PCB Technical - 10 PCB layout and wiring skills

At present, although there are many software can realize PCB Automatic Layout and routing. However, with the continuous improvement of signal frequency, engineers often need to understand the basic principles and skills of PCB layout and wiring, so as to make their design perfect.

The following covers the basic principles and design skills of PCB layout and routing, and answers the difficult questions about PCB layout and routing in the form of question and answer.

1. What problems should be paid attention to when wiring high frequency signals?

1. Impedance matching of signal line;

2. Space isolation from other signal lines;

3. For digital high frequency signal, the effect of differential line is better;

2. When distributing boards, if the wires are dense, there may be more vias. Of course, the electrical performance of the board will be affected. How to improve the electrical performance of the board?

For low-frequency signals, vias do not matter, high-frequency signals as far as possible to reduce vias. If there are many lines, multilayer board can be considered;

3. Is the more decoupling capacitors added to the board, the better?

The decoupling capacitance needs to be added at the right position. For example, it is added at the power supply port of your analog device, and different capacitance values are needed to filter out the stray signals with different frequencies;

4. What is the standard of a good board?

Reasonable layout, sufficient power redundancy of power line, high frequency impedance and simple low frequency wiring.

5. What is the effect of through-hole and blind hole on signal difference? What are the principles of application?

Blind hole or buried hole is an effective method to increase the density of multilayer board, reduce the number of layers and the size of the plate surface, and greatly reduce the number of plated through holes. But in comparison, through-hole is easy to realize in technology and has low cost, so it is generally used in design.

6. When it comes to analog-to-digital hybrid system, some people suggest that the electric layer should be divided, and the ground plane should be covered with copper. Some people also suggest that the electric layer should be divided. Different ground should be connected at the end of the power source, but the return path of the signal will be far away. How to choose an appropriate method for specific application?

If you have a high frequency > 20MHz signal line, and the length and number are relatively large, then you need at least two layers to give this analog high frequency signal. A layer of signal line, a layer of large area, and the signal line layer needs to drill enough via to the ground. The purpose is to:

1. For analog signals, this provides a complete transmission medium and impedance matching;

2. The ground plane isolates analog signals from other digital signals;

3. The ground circuit is small enough because you have drilled a lot of vias and the ground is a large plane.

7. In the circuit board, the signal input plug-in is on the left side of the PCB, and the MCU is on the right side. In the layout, should the voltage stabilizing power supply chip be placed close to the connector (the power IC output 5V goes through a long path to reach the MCU), or the power IC should be placed to the right in the middle (the output 5V line of the power IC will be shorter when it reaches the MCU, but the input power line will pass through a longer section of PCB board) )？ Or better layout?

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 layout should not affect the signal integrity of the analog part

(1) First of all, whether your regulated power supply chip is a clean power supply with small ripple

(2) Whether the analog part and your MCU are one power supply, in the design of high circuit, it is recommended to separate the power supply of analog part and digital part

(3) The digital part of the power supply needs to be considered to minimize the impact on the analog circuit

8. In the application of high-speed signal link, there are analog and digital ground for multiple ASIC. Should we adopt ground division or not? What are the existing criteria? Which is better?

So far, there is no conclusion. In general, you can refer to the manual of the chip. All ADI hybrid chip manuals recommend a grounding scheme, some of which are public ground, some are isolated. It depends on the chip design.

9. When should I consider the equal length of a line? If we want to consider the use of equal length wire, the difference between the length of two signal lines can not be more than? How to calculate?

Difference line calculation idea: if you transmit a sinusoidal signal, your length difference is equal to half of its transmission wavelength, and the phase difference is 180 degrees, then the two signals are completely offset. So the length difference here is a value. By analogy, the signal line difference must be less than this value.

10. What kind of situation is suitable for high-speed serpentine routing? What are the disadvantages? For example, for differential wiring, two groups of signals are required to be orthogonal?