ground is a large plane.
7. Q: In the circuit board, the signal input plug-in is on the left edge of the PCB, and the mcu is on the right. So, in the layout, is it better to place the regulated power supply chip close to the connector (the power supply IC outputs 5V through a longer path to reach the MCU), or to place the power supply IC to the right in the middle (the power supply IC outputs 5V through a shorter line to the MCU, but the input power line passes through a longer PCB)? Or better layout?
A: First of all, is your so-calLED signal input plug-in a simulator? If it is a simulator, it is recommended that your power supply layout should not affect the signal integrity of the analog part as much as possible. Therefore, there are several points to consider:
First of all, whether your regulated power supply chip is a clean, low ripple power supply. For the power supply of the analog part, the requirements for the power supply are relatively high;
Whether the analog part and your MCU are the same power supply. In the design of high circuit, it is recommended to separate the power supply of analog part and digital part;
The power supply for the digital part needs to consider minimizing the impact on the analog circuit.
8. Q: In the application of high-speed signal link, there are analog ground and digital ground for multiple ASICs. Is ground separation or undivided? What are the existing guidelines? Which works better?
A: So far, there is no final conclusion. In general, you can refer to the chip manual. The manual of all mixed chips of ADI recommends a grounding scheme, some of which are recommended for common ground and some for isolation. This depends on the chip design.
9. Q: When should we consider the equal length of lines? If you want to consider the use of isometric lines, what is the difference between the lengths of the two signal lines? How to calculate?
Answer: Difference line calculation idea: if you transmit a sine signal, your length difference is equal to half of its transmission wavelength, and the phase difference is 180 degrees, then the two signals will be completely canceled. So the length difference is a value. By analogy, the difference between signal lines must be less than this value.
10. Q: In which case is the serpentine route suitable for high-speed? Is there any disadvantage? For example, for differential routing, two sets of signals are required to be orthogonal?
Answer: The serpentine wiring has different functions due to different applications:
If the serpentine wiring appears in the computer board, it mainly plays a role in matching the filter inductance and impedance, and improving the anti-interference ability of the circuit. The serpentine wiring in the computer motherboard is mainly used in some clock signals, such as PCI-Clk, AGPCIK, IDE, DIMM, etc.
In general, PCB can be used as inductance coil of radio antenna in addition to filtering inductance. For example, 2.4G walkie talkies are used as inductors.
The wiring length of some signals must be strictly equal. The equal line length of high-speed digital PCB is to keep the delay difference of each signal within a range and ensure the validity of the data read by the system in the same cycle (when the delay difference exceeds one clock cycle, the data in the next cycle will be misread).
For example, there are 13 HUBLinks in the INTELHUB architecture. The frequency of 233MHz is used. They must be strictly equal in length to eliminate hidden dangers caused by time delay. Wire winding is the only solution. Generally, the delay difference is required to be no more than 1/4 clock cycle. The delay difference per unit length is also fixed. The delay is related to the line width, line length, copper thickness, and board structure. However, if the line is too long, the distributed capacitance and inductance will be increased, and the signal quality will be reduced.
Therefore, clock IC pins are generally connected; "Termination, but the serpentine line does not act as an inductor. On the contrary, inductance will shift the higher harmonics in the rising edge of the signal, causing deterioration of the signal quality. Therefore, the serpentine line spacing is required to be less than twice the line width.
The smaller the rise time of the signal, the more susceptible to the influence of distributed capacitance and distributed inductance.
Serpentine routing acts as a LC filter with distributed parameters in some special circuits.
For example, the position of the clock generator should not be close to the external connector as much as possible, the high-speed signal should go through the inner layer as much as possible and pay attention to the continuity of the characteristic impedance matching and the reference layer to reduce reflection, the slope rate of the signal pushed by the device should be as small as possible to reduce the high-frequency component, and when selecting the decoupling/bypass capacitor, pay attention to whether its frequency response meets the requirements to reduce the power layer noise.
In addition, pay attention to the return path of high-frequency signal current to make the loop area as small as possible (that is, the loop impedance as small as possible) to reduce radiation. You can also control the range of high-frequency noise by dividing the stratum, and properly select the grounding point between PCB and housing.
12. Q: What should I pay attention to in the transmission line design of RF broadband PCB? How to properly set the ground hole of the transmission line? Do you need to design the impedance matching by yourself or cooperate with the PCB manufacturer?
A: There are many factors to consider. For example, various parameters of PCB materials, transmission line models established based on these parameters, device parameters, etc. Impedance matching is generally designed according to the data provided by the manufacturer.
13. Q: When analog circuits and digital circuits coexist, for example, half of them are FPGA or SCM digital circuits, and the other half is DAC and related amplifier analog circuits. There are many power supplies with various voltage values. In case of power supplies with voltage values that are used by both digital and analog circuits, can we use the same power supply? What are the skills in wiring and magnetic bead layout?
Answer: It is generally not recommended to use this way. It will be more complex and difficult to debug.
14. Q: Hello, when designing high-speed multilayer PCB, what is the main basis for selecting the packaging of resistor capacitor and other devices? Can you give some examples of commonly used packages.
Answer: 0402 is commonly used by mobile phones; 0603 is commonly used in general high-speed signal modules; The basis is that the smaller the package, the smaller the parasitic parameters. Of course, the same package from different manufacturers has great differences in high-frequency performance.
It is recommended that you use high-frequency special components at key locations.
15. Q: In general, in the design, the signal line or ground wire is used first for the double panel?
Answer: This should be considered comprehensively. The routing should be considered first when considering the layout.
16. Q: What should be paid attention to when designing high-speed multilayer PCB? Can you provide a detailed solution to the problem.
Answer: You should pay attention to the design of your layer, that is, how you divide the signal line, power line, ground line and control line in each layer.
The general principle is that the analog signal and analog signal ground shall at least have a separate layer. A separate layer is also recommended for power supply.
17. Q: Are there any strict technical restrictions on when to use 2-layer, 4-layer and 6-layer boards? (Excluding the volume reason) Is it based on the frequency of the CPU or the frequency of its interaction with external device data?
Answer: The use of Multilayer boards can first provide a complete ground plane, in addition, it can provide more signal layers to facilitate routing.
For applications where the CPU is to control external memory devices, the interaction frequency should be considered. If the frequency is high, the complete ground plane must be guaranteed. In addition, the signal lines should be kept equal length.
18. Q: How to analyze the impact of PCB wiring on analog signal transmission, and how to distinguish whether the noise introduced during signal transmission is caused by wiring or by operational amplifier devices.
Answer: It is difficult to distinguish this. Only PCB wiring can minimize the introduction of additional noise.
19. Q: I have learned about PCB design recently. For high-speed multilayer PCB, what is the appropriate line width of power line, ground line and signal line, and what are the common settings? Can you give an example? For example, how to set the operating frequency at 300Mhz?
Answer: 300 MHz signal must be impedance simulated to calculate line width and distance between line and ground; The line width of the power line needs to be determined according to the size of the current. When mixing signal PCB, the ground usually uses the whole plane instead of the "line", so as to ensure the loop resistance, and there is a complete plane below the signal line.
20. Q: What kind of layout can achieve the heat dissipation effect?
Answer: There are mainly three aspects of heat in PCB:
Heating of electronIC components;
Heating of PCB itself;
Heat from other parts.
Among the three heat sources, the heating capacity of components is the main heat source, followed by the heat generated by pcb board. The heat transferred from outside depends on the overall thermal design of the system, which is not considered temporarily.
The purpose of thermal design is to take appropriate measures and methods to reduce the temperature of components and PCB boards, so that the system can work normally at the appropriate temperature. It is mainly realized by reducing heat and accelerating heat dissipation.
21. Question: Can you explain the relationship between the line width and the size of the matching vias?
Answer: This question is very good. It is hard to say that there is a simple proportional relationship, because their simulations are different. One is surface transmission and the other is ring transmission. You can find a via impedance calculation software on the network, and then keep the impedance of the via consistent with the impedance of the transmission line.
22. Question: In an ordinary pcb controlled by a MCU, but without high current and high speed signals, the requirements are not very high. Is it better to lay a layer of ground wire around the edges of the PCB to wrap the entire PCB?
A: Generally speaking, a complete floor is enough.
1) I know that the AD conversion chip needs to make analog and digital single point connections below, but what if there are multiple AD conversion chips on the board?
2) In multilayer circuit boards, when the multiplexer switches the analog sampling, do you need to separate the analog part from the digital part like the AD conversion chip?
Answer: Several ADCs should be put together as much as possible, and the analog digital connection should be made at a single point below the ADC; It depends on the switching speed between MUX and ADC. Generally, the speed of ADC is higher than MUX, so it is recommended to place it below ADC. Of course, to be on the safe side, you can also put a magnetic bead package under the MUX. During debugging, you can choose where to perform single point connection according to the specific situation.
24. Question: In the conventional network circuit design, some people use to connect several grounding points together. Is there such a usage? Why?
A: I'm not very clear about your question. For a hybrid system, there must be several types of ground, which are always connected together at one point. The purpose of this is to equalize the potential. We need a common ground level for reference.
25. Question: How to effectively handle the analog part and digital part, analog part and digital part in PCB?
Answer: The analog circuit and digital circuit should be placed separately, so that the return current of the analog circuit is in the analog circuit area and the digital circuit is in the digital area, so that the digital circuit will not affect the analog circuit. The starting point of analog and digital processing is similar, and the digital signal cannot flow back to the analog ground.