PCB Technical - Sevev of High Speed PCB Design Guide: How to Reduce RF Effect in PCB Interconnect Design

The interconnection of the circuit board system includes three types of interconnection between the chip to the circuit board, the interconnection within the PCB board, and the PCB and external devices. In the RF design, the electromagnetic characteristics at the interconnection point is one of the main problems faced by the engineering design. This article introduces the various techniques of the above-mentioned three types of interconnection design. The content involves device installation methods, wiring isolation and measures to reduce lead inductance. and many more.

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:

1. 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;

2. 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. At present, the main method to solve the crosstalk problem is to manage the ground plane, to space the wiring and to reduce the lead inductance (stud capacitance). 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. At the recent GHz Interconnect Symposium (www.az.ww.com), the most exciting thing is that the methods for dealing with the ever-increasing number and frequency of I/O have been widely known. 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. An innovative solution was proposed at the meeting, that is, the use of a local wireless transmitter inside the chip to transmit 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:

1. The corners of the transmission line should be 45° to reduce the return loss;

2. Use high-performance insulated circuit boards whose insulation constant values are strictly controlled by level. This method is conducive to effective management of the electromagnetic field between the insulating material and the adjacent wiring.

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

4. 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.

5. 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.

6. 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 (Figure 2). In addition, this highly solderable coating requires fewer leads, which helps reduce environmental pollution.

7. The solder mask can prevent 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.

8. 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. You can also discuss with them the price range you can afford. For example, copper-backed coplanar microstrip design is more economical than stripline design. You can discuss this with them to get better suggestions. Good engineers may not be accustomed to considering cost issues, but their suggestions are also quite helpful. Now try to train young engineers who are unfamiliar with RF effects and lack experience in handling RF effects. This will be a long-term job.

In addition, other solutions can also be adopted, such as improving the computer type to enable it to handle RF effects.

Three, PCB and external device interconnection

It can now be considered that we have solved all the signal management problems on the board and the interconnection of individual discrete components. So how to solve the signal input/output problem from the circuit board to the wire connecting the remote device? Trompeter Electronics, an innovator in coaxial cable technology, is working to solve this problem and has made some important progress (Figure 3). Also, take a look at the electromagnetic field given in Figure 4. 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 issues within the circuit board is not a design issue 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.

This article summarizes:

PCB platform technology needs continuous improvement to meet the requirements of integrated circuit designers. The management of high-frequency signals in PCB design 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 think the bandwidth will be higher and higher, and the use of high-frequency signal technology is a prerequisite for realizing this continuous increase in bandwidth.