PCB Blog - Tips for reducing RF effects in PCB interconnect design

This article will introduce various techniques for the design of the three types of interconnections between the chip-to-board, the interconnection within the PCB board, and the PCB and external devices, including device mounting, wiring isolation, and measures to reduce lead inductance, etc., to help designers reduce RF effects in PCB board interconnect design. The interconnection of the circuit board system includes: chip to circuit board, interconnection within the PCB board, and three types of interconnection between the PCB board and external devices. In RF design, the electromagnetic characteristics at the interconnection point is 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, wiring isolation, and measures to reduce lead inductance etc. 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 or higher. 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 losses have two negative effects:
1) The reflection of the signal 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 basically degrades the signal quality because the shape of the input signal changes.
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. The recommended total return loss at digital system frequencies (usually poor data points) is -25dB, which equates to a VSWR of 1.1. The goal of PCB board design is to be smaller, faster and less expensive. For RFPCB boards, high-speed signals sometimes limit the miniaturization of PCB board designs. Currently, the main solution to the problem of crosstalk is ground plane management, spacing between traces and reducing lead inductance. 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 weak 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 interconnections, such as chip-to-circuit board, interconnection within the PCB, and signal input/output between the PCB and external devices.

1. The interconnection between the chip and the PCB board
Pentium IV and high-speed chips with a large number of input/output 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: The way to deal with the ever-increasing number and frequency of I/Os is well known. The main problem with chip-to-PCB interconnection is that the interconnection density is too high to cause the basic structure of the PCB material to be the limiting factor for interconnection density growth.

2. Interconnection in the PCB
The skills and methods for high-frequency PCB board design are as follows:
2.1 The corner of the transmission line should be 45° to reduce the return loss;
2.2 The high-performance insulating circuit board whose insulation constant value is strictly controlled according to the level shall be adopted. This approach facilitates efficient management of electromagnetic fields between insulating materials and adjacent wiring.
2.3 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 undercuts and cross sections of the wiring shape, and specifying wiring sidewall plating conditions. Overall management of wiring (conductor) geometry and coating surfaces is important to address skin effect issues associated with microwave frequencies and to achieve these specifications.
2.4 There is a tap inductance on the protruding leads, so avoid using leaded components. For high frequency environments, use surface mount components.
2.5 For signal vias, avoid using the via processing (pth) process on sensitive boards 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.
2.6 To provide a rich ground plane. Molded vias are used to connect these ground planes to prevent the effects of 3D electromagnetic fields on the board.
2.7 To choose electroless nickel plating or immersion gold plating process, do not use HASL method for electroplating. This plated surface provides better skin effect for high frequency currents. In addition, this highly solderable coating requires fewer leads, helping to reduce environmental pollution.
2.8 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. Welding dams are generally used as solder mask layers.
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 coplanar microstrip design with copper on the back is more economical than a stripline design, and you can discuss this with them for better advice. of engineers may not be accustomed 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.

3. Interconnection between PCB board and external devices
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? 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. This introduces certain edge effects that need to be understood, predicted and taken into account at design time. Of course, this mismatch also results in return loss, which must be reduced to avoid noise and signal interference. The management of impedance issues within the board is not a design issue that can be ignored. Impedance starts at the surface of the board and travels through a solder joint to the connector, terminating at the coaxial cable. Since impedance varies with frequency, the higher the frequency, the more difficult impedance management becomes. The problem of using higher frequencies to transmit signals over broadband appears to be a major problem in PCB board design.