PCB Tech - Techniques to reduce RF effect in PCB interconnection

The interconnection of the circuit board system includes:

Chip-to-board

PCB interconnection

PCB and external components

In RF design, the electromagnetic characteristics at the interconnection point is one of the main problems faced by engineering design

This article introduces the various techniques of the above-mentioned three types of interconnection design, including device mounting methods, wiring isolation, and measures to reduce lead inductance, and so on. At present, there are signs that the frequency of printed circuit board design is getting higher and higher. As the data rate continues to increase, the bandwidth required for data transmission also promotes the upper limit of the signal frequency to 1GHz or even higher. Although this high-frequency signal technology is far beyond the range of millimeter wave technology (30GHz), it does also involve RF and low-end microwave technology.

The RF engineering design method must be able to deal with the stronger electromagnetic field effects that are usually generated at higher frequency bands. These electromagnetic fields can induce signals on adjacent signal lines or PCB lines, causing unpleasant crosstalk (interference and total noise), and can impair system performance. The return loss is mainly caused by impedance mismatch, and the influence on the signal is the same as the influence caused by additive noise and interference.

There are two negative effects of high return loss:

The signal reflected back to the signal source will increase the system noise, making it more difficult for the receiver to distinguish the noise from the signal;

Any reflected signal will basically degrade the signal quality because the shape of the input signal has changed.

Although the digital system only processes 1 and 0 signals and has very good fault tolerance, the harmonics generated when the high-speed pulse rises will cause the higher the frequency, the weaker the signal.

Although the forward error correction technology can eliminate some negative effects, part of the system bandwidth is used to transmit redundant data, which leads to a decrease in system performance.

A better solution is to let RF effects help rather than detract from signal integrity. It is recommended that the total return loss of the digital system at the highest frequency (usually the poor data point) is -25dB, which is equivalent to a VSWR of 1.1.

The goal of PCB design is smaller, faster and lower cost. For RF PCBs, high-speed signals sometimes limit the miniaturization of PCB designs.

The main method to solve the crosstalk problem is to manage the ground plane, space between wiring and reduce lead inductance.

The main method to reduce the return loss is impedance matching. This method includes effective management of insulating materials and isolation of active signal lines and ground lines, especially between signal lines that have transitioned states and ground.

Since the interconnection point is the weakest link in the circuit chain, in the RF design, the electromagnetic properties at the interconnection point are the main problems faced by the engineering design. Each interconnection point must be investigated and the existing problems must be solved. The interconnection of the circuit board system includes three types of interconnection: the chip to the circuit board, the interconnection within the PCB board, and the signal input/output between the PCB and external devices.

One, the interconnection between the chip and the PCB board

Pentium IV and high-speed chips containing a large number of input/output interconnection points are already available. As far as the chip itself is concerned, its performance is reliable, and the processing rate has been able to reach 1GHz. The main problem of the interconnection between the chip and the PCB is that the interconnection density is too high, which will cause the basic structure of the PCB material to become a factor limiting the growth of the interconnection density. A common solution is to use the local wireless transmitter inside the chip to transmit the data to the adjacent circuit board. Regardless of whether this scheme is effective or not, the participants are very clear: In terms of high-frequency applications, IC design technology is far ahead of PCB design technology.

Two, PCB board interconnection

The skills and methods for high-frequency PCB design are as follows:

The corner of the transmission line should be 45° to reduce the return loss;

A high-performance insulated circuit board with an insulation constant value strictly controlled by level shall be adopted. This method is conducive to effective management of the electromagnetic field between the insulating material and the adjacent wiring.

Protruding leads have tap inductance, so avoid using components with leads. In high frequency environments, it is best to use surface mount components.

For signal vias, avoid using a via processing (pth) process on sensitive boards, because this process will cause lead inductance at the vias. For example, when a via on a 20-layer board is used to connect layers 1 to 3, the lead inductance can affect layers 4 to 19.

To provide a rich ground plane. Use molded holes to connect these ground planes to prevent the 3D electromagnetic field from affecting the circuit board.

To choose electroless nickel plating or immersion gold plating process, do not use HASL method for electroplating. This kind of electroplated surface can provide better skin effect for high frequency current. In addition, this highly solderable coating requires fewer leads, which helps reduce environmental pollution.

The solder mask prevents the flow of solder paste. However, due to the uncertainty of the thickness and the unknown of the insulation performance, the entire surface of the board is covered with solder mask material, which will cause a large change in the electromagnetic energy in the microstrip design. Generally, a solder dam is used as the solder mask.

To improve the PCB design specifications related to high-precision etching. It is necessary to consider that the total error of the specified line width is +/-0.0007 inches, the undercut and cross-section of the wiring shape should be managed, and the plating conditions of the wiring side wall should be specified. The overall management of wiring (wire) geometry and coating surface is very important to solve the skin effect problem related to microwave frequency and realize these specifications. If you are not familiar with these methods, you can consult an experienced design engineer who has been engaged in military microwave circuit board design. For example, a copper-backed coplanar microstrip design is more economical than a stripline design.

Three, PCB and external device interconnection

How to solve the signal input/output problem from the circuit board to the wire connecting the remote device? Trompeter Electronics, an innovator of coaxial cable technology, is working to solve this problem and has made some important progress. Also, take a look at the electromagnetic field given in the figure. In this case, we manage the conversion from microstrip to coaxial cable. In the coaxial cable, the ground layer is interwoven ring-shaped and evenly spaced. In microstrip, the ground plane is below the active line. This introduces certain edge effects, which need to be understood, predicted and considered during design. Of course, this mismatch will also cause return loss, and this mismatch must be minimized to avoid noise and signal interference.

The management of impedance problems in PCB boards is not a design problem that can be ignored. The impedance starts from the surface of the circuit board, then passes through a solder joint to the connector, and finally ends at the coaxial cable. Since impedance varies with frequency, the higher the frequency, the more difficult impedance management is. The problem of using higher frequencies to transmit signals over broadband seems to be the main problem faced in the design.