PCB Blog - Methods to Reduce RF Effects in PCB Board Interconnect Design

The interconnection of PCB board system includes a chip to the circuit board, interconnection within PCB board, and three types of interconnection between PCB board and external devices. In RF design, the electromagnetic characteristics at the interconnection point 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 mounting methods, isolation of wiring, measures to reduce lead inductance, and many more. There are currently signs that the frequency of printed circuit board designs is getting higher and higher. As data rates continue to increase, the bandwidth required for data transfer also pushes the upper limit of signal frequencies to 1 GHz and beyond. This high-frequency signaling technology, while well beyond mmWave technology (30GHz), does involve RF and low-end microwave technology as well.

RF engineering methods must be able to handle the stronger electromagnetic field effects that typically occur at higher frequency frequencies. These electromagnetic fields can induce signals on adjacent signal lines or PCB board traces, cause unwanted crosstalk (interference and total noise), and impair system performance. Return loss is primarily caused by impedance mismatches and can have the same effect on the signal as additive noise and interference. High return loss has two negative effects: 1. Signal reflections back to the source add noise to the system, making it more difficult for the receiver to distinguish the noise from the signal; 2. Any reflected signal basically degrades the signal quality because the input signal shape has changed. Although digital systems are very fault-tolerant because they only deal with 1s and 0s, the harmonics generated when a high-speed pulse rises can cause a weaker signal at higher frequencies. Although forward error correction techniques can eliminate some negative effects, part of the system bandwidth is used to transmit redundant data, resulting in reduced system performance. A better solution is to let RF effects help rather than detract from signal integrity. Returns at recommended digital system frequencies (usually poorer data points)

The total loss is -25dB, which corresponds to a VSWR of 1.1.

The goal of PCB board design is to be smaller, faster, and less expensive. For RF PCB boards, high-speed signals sometimes limit the miniaturization of PCB board designs. At present, the main methods to solve the problem of crosstalk are ground plane management, spacing between wiring, and reducing stud capacitance. The main way to reduce return loss is through impedance matching. This method includes effective management of insulating materials and isolation of active signal lines and ground lines, especially between signal lines and ground where state transitions occur. Since the interconnection point is the weakest link in the circuit chain, in RF design, the electromagnetic properties at the interconnection point are the main problems faced by the engineering design, and each interconnection point should be examined and the existing problems should be solved. The interconnection of the circuit board system includes three types of interconnection: chip to the circuit board, interconnection within the PCB board, and signal input/output between the PCB board and external devices.

Chip-to-PCB Interconnection

The Pentium IV and high-speed chips with a large number of I/O interconnect 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 excitement at the Near-GHz Interconnect Symposium (www.az.ww.com) is that methods for dealing with the growing number and frequency of I/Os are well known. The main problem with chip-to-PCB interconnection is that the interconnection density is so high that the basic structure of the PCB material becomes the limiting factor for interconnection density growth. An innovative solution was presented at the meeting, using a local wireless transmitter inside the chip to transmit data to an adjacent circuit board. Whether or not this solution works, it was clear to the attendees that IC design techniques have far surpassed PCB board design techniques when it comes to high-frequency applications. The skills and methods for high-frequency PCB board design are as follows:

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

2) A high-performance insulating circuit board whose insulation constant value is strictly controlled according to the level should be used. This approach facilitates the efficient management of electromagnetic fields between insulating materials and adjacent wiring.

3) It is necessary to improve the PCB board design specifications for high-precision etching. Consider specifying a total error of +/- 0.0007 inches in line width, managing the undercut and cross-section of the wiring shape, and specifying the wiring sidewall plating conditions. Overall management of wiring (conductor) geometry and coating surface is important to address skin effect issues associated with microwave frequencies and to achieve these specifications.

4) There is a tap inductance in the protruding leads, and the use of components with leads should be avoided. For high-frequency environments, use surface mount components.

5) For signal vias, avoid using the via processing (pth) process on the sensitive board because this process will cause 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) To provide a rich ground plane. These ground planes are connected together with molded holes to prevent the effects of 3D electromagnetic fields on the board.

7) To choose electroless nickel plating or immersion gold plating process, do not use the HASL method for electroplating. This plated surface provides a better skin effect for high-frequency currents. In addition, this highly solderable coating requires fewer leads, helping to reduce environmental pollution.

8) Solder mask prevents the flow of solder paste. However, covering the entire board surface with solder mask material will result in large variations in the electromagnetic energy in the microstrip design due to thickness uncertainty and unknown insulating properties. A solder dam is generally used as a solder mask.

If you are unfamiliar with these methods, consult an experienced design engineer who has worked on military microwave circuit boards. You can also discuss with them the price range you can afford. For example, a copper-backed coplanar microstrip design is more economical than a stripline design, and you can discuss this with them for a better build. 's engineers may not be used to thinking about cost, but their advice can be quite helpful. Trying now to train young engineers who are unfamiliar with RF effects and inexperienced in dealing with RF effects will be a long-term endeavor. In addition, other solutions are available, such as retrofitting the computer with the ability to handle RF effects. It can now be considered that we have solved all the signal management issues on the board and on the interconnection of the various discrete components. So how do you solve the signal input/output problem from the circuit board to the wires that connect to the remote device? Trompeter Electronics, an innovator in coaxial cable technology, is working to solve this problem and has made some important strides. Also, look at the electromagnetic fields given. In this case, we manage the transition between microstrip and coax. In a coaxial cable, the ground planes are interwoven in a ring and evenly spaced. In microstrip, the ground plane is below the active line on PCB board.