Some questions and answers about PCB wiring techniques, worth reading!

How to improve your PCB layout skills? A very important way is to see if you can ask questions and answer them. Alternatively, we can also grow from other people’s questions and answers.

1. I recently studied PCB design. For high-speed multi-layer PCB, what are the appropriate line width settings for power lines, ground lines and signal lines? What are the commonly used settings? Can you give an example? For example, how to set it when the operating frequency is 300Mhz?

Answer: For 300MHz signals, impedance simulation must be done to calculate the line width and the distance between the line and the ground; the line width of the power line needs to be determined according to the size of the current; when the ground is on a mixed-signal PCB, the “line” is generally not used, but the entire plane is used. , so as to ensure that the loop resistance is minimal and there is a complete plane under the signal line.

2. For high-speed PCB, how to avoid via holes during wiring? Do you have any good suggestions?

Answer: For high-speed PCB, it is best to drill fewer vias and add signal layers to solve the need for more vias.

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

Answer: 1) Impedance matching of signal lines; 2) Spatial isolation from other signal lines; 3) For digital high-frequency signals, differential lines will work better.

4. Differential lines generally need to be of equal length. If it is difficult to implement in LAYOUT, are there any other remedies?

Answer: The problem of equal lengths can be solved by running serpentine lines. Now most PCB software can automatically run equal length lines, which is very convenient.

5. When should we consider equal lengths of lines? If we want to consider using equal-length wires, what is the maximum difference in length between the two signal wires? How to calculate?

Answer: Differential line calculation idea: If you transmit a sinusoidal signal, the length difference is equal to half of its transmission wavelength, and the phase difference is 180 degrees. At this time, the two signals are completely canceled. So the length difference at this time is the maximum value. By analogy, the signal line difference must be smaller than this value.

6. In what situation is snake-shaped wiring suitable for high speed? Are there any disadvantages? For example, for differential wiring, the two sets of signals are required to be orthogonal?

Answer: Snake wiring has different functions depending on the application:

(1) If the serpentine trace appears in the computer board, it mainly plays the role of a filter inductor and impedance matching, improving the anti-interference ability of the circuit. The serpentine traces in computer motherboards are mainly used in some clock signals, such as PCI-Clk, AGPCIK, IDE, DIMM and other signal lines.

(2) If used in an ordinary PCB board, in addition to functioning as a filter inductor, it can also be used as an inductor coil for a radio antenna, etc. For example, it is used as an inductor in 2.4G walkie-talkies.

(3) Some signal wiring lengths must be strictly equal. The equal length of high-speed digital PCB boards 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 ( If the delay difference exceeds one clock cycle, the data of the next cycle will be read incorrectly).

(4) Snake wiring plays the role of a distributed parameter LC filter in some special circuits.

7. When there are high-speed logic devices in the circuit, what is the maximum wiring length?

Answer: We are not afraid of long wiring, but we are afraid of asymmetry or relatively large differences, which can easily cause wrong logic due to time delay.

8. How to avoid noise introduced during wiring?

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


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