PCB Tech - Tips for reducing RF effect in PCB interconnection design

This paper introduces various skills of chip to circuit board, interconnection in PCB and interconnection design between PCB and external devices, including device installation, wiring isolation and measures to reduce lead inductance, so as to help designers reduce RF effect in PCB interconnection design.

The interconnection of circuit board system includes chip to circuit board, interconnection in PCB and interconnection between PCB and external devices. In RF design, the electromagnetic characteristics at the interconnection point is one of the main problems faced by engineering design. This paper introduces various skills of the above three types of interconnection design, including device installation methods, wiring isolation and measures to reduce lead inductance.

At present, there are signs that the frequency of printed circuit board design is getting higher and higher. With the continuous growth of data rate, the bandwidth required for data transmission also makes the upper limit of signal frequency reach 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 strong electromagnetic field effects usually generated in the higher frequency band. These electromagnetic fields can induce signals on adjacent signal lines or PCB lines, resulting in annoying crosstalk (interference and total noise) and damaging system performance. The return loss is mainly caused by impedance mismatch, which has the same impact on the signal as additive noise and interference.

High return loss has two negative effects:

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 changes.

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 forward error correction technology can eliminate some negative effects, part of the system bandwidth is used to transmit redundant data, resulting in the reduction of system performance. A better solution is to let the RF effect help rather than damage the integrity of the signal. It is recommended that the total return loss at the digital system frequency (usually poor data points) is - 25dB, which is equivalent to VSWR of 1.1

The goal of PCB design is smaller, faster and lower cost. For rfpcb, high-speed signal sometimes limits the miniaturization of PCB design. At present, the main methods to solve the crosstalk problem are ground plane management, spacing between wiring and reducing 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 line and ground wire, especially between signal line and ground with state jump.

Because the interconnection point is the weak link in the circuit chain, the electromagnetic property at the interconnection point is the main problem faced by the engineering design in RF design. It is necessary to investigate each interconnection point and solve the existing problems. The interconnection of circuit board system includes chip to circuit board, interconnection in PCB and signal input / output between PCB and external devices.

a. Interconnection between chip and PCB

Pentium IV and high-speed chips containing a large number of input / output interconnection points have been available. As far as the chip itself is concerned, its performance is reliable, and the processing rate has reached 1GHz. At the near GHz interconnection seminar, the exciting thing is that the methods to deal with the growing number and frequency of I / O have been widely known. The main problem of interconnection between chip and PCB is that too high interconnection density will lead to the basic structure of PCB material becoming a factor limiting the growth of interconnection density. An innovative solution was proposed at the meeting, that is, the local wireless transmitter inside the chip was used to transmit data to the adjacent circuit board. Whether this scheme is effective or not, the participants are very clear: in terms of high-frequency applications, IC design technology is far from PCB design technology.

b. Interconnection in PCB

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

1. 45 ° angle shall be adopted for the corner of transmission line to reduce back loss

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

3. Improve PCB design specifications for high-precision etching. Consider specifying a total line width error of + / - 0.0007 inches, managing undercut and cross section of wiring shapes, and specifying wiring sidewall plating conditions. The overall management of wiring (wire) geometry and coating surface is very important to solve the problem of skin effect related to microwave frequency and realize these specifications.

4. The protruding lead has tap inductance, and the components with leads shall be avoided. Use surface mount components in high frequency environments.

5. For signal vias, avoid using the via machining (PTH) process on the sensitive board, because this process will lead to lead inductance at the via. 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.

6. Provide abundant ground plane. Molded holes shall be used to connect these grounding layers to prevent the influence of 3D electromagnetic field on the circuit board.

7. Non electrolytic nickel plating or gold dipping process shall be selected, and HASL method shall not be used for electroplating. This electroplated surface can provide better skin effect for high-frequency current (Fig. 2). In addition, this highly weldable coating requires fewer leads, which helps to reduce environmental pollution.

8. The solder resist layer can prevent the flow of solder paste. However, due to the uncertainty of thickness and the uncertainty of insulation performance, the whole plate surface is covered with solder resist material, which will lead to great changes in electromagnetic energy in microstrip design. Solderdam is generally used as the solder resist layer.

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, using copper-backed coplanar microstrip design is more economical than stripline design. You can discuss it with them to get better suggestions. Our engineers may not be used to thinking about cost, but their suggestions are also quite helpful. Now we should try our best to train young engineers who are not familiar with RF effect and lack experience in dealing with RF effect, which will be a long-term work.

In addition, other solutions can be adopted, such as improving the computer type to have RF effect processing capability.

c. PCB interconnection with external devices

It can now be considered that we have solved all the signal management problems on the board and on the interconnection of various 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 progress (Figure 3). In addition, take a look at the electromagnetic field shown in Figure 4 below. In this case, we manage the conversion from microstrip to coaxial cable. In coaxial cable, the ground wire layer is ring interleaved and evenly spaced. In a microstrip, the ground plane is below the active line. This introduces some edge effects that need to be understood, predicted and considered in the design. Of course, this mismatch will also lead to back loss. This mismatch must be reduced to avoid noise and signal interference.

The management of impedance problem in circuit board is not a negligible design problem. The impedance starts from the surface of the circuit board, then passes through a solder joint to the connector and ends at the coaxial cable. Since the impedance varies with frequency, the higher the frequency, the more difficult it is to manage the impedance. The problem of using higher frequency to transmit signals on broadband seems to be the main problem in the PCB design.