The interconnection of circuit board systems includes three types of interconnections: chip to circuit board, PCB internal interconnection, and PCB to external devices. In RF design, the electromagnetic characteristics at the interconnection points are one of the main problems faced by engineering design. This article introduces various techniques for the above three types of interconnection design, including device installation methods, wiring isolation, and measures to reduce lead inductance.

There are currently indications that the frequency of printed circuit board design is increasing. With the continuous growth of data rate, the bandwidth required for data transmission also drives the signal frequency limit to reach 1GHz or even higher. This high-frequency signal technology, although far beyond the range of millimeter wave technology (30GHz), does indeed involve RF and low-end microwave technology.

The RF engineering design method must be able to handle the strong electromagnetic field effects typically generated in higher frequency bands. These electromagnetic fields can induce signals on adjacent signal lines or PCB lines, causing annoying crosstalk (interference and total noise) and damaging system performance. Return loss is mainly caused by impedance mismatch, which has the same impact on the signal as additive noise and interference.

There are two negative effects of high return loss: 1 Signal reflection back to the signal source will increase system noise, making it more difficult for the receiver to distinguish between noise and signal; 2. Any reflected signal will basically reduce the signal quality because the shape of the input signal changes.

Although digital systems only process 1 and 0 signals and have excellent fault tolerance, the harmonics generated during high-speed pulse rise can lead to weaker signals with higher frequencies. Although forward error correction technology can eliminate some negative effects, a portion of the system's bandwidth is used to transmit redundant data, leading to a decrease in system performance. A better solution is to make the RF effect beneficial rather than damaging the integrity of the signal. It is recommended that the total return loss at the highest frequency of the digital system (usually poor data points) be -25dB, which is equivalent to a VSWR of 1.1.

The goal of PCB design is to be smaller, faster, and more cost-effective. For RF PCBs, high-speed signals sometimes limit the miniaturization of PCB design. At present, the main method to solve the problem of crosstalk is to manage the grounding layer, set intervals between wiring, and reduce the lead inductance (study capacitance). The main method to reduce return loss is to perform impedance matching. This method includes effective management of insulation materials and isolation of active signal lines and ground wires, especially when there is a state jump between signal lines and ground.

Due to the fact that interconnection points are the weakest link in the circuit chain, the electromagnetic properties at the interconnection points are the main problem faced by engineering design in RF design. It is necessary to examine each interconnection point and solve the existing problems. The interconnection of circuit board systems includes three types of interconnections: chip to circuit board, intra PCB interconnection, and signal input/output between PCBs and external devices.

1、 Interconnection between chips and PCB boards

Pentium IV and high-speed chips containing a large number of input/output interconnect points have been introduced. As far as the chip itself is concerned, its performance is reliable and the processing speed can already reach 1GHz. At the recent GHz Interconnection Symposium (www.az.ww. com), the most exciting thing is that the methods for dealing with the increasing number and frequency of I/Os have become widely known. The main problem with the interconnection between chips and PCBs is that high interconnect density can lead to the basic structure of PCB materials becoming a limiting factor for the growth of interconnect density. An innovative solution was proposed at the meeting, which is to use local wireless transmitters inside the chip to transmit data to adjacent circuit boards.

Regardless of whether this plan is effective or not, the attendees are very clear: in terms of high-frequency applications, IC design technology is far ahead of PCB design technology.

2、 PCB board interconnection

The techniques and methods for designing high-frequency PCBs are as follows:

1. The corner of the transmission line should be at a 45 ° angle to reduce return loss (Figure 1);

2. High performance insulated circuit boards with strict hierarchical control of insulation constant values should be used. This method is beneficial for effective management of the electromagnetic field between insulation materials and adjacent wiring.

3. It is necessary to improve the PCB design specifications related to high-precision etching. Consider specifying a total line width error of+/-0.0007 inches, managing the undercut and cross-section of the wiring shape, and specifying the plating conditions for the wiring sidewalls. Overall management of the geometry and coating surface of wiring (wires) is crucial for addressing skin effect issues related to microwave frequency and achieving these specifications.

4. If there is a tap inductance in the protruding lead, avoid using components with leads. In high-frequency environments, it is best to use surface mount components.

5. For signal via, it is important to avoid using via machining (PTH) technology on sensitive boards, as this process can lead to the generation of lead inductance at the via. When a through hole on a 20 layer board is used to connect layers 1 to 3, the lead inductance can affect layers 4 to 19.

6. Provide rich grounding layers. Molded holes should be used to connect these grounding layers together to prevent the impact of 3D electromagnetic fields on the circuit board.

7. Choose non electrolytic nickel plating or immersion gold plating process, and do not use HASL method for electroplating. This electroplated surface can provide better skin effect for high-frequency currents (Figure 2). In addition, this high solderability coating requires fewer leads, which helps to reduce environmental pollution.

8. The solder mask layer can prevent the flow of solder paste. However, due to thickness uncertainty and unknown insulation performance, covering the entire board surface with solder mask material will result in significant changes in electromagnetic energy in microstrip design. Generally, a solder dam is used as the solder mask layer.

If you are not familiar with these methods, you can consult experienced design engineers who have worked 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 designs is more economical than strip line designs, and you can discuss this with them to get better suggestions. Excellent engineers may not be accustomed to considering cost issues, but their advice is also quite helpful. It will be a long-term job to train young engineers who are not familiar with RF effects and lack experience in handling them.

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

3、 Interconnection between PCB and external devices

It can now be considered that we have solved all signal management issues 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 committed to solving this problem and has made some significant progress (Figure 3). Also, take a look at the electromagnetic field shown in Figure 4. In this case, we manage the conversion between microstrip and coaxial cables. In coaxial cables, the ground wire layer is ring-shaped interwoven and evenly spaced. In microstrip, the grounding layer is below the active line. This introduces certain edge effects that need to be understood, predicted, and considered during design. Of course, this mismatch can also lead to backloss, and it is necessary to minimize this mismatch to avoid generating noise and signal interference.

The management of impedance issues within circuit boards 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. Due to the variation of impedance with frequency, impedance management becomes more difficult at higher frequencies. The problem of using higher frequencies to transmit signals over broadband appears to be a major challenge in the design.