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Reduce rf effect in pcb interconnect design flow --2

Release Date:2020-03-19

Interconnect in PCB
The techniques and methods for high-frequency PCB design are as follows:

1. The 45 ° angle of the transmission line should be used to reduce the return loss;
2. A high-performance insulated circuit board whose insulation constant value is strictly controlled in layers. This method facilitates the effective management of the electromagnetic field between the insulating material and adjacent wiring.
3. It is necessary to perfect PCB design specifications related to high-precision etching. It is necessary to consider specifying a total line width error of +/- 0.0007 inches, managing the undercut and cross section of the wiring shape, and specifying the wiring sidewall plating conditions. The overall management of wiring (wire) geometry and coating surfaces is important to address the skin-effect issues associated with microwave frequencies and to achieve these specifications.
4. There is a tap inductance on the protruding leads. Avoid using leaded components. In high frequency environments, it is best to use surface mount components.
5. For signal vias, avoid using the pth process on the sensitive board, as this process will cause lead inductance at the vias. For example, when a via on a 20-layer board is used to connect 1 to 3 layers, the lead inductance can affect 4 to 19 layers.
6. Provide a rich ground plane. Molded holes are used to connect these ground planes to prevent 3D electromagnetic fields from affecting the circuit board.
7. To choose non-electrolytic nickel plating or immersion gold plating, do not use HASL plating. This plated surface provides better skin effect for high frequency currents. In addition, this highly solderable coating requires fewer leads and helps reduce environmental pollution.
8. Solder mask prevents solder paste from flowing. However, due to the uncertainty of thickness and the unknown of insulation performance, covering the entire board surface with solder resist material will cause a large change in the electromagnetic energy in the microstrip design. Solder dam is generally used as the solder resist layer.
If you are unfamiliar with these methods, consult an experienced design engineer who has worked in military microwave circuit board design. You can also discuss with them the price range you can afford. For example, copper-backed coplanar microstrip designs are more economical than stripline designs, and you can discuss them with them for better suggestions. Good engineers may not be used to thinking about cost, but their suggestions are also quite helpful. Now it is a long-term job to train young engineers who are not familiar with RF effects and lack experience in dealing with RF effects.
In addition, other solutions can also be used, such as improving the computer type, so that it has RF effect processing capabilities.

PCB interconnection with external devices

Now we can think that we have solved all the signal management problems on the board and on the interconnection of the discrete components. So how to solve the signal input / output problem from the circuit board to the wire connecting the remote device? Trompeter Electronics, the innovator of coaxial cable technology, is working to solve this problem and has made some important advances.
Also, look at the electromagnetic field given in the pcb. In this case, we manage the conversion from microstrip to coaxial cable. In coaxial cables, the ground plane is intertwined in a ring shape and spaced evenly. In microstrip, the ground plane is below the active line. This introduces some edge effects that need to be understood, predicted, and considered during design. Of course, this kind of mismatch will also cause return loss, and this kind of mismatch must be minimized to avoid noise and signal interference.
Managing impedance issues within a board is not a design issue that can be ignored. The impedance starts at the surface of the circuit board, passes through a solder joint to the connector, and ends at the coaxial cable. Because impedance varies with frequency, the higher the frequency, the more difficult it is to manage impedance. The problem of using higher frequencies to transmit signals over broadband appears to be a major problem in design.
Summary of this article
PCB platform technology needs continuous improvement to meet the requirements of integrated circuit designers. The management of high-frequency signals in the PCB design process and the management of signal input / output on the PCB circuit board need continuous improvement. No matter what exciting innovations will happen in the future, I believe that the bandwidth will become higher and higher, and the use of high-frequency signal technology is the prerequisite to achieve this increasing bandwidth.
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