In electronic product design, PCB layout and routing is the most important step, and the quality of PCB layout and routing will directly affect the performance of the circuit. Now, although there are many softwares that can realize PCB automatic layout and routing, as the signal frequency continues to increase, engineers often need to understand the most basic principles and skills of PCB layout and routing, so as to make their designs perfect.
1. What problems should be paid attention to when wiring high-frequency signals?
Answer: Impedance matching of signal lines; spatial isolation from other signal lines; for digital high-frequency signals, the effect of differential lines will be better;
2. When laying out the board, if the wires are dense, there may be more vias, which of course will affect the electrical performance of the board. How to improve the electrical performance of the board?
Answer: For low-frequency signals, vias do not matter, and for high-frequency signals, minimize vias. If there are many lines, multi-layer boards can be considered.
3. Is it better to add more decoupling capacitors on the board?
Answer: The decoupling capacitor needs to be added with the appropriate value at the appropriate position. For example, add it at the power supply port of your analog device, and need to use different capacitor values to filter out spurious signals of different frequencies.
4. What is the standard for a good board?
Answer: The layout is reasonable, the power line power redundancy is sufficient, the high-frequency impedance is impedance, and the low-frequency wiring is simple.
5. What is the difference between through holes and blind holes on the signal? What principles are applied?
Answer: The use of blind or buried holes is an effective way to increase the density of multilayer boards, reduce the number of layers and board size, and greatly reduce the number of plated through holes. But in comparison, the through hole is easy to realize in the process and the cost is low, so the through hole is generally used in the design.
6. When it comes to analog-digital hybrid systems, some people suggest that the electrical layer be divided, and the ground plane adopts a whole piece of copper. How to choose the appropriate method for specific application?
Answer: If you have high-frequency > 20MHz signal lines, and the length and quantity are relatively large, then at least two layers are required for this analog high-frequency signal. One layer of signal lines, one layer of large-area ground, and the signal line layer needs to drill enough vias to the ground. The purpose of this is:
a. For analog signals, this provides a complete transmission medium and impedance matching;
b. The ground plane isolates the analog signal from other digital signals;
c. The ground loop is small enough, because you have drilled a lot of vias, and the ground has a large plane.
7. In the circuit board, the signal input plug-in is on the leftmost edge of the PCB, and the MCU is on the right, so the regulated power supply chip is placed close to the connector during layout (the power supply ic outputs 5V to reach the MCU after a relatively long path. ), or put the power IC to the right of the middle (the output 5V line of the power IC reaches the MCU is relatively short, but the input power line passes through a relatively long PCB board)? Or is there a better layout?
Answer: First of all, is your so-called signal input plug-in an analog device? If it is an analog device, it is recommended that your power supply layout should try not to affect the signal integrity of the analog part. So there are a few points to consider:
a. First of all, is your regulated power supply chip a relatively clean power supply with small ripples? For the power supply of the analog part, the requirements for the power supply are relatively high.
b. Is the analog part and your MCU the same power supply? In the design of high-precision circuits, it is recommended to separate the power supply of the analog part and the digital part.
c. The power supply to the digital part needs to consider minimizing the impact on the analog circuit part.
8. In the application of high-speed signal chains, there are analog and digital grounds for multiple ASICs. Should the grounds be divided or not? What are the existing norms? Which works better?
A: So far, there is no conclusion. In general, you can consult the manual of the chip. The manuals of all ADI hybrid chips recommend you a grounding scheme, some recommend a common ground, and some recommend an isolated ground. It depends on the chip design.
9. What are the most important issues that should be paid attention to when designing high-speed multi-layer PCBs? Can you elaborate on the solution to the problem.
Answer: The most important thing to pay attention to is your layer design, that is, how you divide signal lines, power lines, ground, and control lines in each layer. The general principle is that the analog signal and the analog signal ground must at least have a separate layer. It is also recommended to use a separate layer for the power supply.
10. Are there any strict technical restrictions on when to use 2-layer boards, 4-layer boards, and 6-layer boards? (Except for volume reasons) Is the frequency of the CPU or the frequency of data interaction with external devices the criterion?
Answer: The use of multi-layer boards can firstly provide a complete ground plane, and can also provide more signal layers to facilitate wiring. For applications where the CPU needs to control external storage devices, the frequency of interaction should be considered. If the frequency is high, a complete ground plane must be guaranteed. In addition, the signal lines should be kept equal in length.
11. How to analyze the impact of PCB layout on analog signal transmission, and how to distinguish whether the noise introduced during signal transmission is caused by wiring or op amp devices.
Answer: This is difficult to distinguish, and the extra noise introduced by the wiring can only be minimized through PCB wiring.
12. I recently studied PCB design. For high-speed multilayer PCBs, what is the appropriate line width setting for power lines, ground lines, and signal lines? What are the common settings? Can you give an example? For example, how to set when the working frequency is 300Mhz?
Answer: 300MHz signal must do impedance simulation to calculate the line width and the distance between the line and the ground; the power line needs to determine the line width according to the size of the current. Generally, the "line" is not used in the mixed signal PCB, but the entire plane is used. Only in this way can the loop resistance be minimized, and there is a complete plane under the signal line.
13. What kind of layout can achieve the best cooling effect?
Answer: There are three main sources of heat in PCB:
a. Heating of electronic components;
b. The heating of the PCB itself;
c. Heat from other parts.
Among these three heat sources, components generate the most heat and are the main heat source, followed by the heat generated by the PCB board. The heat input from the outside depends on the overall thermal design of the system and will not be considered for the time being. Then the purpose of thermal design is to take appropriate measures and methods to reduce the temperature of components and PCB board, so that the system can work normally at an appropriate temperature. It is mainly achieved by reducing heat generation and accelerating heat dissipation.
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