How Can You Improve Your PCB Wiring Skills

How to improve your PCB wiring skills? A very important way is to see if you can ask questions and answer them. Or, we can grow from the questions and answers of others.

 

1. For high-speed multi-layer PCB, what is the appropriate line width setting for power cord, ground wire and signal wire? For example, how to set the working frequency at 300MHz?

Answer: 300MHz signal must do impedance simulation calculation of line width and the distance between the line and the ground; Power line needs to be determined by the size of the current line width; In a mixed signal PCB, the “line” is usually not used, but the whole plane is used, so that the loop resistance is minimal and there is a complete plane below the signal line.

 

2. High-speed PCB, how to deal with the hole avoidance in the wiring process, any good suggestions?

Answer: For high-speed PCB, it is better to drill fewer holes and to increase the signal layer to solve the need of adding more holes.

 

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

Answer: 1) Impedance matching of signal line;  2) Spatial isolation from other signal lines;  3) For digital high frequency signals, the difference line will be better.

 

4. Generally, difference lines need to be of equal length. If it is really difficult to realize in the LAYOUT, are there any other remedial measures?

Answer: The problem of equilength can be solved by taking the serpentine line. Now most PCB software can automatically take the equilength line, which is very convenient.

 

5. When should I consider the length of a line? If equilength is to be considered, what is the maximum length difference between two signal lines? How do we calculate it?

Answer: If you send a sinusoidal signal, your length difference is equal to half of its wavelength, and the phase difference is 180 degrees, then the two signals cancel out completely. So the difference in length is at its maximum. Similarly, the signal line difference must be less than this value.

 

6. Snake-like routing in high speed is suitable for what kind of situation? Are there any disadvantages, such as the requirement that the two sets of signals be orthogonal for differential wiring?

Answer: Snake-like wiring, because of the application of different occasions and have different roles: (1) If the snake circuitry appears in the computer board, it mainly plays a role of filtering inductance and impedance matching, and improves the anti-interference ability of the circuit. The serpentine circuitry in the computer motherboard is mainly used in some clock signals, such as PCI-CLK, AGPCIK, IDE, DIMM signal lines.  (2) In the general PCB, in addition to the role of filtering inductance, it can also be used as the inductance coil of the radio antenna, etc. For example, the 2.4G interphone is used as an inductor.  (3) The wiring length of some signals must be strictly equal. The equiline length of high-speed digital PCB board 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 (the data of the next cycle will be misread if the delay difference exceeds one clock cycle).  (4) Serpentine routing acts as an LC filter with distributed parameters in some special circuits.

 

7. What is the maximum wiring length when there are high-speed logic devices in the circuit?

Answer: Wiring is not afraid of long, afraid of asymmetry or a relatively large difference, so easy because of the delay caused by the wrong logic.

 

8. How to avoid the noise introduced during wiring?

Answer: The digital ground and the analog ground should be grounded at a single point, otherwise the digital backflow will flow through the analog ground and cause interference to the analog circuit.