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

There are signs that printed circuit boards are being designed with increasing frequency. As data rates continue to increase, the bandwidth required for data transmission also pushes the signal frequency ceiling to 1GHz or higher. This high frequency signal technology, although far beyond the millimeter wave technology (30GHz), does involve RF and low-end microwave technology.

RF engineering design methods must be able to handle the stronger electromagnetic field effects that are typically generated at higher frequencies. These electromagnetic fields can induce signals on adjacent signal lines or PCB lines, causing undesirable crosstalk (interference and total noise) and harming system performance. Backloss is mainly caused by impedance mismatch, which has the same effect 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 noise of the system, making it more difficult for the receiver to distinguish noise from signal; 2. 2. Any reflected signal will essentially degrade the quality of the signal because the shape of the input signal changes.

Although digital systems are very fault tolerant because they only deal with 1 and 0 signals, the harmonics generated when the pulse is rising at high speed cause the signal to be weaker at higher frequencies. Although forward error correction can eliminate some of the negative effects, part of the system bandwidth is used to transmit redundant data, resulting in performance degradation. A better solution is to have RF effects that help rather than detract from signal integrity. It is recommended that the total return loss at the highest frequency of a digital system (usually a poor data point) be -25dB, equivalent to a VSWR of 1.1.

PCB design aims to be smaller, faster and less costly. For RF PCBS, high-speed signals sometimes limit the miniaturization of PCB designs. At present, the main method to solve the problem of cross-talk is ground 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 insulation materials and isolation of active signal lines and ground lines, especially between the state of the signal line and ground.

Because the interconnect is the weakest link in the circuit chain, in RF design, the electromagnetic properties of the interconnect point is the main problem facing engineering design, each interconnect point should be investigated and the existing problems solved. Circuit board interconnection includes chip-to-circuit board interconnection, PCB interconnection and signal input/output interconnection between PCB and external devices.

Interconnection between chip and PCB board

The PenTIum IV and high-speed chips containing a large number of input/output interconnects are already available. As for the chip itself, its performance is reliable, and the processing rate has been able to reach 1GHz. One of the most exciting aspects of the recent GHz Interconnect symposium (www.az.ww. Com) is that approaches to dealing with the ever-increasing volume and frequency of I/O are well known. The main problem of interconnect between chip and PCB is that the density of interconnect is too high. An innovative solution was presented that uses a local wireless transmitter inside the chip to transmit data to a nearby circuit board.

Whether or not this solution works, it was clear to the attendees that IC design technology is far ahead of PCB design technology for hf applications.

PCB interconnect

The techniques and methods for hf PCB design are as follows:

1. A 45° Angle should be used for the transmission line corner to reduce the return loss (FIG. 1);

2 insulation constant value according to the level of strictly controlled high-performance insulating circuit board. This method is beneficial for effective management of electromagnetic field between insulating material and adjacent wiring.

3. PCB design specifications for high precision etching should be improved. Consider specifying a total line width error of +/-0.0007 inches, managing undercut and cross sections of wiring shapes and specifying wiring side wall plating conditions. Overall management of wiring (wire) geometry and coating surfaces is important to address skin effects related to microwave frequencies and to implement these specifications.

4. There is tap inductance in protruding leads. Avoid using components with leads. For high frequency environments, it is best to use surface mounted components.

5. For signal through holes, avoid using the PTH process on the sensitive plate, as this process can cause lead inductance at the through hole.

6. Provide abundant ground layers. Moulded holes are used to connect these grounding layers to prevent 3d electromagnetic fields from affecting the circuit board.

7. To choose non-electrolysis nickel plating or immersion gold plating process, do not use HASL plating method. This electroplated surface provides a better skin effect for high-frequency currents (Figure 2). In addition, this highly weldable coating requires fewer leads, helping to reduce environmental pollution.

8. Solder resistance layer can prevent solder paste from flowing. However, due to the uncertainty of thickness and unknown insulation performance, covering the entire plate surface with solder resistance material will lead to a large change in electromagnetic energy in microstrip design. Generally, solder dam is used as solder resistance layer.

If you are not familiar with these methods, consult an experienced design engineer who has worked on microwave circuit boards for the military. You can also discuss with them what price range you can afford. For example, it is more economical to use a copper-backed coplanar microstrip design than a strip design. Discuss this with them to get a better idea. Good engineers may not be used to thinking about cost, but their advice can be quite helpful. It will be 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 be adopted, such as improving the computer model to be able to handle RF effects.

PCB interconnect with external devices

We can now assume that we have solved all signal management problems on the board and on the interconnections of discrete components. So how do you 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 on this problem and has made some important progress (Figure 3). Also, 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 cables, the ground layers are interlaced in rings and evenly spaced. In microbelts, the grounding layer is below the active line. This introduces certain edge effects that need to be understood, predicted, and considered at design time. Of course, this mismatch can also lead to backloss and must be minimized to avoid noise and signal interference.

The management of the internal impedance problem is not a design problem that can be ignored. The impedance starts at the surface of the circuit board, passes through a solder joint to the joint, and ends at the coaxial cable. Because 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 appears to be the main design problem.

This paper summarizes

PCB platform technology needs continuous improvement to meet the requirements of IC designers. Hf signal management in PCB design and signal input/output management on PCB board need continuous improvement. Whatever exciting innovations are coming, I think bandwidth is going to get higher and higher, and using high frequency signals is going to be a prerequisite for that growth.